Photosynthetic Organisms as Producers (OCR Gateway GCSE Biology: Combined Science)

• Plants and algae are autotrophs – this means that they can make their own food
  • Autotrophic organisms make complex molecules (that can be used as an energy store) from simple molecules (carbon dioxide and water) 
• Plants and algae make glucose using light energy, water and carbon dioxide in the process of photosynthesis
  • The glucose can be used as a source of energy in cellular respiration or used to make more complex molecules that are used to help the plant or algae grow
  • This makes-up the organism's biomass (mass of living material)
• This is why autotrophs such as plants are called producers in food chains
  • The energy stored in the biomass makes its way through the food chain
  • Photosynthetic organisms help support nearly all life on earth

• Photosynthesis can be defined as the process by which plants manufacture carbohydrates from raw materials using energy from light
• The reactants of photosynthesis are carbon dioxide and water
• The products of photosynthesis are glucose and oxygen
• These reactions can be summarised into a simple equation

Word equation for photosynthesis

Where do the reactants come from and where do the products go?

• This equation can also be shown as a balanced chemical equation
  • Six carbon dioxide molecules combine with six water molecules to make one glucose molecule and six oxygen molecules

The balanced chemical equation for photosynthesis

• Photosynthesis takes place in specialised sub-cellular structures called chloroplasts
  • The chloroplasts contain chlorophyll to help absorb as much light energy as possible
• The leaves of most plants contain specialised mesophyll cells which are packed with chloroplasts
  • This helps maximise the amount of photosynthesis
• Photosynthesis is an endothermic reaction in which energy is transferred from the environment to the chloroplasts by light

Photosynthesis is a two-stage process

• Photosynthesis occurs in two main stages
  • Energy transferred from light is used to split water into oxygen gas and hydrogen ions
  • Carbon dioxide gas taken from the atmosphere then combines with the hydrogen ions to make glucose