What you'll learn
Bioenergetics is the unit of AQA GCSE Combined Science: Trilogy that deals with how organisms get and use energy. It has two halves that mirror each other: photosynthesis, in which plants and algae use light energy to build glucose, and respiration, in which all living organisms break glucose down to release energy for their cells. By the end of this unit you should be able to write and explain both word equations, describe photosynthesis as an endothermic reaction and respiration as an exothermic one, explain how light intensity, carbon dioxide concentration and temperature limit the rate of photosynthesis, apply the inverse square law to light intensity, compare aerobic and anaerobic respiration in animals and in yeast, explain oxygen debt, and describe what happens to metabolic products in the body. Bioenergetics is assessed on Biology Paper 1 and includes the required practical on the effect of light intensity on the rate of photosynthesis.
Key terms and definitions
Photosynthesis — the endothermic reaction in which light energy is used to convert carbon dioxide and water into glucose and oxygen
Endothermic reaction — a reaction that takes in energy from the surroundings
Exothermic reaction — a reaction that transfers energy to the surroundings
Limiting factor — the factor that is in shortest supply and therefore restricts the rate of a reaction
Respiration — the exothermic reaction, occurring continuously in all living cells, that transfers energy from glucose
Aerobic respiration — respiration using oxygen, releasing a relatively large amount of energy per molecule of glucose
Anaerobic respiration — respiration without oxygen, releasing much less energy and producing lactic acid in animals or ethanol and carbon dioxide in yeast
Oxygen debt — the extra oxygen the body needs after exercise to react with accumulated lactic acid and remove it from the tissues
Metabolism — the sum of all the chemical reactions in a cell or in the body
Inverse square law — the relationship in which light intensity is proportional to one divided by the distance squared
Core concepts
The photosynthesis reaction
Photosynthesis takes place in the chloroplasts of plant and algal cells, where the green pigment chlorophyll absorbs light. The word equation is carbon dioxide plus water, in the presence of light, producing glucose plus oxygen.
Photosynthesis is endothermic: energy is transferred from the environment, in this case from light, to the chloroplasts. This is worth stating explicitly because questions often ask for the energy classification and a single word earns the mark.
The carbon dioxide enters the leaf through pores called stomata, and the water is drawn up from the roots through the xylem. The oxygen produced is released as a waste product, though the plant also uses some of it for respiration.
What the plant does with the glucose
The glucose made in photosynthesis is put to five uses, and questions ask for these routinely. Some is used immediately in respiration to release energy. Some is converted into insoluble starch for storage, which is important because starch does not affect the water balance of the cell by osmosis in the way that soluble glucose would. Some is converted into fats and oils for storage, particularly in seeds. Some is used to produce cellulose, which strengthens the cell wall. And some is combined with nitrate ions absorbed from the soil to produce amino acids, which are then built into proteins.
Limiting factors
The rate of photosynthesis is affected by four factors: light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll. At any moment one of these will be the factor in shortest supply, and it is that limiting factor which determines the rate.
The pattern on a graph is recognisable. As light intensity increases from zero, the rate rises steadily because light is limiting. Then the line levels off: light is no longer limiting, and something else has become the constraint, usually carbon dioxide concentration or temperature. If the experiment is repeated at a higher carbon dioxide concentration, the line levels off at a higher rate, which is how you can tell which factor was limiting on the plateau.
Temperature behaves differently from the other two. The rate rises with temperature up to an optimum, but above that the enzymes controlling photosynthesis are denatured, their active sites change shape, and the rate falls sharply. A graph of temperature against rate therefore has a peak rather than a plateau.
The amount of chlorophyll can be reduced by disease, such as tobacco mosaic virus, or by a lack of magnesium ions in the soil, since magnesium is needed to make chlorophyll.
Light intensity and the inverse square law
In the required practical, pondweed is placed at different distances from a lamp and the bubbles of oxygen produced are counted. The light intensity is not proportional to the distance but to one divided by the distance squared. Doubling the distance therefore reduces the light intensity to a quarter, not a half.
This matters because questions give you distances and expect you to compare intensities correctly. It is also worth noting that a lamp warms the water, so a heat shield or a beaker of water between the lamp and the plant is used to keep temperature constant.
Using limiting factors in greenhouses
Commercial growers control the environment so that no factor limits growth. Artificial lighting extends the day and allows growth through winter. Paraffin heaters both warm the greenhouse and release carbon dioxide as they burn, raising two factors at once. Temperature is controlled by heating and by ventilation.
The economic reasoning is part of the specification. A grower must judge whether the increased yield is worth the cost of the heating and lighting. The optimum is the point at which the extra income from the extra yield just exceeds the extra cost, and beyond that point increasing the conditions further loses money.
Aerobic respiration
Respiration occurs continuously in every living cell and is exothermic, transferring energy to the surroundings. The word equation for aerobic respiration is glucose plus oxygen producing carbon dioxide plus water.
Aerobic respiration takes place mainly in the mitochondria, which is why cells that need a lot of energy, such as muscle cells and sperm cells, contain many of them. The energy transferred is used for muscle contraction, for keeping the body temperature steady in mammals and birds, and for building larger molecules from smaller ones.
Anaerobic respiration
When the body cannot supply oxygen quickly enough, muscle cells respire anaerobically. The word equation is simply glucose producing lactic acid. Because the glucose is only partially broken down, far less energy is released per molecule than in aerobic respiration, which is why anaerobic respiration cannot sustain activity for long.
In yeast and plant cells the anaerobic pathway is different: glucose produces ethanol plus carbon dioxide. This is called fermentation and it is the basis of both brewing, which uses the ethanol, and bread making, which uses the carbon dioxide to make the dough rise.
Exercise and oxygen debt
During exercise the heart rate, breathing rate and breath volume all increase, supplying more oxygen and glucose to the muscles and removing carbon dioxide faster. If exercise is vigorous enough that oxygen cannot be supplied fast enough, anaerobic respiration begins and lactic acid builds up, causing muscle fatigue so the muscles stop contracting efficiently.
After exercise, the person continues to breathe heavily to repay the oxygen debt: the extra oxygen needed to react with the accumulated lactic acid and remove it. Blood flowing through the muscles transports the lactic acid to the liver, where it is converted back into glucose.
Metabolism
Metabolism is the sum of all the reactions in a cell or the body, and it links every part of this unit. Reactions that build larger molecules include the conversion of glucose to starch, glycogen and cellulose, the formation of lipids from a molecule of glycerol and three fatty acids, and the synthesis of amino acids from glucose and nitrate ions and then of proteins from amino acids. Reactions that break molecules down include respiration itself and the breakdown of excess protein to urea, which is excreted by the kidneys.
Worked examples
Example 1: Applying the inverse square law (3 marks)
A lamp is moved from 10 cm to 20 cm from a piece of pondweed. Describe the effect on light intensity and predict the effect on the rate of photosynthesis.
Light intensity is proportional to one divided by the distance squared. Doubling the distance from 10 cm to 20 cm therefore reduces the light intensity to one quarter of its original value. Provided light is the limiting factor, the rate of photosynthesis would be expected to fall to approximately a quarter of its original rate.
Example 2: Identifying a limiting factor from a graph (4 marks)
A graph of photosynthesis rate against light intensity rises and then levels off at 20 units. The experiment is repeated at a higher temperature and levels off at 30 units. Explain what this shows.
On the rising part of each line, light intensity is the limiting factor, because increasing it increases the rate. On the plateau, further light makes no difference, so light is no longer limiting. Because raising the temperature raised the height of the plateau, temperature must have been the limiting factor on the original plateau. Increasing it allowed a higher rate before some other factor became limiting.
Example 3: Explaining oxygen debt (4 marks)
Explain why an athlete continues to breathe deeply for several minutes after finishing a race.
During the race the athlete could not supply oxygen to the muscles fast enough, so the muscles respired anaerobically and lactic acid built up. After the race the athlete must repay the oxygen debt, which is the extra oxygen needed to react with this lactic acid and remove it. The lactic acid is also transported in the blood to the liver, where it is converted back into glucose. Deep breathing continues until the lactic acid has been cleared.
Common mistakes and how to avoid them
The commonest error is writing that plants respire only at night. Plants respire continuously, day and night. What changes is that during the day photosynthesis usually happens faster than respiration, so the net movement of gases is the other way round.
Students often write that photosynthesis produces energy. It does not produce energy; it transfers energy from light into the chemical store of glucose. The same applies to respiration, which releases or transfers energy rather than producing it.
Another regular slip is treating light intensity as proportional to distance rather than to one over distance squared. Halving the distance quadruples the intensity.
In limiting factor questions, many students name a factor without explaining the graph. The mark is for connecting the shape to the factor: rising means the factor on the axis is limiting; level means something else is.
Finally, be careful not to give lactic acid as a product of anaerobic respiration in yeast. Yeast produces ethanol and carbon dioxide; lactic acid is the animal pathway.
Exam technique for "Biology: Bioenergetics"
Learn all four word equations cold — photosynthesis, aerobic respiration, anaerobic respiration in muscle, and anaerobic respiration in yeast. They are guaranteed marks and take a minute to check.
When a question asks about the rate of photosynthesis, always identify what is on each axis before interpreting. The phrase "the limiting factor is" should be followed by a reason drawn from the shape of the line, not from general knowledge.
For the required practical, be ready to state the independent variable as distance from the lamp, the dependent variable as the number of bubbles per minute or volume of gas collected, and the control variables as temperature, carbon dioxide concentration from the hydrogencarbonate solution, and the same piece of pondweed.
Comparison questions between aerobic and anaerobic respiration should cover three things: whether oxygen is used, what the products are, and how much energy is transferred relative to the other. Three clear differences reliably score full marks.
Quick revision summary
Photosynthesis is endothermic: carbon dioxide plus water, using light, gives glucose plus oxygen, in the chloroplasts. The glucose is respired, stored as starch or as fats and oils, made into cellulose, or combined with nitrate ions to make amino acids. Rate is restricted by whichever of light intensity, carbon dioxide concentration, temperature or chlorophyll amount is in shortest supply, and light intensity follows an inverse square law. Greenhouses control these factors, balanced against cost. Respiration is exothermic and continuous in all cells: aerobic respiration uses oxygen and gives carbon dioxide and water, releasing much more energy than the anaerobic route. Anaerobic respiration gives lactic acid in muscle and ethanol plus carbon dioxide in yeast. Lactic acid causes fatigue and creates an oxygen debt repaid after exercise, with the liver converting lactic acid back to glucose. Metabolism is the sum of all these reactions.