Cellular respiration is the controlled release of energy from organic compounds in cells to form ATP. During cell respiration, glucose is broken down by glycolysis into pyruvate, with a small yield of ATP. Anaerobic respiration involves converting pyruvate into either lactate or ethanol and carbon dioxide in the cytoplasm, with no additional ATP produced. Aerobic respiration breaks down pyruvate in mitochondria into carbon dioxide and water, yielding a large amount of ATP.

1. 3.7 Cell Respiration
Topic 3: The Chemistry of Life

2. Cellular Respiration
3.7.1 Define cell respiration - the controlled release of energy from
organic compounds in cells to form ATP.
3.7.2 State that, in cell respiration, glucose in the cytoplasm is
broken down by glycolysis into pyruvate, with a small yield of
ATP.

3. Cellular Respiration
3.7.3 Explain that, during anaerobic cell respiration, pyruvate can be
converted in the cytoplasm into lactate, or ethanol and carbon
dioxide, with no further yield of ATP.
Mention that ethanol and carbon dioxide are produced in yeast,
whereas lactate is produced in humans.
3.7.4 Explain that, during aerobic cell respiration, pyruvate can be
broken down in the mitochondrion into carbon dioxide and water
with a large yield of ATP.

4. Cellular Respiration
Most people use the term respiration in reference to breathing.
Biologists use the term in a different way.
To biologists, respiration is:
“The controlled release of energy in the form of ATP
from organic compounds in cells”
To avoid confusion, this process is sometimes called cellular
respiration.
Cellular respiration occurs in every living cell.
It is the only way a cell can obtain energy in a usable form.

5. ATP
ATP stands for adenosine triphosphate.
ATP is the form of chemical energy that is used to fuel energy
consuming biological reactions.
All living things need energy to grow, reproduce, move, repair,
etc.
They obtain this energy from food.
Food can be made in the organism (photosynthesis) or taken in
by eating (heterotrophs).
During cellular respiration the energy in food molecules is
transferred to molecules of ATP.

6. Types of Cellular Respiration
There are TWO main types of Cellular Respiration:
– Anaerobic Respiration.
• Respiration without oxygen
– Aerobic Respiration.
• Respiration with oxygen

7. Glycolysis
Glucose is often the organic compound used in cell respiration.
Glycolysis is the first stage of both anaerobic and aerobic
respiration.
It takes place in the cytoplasm of the cell.
Glucose molecules are broken down into pyruvate molecules
with the release of a small amount of energy (2 ATP).
Actually 4 ATP are produced, but 2 are used in the process giving
a net yield of 2 ATP
Glucose 2 pyruvate + 2 ATP

8. Anaerobic Respiration
When oxygen is not available, anaerobic respiration may occur.
In the absence of oxygen, glucose can only be partly broken
down, the molecules produced will be larger and there will be
less energy available to the cell.
The name given to these anaerobic processes is Fermentation.

9. Anaerobic Respiration
There are generally two types of fermentation:
– Alcohol fermentation:
• Which occurs in plants and yeasts.
– Lactate fermentation:
• Which occurs in animals.
In both of these reactions, no further yield of ATP is produced.
pyruvate
pyruvate
ethanol + carbon dioxide
lactate (lactic acid)

10. Alcohol Fermentation
In a number of organisms, (particularly some plants, yeast and
bacteria) glucose can be broken down to ethanol and carbon
dioxide.
The reaction can be summarised as:
This reaction is the basis of the brewing and baking industries.
The net production of energy is 2 ATP (from the glycolysis stage).
C6H12O6 2C2H5OH + 2CO2 + energy
glucose
carbon
dioxide
energy++ethanol

11. Lactate Fermentation
In animals, respiration may also occur in the absence of oxygen.
This is much less efficient and the lactate (lactic acid) that is
produced is poisonous to the cells.
The reaction can be summarised as:
Lactate can build up in muscles after exercise and cause soreness.
The net production of energy is 2 ATP (from the glycolysis stage).
C6H12O6 + energy2C3H6O3
glucose energy+lactate

12. Aerobic Respiration
Aerobic respiration occurs when oxygen is present.
The pyruvate from glycolysis moves to the Mitochondria.
Inside the mitochondria the pyruvate is broken down into carbon
dioxide and water.
A large amount of ATP is produced (about 36 ATP).
pyruvate carbon dioxide + water + energy

13. Aerobic Respiration
Aerobic respiration can be summarised as:
C6H12O6 + 6O2 6CO2 + 6H20 + energy
glucose + oxygen
carbon
dioxide
+ water + energy

14. Comparing Aerobic and Fermentation
The first stage of fermentation is the same as for aerobic
respiration – glycolysis, which produces 2 pyruvate molecules.
However, in fermentation the pyruvate is converted to ethanol
and CO2 (plants and yeast) or lactate (animals).
This final process occurs in the cytoplasm (as compared to in
the mitochondria in aerobic respiration).
Fermentation does not produce as much ATP as aerobic
respiration because the products are bigger.
Aerobic respiration is 17 times more effective than fermentation
at producing ATP molecules.

15. Comparing Aerobic and Fermentation
Aerobic respiration Anaerobic respiration
Site in
eukaryote
cytoplasm/mitochondria cytoplasm
Reactants glucose and oxygen glucose
Products CO2 and H2O
ethanol and CO2 or
lactate
ATPs per
glucose
36 2

16. IBO guide:
3.7.1 Define cell respiration - the controlled release of energyfrom
organic compounds in cells to form ATP.
3.7.2 State that, in cell respiration, glucose in the cytoplasm is
broken down by glycolysis into pyruvate, with a small yield of
ATP.

17. IBO guide:
3.7.3 Explain that, during anaerobic cell respiration, pyruvate can be
converted in the cytoplasm into lactate, or ethanol and carbon
dioxide, with no further yield of ATP.
Mention that ethanol and carbon dioxide are produced in yeast,
whereas lactate is produced in humans.
3.7.4 Explain that, during aerobic cell respiration, pyruvate can be
broken down in the mitochondrion into carbon dioxide and water
with a large yield of ATP.