Photosynthesis as an endothermic reaction, the factors limiting its rate, the uses of glucose, aerobic and anaerobic respiration, the difference between the two, and how the body responds to exercise.

What this topic is asking

AQA wants you to describe photosynthesis as an endothermic reaction, explain the factors that limit its rate, list the uses of glucose, compare aerobic and anaerobic respiration, and describe how the body responds to exercise including oxygen debt.

Photosynthesis and limiting factors

The word equation is carbon dioxide + water $\rightarrow$ glucose + oxygen, and the balanced symbol equation is $6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$. It is endothermic because energy is transferred from the surroundings (light) to the chloroplasts, where chlorophyll absorbs it. Photosynthesis is the foundation of almost every food chain, so AQA expects you to treat it as the entry point of energy into living systems.

The rate of photosynthesis is limited by three factors: light intensity, carbon dioxide concentration and temperature. The factor in shortest supply at a given moment is the limiting factor, the one whose increase would speed up the rate. On a rate graph the line rises steeply, then plateaus when a different factor takes over as the limit. Temperature is unusual: raising it speeds the rate up to an optimum, but above roughly 45 degrees Celsius the enzymes that catalyse photosynthesis begin to denature, the active sites change shape, and the rate falls sharply. Growers exploit limiting factors commercially: greenhouses add artificial light, paraffin heaters raise both temperature and carbon dioxide, and these costs are weighed against the increased yield.

Uses of glucose

Plants use the glucose made in photosynthesis in five ways that AQA expects you to list precisely:

• Respiration, to release energy for life processes.
• Converting it into cellulose to strengthen cell walls (important in rapidly growing plants).
• Combining it with nitrate ions absorbed from the soil to make amino acids, then proteins.
• Converting it into lipids (oils and fats) for storage in seeds.
• Storing it as starch in leaves, roots and seeds for use when photosynthesis is not happening (for example at night).

Starch is the storage form because it is insoluble, so it does not affect the water balance of cells by osmosis, unlike soluble glucose.

Respiration and exercise

• Aerobic respiration uses oxygen, takes place in the mitochondria, and releases the most energy per glucose molecule: glucose + oxygen $\rightarrow$ carbon dioxide + water, or $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$.
• Anaerobic respiration in muscles releases less energy because glucose is only partly broken down, and produces lactic acid: glucose $\rightarrow$ lactic acid.
• Anaerobic respiration in yeast and plant cells (fermentation) produces ethanol and carbon dioxide, which is the basis of brewing and bread-making.

During exercise the heart rate, breathing rate and breath volume increase to deliver more oxygenated blood (carrying oxygen and glucose) to the muscles and to remove carbon dioxide faster. If the muscles cannot get oxygen quickly enough, they respire anaerobically, lactic acid builds up, and this creates an oxygen debt: the volume of extra oxygen the body needs after exercise to react with the accumulated lactic acid. Blood carries the lactic acid to the liver, where it is converted back to glucose, which is why breathing stays heavy for a while after you stop.