Respiration Notes

Cambridge IGCSE Biology: Revision notes

Key facts

  • Respiration releases energy from organic molecules such as glucose, and the energy is stored in ATP.
  • ATP supplies energy for muscle contraction, protein synthesis, active transport, nerve impulses, cell division, growth and keeping a constant body temperature.
  • The rate of respiration in yeast rises with temperature to an optimum (about 37 to 40 °C), then falls as enzymes denature.
  • Endothermic animals (mammals, birds) respire continuously to stay warm, so they need more food than ectothermic animals.

Respiration and ATP

Respiration releases the energy in glucose and stores it in ATP, which cells spend on life processes.

Glucose is an organic molecule used as fuel. Respiration releases its energy, and the energy is stored in ATP, which is used wherever the cell needs energy. ATP is the 'currency' of the cell. Link energy to specific uses rather than saying a process 'needs energy'.

  1. 1

    Glucose

    An organic molecule used as fuel

  2. 2

    Respiration

    Releases the energy from glucose

  3. 3

    ATP

    Stores and releases energy for cells

  4. 4

    Life processes

    Muscle contraction, protein synthesis, active transport and more

Respiration and energy

Where is the energy released in respiration stored?

Uses of energy

Cells use ATP for movement, building molecules, moving substances, sending nerve impulses and dividing.

Muscle contraction, protein synthesis, active transport, nerve impulses and cell division all depend on ATP from respiration. Active transport moves substances against the concentration gradient and so needs ATP, unlike passive diffusion.

What happens

Muscle contraction:
Filaments slide to shorten the muscle
Protein synthesis:
Amino acids are joined on ribosomes
Active transport:
Substances move against the gradient
Nerve impulses:
Ions are pumped across membranes
Cell division:
DNA is copied and chromosomes move

Energy needed for

Muscle contraction:
Powering the myosin heads
Protein synthesis:
Linking the amino acids
Active transport:
Carrier proteins
Nerve impulses:
The sodium-potassium pump
Cell division:
Each stage of mitosis

Which statement describes active transport?

Temperature and yeast

The rate of respiration rises with temperature to an optimum, then falls as enzymes denature.

Below the optimum, enzyme molecules have more kinetic energy and collide more often, so the rate rises. In this example the rate doubles with each 10 °C rise. Above the optimum, enzymes denature and the rate falls sharply even though glucose is still available. Do not say the rate keeps increasing.

01020304020°C30°C40°CTemperature (°C)Bubbles per minute
Example only: yeast respiration rate at different temperatures

Why does the rate of yeast respiration fall above the optimum temperature?

Energy and body temperature

Endothermic animals use respiration to keep their body temperature constant, so they need more food.

In mammals and birds, heat from respiration keeps the body at a constant temperature, whatever the environment. Reptiles and amphibians take on the temperature of their surroundings. For animals of similar size the endothermic animal uses far more energy and so needs more food.

Endothermic (mammals, birds)

Body temperature:
Kept constant by respiration
Energy used for temperature:
High
Food needed (similar size):
More

Ectothermic (reptiles, amphibians)

Body temperature:
Matches the environment
Energy used for temperature:
None
Food needed (similar size):
Less

Why do endothermic animals need more food than ectothermic animals of similar size?

Try an exam question

Explain why a mammal needs more food than a lizard of the same size.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.