Flashcards · Biology · Photosynthesis
Investigating Light Intensity and Photosynthesis
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What gas do the bubbles from pondweed contain?
Oxygen.
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What is the independent variable in the pondweed practical?
The distance of the lamp from the pondweed.
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What is the dependent variable?
The number of bubbles per minute, or the volume of gas collected.
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Why is sodium hydrogencarbonate added to the water?
To provide carbon dioxide, so it is not the limiting factor.
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Why use a heat shield or water bath?
To stop the temperature rising and affecting the rate.
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Name three control variables.
Any three of the same pondweed, temperature, carbon dioxide level, time and size of beaker.
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Why repeat each distance?
To check reliability and calculate a mean.
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Why is counting bubbles a rough measure?
The bubbles can differ in size.
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What is a better way to measure the gas?
Collect it in a gas syringe or measuring tube and measure the volume.
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What happens to the rate as the lamp is moved further away?
It decreases.
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Why does moving the lamp change the light intensity?
The further away the lamp is, the dimmer the light that reaches the plant.
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(HT) State the inverse square law.
Light intensity is proportional to 1 ÷ distance².
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(HT) What happens to light intensity if the distance doubles?
It falls to one quarter.
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(HT) What happens to light intensity if the distance triples?
It falls to one ninth.
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(HT) What should a graph of rate against 1/d² look like?
A straight line, while light is the limiting factor.
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Name the part marked X and say why it is there. The ruler. It measures the distance between the lamp and the pondweed, which is the independent variable.
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Label the apparatus. 
Lamp, ruler, pondweed, bubbles of oxygen, beaker of water with sodium hydrogencarbonate and thermometer.