In your own words, define or describe what you already know about photosynthesis

Photosynthesis is the process used by plants, algae and certain bacteria to harness energy from sunlight and turn it into chemical energy. Here, we describe the general principles of photosynthesis and highlight how scientists are studying this natural process to help develop clean fuels and sources of renewable energy.

Contents

Types of photosynthesis

There are two types of photosynthetic processes: oxygenic photosynthesis and anoxygenic photosynthesis. The general principles of anoxygenic and oxygenic photosynthesis are very similar, but oxygenic photosynthesis is the most common and is seen in plants, algae and cyanobacteria.

During oxygenic photosynthesis, light energy transfers electrons from water (H2O) to carbon dioxide (CO2), to produce carbohydrates. In this transfer, the CO2 is “reduced,” or receives electrons, and the water becomes “oxidized,” or loses electrons. Ultimately, oxygen is produced along with carbohydrates.

Oxygenic photosynthesis functions as a counterbalance to respiration by taking in the carbon dioxide produced by all breathing organisms and reintroducing oxygen to the atmosphere.

On the other hand, anoxygenic photosynthesis uses electron donors other than water. The process typically occurs in bacteria such as purple bacteria and green sulfur bacteria, which are primarily found in various aquatic habitats.

“Anoxygenic photosynthesis does not produce oxygen — hence the name,” said David Baum, professor of botany at the University of Wisconsin-Madison. “What is produced depends on the electron donor. For example, many bacteria use the bad-eggs-smelling gas hydrogen sulfide, producing solid sulfur as a byproduct.”

Though both types of photosynthesis are complex, multistep affairs, the overall process can be neatly summarized as a chemical equation.

Oxygenic photosynthesis is written as follows:

6CO2 + 12H2O + Light Energy → C6H12O6 + 6O2 + 6H2O

Here, six molecules of carbon dioxide (CO2) combine with 12 molecules of water (H2O) using light energy. The end result is the formation of a single carbohydrate molecule (C6H12O6, or glucose) along with six molecules each of breathable oxygen and water.

Similarly, the various anoxygenic photosynthesis reactions can be represented as a single generalized formula:

CO2 + 2H2A + Light Energy → + 2A + H2O

The letter A in the equation is a variable and H2A represents the potential electron donor. For example, A may represent sulfur in the electron donor hydrogen sulfide (H2S), explained Govindjee and John Whitmarsh, plant biologists at the University of Illinois at Urbana-Champaign, in the book “Concepts in Photobiology: Photosynthesis and Photomorphogenesis” (Narosa Publishers and Kluwer Academic, 1999).

Plants need energy from sunlight for photosynthesis to occur. (Image credit: )

The photosynthetic apparatus

The following are cellular components essential to photosynthesis.

Pigments

Pigments are molecules that bestow color on plants, algae and bacteria, but they are also responsible for effectively trapping sunlight. Pigments of different colors absorb different wavelengths of light. Below are the three main groups.

  • Chlorophylls: These green-colored pigments are capable of trapping blue and red light. Chlorophylls have three subtypes, dubbed chlorophyll a, chlorophyll b and chlorophyll c. According to Eugene Rabinowitch and Govindjee in their book “Photosynthesis”(Wiley, 1969), chlorophyll a is found in all photosynthesizing plants. There is also a bacterial variant aptly named bacteriochlorophyll, which absorbs infrared light. This pigment is mainly seen in purple and green bacteria, which perform anoxygenic photosynthesis.
  • Carotenoids: These red, orange or yellow-colored pigments absorb bluish-green light. Examples of carotenoids are xanthophyll (yellow) and carotene (orange) from which carrots get their color.
  • Phycobilins: These red or blue pigments absorb wavelengths of light that are not as well absorbed by chlorophylls and carotenoids. They are seen in cyanobacteria and red algae.

Plastids

Photosynthetic eukaryotic organisms contain organelles called plastids in their cytoplasm. The double-membraned plastids in plants and algae are referred to as primary plastids, while the multiple-membraned variety found in plankton are called secondary plastids, according to an articlein the journal Nature Education by Cheong Xin Chan and Debashish Bhattacharya, researchers at Rutgers University in New Jersey.

Plastids generally contain pigments or can store nutrients. Colorless and nonpigmented leucoplasts store fats and starch, while chromoplasts contain carotenoids and chloroplasts contain chlorophyll, as explained in Geoffrey Cooper’s book, “The Cell: A Molecular Approach” (Sinauer Associates, 2000).

Photosynthesis occurs in the chloroplasts; specifically, in the grana and stroma regions. The grana is the innermost portion of the organelle; a collection of disc-shaped membranes, stacked into columns like plates. The individual discs are called thylakoids. It is here that the transfer of electrons takes place. The empty spaces between columns of grana constitute the stroma.

Chloroplasts are similar to mitochondria, the energy centers of cells, in that they have their own genome, or collection of genes, contained within circular DNA. These genes encode proteins essential to the organelle and to photosynthesis. Like mitochondria, chloroplasts are also thought to have originated from primitive bacterial cells through the process of endosymbiosis.

“Plastids originated from engulfed photosynthetic bacteria that were acquired by a single-celled eukaryotic cell more than a billion years ago,” Baum told Live Science. Baum explained that the analysis of chloroplast genes shows that it was once a member of the group cyanobacteria, “the one group of bacteria that can accomplish oxygenic photosynthesis.”

In their 2010 article, Chan and Bhattacharya make the point that the formation of secondary plastids cannot be well explained by endosymbiosis of cyanobacteria, and that the origins of this class of plastids are still a matter of debate.

Antennae

Pigment molecules are associated with proteins, which allow them the flexibility to move toward light and toward one another. A large collection of 100 to 5,000 pigment molecules constitutes “antennae,” according to an article by Wim Vermaas, a professor at Arizona State University. These structures effectively capture light energy from the sun, in the form of photons.

Ultimately, light energy must be transferred to a pigment-protein complex that can convert it to chemical energy, in the form of electrons. In plants, for example, light energy is transferred to chlorophyll pigments. The conversion to chemical energy is accomplished when a chlorophyll pigment expels an electron, which can then move on to an appropriate recipient.

Reaction centers

The pigments and proteins, which convert light energy to chemical energy and begin the process of electron transfer, are known as reaction centers.

The photosynthetic process

The reactions of plant photosynthesis are divided into those that require the presence of sunlight and those that do not. Both types of reactions take place in chloroplasts: light-dependent reactions in the thylakoid and light-independent reactions in the stroma.

Light-dependent reactions (also called light reactions): When a photon of light hits the reaction center, a pigment molecule such as chlorophyll releases an electron.

“The trick to do useful work, is to prevent that electron from finding its way back to its original home,” Baum told Live Science. “This is not easily avoided, because the chlorophyll now has an ‘electron hole’ that tends to pull on nearby electrons.”

The released electron manages to escape by traveling through an electron transport chain, which generates the energy needed to produce ATP (adenosine triphosphate, a source of chemical energy for cells) and NADPH. The “electron hole” in the original chlorophyll pigment is filled by taking an electron from water. As a result, oxygen is released into the atmosphere.

Light-independent reactions (also called dark reactions and known as the Calvin cycle): Light reactions produce ATP and NADPH, which are the rich energy sources that drive dark reactions. Three chemical reaction steps make up the Calvin cycle: carbon fixation, reduction and regeneration. These reactions use water and catalysts. The carbon atoms from carbon dioxide are “fixed,” when they are built into organic molecules that ultimately form three-carbon sugars. These sugars are then used to make glucose or are recycled to initiate the Calvin cycle again.

This June 2010 satellite photo shows ponds growing algae in southern California. (Image credit: PNNL, QuickBird satellite)

Photosynthesis in the future

Photosynthetic organisms are a possible means to generate clean-burning fuels such as hydrogen or even methane. Recently, a research group at the University of Turku in Finland, tapped into the ability of green algae to produce hydrogen. Green algae can produce hydrogen for a few seconds if they are first exposed to dark, anaerobic (oxygen-free) conditions and then exposed to light The team devised a way to extend green algae’s hydrogen production for up to three days, as reported in their 2018 study published in the journal Energy & Environmental Science.

Scientists have also made advances in the field of artificial photosynthesis. For instance, a group of researchers from the University of California, Berkeley, developed an artificial system to capture carbon dioxide using nanowires, or wires that are a few billionths of a meter in diameter. The wires feed into a system of microbes that reduce carbon dioxide into fuels or polymers by using energy from sunlight. The team published its design in 2015 in the journal Nano Letters.

In 2016, members of this same group published a study in the journal Science that described another artificial photosynthetic system in which specially engineered bacteria were used to create liquid fuels using sunlight, water and carbon dioxide. In general, plants are only able to harness about one percent of solar energy and use it to produce organic compounds during photosynthesis. In contrast, the researchers’ artificial system was able to harness 10 percent of solar energy to produce organic compounds.

Continued research of natural processes, such as photosynthesis, aids scientists in developing new ways to utilize various sources of renewable energy. Seeing as sunlight, plants and bacteria are all ubiquitous, tapping into the power of photosynthesis is a logical step for creating clean-burning and carbon-neutral fuels.

Additional resources:

  • University of California, Berkeley: Photosynthetic Pigments
  • Arizona State University: An Introduction to Photosynthesis and Its Applications
  • University of Illinois at Urbana-Champaign: What Is Photosynthesis?

Photosynthesis for Kids

What is Photosynthesis?

The word photosynthesis can be separated to make two smaller words:

“photo” which means light

“synthesis” which means putting together

Plants need food but they do not have to wait on people or animals to provide for them. Most plants are able to make their own food whenever they need it. This is done using light and the process is called photosynthesis.

Photosynthesis is the process by which plants make their own food. We will add more details to this definition after making a few things clear as you will see below.

What is needed for Photosynthesis?

To make food, plants need not just one but all of the following:

  • carbon dioxide
  • water
  • sunlight

Let’s take a look at how these are collected by plants.

  • Carbon dioxide from the air passes through small pores (holes) in the leaves. These pores are called stomata.
  • Water is absorbed by the roots and passes through vessels in the stem on its way to the leaves.
  • Sunlight is absorbed by a green chemical in the leaves.

What happens during Photosynthesis?

The photosynthesis process takes place in the leaves of plants. The leaves are made up of very small cells. Inside these cells are tiny structures called chloroplasts. Each chloroplast contains a green chemical called chlorophyll which gives leaves their green color.

  • Chlorophyll absorbs the sun’s energy.
  • It is this energy that is used to split water molecules into hydrogen and oxygen.
  • Oxygen is released from the leaves into the atmosphere.
  • Hydrogen and carbon dioxide are used to form glucose or food for plants.

Some of the glucose is used to provide energy for the growth and development of plants while the rest is stored in leaves, roots or fruits for later use by plants.

Here is the process in greater detail:

Photosynthesis occurs in two stages commonly known as Light dependent Reactions and the Calvin Cycle.

Light dependent Reactions

Light dependent reactions occur in the thylakoid membrane of the chloroplasts and take place only when light is available. During these reactions light energy is converted to chemical energy.

  • Chlorophyll and other pigments absorb energy from sunlight. This energy is transferred to the photosystems responsible for photosynthesis.
  • Water is used to provide electrons and hydrogen ions but also produces oxygen. Do you remember what happens to the oxygen?
  • The electrons and hydrogen ions are used to create ATP and NADPH. ATP is an energy storage molecule. NADPH is an electron carrier/donor molecule. Both ATP and NADPH will be used in the next stage of photosynthesis.

Details about the flow of electrons through Photosystem II, b6-f complex, Photosystem I and NADP reductase have not been included here but can be found under The Process of Photosynthesis in Plants.

The Calvin Cycle

The Calvin Cycle reactions occur in the stroma of the chloroplasts. Although these reactions can take place without light, the process requires ATP and NADPH which were created using light in the first stage. Carbon dioxide and energy from ATP along with NADPH are used to form glucose.

More details about the formation of sugars can be found under the Process of Photosynthesis in Plants.

What have you learned so far?

You already know that plants need carbon dioxide, water and sunlight to make their food. You also know that the food they make is called glucose. In addition to glucose, plants also produce oxygen. This information can be written in a word equation as shown below.

The equation below is the same as the one above but it shows the chemical formula for carbon dioxide, water, glucose and oxygen.

Now back to the definition… Earlier you learned that photosynthesis is the process by which plants make their own food. Now that we know what plants need to make food, we can add that information as shown below.

Photosynthesis is the process by which plants make their own food using carbon dioxide, water and sunlight.

What does Photosynthesis produce?

Photosynthesis is important because it provides two main things:

  • food
  • oxygen

Some of the glucose that plants produce during photosynthesis is stored in fruits and roots. This is why we are able to eat carrots, potatoes, apples, water melons and all the others. These foods provide energy for humans and animals.

Oxygen that is produced during photosynthesis is released into the atmosphere. This oxygen is what we breathe and we cannot live without it.

While it is important that photosynthesis provides food and oxygen, its impact on our daily lives is far more extensive. Photosynthesis is so essential to life on earth that most living organisms, including humans, cannot survive without it.

All of our energy for growth, development and physical activity comes from eating food from plants and animals. Animals obtain energy from eating plants. Plants obtain energy from glucose made during photosynthesis.

Our major sources of energy such as natural gas, coal and oil were made millions of years ago from the remains of dead plants and animals which we already know got their energy from photosynthesis.

Photosynthesis is also responsible for balancing oxygen and carbon dioxide levels in the atmosphere. Plants absorb carbon dioxide from the air and release oxygen during the process of photosynthesis.

Photosynthesis

plants: photosynthesisThe location, importance, and mechanisms of photosynthesis.Encyclopædia Britannica, Inc.See all videos for this article

Photosynthesis, the process by which green plants and certain other organisms transform light energy into chemical energy. During photosynthesis in green plants, light energy is captured and used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds.

Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.Encyclopædia Britannica, Inc. Top Questions

Why is photosynthesis important?

Photosynthesis is critical for the existence of the vast majority of life on Earth. It is the way in which virtually all energy in the biosphere becomes available to living things. As primary producers, photosynthetic organisms form the base of Earth’s food webs and are consumed directly or indirectly by all higher life-forms. Additionally, almost all the oxygen in the atmosphere is due to the process of photosynthesis. If photosynthesis ceased, there would soon be little food or other organic matter on Earth, most organisms would disappear, and Earth’s atmosphere would eventually become nearly devoid of gaseous oxygen.

What is the basic formula for photosynthesis?

The process of photosynthesis is commonly written as: 6CO2 + 6H2O → C6H12O6 + 6O2. This means that the reactants, six carbon dioxide molecules and six water molecules, are converted by light energy captured by chlorophyll (implied by the arrow) into a sugar molecule and six oxygen molecules, the products. The sugar is used by the organism, and the oxygen is released as a by-product.

Read more below: General characteristics: Overall reaction of photosynthesis

Which organisms can photosynthesize?

The ability to photosynthesize is found in both eukaryotic and prokaryotic organisms. The most well-known examples are plants, as all but a very few parasitic or mycoheterotrophic species contain chlorophyll and produce their own food. Algae are the other dominant group of eukaryotic photosynthetic organisms. All algae, which include massive kelps and microscopic diatoms, are important primary producers. Cyanobacteria and certain sulfur bacteria are photosynthetic prokaryotes, in whom photosynthesis evolved. No animals are thought to be independently capable of photosynthesis, though the emerald green sea slug can temporarily incorporate algae chloroplasts in its body for food production.

It would be impossible to overestimate the importance of photosynthesis in the maintenance of life on Earth. If photosynthesis ceased, there would soon be little food or other organic matter on Earth. Most organisms would disappear, and in time Earth’s atmosphere would become nearly devoid of gaseous oxygen. The only organisms able to exist under such conditions would be the chemosynthetic bacteria, which can utilize the chemical energy of certain inorganic compounds and thus are not dependent on the conversion of light energy.

Energy produced by photosynthesis carried out by plants millions of years ago is responsible for the fossil fuels (i.e., coal, oil, and gas) that power industrial society. In past ages, green plants and small organisms that fed on plants increased faster than they were consumed, and their remains were deposited in Earth’s crust by sedimentation and other geological processes. There, protected from oxidation, these organic remains were slowly converted to fossil fuels. These fuels not only provide much of the energy used in factories, homes, and transportation but also serve as the raw material for plastics and other synthetic products. Unfortunately, modern civilization is using up in a few centuries the excess of photosynthetic production accumulated over millions of years. Consequently, the carbon dioxide that has been removed from the air to make carbohydrates in photosynthesis over millions of years is being returned at an incredibly rapid rate. The carbon dioxide concentration in Earth’s atmosphere is rising the fastest it ever has in Earth’s history, and this phenomenon is expected to have major implications on Earth’s climate.

Requirements for food, materials, and energy in a world where human population is rapidly growing have created a need to increase both the amount of photosynthesis and the efficiency of converting photosynthetic output into products useful to people. One response to those needs—the so-called Green Revolution, begun in the mid-20th century—achieved enormous improvements in agricultural yield through the use of chemical fertilizers, pest and plant-disease control, plant breeding, and mechanized tilling, harvesting, and crop processing. This effort limited severe famines to a few areas of the world despite rapid population growth, but it did not eliminate widespread malnutrition. Moreover, beginning in the early 1990s, the rate at which yields of major crops increased began to decline. This was especially true for rice in Asia. Rising costs associated with sustaining high rates of agricultural production, which required ever-increasing inputs of fertilizers and pesticides and constant development of new plant varieties, also became problematic for farmers in many countries.

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A second agricultural revolution, based on plant genetic engineering, was forecast to lead to increases in plant productivity and thereby partially alleviate malnutrition. Since the 1970s, molecular biologists have possessed the means to alter a plant’s genetic material (deoxyribonucleic acid, or DNA) with the aim of achieving improvements in disease and drought resistance, product yield and quality, frost hardiness, and other desirable properties. However, such traits are inherently complex, and the process of making changes to crop plants through genetic engineering has turned out to be more complicated than anticipated. In the future such genetic engineering may result in improvements in the process of photosynthesis, but by the first decades of the 21st century, it had yet to demonstrate that it could dramatically increase crop yields.

Another intriguing area in the study of photosynthesis has been the discovery that certain animals are able to convert light energy into chemical energy. The emerald green sea slug (Elysia chlorotica), for example, acquires genes and chloroplasts from Vaucheria litorea, an alga it consumes, giving it a limited ability to produce chlorophyll. When enough chloroplasts are assimilated, the slug may forgo the ingestion of food. The pea aphid (Acyrthosiphon pisum) can harness light to manufacture the energy-rich compound adenosine triphosphate (ATP); this ability has been linked to the aphid’s manufacture of carotenoid pigments.

What Is Photosynthesis: Chlorophyll And Photosynthesis For Kids

What is chlorophyll and what is photosynthesis? Most of us already know the answers to these questions but for kids, this can be unchartered waters. To help kids gain a better understanding of the role of chlorophyll in photosynthesis in plants, keep reading.

What is Photosynthesis?

Plants, just like humans, require food in order to survive and grow. However, a plant’s food looks nothing like our food. Plants are the greatest consumer of solar energy, using power from the sun to mix up an energy rich meal. The process where plants make their own food is known as photosynthesis.

Photosynthesis in plants is an extremely useful process whereby green plants take up carbon dioxide (a toxin) from the air and produce rich oxygen. Green plants are the only living thing on earth that are capable of converting the sun’s energy into food.

Almost all living things are dependent upon the process of photosynthesis for life. Without plants, we would not have oxygen and the animals would have nothing to eat, and neither would

we.

What is Chlorophyll?

The role of chlorophyll in photosynthesis is vital. Chlorophyll, which resides in the chloroplasts of plants, is the green pigment that is necessary in order for plants to convert carbon dioxide and water, using sunlight, into oxygen and glucose.

During photosynthesis, chlorophyll captures the sun’s rays and creates sugary carbohydrates or energy, which allows the plant to grow.

Understanding Chlorophyll and Photosynthesis for Kids

Teaching children about the process of photosynthesis and the importance of chlorophyll is an integral part of most elementary and middle school science curriculums. Although the process is quite complex in its entirety, it can be simplified enough so that younger children can grasp the concept.

Photosynthesis in plants can be compared with the digestive system in that they both break down vital elements to produce energy that is used for nourishment and growth. Some of this energy is used immediately, and some is stored for later use.

Many younger children may have the misconception that plants take in food from their surroundings; therefore, teaching them the process of photosynthesis is vital to them grasping the fact that plants actually gather the raw ingredients necessary to make their own food.

Photosynthesis Activity for Kids

Hands-on activities are the best way to teach kids how the process of photosynthesis works. Demonstrate how the sun is necessary for photosynthesis by placing one bean sprout in a sunny location and one in a dark location.

Both plants should be watered regularly. As students observe and compare the two plants over time, they will see the importance of sunlight. The bean plant in the sun will grow and thrive while the bean plant in the dark will become very sickly and brown.

This activity will demonstrate that a plant cannot make its own food in the absence of sunlight. Have children sketch pictures of the two plants over several weeks and make notes regarding their observations.

Factors Influencing Leaf Chlorophyll Content in Natural Forests at the Biome Scale

Introduction

Photosynthesis is the most important source of energy for plant growth (Mackinney, 1941; Baker, 2008), because chlorophyll (Chl) represents an important pigment for photosynthesis. The photosynthetic reaction is mainly divided into three steps: (1) primary reaction, (2) photosynthetic electron transport and photophosphorylation, and (3) carbon assimilation. Chlorophyll a (Chl a) and chlorophyll b (Chl b) are essential for the primary reaction. Chl a and Chl b absorb sunlight at different wavelengths (Chl a mainly absorbs red-orange light and Chl b mainly absorbs blue-purple light), leading to the assumption that the total amount leaf chlorophyll content (Chl a+b) and allocated ratio (Chl a/b) directly influence the photosynthetic capacity of plants. This assumption has been verified by a controlled experiment using several plant species (Croft et al., 2017). However, to date, it is unclear how leaf Chl content varies among plant species, plant functional groups (PFGs), and communities in natural forests, especially at a large scale.

When considering on the importance of Chl for photosynthesis, plants in the natural community should optimize light absorption and photosynthesis by adjusting the content and ratios of Chl to enhance growth and survival at the long-term evolutionary scale. Certain factors might influence Chl levels. From the perspective of phylogeny, stable traits are the results of long-term adaption and evolution to the external environments. If Chl was a stable trait, the length of evolutionary history should have a constraining effect on it. In other words, Chl should be influenced by phylogeny. Some studies have demonstrated that phyletic evolution significantly influences certain leaf traits, such as element content and wood traits (He et al., 2010; Zhang et al., 2011; Liu et al., 2012; Zhao et al., 2014). If Chl is really influenced by phylogeny, the rule of Chl should be used only after excluding phylogenetic effects by using phylogenetically independent contrasts in future studies.

Alternatively, plants inevitably adjust their own traits to adapt to different environments. Therefore, it is widely considered that plants should adjust Chl (Chl a, Chl b, Chl a+b, and Chl a/b) to adapt to a given environment and optimize photosynthesis. Thus, climate and soils should play important roles in regulating Chl, especially at a large scale. Chlorophyll synthesis requires many elements from soils; thus, soils should influence Chl (Fredeen et al., 1990). The synthesis of chlorophyll needs through a series of enzymatic reactions, with the temperatures that are too high or low inhibiting the enzyme reaction, even destroying the original chlorophyll. The optimum temperature of general plant chlorophyll synthesis is 30°C, DVR enzyme activity peaking at 30°C (Nagata et al., 2005). Thus, temperature also influences chlorophyll synthesis (Wolken et al., 1955). Precipitation might affect the photochemical activity of chloroplasts (Zhou, 2003), with water being the medium used for transporting nutrients in plants, as mineral salts must be dissolved in water to be absorbed by plants. Consequently, chlorophyll synthesis and water are closely related. The lack of water in leaves influences the synthesis of chlorophyll and promotes the decomposition of chlorophyll, and accelerating leaf yellowing. There is also indirect evidence that Chl is jointly controlled by climate and soils; thus, Chl might be an indicative trait for characterizing how plants respond to climate change.

A major challenge is establishing how to link traits and functioning in natural ecosystems; yet, such knowledge is important to predict how ecosystem functioning varies with changing environments (Garnier and Navas, 2012; Reichstein et al., 2014). Because of the importance of Chl on photosynthesis, scientists assumed that a relationship between Chl and gross primary production (GPP) exists, using this relationship for model optimization. For instance, Croft et al. (2017) found that, for several plant species, it is better to use Chl as a proxy to replace photosynthetic efficiency, than the traditional substitute-leaf N content, when establishing models of GPP in forest communities. This approach was innovative and exciting for scientists involved in physiological ecology and macroecology. The reliability of leaf Chl content represents the maximum rate of carboxylation (Vmax), and has the potential for use as a proxy in models in the future (Croft et al., 2017). However, it is difficult to obtain data on Chl in natural communities, especially at a large scale.

In this study, we investigated Chl across 823 plant species from nine forests, extending from tropical forests to cold-temperate forests in eastern China. Using these data, we explored variation in Chl, latitudinal patterns, and underlying influencing factors (climate, soil, and phylogeny). The main objectives of this study were to: (1) investigate how Chl varies in natural forests among different plant species, PFGs, and communities; (2) determine whether there is a latitudinal pattern in Chl; (3) identify the main factors influencing Chl (phylogeny, climate, and soil); and (4) provide large-scale evidence supporting the concept that Chl is a reliable proxy of Vmax in global ecological models.

Materials and Methods

Site Description

The North-South Transect of Eastern China (NSTEC) is the fifteenth standard transect of the International Geosphere-Biosphere Program(IGBP), which is a unique forest belt mainly driven by thermal gradient and encompasses almost all forest types found in the northern hemisphere (Canadell et al., 2002). In this study, nine natural forests along the NSTEC were selected along the 4,200-km transect, including: Jiangfeng (JF), Dinghu (DH), Jiulian (JL), Shennong (SN), Taiyue (TY), Dongling (DL), Changbai (CB), Liangshui (LS), and Huzhong (HZ) (Figure S1). These nine forests span cold-temperate, temperate, subtropical, and tropical forests from 18.7 to 51.8°N. The mean annual temperature (MAT) of these forests ranges from −4.4 to 20.9°C, while mean annual precipitation (MAP) ranges from 481.6 to 2449.0 mm. Soils vary from tropical red soils with low organic matter to brown soils with high organic matter (Song et al., 2016). Detailed geographical information of the region is presented in Table S1.

Field Sampling

The field survey was conducted between July and August of 2013. First, four experimental plots (30 × 40 m) were selected in each forest. The geographical details, plant species composition, and community structure were determined for each plot. Trees were measured within the 30 × 40 m plots. Next, four quadrats (5 × 5 m) of shrubs and four plots (1 × 1 m) of herbs were setup at the four corners of each experimental plot. All common species were identified in each plot, including trees, shrubs, and herbs. Subsequently, we collected more than 30 pieces of leaves from each species, of which four pieces were randomly selected to cut up and determine chlorophyll content from each plant species within the plot. We chose mature and healthy trees, and collected fully expanded, sun-exposed leaves from four individuals of each plant species, which were considered as four repetitions (Zhao et al., 2016). Overall, the leaves of 823 plant species were collected. Soil samples (0–10 cm depth) were randomly collected from 30 to 50 points using a soil sampler (6-cm diameter) in each plot and were combined to form one composite sample in each plot (Tian et al., 2016; Wang et al., 2016; Li et al., 2018).

Measurement of Leaf Chlorophyll Content

Fresh leaves were cleaned to remove soil and other contaminants, and 0.1 g of fresh leaves was used to extract chlorophyll using 95% ethanol, with four replicates for each species of plants. The Chl content (Chl a and Chl b) of the filtered solution was measured using the classical spectrophotometric method with a spectrophotometer (Pharma Spec, UV-1700, Shimadzu, Japan) (Mackinney, 1941; Li et al., 2018).

According to the Lambert-Beer law, the relationship between concentration and optical density is:

D665=83.31 Ca+18.60 Cb (1) D649=4.54 Ca+44.24 Cb (2) G=Ca+Cb (3)

where D665 and D649 are the optical densities of the chlorophyll solution at wavelengths 665 and 649 nm; Ca, Cb, and G are the concentrations of Chl a, Chl b, and total Chl, respectively (g L−1); 83.31 and 18.60 are the specific absorption of Chl a and Chl b at a wavelength of 665 nm; and 24.54 and 44.24 are the specific absorption of Chl a and Chl b at a wavelength of 649 nm.

Chl a (mg g-1)=Ca×50/(1000×0.1) (4) Chl b (mg g-1)=Cb×50/(1000×0.1) (5)

Then, Chl a+b and Chl a/b were calculated (mg g−1, leaf fresh mass, FM) as:

Chl a+b (mg g-1)=G×50/(1000×0.1) (6) Chl a/b=Chl aChl b (7)

Measurement of Soil Parameters

Soil samples were acidified with HNO3 and HF overnight. Then, the samples were digested using a microwave digestion system (Mars X Press Microwave Digestion system, CEM, Matthews, NC, USA) before analyzing soil total phosphorus (STP) content.

Elemental analyzer (Vario Analyzer, Elementar, Germany) was used to measure soil organic carbon (SOC) and soil total nitrogen (STN). Soil pH was determined using a pH meter (Mettler Toledo Delta 320, Switzerland) by using a slurry of soil and distilled water (1: 2.5) (Zhao et al., 2014).

Climate Data

The primary climate variables, including mean annual temperature (MAT) and mean annual precipitation (MAP), were extracted from the climate dataset at a 1 × 1 km spatial resolution. The data were collected at 740 climate stations of the China Meteorological Administration, from 1961 to 2007, using the interpolation software ANUSPLIN (He et al., 2014).

Phylogeny

Eight hundred and twenty-three species of plants were used to construct a phylogenetic tree at the species and family levels. First, we checked the species information in the Plant List (http://www.theplantlist.org/), and confirmed the correct Latin names of each species. Second, we defined a reference phylogenetic tree by using the “phytools” package in R and S. Phylo Maker (Qian and Jin, 2016), and generated the phylogenetic tree in the MAGA 5.1. Finally, using the algorithm BLADJ provided by the software Phylocom (Webb et al., 2008), according to molecular and fossil dating data (Wikström et al., 2001), we calculated the time of each node in the phylogenetic tree (in one million, m letters years, MY). At the family level, according to the evolution of angiosperm families provided by molecular and fossil dating data, we identified the evolution of angiosperm families.

Statistical Analysis

Plant species were divided by plant functional groups (PFGs, trees, shrubs, and herbs), growth forms (coniferous or broadleaved tree), and leaf types (evergreen or deciduous tree). One–way analysis of variance (ANOVA) with the post-hoc Duncan’s multiple comparison was used to test the differences in Chl for different sites, PFGs, and leaf types. We used regression analyses to explore latitudinal patterns of Chl. To analyze the factors influencing Chl, we calculated Spearman’s rank correlation coefficients for 823 plant species across sites, PFGs, and leaf types. We then used redundancy analysis (RDA) to analyze the relative influences of climate, soil, and the interspecific variation of species.

The strength of the phylogenetic signal in Chl a, Chl b, Chl a+b, Chl a/b across plant species was quantified using Blomberg’s K statistics, which tests whether observed traits vary across a phylogeny (Blomberg et al., 2003). We tested the significance of this phylogenetic signal by comparing the actual system to a null model without phylogenetic structure. If the real value of the phylogenetic signal in the trait was greater than 95% that of the null model (P < 0.05), the phylogenetic signal was considered significant, and vice versa. The phylogenetic signal was quantified and tested using the “picante” package in R.

Results

FIGURE 1

Figure 1. Statistics of leaf chlorophyll content for the nine study forests along the North-South Transect of Eastern China (NSTEC). (A–D) were calculated in Chl a, Chl b, Chl a+b, Chl a/b, respectively. The black lines across the boxes are median values and red points represent the means. HZ, Huzhong; LS, Liangshui; CB, Changbai; DL, Dongling; TY, Taiyue; SN, Shennongjia; JL, Jiulian; DH, Dinghu; JF, Jianfengling. Numbers in brackets represent the number of sample species in the specific site. Same letters denote no significant difference among the nine sites (P < 0.05).

TABLE 1

Table 1. Statistics of leaf chlorophyll content (mg g−1) for different life forms in forests.

TABLE 2

Table 2. Statistics of leaf chlorophyll content (mg g−1) for different growth forms in forests.

TABLE 3

Table 3. Statistics of leaf chlorophyll content (mg g−1) for different leaf shapes in forests.

To quantify the variance of different groups (sites, life forms), interpretation ratios were calculated within groups and among groups. The variance of Chl a, Chl b, Chl a+b, and Chl a/b was mainly explained by within-site variation, with only a small portion arising from among-site variation (Figure 2A). Similar results were obtained for different plants with respect to life forms and leaf types (Figures 2B–D). Chl a, Chl b, and Chl a+b increased with increasing latitude; however, this trend was weak (all Ps < 0.01, R2 = 0.02) (Figures 3A–C). In comparison, there was not obvious latitudinal pattern on Chl a/b (Figure 3D). The contents of Chl a, Chl b, and Chl a+b in leaves were negatively correlated with MAT and MAP (all P’s < 0.01), and no significant relationships between Chl a/b and both MAT and MAP were observed (Figures S2, S3).

FIGURE 2

Figure 2. Partitioning the total spatial variance of leaf chlorophyll content in the nine forests (A), different life forms (B), different growth forms (C), and different leaf shapes (D).

FIGURE 3

Significant phylogenetic signals were observed for Chl a within life forms, communities, and across the whole transect, except for shrubs and conifer trees (Table 4). The K-values were approximately equal to 0. Significant phylogenetic signals were observed for Chl b within life forms and communities, and across the whole transect, except for shrubs and conifer trees. K-values were approximately equal to 0. There were no significant phylogenetic signals in Chl a/b (Table 4). Across different sites, there were no significant phylogenetic signals in Chl a+b at most sites (Table 5). Significant phylogenetic signals were observed for Chl a+b at some sites; however, all K-values were approximately equal to 0. In other words, the phylogenetic signals were too weak for Chl a, Chl b, Chl a+b, and Chl a/b (Figures S4, S5).

TABLE 4

Table 4. Strength of the phylogenetic signal in chlorophyll traits for different growth forms.

TABLE 5

Table 5. Strength of the phylogenetic signal in chlorophyll traits for each of the nine forests.

Across all species, Chl a was negatively correlated with MAT and MAP (Ps < 0.01) and positively correlated with SOC, STN, and STP (Ps < 0.01). Chl b and Chl a+b were negatively correlated to MAT and MAP (Ps < 0.01), and positively correlated to SOC, STN, and STP. Chl a/b was negatively correlated to SOC and STP (Table S3). Environmental factors affected the Chl of PFGs and leaf types differently (Tables S4–S6). Environmental factors had weak influences on Chl a, Chl b, Chl a+b, and Chl a/b. Furthermore, more than 80% of the total variation of Chl a, Chl b, and Chl a+b could be explained by the interspecific variation in different life forms, forest communities, except for shrubs and conifer trees. For conifer trees, these Chl parameters explained 45% of variation. The contributions of climate and soil factors to the total variance of Chl along the transect were very low, less than 1% for all parameters (Table S7).

Discussion

Significant Differences in Chl Among Different Species, Life Forms, and Communities

Large variation in Chl was observed among the 823-plant species in the natural forest communities. Across all plants, there were significant differences of Chl among different species, life forms, and communities. The coefficient of variations of Chl a, Chl b, Chl a+b, and Chl a/b were 0.41, 0.43, 0.41, and 0.14, respectively. The coefficient of variations of Chl a/b was relatively smaller than that of Chl a, Chl b, and Chl a+b. Although interspecific differences in Chl a and Chl b were very big, there should be a strong linear relationship between Chl a and Chl b (Li et al., 2018), lowering the variability of Chl a/b. Furthermore, the coefficients of variation of Chl a, Chl b, and Chl a+b were also large. Less than 10% of the total variation was among groups when all species were divided into diverse groups by sites, life forms, and leaf types. Especially for Chl a/b, total variation was close to zero. Interspecific variation might explain more than 80% of the total variations of Chl, which further confirms the presence of the larger interspecific variation of Chl.

Weak Increasing Latitudinal Pattern of Chl in Natural Forests

Although there were significant latitudinal patterns for Chl a, Chl b, and Chl a+b, the R2s were very weak. Chl a/b had no significant latitudinal pattern. The correlations between Chl contents and both MAT and MAP were weak. This phenomenon might be explained by a large amount of Chl in the forest communities being redundant, some of the plant chlorophyll is not involved in the photosynthetic reaction. Therefore, chlorophyll content is not necessarily linked to this reaction, even though temperature and water are the important factors in the synthesis of chlorophyll. Alternatively, high interspecific variation might have led to a weak latitudinal pattern in our study, with the highest coefficient of variation of Chl reaching 0.52. Yet, interspecific species variation could not be explained by environmental differences between the scale of the study and the observed weak latitudinal patterns. Furthermore, chlorophyll content might be affected by community structure, because the light shading effect on vertical structure might change Chl. Some control experiments found that shading effect might affect plant Chl content, and Chl a/b may also decrease.

With the development of the molecular clock theory (molecular evolution speed constancy) and fossil dating data, researchers found that phylogenic history play a decisive role for some plant traits (Comas et al., 2014; Kong et al., 2014; Li et al., 2015). Some plant traits (i.e., N and P content and calorific values) are strongly affected by the phylogeny of plant species (Stock and Verboom, 2012; Chen et al., 2013; Song et al., 2016). In contrast, in our study, although Chl in the overall and different life forms had significant phylogenetic signals, the phylogenetic signal K values were close to zero. Theoretically, these traits were deemed as a strong phylogenetic signal if the K value > 1. Previous studies have demonstrated that if a plant trait has a significant phylogenetic signal, it could be considered as a conservative trait. Traits also perform more similarly when the genetic relationship of different species is closer (Felsenstein, 1985), and vice versa. Therefore, our results showed that Chl is almost not influenced by phylogeny in forests at a large scale.

In theory, as an important photosynthetic pigment of plants, Chl is widely influenced by the environment. Unexpectedly, our results showed that both climate and soil factors had very small influences on Chl, while interspecific variation was the main factor influencing the spatial patterns of Chl from tropical to cold-temperate forests. Through the redundancy analysis, we found that climate and soil factors were not the main factors influencing Chl, because their independent effects were less than 1%. However, interspecific variation did explain more than 80% of Chl. Thus, our results support that plants adapt to the environment by adjusting Chl. Because of the small plasticity of Chl, the variability of Chl in single species had a certain threshold. Above this threshold, some plants would be replaced by others because of the environmental filter. This hypothesis requires to be verified in natural forests in future. Some studies have demonstrated that the variability of leaf traits is influenced by a combination of species, climate, and soil factors (Reich et al., 2007; Han et al., 2011; Liu et al., 2012).

At a large scale, the patterns of leaf traits are not obvious along the changing environmental gradient, with greater variability occurring in species that coexist (Freschet et al., 2011; Onoda et al., 2011; Moles et al., 2014). Twenty-percent of spatial variation in leaf economic traits (specific leaf area, leaf longevity, photosynthetic rate, and leaf N and P content) is associated with the coexistence of different plant species (Wright et al., 2004; Freschet et al., 2011). Furthermore, previous control experiments showed that Chl is significantly associated with temperature and moisture (Yamane et al., 2000; Yin et al., 2006). However, the environmental factors used in this study were MAT and MAP, it would be an interesting idea to obtain the real-time data of chlorophyll and temperature and precipitation for future work. For the relationship between Chl and soil nutrients, we found that soils explain a small portion of total variation, because important elements are prioritized for important organs. As an important photosynthetic pigment, leaves should receive more nutrient elements for Chl, even if the low content of nutrient elements in soils has a small effect on Chl. Therefore, future research should focus on understanding how soils and the climate influence or optimize ecosystem functioning through a combination of element allocation. In addition, light is an important factor for chlorophyll synthesis, however in our analyzes light was not taken into account. To my knowledge, radiation could be measured using satellite data, and light extinction within forest canopies can be modeled. These would be potential next steps for our research.

Chl Should be Cautiously Used as a Proxy for Simulating GPP in Natural Communities

The main purpose of most studies on Chl in natural communities has been to establish a link between Chl and ecosystems functioning, because Chl is widely considered an important factor influencing leaf photosynthetic capacity (Singsaas et al., 2004). Using four deciduous tree species sampled across three growing seasons, Croft et al. (2017) showed that, compared with leaf N content, Chl serves as a better proxy for leaf photosynthetic capacity. Furthermore, Chl can be modeled accurately from remotely sensed data (Croft et al., 2013); thus, this important parameter has been used to model leaf photosynthetic capacity at the global scale (Jacquemoud and Baret, 1990).

However, our results showed that significant differences in Chl occur among coexisting species, functional groups, and communities. Furthermore, the vertical structure of the plant community generated strong variation in the light environment, which might result in the accumulation of redundant Chl. In addition, our previous studies have also found that Chl showed a weak correlation with GPP in the communities (Li et al., 2018). Therefore, the photosynthetic capacity in a given natural forest community could be overestimated by using Chl. In other words, when using Chl as a proxy of GPP in the natural community, the outputs should be treated with caution. While it is a clever concept to optimize models using Chl data derived through remote sensing technology, more research is required to link Chl and GPP in the natural community in a way that is both representative and informative.

Conclusions

Significant variation in Chl was observed among different plant species, functional groups, and communities. Unexpectedly, Chl showed a very weak latitudinal pattern from tropical monsoon forests to cold-temperate coniferous forests, because of significant variation among coexisting species. This interspecific variation was the main factor affecting Chl, rather than soil and climate. Because of this “fuzzy regulation” of Chl in the natural community, caution should be taken when integrating Chl in ecological models. In conclusion, this approach should only being used if scientists are able to link Chl with ecosystem functioning in natural forest communities objectively in future studies.

Authors Contributions

NH and JH conceived the ideas and designed methodology; YL, LX, CL, JZ, QW, XZ, and XW collected the data; NH and YL analyzed the data; YL, NH, and JH led the writing of the manuscript. All authors contributed critically to the drafts and gave final approval for publication.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This work was supported by the National Key Research Project of China (2017YFC0504004, 2016YFC0500202), the National Natural Science Foundation of China (31000263, 611606015, 31290221), STS of Chinese Academy of Sciences (KFJ-SW-STS-167), and the program of Youth Innovation Research Team Project (LENOM2016Q0005).

Supplementary Material

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Making Food

What is photosynthesis?
Photosynthesis is the process by which plants make food from light, water, nutrients, and carbon dioxide. What is chlorophyll?
Chlorophyll is the green pigment, or color, found in plants that helps the plant make food.

Plants are very important to us. All food people eat comes directly or indirectly from plants.

Directly from plants: Indirectly from plants:
For example, apples come from an apple tree. The flour used to make bread comes from a wheat plant. Steak comes from a cow, and we all know that cows are animals, not plants, right? But what does the cow eat? It eats grass and grains—PLANTS!

So all the foods we eat come from plants. But what do plants eat? They make their own food!

What Do Plants Need to Make Food?

Plants need several things to make their own food.
They need:

  • chlorophyll, a green pigment found in the leaves of plants (see the layer of chlorophyll in the cross-section of a leaf below)

  • light (either natural sunlight or artificial light, like from a light bulb)
  • carbon dioxide (CO2)(a gas found in the air; one of the gases people and animals breathe out when they exhale)
  • water (which the plant collects through its roots)
  • nutrients and minerals (which the plant collects from the soil through its roots)

Plants make food in their leaves. The leaves contain a pigment called chlorophyll, which colors the leaves green. Chlorophyll can make food the plant can use from carbon dioxide, water, nutrients, and energy from sunlight. This process is called photosynthesis.

During the process of photosynthesis, plants release oxygen into the air. People and animals need oxygen to breathe.

Disclaimer/Credits Copyright © 2009 Missouri Botanical Garden

Chlorophyll is responsible for the lush green hues of many plants.

Why do some plants appear green?

Green plants are green because they contain a pigment called chlorophyll. Chlorophyll absorbs certain wavelengths of light within the visible light spectrum. As shown in detail in the absorption spectra, chlorophyll absorbs light in the red (long wavelength) and the blue (short wavelength) regions of the visible light spectrum. Green light is not absorbed but reflected, making the plant appear green.

Chlorophyll is found in the chloroplasts of plants. There are various types of chlorophyll structures, but plants contain chlorophyll a and b. These two types of chlorophyll differ only slightly, in the composition of a single side chain.

Absorption spectra showing how the different side chains in chlorophyll a and chlorophyll b result in slightly different absorptions of visible light. Light with a wavelength of 460 nm is not significantly absorbed by chlorophyll a, but will instead be captured by chlorophyll b, which absorbs strongly at that wavelength. The two kinds of chlorophyll in plants complement each other in absorbing sunlight. Plants are able to satisfy their energy requirements by absorbing light from the blue and red parts of the spectrum. However, there is still a large spectral region between 500 and 600 nm where chlorophyll absorbs very little light, and plants appear green because this light is reflected.

What is chlorophyll?

Chlorophyll is a compound that is known as a chelate. A chelate consists of a central metal ion bonded to a large organic molecule, composed of carbon, hydrogen, and other elements such as oxygen and nitrogen.

Chlorophyll has magnesium as its central metal ion, and the large organic molecule to which it bonds is known as a porphyrin. The porphyrin contains four nitrogen atoms bonded to the magnesium ion in a square planar arrangement. Chlorophyll occurs in a variety of forms.

The structure of chlorophyll a.

Chlorophyll does not contain chlorine as the name might suggest; the chloro- portion stems from the Greek chloros, which means yellowish green. The element chlorine derives its name from the same source, being a yellowish-green gas.

How do birds and animals see plants?

Vegetation will not appear to animals as it does to us. Although our color perception is the most advanced amongst mammals, humans have less effective color vision than many birds, reptiles, insects and even fish. Humans are trichromats, sensitive to three fundamental wavelengths of visible light. Our brains interpret color depending on the ratio of red, green and blue light. Some insects are able to see ultraviolet light. Birds are tetrachromatic, able to distinguish four basic wavelengths of light, sometimes ranging into ultraviolet wavelengths, giving them a far more sensitive color perception.

It is hard for us to imagine how the world appears to birds, but they will certainly be able to distinguish more hues of green than we do, and so are far more able to distinguish between types of plants. We can speculate that this is of great benefit when choosing where to feed, take shelter and rear young. Aquatic creatures, from fish to the hyperspectral mantis shrimp (which distinguishes up to twelve distinct wavelengths of light) are uniquely tuned to the colors of their environment. The pages on animals include more information on the variety of color vision in the animal kingdom.

The vivid colors of fall leaves emerge as yellow and red pigments, usually masked by chlorophyll, are revealed by its absence. Chlorophyll decomposes in bright sunlight, and plants constantly synthesize chlorophyll to replenish it. In the fall, as part of their preparation for winter, deciduous plants stop producing chlorophyll. Our eyes are tuned to distinguish the changing colors of the plants, which provide us with information such as when fruits are ripe and when the seasons are starting to change.

Apart from coloring, has chlorophyll any other role?

The green color of chlorophyll is secondary to its importance in nature as one of the most fundamentally useful chelates. It channels the energy of sunlight into chemical energy, converting it through the process of photosynthesis. In photosynthesis, chlorophyll absorbs energy to transform carbon dioxide and water into carbohydrates and oxygen. This is the process that converts solar energy to a form that can be utilized by plants, and by the animals that eat them, to form the foundation of the food chain.

Chlorophyll is a molecule that traps light – and is called a photoreceptor.

Photosynthesis

Photosynthesis is the reaction that takes place between carbon dioxide and water, catalysed by sunlight, to produce glucose and a waste product, oxygen. The chemical equation is as follows:

Glucose can be used immediately to provide energy for metabolism or growth, or stored for use later by being converted to a starch polymer. The by-product oxygen is released into the air, and breathed in by plants and animals during respiration. Plants perform a vital role in replenishing the oxygen level in the atmosphere.

In photosynthesis, electrons are transferred from water to carbon dioxide in a reduction process. Chlorophyll assists in this process by trapping solar energy. When chlorophyll absorbs energy from sunlight, an electron in the chlorophyll molecule is excited from a lower to a higher energy state. The excited electron is more easily transferred to another molecule. A chain of electron-transfer steps follows, ending when an electron is transferred to a carbon dioxide molecule. The original chlorophyll molecule is able to accept a new electron from another molecule. This ends a process that began with the removal of an electron from a water molecule. The oxidation-reduction reaction between carbon dioxide and water known as photosynthesis relies on the aid of chlorophyll.

There are actually several types of chlorophyll, but all land plants contain chlorophyll a and b. These 2 types of chlorophyll are identical in composition apart from one side chain, composed of a -CH3 in chlorophyll a, while in chlorophyll b it is -CHO. Both consist of a very stable network of alternating single and double bonds, a structure that allows the orbitals to delocalize, making them excellent photoreceptors. The delocalised polyenes have very strong absorption bands in the visible light spectrum, making them ideal for the absorption of solar energy.

The chlorophyll molecule is highly effective in absorbing sunlight, but in order to synthesize carbohydrates most efficiently, it needs to be attached to the backbone of a complex protein. This protein provides exactly the required orientation of the chlorophyll molecules, keeping them in the optimal position that enables them to react efficiently with nearby CO2 and H2O molecules. This bacterial photoreceptor protein forms the backbone for a number of chlorophyll molecules.

The basic structure seen in the chlorophyll molecule recurs in a number of molecules that assist in biochemical oxidation-reduction reactions, because it is ideally suited to promote electron transfer. Heme consists of a porphyrin similar to that in chlorophyll with an iron (II) ion at its center. Heme is bright red, the pigment that characterizes red blood. In the red blood cells of vertebrates, heme is bound to proteins to form hemoglobin. Oxygen enters the bloodstream in the lungs, gills or other respiratory surfaces and combines with hemoglobin. This oxygen is carried round the body of the organism in the bloodstream and released in the tissues. Hemoglobin in the muscle cells is known as myoglobin, a form that enables the organism to store oxygen as an electron source, ready for energy-releasing oxidation-reduction reactions.

Commercial pigments

Chlorophyll is a pigment that causes a green colour. Chlorophyll as a green dye has been used commercially in processed foods, toothpaste, soaps and cosmetics. Commercial pigments with structures similar to chlorophyll have been produced in a range of colors. In some, the porphyrin is modified, for example by replacing the chlorine atoms with hydrogen atoms. In others, different metal ions may be present. Phthalocyanine is a popular bright blue pigment with a copper ion at the center of the porphyrin.

Phthalocyanine is a blue pigment.

Green plants have the ability to make their own food. They do this through a process called photosynthesis, which uses a green pigment called chlorophyll. A pigment is a molecule that has a particular color and can absorb light at different wavelengths, depending on the color. There are many different types of pigments in nature, but chlorophyll is unique in its ability to enable plants to absorb the energy they need to build tissues.

Chlorophyll is located in a plant’s chloroplasts, which are tiny structures in a plant’s cells. This is where photosynthesis takes place. Phytoplankton, the microscopic floating plants that form the basis of the entire marine food web, contain chlorophyll, which is why high phytoplankton concentrations can make water look green.

Chlorophyll’s job in a plant is to absorb light—usually sunlight. The energy absorbed from light is transferred to two kinds of energy-storing molecules. Through photosynthesis, the plant uses the stored energy to convert carbon dioxide (absorbed from the air) and water into glucose, a type of sugar. Plants use glucose together with nutrients taken from the soil to make new leaves and other plant parts. The process of photosynthesis produces oxygen, which is released by the plant into the air.

Chlorophyll gives plants their green color because it does not absorb the green wavelengths of white light. That particular light wavelength is reflected from the plant, so it appears green.

Plants that use photosynthesis to make their own food are called autotrophs. Animals that eat plants or other animals are called heterotrophs. Because food webs in every type of ecosystem, from terrestrial to marine, begin with photosynthesis, chlorophyll can be considered a foundation for all life on Earth.

Objective

Role of the colour of light during Photosynthesis

Did you know that the colour of light plays an important role during photosynthesis? Yes, it does. Plants use only certain colours from light for the process of photosynthesis. The chlorophyll absorbs blue, red and violet light rays. Photosynthesis occurs more in blue and red light rays and less, or not at all, in green light rays.

The light that is absorbed the best is blue, so this shows the highest rate of photosynthesis, after which comes red light. Green light cannot be absorbed by the plant, and thus cannot be used for photosynthesis. Chlorophyll looks green because it absorbs red and blue light, making these colours unavailable to be seen by our eyes. It is the green light which is not absorbed that finally reaches our eyes, making the chlorophyll appear green.

Factors affecting Photosynthesis

For a constant rate of photosynthesis, various factors are needed at an optimum level. Here are some of the factors affecting photosynthesis.

  • Light Intensity:An increased light intensity leads to a high rate of photosynthesis and a low light intensity would mean low rate of photosynthesis.
  • Concentration of CO2: Higher carbon dioxide concentration increases the rate of photosynthesis. Normally the carbon dioxide concentration of 0.03 to 0.04 percent is sufficient for photosynthesis.
  • Temperature:An efficient photosynthesis requires an optimum temperature range between 25 to 35oC.
  • Water: Water is an essential factor for photosynthesis. The lack of water also leads to a problem for carbon dioxide intake. If water is scarce, the leaves refuse to open their stomata to keep water they have stored inside.
  • Polluted Atmosphere:The pollutants and gases (impure carbon) settle on leaves and block the stomata, making it difficult to take in carbon dioxide. A polluted atmosphere can lead to a 15 percent decrease in the rate of photosynthesis.

Learning Outcomes

  • Students understand the concept that light is necessary for photosynthesis.
  • Students understand the principle of photosynthesis and the factors affecting photosynthesis.
  • Students will be able to do the experiment more accurately in the real lab once they understand the steps through the animation and simulation.

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Any opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of UK Essays.

Light has a particulate nature and a wave nature. Light represents that part of the radiant energy having wavelengths visible to the naked eye, approximately 390 to 760 nm. This is a very narrow region of the electromagnetic spectrum. The particulate nature of light is usually expressed in statements that light comes in quanta or photons, discrete packets of energy, each having a specific associated wavelength. In other words, light can be defined as electromagnetic energy propagated in discrete corpuscles called quanta or photon. As the energetics of chemical reactions are usually described in terms of kilocalories per mole of the chemicals (1 mole = 6.02 x 1023 molecules). Therefore, light energies are usually described in terms of kilocalories per mole quantum or per einstein (1 mole quantum or 1 einstein = 6.02 x 1023 quanta).

The colour of the light is determined by the wavelength (λ) of the light radiation. At any given wavelength, all the quanta have the same energy. The energy (E) of a quantum is inversely proportional to its wavelength. Thus the violet and blue wavelengths are more energetic than the longer orange and red ones. Therefore, the energy of blue light (λ = 420 nm or mµ) is in the order of 70 K-cal/einstein and that of red light (λ = 690 nm or mµ) about 40 K-cal/einstein. The symbol commonly used for quantum, hv, is derived from this relationship. In any wave propagation, the frequency (v) is inversely proportional to the wavelength. Since E α 1/ λ, then E α V. Plank’s constant (h) converts this to an equation E = hv. Thus hv, used to designate a quantum, refers to the energy content of the quantum.

A fundamental principle of light absorption, often called as Einstein law, is that any pigment (coloured molecule) can absorb only one photon at a time and that this photon causes the excitation of one electron. Specific valence (bonding) electrons in stable ground state orbitals are then usually exited and each electron can be driven away from the positively charged nucleus for a distance corresponding to an energy exactly equal to the energy of the photon absorbed (Fig. 5-10). The pigment molecule is then in an excited state and it is this excitation energy that is used in photosynthesis. The relationship between the energies of light, both as calories per mole quanta (per Einstein) and as E ‘O values and the energies required to conduct certain reactions is shown in table 5-2. It is evident that energy of a red quantum is just sufficient to raise an electron from OH- to the reducing level of H2; a uv quantum contains nearly twice this amount of energy. Thus, there is enough energy in a quantum of light (barely enough in a red quantum) to split water.

EMERSON EFFECT

By experiments, it appears that the high energy of blue light absorbed by chlorophyll is not used efficiently. The basic requirement is for a basic number of quanta. Therefore, the energy of the quanta is unimportant provided they can be absorbed by the chlorophyll. Red quanta (40 Kcal/einstein) are as effective as blue quanta (70 Kcal/einstein), the extra energy of the blue quanta is wasted. Presumably if a quantum is of the appropriate wavelength to be absorbed, it will be effective. However, an important exception to this behavior is the so called red drop, a decided decrease in efficiency found in many organisms at the far red end of the absorption spectrum, usually over 685 nm. Emerson, working with an algal system found that two pigment systems and two light reactions participated in photosynthesis. When exposed to a wavelength more than 680 nm, a specific rate of photosynthesis was observed. Likewise when exposed to a wavelength less than 680 nm a little effect on photosynthesis resulted. However, when the system was exposed to light of both the wavelengths at the same time, the effect on photosynthesis exceeded the sum of the two effects caused separately. Thus Emerson concluded that the efficiency of red light at a wavelength of about 700 nm could be increased by adding shorter wavelength light (650 nrn). This proved that the rate of photosynthesis in light of the two wavelengths together was greater than the added rates of photosynthesis in either alone. This is known as the Emerson effect after its inventor. This provided the ground that the two pigment systems worked in cooperation with each other. The resultant increase in photosynthesis was due to synergism (Fig. 5-13).

FACTORS AFFECTING PHOTOSYNTHESIS

Several external and internal factors influence photosynthesis. Of the external factors, influencing photosynthesis, light quality and intensity, CO2 concentration, temperature, oxygen, concentration of water, wind speed and nutrient level, are most important. The internal factors include chlorophyll contents, stomatal behaviour, leaf water content and enzymes. Morphology of the plants also influences photosynthesis. Most of the internal factors are influenced by the external factors. However, several of these interact to influence the rate of photosynthesis. For instance, increase in CO2, concentration enhances photosynthesis but such an increase may also cause closure of stomata. Therefore, no net increase in photosynthetic rate is observed. In summary, it may be understood that no single factor should be taken in account to explain an increase in photosynthesis. Certain specific factors that affect photosynthetic pathways are briefly discussed as under:

Temperature

As described earlier, Blackmann was the first to recognize the interrelations between light intensity and temperature. When CO2 light and other factors are not limiting, the rate of photosynthesis increases with a rise in temperature between the physiological range of 5.35°C. Between 25-30°C photosynthesis usually has Q10 of about 2. Certain organisms can continue CO2 fixation at extraordinary extremes of temperature some conifers at -20oC and algae that inhabits hot springs, a temperatures in excess of 50°C. But in most plants, photosynthesis ceases or declines sharply beyond the physiological limit. Because above 40°C there is an abrupt fall in the rate and the tissues die. High temperatures, cause inactivation of enzymes thus affecting the enzymaticaily controlled dark reactions of photo­synthesis.

Temperature range at which optimum photosynthesis can occur varies with the plant species e.g. some lichens can photosynthesize at 20°C while conifers can assimilate at 35°C.

In nature the maximum rate of photosynthesis due to temperature is not realized because light or CO2 or both are limiting. The response curve of net photosynthesis to temperature is different from those of light and CO2. It shows minimum, optimum and maximum temperatures. Between the C3 and C4 plants, the former species have optimal rates from 20-25°C while the latter from 35-40°C. Similarly, temperature also influences the light (optimum at 30-35°C) and dark respiration (optimum at 40-45°C).

Oxygen

Oxygen affects photosynthesis in several ways. Certain of the photosynthetic electron carriers may transfer electrons to oxygen, and ferredoxin in particular appears to be sensitive to O2. In bright light, high oxygen leads to irreversible damage to the photosynthetic system, probably by the oxidation of pigments. Carotenes in chloroplasts tend to protect chlorophylls from damage by solarization. The reaction of RuBP-case provides the most important site of O2 effect on photosyn­thesis. Oxygen competitively and reversibly inhibits the photosynthesis of C3 plants over all concen­trations of CO2; at high O2 (80% or over) irreversible inhibition also takes place. On the other hand, C4 plants do not release CO2 in photorespiration, therefore, photosynthesis in them is not affected until very high concentrations are reached which cause irreversible damage to the photosynthetic system (Fig. 5-23).

Carbon dioxide concentrations

Under field conditions, CO2 concentration is frequently the limiting factor in photosynthesis. The atmospheric concentration of about 0.033% (330 ppm) is well below C O2 saturation, for most plants. Some do not saturate until a concentration of 10 to 100 times this is reached. Characteristic CO2 saturation curves are shown in (Fig. 5-24). Photosynthesis is much affected by CO2 at low concentrations but is more closely related to light intensity at higher concentrations. At reduced CO2 concentrations the part of carbon may change dramatically because glycolate production results due to increased relative level of 02.

As CO2 concentration is reduced, the rate of photosynthesis slows until it is exactly equal to the rate of photorespiration. This CO2 concentration at which CO2 uptake and out put are equal, is called the CO2 compensation point. The CO2 compensation point of C4 plants, which do not release CO2 in photorespiration, is usually very low (i.e. from 2-5 ppm CO2).

Light

The photosyntheticaliy active spectrum of light is between 400-700 nm. Green light (550 nm) plays no important role in photosynthesis. Light supplies the energy for the process and varies in intensity, quality and duration.

Intensity

When CO2 and temperature are not limiting and light intensities are low, the rate of photosynthesis increases with an increase in its intensity. At a point saturation may be reached, when further increase in light intensity fails to induce any increase in photosynthesis. Optimum or saturation intensities may vary with different plant species e.g. C3 and C4 plants. The former become saturated at levels considerably lower than full sunlight but the later are usually not saturated at full sunlight.

When the intensity of light falling on a photosynthesizing organ is increased beyond a certain point, the cells of that organ become vulnerable to chlorophyll catalyzed photooxidations. Conse­quently these organs begin to consume O2 instead of CO2 and the CO2 is released. Photooxidation is maximal when O2 is present or carotenoids are absent or CO2 concentration is low.

Duration

Generally a plant will accomplish more photosynthesis when exposed to long periods of light. Uninterrupted and continuous photosynthesis for relatively long periods of time may be sustained without any visible damage to the plant. If the light source is removed, the rate of CO2 fixation falls to zero immediately.

The light compensation point is that at which photosynthesis equals respiration and no net gas exchange occurs. The light compensation point of shade tolerant plants is much lower than that of sun plants.

Water

Water is an essential raw material in carbon assimilation. Less than 1% of water absorbed by a plant is used in photosynthesis. Thus decrease of water of the soil from field capacity to permanent wilting percentage (PWP) results in decreased photosynthesis. The inhibitory effect is primarily due to dehydration of protoplasm and also closure of stomata. The removal of water from the protoplasm also affects its colloidal state, impairs enzymatic efficiency, inhibits vital processes like respiration, photosynthesis etc.

The synthesis oforganic compound from carbon dioxide and water (with the reiease of oxygen)using light energy absorbed by chlorophyll is called as photosynthesis. Or through photosynthesis light energy is captured n then that enegy is converted into chemical energy and that energy is the need of organism to survive.plants are autotrophs and they get energy from sun light and they assemble the organic molecules from inorganic resources and this is the reason that’s why it is called as photosynthesis.it is a greek word PHOTO means light and SYNTHESIS means to put together.

Ecological considerations in photosynthesis:

Ecological consideration means the effect of light ,CO2, water etc. Chlorophyll is not the only pigment found in chloroplasts. There is also a family of orange and yellow pigments called carotenoids. Carotenoids include the carotenes, which are orange, and the xanthophylls, which are yellow. The principal carotene in chloroplasts is beta-carotene, which is located in the chloroplasts along with chlorophyll. At one time, the carotenoids were considered accessory pigments-it was believed that light energy absorbed by carotenoids was transferred to the chlorophylls for use in photosynthesis. It now appears that carotenoids have little direct role in photosynthesis, but function largely to screen the chlorophylls from damage by excess light (see Chapter 6). Carotenoid pigments are not limited to leaves, but are widespread in plant tissues. The color of carrot roots, for example, is due to high concentrations of beta-carotene in the root cells and lycopene, the red-orange pigment of tomatoes, is also a member of the carotenoid family. Lycopene and betacarotene are important because of their purported health benefits. Beta-carotene from plants is also the principal source of vitamin A, which plays an important role in human vision. Lycopene is an antioxidant that may help protect against a variety of cancers. Carotenes and xanthophylls are also responsible for the orange and yellow colors in autumn leaves. In response to shortening day length and cooler temperatures, the chloroplast pigments begins to break down. Chlorophyll, which normally masks the carotenoids, breaks down more rapidly than the carotenoids and the carotenoids are revealed in their entire autumn splendor. The red color that appears in some leaves at this time of the year is due to water-soluble anthocyanins, whose synthesis is promoted by the same conditions that promote the breakdown of chlorophyll. known as CO2 fertilization. In practice, the CO2 content may be increased by 150-200 ppm to a total of perhaps 1.5 times atmospheric levels, although some foliage plant growers may supplement with CO2 up to a total of 700-1,000 ppm.

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