Summarise the inputs, outputs and locations of photosynthesis and of aerobic and anaerobic cellular respiration

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

1. 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.
2. 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:

The two processes drive the global carbon and oxygen cycles and underlie all biological energy flow (see energy flow and trophic relationships).

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).

Examples in context

Example 1. Sugarcane photosynthesis on the Burdekin floodplain. Queensland sugarcane (Saccharum officinarum) grown on the Burdekin floodplain is a C4 plant: mesophyll chloroplasts fix CO2 onto phosphoenolpyruvate, then bundle-sheath chloroplasts run the Calvin cycle on concentrated CO2. The summary equation is $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$, but the spatial separation lets sugarcane fix carbon efficiently at 35 degrees Celsius when C3 photorespiration would dominate. Burdekin cane reaches dry-matter productivity above 80 t/ha/year, among the world's highest. The example shows light reactions on thylakoid membranes and the Calvin cycle in the stroma operating in two different cell types.

Example 2. Marathon runner anaerobic respiration in Brisbane heat. During the Brisbane Marathon in August, runners reach a lactate threshold around 80 percent VO2 max when oxygen supply lags ATP demand. Aerobic respiration (yielding approximately 30 to 32 ATP per glucose) in mitochondria continues, but extra glycolysis in the cytosol produces pyruvate faster than the Krebs cycle can consume it. Pyruvate is reduced to lactate by lactate dehydrogenase, regenerating NAD+ so glycolysis can continue: yield drops to 2 ATP per glucose. Blood lactate rises from 1 to 12 mmol/L, contributing to muscle fatigue. The example contrasts aerobic and anaerobic respiration with quantitative ATP yields.

Try this

Q1. Write the summary equation for aerobic cellular respiration and state the location and ATP yield of each of glycolysis, Krebs cycle and oxidative phosphorylation. [4 marks]

• Cue. $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$. Cytosol 2 ATP; matrix 2 ATP; inner membrane 26 to 28 ATP.

Q2. A leaf was placed in a sealed chamber under increasing light intensities and CO2 fixation was measured at 0, 5, 15 and 35 percent full sunlight, giving 0, 8, 18 and 22 micromoles CO2 per minute. Identify the limiting factor at low light and at high light, and predict the trend if temperature rises from 20 to 35 degrees Celsius. [3 marks]

• Cue. Light-limited at low intensity; CO2 or enzymes at high. Higher temperature accelerates Calvin cycle to a point.

Q3. Compare aerobic and anaerobic respiration. (a) State the location and final electron acceptor of each. (b) Explain why anaerobic yield is so much lower. (c) Justify why yeast brewing of XXXX beer favours anaerobic conditions. [2+2+2 marks]

• Cue. (a) Aerobic: mitochondria, O2. Anaerobic: cytosol, pyruvate (or NAD+ regeneration). (b) Only substrate-level ATP; no ETC. (c) Produces ethanol, the desired product.