Lesson focus: Leaf tissues balance light capture and gas exchange with water transport and water-loss control.
Relate leaf structures to photosynthesis and explain how stomata create a trade-off between carbon dioxide uptake and water loss.
Build on earlier learning
PhotosynthesisThe previous lesson established the inputs of photosynthesis. Leaf structure determines how those inputs reach photosynthetic cells.
Practical
Counting stomata
- Paint clear nail varnish on the underside of a leaf and let it dry.
- Peel it off with clear tape and stick it to a slide.
- Count the stomata in the field of view at high power.
Record: Stomata counts for upper and lower surfaces.
You will be successful when you can…
- Identify major tissues in a leaf cross-section.
- Link palisade cells to light absorption.
- Explain xylem and phloem roles.
- Explain stomatal control as a trade-off.
Words that carry the science
- palisade mesophyll
- A leaf tissue of closely packed, chloroplast-rich cells near the upper surface.
- spongy mesophyll
- A leaf tissue with air spaces that aid gas diffusion.
- stoma
- A pore in the epidermis through which gases and water vapour move.
- guard cell
- A specialised cell that changes shape to alter stomatal aperture.
- xylem
- Vascular tissue that transports water and mineral ions and provides support.
- phloem
- Vascular tissue that transports dissolved assimilates such as sucrose.
Relate each leaf tissue to transport or exchange
01A broad, thin blade supports exchange
A broad lamina provides a large area for intercepting light. Its small diffusion distance helps carbon dioxide reach mesophyll cells. A transparent cuticle and upper epidermis allow light to reach the palisade mesophyllThe layer of tall, tightly packed cells near the top of a leaf, where most photosynthesis happens..
Palisade cells contain many chloroplasts and are commonly positioned near the upper surface. spongy mesophyllA layer of loosely packed cells in a leaf with air spaces between them so gases can move. air spaces increase internal exchange surfaces and connect with stomata.
02Vascular bundles connect the leaf to the plant
xylemTubes in a plant that carry water and minerals up from the roots. delivers water and mineral ions from roots. phloemTubes in a plant that carry sugars made in the leaves to other parts of the plant. transports assimilates, commonly sucrose, between sources and sinks. Veins also help support the lamina.
Water evaporates from moist mesophyll surfaces and diffuses through stomata. This transpiration contributes to water movement but can also dehydrate the plant.
03Guard cells regulate a compromise
When guard cellOne of a pair of cells around a stoma (a pore in a leaf) that change shape to open and close it. gain water and become turgid, their geometry can open the pore. When they lose water, the pore narrows. Opening supports carbon dioxide uptake but also increases water-vapour loss.
Many terrestrial broad leaves have more stomata on the lower surface, reducing exposure to sun and moving air, but this pattern has important exceptions among habitats and species.
Gas exchange in a leaf
Gases move in and out of a leaf through tiny pores called stomata, mostly on the underside. Guard cells open the stomata in the light and close them in the dark. In the light, photosynthesis takes in carbon dioxide faster than respiration gives it out.
Blue: gas diffusing in. Red: gas diffusing out. Two guard cells surround each stoma. Air spaces in the spongy layer let gases reach every cell.
Controls
Explain a leaf adaptation
Explain why palisade mesophyll is commonly near the upper leaf surface.
- Light usually reaches the upper surface first.
- The upper epidermis is thin and transparent.
- Palisade cells contain many chloroplasts with chlorophyll.
- Positioning these cells near incoming light increases the chance of absorption before light is scattered or absorbed elsewhere.
How to make the reasoning visible: A complete answer links position, chloroplast abundance and light absorption to photosynthesis.
Interpret a stomatal-print data set
The counts are illustrative means from equal microscope fields.
| Leaf surface | Mean stomata per field | Likely consequence | Cautious interpretation |
|---|---|---|---|
| Upper | 18 | Some gas exchange | Not zero; species-specific pattern |
| Lower | 82 | Greater potential exchange | May reduce exposure compared with upper surface |
| Lower after dry treatment | 46 | Lower potential conductance | Could reflect development or closure visibility |
| Upper after dry treatment | 15 | Small change | Needs repeats and matched leaves |
Why must microscope field area be kept the same?
Why does a stomatal count not directly measure photosynthesis?
Check your understanding
Answer from memory, then check against the suggested answer.
Which tissue usually contains the most chloroplasts?
What does xylem transport?
What is a stoma?
Predict one advantage and one disadvantage of closing stomata during hot, dry weather.
Explain why a floating aquatic leaf may have stomata mainly on its upper surface.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
The internal structure of a leaf is adapted for photosynthesis.
Describe how light reaches the chloroplasts and is absorbed.
(2)Explain how the leaf obtains carbon dioxide and water.
(3)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., name each structure and immediately state how it supports an input or process.