Cellular Respiration, revision notes

Overview

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.

Energy, ATP and why every cell respires (Content Standard 7.1)

Content Standard 7.1 treats cellular respiration as the process that releases the energy stored in the chemical bonds of glucose so the cell has a usable supply. Respiration does not create energy; it transfers the chemical energy already held in glucose into adenosine triphosphate (ATP), a small molecule that acts as the cell's immediate energy currency.

ATP is broken down to ADP and a phosphate group wherever energy is needed, releasing that energy at once, and is rebuilt using further energy from respiration, so a living cell is never left without a supply.

This released energy powers the activities that keep an organism alive: the contraction of muscles for movement, the active transport of ions and molecules across cell membranes, the building of large molecules such as proteins during growth and repair, the transmission of nerve impulses, and the generation of heat that keeps a mammal's body temperature stable. A question that asks what respiration is 'for' is testing these uses, not the word equation on its own.

Every cell begins respiration the same way, whether oxygen is present or not. Glucose is first broken down in the cytoplasm through glycolysis, which releases a small amount of energy without needing oxygen.

What happens to the product of glycolysis next depends entirely on the oxygen supply, and this single branching point is the key idea that ties the whole chapter together.

Aerobic respiration (Content Standard 7.2)

Aerobic respiration breaks glucose down completely using oxygen and releases a large amount of energy. The word equation is: glucose + oxygen → carbon dioxide + water + energy.

After glycolysis in the cytoplasm, the energy-rich stages take place inside the mitochondria, which is why cells with a high energy demand, such as muscle and liver cells, contain a large number of mitochondria.

Because glucose is taken apart fully into carbon dioxide and water, aerobic respiration extracts far more energy from each glucose molecule than any oxygen-free pathway can. This high yield is the reason aerobic respiration is the body's main energy source at rest and during gentle activity, when the lungs and blood can deliver oxygen fast enough to meet the demand of the tissues.

In the exam, a full-mark answer writes the equation with every reactant and product correct, places oxygen on the left, and names the mitochondria as the main site. Leaving out oxygen, or writing the products as only carbon dioxide with no water, is a frequent way marks are lost.

Anaerobic respiration and fermentation (Content Standard 7.3)

When a cell cannot be supplied with enough oxygen, it respires anaerobically, meaning without oxygen. In human muscle during vigorous exercise the glucose is only partly broken down: glucose → lactic acid + a small amount of energy.

The lactic acid accumulates in the muscle, lowers its pH and contributes to the fatigue felt during and shortly after hard exercise.

Yeast carries out anaerobic respiration by fermentation: glucose → ethanol + carbon dioxide + a small amount of energy. This reaction is used directly in industry.

In baking, the carbon dioxide produced makes bread dough rise and gives the loaf its texture; in brewing, the ethanol produced from sugar is the alcohol present in beer and wine.

Both anaerobic pathways release only a small amount of energy because glucose is only partly broken down, so its products still hold a large amount of chemical energy that has not been released. The two pathways share the same first step, glycolysis, but differ in their end products: lactic acid in human muscle, ethanol and carbon dioxide in yeast.

The three pathways compared

The clearest way to hold the chapter together is to set the three respiration pathways side by side. The table below compares whether oxygen is needed, where the pathway takes place, what it produces, how much energy it releases and the word equation for each.

Oxygen debt, muscle fatigue and recovery

During vigorous exercise the muscles' demand for energy can exceed what the oxygen supply allows, so the cells switch partly to anaerobic respiration and produce lactic acid faster than the body can remove it. This shortfall in oxygen is called the oxygen debt: the extra oxygen the body will later need to deal with the accumulated lactic acid.

When exercise stops, breathing rate and heart rate stay raised for a time so that extra oxygen can be taken in and carried to the muscles and liver, where the lactic acid is broken down. This is why a person keeps panting after a hard sprint has finished rather than while sprinting, and it is the repayment of the oxygen debt, not exercise itself, that the deep breathing afterwards represents.

A precise answer sets out the sequence in order: vigorous exercise, oxygen supply falls short of demand, anaerobic respiration, lactic acid builds up, oxygen debt, and repayment after exercise when breathing and heart rate remain high. Describing the events out of order, or claiming the debt is repaid during the exercise, is a common source of lost marks.

Investigating respiration in the laboratory

Respiration is investigated by detecting one of its products or measuring one of its reactants, and Paper 3 often asks students to design or interpret such an experiment. Limewater turns cloudy (milky) when the carbon dioxide released by respiring seeds or small organisms is bubbled through it.

A thermometer inside an insulated (vacuum) flask detects the heat released by germinating seeds. A respirometer measures the volume of oxygen consumed over time, usually with soda lime or potassium hydroxide to absorb the carbon dioxide so that any change in gas volume is due to oxygen uptake alone.

A valid experiment always includes a control. In the germinating-seed experiments the control is an identical set-up using seeds that have been boiled and then cooled, because boiled seeds are dead and do not respire, so any cloudiness of the limewater or rise in temperature in the living set-up can be attributed to respiration rather than to the apparatus.

When answering a Paper 3 question on this topic, state the manipulated variable, the responding variable and at least one controlled variable, and describe precisely what change in the limewater, the thermometer reading or the gas volume would count as a positive result. Marks are given for this structure, not for a vague statement that respiration 'happens'.

Key concepts to master

• 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.

Quick recall checklist

1. Can you define and explain Energy and ATP?
2. Can you define and explain Aerobic respiration?
3. Can you define and explain Anaerobic respiration in humans?
4. Can you define and explain Fermentation in yeast?
5. Can you define and explain Comparison?
6. Can you define and explain Oxygen debt?
7. Can you define and explain Where respiration happens?
8. Can you define and explain Uses of energy from respiration?
9. Can you define and explain Investigating respiration?
10. Can you define and explain Applications of fermentation?

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