Limiting Factors of Photosynthesis (A-level Biology)

Limiting Factors of Photosynthesis

Factors Affecting Photosynthesis

A limiting factor is something which determines the efficiency and rate of the reaction due to their availability. In this case, the rate of photosynthesis determines how much triode phosphate, and eventually glucose and starch, a plant can produce.

The table below lists certain factors photosynthesis requires to occur.

A lack of availability of any of these factors can cause problems for a plant, slowing down the speed of photosynthesis, preventing plants from producing enough food stores. This will lead to a fall in plant growth, eventually dying as it is unable to meet its energetic needs.

Below we will look into some of the above factors which limit photosynthesis in more detail:

1. Carbon Dioxide

This is a key factor affecting photosynthesis. 

• Low CO₂ affects the Calvin Cycle. If CO₂ levels are low, rubisco cannot convert RuBP to GP in step one of the Calvin Cycle. This leads to accumulation of RuBP and an overall slowing of the Calvin Cycle, which results in a fall in the production of TP/GALP.

• High CO₂ can cause stomata to close. However, if CO₂ rises too high, then some stomata begin to close, which can lead to less CO₂ uptake by the plant.

• The ideal CO₂ concentration is 0.4%. The atmospheric CO₂ concentration is 0.04%. Increasing this by ten times to 0.4% will increase the rate of photosynthesis.

2. Water Supply

This is another important factor affecting photosynthesis

• Water is needed for photolysis in the light-dependent stage. Low levels of water prevent efficient photolysis occurring during the light dependent reactions. This will disrupt production of ATP and reduced NADP, both of which are needed for the Calvin Cycle.

• Water vapour and soil are sources of water. Plants can obtain water through their stomata from water vapour in the air, and also by absorbing water from soil via their roots.

3. Light Intensity

• Light is needed for the light-dependent stage. Light energy is needed to excite the electrons and for photolysis of water in the light-dependent stage. Without light, there would be little ATP and reduced NADP produced for the Calvin Cycle.

• Certain wavelengths of light are absorbed. Light is absorbed by the photosynthetic pigments (e.g. chlorophyll a), as we learnt before. Green light is not absorbed but is instead reflected.

4. Temperature

• Low temperatures reduce the rate of enzyme action. Many of the reactions in both the light dependent and independent reactions are enzyme-driven processes. Enzymes require an optimal temperature to function. In low temperatures, there is not enough kinetic energy in the chloroplast to allow for effective collisions between enzymes and their substrates. This leads to a fall in the rate of photosynthesis and TP production.

• Very high temperatures also reduce the rate of enzyme action. At high temperatures, enzymes can become denatured. Also, stomata tend to close at high temperatures to avoid water loss, which means less CO₂ is available for photosynthesis.

Limitating Factors of Photosynthesis: Light Intensity and CO₂ Concentration

We can also look at a combination of factors together:

• In the above graph, temperature is constant, so the effect of light intensity and carbon dioxide concentration on photosynthesis is shown.

• As you increase light intensity, rate increases until we reach a plateau, after which light is no longer the limiting factor.

• As you increase CO₂ (shift from 0.05% to 0.5%), the rate increases. Also, the light intensity effect plateaus at a higher level, so light intensity is a limiting factor for longer.

Agricultural Factors

Greenhouses can optimise farming conditions. Greenhouses have artificially made conditions in order to farm more efficiently.

Greenhouses control temperature:

• • • • Glass – greenhouses are made of glass in order to trap the sun’s heat, via the greenhouse effect, in the summer to increase the speed of photosynthesis.

• • • • Blinds – they have blinds that are used if it is too hot to stop the denaturing of enzymes.

• • • • Heaters – during the cold months, farmer use heaters inside the greenhouse in order to artificially provide heat to keep plants photosynthesising year round.

• Greenhouses control CO₂ concentration. Greenhouses can burn paraffin heaters to give off carbon dioxide for photosynthesis, whilst  heating the greenhouse.

• Greenhouses control light intensity. Light can be artificially provided (i.e. lamps) to provide the energy for photosynthesis. Greenhouses tend to have glass to allow maximum sunlight through.

• Conditions must be carefully monitored. This is an intensive process, so it must be done as efficiently as possible to provide the greatest amount of produce for the money put in.

In conclusion, we have looked at photosynthesis and factors affecting photosynthesis in great detail. 

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