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Photosynthesis and respiration

Photosynthesis

Photosynthesis is the process whereby plants, in the presence of light, manufacture their food and build stores of energy.

This phenomenon takes place in plant cells that contain chlorophyll, a pigment that gives plants their green colour. Most of the chlorophyll in a plant is found in its leaves.

Chlorophyll captures light energy and uses it to produce carbohydrates (sugars) from carbon dioxide (CO2) and water. This reaction also produces oxygen, which is released into the atmosphere.

The reaction can be described as follows:

Photosynthesis equation

The carbohydrates produced by photosynthesis are used by the plant in several ways:

  • as a source of immediate energy (through respiration) in order to function, grow, reproduce, absorb nutrients, etc.;
  • for energy reserves. For example, before winter, the plant stores sugars in the form of starch; these reserves enable the plant to survive the winter and start functioning again quickly in spring;
  • for the formation of plant tissues. Sugars can be transformed (proteins, lipids or complex sugars) to produce leaves, wood, flowers, fruits, roots, etc.

Photosynthesis is influenced by several factors, including:

  • temperature: the optimum temperature is between 20 and 35°C. Photosynthesis ceases at temperatures below 0°C because of the slowing of the plant's physiology (e.g. leaves drop off and water absorption is reduced);
  • the concentration of carbon dioxide (CO2) in the air. An atmosphere rich in CO2 promotes photosynthesis;
  • intense light : the more light there is, the more effective the chlorophyll will be. Photosynthesis will be less intense under a cloud cover than in bright sunshine;
  • leaf surface area exposed to light. The lower leaves of hardwood trees often grow larger and thinner than those higher up in order to compensate for the lack of light. The leaves of conifers are very small in order to achieve better frost resistance, but they are also very numerous;
  • the availability of water in the soil: the plant must transpire in order to absorb carbon dioxide. If water becomes scarce, the plant reduces transpiration and slows photosynthesis.
Photosynthesis and Respiration illustration

Respiration

Respiration is the opposite of photosynthesis. It consumes oxygen (oxidation of sugars) and releases carbon dioxide (CO2) and water.

Respiration equation

Respiration is common to all living organisms, whether plants, animals or micro-organisms. Respiration enables both autotrophs and heterotrophs to obtain energy from carbohydrates. This energy is necessary for growth, movement and all vital functions.

The following are a number of factors that affect respiration in plants:

  • temperature: respiration falls to a minimum at temperatures below 0°C, and is at a maximum at 45-50°C;
  • the plant's developmental stage : respiration increases during the flowering stage of trees;
  • the type of plant: respiration is less significant in woody than herbaceous plants.

Plants respire both during the day and at night. However, photosynthesis takes place during the day only, in the presence of light.