Bioenergetics is the study of how living organisms get and use energy. Plants and algae make their own food by photosynthesis, transferring energy from light into the chemical store of glucose. Every living cell, including plant cells, then breaks down glucose by respiration to release the energy needed for life processes. Together these two reactions move energy and carbon between organisms and the environment.
Photosynthesis is an endothermic reaction: energy is transferred from the environment (light) to the chloroplasts of a plant or algal cell. Light energy is absorbed by the green pigment chlorophyll.
The word equation is:
carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll)
The balanced symbol equation (Higher tier should be able to recognise it) is:
6CO2 + 6H2O → C6H12O6 + 6O2
Carbon dioxide enters leaves through the stomata, water is drawn up from the roots, and the oxygen produced is released as a waste product.
The rate of photosynthesis is affected by several environmental factors. A limiting factor is the factor in shortest supply that holds back the rate — increasing it will increase the rate until something else becomes limiting. The main limiting factors are:
On a graph, the rate rises then levels off (plateaus) when another factor becomes limiting. Higher tier students should understand that these factors interact and that any one of them may be the limiting factor at a given time.
Light intensity is inversely proportional to the square of the distance from the light source. This is described by the inverse square law: light intensity ∝ 1 / distance2.
This means doubling the distance between a lamp and a plant reduces the light intensity to one quarter, not one half. Higher tier students should be able to use this relationship to compare or predict light intensity at different distances when investigating the effect of light on photosynthesis.
The glucose produced in photosynthesis is used by the plant in several ways:
Starch is a good storage molecule because it is insoluble and so does not affect the water balance (osmosis) of cells.
Growers can artificially control the environment in a greenhouse to remove limiting factors and increase the rate of photosynthesis, so crops grow faster and yields are higher. They may add:
However, all of this costs money. A grower must balance the cost of providing optimum conditions (fuel, equipment, electricity) against the extra income from a larger, faster crop. The aim is to gain the maximum yield as economically as possible, where the value of the increased yield is greater than the cost of creating the conditions.
Respiration happens continuously in every living cell to supply the energy needed for life processes. It is an exothermic reaction because it transfers energy to the environment.
Aerobic respiration uses oxygen and releases a large amount of energy from glucose. It mainly takes place in the mitochondria.
Word equation:
glucose + oxygen → carbon dioxide + water
Symbol equation (Higher tier):
C6H12O6 + 6O2 → 6CO2 + 6H2O
The energy released is used for movement (muscle contraction), keeping warm in mammals and birds, building larger molecules from smaller ones, and active transport.
If there is not enough oxygen, cells can respire anaerobically (without oxygen). This releases much less energy per glucose molecule than aerobic respiration because the glucose is only partly (incompletely) broken down.
In muscle cells:
glucose → lactic acid
In plant and yeast cells the process is called fermentation:
glucose → ethanol + carbon dioxide
Fermentation in yeast is economically important: it is used in making bread (the carbon dioxide makes dough rise) and in brewing alcoholic drinks (the ethanol).
During exercise the muscles contract more often, so they need more energy from increased respiration. The body responds by increasing breathing rate and breath volume, and increasing heart rate, to supply more oxygen and glucose to the muscles and remove the extra carbon dioxide.
If exercise is hard, the muscles cannot get enough oxygen, so they begin to respire anaerobically, producing lactic acid. The build-up of lactic acid causes muscle fatigue (the muscles stop contracting efficiently).
This creates an oxygen debt (sometimes called excess post-exercise oxygen consumption): the amount of extra oxygen the body needs after exercise to react with and remove the accumulated lactic acid. This is why you keep breathing hard after stopping. (Higher tier) In the liver, lactic acid is transported in the blood and converted back into glucose, repaying the oxygen debt.
Metabolism is the sum of all the chemical reactions in a cell or the body. The energy transferred by respiration powers these reactions, which build large molecules from smaller ones and break large molecules down. Examples include:
Practise exam-style questions on this topic.