Anaerobic respiration in animals
and oxygen debt
When doing vigorous exercise your
body cannot supply enough oxygen to your muscles for 100% aerobic
If there is a lack of oxygen
('anaerobic' means 'without oxygen')
you cannot oxidise the glucose sugar completely, as in the case of aerobic
respiration - the greatly simplified equation for the incomplete breakdown of
glucose ===> lactic acid
(structure of lactic acid
is CH3CH(OH)COOH, a
carboxylic acid with an alcohol group)
This anaerobic reaction only partially
breaks down the glucose to lactic acid in animals and some
(i) The waste product
is lactic acid, not carbon dioxide and water, as
in aerobic respiration.
(ii) The products are
different in plants and some microorganisms (see next
This is not as efficient as aerobic
respiration and considerably less ATP is formed,
reducing the potential energy
You only make 2 molecules
of ATP per molecule of glucose (much less than 1/10th of ATP
from aerobic respiration).
BUT, it does enable cells
to keep functioning if there is a lack of oxygen and
enough energy can be released to keep a cell alive!
A much less efficient process
of transferring energy from the glucose chemical energy store.
Anaerobic respiration takes
place in the cytoplasm plant and animal cells and some
(a) In human cells, when
you do vigorous exercise, you body can't supply enough
oxygen, so the cells use anaerobic respiration too.
(b) If plant root cells
are growing in waterlogged ground, there is little oxygen
available, so they must respire aerobically.
(c) If bacteria cells get
under your skin where there is little oxygen, they can still
survive by using anaerobic respiration.
In animals, if the physical activity is
intense and prolonged you get the 'cramps' pains due to the build up of lactic acid,
which can be painful as anaerobic respiration kicks in due to lack of oxygen.
With anaerobic respiration you do get the build up of lactic acid in the muscles
because it is biochemically more difficult to oxidise and release energy.
As anaerobic respiration produces a
build up of lactic acid in the muscles, you also get an
oxygen debt in the
muscles which can be painful e.g. you suffer from 'cramps'.
However, anaerobic respiration
has the advantage of enabling the body to keep going for a limited time,
even if you are a bit short of oxygen!
This can be important in an
emergency situation when you need to use your muscles more than
understand anaerobic respiration results in an oxygen debt that has to be repaid in
order to oxidise lactic acid to carbon dioxide and water.
Know and understand
that if muscles are subjected to
long periods of vigorous activity they become fatigued, ie they stop
The more vigorous the
exercise, the more energy you need and you need to increase your
rate of respiration.
You need to breathe at a
faster rate and take in larger volumes of air for the oxygen
needed to sustain this increase in rate of respiration.
Your heart rate increases
to get the oxygenated blood to your muscles and
simultaneously remove the carbon dioxide efficiently too.
When your exercise is
really vigorous there is not enough oxygen for
aerobic respiration so your body responds by using
anaerobic respiration too.
respiration is not as energy efficient in
transferring energy as aerobic respiration and if the
exercise is prolonged you become fatigued.
Know that one cause of muscle fatigue is the
lactic acid in the muscles from anaerobic respiration even though the
blood flowing through the muscles removes the lactic acid, oxygen is used up to oxidise
lactic acid to carbon dioxide and water.
You get this extra oxygen for the aerobic
respiration of the lactic acid from deeper faster breathing.
(3B) More on oxygen debt an the build-up
of lactic acid
During vigorous exercise the heart, lungs and limb
muscles begin struggle to keep up with what you want your body wants to do
you can keep your muscles going longer using anaerobic respiration, at least up to a point
of total fatigue - like just about staggering over the line at the end of a
Anaerobic respiration: glucose ===> lactic acid
Unfortunately, when your body
starts to use anaerobic respiration, you are building up lactic
acid an 'oxygen
The oxygen debt is the
quantity of oxygen your body needs to react with the build
up of lactic acid in the cells and remove it by oxidation to
carbon dioxide and water (as happens with aerobic
respiration of glucose) and replace the body's reserve of
oxygen in the bloodstream and cells.
This results in muscle
fatigue and stop contracting efficiently.
This means your body has
to be repaid with the oxygen that the muscles didn't get for
complete aerobic respiration - your lungs, heart rate and
bloodstream couldn't keep up with demands of aerobic
The body has a low
tolerance to lactic acid, which must be removed.
The lactic acid is
taken to the liver by blood and either completely
oxidised to carbon dioxide and water or converted back
into glucose and then glycogen - but this takes time and
This means, even when you stop
doing a vigorous exercise, you have to continue to breathe quite deeply to repay
this oxygen debt, transfer oxygen to the cells and oxidise the lactic acid
in them to the harmless waste products
of carbon dioxide and water.
Repaying the oxygen
debt may take hours to complete and even days after
running a marathon!
As long as your body detects
higher than normal levels of carbon dioxide or lactic acid your breathing
rate and pulse rate will stay higher than normal until their levels are
reduced to normal, i.e. when all the excess lactic acid has been oxidised to
carbon dioxide and water.
Note: Your body does
have another way of reducing high levels of lactic acid and
The blood flow through your muscles transports
lactic acid to the liver where it is converted back to glucose -
chemically the opposite reaction to anaerobic respiration.
Regular aerobic exercise can
reduce your risk from some non-communicable diseases.
Keeping healthy - diet and exercise
Keeping healthy - non-communicable diseases
- risk factors for e.g. cancers
Keywords, phrases and learning objectives for this section on respiration
Be able to describe why the switch to anaerobic respiration
(from aerobic respiration) in animals and the build up of an oxygen debt of lactic acid
leading to cramps and muscle fatigue.
Know that in animal aerobic respiration, lactic acid
is formed from glucose.
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