Leaf Structure and Function
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.
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Content standards in this chapter
- 17.1 The Structure of a Leaf
- 17.2 The Leaf as the Main Organ for Gaseous Exchange
- 17.3 The Leaf as the Main Organ for Transpiration
- 17.4 Main Organ for Photosynthesis
- 17.5 Compensation Point
Key concepts
- 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.
How this chapter is examined
SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.
Common exam angles
- Labelling a cross-section of a leaf and giving the function of each part.
- Linking a leaf adaptation to photosynthesis, gas exchange or transpiration.
- Explaining the compensation point.
- Explaining how guard cells open and close a stoma.
- Relating a leaf vein to the two transport tissues it contains and their functions.
- Identifying the limiting factor on a graph of photosynthesis rate against light intensity.
- Explaining why most of the water absorbed by roots is lost through transpiration rather than used in photosynthesis.
- Describing how a potometer estimates the rate of transpiration.
Common mistakes
What students write: Saying photosynthesis happens most in the spongy mesophyll.
What earns the mark: Most photosynthesis is in the palisade mesophyll, which has the most chloroplasts and is near the top.
What students write: Writing that stomata are mainly on the upper surface.
What earns the mark: In most leaves stomata are more numerous on the lower surface, which reduces water loss.
What students write: Confusing the cuticle with the epidermis.
What earns the mark: The cuticle is a waxy waterproof layer on top of the epidermis; the epidermis is the layer of cells beneath it.
What students write: Saying no gases are exchanged at the compensation point.
What earns the mark: At the compensation point photosynthesis and respiration both occur, but their gas exchanges cancel out, so there is no net exchange.
What students write: Saying guard cells are ordinary epidermal cells.
What earns the mark: Guard cells are specialised cells with unevenly thickened walls and chloroplasts, unlike the surrounding ordinary epidermal cells, which lack chloroplasts and do not control pore size.
What students write: Mixing up which tissue in a vein carries water and which carries sugar.
What earns the mark: Xylem carries water and minerals upward from the roots; phloem carries the sugars made in photosynthesis away from the leaf to other parts of the plant.
What students write: Saying the cuticle absorbs water for the leaf.
What earns the mark: The cuticle is a waterproof waxy layer whose function is to reduce water loss from the leaf surface, not to absorb water.
What students write: Assuming light intensity always limits the rate of photosynthesis.
What earns the mark: Light intensity limits the rate only up to a point; beyond that, another factor such as carbon dioxide concentration or temperature becomes limiting instead.
What students write: Assuming most of the water absorbed by roots is used directly in photosynthesis.
What earns the mark: Only a very small fraction of absorbed water is used in photosynthesis; nearly all of it is lost through transpiration.
What students write: Saying a potometer measures the rate of photosynthesis.
What earns the mark: A potometer measures the rate of water uptake by a shoot, which is used as an estimate of the rate of transpiration, not photosynthesis.
What students write: Describing the midrib only as 'the middle part' of the leaf without stating its function.
What earns the mark: The midrib contains the main vascular bundle of the leaf and gives the lamina mechanical support, in addition to transporting water and sugars.
Study this chapter
Structures in this chapter
Experiments in this chapter
Frequently asked questions
Why does most photosynthesis happen in the palisade mesophyll?
Why are stomata mostly on the lower surface of a leaf?
What is the compensation point?
How do guard cells open and close a stoma?
What is the difference between xylem and phloem in a leaf?
Why does a plant lose so much more water than it actually uses?
What does a potometer actually measure?
What is the function of the midrib and petiole?
Source:SRC-DSKP-EN, SRC-FORMAT
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