Download Anaerobic Respiration - Deans Community High School

Chapter 4
Chemistry of Respiration
Respiration is the process by which chemical energy is released from food by
oxidation. It occurs in every living cell and involves the regeneration of the high
energy ATP by a complex series of metabolic reactions.
1. Glycolysis
Occurs in the cytoplasm of a living cell and does not require oxygen.
A molecule of 6-carbon glucose is broken down by a series of enzyme-controlled
steps to form 2 molecules of 3-carbon pyruvic acid. This process needs 2 molecules
of ATP to start it but later in the process, 4 molecules of ATP are formed so there is
a net gain of 2 ATP.
Hydrogen released from the respiratory substrate becomes temporarily bound to a
co-enzyme (NAD) which acts as a hydrogen acceptor and carrier to transfer
hydrogen to the cytochrome system.
NAD + H2
NADH2
[If no oxygen is present, anaerobic respiration now occurs and this will be covered
later. Here we consider the aerobic pathway].
2. Kreb’s Cycle
The pyruvic acid diffuses into the matrix of a mitochondrion and is converted into
acetyl CoA , a compound with 2 carbon atoms involved in the respiration reaction.
Hydrogen released by this process is again bound to NAD to form NADH2 and is
carried to the cytochrome system. The carbon lost is given off as carbon dioxide.
Each molecule of this 2-carbon compound reacts with a 4-carbon compound present
in the matrix of the mitochondrion, to form 6-carbon citric acid. This is gradually
converted back to the 4-carbon compound by a cyclic series of enzyme-controlled
reactions.
Decarboxylases are enzymes which control the release of carbon from compounds
to form carbon dioxide which diffuses out of the cell as a waste product.
Look at the diagram on Page 25 Torrance and note where this occurs :
Dehydrogenases are enzymes which control the release of hydrogen.
Notice where this is occurring in the pathway on Page 25 – at every stage except the
formation of citric acid.
Released hydrogen is immediately bound to the hydrogen acceptor NAD and carried
as NADH2 to the cytochrome system (attached to the cristae of the mitochondria).
NAD + H2
NADH2
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Chapter 4
3. Cytochrome System
At 6 points along the above pathways, hydrogen is released and taken to the
cytochrome system by reduced co-enzymes (hydrogen carriers). The cytochrome
system is a chain of hydrogen carriers (electron carriers), each attached to a crista in
the mitochondrion.
Transfer of hydrogen from each NADH2, along the cytochrome system releases
enough energy to produce three molecules of ATP. This process is called oxidative
phosphorylation and generates 36 ATP from 1 molecule of glucose.
Oxygen is the final hydrogen acceptor. Hydrogen and oxygen combine under the
action of the enzyme cytochrome oxidase to form water. If this oxygen is not present
to act as the final acceptor, the hydrogen cannot pass through the system and the
oxidation process cannot take place beyond glycolysis.
In total, Aerobic Respiration produces 38 molecules of ATP


2 from glycolysis
38 from the cytochrome system.
See Aerobic summary diagram p 27 Torrance Fig 4.3
Alternative Respiratory Substrates
Fat
During digestion, fats in the diet are broken down to fatty acids and glycerol.
In the matrix of mitochondrion, fatty acids are converted to the acetyl CoA. The
Kreb’s cycle then continues as before. Fat liberates more than double the energy
released by the same mass of carbohydrate.
Protein
During digestion, proteins in the diet are broken down to amino acids which can also
act as respiratory substrates. The amino acid alanine can be converted to pyruvic
acid allowing it to enter the pathway and release energy. A certain amount of energy
is always derived from excess dietary protein, but tissue protein is only used as a
source of energy during prolonged starvation.
Experiments
Investigating the action of dehydrogenase enzyme in yeast p 27
Measuring rate of respiration p 28
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Chapter 4
Anaerobic Respiration
As oxygen is not available, Kreb’s cycle and the hydrogen transfer system cannot
function in any of the cell’s mitochondria.
Each glucose molecule is broken down to 2 molecules of pyruvic acid during
glycolysis and this partial breakdown of sugar in the absence of oxygen yields only 2
molecules of ATP. Anaerobic respiration is therefore less efficient compared with
aerobic respiration which yields 38 molecules ATP per glucose molecule.
Following glycolysis, an alternative metabolic pathway takes place in the cell’s
cytoplasm. This pathway id different in plants and in animals.
In Plants (Remember yeast is a fungus and considered a plant)
This process could take place in waterlogged roots and yeast cells.
In Animals
e.g. in skeletal muscle cells
During the formation of lactic acid, the body accumulates an oxygen debt. This is
repaid when oxygen becomes available and lactic acid is converted back to pyruvic
acid which then enters the aerobic pathway.
Most living cells thrive in oxygen and respire aerobically. They resort to anaerobic
respiration only to obtain a little energy for survival while oxygen is absent.
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