Cellular respiration - TCE Biology (Tasmania)
“Respiration: the role of mitochondria in aerobic respiration; the main stages of anaerobic and aerobic respiration (glycolysis, the Krebs cycle and the electron transport chain); inputs and outputs as balanced net chemical equations; factors affecting rate of reaction (temperature, glucose concentration, oxygen concentration and pH)”
Cellular respiration releases energy from glucose as ATP. Aerobic respiration uses oxygen: glycolysis in the cytoplasm splits glucose into pyruvate (2 ATP), the Krebs cycle in the mitochondrial matrix releases carbon dioxide (2 ATP), and the electron transport chain on the inner mitochondrial membrane (the folded cristae) produces most of the ATP, for a total of about 36 to 38 ATP: C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP. Without oxygen, only glycolysis runs, and pyruvate becomes lactic acid in animals, or ethanol and carbon dioxide in plants and yeast, giving 2 ATP. The rate depends on temperature, glucose concentration, oxygen concentration and pH.
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What this dot point is asking
Respiration is examined under criterion 5 (Section B of the TASC exam). You need the names and locations of the stages, the balanced equations for aerobic and anaerobic respiration, ATP yields and the factors affecting rate. Individual enzyme pathways and the carriers NAD and FAD are excluded.
The mitochondrion
Mitochondria are the site of the aerobic stages of respiration. Each has a double membrane: the inner membrane is folded into cristae, which increase the surface area for the reactions of the electron transport chain. The fluid inside is the matrix, where the Krebs cycle takes place.
The stages of aerobic respiration
| Stage | Where | What happens | ATP per glucose |
|---|---|---|---|
| Glycolysis | Cytoplasm | Glucose (6 carbons) is broken down into two pyruvate molecules (3 carbons each); no oxygen needed | 2 (net) |
| Krebs cycle | Mitochondrial matrix | Pyruvate is converted to acetyl coenzyme A, which enters the cycle; carbon dioxide is released | 2 |
| Electron transport chain | Inner mitochondrial membrane (cristae) | Hydrogen from the earlier stages is used to make most of the ATP; oxygen is the final acceptor and water is formed | Most of the total |
The total is about 36 to 38 ATP per glucose molecule (TASC's course document gives 36 to 38; its exam specifications accept 34 to 38).
Anaerobic respiration
When oxygen is not available, the electron transport chain and Krebs cycle stop, and only glycolysis continues in the cytoplasm. Pyruvate is converted to:
- lactic acid in animals (for example, muscles during intense exercise):
- ethanol and carbon dioxide in plants and yeast (fermentation):
Anaerobic respiration releases far less ATP (2 per glucose) because glucose is only partly broken down.
Glycolysis: 2 ATP (cytoplasm). Krebs cycle: 2 ATP (matrix). Electron transport chain: most of the ATP (inner membrane). Aerobic total: about 36 to 38. Anaerobic total: 2.
Factors affecting the rate
- Temperature: respiration is enzyme-controlled, so the rate rises to an optimum and then falls as enzymes denature.
- Glucose concentration: more substrate increases the rate until enzymes are working at their maximum.
- Oxygen concentration: more oxygen increases aerobic respiration; without oxygen, cells switch to anaerobic respiration.
- pH: each enzyme has an optimum pH; extreme pH denatures enzymes.
Germinating seeds
A respirometer contains germinating peas and a chemical that absorbs carbon dioxide. A coloured liquid in the attached tube moves towards the peas over 20 minutes.
- The peas use oxygen in aerobic respiration.
- The carbon dioxide they release is absorbed, so the gas volume in the tube decreases and the liquid moves towards the peas.
- The distance moved per minute is a measure of the rate of oxygen consumption.
- A control tube with glass beads instead of live peas checks that the movement is caused by respiration, not by changes in temperature or pressure.
Marker's note: state what is being measured (oxygen used, not carbon dioxide produced) and why the liquid moves in that direction.
- Putting the Krebs cycle and electron transport chain in the wrong places
- Krebs cycle: matrix. Electron transport chain: inner membrane (cristae).
- Saying anaerobic respiration in humans makes ethanol or carbon dioxide
- Human muscle makes lactic acid only.
- Saying glycolysis happens in the mitochondrion
- It happens in the cytoplasm and does not need oxygen.
- Forgetting ATP in the equation
- The course asks for balanced net equations with ATP shown.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original6 marksCompare aerobic respiration with anaerobic respiration in human muscle cells, referring to where each occurs, the products formed and the amount of ATP produced per glucose molecule.Show worked answer →
Two marks for each point of comparison.
- Location
- Aerobic respiration begins with glycolysis in the cytoplasm, then continues in the mitochondria (Krebs cycle in the matrix, electron transport chain on the inner membrane). Anaerobic respiration occurs only in the cytoplasm.
- Products
- Aerobic respiration produces carbon dioxide and water. Anaerobic respiration in muscle produces lactic acid, with no carbon dioxide.
- ATP
- Aerobic respiration yields about 36 to 38 ATP per glucose (most from the electron transport chain). Anaerobic respiration yields only the 2 ATP from glycolysis.
Original4 marksYeast is mixed with glucose solution in a sealed flask at 30 degrees Celsius. Write the balanced equation for the respiration that occurs once the oxygen is used up, and explain why the rate of carbon dioxide production would fall if the flask were moved to 70 degrees Celsius.Show worked answer →
Equation (2 marks). C6H12O6 → 2C2H5OH + 2CO2 (+ 2 ATP): anaerobic respiration (fermentation) producing ethanol and carbon dioxide.
Explanation (2 marks). Respiration is a series of enzyme-controlled reactions. At 70 degrees Celsius the enzymes are denatured: their active sites change shape so substrates can no longer bind, so the reactions slow or stop and less carbon dioxide is produced.