CIE A Level Biology (9700) exams from 2022

Revision Notes

12.1.1 Energy

The Need for Energy

  • Living organisms are composed of cells, and within each cell, many activities and processes are constantly being carried out to maintain life
  • Work in a living organism requires energy and usable carbon compounds

Essential work within organisms table

Essential Work Within Organisms Table, downloadable AS & A Level Biology revision notes

The source of energy & materials

  • For nearly all organisms the sun is the primary source of energy
  • The reactions of photosynthesis store energy in organic molecules
    • Light energy from the sun is transformed into chemical potential energy in the synthesis of carbohydrates
    • The carbohydrates formed are then used in the synthesis of ATP (from their breakdown) or are combined and modified to form all the usable organic molecules that are essential for all metabolic processes within the plant
    • Photosynthesis is carried out by the first organism in a food chain, such as plants and some other small organisms
  • Respiration in all living cells releases energy from the breakdown of organic molecules
  • Respiration involves the transfer of chemical potential energy from nutrient molecules (such as carbohydrates, fats and proteins) into a usable energy form (through the synthesis of ATP) that can be used for work within an organism

Glucose equations

glucose + oxygen →  carbon dioxide + water + energy

C6H1206 + 6 O2 →  6 CO2 + 6 H20 + 2870kJ

  • Autotrophs are organisms that are able to synthesise their own usable carbon compounds from carbon dioxide in the atmosphere through photosynthesis
  • Heterotrophs don’t have this ability. They require a supply of pre-made usable carbon compounds which they get from their food

Exam Tip

According to the laws of thermodynamics, energy cannot be created or destroyed; it is transformed from one form into another. Be careful not to say that energy is “created” when talking about photosynthesis and respiration.

You may also be expected to name examples of energy-requiring reactions in organisms:

  • The sodium-potassium pump that is found on many cell membranes is a great example of active transport. Three sodium ions are taken out of the cell while two potassium ions are taken in, both against their respective concentration gradients
  • The movement and contraction of muscles also requires substantial amounts of energy

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Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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