A leaf is a food factory only a fraction of a millimetre thick. Its broad, thin shape catches light and keeps diffusion distances short, and each layer inside, from the waxy cuticle to the stomata, has a job in getting light, carbon dioxide and water to the chloroplasts and the products away.
Most leaves have a large surface area, to absorb as much light as possible, and are thin, so carbon dioxide has only a short distance to diffuse to the mesophyll cells and light can reach all of them. Inside, palisade cells packed with chloroplasts sit near the upper surface, air spaces in the spongy mesophyll connect to the stomata for gas exchange, and veins bring water in the xylem and carry sucrose away in the phloem.
Two features of the whole leaf matter before you look inside. Both are adaptations that make photosynthesis faster.
Say what the feature does for photosynthesis: "large surface area to absorb more light", "thin, so a short diffusion distance for carbon dioxide". Just writing "large surface area" scores nothing.
Rotate the block cut from a leaf in the sim and click each tissue, or pick it from the list. The chosen tissue lights up and the rest fade, with an explanation of how it helps photosynthesis.
| Structure | Feature | How it helps photosynthesis |
|---|---|---|
| Waxy cuticle | thin, waxy, transparent | reduces water loss; lets light through |
| Upper epidermis | one layer of transparent cells, no chloroplasts | light passes straight through to the palisade |
| Palisade mesophyll | tall, closely packed cells near the top, many chloroplasts | absorbs most light; most photosynthesis happens here |
| Chloroplasts | contain chlorophyll | absorb light energy |
| Spongy mesophyll | rounded, loosely packed cells, moist surfaces | gases dissolve and diffuse into and out of cells |
| Air spaces | connected spaces between spongy cells | large internal surface; carbon dioxide diffuses quickly to every cell |
| Guard cells and stomata | pairs of guard cells around pores, mostly in the lower epidermis | open to let carbon dioxide in and oxygen out; close to save water |
| Xylem (in vascular bundle) | vessels in the vein | brings water (a raw material) and mineral ions |
| Phloem (in vascular bundle) | tubes in the vein | carries sucrose made from the glucose away |
The upper epidermis cells have no chloroplasts, but guard cells do. And in a vascular bundle in a leaf, the xylem is on the upper side and the phloem on the lower side.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the leaf sim, which tissue is described as tall, closely packed cells full of chloroplasts?
According to the sim, what is carried away from the leaf in the phloem?
In the sim, the stomata with their guard cells are found in the:
Why does the sim say the upper epidermis has no chloroplasts?
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Most leaves are thin. How does this help photosynthesis?
Which feature of the spongy mesophyll helps gas exchange?
Which structure brings the water used in photosynthesis into the leaf?
Why are the palisade cells near the upper surface of the leaf?
This skill is now recorded on your profile. Keep the chain going.