Photosynthesis & Respiration (Cambridge (CIE) AS Environmental Management)

Photosynthesis

• Photosynthesis is a series of chemical reactions that occurs in producers such as plants and algae

• Photosynthesis converts light energy into chemical energy which is then stored in the biomass of producers

• The light energy is used to split strong bonds in water molecules (H₂O), releasing hydrogen and oxygen

• Oxygen is released into the atmosphere as a waste product

• Hydrogen is combined with carbon dioxide to produce glucose 

  • Chemical energy is stored within the bonds in glucose molecules; glucose can therefore function as a fuel for respiration

• The word equation for photosynthesis is:

carbon dioxide + water → glucose + oxygen

• The balanced chemical equation for photosynthesis is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Where does photosynthesis take place?

• The inputs and outputs are:

  • Inputs: sunlight as energy source, carbon dioxide, and water

  • Processes: inside chloroplasts, chlorophyll captures certain visible wavelengths of sunlight energy and stores this as chemical energy

  • Outputs: glucose and oxygen

• Photosynthesis produces the raw material for producing biomass

  • The glucose produced during photosynthesis is used as an energy source for the plant but also as the basic starting material for other organic molecules (e.g. cellulose and starch)

• In ecosystems where sunlight and water is available, the process of photosynthesis enables plants to synthesise organic compounds (glucose and other sugars) from carbon dioxide

• Most of these sugars synthesised by plants are used by the plant as respiratory substrates

Limiting Factors of Photosynthesis

• In order to photosynthesise, a plant needs light, water and carbon dioxide

  • Their availability can affect how much and how quickly photosynthesis occurs (i.e. the rate of photosynthesis)

• Plants do not often have unlimited supplies of their raw materials, so their rate of photosynthesis is limited by whatever factor is the lowest at that time

  • This is known as a limiting factor

• For example, the more carbon dioxide that is present, the faster the photosynthesis reaction can occur

  • This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply
    

• Similarly, the more light a plant receives, the faster the rate of photosynthesis

  • This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply

Respiration

• Respiration is the conversion of organic matter into carbon dioxide and water in all living organisms, releasing energy

• Aerobic respiration is a chemical reaction that converts glucose into cellular energy in the presence of oxygen

  • The process of aerobic respiration in cells breaks down glucose molecules and releases energy, carbon dioxide and water

• The word equation for aerobic respiration is:

glucose + oxygen → carbon dioxide + water

• The balanced chemical equation for aerobic respiration is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O