Biology: Energy Transfers in Organisms
Energy transfers in organisms is the A-level Biology topic covering the biochemical detail of aerobic respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation), anaerobic respiration in mammals and in yeast, respiratory substrates and the respiratory quotient (RQ), and how energy transfer efficiency between trophic levels is calculated. It extends the overview met in Energy Transfers in and between Organisms with the reaction-by-reaction mechanism and the respirometer required practical that Year 13 papers test in depth.
Before you start
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Method
- Work through the four stages of aerobic respiration in order (glycolysis in the cytosol; the link reaction and Krebs cycle in the mitochondrial matrix; oxidative phosphorylation on the inner mitochondrial membrane, the cristae), stating the net ATP, reduced coenzyme (NAD or FAD) and CO2 produced at each stage.
- For chemiosmosis questions, describe the sequence in order: reduced NAD/FAD is oxidised at the electron transport chain, the energy released pumps protons into the intermembrane space, the resulting proton gradient drives protons back through ATP synthase, and this releases the energy that phosphorylates ADP to ATP.
- Distinguish anaerobic respiration in mammals (pyruvate converted to lactate, regenerating NAD, no further ATP made) from anaerobic respiration in yeast and plants (pyruvate converted to ethanol and CO2, regenerating NAD), and state that only the 2 net ATP from glycolysis are produced in either case.
- For respiratory quotient questions, use RQ = volume of carbon dioxide produced / volume of oxygen consumed, and link the value to the respiratory substrate being used (carbohydrate approximately 1.0, lipid approximately 0.7, protein approximately 0.9).
- For the respirometer required practical, identify soda lime's role (it absorbs the CO2 produced, so any volume change measured is due to O2 uptake alone) and the role of a control tube (it corrects for volume changes caused by atmospheric pressure or temperature rather than by respiration).
- For energy-transfer-efficiency questions, use percentage efficiency = (energy transferred to the next trophic level / energy available at the level before) x 100, and explain energy losses as heat from respiration and as matter that is not consumed, not digested (egested), or lost before being eaten (through death or excretion).
Worked example
A student used a respirometer to investigate respiration in germinating pea seeds at 20 degrees C. With soda lime present in the respirometer, the meniscus in the manometer moved 42 mm towards the seeds in 10 minutes. The apparatus was calibrated so that 1 mm of meniscus movement corresponds to 0.02 cm^3 of gas. Calculate the rate of oxygen uptake, in cm^3 per minute.
- Convert the meniscus movement to a volume: 42 mm x 0.02 cm^3 per mm = 0.84 cm^3.
- With soda lime present, all the CO2 produced by respiration is absorbed, so the volume change measured is entirely due to O2 being taken up by the seeds (the meniscus moves towards the respiring organism as the gas volume falls).
- Divide the volume by the time taken to find the rate: 0.84 cm^3 / 10 minutes = 0.084 cm^3 per minute.
- Final answer: rate of oxygen uptake = 0.084 cm^3 per minute (8.4 x 10^-2 cm^3/min).
Practice questions
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Q1State the two reduced coenzymes produced during the Krebs cycle that go on to be used in oxidative phosphorylation.Show answer
Answer: Reduced NAD (NADH) and reduced FAD (FADH2).
Q2State where in the mitochondrion oxidative phosphorylation takes place.Show answer
Answer: Across (on) the inner mitochondrial membrane, the cristae.
Q3A yeast culture is respiring anaerobically. State the two products of this reaction, other than the coenzyme that is regenerated.Show answer
Answer: Ethanol and carbon dioxide.
Q4Define the term respiratory quotient.Show answer
Answer: The volume of carbon dioxide produced divided by the volume of oxygen consumed, in the same time and under the same conditions (RQ = volume CO2 / volume O2).
Q5A sample of respiring tissue has an RQ of 0.7. Identify the respiratory substrate being used.Show answer
Answer: Lipid (fat).
Q6Explain why soda lime is included in a respirometer used to measure the rate of oxygen uptake.Show answer
Answer: Soda lime absorbs (removes) the carbon dioxide produced by respiration, so the only gas volume change recorded is due to oxygen being used up.
Q7In an ecosystem, primary consumers had a gross energy intake of 18500 kJ per m^2 per year, of which 11200 kJ per m^2 per year was lost in egestion and excretion (faeces and urine) and 5900 kJ per m^2 per year was lost as heat in respiration. Calculate the energy available to secondary consumers (the net production of the primary consumers).Show answer
Answer: 1400 kJ per m^2 per year (18500 - 11200 - 5900 = 1400).
Q8Explain why the efficiency of energy transfer is usually higher for secondary consumers (carnivores) than for primary consumers (herbivores).Show answer
Answer: Herbivores cannot digest all the plant material they eat (for example cellulose in cell walls), so proportionally more energy is lost undigested (egested); carnivores eat more digestible, energy-dense animal tissue, so proportionally less energy is lost this way.
Exam-style questions
Written in the style of a A Level Science exam paper, with a full mark scheme.
Outline the role of the electron transport chain in the production of ATP during oxidative phosphorylation.
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A sprinter's leg muscles and a sample of yeast were both allowed to respire anaerobically. (a) State the products of anaerobic respiration in the sprinter's muscle cells, other than the coenzyme regenerated. (b) State the products of anaerobic respiration in the yeast cells, other than the coenzyme regenerated. (c) Explain why anaerobic respiration produces far less ATP per glucose molecule than aerobic respiration.
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A respirometer was set up to measure the rate of oxygen uptake by blowfly larvae at 15 degrees C. A control tube containing an equal volume of inert glass beads replaced the larvae, and soda lime was present in both tubes. In the experimental tube, the meniscus moved 18 mm towards the larvae in 5 minutes; in the control tube, the meniscus moved 2 mm in the same direction over the same time. The calibrated scale reads 0.01 cm^3 of gas per mm of movement. (a) Explain the purpose of the control tube in this investigation. (b) Calculate the corrected rate of oxygen uptake by the larvae, in cm^3 per minute. (c) Suggest one variable, other than temperature, that should be controlled to make this a fair test.
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