Investigating Photosynthesis

Question 1

1 Explain why the apparatus needs to be airtight. 2 Explain why the temperature of the water bath needs to be kept constant.

3 Suggest an advantage of providing an additional source of carbon dioxide.

4 Suggest a reason for carrying out the experiment in a room that is dark except for the light source.

5 Sugg why the plant is kept in the dark before the experiment begins

6 Suggest why measuring the volume of gas produced by the plant in this experiment may not be an accurate measure of photosynthesis.

Answer 1

1. Because any air escaping from or entering the apparatus will respectively decrease or increase the volume of gas measured, which will give an unreliable result (1)
2. So that any changes in the rate of photosynthesis can be said to be the result of changes in light intensity and not changes in temperature (1)
3. To ensure there is sufficient CO2 and so does not limit the rate of photosynthesis (1)
4. To prevent other light falling on the plant as this may fluctuate and will affect intensity and hence the rate of photosynthesis, leading to an unreliable result. (1)
5. To prevent photosynthesis and to allow any oxygen produced before the experiment begins to disperse (1)
6. Because the volume of oxygen produced will be less than that produced by photosynthesis as some of the oxygen will be used in cellular respiration/ dissolved oxygen (and other gases) may be released from or absorbed by the water. (1)
   Total =6 marks

Question 2

Light intensity
Describe

Answer 2

The rate of photosynthesis is directly proportional to light intensity and then increasing light intensity further has no effect – the graph levels out by B

Question 3

Light intensity
Explain

Answer 3

Between points A & B the light intensity is the limiting factor, so as light intensity increases so does the rate of photosynthesis. Point B is the saturation point – increasing the light intensity after this point makes no difference, another factor becomes limiting eg: Temperature

Question 4

Carbon dioxide concentration
Describe

Answer 4

The rate of photosynthesis is directly proportional to concentration and then increasing the carbon dioxide concentration further has no effect – the graph levels out

Question 5

Carbon dioxide concentration
Explain

Answer 5

Initially the carbon is the limiting factor, so as light intensity increases so does the rate of photosynthesis. Point B is the saturation point – increasing the light intensity after this point makes no difference, another factor becomes limiting eg: Temperature

Question 6

Temperature
Describe

Answer 6

The rate of photosynthesis is directly proportional to temperature, then and then gradually slows reaching a maximum at the optimum temperature (25’C). Above the optimum increasing the temperature further decreases the rate.

Question 7

Temperature
Explain

Answer 7

Initially the temperature is the limiting factor. As temperature increases, the particles have more kinetic energy, more successful collision between substrate and enzyme active sites result in more products formed. The rate of photosynthesis increases.
Above the optimum temperature the enzymes begin to denature – hydrogen and ionic bonds break, the tertiary structure changes and substrate can no longer bind to the active site to form enzyme-substrate complexes, so the rate of photosynthesis falls.

Question 8

Why does temperature affect photosynthesis?

Answer 8

Photosynthesis involves enzymes (e.g. ATP synthase, rubisco). If the temperature falls below 10°C then the enzymes become inactive (due to low K.E), but if the temperature is greater than 45°C enzymes begin to denature. At high temperatures the stomata close to avoid losing too much water. This causes photosynthesis to slow down because less CO2 enters the leaf

Question 9

Limiting Factors
At any given moment, the rate of a physiological process is limited (slowed down)by the factor that is at its least favourable value.

In the case of photosynthesis this could be

Answer 9

Light – intensity, wavelength or photoperiod
Carbon Dioxide - concentration (too high and it is toxic)
Temperature - plants have a lower Optimum than mammals why?

Question 10

Stomata open and close.
If the carbon dioxide concentration is too high, the stomata will close.
Explain why this would be a problem.

Answer 10

Less carbon dioxide for photosynthesis.

Question 11

Water

Answer 11

Plants also need water- too little and photosynthesis (photolysis)stops, hydrolysis reactions cannot take place; cells become plasmolysed and the plant wilts.
Too much and soil becomes waterlogged reducing the uptake of minerals

Question 12

There is less oxygen in waterlogged soil. Roots are unable to respire aerobically. This means that

Answer 12

there is less ATP available for the active transport of minerals into the roots.

Question 13

PPQ

Answer 13