What is respiration?
All living organisms need a constant supply of energy.
Respiration is the process of releasing energy through
the breakdown of nutrients such as glucose.
Respiration can be aerobic (uses oxygen) or anaerobic
(does not use oxygen). Aerobic respiration takes place
continuously in both plants and animals.
iGCSE SCIENCE: respiration
Where does
respiration occur?
Respiration is a series of chemical reactions controlled
by enzymes. Most of the reactions for aerobic
respiration occur inside mitochondria.
Mitochondria are structures inside plant and animal
cells. They contain highly-folded inner membranes
which are embedded with enzymes. These enzymes
catalyse the reaction that releases energy from
glucose.
iGCSE SCIENCE: respiration
How is energy from
respiration used?
The energy released during respiration is used to carry
out life processes.
process
building larger molecules from smaller ones active transport
enabling muscle contraction
maintaining body temperature
organism
plants and animals
plants and animals
animals
mammals and birds
iGCSE SCIENCE: respiration
Aerobic respiration
Aerobic respiration uses oxygen to
break down glucose.
Aerobic respiration can be described as the process
of releasing energy through the oxidation of glucose
molecules.
Aerobic respiration releases lots of energy from each
glucose molecule by breaking it down completely into
carbon dioxide and water.
iGCSE SCIENCE: respiration
Reactants and products
of aerobic respiration
Glucose and oxygen are the reactants of aerobic
respiration. Carbon dioxide and water are the products.
Energy is released during the reaction.
reactants
products
glucose
carbon dioxide
oxygen
water
Plants obtain glucose from photosynthesis. Animals
obtain glucose from food. Oxygen is obtained through
gas exchange surfaces such as leaves and lungs.
iGCSE SCIENCE: respiration
C6H12O6 + 6O2 >
During aerobic respiration, glucose and oxygen are
converted into carbon dioxide and water. This releases
the chemical energy stored in glucose molecules.
glucose + oxygen
> carbon dioxide + water (+ energy)
The energy is transferred to small molecules called
ATP (adenosine triphosphate), which cells use as an
immediate source of energy.
iGCSE SCIENCE: respiration
Anaerobic respiration
Symbol equation for
aerobic respiration
Word equation for
aerobic respiration
Anaerobic respiration takes place
without oxygen.
6CO2 + 6H2O
One molecule of glucose (C6H12O6) and six molecules
of oxygen (O2) react to form six molecules of carbon
dioxide (CO2) and six molecules of water (H2O).
iGCSE SCIENCE: respiration
Anaerobic respiration is the incomplete breakdown of
glucose. This process releases much less energy per
glucose molecule than aerobic respiration and produces
lactic acid.
Anaerobic respiration occurs when cells have
insufficient oxygen for aerobic respiration to occur, for
example, during strenuous exercise.
iGCSE SCIENCE: respiration
Word equation for
anaerobic respiration
During anaerobic respiration in human cells, glucose
is partially broken down into lactic acid.
glucose >
lactic acid (+ energy)
Anaerobic respiration releases much less energy per
glucose molecule than aerobic respiration.
iGCSE SCIENCE: respiration
Muscle fatigue
Lactic acid builds up in muscle cells and prevents the
muscles from contracting efficiently. The build-up of
lactic acid can cause fatigue, pain and cramping.
Lactic acid is removed from the muscles by the blood.
iGCSE SCIENCE: respiration
Oxygen debt
The lactic acid produced during anaerobic respiration
needs to be broken down into carbon dioxide and
water. Oxygen is needed for this process to occur.
The oxygen needed to break down the lactic acid is
called the oxygen debt.
iGCSE SCIENCE: respiration
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iGCSE SCIENCE: respiration