2.5.3Structure and Function in Living Organisms
Leaf: Structure and Adaptations
5 min read 9 parts Lesson 16 of 55 1 quiz
Section 1 of 9The leaf as an organ
A leaf is an organ whose main function is photosynthesis.
The leaf as an organ
It is thin and flat, giving a large surface area to absorb light and a short diffusion distance for gases.
Its tissues are arranged in layers,
- from the top downwards: the upper epidermis, the palisade mesophyll, the spongy mesophyll with air spaces, the lower epidermis with stomata,
- and veins running through the middle.
Section 2 of 9Tissues and their adaptations
| Tissue | Structure | Function and adaptation |
|---|---|---|
| Waxy cuticle | Waterproof layer on the outside | Reduces water loss by evaporation |
| Upper epidermis | Thin, transparent layer of cells | Protects the leaf and lets light through to the cells below |
| Palisade mesophyll | Tall, tightly packed cells with many chloroplasts, near the top | Main site of photosynthesis, as it receives the most light |
| Spongy mesophyll | Rounded cells with large air spaces between them | Allows gases to diffuse through the leaf to the cells; some photosynthesis |
| Lower epidermis with stomata | Contains pores and guard cells | Gas exchange: carbon dioxide in, oxygen and water vapour out |
| Xylem and phloem in veins | Vascular bundles | Bring water and take away sugar; the veins support the leaf |
Section 3 of 9Stomata and guard cells
Stomata are small pores, mostly on the underside of the leaf, where it is cooler and less water is lost.
Guard cells
Each stoma is surrounded by two guard cells.
When water enters the guard cells by osmosis, they swell and become turgid.
Their inner walls are thicker, so they bend and curve apart, opening the pore.
When they lose water, they become flaccid, and close.
This opening and closing controls gas exchange and water loss.
Stomata are usually open in daylight so that carbon dioxide can enter for photosynthesis,
- and close at night or when the plant is short of water, to avoid wilting.
Because a closed stoma stops carbon dioxide entering, hot dry conditions can slow photosynthesis.
Section 4 of 9Gas exchange and diffusion
Carbon dioxide diffuses into the leaf through the stomata and moves through the air spaces of the spongy mesophyll to the palisade cells, down a concentration gradient maintained because palisade cells use it up.
Gas exchange and diffusion
Oxygen produced in photosynthesis diffuses out the other way.
The cells are covered by a thin film of water, in which gases dissolve before entering.
The large surface area of the mesophyll cells in contact with air spaces speeds up this diffusion, a feature you can compare with alveoli in lungs.
Section 5 of 9Leaf adaptations for different environments
Leaf structure varies with habitat.
Leaf adaptations for different environments
Plants of dry places have thicker cuticles and fewer or sunken stomata,
- which reduces water loss,
- while plants floating on water have stomata on the upper surface,
- because the lower surface is in contact with the water.
Shade plants tend to have thinner leaves and larger surface area to catch weak light.
More about these adaptations appears in the lesson on extreme conditions.
Section 6 of 9Drawing and calculating from leaf sections
Questions often provide a micrograph of a leaf cross-section and ask you to label tissues or calculate magnification.
Drawing and calculating from leaf sections
To identify the palisade layer, look for elongated cells just below the upper epidermis with dark green chloroplasts; spongy mesophyll cells are rounder with gaps.
Magnification = size of image / size of real object,
- so if a palisade cell measures 30 mm in the image and its real length is 0.06 mm, magnification = 30 / 0.06 = x500.
- Remember to convert units consistently.
Stomatal density, the number of stomata per mm2, can be counted from nail varnish impressions of the leaf surface under the microscope.
Different leaf layers also affect the rate of photosynthesis in the practical context.
1
Because the upper epidermis is transparent and has no chloroplasts (except guard cells), light passes straight through.
2
The cuticle is thicker on the upper surface, where the Sun heats the leaf.
3
The lower epidermis has many stomata,
- which can open and close;
- the opening is controlled by the water content of guard cells and by light level and carbon dioxide concentration.
4
If the plant is losing too much water, a hormone signal causes the guard cells to lose water, so the stomata close.
5
This protects the plant from wilting but reduces photosynthesis.
6
The cooling effect of evaporation from the leaf also helps keep the leaf from overheating.
7
The veins branch through the leaf so that every cell is close to a vessel, meaning that water and sugar travel only a short distance by diffusion.
Section 7 of 9Exam-style questions
Explain why most chloroplasts are in the palisade layer. [2]2 marks
The palisade layer is near the upper surface and receives most light [1], so the chloroplasts there can absorb the most light for photosynthesis [1].
Describe how guard cells open a stoma. [3]3 marks
Water enters the guard cells by osmosis [1] so they become turgid [1]. The inner wall is thicker, so the cells bend apart and open the pore [1].
Explain why the leaf has air spaces in the spongy mesophyll. [2]2 marks
The air spaces allow carbon dioxide to diffuse quickly to cells through the leaf [1], and oxygen to diffuse out; they also give a large surface area for gas exchange [1].
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Palisade mesophyll | The layer of tall cells with many chloroplasts where most photosynthesis occurs. |
| Spongy mesophyll | A layer of loosely packed cells with air spaces. |
| Stoma | A pore in the leaf epidermis for gas exchange. |
| Guard cell | One of a pair of cells that control the opening of a stoma. |
| Cuticle | A waxy waterproof layer on the leaf surface. |
| Turgid | Swollen and firm because of water pressure inside the cell. |
| Epidermis | The outer layer of cells of a leaf. |
Section 9 of 9Exam technique
1
Saying stomata are in the cuticle. They are pores in the epidermis.
2
Stating that guard cells open when they lose water.
3
Saying that all the leaf cells have the same number of chloroplasts. Palisade cells have the most.
4
Thinking oxygen enters the leaf in photosynthesis. Oxygen leaves; carbon dioxide enters.