Gas exchange in plants and the opening of stomata – Gas Exchange in Animals and Plants (Senior 5 Biology) | Ganzaa

Gas Exchange in Animals and Plants

Lessons

Senior 5 · Biology

Gas Exchange in Animals and Plants

5 lessons

Lesson 5 of 5

Gas exchange in plants and the opening of stomata

25 min

A peel of nail varnish

Paint a thin patch of clear nail varnish on the underside of a leaf, let it dry, and peel it off with clear tape. Under a microscope the peel shows a print of the leaf surface: jigsaw-shaped epidermal cells and, scattered among them, hundreds of tiny pores, each between two bean-shaped cells. These are stomata, the doors through which a leaf exchanges gases with the air. This lesson shows how plants exchange gases without lungs or breathing movements, and how a stoma opens and closes.

Definition

Guard cells: a pair of specialised epidermal cells that surround a stoma and change shape to open or close it.

Lenticel: a patch of loosely packed cells in the bark of a woody stem or root through which gases diffuse.

Compensation point: the light intensity at which a plant's rate of photosynthesis equals its rate of respiration, so there is no net exchange of oxygen or carbon dioxide.

Abscisic acid (ABA): a plant hormone, made especially when a plant is short of water, that makes stomata close.

Gas exchange in a flowering plant

Plants need no ventilation and no blood to carry gases, for three reasons: their demand for oxygen is low, because their metabolic rate is low; every living cell is close to an air space; and gases diffuse about ten thousand times faster in air than in water.

Leaves: the spongy mesophyll has large air spaces, so the wet cell walls inside a leaf give a huge internal surface for exchange. The leaf is thin, so diffusion distances are short. Gases enter and leave through the stomata.
Stems and roots: woody stems exchange gases through lenticels. Roots take oxygen from the air between soil particles, which is why a waterlogged field, where water fills those spaces, can kill a crop's roots.

Which way the gases move depends on the balance between photosynthesis and respiration. Respiration goes on all the time; photosynthesis only in the light.

Condition	Net carbon dioxide movement	Net oxygen movement
Darkness	Out of the leaf (respiration only)	Into the leaf
Dim light, at the compensation point	None	None
Bright light	Into the leaf (photosynthesis faster than respiration)	Out of the leaf

Worked example 1: a hydrogencarbonate indicator experiment

Hydrogencarbonate indicator is red when in equilibrium with normal air, turns yellow as carbon dioxide rises, and turns purple as carbon dioxide falls. Three sealed tubes each hold indicator and a leaf, on a platform so that the leaf does not touch the liquid. After 2 hours: tube A (dark) is yellow; tube B (dim light) is still red; tube C (bright light) is purple. A fourth tube, D, with no leaf, in bright light, stays red. Explain the results.

A: in the dark the leaf only respires, so it releases carbon dioxide: the indicator turns yellow.
B: photosynthesis uses carbon dioxide exactly as fast as respiration makes it: the leaf is at its compensation point, so there is no net change.
C: photosynthesis is faster than respiration, so the leaf takes in more carbon dioxide than it releases: the indicator turns purple.
D is the control: it shows that light alone does not change the indicator, so the changes in A to C are caused by the leaf.

The structure of a stoma

Two guard cells, kidney-shaped in most flowering plants and dumbbell-shaped in grasses, surround the pore.
The wall next to the pore (the inner wall) is thick and stiff; the outer wall is thin and stretchy. Bands of cellulose microfibrils run around each guard cell like hoops, so it cannot get fatter, only longer.
So when a guard cell takes in water and swells, its thin outer wall stretches more than its thick inner wall, and the cell bends outwards like a bow: the pore opens. When it loses water, the cells straighten and the pore closes.
Unlike other epidermal cells, guard cells contain chloroplasts and many mitochondria, and their membranes have many proton pumps and ion channels.

The epidermis of a geranium leaf at a total magnification of ×400, photographed through a microscope. Each stoma is a pair of bean-shaped guard cells around a pore: some pores are wide open, as dark lens-shaped gaps, while others are almost closed. The surrounding epidermal cells have wavy walls, and green mesophyll tissue shows at the top. Photo: BioGenZambrano, CC BY-SA 4.0, via Wikimedia Commons

How do guard cells take in water? Two theories

Every theory must explain how the water potential of the guard cells falls in the light, so that water enters them by osmosis.

1. The starch–sugar hypothesis (older)

Starch is insoluble and has no effect on water potential; sugar is soluble and lowers it. The hypothesis, from the early twentieth century, says: in the light, photosynthesis in the guard cells uses carbon dioxide, so the pH in the cells rises; at high pH an enzyme (phosphorylase) converts starch to sugar, the water potential falls, water enters and the stoma opens. In the dark, respiration makes the cells more acidic, sugar is turned back into starch, and the stoma closes.

Problems: stomata open within minutes, faster than starch can be converted; the guard cells of some plants, such as onion, contain no starch; and the measured changes in sugar are too small to explain the changes in water potential.

2. The potassium ion (proton pump) theory (accepted today)

When the stoma opens, the concentration of potassium ions in the guard cells rises several times over, which has been measured directly; and substances that stop ATP production stop opening. Both observations support the theory. To close a stoma, the steps reverse: in the dark, or when ABA signals that the plant is short of water, potassium and other ions leave the guard cells, the water potential rises, water leaves by osmosis, and the cells become flaccid.

Stomatal closure saves water but stops carbon dioxide entering, so a plant in a drought must choose between wilting and starving. This is why yields of maize and beans fall in a dry season even before the plants wilt.

Worked example 2: stomatal density from a peel

A student views a nail-varnish peel at ×400. The field of view is a circle of diameter 0.40 mm. She counts a mean of 24 stomata per field on the lower surface and 6 on the upper surface. Calculate the stomatal density of each surface, in stomata per mm², and comment.

Radius $= 0.20\,\mathrm{mm}$ ; area of the field $= \pi r^2 = \pi \times 0.20^2 = 0.126\,\mathrm{mm^2}$ .
Lower surface: $\frac{24}{0.126} = 191$ stomata per mm².
Upper surface: $\frac{6}{0.126} = 48$ stomata per mm².
The lower surface has $\frac{191}{48} \approx 4$ times as many stomata.

Comment: the lower surface is shaded and cooler, so having most stomata there lets the leaf take in CO₂ while losing less water. The count should be repeated in several fields to get a reliable mean.

Warning

Common mistakes and safety

"Plants respire only at night." They respire all the time; at night respiration is simply not masked by photosynthesis.
"Guard cells open the stoma when they lose water." They open when they gain water and become turgid.
"Potassium ions enter by osmosis." Ions enter through channels, driven by the gradient set up by proton pumps; it is water that enters by osmosis.
Safety: nail varnish is flammable and its vapour is irritant: use it in a ventilated room, away from flames.

OtterBioTutor: "How the Stoma Opens & Closes, Effect of Abscisic Acid" (Cambridge AS & A Level Biology). Watch for the proton pumps, the entry of potassium ions and the role of ABA in closing, and compare each step with Figure 10.

Tip

Memory hook for opening: "H⁺ out, K⁺ in, Water in, stoma Open": H-K-W-O. For closing, run it backwards, starting with ABA.

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Practice: gas exchange in plants and stomata

Give three reasons why flowering plants need no ventilation system.
Describe the net exchange of oxygen and carbon dioxide by a leaf (a) in the dark, (b) in bright light.
Explain how the uneven thickness of the guard cell walls makes a stoma open when the cells become turgid.
State two pieces of evidence against the starch–sugar hypothesis.
Outline the potassium ion theory of stomatal opening.
A field of view of diameter 0.50 mm contains 18 stomata. Calculate the stomatal density per mm².
Explain why waterlogged soil can kill the roots of a bean crop.
Suggest why the stomata of a maize plant close on a hot, dry afternoon, and explain the effect on photosynthesis.

Show answersHide answers

Any three: low metabolic rate and low oxygen demand; every cell is near an air space; leaves are thin; gases diffuse quickly through air; a large internal surface of mesophyll.
(a) Carbon dioxide out, oxygen in (respiration only). (b) Carbon dioxide in, oxygen out (photosynthesis faster than respiration).
The thin outer wall stretches more than the thick inner wall, and the hoops of cellulose stop the cell getting fatter, so it lengthens and bows outwards, pulling the inner walls apart.
Any two: stomata open faster than starch can be converted to sugar; some guard cells, as in onion, contain no starch; sugar changes are too small to explain the change in water potential.
Light activates proton pumps that use ATP to pump H⁺ out; the cell interior becomes more negative; K⁺ channels open and K⁺ enters, with Cl⁻ and malate balancing the charge; the water potential falls; water enters by osmosis; the cells become turgid and bow apart.
Area $= \pi \times 0.25^2 = 0.196\,\mathrm{mm^2}$ ; density $= \frac{18}{0.196} \approx 92$ per mm².
Water fills the air spaces in the soil, so the roots lack oxygen for aerobic respiration; they cannot make enough ATP for active uptake of ions, and anaerobic respiration produces toxic ethanol.
The plant loses water faster than its roots can replace it, so ABA is made and causes the guard cells to lose K⁺ and water and close. Closed stomata stop CO₂ entering, so photosynthesis slows.

Key points

Plants exchange gases by diffusion through stomata and lenticels; the mesophyll air spaces give a large, moist internal surface.
Net exchange depends on photosynthesis versus respiration; at the compensation point there is none.
Guard cells have thick inner walls and radial cellulose hoops, so turgid cells bow apart and open the pore.
Accepted mechanism: light → proton pumps → K⁺ in → water potential falls → water in → open; ABA reverses it.

1A peel of nail varnish

2Gas exchange in a flowering plant

3The structure of a stoma

4How do guard cells take in water? Two theories