Photosynthesis

Where it happens

Photosynthesis takes place in the chloroplasts, which contain the green pigment chlorophyll. In a leaf, most chloroplasts are in the palisade mesophyll near the upper surface, where they receive the most light.

Inside each chloroplast there are two working regions. The light reaction runs on the stacked membranes called grana, where chlorophyll is held, while the dark reaction runs in the fluid stroma that surrounds them.

The leaf supports both regions: stomata on the lower surface admit carbon dioxide, air spaces in the spongy mesophyll let the gas diffuse to every cell, xylem in the veins delivers water, and phloem carries the sugar away to the rest of the plant.

Inputs and outputs

• Input: carbon dioxide, which enters the leaf through the stomata and diffuses through the air spaces to the mesophyll cells.
• Input: water, absorbed by the roots and carried up the xylem to the leaf.
• Input: light energy, absorbed by chlorophyll in the grana of the chloroplast.
• Output: glucose, the first stable sugar formed, later converted to sucrose for transport or starch for storage.
• Output: oxygen, released as a by-product of splitting water and diffused out through the stomata.
• Intermediates: ATP and hydrogen carriers made in the light reaction and used up in the dark reaction.

The steps

1. Carbon dioxide diffuses through the stomata into the air spaces and then into the mesophyll cells; water arrives through the xylem.
2. Chlorophyll in the grana absorbs light energy, mainly red and blue wavelengths.
3. In the light reaction, that energy splits water molecules (photolysis) into hydrogen and oxygen.
4. The oxygen is released as a by-product; the hydrogen is picked up by a carrier molecule.
5. Some of the absorbed energy is used to build ATP from ADP and phosphate.
6. In the dark reaction, in the stroma, carbon dioxide is fixed and combined with the hydrogen using the ATP, producing glucose.
7. Glucose is either respired at once, converted to sucrose for transport in the phloem, or stored as starch grains in the chloroplast.

Why it matters and how it is controlled

Photosynthesis is the starting point of almost every food chain and the source of the oxygen animals breathe. Its rate is affected by light intensity, carbon dioxide concentration and temperature.

The rate is controlled by whichever factor is in shortest supply, called the limiting factor. On a dim morning, light intensity limits the rate; raising the light raises the rate until carbon dioxide, at only about 0.04 percent of the air, becomes the new limit.

Temperature matters because the dark reaction is enzyme-driven: the rate rises up to an optimum and then falls sharply as the enzymes denature. Farmers use this knowledge in greenhouses by adding carbon dioxide, heating and artificial lighting so that no single factor holds the crop back.

How it is examined

You may be asked for the word equation, to state the raw materials and products, to explain how a factor changes the rate, or to interpret a rate-of-photosynthesis graph.

Practical questions usually build on 2 classic set-ups. The first counts oxygen bubbles from a water plant such as Hydrilla as a lamp is moved to different distances, which tests your handling of the manipulated and responding variables.

The second is the starch test: a plant is kept in the dark for 2 days to remove stored starch (destarching), one leaf is partly covered with foil, and after a day in light the leaf is boiled, decolourised in warm ethanol and flooded with iodine solution. Only the uncovered region turns blue-black, showing that light is needed for starch to form.

Common misconceptions

Worked exam-style question

Question. A student placed a sprig of Hydrilla in a beaker of water containing sodium hydrogen carbonate and counted the bubbles released per minute with a lamp at 10 cm, 20 cm, 40 cm and 80 cm from the plant. The counts were 42, 30, 12 and 4 bubbles per minute.

(a) Name the gas in the bubbles and the process that produced it. (b) Explain why the count fell as the lamp was moved away.

(c) State why sodium hydrogen carbonate was added to the water. (d) Suggest why the count at 10 cm would stop rising if the lamp were moved closer still.

Model answer. (a) The gas is oxygen, produced by photosynthesis. (b) Moving the lamp away lowers the light intensity reaching the plant, so chlorophyll absorbs less light energy, less water is split in the light reaction, and the rate of photosynthesis falls, releasing fewer oxygen bubbles.

(c) Sodium hydrogen carbonate supplies carbon dioxide so that it does not become the limiting factor. (d) At very high light intensity, light is no longer limiting; the rate is now held back by another factor such as carbon dioxide concentration or temperature, so extra light produces no further rise.

Source:SRC-DSKP-EN

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