2: Investigating Limiting Factors

Question 1

Define ‘limiting factor’.

Answer 1

factor (in a process e.g. photosynthesis) that is nearest to its minimum - (the factor that restricts the process)

Question 2

State the possible limiting factors for photosynthesis?

Answer 2

• temperature
• light intensity
• carbon dioxide concentration

Question 3

State the principles which should be remembered when designing an experiment to investigate the effect of a limiting factor on photosynthesis.

Answer 3

1. only one lim factor investigated at a time (this is IV)
2. suitable range for IV: from lowest possible level to a level at which the factor is no longer limiting
3. accurate method of measuring rate of photosynthesis (the DV). usually a measure of oxygen production per unit time
4. methods must be devised to keep all factors constant (the control variables)

Question 4

How to investigate temperature as limiting factor?

Answer 4

• method of varying factor: place pondweed in water in a thermostatically controlled water bath or on a hot plate to vary the temperature
• suggested range: 5°C to 45°C; in 5 or 10°C intervals
• controlling the factor: set thermostat to 25°C and keep it there throughout experiment

Question 5

How to investigate light intensity as limiting factor?

Answer 5

• method of varying factor: move light source to different distances and measure light intensity with a lux meter (light intensity = 1/(distance^2) )
• suggested range: 4,5,7,10 and 14cm and no light gives a good range of intensities
• controlling the factor: keep the light source at a constant distance, such as 5cm

Question 6

How to investigate carbon dioxide concentration as limiting factor?

Answer 6

• method of varying factor: start with boiled, cooled water (no CO2) then add measured quantities of NaHCO3 to increase the CO2 concentration
• suggested range: 0 to 50 mmol dm^-3 in 10mmol dm^-3 intervals
• controlling the factor: add enough NaHCO3 to give a high CO2 concentration (50 mmol dm^-3)

Question 7

Outline effect of light intensity on rate of photosynthesis.

Answer 7

at low light intensity:

• rate of photolysis is limited
• this limits the production of ATP and high energy electrons
• this limits the conversion of CO2 into glucose
• therefore, low light intensities limit the production of glucose and other useful substances
• at high light intensities some other factor is limiting photosynthesis
• (unless a plant is heavily shaded, or the sun is rising or setting, light intensity is not usually the lim factor)

Question 8

Outline the effect of CO2 concentration on photosynthesis.

Answer 8

• below 0.01% CO2 the enzyme used to fix CO2 (rubisco) is not effective and in many plants there is no net photosynthesis
• between 0.01 - 0.04% the concentration of CO2 is the lim factor because the rate of successful collisions between CO2 molecules and active site of enzyme that fixes it is still lower than any of other steps in photosynthesis.
• ATP and high energy electrons are not used as rapidly as they are produced, which restricts further photolysis and therefore oxygen production
• at very high CO2 concentrations some other factor is limiting

Question 9

Outline effect of temperature on photosynthesis.

Answer 9

at low temperatures:

• all of enzymes that catalyse the conversion of CO2 into carbohydrate work slowly
• below 5°C there is little or no photosynthesis in many plants
• at temperature above 30°C the enzyme used to fix carbon dioxide (rubisco) is decreasingly effective, even though it has not been denatured
• temperature is therefore the limiting factor at both low and high temperatures, with the low rate of use of ATP and high energy electrons restricting further photolysis and therefore oxygen production
• at intermediate temperatures, some other factor is limiting

Question 10

Know how to deduce the limiting factor from a graph

Answer 10

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