6.1.1. Photosynthesis: principles and limiting factors

Photosynthesis is:

• the process by which plants manufacture carbohydrates from raw materials 
• using energy from light

1. Green plants take in CO₂ through leaves by diffusion.
2. Water is absorbed through roots by osmosis and is transported up xylem vessels to leaves.
3. Chloroplasts, containing chlorophyll*, trap light energy. This energy is used to 
• break up water molecules
• bond hydrogen (H+) and CO₂ --> glucose
4. Glucose --> sucrose (storage).
5. O₂ released as waste product, or used by plant for respiration.

*Chlorophyll transfers light energy into chemical energy in molecules, for the synthesis of carbohydrates.

• glucose is formed
• glucose is converted to sucrose for translocation around plants
• sucrose 
• converted to starch for storage
• starch is insoluble and causes no osmotic problems
• converted to cellulose for cell walls
• source of energy

Factors affecting photosynthesis

• supply of raw materials - CO₂ and water
• quantity of sunlight (light intensity) - which provides energy for the reactions
• temperature - affects enzyme activity

Limiting factor: something present in the environment in such short supply that it restricts life processes.

1. Light intensity

In the dark, a plant cannot photosynthesise at all. In dim light, it can photosynthesise slowly. 

As light intensity increases, the rate of photosynthesis increases, until the plant is photosynthesising as fast as it can. 

At this point, even if the light becomes brighter, the plant cannot photosynthesise any faster. Here, light is no longer the limiting factor.

2. CO₂ concentration

The more carbon dioxide a plant is given, the faster it can photosynthesise. 

Once the carbon dioxide concentration reaches a certain level, there is no further increase in the rate of photosynthesis.

* if stomata are closed on a hot day to prevent too much water loss, this also limits the diffusion of CO₂ for the plant to use

3. Temperature

• Some reactions involved in photosynthesis are catalysed by enzymes, which are affected by temperature. 
• Plant enzymes have a much lower optimum temperature than mammalian enzymes.
• Plants photosynthesise faster on warm days.

For the original posts, click below:

• What is photosynthesis?
  
• Limiting factors in photosynthesis

For past papers, click here.