Photosynthesis
Senior 2 · Biology
Photosynthesis
Lesson 3 of 4
22 min
Let's begin
A leaf is a solar-powered food factory. Look at a banana leaf: it is wide and very thin, held out flat to face the sky, and threaded with veins. None of this is accidental. Each feature helps the leaf capture light, take in carbon dioxide, receive water and send food away.
Most leaves have a flat blade called the lamina, joined to the stem by a stalk called the petiole. A midrib runs down the middle and branches into veins, which carry water in and food out.
Seen in cross-section (a thin slice cut across the leaf), a typical leaf has these layers, from top to bottom:
| Layer | Feature | How it helps photosynthesis |
|---|---|---|
| Cuticle | thin, waxy, transparent layer | reduces water loss but lets light through |
| Upper epidermis | one layer of transparent cells with no chloroplasts | protects the leaf and lets light pass to the cells below |
| Palisade mesophyll | tall, column-shaped cells, tightly packed near the upper surface, full of chloroplasts | the main site of photosynthesis; absorbs most of the light |
| Spongy mesophyll | rounded, loosely packed cells with large air spaces | air spaces let carbon dioxide and oxygen diffuse through the leaf |
| Lower epidermis | contains many stomata, each between two guard cells | stomata let carbon dioxide in and oxygen out; guard cells open and close them |
| Vein (vascular bundle) | xylem and phloem | xylem brings water and minerals; phloem carries sugars away |
Guard cells are the only epidermal cells with chloroplasts.
Worked example
Explain why most stomata are on the lower surface of a leaf. Reasoning: the upper surface faces the sun and is hotter, so water would evaporate faster through stomata there. Placing stomata underneath lets gases be exchanged while losing less water.
"Chlorophyll" is really a mixture of pigments. You can extract and separate them in the laboratory:
You usually see an orange-yellow band near the top (carotene), a yellow band (xanthophyll), a blue-green band (chlorophyll a) and a yellow-green band (chlorophyll b) nearest the start.
Worked example
Each pigment has an Rf value = distance moved by the pigment ÷ distance moved by the solvent. The solvent front has moved 8.0 cm and a blue-green band has moved 4.0 cm. Rf = 4.0 ÷ 8.0 = 0.50. Rf has no units and is always less than 1.
Tip
Palisade = Packed with chloroplasts, near the top where the light is. Spongy = Spaces for gases.
In "adaptation" questions, always link a feature to its function: "thin, so carbon dioxide diffuses a short distance", not just "it is thin".
Why do palisade cells lie just below the upper epidermis?
Solution: That is where the light is strongest. Their many chloroplasts can absorb as much light as possible for photosynthesis.
State two ways the spongy mesophyll is adapted for gas exchange.
Solution: Its cells are loosely packed with large air spaces, so gases diffuse quickly through the leaf; its cells have moist surfaces, so carbon dioxide can dissolve and diffuse into them.
In a chromatogram the solvent moved 7.5 cm and the carotene band moved 7.2 cm. Calculate the Rf of carotene.
Solution: Rf = 7.2 ÷ 7.5 = 0.96. Carotene travels almost as far as the solvent.
Why must the starting line be drawn in pencil and kept above the solvent level?
Solution: Pencil (graphite) does not dissolve, so it cannot run and mix with the pigments. If the spot were below the solvent surface, the pigments would dissolve into the solvent in the jar instead of moving up the paper.
Remember
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
