biology

Guard cell

Guard cell is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Guard cell rather than just read about it. In short: Guard cells are specialized cells in the epidermis of leaves, stems and other organs of land plants that are used to control gas exchange. They are produced in pairs with a gap between them that forms a stomatal pore.

Key takeaways

Guard cell belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Guard cell to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Guard cell from memory before moving on to harder problems.

Reference excerpt

Guard cells are specialized cells in the epidermis of leaves, stems and other organs of land plants that are used to control gas exchange. They are produced in pairs with a gap between them that forms a stomatal pore. The stomatal pores are largest when water is freely available and the guard cells become turgid, and closed when water availability is critically low and the guard cells become flaccid. Photosynthesis depends on the diffusion of carbon dioxide (CO2) from the air through the stomata into the mesophyll tissues. Oxygen (O2), produced as a byproduct of photosynthesis, exits the plant via the stomata. When the stomata are open, water is lost by evaporation and must be replaced via the transpiration stream, with water taken up by the roots. Plants must balance the amount of CO2 absorbed from the air with the water loss through the stomatal pores, and this is achieved by both active and passive control of guard cell turgor pressure and stomatal pore size.

Guard cell function Guard cells are cells surrounding each stoma. They help to regulate the rate of transpiration by opening and closing the stomata. Light is the main trigger for the opening or closing. Guard cells are bean-shaped, with a thicker inner side (i.e. the side closest to the pore) and a thinner outer side As water enters the cell, the thin side bulges outward more than the thicker side does. Thus the guard cell bends outwards and the stoma opens. Guard cells contain phototropin proteins which are serine and threonine kinases with blue-light photoreceptor activity. Phototrophins contain two light, oxygen, and voltage sensor (LOV) domains, and are part of the PAS domain superfamily. The phototropins trigger many responses such as phototropism, chloroplast movement and leaf expansion as well as stomatal opening. Not much was known about how these photoreceptors worked prior to around 1998. The mechanism by which phototropins work was elucidated through experiments with broad bean (Vicia faba). Immunodetection and far-western blotting showed blue light excites phototropin 1 and phototropin 2, causing protein phosphatase 1 to begin a phosphorylation cascade, which activates H+-ATPase, a pump responsible for pumping H+ ions out of the cell. The phosphorylated H+-ATPase allows the binding of a 14-3-3 protein to an autoinhibitory domain of the H+-ATPase at the C terminus. Serine and threonine are then phosphorylated within the protein, which induces H+-ATPase activity. The same experiment also found that upon phosphorylation, a 14-3-3 protein was bound to the phototropins before the H+-ATPase had been phosphorylated. In a similar experiment, researchers concluded that the binding of 14-3-3 protein to the phosphorylation site is essential for the activation of plasma membrane H+-ATPase activity. This was done by adding phosphopeptides such as P-950, which inhibits the binding of 14-3-3 protein, to phosphorylated H+-ATPase and observing the amino acid sequence. As protons are being pumped out, a negative electrical potential was formed across the plasma membrane. This hyperpolarization of the membrane allowed the accumulation of charged potassium (K+) ions and chloride (Cl−) ions, which in turn, increases the solute concentration causing the water potential to decrease. The negative water potential allows for osmosis to occur in the guard cell, so that water enters, allowing the cell to become turgid. Opening and closure of the stomatal pore is mediated by changes in the turgor pressure of the two guard cells. The turgor pressure of guard cells is controlled by movements of large quantities of ions and sugars into and out of the guard cells. Guard cells have cell walls of varying thickness(its inner region, adjacent to the stomatal pore is thicker and highly cutinized) and differently oriented cellulose microfibers, causing them to bend outward when they are turgid, which in turn, causes stomata to open. Stomata close when there is an osmotic loss of water, occurring from the loss of K+ to neighboring cells, mainly potassium (K+) ions.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Guard cell

Start with the simplest possible case. Write down what Guard cell claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Guard cell before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Guard cell ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Guard cell

In research: Guard cell appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Guard cell in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Guard cell is common in secondary-school and first-year university syllabi. It links to neighbouring topics Plant cells, so understanding it makes those chapters shorter.
In everyday life: Look for Guard cell outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Guard cell in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Guard cell means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Guard cell out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Guard cell in simple terms?

Why does Guard cell matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Guard cell?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Guard cell.