Photosynthesis makes glucose

Plants and algae trap light energy and use it to build glucose. It is an endothermic reaction — energy is transferred from the environment to the chloroplasts by light.

Watch out: photosynthesis is endothermic (it takes in energy) and it makes glucose — it does not "release energy". Don't muddle it with respiration.

Is photosynthesis endo- or exothermic?

Complete the equation

What the plant does with glucose

The glucose made in photosynthesis is used in five ways:

• Used for respiration to release energy.
• Converted into insoluble starch for storage.
• Used to make fat or oil for storage (especially in seeds).
• Used to make cellulose, which strengthens the cell wall.
• Used to make amino acids for protein synthesis (using nitrate ions from the soil).

Why is glucose used?

Tap a use, then tap the box for its purpose.

What limits the rate?

Four things affect how fast photosynthesis happens. Whichever one is in shortest supply is the limiting factor — the one holding the rate back:

• Light intensity — more light, faster rate (up to a point).
• Carbon dioxide concentration — a raw material.
• Temperature — speeds enzymes up, but too hot denatures them and the rate falls.
• Amount of chlorophyll — needed to absorb the light (less if a plant is diseased or short of magnesium).

Reading graphs: where the line rises, the factor on the x-axis is limiting; where it levels off, a different factor has become the limiting one.

Which factor is limiting?

Light intensity & pondweed

You investigate how light intensity affects the rate of photosynthesis using an aquatic plant such as pondweed (Elodea). As it photosynthesises it gives off oxygen bubbles — count the bubbles (or measure the gas) in a fixed time.

Fair test: keep CO₂ (often added as sodium hydrogencarbonate) and temperature constant — use a water bath/heat shield so the lamp's heat doesn't change the temperature.

The inverse square law for light

Light intensity is inversely proportional to the square of the distance from the lamp. Move the lamp twice as far away and the plant gets only a quarter of the light:

(HT) In a greenhouse, growers raise light, CO₂ and temperature together to push up the rate — but only as far as it stays profitable. The limiting factor decides which is worth adding.

Your turn — inverse square law

Respiration releases energy

Respiration happens continuously in every living cell, transferring the energy needed for life processes. It is an exothermic reaction. Aerobic respiration uses oxygen and mostly takes place in the mitochondria.

The energy released is used for: building larger molecules from smaller ones, movement, and (in mammals and birds) keeping warm.

Watch out: respiration is not the same as breathing, and it does not only happen during exercise — it goes on in all living cells, all the time.

The aerobic equation

Respiring without oxygen

Anaerobic respiration transfers energy without oxygen. The oxidation of glucose is incomplete, so much less energy is released than in aerobic respiration. The products differ between animals and plants/yeast:

Watch out: anaerobic respiration releases less energy per glucose because the glucose is not fully broken down.

Aerobic vs anaerobic

Exercise & oxygen debt

During exercise muscles need more energy, so the body reacts:

• Heart rate rises — pumps blood faster.
• Breathing rate and breath volume rise — to take in more oxygen and remove CO₂.
• These supply the muscles with more oxygenated blood.

If oxygen still can't be supplied fast enough, muscles switch to anaerobic respiration. The incomplete oxidation of glucose builds up lactic acid and creates an oxygen debt. Long, vigorous activity causes muscle fatigue.

(HT) Blood carries the lactic acid to the liver, where it is converted back into glucose. The oxygen debt is the extra oxygen the body needs after exercise to react with (and remove) the accumulated lactic acid.

Oxygen debt

Photosynthesis or respiration?

Tap which process each statement describes.

Metabolism: the sum of all reactions

Metabolism is the sum of all the reactions in a cell or the body. The energy from respiration drives these continual, enzyme-controlled reactions that both build up and break down molecules.

• Converting glucose into starch, glycogen and cellulose.
• Forming a lipid from one molecule of glycerol and three fatty acids.
• Using glucose and nitrate ions to form amino acids, then proteins.
• Respiration itself.
• Breaking down excess proteins to form urea for excretion.

Metabolism covers both synthesis (building larger molecules) and breakdown (e.g. excess protein → urea) — all powered by the energy respiration transfers.

Using glucose

The key facts to know

You've covered all of AQA 4.4 — photosynthesis, respiration and metabolism. Press Finish to see your score.