POPULATION AND ENVIRONMENT
3.4.5
Calculating the efficiency of Photosynthesis
Only a very small amount of the total visible light energy can be converted to
chemical potential energy in substances produced by photosynthesis. The rate at
which a plant is able to produce organic substances as a result of photosynthesis
is called its GROSS PRODUCTIVITY. This measured as an increase in either
Dry mass in units such as g/m2/day or
Chemical potential energy in the organic substances in units such as kJ/m2/day
Some of the substances formed during photosynthesis are not used to form new
cells and tissues but are used in respiration. The difference between gross
productivity and respiration is NET PRODUCTIVITY. This represents the
amount of food available to the next trophic level. It is important in agriculture
as it gives an indication of the potential yield of a crop.
NET PRODUCTIVITY=GROSS PRODUCTIVITY-RESPIRATION
Heather is good for studying efficiency of
photosynthesis because:
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

Efficiency of photosynthesis = chemical potential energy in heather plants
total amount of light energy falling on heather
To find the top line of the equation we need to multiply the chemical potential
energy in 1g of heather by the biomass of heather produced in g/m2/year
1.
2.
3.
POPULATION AND ENVIRONMENT
3.4.5
Results
The amount of energy needed to raise the temperature of 1cm3 of water by 1ºC
is 4.2J. Calculate the amount of energy released by burning 1 g of heather.
Do you think the figure you have calculated is an overestimate or an
underestimate? Explain.
POPULATION AND ENVIRONMENT
3.4.5
1.
2.
a)
b)
c)
Explain how you would use the graph to calculate the mean annual increase in dry
biomass of the heather plants.
What is the evidence from the graph that the heather did not increase in
biomass by the same amount each year?
Suggest why the heather did not increase by the same amount in biomass each
year.
The figure calculated underestimates the mean annual increase in total biomass
of the heather plants. Suggest why.
Use all the figures to calculate the efficiency of photosynthesis in these
heather plants.