← Unit 1: Cells and multicellular organisms
Topic 1: Cells as the basis of life
Summarise the inputs, outputs and locations of photosynthesis and of aerobic and anaerobic cellular respiration
A focused answer to the QCE Biology Unit 1 dot point on photosynthesis and respiration. Writes the balanced word and chemical equations for photosynthesis and aerobic respiration, locates each in chloroplasts and mitochondria, and compares anaerobic respiration in animals (lactic acid) and yeast (ethanol).
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What this dot point is asking
QCAA wants you to write the equations for photosynthesis and respiration, identify where each occurs in the cell, and compare the two anaerobic pathways. You should be able to use the equations to reason about energy flow and the carbon-oxygen cycle.
The answer
Photosynthesis and cellular respiration are the two pillars of cellular energy metabolism. Photosynthesis captures light energy in glucose; respiration releases that energy as ATP.
Photosynthesis
Word equation. Carbon dioxide + water + (light energy) → glucose + oxygen.
Balanced chemical equation. 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2.
Where. Chloroplasts of plant mesophyll cells, algae and cyanobacteria (the latter in their thylakoid membranes only).
Two stages.
- Light-dependent reactions. Occur on the thylakoid membranes. Light energy excites electrons in chlorophyll, which splits water (photolysis) releasing O2, electrons and protons. The electrons drive ATP and NADPH production.
- Light-independent reactions (Calvin cycle). Occur in the stroma. ATP and NADPH from the light-dependent stage power the fixation of CO2 into glucose.
QCAA Unit 1 only requires the overall equation, the two stages by name and the chloroplast location. Detail of the Calvin cycle is not assessed.
Aerobic cellular respiration
Word equation. Glucose + oxygen → carbon dioxide + water + energy (ATP).
Balanced chemical equation. C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + (~36 to 38) ATP.
Where. Two locations.
- Cytosol. Glycolysis: glucose to 2 pyruvate, net 2 ATP and 2 NADH.
- Mitochondrion. Link reaction (pyruvate to acetyl-CoA), Krebs cycle (matrix) and the electron transport chain (inner membrane / cristae) generate most of the ATP via oxidative phosphorylation.
Why oxygen matters. O2 is the final electron acceptor at the end of the electron transport chain, forming water. Without O2 the chain backs up, NADH cannot be reoxidised and aerobic respiration halts.
Anaerobic respiration (fermentation)
When O2 is unavailable, cells regenerate NAD+ from NADH so that glycolysis can continue to make a small amount of ATP. Two pathways are examined.
Lactic acid fermentation (animals and some bacteria).
- Glucose → 2 lactic acid + 2 ATP.
- Pyruvate accepts the electrons from NADH, becoming lactic acid (lactate).
- Occurs in vertebrate muscle during intense exercise. The lactic acid is later carried in the blood to the liver where it is oxidised back to pyruvate (the Cori cycle) once oxygen is available; this generates the post-exercise oxygen debt.
Alcoholic fermentation (yeast and some plants).
- Glucose → 2 ethanol + 2 CO2 + 2 ATP.
- Pyruvate is decarboxylated to acetaldehyde, then reduced to ethanol.
- The basis of brewing, wine-making and bread leavening.
Both pathways yield only 2 ATP per glucose compared with around 36 to 38 from aerobic respiration. They are emergency or environmental adaptations.
Energy in context: ATP
ATP is the cell's energy currency. Hydrolysis of one phosphate bond releases about 30 kJ per mol, used to power active transport, biosynthesis, muscle contraction and many other processes. Cells regenerate ATP from ADP and Pi continuously using energy from respiration (or photosynthesis in chloroplasts).
The reciprocal relationship
Photosynthesis and aerobic respiration are inverse:
| Photosynthesis | Aerobic respiration | |
|---|---|---|
| Inputs | CO2, H2O, light | Glucose, O2 |
| Outputs | Glucose, O2 | CO2, H2O, ATP |
| Location | Chloroplast | Cytosol and mitochondrion |
| Energy flow | Light to chemical | Chemical to ATP |
The two processes drive the global carbon and oxygen cycles and underlie all biological energy flow (see energy flow and trophic relationships).
Common traps
Unbalanced equations. Always include the 6 in front of CO2, H2O and O2 to balance carbon, hydrogen and oxygen atoms.
Putting glycolysis in the mitochondrion. Glycolysis is cytosolic. The pyruvate produced is then imported into the mitochondrion.
Calling anaerobic respiration "without respiration". Anaerobic respiration still uses glycolysis (the first stage of cellular respiration) but no oxygen for the later stages.
Confusing the two fermentation end products. Animals make lactic acid (lactate). Yeast makes ethanol and CO2.
Cross-link to Year 12 assessment
Energy metabolism underpins Unit 3 ecosystem energy flow (IA1 stimulus often includes productivity and respiration data), Unit 3 IA2 student experiments using yeast fermentation rates to investigate enzyme effects, and Unit 4 cellular biology underpinning genetic and biotech contexts (IA3).
In one sentence
Photosynthesis (6 CO2 + 6 H2O + light → C6H12O6 + 6 O2) occurs in chloroplasts, and aerobic respiration (C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + ATP) occurs in the cytosol and mitochondria; without oxygen, animals ferment glucose to lactic acid and yeast ferment it to ethanol and CO2, each yielding only 2 ATP per glucose.
Past exam questions, worked
Real questions from past QCAA papers on this dot point, with our answer explainer.
2023 QCAA style4 marksWrite the balanced chemical equations for photosynthesis and aerobic respiration. Identify the cellular location of each and state the relationship between them.Show worked answer →
A 4-mark answer needs both equations, the two locations and the inverse relationship.
Photosynthesis. 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2. Location: chloroplasts of photosynthetic eukaryotes (mesophyll cells in leaves).
Aerobic respiration. C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + ATP (approximately 36 to 38 ATP per glucose). Location: cytosol (glycolysis) and mitochondria (link reaction, Krebs cycle, electron transport chain).
Relationship. The two processes are reciprocal: the products of one are the inputs of the other. Photosynthesis stores light energy as chemical energy in glucose; aerobic respiration releases that energy in the form of ATP. Together they cycle carbon and oxygen through the biosphere.
Markers reward balanced equations, both locations, and a clear inverse-relationship statement.
2022 QCAA style3 marksCompare anaerobic respiration in human muscle cells and in yeast.Show worked answer →
A 3-mark answer needs both end products, ATP yields and a shared feature.
In human muscle cells. Glucose to lactic acid (lactate) via glycolysis followed by lactate fermentation. Net 2 ATP per glucose. Occurs during intense exercise when oxygen demand outstrips supply. Lactic acid contributes to muscle fatigue and is later oxidised back to pyruvate when oxygen is available.
In yeast. Glucose to ethanol and carbon dioxide via glycolysis followed by alcoholic fermentation. Net 2 ATP per glucose. Used industrially in brewing and bread-making.
Shared feature. Both pathways regenerate NAD+ from NADH, allowing glycolysis to continue in the absence of oxygen. Both yield only 2 ATP per glucose, far less than aerobic respiration.
Markers reward both end products and the parallel NAD+ regeneration role.
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