The role of ATP, the light-dependent and light-independent reactions of photosynthesis, and the factors that limit the rate.

What this dot point is asking

WJEC wants you to explain the role of ATP, describe the light-dependent and light-independent reactions of photosynthesis and where they occur, and explain how light intensity, carbon dioxide concentration and temperature limit the rate.

The role of ATP

The light-dependent reactions

These occur in the thylakoid membranes, which are stacked into grana to give a large surface area. Light excites electrons in chlorophyll; the excited electrons pass along an electron transport chain of carriers. The energy released pumps protons into the thylakoid space, and they flow back through ATP synthase, driving photophosphorylation (making ATP) by chemiosmosis. NADP is reduced to reduced NADP. Photolysis of water replaces the lost electrons and releases oxygen and protons: $2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4e^- + \text{O}_2$.

The light-independent reactions (Calvin cycle)

It takes three turns of the cycle (fixing three $\text{CO}_2$) to make one molecule of TP available for synthesis, and six turns to build one glucose, which is why the cycle must keep regenerating RuBP.

Limiting factors

The rate of photosynthesis is limited by whichever factor is in shortest supply: light intensity, carbon dioxide concentration or temperature. Raising the limiting factor increases the rate until another factor takes over as limiting; at high temperatures the enzymes, especially rubisco, denature and the rate falls. Growers exploit this in glasshouses by adding $\text{CO}_2$ and light to push the rate up.

Examples in context

Example 1. Commercial glasshouses. Tomato growers burn fuel to raise both temperature and $\text{CO}_2$ concentration and use artificial lighting, deliberately removing every limiting factor in turn so photosynthesis runs near its maximum. Yields rise sharply, a direct commercial application of limiting-factor theory.

Example 2. The Hill reaction. Robert Hill showed that isolated chloroplasts could reduce a dye and release oxygen in the light without any carbon dioxide present. This separated the light reactions from carbon fixation and is the classic evidence that oxygen comes from the photolysis of water, not from carbon dioxide.

Try this

Q1. Name the products of the light-dependent reactions used in the Calvin cycle. [1 mark]

• Cue. ATP and reduced NADP.

Q2. Explain why increasing light intensity beyond a certain point no longer increases the rate of photosynthesis. [2 marks]

• Cue. Another factor, such as carbon dioxide concentration or temperature, has become the limiting factor.

Q3. A plant's rate of photosynthesis rises from $12$ to $30 \text{ arbitrary units}$ when $\text{CO}_2$ is added. Calculate the percentage increase. [2 marks]

• Cue. $\frac{30 - 12}{12} \times 100 = \frac{18}{12} \times 100 = 150\%$.