1 of 24© Boardworks Ltd 2009. 2 of 24© Boardworks Ltd 2009

1 of 24 © Boardworks Ltd 2009



What are limiting factors?

The law of limiting factors states that:

Factors that can limit the rate of photosynthesis include:

At any given moment, the rate of a physiological process is limited by the factor that is at its least favourable value.

This factor is called a limiting factor because it limits the rate at which the process can take place. Changing the levels of other factors will not alter the rate of the process.

CO2 concentration.

temperature

light intensity


How to control limiting factors


Apparatus for the experiments


Are we really measuring photosynthesis?

Oxygen is produced in the light-dependent stage of photosynthesis. However, at all light intensities, plants respire. Respiration uses oxygen.

In other words, the amount of oxygen produced by the plant is excess to that which the plant uses for respiration.

When the rate of photosynthesis is determined by measuring the amount of oxygen formed, what is actually being measured is the net release of oxygen, which represents how much the rate of photosynthesis exceeds the rate of respiration.

Respiration: C6H12O6 + 6O2 6H2O + 6CO2


True or false?



Effect of light intensity


Plotting light intensity

Volume of oxygen can be plotted against light intensity instead of distance. This can be measured with a lightmeter as the lamp is moved closer. A graph like this one is obtained:

What relationship does this graph show between light intensity and oxygen and why could this be? What does this mean in terms of limiting factors?

volume of oxygen

produced in 5 mins (mm3)

light intensity (lux)


Comparing different types of plant

The graph shows the growth of two types of crop at varying light intensities.

light intensity(arbitrary units)

rate

of

ph

oto

syn

thes

is(a

rbit

rary

un

its)

crop B

crop AHow is the growth of the two crops similar?

From this data, suggest which crop would be better suited to grow in a sunny climate, which in a cloudy climate and why.

What other issues might these climates create?



Why is temperature important?

The light-independent reaction of photosynthesis is controlled by enzymes. Temperature affects enzyme reactions.

As temperature increases, collision frequency between reactant particles and between reactant and enzyme increases. This increases the rate of reaction up to the optimum temperature.

Beyond the optimum temperature however, enzymes begin to be denatured. Their tertiary structure breaks down, changing the shape of the active site so that reactant molecules no longer fit.

up to optimum temperature

enzyme denatured at high temperature


Effect of temperature


Fill in the missing words



Atmospheric carbon dioxide

Carbon dioxide is essential for the photosynthesis process. It is used during the light-independent stage to carboxylate ribulose bisphosphate.

The normal concentration for atmospheric carbon dioxide varies between 0.03 % and 0.04 %.

Plants are adapted to survive in varying concentrations of CO2, but the higher the value, the higher the rate of photosynthesis.

Plants may therefore be able to remove some of the CO2

released by human activity from the air.


Effect of carbon dioxide concentration


True or false?



Glossary


What’s the keyword?


Limiting factors and the reactions


Multiple-choice quiz

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1 of 24© Boardworks Ltd 2009. 2 of 24© Boardworks Ltd 2009