Explain how photosynthesis uses light energy to convert carbon dioxide and water into glucose

What this dot point is asking

You need to explain the purpose of photosynthesis, write its overall equation, describe the two stages and where they occur, and explain the factors that limit its rate.

Why photosynthesis matters

Photosynthesis is how plants, algae and some bacteria capture light energy and store it as chemical energy in glucose. It is the foundation of almost all food chains and the source of atmospheric oxygen. The overall equation is:

carbon dioxide + water gives glucose + oxygen (using light energy)

It takes place in chloroplasts, where the green pigment chlorophyll absorbs light. Chloroplasts have internal membranes (thylakoids stacked into grana) surrounded by a fluid stroma.

The two stages

Light-dependent reactions

These occur in the thylakoid membranes and require light:

• Chlorophyll absorbs light energy.
• Water is split (photolysis), releasing oxygen as a by-product.
• The energy is used to produce ATP and NADPH (energy-carrying molecules).

Light-independent reactions (Calvin cycle)

These occur in the stroma and do not directly need light:

• The ATP and NADPH from the light-dependent stage provide energy.
• Carbon dioxide is fixed and combined to build glucose.

The two stages are linked: the light-dependent stage supplies the energy carriers that the light-independent stage spends. If the light is switched off, the light-dependent reactions stop almost immediately, so ATP and NADPH are no longer made; the Calvin cycle then halts within seconds because it has run out of the energy carriers it depends on. This dependency is why even the "light-independent" reactions cannot continue indefinitely in the dark.

Chloroplast structure and function

The chloroplast is built for its two-stage job, and SACE rewards linking structure to function. The thylakoid membranes are stacked into grana, giving a very large surface area packed with chlorophyll and the protein complexes of the light-dependent reactions - the more membrane, the more light can be harvested. The fluid stroma surrounding the thylakoids holds the enzymes of the Calvin cycle, keeping carbon fixation close to the ATP and NADPH supply. A double outer membrane encloses the whole organelle, and like the mitochondrion the chloroplast carries its own circular DNA and 70S ribosomes, evidence cited for the endosymbiotic theory.

Why limiting-factor logic works

At any moment the rate of photosynthesis is set by whichever resource is in shortest supply. Increasing a factor that is not currently limiting has no effect, which is the trap most graph questions are built around. On a typical curve the rate rises steeply while the plotted factor is limiting, then flattens into a plateau once a different factor takes over. To lift the plateau you must raise the new limiting factor. Commercial glasshouses exploit this directly: growers raise carbon dioxide concentration and keep temperature near the enzyme optimum so that light becomes the only thing limiting growth, maximising yield. Past papers frequently ask students to read off the limiting factor at a labelled point on the graph and justify the choice, so always name the factor and state why it is limiting at that point.

Limiting factors

The rate of photosynthesis is controlled by whichever factor is in shortest supply, the limiting factor:

• Light intensity. More light increases the rate until another factor becomes limiting; in darkness, photosynthesis stops.
• Carbon dioxide concentration. More carbon dioxide increases the rate up to a point.
• Temperature. The reactions are enzyme-controlled, so the rate rises with temperature to an optimum, then falls as enzymes denature.