# Light intensity and photosynthetic rate relationship

### BBC Bitesize - GCSE Combined Science - What happens during photosynthesis? - OCR 21C - Revision 5

Increasing the light intensity increases the rate of photosynthesis, until some other At low light intensities the increase in the rate of photosynthesis is linear. Relationships between leaf anatomy and photosynthesis were examined. . concentration, photosynthetic rates were determined at light intensities up to levels. This study was undertaken to determine the relationship between of light intensity and rate of photosynthesis in spinach leaves. Rate of photosynthesis was.

If oxygen production or carbon dioxide uptake is used as a measure of photosynthetic rate, the graphs are slightly different.

The line does not go through the origin. This is because oxygen production and carbon dioxide uptake are affected by respiration as well as photosynthesis. Respiration uses oxygen and produces carbon dioxide. Below is a graph measuring the rate of photosynthesis plotted on carbon dioxide against light intensity: This graph shows that with no light, the carbon dioxide uptake is below zero point A on the graph. With no light, there is no photosynthesis. So the rate of respiration is greater than the rate of photosynthesis.

So this means there is an overall excess of carbon dioxide produced during respiration. An excess is the same as a negative uptake.

As the light intensity increases the rate of photosynthesis follows point B on the graph.

## RELATION BETWEEN LIGHT INTENSITY AND RATE OF PHOTOSYNTHESIS OF LOBLOLLY PINE AND CERTAIN HARDWOODS

This means more carbon dioxide is being absorbed by the plant for photosynthesis. So this means more carbon dioxide uptake occurs. The graph shows this increase in carbon dioxide uptake.

There will come a point when the rate of photosynthesis has increased to match the rate of respiration. This is called the compensation point point C on the graph. This is shown on the graph as the point the line crosses zero on the x-axis. Beyond the compensation point, further increases in light intensity cause an increase in photosynthesis.

This means the rate of photosynthesis increases beyond the rate of respiration point D. Repeat steps 4 and 5, with increasing distances away from the light source e. Note that doubling the the distance does not half the intensity, rather quarters it.

### RELATION BETWEEN LIGHT INTENSITY AND RATE OF PHOTOSYNTHESIS OF LOBLOLLY PINE AND CERTAIN HARDWOODS

This indicates that the rate of photosynthesis increases with light intensity. However, at sufficiently high levels of light intensity, the rate oxygen evolution remains constant.

Use the Waterweed Simulato r to manipulate the light intensity, wavelength and carbon dioxide and see their effect on the rate of photosynthesis. Why is sodium hydrogen carbonate solution used instead of water? Sodium hydrogencarbonate increases carbon dioxide availability, and ensures that carbon dioxide is not a limiting factor for the the reaction. Plain water water may not contain enough oxygen so that the maximum rate of photosynthesis can occur. Tap water may be used to make the solution, but only after it has been left to stand overnight, so that it chlorine content is reduced.

Chlorine tends to inhibit the rate of photosynthesis. Detergent may be added to reduce surface tension. This will allow bubbles to escape freely.

When surface tension is high, bubbles are of varied sizes. The power of the lamp may be varied e.

## What happens during photosynthesis? - OCR 21C

Why was the laboratory darkened? When counting bubbles, the bubbles may be of varied sizes, leading to inaccurate conclusions about the rate of oxygen production. If the rate of bubbling is very high, then there may be errors in counting. Another source of inaccuracy is the presence of other light sources, of which intensities may be varied by shifting shadows.

• Photosynthesis

How can we prove that oxygen was the gas produced? A glowing splint should be placed near the mouth of the test tube, where the gas was collected. If oxygen is present the splint is re-ignited into a flame. If the gas which was produced was analyzed and found not to be pure oxygen, how would you account for this?