Photosynthesis as an endothermic reaction in chloroplasts, the word and symbol equations, the uses of glucose made by the plant, limiting factors (light intensity, carbon dioxide concentration, temperature), and the inverse square relationship between light intensity and distance.

What this dot point is asking

OCR wants you to describe photosynthesis as an endothermic reaction in the chloroplasts, give its equations, state how the plant uses the glucose, explain the three limiting factors, and use the inverse square relationship between light intensity and distance.

Photosynthesis is endothermic

The word equation is:

The balanced symbol equation is:

Notice that this is the reverse of the equation for aerobic respiration. Photosynthesis stores energy in glucose; respiration releases it again.

The uses of glucose

The glucose a plant makes is used in several ways:

• Respiration, to release energy for the plant's own processes.
• Converted to insoluble starch for storage (insoluble so it does not affect osmosis or dissolve away).
• Used to make cellulose for strengthening cell walls.
• Used to make amino acids, and then proteins, which also requires nitrate ions absorbed from the soil.
• Converted to oils and fats for storage, especially in seeds.

Limiting factors

The rate of photosynthesis depends on the conditions. A limiting factor is the factor in shortest supply that limits the rate of the process. The three limiting factors are:

• Light intensity. More light gives a faster rate, until another factor becomes limiting.
• Carbon dioxide concentration. More carbon dioxide gives a faster rate, until another factor becomes limiting.
• Temperature. A higher temperature speeds up the enzyme-controlled reactions of photosynthesis, up to an optimum. Above the optimum the enzymes denature and the rate falls.

On a graph, when the rate is rising with a factor, that factor is limiting; when the line levels off, some other factor has become limiting (or, for temperature, the enzymes are denaturing).

The inverse square law for light

Light intensity does not fall steadily with distance: it follows an inverse square relationship. The intensity is proportional to $\dfrac{1}{d^2}$, where $d$ is the distance from the light source.

This means that if you double the distance, the intensity falls to a quarter ($\dfrac{1}{2^2} = \dfrac{1}{4}$); if you treble it, the intensity falls to a ninth. This is why the rate of photosynthesis drops so quickly when a lamp is moved away from a plant in the light intensity practical.

The photosynthesis practical (PAG B4)

A common required practical investigates the effect of light intensity on the rate of photosynthesis using pondweed (such as Elodea). You count the oxygen bubbles released per minute (or measure the volume of gas collected) at different distances from a lamp, using the distance to vary the light intensity. Important control variables are temperature (use a water bath or beaker of water as a heat shield) and carbon dioxide concentration (add sodium hydrogencarbonate). Exam questions test the method, the control variables, and applying the inverse square law to convert distance into relative light intensity.