5.2.1(a) - Photosynthesis and respiration interrelationship

Linked Processes

Photosynthesis and aerobic respiration are connected, but they are not simply the same reaction running backwards. They happen in different cell structures, involve different enzyme-controlled pathways, and transfer energy in different directions.

The relationship in this lesson is the overview relationship between raw materials and products:

• photosynthesis uses carbon dioxide and water, and produces glucose and oxygen
• aerobic respiration uses glucose and oxygen, and produces carbon dioxide and water

Plants, algae and some bacteria can photosynthesise. Living plant, animal and microbial cells transfer chemical energy to ATP through respiration or related energy-releasing pathways. A green plant is therefore not a "photosynthesis-only" organism: its cells respire as well.

[DIAGRAM: photosynthesis_respiration_interrelationship: Lesson 121: Photosynthesis and respiration interrelationship - diagram 01; asset_slug: 121_m05_2_1_photosynthesis_and_respiration_interrelationship__diagram_01; recommended_method: drawn_biology; description: Deterministic drawn flow diagram linking photosynthesis and aerobic respiration through glucose, oxygen, carbon dioxide and water, with separate arrows for light energy input and ATP-linked energy release.]

Raw Materials And Products

A raw material is a substance used up as a reactant in a process. A product is a substance made by that process. The key OCR idea is that a product of one of these processes can be a raw material for the other.

The simplified equation for photosynthesis is:

The simplified equation for aerobic respiration is:

These equations are useful for ratios. In the simplified photosynthesis equation, 6 molecules of carbon dioxide are linked with 1 molecule of glucose and 6 molecules of oxygen. That means the carbon dioxide:glucose ratio is 6:1, and the carbon dioxide:oxygen ratio is 1:1.

Glucose in the equation is a simplified summary product. In real plant cells, photosynthetic products may be converted into sucrose, starch, cellulose, lipids, amino acids or other organic molecules. For this specification point, the important point is still the same: photosynthesis builds organic molecules using carbon dioxide and water, while respiration can break organic molecules down.

The equations also conserve atoms: one glucose contains the six carbon atoms supplied by six carbon dioxide molecules. A mole is a counting unit, so the same ratios apply to molecules, moles and millimoles. For example, $0.20\,\mathrm{mmol}$ glucose corresponds to $1.20\,\mathrm{mmol}$ carbon dioxide, or $1.20\times10^{-3}\,\mathrm{mol}$. Never compare a value in mol with one in mmol without converting it.

Estimate before calculating: fixing 0.98 mmol glucose needs a little under 6 mmol CO₂ because the ratio is 6:1. The exact result, 5.88 mmol, fits that estimate; 0.163 mmol would indicate an inverted ratio.