Photosynthesis transfers energy into chemical stores, and respiration releases energy from them. Together they supply the energy an organism needs.
Photosynthesis is endothermic: light energy is transferred into the chemical energy stores of glucose and the other organic molecules made from it.
carbon dioxide + water → glucose + oxygen, or 6CO2 + 6H2O → C6H12O6 + 6O2.
Four things affect the rate: light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll.
Any one of them can be the limiting factor, meaning the factor that restricts the rate. Increasing it increases the rate until another factor becomes limiting. On a graph the rate rises as that factor increases and then levels off when something else becomes limiting. Temperature behaves differently at the top end: too high and the enzymes denature, so the rate falls rather than plateauing.
Greenhouses can increase the rate of photosynthesis by controlling conditions such as temperature, light intensity and carbon dioxide concentration, provided the increased yield justifies the cost.
In practice the factors interact, and which one is limiting can change as environmental conditions change during the day.
For an approximately point-like source, light intensity is approximately inversely proportional to the square of the distance: move a lamp twice as far away and the intensity falls to about a quarter, three times as far and to about a ninth. This matters when planning a practical, because the distances are not proportional to the effect.
The glucose made in photosynthesis is used for respiration; converted into starch for storage, because starch is insoluble and so does not affect osmosis; converted into cellulose for cell walls; converted into lipids as a store in seeds; and combined with nitrate ions absorbed from the soil to make amino acids and then proteins.
Respiration is exothermic and takes place in living cells. In eukaryotic cells, most of the reactions of aerobic respiration happen in the mitochondria. The energy released is used for movement, for keeping warm, and for building larger molecules from smaller ones.
Aerobic: glucose + oxygen → carbon dioxide + water.
Anaerobic in muscle: glucose → lactic acid. Anaerobic in yeast (fermentation): glucose → ethanol + carbon dioxide, which is the basis of brewing and baking.
Anaerobic respiration releases much less energy per glucose molecule because the glucose is only partly broken down.
During exercise the heart rate, breathing rate and breath depth all increase, to supply more oxygen and glucose to the muscles and remove more carbon dioxide.
If exercise is vigorous enough that oxygen cannot be supplied fast enough, muscles respire anaerobically and lactic acid builds up, which is associated with fatigue. The oxygen debt is the extra oxygen required after exercise as the body recovers from anaerobic respiration and removes the lactic acid produced. Blood carries some of it to the liver, where it is converted back into glucose.
Metabolism is the sum of all the reactions in a cell or in the body, both the building up of larger molecules and the breaking down of them.
It includes converting glucose to starch, glycogen and cellulose; forming lipids from glycerol and fatty acids; using glucose and nitrate ions to form amino acids and then proteins; respiration; and breaking down excess proteins to form urea for excretion.
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