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Respiration
3.6
Living things need energy
Plants and animals need energy to stay alive. Energy is used by:
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all organisms to build up large molecules from smaller ones (for
example, building up proteins from amino acids or making starch
from glucose needs energy)
animals to contract muscles during movement
mammals and birds to keep their body temperature steady in
colder surroundings
plants to make amino acids from sugars and nitrates.
Releasing energy
Energy, in the form of glucose, is made available to cells by the
breakdown of food molecules. The process of releasing energy from
glucose is called respiration.
Question
 The food store in a seed provides
the energy to build large molecules
during growth.
a What sorts of foods would be broken down to give the glucose required for
respiration?
The energy in glucose can be released in a single reaction. When this
happens heat energy is released as glucose burns. This is combustion.
During respiration glucose is broken down gradually by a series of
reactions, each catalysed by a different enzyme. This releases energy in
small amounts so that it can be used by cells.
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 The gradual breakdown of glucose enables cells to use the energy released.
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 Energy is released as heat when glucose
breaks down rapidly during combustion.
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B2
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 Energy can be used by cells when glucose
is broken down gradually during respiration.
Using oxygen
Respiration which uses oxygen is called aerobic respiration. During
aerobic respiration reactions take place which use glucose and oxygen
to release energy. These reactions are summarised by the equation:
glucose + oxygen → carbon dioxide + water + energy
Powering cells
Most of the reactions in aerobic respiration take place inside the
mitochondria found in the cytoplasm of cells. Cells such as muscle
cells, which need lots of energy contain large numbers of mitochondria.
Because mitochondria release the energy needed by cells they are called
the ‘powerhouse’ of the cell.
Question
glucose
b Nerve tissue contains many
mitochondria. Suggest why
this type of tissue needs many
mitochondria.
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P
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T
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N
energy
mitochondrion
Question
 Cells using a lot of energy need a lot of
mitochondria. This shows muscle tissue.
oxygen
Building large molecules
Cells use energy to synthesise large molecules such as proteins. Proteins
are made by joining amino acids together to form long chains. Amino
acids produced by the digestion of proteins are absorbed into the blood.
They are transported around the body by the blood for use by cells to
make the proteins they need. The joining of amino acids is catalysed
by enzymes found inside cells. Protein synthesis needs energy which is
provided by respiration.
amino
acid
enzyme
amino
acid
+ energy
amino acids
bonded
together
Key points
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amino acids attach to enzyme
Animals get the amino acids they need from food. Plants make all their
own amino acids. Nitrate ions in the soil are absorbed by roots. In the
root cells, enzymes convert nitrates and sugars into amino acids using
enzymes and energy from respiration.
nitrates + sugars +
c Food provides the amino acids
needed to make body proteins.
Describe the steps involved in:
(i) getting amino acids into the
body
(ii) using the amino acids to make
proteins.
energy
from respiration→ amino acids
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In aerobic respiration oxygen
and glucose are used and
energy is released.
Most aerobic respiration takes
place inside mitochondria.
Some of the energy released
in respiration is used to make
larger molecules from smaller
ones, to enable muscles
to contract and to keep
temperature steady.
Some of the energy released
in respiration in plants is used
to make amino acids and
proteins.
Enzymes and homeostasis
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Homeostasis
3.7
Staying the same
Even though the temperature of the air around
you might be freezing in winter and very hot in
summer your core body temperature stays at 37 °C.
Keeping conditions steady inside the body is called
homeostasis. For example, your body temperature,
the amount of glucose in your blood and the amount
of water and ions in your body are all kept at a
steady level. These conditions are being adjusted all
the time by your body to prevent any big changes.
Homeostasis helps your cells to work as efficiently
as possible. The chemical reactions in cells are
controlled by enzymes. Enzymes work best in
particular conditions, so keeping the conditions
at a steady level provides enzymes with the best
working environment.
Getting rid of waste
Chemical reactions in body cells produce waste.
This waste includes carbon dioxide and urea.
The diagram shows the organs that remove waste
products from your body.
liver
breaks down excess
amino acids and
forms urea
kidneys
remove urea
as urine
lungs
remove carbon dioxide
produced in respiration
 Although there may be very large temperature changes around
you, your core body temperature stays very steady.
bladder
stores urine before
it is removed
by urinating
Your lungs play a part in homeostasis by keeping
the concentrations of oxygen and carbon dioxide at
the best level for respiration. The rate of respiration
inside muscle cells increases when you exercise.
More waste carbon dioxide will be produced. When
this happens you change your breathing to get rid of
carbon dioxide more quickly. The graph shows the
amount of air taken in and out of the lungs at rest
and during exercise.
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B2
Volume of air breathed
in and out (cm3/minute)
exercise starts
exercise ends
2000
1500
1000
500
0
0
1
2
3
4
5
Time (minutes)
6
7
8
Questions
blood flowing to the
kidneys contains urea
a Calculate the increase in the volume of air breathed in
and out each minute after 3 minutes of exercise.
b The air that is breathed out contains 4% carbon dioxide.
Calculate the volume of carbon dioxide breathed out
each minute (i) at rest (ii) after 3 minutes of exercise.
Your body needs amino acids for building new cells.
You get these amino acids by digesting the protein in
food. Your body is unable to store the excess amino
acids it does not use. Instead, these are broken down
in the liver to make urea. The urea is taken from the
liver to the kidneys where it is converted into urine.
Urine is stored in the bladder before being removed
from the body.
Controlling water and ions
Your body cells need a constant amount of water.
If the amount of water is not controlled then
too much water may move in or out of cells and
damage them. The diagram here shows how water
is lost and gained by your body each day.
Your body has to balance the amount of water it takes
in with the amount it loses. Sweating helps to cool
your body down, so on a hot day more water is lost
as sweat. When you lose water you become thirsty so
you take in more water by drinking more fluids.
When you sweat you lose ions as well as water.
Sweat contains about 0.15 g of salt (sodium chloride)
per 100 cm3 water. Sports drinks help to replace
both the water and the salt.
blood flowing away from
the kidneys contains
very little urea
urea is removed from
the kidneys in urine
urine is stored in
the bladder
a ring of muscle keeps
the bladder closed until
you urinate
 The kidneys convert urea to urine.
water gain
water loss
food
900 cm3
exhaled air
250 cm3
drink
respiration
300 cm3
skin
600 cm3
urine
1400 cm3
faeces
150 cm3
Question
c The body balances water losses with water gains. Use
the information in the diagram to calculate:
(i) the volume of water taken in by drinking
(ii) the percentage of water lost via the skin.
Key points
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 When you sweat you lose both water and salt. This is why
athletes drink sports drinks – to replace both salt and water.
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Waste products include carbon dioxide and urea.
Carbon dioxide is formed in respiration; urea is
formed from excess amino acids.
Carbon dioxide is removed from the lungs.
Urea is produced in the liver and removed as urine
from the kidneys.
Internal conditions, including water and ion
content, are controlled at a steady level.
Enzymes and homeostasis
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