Cellular Respiration
Respiration releases energy from glucose for the cell to use, stored briefly as ATP. This chapter covers aerobic respiration, anaerobic respiration (fermentation) in humans and in yeast, and how they compare.
It is easy to confuse respiration with breathing, respiration is the chemical release of energy inside cells.
Respiration begins the same way in every cell, whether oxygen is available or not: glucose is first broken down in the cytoplasm through glycolysis, releasing a small amount of energy. What happens to the product of glycolysis afterwards depends on oxygen supply, if oxygen is available, it passes into the mitochondria for the aerobic stages that release far more energy; if oxygen is short, it is converted into lactic acid (in humans) or ethanol and carbon dioxide (in yeast) instead.
This shared first step explains why even an anaerobic organism can extract some usable energy from glucose without any oxygen at all, and why the amount of energy obtained afterwards depends entirely on whether the aerobic stages in the mitochondria can proceed.
Because Paper 3 often asks students to design or interpret an experiment on respiration, this chapter also covers how the process is investigated practically, using limewater to detect the carbon dioxide released, a thermometer or insulated flask to detect the heat released, or a respirometer to measure the oxygen used by germinating seeds or small invertebrates. A well-answered Paper 3 question on this topic states the independent variable, the dependent variable, at least one controlled variable, and describes precisely what change in the limewater, thermometer reading or gas volume would count as a positive result.
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Content standards in this chapter
Key concepts
- Energy and ATP
- Respiration transfers energy stored in the chemical bonds of glucose into ATP, a small molecule that acts as the cell's immediate, usable form of energy. ATP is broken down wherever energy is needed in the cell, releasing that energy instantly, then rebuilt using more energy from respiration, so the cell is never short of an energy supply.
- Aerobic respiration
- Aerobic respiration completely breaks down glucose using oxygen, producing carbon dioxide, water and a large amount of energy: glucose + oxygen gives carbon dioxide + water + energy. It takes place mainly in the mitochondria and is far more efficient than any anaerobic pathway, which is why muscle and liver cells contain especially large numbers of mitochondria.
- Anaerobic respiration in humans
- When the oxygen supply cannot meet a muscle's demand during vigorous exercise, human cells respire anaerobically: glucose gives lactic acid + a small amount of energy. Lactic acid accumulates in the muscle, lowers its pH, and contributes to the fatigue and cramp felt during and shortly after hard exercise.
- Fermentation in yeast
- Yeast cells respire anaerobically by fermentation, breaking glucose down into ethanol and carbon dioxide while releasing a small amount of energy: glucose gives ethanol + carbon dioxide + energy. This reaction is exploited industrially, the carbon dioxide makes bread dough rise during baking, and the ethanol is the basis of beer and wine production.
- Comparison
- Aerobic respiration releases far more energy from each glucose molecule than either anaerobic pathway, because it breaks glucose down completely into carbon dioxide and water rather than only partially into lactic acid or ethanol, both of which still contain a large amount of unreleased chemical energy.
- Oxygen debt
- During vigorous exercise, lactic acid builds up faster than it can be removed, creating an oxygen debt. After exercise stops, breathing and heart rate stay raised so that extra oxygen can be taken in to break down the accumulated lactic acid, which is why a person keeps panting after a hard sprint has finished.
- Where respiration happens
- Every cell begins respiration the same way: glycolysis breaks glucose down in the cytoplasm, releasing a small amount of energy without needing oxygen. If oxygen is available, the products of glycolysis then pass into the mitochondria, where the aerobic stages take place and release most of the total energy obtained from the glucose molecule.
- Uses of energy from respiration
- The ATP produced by respiration powers almost every activity of a living organism, including muscle contraction for movement, active transport of substances across cell membranes, the biosynthesis of large molecules such as proteins during growth, transmission of nerve impulses, and the generation of heat that keeps body temperature stable in warm-blooded animals.
- Investigating respiration
- Respiration is commonly investigated by detecting one of its products or measuring one of its reactants: limewater turns milky when carbon dioxide from respiring seeds or organisms is bubbled through it, a thermometer or insulated flask can detect the heat released by germinating seeds, and a respirometer measures the volume of oxygen consumed over time.
- Applications of fermentation
- Fermentation by yeast is used directly in the food industry: in baking, the carbon dioxide produced makes bread dough rise and gives bread its texture, while in brewing, the ethanol produced from sugar is the alcohol present in beer and wine. Both processes rely on exactly the same anaerobic pathway taught in this chapter.
How this chapter is examined
SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.
Common exam angles
- Writing the word equations for aerobic and anaerobic respiration, including the correct reactants and products for each pathway.
- Comparing respiration in humans and yeast, including where each pathway occurs and what limits it.
- Explaining an experiment on respiration or fermentation (Paper 3), including a suitable control and expected result.
- Explaining how limewater or a thermometer is used to show that respiration is releasing carbon dioxide or heat.
- Explaining why vigorous exercise leads to muscle fatigue and cramp in terms of lactic acid and oxygen debt.
- Interpreting a graph or table of oxygen consumption, carbon dioxide production or heat released during germination or fermentation.
Common mistakes
What students write: Saying respiration is the same as breathing.
What earns the mark: Breathing is the physical movement of air; respiration is the chemical release of energy from glucose inside cells, and one can continue for a short time even if the other stops.
What students write: Writing that anaerobic respiration releases more energy.
What earns the mark: Aerobic respiration releases far more energy per glucose molecule than anaerobic respiration, because glucose is broken down completely rather than only partially.
What students write: Giving lactic acid as the product of yeast fermentation.
What earns the mark: Yeast produces ethanol and carbon dioxide; lactic acid is the product in human muscles, not in yeast cells.
What students write: Forgetting oxygen in the aerobic equation.
What earns the mark: Aerobic respiration requires oxygen: glucose + oxygen gives carbon dioxide + water + energy, and leaving out oxygen makes the equation incorrect.
What students write: Saying respiration happens only in the mitochondria.
What earns the mark: Glycolysis, the first stage of respiration, happens in the cytoplasm of every cell; only the later aerobic stages take place inside the mitochondria. A question that asks for the location of a named stage is testing exactly this distinction between the cytoplasm and the mitochondria.
What students write: Assuming anaerobic respiration only happens when oxygen is completely absent.
What earns the mark: Anaerobic respiration happens whenever the oxygen supplied is not enough to meet a cell's demand, which can occur even while some oxygen is still present in the blood. This is why a muscle can start producing lactic acid within seconds of sprinting, well before the blood oxygen level has actually fallen to zero.
What students write: Writing that respiration creates energy.
What earns the mark: Respiration does not create energy; it releases energy already stored in the chemical bonds of glucose, in line with the principle that energy cannot be created or destroyed.
What students write: Believing the oxygen debt is repaid during exercise itself.
What earns the mark: The oxygen debt is repaid after exercise stops, when breathing and heart rate remain raised to supply the extra oxygen needed to break down accumulated lactic acid.
Study this chapter
Processes in this chapter
Experiments in this chapter
Frequently asked questions
What is the difference between respiration and breathing?
How does anaerobic respiration differ in humans and yeast?
Why does aerobic respiration release more energy?
Why do muscles feel sore or cramp after vigorous exercise?
Where inside a cell does respiration actually take place?
Source:SRC-DSKP-EN, SRC-FORMAT
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