Limiting Factors

What is a limiting factor?

A factor in short supply that restricts the rate of a process. Increasing it increases the rate until another factor becomes limiting.

Name three factors that affect the rate of photosynthesis.

Light intensity, CO₂ concentration and temperature. Water availability and chlorophyll content can also matter.

Why does a rise in temperature increase the rate of photosynthesis, up to a point?

Enzyme-controlled reactions (e.g. RuBisCO) speed up as kinetic energy increases. Above the optimum, enzymes denature and the rate falls.

Why does the rate plateau as light intensity increases?

Another factor, such as CO₂ concentration or temperature, has become limiting. The enzymes or pigments are also working at maximum capacity.

Why are CO₂ and temperature often limiting in greenhouses?

Light is usually adequate, so growers add CO₂ and control temperature to increase yields.

How can the rate of photosynthesis be measured in an aquatic plant such as Elodea?

Count the oxygen bubbles released per unit time, or collect the gas and measure its volume with a gas syringe or capillary tube. Vary one factor, such as light intensity (by changing the lamp’s distance), and control the others.

Why is a water bath or heat shield used in these experiments?

To keep the temperature constant, since a lamp gives off heat that would otherwise affect the enzyme activity.

Why might measuring bubbles be inaccurate?

Bubble size varies, some oxygen dissolves, and some oxygen is used in respiration. A gas syringe measuring volume is more accurate.

How can DCPIP be used to investigate the light-dependent reaction?

DCPIP is a blue dye that is reduced and turns colourless when it accepts electrons from the light-dependent reaction, in place of NADP. The time taken to decolourise, measured with a colorimeter, indicates the rate.

Why is the chloroplast extract kept in ice-cold, isotonic, buffered solution?

Cold slows enzyme activity that could damage the chloroplasts, isotonic prevents osmotic damage, and a buffer maintains a constant pH.

What controls would you use in a DCPIP experiment?

A tube with chloroplasts kept in the dark (or wrapped in foil) and a tube with no chloroplasts. DCPIP should stay blue in both.