Investigating photosynthesis, limiting factors and minerals

Last time

Before we start: last time, in Temperature, pH and enzyme activity, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

What slows a plant down?

Farmers in the Netherlands grow tomatoes in greenhouses with extra light, warmth and carbon dioxide. They are removing whatever is holding photosynthesis back. That is the idea of a limiting factor, and it explains every photosynthesis graph.

Limiting factor

A limiting factor is something present in short supply that limits the rate of a process. For photosynthesis there are three: light intensity, carbon dioxide concentration and temperature. Whichever is in shortest supply controls the rate. Increasing the others makes no difference.

Reading the graph

Here is rate against light intensity. Where the line rises, light IS the limiting factor: more light, faster rate. Where it levels off, light is no longer limiting. Something else, such as carbon dioxide or temperature, is now limiting.

Each factor

Light: provides the energy, absorbed by chlorophyll. Carbon dioxide: it is a raw material, built into glucose. Temperature: photosynthesis is controlled by enzymes, which work faster when warmer, up to their optimum.

Measuring the rate

Put pondweed in water with a little sodium hydrogencarbonate to supply carbon dioxide. Count the bubbles of oxygen given off per minute, or collect the gas and measure its volume. Move a lamp to different distances to change the light intensity, keeping the temperature constant.

Light and distance

Light intensity falls quickly as the lamp moves away. Halve the distance and the light intensity becomes four times as great. Light intensity is proportional to one over the distance squared.

Hydrogencarbonate indicator

Hydrogencarbonate indicator is red at normal carbon dioxide levels. When carbon dioxide is removed by photosynthesis, it turns purple. When carbon dioxide is added by respiration, it turns yellow.

Mineral ions

Plants absorb mineral ions from the soil through their root hairs. Nitrate ions are needed to make amino acids, and so proteins. Without them, growth is stunted and older leaves turn yellow. Magnesium ions are needed to make chlorophyll. Without them, leaves turn yellow between the veins: chlorosis.