S4.1.7 Chapter 7

Testing photosynthesis

How can you measure how fast a plant makes oxygen?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Weigh the plant before and after.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Measure how green its leaves are.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Count the bubbles of gas that a water plant gives off in a minute.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Count oxygen bubbles from a water plant at different distances from a lamp
Plan a fair test of how light changes the bubble rate

Photosynthesis gives off oxygen. A water plant in the light makes oxygen under the water, and the gas rises as small bubbles. If you count the bubbles in a minute, you have a number that stands for the rate.

Scientists ask how light changes the rate. They use a lamp and move it nearer to the plant and further away. A water plant that scientists often use is called Elodea. Any water plant from a pond or an aquarium can be used.

Draw it first

The pondweed at three distances from the lamp

1 The lamp is 10 cm from the pondweed. Bubbles rise fast: 8 in a minute.
2 At 20 cm the plant gets less light. There are 5 bubbles in a minute.
3 At 30 cm there is less light again, and only 3 bubbles in a minute.

The plant is cut at its stem and placed upside down in the funnel, so the gas collects in the tube. The bubbles are mostly oxygen from photosynthesis. A pinch of baking soda in the water gives the plant plenty of carbon dioxide. So carbon dioxide is less likely to limit the rate.

Now think about a fair test. In a fair test you change one thing only, and keep everything else the same. The thing you change is the independent variable. The thing you measure is the dependent variable. The things you keep the same are control variables.

The lamp distance is the independent variable. The bubble count is the dependent variable. The rest are control variables.

One thing needs care. A lamp gives out heat as well as light. Heat would warm the water, and warmth also speeds up photosynthesis. Then the test would not change only the light. Use a cool LED lamp, or put a beaker of water between the lamp and the plant to take up the heat.

Counting bubbles is not exact. You may miss one, or count a little more at times. So you count three times at each distance, and take the mean. Wait a few minutes after each move, so that the plant adjusts to the new light.

Bubbles counted in one minute, three times at each distance. The last column is the mean of the three.

Worked example

At 20 cm the three counts are 5, 6 and 4 bubbles per minute. What is the mean?

Mean = total ÷ number of counts.
Total = 5 + 6 + 4 = 15.
15 ÷ 3 = 5.

Answer: 5 bubbles per minute

Now draw a graph of the means. Put the distance, which you changed, along the bottom. Put the bubbles per minute, which you measured, up the side.

The further the lamp is, the fewer the bubbles. The line falls, but not in a straight line.

For a small bare bulb that shines in all directions, light does not fall in step with distance. It falls much faster. At twice the distance the light spreads over four times the area, so the plant gets a quarter of the light. A torch with a reflector does not follow this rule closely, so use it only as a guide.

Worked example

The lamp is moved from 10 cm to 20 cm away. What percentage of the light reaches the plant now?

For a small bare bulb, the light on the plant = (old distance ÷ new distance) squared.
10 ÷ 20 = 0.5.
0.5 × 0.5 = 0.25.
0.25 is 25%, so the plant gets 25% as much light.

Answer: 25% as much light

The light on the plant as a percentage of the light at 10 cm. The bubbles fall more slowly than the light. In bright light something other than light, such as the temperature, starts to limit the rate. The percentages are rounded to the nearest whole number.

Sometimes one count is far from the others, such as 14 when the rest are 8 and 7. It may be a miscount. A result like this is anomalous. It is best to repeat that count. If you cannot, leave it out and say so.

Worked example

At 10 cm the counts are 8, 7 and 14 bubbles per minute. The 14 is probably a miscount, so it is left out. What is the mean of the other two?

14 is much higher than 8 and 7, so it is anomalous. Leave it out.
Total of the other two = 8 + 7 = 15.
15 ÷ 2 = 7.5.
The mean is 7.5 bubbles per minute.

Answer: 7.5 bubbles per minute

A water plant may be hard to find. If you have none, there are two ways to see the same idea. Put a fresh green leaf under water in a glass with a pinch of baking soda. Stand the glass in bright sunlight, and look for small bubbles on the leaf. Then move it into shade and compare.

If you have a small syringe, without a needle, you can use leaf discs. Cut small circles from a leaf with the end of a clean drinking straw. Draw the air out of them with the syringe in baking soda water, and they sink. In the light they make oxygen and float. The less time they take to float, the faster the photosynthesis.

Try it

Count the bubbles

You need: a water plant from a pond or an aquarium, or a fresh leaf, a clear glass or jar, water and a pinch of baking soda, a battery lamp or LED torch, a ruler, a clock with a second hand, two more glasses of water.

Be safe: Keep mains electricity away from water. Use a battery torch or an LED lamp with a dry cable on a dry shelf. A hot lamp can burn you, so do not touch it. Do not drink the water. Return the water plant to its pond or aquarium afterwards. Wash your hands after touching pond water or the plant.

Put water and a pinch of baking soda in the glass. Stir it. Put in the plant, or the leaf. Fill one more glass the same way, with no plant in it. Stand it beside the first glass, the same distance from the torch. Bubbles in both glasses are air from the warming water. Only extra bubbles on the plant come from photosynthesis.
Put the torch 10 cm from the glass. Put the last glass of plain water between them. Wait three minutes.
Count the bubbles that rise from the plant in one minute. Do this three times.
Move the torch to 20 cm, and then 30 cm. Wait three minutes each time and count three times.
Work out the mean at each distance. Plot the means on a graph.

What you will see: The bubbles are fewer when the torch is further away. The numbers in the picture are from a test like this. Your numbers will not match, but the pattern will.

If you cannot try it, this is what happens.

A wrong idea many people have

The bubbles in the test are carbon dioxide.

Why it feels right: Gas bubbles are often carbon dioxide, and carbon dioxide is the gas plants are known for taking in.

The test that breaks it: Scientists collect the gas in the test tube and put a glowing splint into it. The splint bursts into flame, which is the test for oxygen. Carbon dioxide would put it out. This is done in a lab with a flame, not at home.

What is true: The bubbles are mostly oxygen, made by photosynthesis. The plant takes in carbon dioxide.

Key words

fair test: a test where only one thing changes and everything else is kept the same.
independent variable: the thing you change.
dependent variable: the thing you measure.
control variables: the things you keep the same.
mean: the total of the counts divided by how many counts there are.
anomalous: far from the other results, probably because of a mistake.

Back to your guess

The answer is C: Count the bubbles of gas that a water plant gives off in a minute.

A water plant in bright light gives off small bubbles of oxygen. Counting the bubbles in a set time is a way to measure the rate of photosynthesis.

Keep it

Copy this into your exercise book.

Count the bubbles per minute at several lamp distances. Change only the distance. Keep the plant, water, time and heat the same. Count three times and take the mean. Closer lamp, more light, more bubbles.

Draw: A lamp, a ruler and a beaker of pondweed with bubbles. Then a graph of distance against bubbles per minute.