How do cells release energy from glucose, and what happens when oxygen runs out?
Respiration as the release of energy from glucose to make ATP, the word equation and stages of aerobic respiration including glycolysis, and the fermentation pathways to lactate in animals and to ethanol and carbon dioxide in plant and yeast cells.
An SQA National 5 Biology answer on respiration, covering the release of energy from glucose to make ATP, the word equation and stages of aerobic respiration including glycolysis, and the fermentation pathways to lactate in animals and to ethanol and carbon dioxide in plant and yeast cells.
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What this dot point is asking
The SQA wants you to explain that respiration releases energy from glucose to make ATP, give the word equation and stages of aerobic respiration (including glycolysis and the role of the mitochondria), and describe the fermentation pathways that happen without oxygen: to lactate in animal cells and to ethanol and carbon dioxide in plant and yeast cells.
The purpose of respiration and ATP
Cells use the energy in ATP for jobs such as muscle contraction, cell division, protein synthesis and transmission of nerve impulses.
Aerobic respiration
Aerobic respiration happens in two stages:
- Glycolysis takes place in the cytoplasm. Glucose is broken down into pyruvate, releasing enough energy to make two ATP. Glycolysis does not need oxygen.
- When oxygen is present, the pyruvate is broken down further in the mitochondria into carbon dioxide and water, releasing a large number of ATP.
This is why cells that need a lot of energy, such as muscle cells, contain many mitochondria.
Fermentation: respiration without oxygen
When there is no oxygen, the pyruvate from glycolysis cannot enter the mitochondria, so a different pathway, called fermentation, takes over. Glycolysis still happens, so two ATP are still made, but no further ATP is released.
Because fermentation only gives the two ATP from glycolysis, it releases far less energy than aerobic respiration, which is why organisms rely on aerobic respiration whenever oxygen is available.
Examples in context
Example 1. Muscle fatigue during a sprint. During a hard sprint, the muscles use oxygen faster than the blood can deliver it. The muscle cells switch to fermentation, producing lactate, which builds up and contributes to fatigue. Afterwards, with oxygen restored, the lactate is converted back, which is why you keep breathing hard for a while after stopping.
Example 2. Yeast in brewing and baking. Brewers and bakers use yeast fermenting without oxygen. In brewing, the ethanol produced is the alcohol in beer and wine. In baking, the carbon dioxide produced makes the dough rise. Both rely on yeast converting pyruvate to ethanol and carbon dioxide.
Try this
Q1. State where glycolysis takes place in the cell. [1 mark]
- Cue. In the cytoplasm.
Q2. Name the product of fermentation in animal cells when no oxygen is available. [1 mark]
- Cue. Lactate.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style3 marksDescribe aerobic respiration, including the word equation and where each stage takes place.Show worked answer →
A 3-mark describe answer needs the equation, the two stages and their locations.
The word equation is: glucose + oxygen produces carbon dioxide + water + energy (ATP).
The first stage, glycolysis, takes place in the cytoplasm, where glucose is broken down into pyruvate, releasing enough energy to make two ATP.
In the presence of oxygen, the pyruvate is broken down in the mitochondria into carbon dioxide and water, releasing a large number of ATP.
Markers reward (1) the correct word equation, (2) glycolysis in the cytoplasm producing pyruvate and two ATP, and (3) the breakdown of pyruvate in the mitochondria releasing much more ATP.
SQA N5 style3 marksCompare what happens to pyruvate in animal cells and in yeast cells when no oxygen is available.Show worked answer →
This compares the two fermentation pathways, so describe each and note that one is reversible.
In animal cells with no oxygen, pyruvate is converted into lactate. This pathway is reversible: when oxygen returns, the lactate can be converted back.
In yeast (and plant) cells with no oxygen, pyruvate is converted into ethanol and carbon dioxide. This pathway is irreversible.
Both pathways still produce only the two ATP from glycolysis, far less than aerobic respiration.
Markers reward (1) lactate in animals, (2) ethanol and carbon dioxide in yeast or plants, and (3) the comparison of reversible against irreversible, or the low ATP yield.
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Sources & how we know this
- SQA National 5 Biology Course Specification — SQA (2019)