How do cells release the energy stored in glucose?
Explain how aerobic and anaerobic respiration release energy from glucose to produce ATP
Cellular respiration releases energy from glucose to make ATP; aerobic respiration uses oxygen for a high ATP yield, while anaerobic respiration releases less and produces lactate or ethanol.
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What this dot point is asking
You need to explain the purpose of respiration, write its overall equation, compare aerobic and anaerobic pathways, and identify where they occur and what they produce.
Why cells respire
Cells need a constant supply of usable energy for processes like active transport, muscle contraction and synthesis. Cellular respiration transfers the chemical energy in glucose to ATP (adenosine triphosphate), which cells use directly. Respiration is a controlled, stepwise release of energy, not a single burst.
Aerobic respiration
Aerobic respiration uses oxygen and gives the highest ATP yield. Its overall word and balanced equations are:
glucose + oxygen gives carbon dioxide + water + energy (ATP)
It happens in stages:
- Glycolysis in the cytoplasm splits glucose into two pyruvate molecules, producing a small amount of ATP. This step does not need oxygen.
- The remaining stages (the Krebs cycle and electron transport chain) occur in the mitochondria and require oxygen. They release most of the ATP.
The large surface area of the inner mitochondrial membrane (cristae) supports these reactions, which is why active cells have many mitochondria.
Anaerobic respiration
When oxygen is unavailable or in short supply, cells use anaerobic respiration. It begins with glycolysis (the only ATP-producing step), so the yield is low, just 2 ATP per glucose. The pathway differs between organisms:
- In animal cells (and human muscle during intense exercise): pyruvate is converted to lactate (lactic acid). Build-up of lactate contributes to muscle fatigue and is later broken down when oxygen returns.
- In yeast and plant cells: pyruvate is converted to ethanol and carbon dioxide in fermentation, used in brewing and baking.
Anaerobic respiration releases less energy because glucose is not fully broken down.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 SACE Stage 22 marksThe energy-releasing process in a bacterium at an oxygen concentration of 0% also occurs in plant cells in the absence of oxygen. Write a balanced chemical equation for this process.Show worked answer →
The process is anaerobic respiration (fermentation). In plant and yeast cells this is alcoholic fermentation:
C6H12O6 to 2C2H5OH + 2CO2
(glucose to ethanol + carbon dioxide). One mark is for the correct reactant and products, one for balancing. A word equation, glucose to ethanol + carbon dioxide, is also accepted.
2019 SACE Stage 22 marksExplain how bacteria use ATP in order to grow.Show worked answer →
For 2 marks, link ATP to energy-requiring processes of growth.
ATP is the cell's energy currency. When ATP is broken down to ADP and inorganic phosphate, energy is released.
Bacteria use this released energy to drive the energy-requiring (anabolic) processes of growth, such as synthesising proteins, building new cell membrane and wall material, and replicating DNA, all of which are needed to increase in size and divide.
2018 SACE Stage 22 marksWrite a balanced chemical equation for the reaction that uses oxygen and occurs in spinach leaves (aerobic respiration).Show worked answer →
The oxygen-using reaction is aerobic respiration:
C6H12O6 + 6O2 to 6CO2 + 6H2O
(glucose + oxygen to carbon dioxide + water). One mark is for the correct reactants and products, one for correct balancing.