What is a stomata?

Photosynthesis is a multi-step process that requires sunlight, carbon dioxide (which is low in energy), and water as substrates (Figure 1). After the process is complete, it releases oxygen and produces glyceraldehyde-3-phosphate (GA3P), simple carbohydrate molecules (which are high in energy) that can subsequently be converted into glucose, sucrose, or any of dozens of other sugar molecules. These sugar molecules contain energy and the energized carbon that all living things need to survive.

Figure 1. Photosynthesis uses solar energy, carbon dioxide, and water to produce energy-storing carbohydrates. Oxygen is generated as a waste product of photosynthesis.

The following is the chemical equation for photosynthesis (Figure 2):

Figure 2. The basic equation for photosynthesis is deceptively simple. In reality, the process takes place in many steps involving intermediate reactants and products. Glucose, the primary energy source in cells, is made from two three-carbon GA3Ps.

Although the equation looks simple, the many steps that take place during photosynthesis are actually quite complex. Before learning the details of how photoautotrophs turn sunlight into food, it is important to become familiar with the structures involved.

In plants, photosynthesis generally takes place in leaves, which consist of several layers of cells. The process of photosynthesis occurs in a middle layer called the mesophyll. The gas exchange of carbon dioxide and oxygen occurs through small, regulated openings called stomata (singular: stoma), which also play roles in the regulation of gas exchange and water balance. The stomata are typically located on the underside of the leaf, which helps to minimize water loss. Each stoma is flanked by guard cells that regulate the opening and closing of the stomata by swelling or shrinking in response to osmotic changes.

In all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a chloroplast. For plants, chloroplast-containing cells exist in the mesophyll. Chloroplasts have a double membrane envelope (composed of an outer membrane and an inner membrane). Within the chloroplast are stacked, disc-shaped structures called thylakoids. Embedded in the thylakoid membrane is chlorophyll, a pigment (molecule that absorbs light) responsible for the initial interaction between light and plant material, and numerous proteins that make up the electron transport chain. The thylakoid membrane encloses an internal space called the thylakoid lumen. As shown in Figure 3, a stack of thylakoids is called a granum, and the liquid-filled space surrounding the granum is called stroma or “bed” (not to be confused with stoma or “mouth,” an opening on the leaf epidermis).

Practice Question

Figure 3. Photosynthesis takes place in chloroplasts, which have an outer membrane and an inner membrane. Stacks of thylakoids called grana form a third membrane layer.

On a hot, dry day, plants close their stomata to conserve water. What impact will this have on photosynthesis?

Show Answer Levels of carbon dioxide (a necessary photosynthetic substrate) will immediately fall. As a result, the rate of photosynthesis will be inhibited.

Two Parts of Photosynthesis

Photosynthesis takes place in two sequential stages: the light-dependent reactions and the light independent-reactions. In the light-dependent reactions, energy from sunlight is absorbed by chlorophyll and that energy is converted into stored chemical energy. In the light-independent reactions, the chemical energy harvested during the light-dependent reactions drive the assembly of sugar molecules from carbon dioxide. Therefore, although the light-independent reactions do not use light as a reactant, they require the products of the light-dependent reactions to function. In addition, several enzymes of the light-independent reactions are activated by light. The light-dependent reactions utilize certain molecules to temporarily store the energy: These are referred to as energy carriers. The energy carriers that move energy from light-dependent reactions to light-independent reactions can be thought of as “full” because they are rich in energy. After the energy is released, the “empty” energy carriers return to the light-dependent reaction to obtain more energy. Figure 4 illustrates the components inside the chloroplast where the light-dependent and light-independent reactions take place.

Figure 4. Photosynthesis takes place in two stages: light dependent reactions and the Calvin cycle. Light-dependent reactions, which take place in the thylakoid membrane, use light energy to make ATP and NADPH. The Calvin cycle, which takes place in the stroma, uses energy derived from these compounds to make GA3P from CO2.

Photosynthesis at the Grocery Store

Figure 5. Foods that humans consume originate from photosynthesis. (credit: Associação Brasileira de Supermercados)

Major grocery stores in the United States are organized into departments, such as dairy, meats, produce, bread, cereals, and so forth. Each aisle (Figure 5) contains hundreds, if not thousands, of different products for customers to buy and consume.

Although there is a large variety, each item links back to photosynthesis. Meats and dairy link because the animals were fed plant-based foods. The breads, cereals, and pastas come largely from starchy grains, which are the seeds of photosynthesis-dependent plants. What about desserts and drinks? All of these products contain sugar—sucrose is a plant product, a disaccharide, a carbohydrate molecule, which is built directly from photosynthesis. Moreover, many items are less obviously derived from plants: for instance, paper goods are generally plant products, and many plastics (abundant as products and packaging) can be derived from algae or from oil, the fossilized remains of photosynthetic organisms. Virtually every spice and flavoring in the spice aisle was produced by a plant as a leaf, root, bark, flower, fruit, or stem. Ultimately, photosynthesis connects to every meal and every food a person consumes.

In Summary: An Overview of Photosynthesis

The process of photosynthesis transformed life on Earth. By harnessing energy from the sun, photosynthesis evolved to allow living things access to enormous amounts of energy. Because of photosynthesis, living things gained access to sufficient energy that allowed them to build new structures and achieve the biodiversity evident today.

Only certain organisms, called photoautotrophs, can perform photosynthesis; they require the presence of chlorophyll, a specialized pigment that absorbs certain portions of the visible spectrum and can capture energy from sunlight. Photosynthesis uses carbon dioxide and water to assemble carbohydrate molecules and release oxygen as a waste product into the atmosphere. Eukaryotic autotrophs, such as plants and algae, have organelles called chloroplasts in which photosynthesis takes place, and starch accumulates. In prokaryotes, such as cyanobacteria, the process is less localized and occurs within folded membranes, extensions of the plasma membrane, and in the cytoplasm.

In order for plants to produce energy and maintain cellular function, their cells undergo the highly intricate process of photosynthesis. Critical in this process is the stoma. Stomata (multiple stoma) are located on the outermost cellular layer of leaves, stems, and other plant parts. An open stoma facilitates the process of photosynthesis in three ways. First, it allows light to enter the intercellular matter and trigger the process. Second, it allows for the uptake of carbon dioxide, a key chemical in producing plant energy. Third, it allows for oxygen to be expelled into the outside environment, a byproduct of photosynthesis that is no longer needed by the cell.

While an open stoma is necessary for the plant to undergo photosynthesis, it comes with a negative side effect: water loss. Over 95% of a plant’s water loss occurs through the stoma via water vapor. Therefore, a delicate balance must be maintained that allows light and gases to pass between cells, and does not put the plant at risk for dehydration.

This problem is mitigated with guard cells. Guard cells are a pair of two cells that surround each stoma opening. To open, the cells are triggered by one of many possible environmental or chemical signals. These can include strong sunlight or higher than average levels of carbon dioxide inside the cell. In response to these signals, the guard cells take in sugars, potassium, and chloride ions (i.e., solutes) through their membranes. An increase in solutes induces an influx of water across the guard cell membrane. As the volume of the guard cells increase, they “inflate” into two kidney-bean-like shapes. As they expand, they reveal the stoma opening in the center of the two guard cells (similar to a hole in the center of a doughnut). Once fully expanded, the stoma is open and gases can move between the cell and external environment.

The stoma’s pore closes in the opposite manner. Excess loss of water through the stoma, such as during a drought, triggers chemical reactions that signal water and ions to leave the guard cells. As solutes exit the guard cells, the pair “deflates,” subsequently closing the stoma like two flat balloons.

This summary was contributed by Allison Miller.

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What are the 3 functions of stomata

Sometimes these divided parts function as separate leaves. Guard cells do so by controlling the size of the pores also called stomata. Mesophyll: This forms. They help to regulate the rate of transpiration by opening and closing the stomata . To understand how they function, study the following figures. As you look at. Two functions of stomata: (i) It helps in breathing of the plants. (ii) It helps in exchange of gases which takes place inside the plant cells.

Stomata (1 of 3) Function. Image caption: Carbon dioxide enters, while water and oxygen exit, through a leaf’s stomata. Stomata control a tradeoff for the plant: they allow carbon dioxide in, but they also let precious water escape. Did you know that plants ‘breathe’ through their leaves? Tiny openings called stomata allow plants to exchange gases necessary for cellular. What are the 3 functions of stomates. a. CO2 goes in b. O2 goes out c. Takes water in and out. Why are most of the stomates on the bottom of the leaf?.

Hey, Ur answer is here, The 3 functions of stomata are – <>It help in the loss of water in the form of water vapors from the leaves thereby. Stomata are tiny openings or pores in plant tissue that allow for gas exchange. Stomata are typically found in plant leaves but can also be found in some stems. The following are the three functions of stomata> 1. It helps in gaseous exchange of air 2. It also facilitates the process of transpiration.

Specialized cells known as guard cells surround stomata and function to open and an unequal number of subsidiary cells (three) surrounding each stoma. Answer: Functions of the stomata: (i) They allow Name the structures that control the opening and closing of mismoalarans.tk 2 3 Answer(s). Two functions of stomata: (i) It helps in breathing of the plants. (ii) It helps in exchange of gases which takes place inside the plant cells.

Guard Cells
Definition, Function, Structure of Stomata on Plants

Essentially, guard cells are two bean-shaped cells that surround a stoma. As epidermal cells, they play an important role in gaseous exchange in and out of plant leaves by regulating the opening and closing of pores known as a stoma. In addition, they are the channels through which water is released from leaves to the environment.

As such, guard cells play a crucial role in photosynthesis by regulating the entry of materials necessary for the process. Apart from regulating gaseous exchange (as well as water release from leaves), they have also been shown to contain chloroplasts which also make them a site of photosynthesis.

Some of the factors that influence guard cell activities include:

  • Humidity
  • Temperature
  • Light
  • Carbon dioxide
  • Potassium ions
  • Hormones

* In Greek, the word “stoma” means mouth.

* Although stomata are commonly found in plant leaves, they can also be found in the stems.

Structure of the Guard Cells

As mentioned, guard cells are bean/kidney-shaped cells located on plant epidermis. As such, they, like trichomes and pavement cells, are also epidermal cells.

Between each pair of guard cells is a stoma (a pore) through which water and gases are exchanged. The opening and closing of these pores (collectively known as stomata) is made possible by the thickening and shrinking of guard cells on the epidermis.

* The number of stomata on a plant leaf/organ is highly dependent on the type of plant as well as its habitat.

Ultrastructure of Guard Cells

In different types of plants, guard cells have been shown to contain varying amounts of the typical cell organelles (among other structures) with some unique characteristics. For instance, as compared to the rest of a leaf, the cuticle of guard cells is more permeable to water vapor which in turn influences their activities/functions.

Guard cells have also been shown to have numerous ectodesmata. Here, the cuticle has also been shown to be more permeable to various polar substances. This is particularly important given that it is the concentration of these substances that influence the thickening and shrinkage of guard cells.

* On guard cells, the cuticle tends to be thicker on the outer parts.

* Cuticle permeability is also dependent on its chemical composition.

In young and developing guard cells, pectin and cellulose are gradually deposited into the plasmodesmata (a thin layer of cytoplasm). However, it disappears as guard cells mature while the few that are retained are devoid of any function. There are also perforations on their walls that allow relatively large organelles to pass. For instance, plastids and mitochondria can pass through these perforations.

Various components can also be found in different types of guard cells in varying amounts and orientation.

In dumbbell-shaped guard cells, fibrils are radially in the outer wall. This orientation, however, may change with the thickening and shrinking of the cells. Apart from fibrils and microfibrils, a number of other substances have been identified in various guard cells.

In Zea mays, for instance, lignin has been identified in addition to cellulose. On the other hand, pectin has been identified in the guard cells of many plants.

Some of the organelles found in guard cells include:

· Microtubules – serve to orient cellulose microfibrils. They also contribute to the building and development of guard cells.

· Endoplasmic reticulum – The high amounts of rough endoplasmic reticulum present in guard cells are involved in protein synthesis. Apart from protein synthesis, ER is also involved in the formation of vacuoles and vesicles.

· Lysosomes – contain a number of molecules that contribute to the well functioning of the cell. These include; lipases, endopeptidases, phosphates, and DNAse.

· Lipid droplets – in guard cells are the intermediates in the synthesis of wax and cutin

· Nuclei – are centrally located in guard cells. They have been shown to change their general shape with shapes with the opening and closing of the stoma.

· Plastids – In guard cells, such plastids as chloroplasts vary in number from one plant to another. While some of these plastids may be poorly developed, others are well developed and capable of such functions as photosynthesis. In guard cells with functional chloroplasts, high amounts of starch during the night

· Mitochondria – High amounts of mitochondria can be found in guard cells (compared to mesophyll cells) which is evidence of high metabolic activities.


Basically, stomata refers to both the pore (stoma) and the guard cells that surround them on the epidermis. Surrounding the guard cells are subsidiary cells that have been used to classify the different types of stomata.

While the stoma (pore/opening) is the channel through which gases enter the air spaces in leaves, opening, and closing of these openings is regulated by guard cells located on the epidermis.

Classification of Stomata

Generally, stomata are classified based on distribution and structure.

Types of stomata based on distribution/placement:

· Water lily type – are located on the upper epidermis of leaves. They can be found in many aquatic plants such as the water lily.

· Apple type (mulberry type) – are stomata that are typically found on the lower surface of leaves. As such, they can be found in such plants as walnut, apples, and peach among others.

· Potato type – A majority of these stomata can be found on the lower surface of leaves while a few may be found on the upper surface. As such, they are typically found in amphistomatic and anisostomatic leaves (e.g. potato, tomato, cabbage, etc.)

· Oat type – are found in isostomatic leaves (where stomata are distributed on the upper and lower surface of the leaves)

· Potamogeton type – are either absent or non-functional as is the case in submerged aquatic plants.

Based on Structure

· Anomocytic – A small number of subsidiary cells surround the stomata. For the most part, these cells (subsidiary cells) are identical to the other epidermal cells.

· Cruciferous – The stoma is surrounded by three types of subsidiary cells that vary in size.

· Paracytic – The stoma is surrounded by two cells (subsidiary) that are arranged in a parallel manner to the axis of the guard cells.

· Graminaceous – Here, the guard cells are dumbbell-shaped. With subsidiary cells arranged parallel to them.

· Diacytic – The stoma in this classification is two guard cells. The wall of the subsidiary cells surrounding the stoma is at a right angle to the guard cells.

· Cyclocytic – Here, a minimum of four subsidiary cells surround the guard cell.

* 80 to 90 percent of transpiration occurs through the stomata. Water is also lost through lenticular and cuticular transpiration.

* Only a small amount of water absorbed (about 2 percent) is used for photosynthesis in plants.


Guard cells have a number of adaptations that contribute to their functions.

These include:

They have perforations through which solutes and water enter or leave the cells – This is one of the most important adaptations of the guard cells. This is because the movement of solutes and water in and out of guard cells cause them to shrink or swell which in turn results in the closing or opening of the stoma/pore through which water and gases are exchanged.

They contain chloroplasts – Although they do not contain as many chloroplasts as mesophyll cells, guard cells have been shown be the only epidermal cells with chloroplast. As such, guard cells of soma plants are photosynthetic sites where sugars and energy are produced. However, it is worth noting that chloroplast is either absent or inactive in some guard cells.

They contain hormone receptors – allowing them to respond appropriately to changes in their environment. For instance, water scarcity in the soil causes the release of a hormone (abscisic acid (ABA)). This hormone is transported from the root cells to the receptors on guard cells which in turn causes the guard cells to close the stoma in order to prevent excessive water loss.

Bean/kidney-shape – The shape of guard cells is convenient for the closing and opening of the stoma to regulate gaseous exchange and release of water.

Guard cells are surrounded by a thin, elastic outer wall – contributes to the movement of water and solutes in and out of the cell.

Location – Depending on the habitat, guard cells may be located on the upper or lower surface of the leaf. This regulates the amount of water lost to the environment. For instance, in most aquatic plants, guard cells, and thus the stomata, are located on the upper surface of the leaf which allows for more water to be released into the environment. However, for plants in hotter/dry areas, these cells are located on the lower surface of the leaf and tend to be fewer in number.

Closing and Opening Mechanism

One of the most important functions of guard cells is to control the closing and opening of the stoma/pores. While the opening of these pores allows water to be released into the environment, it also allows carbon dioxide to enter the cell for photosynthesis (as well as the release of oxygen into the environment). For this reason, guard cells play a crucial role in photosynthesis.

Based on a number of studies, such factors as light intensity and hormones have been shown to influence the swelling or shrinkage of guard cells and thus the opening and closing of the pores. Here, with regards to pore opening, these factors influence water uptake into the cell causing the guard cells to inflate. This inflation/swelling results in the opening of the pores which in turn allows for gaseous exchange (as well as the release of water/transpiration).

While the process sounds to be a simple one, the signaling pathway that influences guard cell activities is yet to be fully understood. For this reason, a number of theories have been presented (and refuted) to describe the entire process/mechanism. Regardless, several aspects are well understood and will be highlighted in this section.

Theories aimed at explaining the movement of water in and out of guard cells include:

· The pH theory – An increase in the concentration of hydrogen ions causes a decrease in pH which in turn results in the conversion of glucose-1-phosphate to starch.

· Starch-sugar theory – Conversion of starch to sugar causes the osmotic potential to increase thus drawing water into the guard cells.

· Proton-potassium pump theory – Through a sequence of events, potassium ions are transported into the guard cells during the day increasing solute concentration and drawing water into the cell.

· Active K+ transport theory – An increase in potassium ions is caused by the conversion of starch to phosphoenolpyruvate and consequently malic acid.

Carbon Dioxide Sensing and Signaling

One of the factors that influence the swelling and shrinkage of guard cells is carbon dioxide concentration. In cases of high carbon dioxide concentration in the atmosphere, studies have shown anion channels to be activated causing potassium ions to move out of the cells. At the same time, chloride is released from the cells ultimately reusing in the depolarization of the membrane.

With solutes moving out of the cell, their concentration out of the cell increases as compared to that inside the cell. As a result, water is forced out of the cell through osmosis. In turn, this causes the cell to shrink and close the aperture/pore.

* Malate is suggested to be an intermediate effector between the gas (carbon dioxide) and activation of the channel.

* At low partial pressure of carbon dioxide in the atmosphere, the reverse occurs.

Abscisic Acid (ABA) Sensing and Signaling

In different types of plants, ABA (a plant hormone) has a number of functions ranging from controlling the germination of seeds to its impact on guard cells.

In such environmental conditions as drought or increased salinity in soil, roots have been shown to produce this hormone in higher amounts. The detection of this hormone by guard cells causes changes in the intake or removal of ions from the cells which in turn causes the opening or closing of the stoma. Here, a subunit of Mg-chelatase was shown to bind the hormone and thus serve as the intermediate.

In instances of high amounts of ABA, the efflux of anions as well as potassium through the channels occurs. At the same time, importation of potassium ions is inhibited which prevents the ions from moving into the cell (this would otherwise cause a high concentration of solutes in the cell). With high solute concentration outside the cell, water is forced out through osmosis, which in turn reduces turgor pressure of the guard cells. In turn, this causes the aperture to close, preventing the cells to lose any more water.

* Under normal environmental conditions, stomata open during the day to allow for intake of carbon dioxide and close at night when light-independent reactions (photosynthetic reactions) take place.

* At night, water enters the subsidiary cells from the guard cells which causes them to become flaccid (reducing turgor pressure in guard cells) and thus causing stoma to be closed.

See also Mesophyll Cells and Meristem Cells.

Return to studying Leaf Structure under the Microscope

Return from Guard Cells to MicroscopeMaster home

Cecie Starr. (1991). Biology: Concepts and Applications.

June M. Kwak, Pascal Mäser, Julian I. Schroeder. (2009). The Clickable Guard Cell, Version II: Interactive Model of Guard Cell Signal Transduction Mechanisms and Pathways.

J. M. Whatley. (1971). The Untrastructure of Guard Cells of Phaseolus Vulgaris.

Mareike Jezek and Michael R. Blatt. (2017). The Membrane Transport System of the Guard Cell and Its Integration for Stomatal Dynamics.

Sallanon Huguette, Daniel Laffray, and Alain Coudret. (1993). Structure, ultrastructure and functioning of guard cells of in vitro rose plants. ResearchGate.


New! Comments

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Gases enter and leave the leaf through stomata, which actively control the gas exchange of CO2 and H2O, for example, between the plant and the atmosphere.

Plants need to keep the stomata as open as possible to obtain atmospheric carbon dioxide but at the same time, they need to control the loss of water via transpiration. Therefore, plants actively optimise the status of stomata to gain carbon as much as possible but not to lose too much water.

Pairs of specialized guard cells form the stomata and control the opening of the small pore in between the guard cells. Plants physiologically control the opening and closing of stomata by accumulation of solutes in the guard cells. If guard cells are swollen, the stoma is open. The movement of water out of the guard cells results in closing of stoma. Stomata react to changes in light intensity (PAR), air humidity (VPD) and soil moisture (REW), for example.

In many plants, stomata are located in the lower leaf surface, whereas conifers have stomata on both needle surfaces. A leaf can have hundreds to thousands of stomata. The stomatal density is highest in leaves developed at high light, low CO2 and moist environments. When fully open, the stomatal pore is app. 5 µm wide and 10 µm long.

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