B2: Photosynthesis

Millions of years ago there was hardly any oxygen in the air. Oxygen was first made when bacteria evolved that could photosynthesise. Gradually, over millions of years, the amount of oxygen in the air has gone up. Now more than 20% of the air is oxygen. It has all been made by bacteria, algae and plants.

In photosynthesise, light energy is stored in glucose molecules.

The energy is transferred to animals when they eat plants.

Glucose can be converted into starch and stored for later use. Starch molecules are big and, unlike glucose, cannot diffuse out of cells.

Light energy is absorbed by chlorophyll found in the chloroplasts of plant cells.

Chlorophyll looks green because it absorbs the blue and red parts of the spectrum and reflects the green part.

The equation for photsynthesis is:

The process for photosynthesis is:

1 Carbon dioxide is taken in by the leaves, and water is taken up by the roots

2 The chlorophyll traps the light energy needed for photosynthesis

3 The energy is used to convert the carbon dioxide and water into glucose (a sugar).

By testing leaves with iodine solution (the iodine test for starch) we can identify the starch in the leaf and show that photosynthesis has ocurred. Variegated leaves have patches of green (with chlorophyll) and white (without chlorophyll). Only the green patches will turn blue-black to show that starch has been made.

A limiting factor is something that is in short supply and therefore stops a process from happening faster.

Light intensity - If there is not enough light, there is not enough energy to activate the chlorophyll.

Carbon dioxide - If there is not enough carbon dioxide, the plant is in shortage of the raw material it needs to make glucose.

Temperature - Photosynthesis reactions are controlled by enzymes, which have an optimum temperature. The lower the temperature, the slower the rate of photosynthesis and the slower the growth. Above a certain temperature, the enzymes are denatured and do not work, so photosynthesis stops completely.

Magnesium is needed for making chlorophyll.

Nitrates are needed for making proteins.

Plants absorb nitrate ions as well as magnesium from the soil.

The glucose produced by photosynthesis may be:

• converted into insoluble starch for storage
• used for respiration
• converted into fats and oils for storage
• used to produce cellulose which strengthens cell walls
• used to produce proteins