Leaf Architecture
Cut a leaf in half and every layer has a job. Label the tissues, match them to their functions, and see how a leaf is engineered to catch light, swap gases and save water.
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Leaf Architecture
The Energy Factory showed you what photosynthesis does. Now look at the building it happens in. Slice a leaf across and you find layers, each shaped for a job. A leaf has to do three things at once: catch as much light as possible, swap gases with the air, and avoid losing too much water. Its structure is a neat compromise between all three.
From top to bottom
Working down through a leaf. Every layer is placed where it is for a reason, and the reason is what the marks are for:
Label the leaf cross-section
Drag each label to the right layer. Top to bottom the leaf runs cuticle, epidermis, palisade, spongy, then the stomata on the lower surface.
Match each tissue to its job
- Palisade mesophyll
- Spongy mesophyll
- Waxy cuticle
- Guard cells
- Packed with chloroplasts near the top to absorb the most light
- Air spaces that let gases diffuse to and from the cells
- A waterproof coat that reduces water loss from the surface
- Open and close the stomata to control gas exchange and water loss
Match structure to job
The palisade mesophyll sits near the top of the leaf because its cells are packed with _____, so that position catches the most light. The spongy mesophyll below it is loosely packed with _____, which let gases diffuse to and from the cells. Over the whole leaf a waxy _____ reduces water loss, and it is transparent so it does not block the _____ getting in.
Swapping gases
Photosynthesis needs carbon dioxide and releases oxygen. These gases move in and out through the stomata by diffusion. Guard cells on either side of each pore change shape to open the stoma in the light and close it in the dark, or when the plant is short of water. Open stomata also let water vapour escape, which is the unavoidable price of letting CO2 in.
Top or bottom?
Most stomata are on the LOWER surface of a leaf. Why is that an advantage?
- The underside is shadier and cooler, so less water vapour is lost through the open pores
- So they can absorb more light directly
- So rain can pour straight into them
- To make the underside look greener
Built for the desert
A xerophyte is a plant adapted to very dry places, like a cactus. Its adaptations all serve ONE purpose: reducing water loss. Select ALL THREE genuine xerophyte adaptations.
- A very thick waxy cuticle
- Few, sunken stomata
- Leaves reduced to spines with a small surface area
- Broad, thin leaves with a huge surface area
- No stomata at all, so it never needs carbon dioxide
- A thin cuticle to let water evaporate faster
The plumbing
Running through the leaf are veins, or vascular bundles. Two vessels, and questions here always turn on telling them apart: what is carried, which way, and what the movement is called.
Xylem or phloem?
Sugars made in the leaf need to reach a growing root tip. Which vessel carries them, and what is the movement called?
- Phloem, by translocation
- Xylem, by translocation
- Xylem, by transpiration
- Phloem, by transpiration
Leaf summary
Light passes through the transparent upper epidermis to the _____ mesophyll, whose cells are packed with chloroplasts. Gases diffuse through air spaces in the _____ mesophyll and in and out through pores called _____, which are controlled by guard cells. Water travels up the _____ while sugars are moved around the plant in the phloem.