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: - **Waxy cuticle**: a clear waterproof coat that cuts water loss. - **Upper epidermis**: transparent, so light passes straight through. - **Palisade mesophyll**: tall cells packed with chloroplasts, right near the top to grab the light. - **Spongy mesophyll**: loosely packed cells with **air spaces** so gases diffuse easily. - **Lower epidermis**: the bottom layer, dotted with pores. - **Stomata**: those pores, opened and closed by **guard cells**, mostly on the underside.
Label the leaf cross-section
An interactive activity.
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
Why up top?
Why is the palisade mesophyll found near the TOP of the leaf rather than the bottom?
- It is packed with chloroplasts, so being near the top means it absorbs the most light
- So it can lose water more quickly
- Because that is where carbon dioxide enters the leaf
- To give the leaf its strength and hold it flat
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, that carry things in and out: - **Xylem** carries **water and mineral ions UP** from the roots. It is a one-way delivery of water for photosynthesis and to replace what transpiration loses. - **Phloem** carries **dissolved sugars** (made in the leaf) to wherever they are needed or stored. This movement is called **translocation**, and it can go in any direction.
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.