6.1 Photosynthesis
Chlorophyll is a green pigment found in chloroplasts. It absorbs light energy and transfers it into chemical energy for the formation of glucose and other carbohydrates.
Carbon dioxide enters the leaf through the stomata, water comes up from the roots in the xylem, and oxygen leaves through the stomata.
What happens to the glucose
| Use | Detail |
|---|---|
| respiration | glucose is broken down to release energy |
| stored as starch | an insoluble energy store (in leaves, roots and seeds) |
| cellulose | built into cell walls |
| sucrose | transported through the phloem to other parts of the plant |
| other substances | with nitrate ions, used to make amino acids and then proteins; also fats and oils |
Investigating photosynthesis
To show that starch has been made, first destroy the leaf's enzymes and remove its colour: dip the leaf in boiling water (to kill the cells), then in hot ethanol (to remove the chlorophyll), rinse it in water, and add iodine solution. A leaf that photosynthesised turns blue-black.
| To show that this is needed | Experiment | Control |
|---|---|---|
| chlorophyll | a variegated leaf (green and white parts): only the green parts turn blue-black | the green part, compared with the white part |
| light | cover part of a leaf with black paper or foil: the covered part stays brown | the uncovered part of the same leaf |
| carbon dioxide | put a plant in a sealed jar with soda lime or potassium hydroxide to absorb the carbon dioxide: the leaf stays brown | a plant in a jar without soda lime |
Plants must first be destarched (kept in the dark for 24 to 48 hours) so that any starch found has been made during the experiment.
The rate of photosynthesis: limiting factors
The rate of photosynthesis depends on light intensity, carbon dioxide concentration and temperature.
| Factor | Effect |
|---|---|
| light intensity | the rate rises as light intensity increases, then levels off when another factor becomes limiting |
| carbon dioxide concentration | the rate rises with carbon dioxide concentration until it levels off |
| temperature | the rate rises with temperature (enzymes work faster) up to an optimum of about –, then falls as the enzymes are denatured |
Using a limiting factor. In a greenhouse, growers add extra light, heating and carbon dioxide, and the one that is limiting is the one that should be increased.
Investigating the rate with a submerged aquatic plant such as Elodea (pondweed): count the bubbles of oxygen given off in a given time, or collect the gas. Vary the light intensity by changing the distance of a lamp (a lamp closer means higher light intensity), the carbon dioxide concentration by adding sodium hydrogencarbonate to the water, and the temperature using a water bath.
Hydrogencarbonate indicator can also be used. It changes colour as the level of carbon dioxide changes: it is red at normal carbon dioxide levels, turns yellow when carbon dioxide is high (as in respiration) and purple when carbon dioxide is low (when the plant photosynthesises).
Light is no longer limiting, so the rate is limited by another factor: carbon dioxide concentration or temperature.
6.2 Leaf structure
Leaves are adapted for photosynthesis: they have a large surface area to absorb light and are thin, so that carbon dioxide only has a short distance to diffuse and light can reach all the cells.
| Structure | Adaptation and function |
|---|---|
| cuticle (waxy layer) | waterproof, reduces water loss by evaporation from the surface |
| upper epidermis | a thin, transparent layer that lets light through to the cells beneath |
| palisade mesophyll | tall cells packed with chloroplasts, near the top of the leaf where most light falls; the main site of photosynthesis |
| spongy mesophyll | rounded cells with some chloroplasts and air spaces between them, so that gases can diffuse through the leaf |
| air spaces | allow carbon dioxide to diffuse to the cells and oxygen out; water vapour leaves through them |
| stomata and guard cells | pores (stomata) in the lower epidermis for gas exchange; each is opened or closed by a pair of guard cells |
| vascular bundle (xylem and phloem) | xylem brings water to the leaf; phloem carries away sucrose and amino acids |
| lower epidermis | contains most of the stomata |
Most chloroplasts are in the palisade layer, because it is nearest to the light. The stomata are on the underside, which is cooler and out of direct sunlight, so less water is lost.
6.3 Mineral nutrition
Plants take up mineral ions from the soil through their roots. Two important ones:
- Nitrate ions are needed to make amino acids, which are joined together to make proteins. Without nitrate, a plant has stunted growth and its older leaves turn yellow.
- Magnesium ions are needed to make chlorophyll. Without magnesium, the leaves turn yellow (chlorosis) because the plant cannot make enough chlorophyll to photosynthesise.
Practise this chapter
Every past-paper question on these topics, with the mark scheme.