Leaf Structure and Function, revision notes

Overview

The leaf is the plant's main organ for photosynthesis, gas exchange and transpiration. This chapter covers the internal structure of a leaf and how each part is adapted to its function.

It also introduces the compensation point, where photosynthesis and respiration balance.

Guard cells control the opening and closing of each stoma: when guard cells take up water and become turgid, their unevenly thickened cell walls bow outward and pull the pore open; when they lose water and become flaccid, the pore closes. This mechanism links a leaf's water status directly to how much gas exchange and transpiration is possible at any moment.

The vein running through a leaf contains xylem, which brings water and dissolved minerals up from the roots, and phloem, which carries the sugars made by photosynthesis away to other parts of the plant; both tissues are therefore essential to keeping the mesophyll cells supplied and productive.

Leaves lose water far faster than they use it: typically less than one percent of the water a plant absorbs through its roots is actually used in photosynthesis or built into new plant tissue, while almost all the rest evaporates from the mesophyll and diffuses out through the stomata as transpiration.

A potometer is commonly used to estimate the rate of transpiration indirectly, by measuring how fast a plant shoot takes up water over time; because almost all the water taken up is eventually lost as vapour, the rate of water uptake gives a good estimate of the rate of transpiration under a given set of conditions.

The midrib and petiole, though mostly made of vascular and supporting tissue rather than photosynthetic cells, keep the lamina held up and angled toward the light; without this support the thin blade would collapse and overlap with neighbouring leaves, reducing the total light each leaf could intercept.

17.1 Structure of a Leaf

A dicotyledonous leaf is built from distinct tissue layers, each suited to a different job. Working from the top surface downward: a waxy cuticle covers the upper epidermis, a single layer of tall palisade mesophyll cells sits just beneath it, an irregular layer of spongy mesophyll with large air spaces lies below that, and a lower epidermis containing most of the stomata forms the base.

A network of veins, each containing xylem and phloem, runs through the mesophyll.

Because a leaf's tissues are all thin and close to a surface, gases and light reach every cell within a short diffusion distance. This internal layout is what makes the leaf the plant's main organ for gaseous exchange, transpiration and photosynthesis at the same time, and it is why exam questions on this chapter almost always start from a labelled cross-section.

Exam questions frequently present an unlabelled diagram of a leaf cross-section and ask candidates to identify a named tissue, state one visible feature that identifies it, and give its function in a single answer; marks are often lost when a candidate names the tissue correctly but forgets the identifying feature, such as the density of chloroplasts in the palisade layer or the large air spaces in the spongy layer.

17.2 Main Organ for Gaseous Exchange

The leaf exchanges carbon dioxide and oxygen with the atmosphere mainly through its stomata. Each stoma is a pore between a pair of guard cells; when the guard cells are turgid, their unevenly thickened walls bow apart and the pore opens, and when they become flaccid the pore closes.

Below the pore, the large air spaces of the spongy mesophyll act as a shared reservoir so that gases can diffuse quickly between every mesophyll cell and the outside air.

Placing most stomata on the lower, shaded surface keeps gaseous exchange efficient while limiting excessive water loss, since the lower surface receives less direct sunlight and heat than the upper surface. A leaf therefore balances two competing needs at the same pore: letting carbon dioxide in for photosynthesis while keeping water loss from evaporation under control.

Stomatal density, the number of stomata per unit leaf area, differs between species and even between the upper and lower surface of the same leaf. A plant growing in bright, dry conditions typically has a lower stomatal density to limit water loss, while a plant in shaded, humid conditions can afford a higher stomatal density because the pressure to conserve water is lower.

17.3 Main Organ for Transpiration

Transpiration is the loss of water vapour from a plant's surface, and most of it occurs through the leaf stomata. Water travels from the root, up the xylem in the vein, into the mesophyll cell walls, where it evaporates into the intercellular air spaces before diffusing out through open stomata.

Only a very small fraction of the water absorbed by the roots, typically under one percent, is actually used in photosynthesis or built into new plant tissue; nearly all of it is eventually lost this way.

The rate of transpiration rises with higher temperature, higher light intensity, lower humidity and stronger air movement, because each of these steepens the concentration gradient of water vapour between the leaf's air spaces and the surrounding air, or removes the thin, humid boundary layer that otherwise slows further diffusion. A potometer is the standard apparatus for estimating this rate indirectly: it measures how fast a cut shoot takes up water, which closely tracks the rate at which the shoot loses water as vapour.

The pathway water follows after leaving the xylem, evaporating from the mesophyll cell walls into the intercellular spaces before diffusing out through the stomatal pore, is sometimes tested as a four- or five-step sequence that candidates must place in the correct order; the two steps most often confused are evaporation from the cell wall surface, which happens inside the leaf, and diffusion through the stomatal pore, which is the final step into the outside air.

17.4 Main Organ for Photosynthesis

The palisade mesophyll is the leaf's main site of photosynthesis because its cells are packed with chloroplasts and positioned near the upper surface, where light intensity is greatest before it is absorbed or scattered by tissue above. A leaf's broad, thin shape also gives it a large surface area relative to its volume, so a high proportion of its cells can be reached by both light and carbon dioxide without materials having to travel far.

The rate of photosynthesis responds to light intensity, carbon dioxide concentration and temperature together, but only one of these limits the rate at any given moment. As light intensity rises from darkness, the rate of photosynthesis increases roughly in proportion, showing that light is the limiting factor; beyond a certain intensity the graph levels off, showing that another factor, commonly carbon dioxide concentration or temperature, has become limiting instead.

A graph of the rate of photosynthesis against light intensity is one of the most frequently drawn graphs in this chapter, and candidates are commonly asked to read off the light compensation point, identify the limiting factor in a labelled region, or predict how the curve would shift if carbon dioxide concentration or temperature were increased instead of light intensity.

17.5 Compensation Point

The compensation point is the light intensity at which a plant's rate of photosynthesis exactly equals its rate of respiration. At this point the carbon dioxide released by respiration is entirely reabsorbed by photosynthesis, and the oxygen released by photosynthesis is entirely used up by respiration, so there is no net exchange of either gas with the surrounding air.

Below the compensation point, a plant is a net absorber of oxygen and a net releaser of carbon dioxide, because respiration outpaces photosynthesis; above it, the plant is a net releaser of oxygen and a net absorber of carbon dioxide. Across a full day, a plant typically spends only the darkest hours below its compensation point, since sunlight during the day normally keeps light intensity above this threshold for most healthy, unshaded leaves.

Because the compensation point marks the boundary between net carbon dioxide absorption and net carbon dioxide release, a plant kept permanently below its compensation point, for example in very deep shade, will gradually lose more organic matter through respiration than it gains through photosynthesis and cannot sustain long-term growth, which is why the light compensation point also helps explain why some plants cannot survive in heavily shaded habitats.

Key concepts to master

• Leaf structure, From top to bottom: waxy cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, lower epidermis with stomata, and veins.
• Main organ for gaseous exchange, Stomata let carbon dioxide in and oxygen out; the spongy mesophyll has air spaces for diffusion.
• Main organ for transpiration, Water evaporates from the mesophyll and diffuses out through the stomata.
• Main organ for photosynthesis, The palisade mesophyll near the top is packed with chloroplasts to trap light.
• Adaptations, A broad, thin blade, many chloroplasts near the top, and stomata mainly on the lower surface make the leaf efficient.
• Compensation point, The light intensity at which the rate of photosynthesis exactly equals the rate of respiration, so there is no net gas exchange.
• Guard cells, A pair of guard cells surrounds each stoma; when turgid, their unevenly thickened walls curve the pair apart to open the pore, and when flaccid, the pore closes, regulating both gas exchange and water loss.
• Xylem and phloem in the leaf, Leaf veins contain xylem, which transports water and mineral ions from the roots to the mesophyll, and phloem, which transports the sugars produced by photosynthesis to other parts of the plant.
• Cuticle, A waxy, waterproof layer covering the upper, and to a lesser extent lower, epidermis that reduces uncontrolled water loss through the leaf surface itself, forcing most water loss to occur through the stomata instead.
• Rate of photosynthesis and light intensity, As light intensity increases from zero, the rate of photosynthesis increases, first limited by light itself, then eventually limited by another factor such as carbon dioxide concentration or temperature, at which point further increases in light no longer raise the rate.
• Surface area to volume ratio, A leaf's broad, flat shape gives it a large surface area relative to its volume, which increases the area available for light absorption, gas exchange and water loss, all of which occur across the leaf's surfaces rather than through its bulk.
• Boundary layer and humidity, A thin, still layer of humid air can build up just outside the stomata; wind or air movement disturbs this layer and increases the rate of transpiration by maintaining a steeper water vapour concentration gradient between the leaf and the surrounding air.
• Midrib and petiole, The midrib is the thickened central vein running the length of the leaf, and the petiole is the stalk joining the leaf to the stem; both contain vascular tissue and give the thin lamina mechanical support so it can be held at an angle to the light.

Quick recall checklist

1. Can you define and explain Leaf structure?
2. Can you define and explain Main organ for gaseous exchange?
3. Can you define and explain Main organ for transpiration?
4. Can you define and explain Main organ for photosynthesis?
5. Can you define and explain Adaptations?
6. Can you define and explain Compensation point?
7. Can you define and explain Guard cells?
8. Can you define and explain Xylem and phloem in the leaf?
9. Can you define and explain Cuticle?
10. Can you define and explain Rate of photosynthesis and light intensity?
11. Can you define and explain Surface area to volume ratio?
12. Can you define and explain Boundary layer and humidity?
13. Can you define and explain Midrib and petiole?

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