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Surface area to volume ratio and the need for a specialised exchange surface in organisms
All animals need to maintain a supply of the materials they need (for example nutrients and oxygen, and for the
removal of waste products of metabolism). In single-celled organisms (unicellular) these needs can be met purely by
diffusion. This is because the distances are short, mere nanometres or millimetres in length. They have a relatively large
surface area in comparison with their size, so exchange can happen across their cell surface.
However, larger multicellular organisms cannot rely upon diffusion. The distances that diffusion would have to take
place over (the diffusion paths) are too great and the surface area is small in comparison to larger animals’ sizes.
Therefore, they need specialist exchange surfaces and a transport system to deliver materials to and from exchange
surfaces and satisfy the demands created by high activity levels.
The size of an animal is what affects its surfacearea-to-volume ratio (SA:Vol). This is because as
the size of an organism increases, its volume will
increase at a much faster rate (cubic) than its
surface area (squared). This means that as the size
of the organism increases, the SA:Vol ratio
decreases more and more.
1cm
2cm
2
SA: 6cm
3
Vol: 1cm
SA:Vol = 6
3cm
2
SA: 24cm
3
Vol: 8cm
SA:Vol = 3
2
SA: 54cm
3
Vol: 27cm
SA:Vol = 2
The three cubes to the left are increasing in size
from a length of 1cm, to 2cm, to 3cm. As you can
see, the surface area does not increase as rapidly
as volume, therefore the SA:Vol ratio quickly
drops from 6 to 3 to 2, and would continue to.
EXCHANGE SURFACES
All good exchange surfaces have certain features in common:
 a large surface area to provide more space for molecules to pass through (this is often achieved by folding the
walls and membranes of the surface for exchange)
 a thin barrier to the surface to reduce the diffusion path
 a fresh supply of molecules on one side of the barrier to keep to the concentration high
 removal of required molecules on the other side to keep the concentration low
The latter three points above help to maintain a steep concentration gradient. Certain exchange surfaces also use an
active transport mechanism to increase the rate of exchange or to allow exchange where not otherwise possible.
Examples of specialised exchange surfaces include the root hairs of plants, where long hair-like extensions increase the
surface area, and they have specialised features to help with the absorption of water and minerals. However, not all
exchange surfaces are present at the surface of a large organism, many are found in the organs where substances are
removed from the transport system, for example, the alveoli in the lungs.
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The lungs as an organ of exchange of materials
The mammalian respiratory system consists of the airways and lung tissue. Air
is most often breathed through the nose because then it is warmed,
moistened and filtered.
Air travels through the breathing tubes: trachea, bronchi and bronchioles,
and ends up in the air sacs or alveoli. The alveoli are a specially-adapted area
2
for exchange, with over 700 million in a human making 70m in total. They are
made of squamous epithelia and are vascularised (richly supplied with blood).
The lungs are in the chest cavity and are protected by the ribs which surround them. Each lung is covered by pleural
membranes which secrete a lubricating fluid, allowing the lungs to inflate and deflate without rubbing against the walls
of the ribcage. There are intercostal muscles in between the ribs. The external intercostal muscles can contract to raise
the ribcage, and the internal muscles can contract to lower the ribcage. There is a muscular diaphragm separating the
lungs from the abdomen, which is usually domed shape, but it flattens when it contracts.
Essentially only different in size, the trachea and the bronchi are structurally similar. The inner surface is covered in
ciliated epithelia which have a rhythmic wave-like movement. Goblet cells are also present which secrete mucus
containing glycoproteins and lysozyme (the enzyme in lysosome which causes lysis – the breakdown of bacteria).
Smooth Muscle found around many internal
organs, these muscles are
under involuntary control
There are rings of cartilage, which is strong and flexible helping to hold the airways
open during inhalation. In the trachea, the cartilage bands are incomplete (Cshaped). This allows food to pass down the oesophagus which runs behind the
trachea. Inside the cartilage is loose tissue which is made of smooth muscle.
Gases pass both ways through the thin walls of the alveoli. Oxygen passes from
the air in the alveoli to the blood in the capillaries. Carbon dioxide passes from
the blood to the air in the alveoli.
For diffusion to be rapid, a steep diffusion gradient is needed, as was one of the
criteria for a good exchange surface, outlined in 2.1 Special Surfaces for
Exchange. This means a high concentration of molecules is needed on the supply
side, and a low concentration on the demand side. This is achieved via the
action of the blood transport system and breathing movements (ventilation).
The blood brings carbon dioxide from the tissues to the lungs. This ensures that
the concentration of carbon dioxide in the blood is higher than that in the air of
the alveoli. It also carries oxygen away from the lungs. This ensures that the
concentration of oxygen in the blood is kept lower than the concentration in the
air inside the alveoli. The heart pumps the blood along the pulmonary artery to
the lungs. In the lungs, the artery divides up to form finer vessels. These eventually carry blood into tiny capillaries that
are only just wide enough for a red blood cell to squeeze through. These capillaries lie over the surface of the alveoli, as
shown in the diagram.
The breathing movements of the lungs ventilate them. They replace the used air with fresh air. This brings more oxygen
into the lungs and ensures that the concentration of oxygen in the air of the alveolus remains higher than the
concentration in the blood. Ventilation also removes air containing carbon dioxide from the alveoli. This ensures that
the concentration of carbon dioxide in the alveoli remains lower than that in the blood.
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This constant supply of gas to one side of the exchange
surface and its removal from the other side ensures that
diffusion, and therefore exchange, can continue.
When a human breathes in (inspiration)…
 diaphragm contracts to become flattened and pushes
digestive organs down
 external intercostal muscles contract to raise ribs
 volume of chest cavity increases
 air moves into the lungs
When a human breathes out (expiration)…
 diaphragm relaxes and is pushed up by displaced
organs underneath
 external intercostal muscles relax and ribs fall
 volume of chest cavity decreases
 air moves out of the lungs
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The different elements of lung volume involved in respiration
When you rest, air moves in and out of your lungs about twelve times a minute. Each breath refreshes some of the air in
your lung and removes some of the carbon dioxide generated by your body. When you exercise, or are frightened, you
breathe more deeply, and more oxygen-rich air enters the blood and more carbon dioxide is removed with each breath.
A spirometer consists of a chamber filled with oxygen that floats on a tank of water. A person breathes from a
disposable mouthpiece attached to a tube connected to the chamber of oxygen. Breathing in takes oxygen away from
the chamber, which then sinks down. Breathing out then pushes air into the chamber causing it to float.
ELEMENTS OF LUNG VOLUME
6
Expiratory
reserve
volume
5
Tidal volume
4
Volume/dm
3
Vital
capacity
3
Inspiratory
reserve
volume
Total
lung
capacity
2
1
Residual
volume
0
Time
3
Tidal volume is the volume of air moved in and out of the lungs with each breath at rest. It is usually around 0.5dm and
provides the body with enough oxygen for resting needs and removes enough carbon dioxide to maintain safe levels
Vital capacity is the largest volume of air that can be moved into and out of the lungs in any one breath. It is usually
3
around 5dm but varies depending on gender, age, size, height and exercise habits
Residual volume is the volume of air that always remains in the lungs, even after the biggest possible exhalation. It is
3
usually around 1.5dm
Inspiratory reserve volume is how much more air can be breathed in (inspired) over and above the normal tidal volume
when you take a big breath. This reserve is called upon when you exercise
Expiratory reserve volume is how much more air can be breathed out (expired) above the tidal volume. This is smaller
than the Inspiratory reserve volume
Dead space is the amount of air in the bronchioles, bronchi and trachea. There is no gas exchange between this air and
the blood
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BREATHING RATE
1
2
3
Volume/dm
3
4
5
6
This is easy to calculate using a spirometer trace, like the one here or on
the pervious page. The trace will consist of a series of peaks and troughs.
‘Peak-to-peak’ or ‘trough-to-trough’ is one breath, so all you need to do is
simply count the number of breaths per minute (or per 30 seconds and
double the answer). You can also calculate pulmonary ventilation. This is
the total air breathed per minute:
pulmonary ventilation
=
tidal volume
x
breathing rate
Time
The air breathed into a spirometer has carbon dioxide removed from it. This is because there is soda lime present to
absorb the carbon dioxide, as if someone was to breathe into the spirometer for a long time without having the carbon
dioxide removed, the levels of carbon dioxide which built up would be dangerous. Because of this, the volume of gas
decreases. The decrease is equivalent to oxygen consumption. We calculate the gradient on a spirometer trace to
obtain the oxygen consumption rate.
during exercise
3
Volume/dm
Volume/dm
3
at rest
oxygen
used
oxygen
used
Time
Note that the gradient of oxygen usage for the person
during exercise is a lot steeper than at rest.
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Time
The different elements of lung volume involved in respiration
A spirometer can produce a trace which shows the amount of air in the lungs over a certain period of time.
6
5
D
A
4
Volume/dm
3
E
3
F
B
2
1
C
0
Time
Name the elements of lung volume, named A to F in the trace, and explain what they mean.
A represents ……………………………… which is ………………………………………………………
…………………………………………………………………………………………………………...
B represents ……………………………… which is ………………………………………………………
…………………………………………………………………………………………………………...
C represents ……………………………… which is ………………………………………………………
…………………………………………………………………………………………………………...
D represents ……………………………… which is ………………………………………………………
…………………………………………………………………………………………………………...
E represents ……………………………… which is ………………………………………………………
…………………………………………………………………………………………………………...
F represents ……………………………… which is ………………………………………………………
…………………………………………………………………………………………………………...
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Questions on Units 2.1 – 2.3 on Surface Exchange
1
The diagram below shows two cubes, A and B which represent the organisms A and B.
Organism A
Organism B
2cm
4cm
(a) Complete the table below to identify the surface area, volume and surface-area-to-volume ratio
for cube B
Cube
A
Surface area
2cm x 2cm = 4cm2
4cm2 x 6 = 24cm2
Volume
2cm x 2cm x 2cm = 8cm3
Surface-area-to-volume ratio
24:8 = 3:1
B
(2 marks)
(b) Which of the organisms, A or B, is most likely to require a specialised transport system?
Explain your answer.
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
(3 marks)
Total: 5 marks
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2
The diagram below shows an alveolus and a connected blood capillary.
Alveolus
Blood capillary
Cell X
(a) (i)
Name the cell marked X on the diagram.
……………………………………………………………………………………………
(1 mark)
(ii) On the diagram there is an arrow marked. This is showing the movement of a molecule from
the alveolus to cell X in the blood capillary.
Explain the process by which the molecule moves into cell X from the alveolus.
……………………………………………………………………………………………
……………………………………………………………………………………………
.……………………………………………………………………………………………
(2 marks)
(b) State three features of the lining of the alveolus which are specialised to aid its functioning.
1 ……………………………………………………………………………………………….
…………………………………………………………………………………………………
2 ……………………………………………………………………………………………….
…………………………………………………………………………………………………
3 ……………………………………………………………………………………………….
…………………………………………………………………………………………………
(3 marks)
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(c) Explain why mammals have large numbers of alveoli in the lungs
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
(3 marks)
Total: 9 marks
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3
The inside of a bronchiole is lined with tissue like the one shown below.
Tissue Y
Cell Z
(a) (i)
Muscle
layer
Name tissue Y.
.……………………………………………………………………………………………
(1 mark)
(ii) Explain why tissue Y is regarded as a tissue.
……………………………………………………………………………………………
……………………………………………………………………………………………
.……………………………………………………………………………………………
(2 marks)
(iii) Tissue Y includes numerous cell types, including cell Z.
Name cell Z.
.……………………………………………………………………………………………
(1 mark)
(iv) State and explain the function of cell Z.
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
.……………………………………………………………………………………………
(3 marks)
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(b) The diagram also shows a layer of muscle. Name this type of muscle and explain its purpose in the
internal lining of the bronchiole.
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
(3 marks)
(c) Using the diagram on the previous page, identify one characteristic of either cell from tissue Y
which shows that the tissue cells are eukaryotic.
…………………………………………………………………………………………………
…………………………………………………………………………………………………
(1 mark)
Total: 11 marks
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The trace below shows the volume of air within the lungs of a student over a period of time.
6
5
T
R
U
S
V
W
3
4
Volume/dm
4
3
2
1
0
Time
(a) Name the piece of equipment which may have been used to produce this trace.
…………………………………………………………………………………………………
(1 mark)
(b) With reference to the trace, and all points R, S, T, U, V and W among the trace, describe the
breathing of the student as shown in the trace.
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
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…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
(7 marks)
Total: 8 marks
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Features of a transport system and types of circulatory system
As covered in 2.1 Special Surfaces for Exchange, the size and surface-area-to-volume ratio have a big impact on the
need for a transport system. Single-celled organisms do not need a specialised transport system because materials can
be transported via simple diffusion, whereas larger animals require a more elaborate transport system.
Another factor affecting the need for a specialised system is the level of activity. Animals need energy from food so that
they can move around, and releasing this energy from food requires oxygen. If an animal is very active, it is clearly going
to need a decent supply of nutrients and oxygen to supply the energy to cater for its movement. Animals which keep
themselves warm (e.g. mammals) need even more energy.
An effective transport system will include:
 a fluid or medium to carry nutrients and oxygen around the body (blood)
 a pump to create pressure that will push the fluid around the body (heart)
 exchange surfaces that enable oxygen and nutrients to enter the blood and leave it again where they are needed
More efficient transport systems also have:
 tubes or vessels to carry the blood
 two circuits – one to pick up oxygen and another to deliver it to the places it is needed in the body
Heart
Body
Gills
Heart
Body
Lungs
Fish are organisms which have a single circulatory
system. In fish, the blood flows from the heart to
the gills and then onto the body, and back to the
heart. This is shown in the left-hand flow chart.
Mammals have a circulation which uses two
circuits; this is a double circulatory system. One
carries blood to the lungs to pick up oxygen (this is
the pulmonary circulation) and the other carries
the oxygenated blood around the body to the
tissues where it’s needed (systemic circulation).
The mammalian heart (see 2.6 The Mammalian
Heart) is adapted to form two pumps, one for each
circuit. Blood flows through the heart twice for each
circulation of the body. This is shown in the righthand flow chart.
There are advantages attached to a double circulatory system:
 Blood can be maintained at a higher pressure in the systemic circuit, so it is delivered more quickly
 A slightly lower pressure can be maintained in the pulmonary circuit to prevent damage to capillaries of the lungs
For an animal to be active, it needs a good supply of oxygen and nutrients (such as glucose, amino acids and fatty acids)
and the rapid removal of carbon dioxide. In mammals, blood is never released into the body cavity, it is always confined
within blood vessels. This is called a closed circulatory system. However, many animals, including all insects, have an
open circulatory system. This means that the blood isn’t always held within vessels, instead, it is free to circulate the
body cavity, so any cells, tissues and organs of the organism are bathed in blood.
In an insect, there is a muscular pumping organ much like a heart. This is a long tube that lies under the dorsal (upper)
surface of the insect. Blood from the body enters the heart through pores called ostia. The heart then pumps the blood
towards the head via peristalsis. At the forward end of the heart (nearest the head), the blood simply pours out into the
body cavity.
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The open circuit works for insects because they are small, so the blood doesn’t have to travel far, and because they do
not rely on blood for the transport of oxygen and carbon dioxide – they have a separate transport system for this.
Larger organisms need the blood to transport these, so a closed circuit is necessary. In these organisms, the blood
always remains inside blood vessels, and a separate fluid (tissue fluid) bathes the organs and tissues.
BLOOD VESSELS
Blood flows through a series of vessels. Each one is specially adapted for its function, which is affected by its distance
from the heart. Each vessel has an inner layer called endothelium which is one cell thick. It is smooth to reduce the
friction of the blood flow.
Lumen
Endothelium
Lumen
Elastic fibres
Endothelium
Smooth muscle
Collagen fibres
Arteries (left diagram) carry blood away from the heart. They have a small lumen to maintain a high pressure and
relatively thick walls containing collagen to give it strength and to make the walls able to withstand the pressure. There
is also elastic tissue present which can stretch and then recoil the artery when the heart pumps; and smooth muscle
which contract to narrow the size of the lumen. The endothelium is folded, and unfolds when the artery stretches.
Veins (centre diagram) carry blood back to the heart. The blood is at a relatively low pressure, and so the walls are not
as thick as in arteries. The lumen is significantly larger, to ease the flow of blood. The levels of collagen fibres, smooth
muscle and elastic fibres are lower, because a vein does not need to stretch and do not actively constrict to reduce
blood flow. Veins contain valves to prevent the backflow of blood in the opposite direction. The movement of blood in
the veins is enables by the contraction of muscles surrounding the veins. For example, in the legs, it is the leg muscles
which allow the blood to flow upwards in the veins by contracting (NOT the muscle in the vein wall, but the skeletal
muscles surrounding them).
Capillaries (right diagram) have very thin walls. This creates a short diffusion pathway. The endothelium is composed of
squamous (pavement) epithelial cells which have small gaps called fenestrations separating the cells. Having a small
lumen means that all the red blood cells are constantly in contact with the vessel wall, to allow for quicker diffusion of
oxygen from the oxyhaemoglobin (see 2.8 Carriage of Oxygen). There is no muscle tissue or elastic tissue in capillaries.
The vessels come in packs of many because the high pressure from arteries needs to be evenly spread out. Therefore, it
is not a huge concern when one capillary becomes broken or damaged.
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The differences between blood and tissue fluid and the formation of tissue fluid and lymph
Blood consists of blood cells in a watery fluid called plasma which contains many dissolved substances including oxygen,
carbon dioxide, salts, glucose, fatty acids, amino acids, hormones and plasma proteins. The cells include red blood cells
(erythrocytes), various types of white blood cell (leucocytes) and fragments called platelets.
Tissue fluid is similar to blood, but doesn’t contain most of the cells found in blood, nor does it contain plasma proteins.
The role of tissue fluid is to transport and nutrients from the blood to the cells, and to carry carbon dioxide and other
wastes back to the blood.
When an artery reaches the tissues it branches out into smaller
arterioles, then into a network of capillaries which eventually link up
with venules to carry blood back to the veins. So blood flowing into
an organ or tissue is contained in the capillaries. At the arterial end of
the capillaries, the blood is under high pressure due to the
contraction of the heart muscle – this is called hydrostatic pressure.
It tends to push blood fluid out of the capillaries. The fluid leaves via
the tiny gaps in capillary walls.
The fluid which leaves the blood consists of plasma with dissolved
nutrients and oxygen. Red blood cells, platelets, plasma proteins
and most white blood cells remain in the blood – they are too large
to be pushed through the gaps. The resultant fluid is tissue fluid.
This fluid surrounds body cells, so exchange of gases and nutrients
can occur across the cell surface membranes. This exchange occurs
via diffusion and facilitated diffusion (see 1.8 Movement Across
Cell Membranes).
Artery
Tissue fluid
surrounding cells
Body cells
Lymphatic
system
Excess tissue
fluid drains
into lymphatic
system
Plasma forces
out of capillary
to become
tissue fluid
Capillary
Tissue fluid returns
to capillary
Vein
The hydrostatic pressure of the blood is not the only force acting on the fluid. The tissue fluid itself has some hydrostatic
pressure which will push the fluid back into the capillaries. Both the blood and tissue fluid contain solutes, giving them a
negative water potential (see 2.11 Water Potential). The water potential of the tissue fluid is less negative than that of
the blood. This means that water tends to move back into the blood from the tissue fluid via osmosis, down the water
potential gradient.
Arteriole end
Direction of
blood flow
Venule end
4.3
-3.3
-1.3
1.1
Net outflow is
1.2kPa
Hydrostatic pressure
Effect of water potential
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1.6
-3.3
-1.3
1.1
Net inflow is
-1.5kPa
The diagram on the previous page shows the hydrostatic and osmotic pressures acting on a single capillary. At the
venous end, the blood has lost its hydrostatic pressure. The combined effect of the hydrostatic pressure in the tissue
fluid and the osmotic force of the plasma proteins are sufficient to move fluid back into the capillary. It carries with it
any waste substances such as carbon dioxide that have left the cells.
LYMPH
Not all tissue fluid returns to capillaries. Some is drained away into the lymphatic system. This consists of a number of
vessels (tubes) that are like capillaries. They start in the tissues and drain the excess fluid into larger vessels, which
eventually rejoin the blood system in the chest cavity.
Lymph fluid is similar to tissue fluid and contains the same solutes. There will be less oxygen and fewer nutrients, as
they have been absorbed by body cells. There will therefore be more carbon dioxide and other wastes from the body
cells. Lymph also has a more fatty material that has been absorbed from the intestines. The main difference between
tissue fluid and lymph is that lymph contains many lymphocytes produced in the lymph nodes. The lymph nodes are
swellings found along the lymphatic system. They filter any bacteria and foreign material from the fluid. The
lymphocytes engulf and destroy these bacteria and foreign particles. This is part of the immune system.
The table below outlines the differences between blood, tissue fluid and lymph:
Feature
Blood
Tissue fluid
Lymph
Cells
Erythrocytes,
leucocytes and
platelets
Some leucocytes
Lymphocytes
Proteins
Hormones and
plasma proteins
Some hormones, proteins
secreted by body cells
Few proteins
Glucose
More (80-120mg
3
per 100cm )
None
Less
Amino acids
More
Less (absorbed by body cells)
Less
Oxygen
More
Less (absorbed by body cells)
Less
Carbon dioxide
Less
More (released by body cells)
More
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The structure and function of the mammalian heart
The mammalian heart is a muscular double pump. The right side pumps deoxygenated blood to the lungs, to be
oxygenated. The left side pumps the oxygenated blood to the rest of the body. The heart squeezes the blood, which
puts it under pressure forcing it along arteries.
The heart is made mainly of dark red muscle, which surrounds the two main pumping chambers: the ventricles. Above
the ventricles are two thin-walled chambers: the atria (plural of atrium). Whilst in diagrams these may be drawn as
fairly big, atria are very small in size and are easy to overlook. There are coronary arteries which lay over the heart
surface to provide the heart muscle itself with oxygenated blood, as hard-working muscles. These arteries are of
particular importance to the wellbeing of both the heart and mammal as a whole. If blood flow to the heart is restricted
it may cause angina or a heart attack (myocardial infarction). There are a number of larger veins and arteries at the
very top of the heart which carry the blood into and out of the heart.
aorta
pulmonary
artery
vena cava
pulmonary
vein
semilunar
valve
left atrium
right
atrium
right
atrioventricular
valve
right
ventricle
tendinous
cords
papillary
muscle
semilunar
valve
left
atrioventricular
valve
left
ventricle
ventricular
septum
The heart is split into four chambers:
the two atria and two ventricles.
Deoxygenated blood flows from the
vena cava into the right atrium.
Oxygenated blood from the lungs
flows from the pulmonary vein into
the left atrium.
From the atria, blood flows down
through atrioventricular valves into
the ventricles. These are pocket
tissues which fill up with blood and
remain closed whenever the
ventricles contract. This ensures the
blood flows upwards into the major
arteries and not back into the atria.
Inside the ventricles are tendinous
cords, which attach the valves to the
walls of the ventricle, preventing the
valves from turning inside out, which
would also allow the backflow of
blood back into the atria.
A wall of muscle (called the ventricular septum) separates the ventricles from each other. This ensures that the
oxygenated blood in the left side of the heart and the deoxygenated blood in the right side are kept separate.
Deoxygenated blood leaving the right ventricle flows into the pulmonary artery leading to the lungs. Oxygenated blood
leaving the left ventricle flows into the aorta. This carries blood to a wide series of arteries which supply the rest of the
body with blood. At the base of the major arteries, where they exit the heart, are semilunar valves which prevent the
blood returning the heart as the ventricles relax.
BLOOD PRESSURE
The muscle of each chamber contracts to create an increased pressure in the blood. The higher the pressure created in
the heart, the further it will push the blood. The muscle of the atria wall is very thin. This is because their only function
is to push the blood into the ventricles, so there is no need for a very high pressure.
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However, the walls of the right ventricle are thicker. This enables the right ventricle to pump the blood out of the heart.
The left ventricle has walls which are even thicker than the right, often two or three times thicker. This is for several
reasons. Mainly, this is because the left ventricle needs to pump the oxygenated blood all around the body, whereas the
right ventricle only needs to pump the deoxygenated blood round to the lungs, which are situated alongside the heart
in the chest cavity, so the distance it needs to be pumped is a lot smaller. Another reason is that the lungs contain a
network of very fine capillaries which are in close contact with the alveoli. The alveoli walls are very thin, and therefore
the capillaries are not supported and could therefore burst if there were a too high pressure, which also helps to explain
the lower pressure in the right ventricle.
The diagram below shows pressure changes within the vascular system during both circuits of the blood:
Pulmonary circulation
Blood pressure
Systemic circulation
Arteries → Arterioles
Capillaries
Venules → Veins Arteries → Arterioles Capillaries
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Venules → Veins
The structure and function of the mammalian heart
The heart is a muscular double pump which pumps two separate circuits of blood around the body. This is a double
circulatory system. It pumps deoxygenated blood to the lungs where it can pick up oxygen and become oxygenated; and
the other circuit pumps this oxygenated blood to the rest of the body where it is needed.
A
Fill in the spaces in the diagram
………………………………
………………………………
………………………………
………………………………
………………………………
………………………………
………………………………
………………………………
………………………………
………………………………
………………………………
………………………………
………………………………
B
………………………………
………………………………
Using the information from 2.6 The Mammalian Heart, fill in the gaps in the text below
Deoxygenated blood flows into the right atrium from the ……………………… and oxygenated blood into the
left atrium from the ……………………… from where the blood travels through the ………………………
valves into the ventricles. Deoxygenated blood which leaves the ……………………… flows through the
……………………… going to the lungs. Oxygenated blood flows through the ……………………… from
the left ventricle. At the apex (base) of the heart lie ……………………… valves which prevent blood from
returning to the heart after the ventricles relax.
C
Delete the words in the following text as appropriate. The first has been done for you
The muscular walls of the atria are very (thin/thick) because it only needs to push blood through the
(atrioventricular/semilunar) valves into the ventricles. The walls of these chambers are (thin/thick), which allows the
right ventricle to pump blood (into/out of) the heart. The (left/right) ventricle has thicker walls than the (left/right)
ventricle, because it needs to pump (oxygenated/deoxygenated) blood all around the body
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The cycle sequence of the heart beat and control of the cardiac cycle of the mammalian heart
The cardiac cycle refers to the sequence of events of a heartbeat. The pumping of the heart consists of alternate
contractions (systole) and relaxations (diastole). During each complete cycle, each chamber of the heart undergoes a
systole and a diastole.
During atrial systole the atria contract, which pushes blood into the
currently relaxed ventricles. The ventricles contract and push blood
out through the arteries in ventricular systole. At this stage, the
atrioventricular valves close to prevent blood flowing back into the
atria.
Atrial
Systole
Diastole
Ventricular
Systole
Atrial systole: Both atria contract.
Blood flows from the atria into the
ventricles. Backflow of blood into
the vein is prevented by closure of
the valves in the veins
Atrial diastole happens whenever atrial systole is not happening, and
similarly ventricular diastole happens whenever ventricular systole is
not currently happening. When these two diastoles overlap, the
period of the cycle is generally named diastole. Here, all chambers are
relaxed and blood flows into the atria (and some into ventricles). The
semilunar valves prevent the back flow of blood into the ventricles.
Ventricular systole: Both ventricles
contract. The atrio-ventricular valves
are pushed shut by pressurised
blood in the ventricles. Blood flows
from the ventricles into the arteries
semilunar
valves open
Pressure (kPa)
semilunar
valves close
atrioventricular
valves close
atrioventricular
valves open
Time
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Diastole: Atria and ventricles relax.
The semilunar valves have been
pushed shut. Blood flows from the
veins through the atria and into
the ventricles
KEY:
Left atrium
Left ventricle
Aorta
The graph on the previous page shows pressure changes in the heart throughout the various stages in the cardiac cycle.
To understand whereabouts the three stages happen in the graph, note that:
 Atrial systole runs until the atrioventricular valves close
 Ventricular systole runs from when the atrioventricular valves close to when the semilunar valves close
 Diastole runs from when the semilunar valves close to the beginning of atrial systole
Heart muscle is very different from other muscles in the body. It is myogenic. This means that it naturally contracts and
relaxes and does not need to receive nerve impulses to make it contract. Cardiac muscle can contract indefinitely
without suffering fatigue. It is also involuntary, in that it does not have to (and indeed cannot be) controlled voluntarily.
Heart muscle will contract even in the absence of nervous signal. However, signals from the brain are used to control
the rate at which these contractions occur. The cardiac cycle is initiated in a specialised patch of muscle in the wall of
the right atrium, called the sinoatrial node or SAN (also known as a pacemaker). This is a small patch of tissue which
generates electrical activity. The SAN initiates a wave of excitation at regular intervals. The wave quickly passes through
the walls of both atria, travelling along the membranes of the muscle tissue, causing the atria to contract. This is what
causes atrial systole.
At the base of the atria lies a disc of tissue which cannot conduct the excitation, so it cannot spread directly to the
ventricular walls. There is another node at the top of the ventricular septum (see 2.6 The Mammalian Heart) called the
atrioventricular node or AVN. This is the only route through the disc of non-conducting tissue which the wave can take,
although there is a slight delay for the wave of excitation at this second node. This time allows for the atria to finish
contracting and for blood to fill the ventricles before they contract.
Atria
walls
SAN
AVN
Purkyne
tissue
Ventricle
walls
After the delay, the wave moves from the AVN and
spreads down the specialised conducting tissue, this is
called the Purkyne tissue. This tissue runs all the way
down the septum. When the wave reaches the bottom
of the septum it spreads all over the ventricular walls,
starting from the base (apex) of the ventricles. This
causes them to contract from the bottom upwards,
which pushes the blood in the ventricles upwards
towards the major arteries at the top of the heart.
ELECTROCARDIOGRAMS
An electrocardiogram (or ECG) is used to record the electrical activity within the heart. A typical ECG for a normal
patient would identify a pattern of peaks and troughs which we give the letters P, Q, R, S and T.
R
T
P
The P wave represents the contraction from the
electrical excitation sweeping over the atria walls.
The QRS complex represents the wave of excitation
spreading through the ventricles walls.
Q
S
The T wave indicates diastole in the ventricles.
In order to obtain an ECG, a number of sensors are attached to the skin. Some of the electrical wave of excitation
spread through the muscular walls of the heart are passed onto the skin, where the sensors pick up on them and record
them as a trace like the one shown above.
The shape of an ECG can help to indicate when part of the heart muscle is not healthy. It can show if it has arrhythmia
(irregular heartbeat), if it is in fibrillation (uncoordinated beating) or has suffered a myocardial infarction (heart attack).
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This trace shows a normal PQRS trace. Such a trace
might be used for measuring the relative lengths of
the events involved in the cardiac cycle, for
example, contracting time for Q - T, filling time for
T – Q, calculating heart rate, etc…
Tachycardia – where heart rate is fast, such as
during exercise, stress, etc. Depending on the
cause, treatment may be stress management, giving
up smoking or treatment with beta blockers to slow
down heart rate
Bradycardia – pattern of activity is normal, but slow
and the intervals between each heartbeat are long.
Could be caused by beta blockers or tranquilisers,
or may just be from an elite athlete. There is a risk
of blood clot and stagnation
Fibrillation – the heart is contracting but in an
uncoordinated and irregular way. When a strong
electric current is passed through a heart
experiencing fibrillation, it will stop the heart for a
few seconds, hopefully returning it to its normal
pace when the heart restarts
The heart muscle cells respire fatty acids and must have a continuous supply of oxygen because fat can only be respired
aerobically. So a blood clot in the coronary artery starves part of the heart muscle of oxygen and those cells die. This is a
heart attack.
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The role of haemoglobin in the carriage of oxygen within the body
Oxygen is transported around the body within erythrocytes (red blood cells), which contain the protein haemoglobin
(Hb). When haemoglobin binds (reversibly) with oxygen, it becomes oxyhaemoglobin.
haemoglobin + oxygen ⇌ oxyhaemoglobin
Haemoglobin consists of four subunits, each composed of a polypeptide (protein) chain and a haem (non-protein)
2+
group. The haem group contains a single iron ion (Fe ) which can attract and hold an oxygen molecule (it has an
affinity, or attraction, to the oxygen molecule). Each haemoglobin molecule can therefore hold four oxygen molecules.
The diagram shows where oxygen is absorbed (the lungs), and
delivered to (the rest of the body). In the lungs, the molecules of
oxygen diffuse into the blood plasma and enter the erythrocytes
where they are picked up by haemoglobin. Body tissues require the
oxygen to respire, so oxyhaemoglobin must be able to release the
oxygen – this is called dissociation.
Lungs
LA
RV
LV
If you were to plot the partial pressure of oxygen against the
percentage saturation with oxygen, you would get an S-shaped
(sigmoid) curve, not a straight line. This is because at a low
pressure, haemoglobin will not bind with oxygen as readily, so
there is a low affinity for oxygen. The steepness of the sigmoid
comes from the increase in pressure causing an increase in affinity
for oxygen, and it eventually levels off when a haemoglobin
molecule has three oxygen molecules, because it then becomes
more difficult to attract the fourth.
Rest of
body
Mammalian haemoglobin is well adapted to
transporting oxygen around the body. The
partial pressure of oxygen (also called oxygen
tension) in the lungs is sufficient enough to
achieve almost 100% saturation.
The oxygen tension in the respiring tissues
around the body is sufficiently low, which causes
oxygen to dissociate readily from the
haemoglobin.
The graph to the right shows the relationship
between partial pressure of oxygen and the
percentage saturation, and its sigmoid curve.
The graph is given the name of the oxygen
dissociation curve.
100
90
Percentage saturation with oxygen
RA
The ability for haemoglobin to take up and release oxygen is
dependent on the amount of oxygen in surrounding tissues. The
amount of oxygen is measured by its relative pressure it contributes
to a mixture of gases. This is called the partial pressure (pO2).
80
70
60
50
40
30
20
10
0
0
2
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10
6
8
4
Partial pressure of oxygen (kPa)
12
14
FOETAL HAEMOGLOBIN
100
A mammalian foetus does not have functioning
lungs, and so the haemoglobin of a foetus must
have a higher affinity for oxygen so that it can
absorb oxygen from an environment where
adult haemoglobin will release it.
90
Percentage saturation with oxygen
Foetal haemoglobin
80
Maternal haemoglobin
70
In the placenta, foetal haemoglobin must
absorb oxygen from the mother’s blood. This
reduces the partial pressure of oxygen, which
causes the maternal haemoglobin to release
oxygen.
60
50
40
Because foetal haemoglobin has a higher affinity
for oxygen than maternal, the curve is slightly to
the left, as shown in the graph.
30
20
10
0
0
2
10
6
8
4
Partial pressure of oxygen (kPa)
12
14
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How haemoglobin is involved in the transport of carbon dioxide
All respiring tissues release carbon dioxide as they respire. There are three ways in which this can be removed:
 it can dissolve directly into the blood plasma
 it can combine with haemoglobin to form a compound known as carbaminohaemoglobin
 it can form hydrogen carbonate ions
The majority of carbon dioxide (around 85%) is transported in the latter way. This is a fairly complicated process:
Carbon dioxide diffuses into the blood and enters red blood cells, where it combines with water to form carbonic acid –
this reaction is catalysed by the enzyme catalyst carbonic anhydrase.
CO2
+
→
H2O
H2CO3
The carbonic acid (H2CO3) is very unstable, and so it easily dissociates to release hydrogen ions (protons) and hydrogen
carbonate ions.
H2CO3
→
H
+
+
-
HCO3
-
The hydrogen carbonate ions (HCO3 ) diffuse out of the red blood cells. As these ions have a negative charge, this means
that there is a positive charge inside the cell as a result. Therefore, chloride ions (Cl ) which also have a negative charge
move into the blood cell to balance out the charges, returning it to neutral. This is called the chloride shift.
To prevent the hydrogen ions (which are protons) causing the blood cell to become very acidic, hydrogen ions are taken
+
up by haemoglobin to produce haemoglobinic acid (HHb ).
OXYHAEMOGLOBIN
Haemoglobin carries oxygen in the form of oxyhaemoglobin. When this molecule dissociates, oxygen is released. The
hydrogen ions which are released from the dissociation of carbonic acid compete for the space oxygen takes up on the
haemoglobin molecule. So when carbon dioxide is present, hydrogen ions displace oxygen molecules on the
haemoglobin. As a result, the higher the potential pressure of carbon dioxide, the more oxygen is released from the
oxyhaemoglobin, because there is a lower affinity for oxygen.
100
Where there are heavily respiring tissues, such as
the muscles, there will be more carbon dioxide
present. Therefore, there will be more hydrogen
ions produced in the red blood cells, making the
oxyhaemoglobin release more oxygen. This shift
as shown in the graph is called the Bohr effect.
Percentage saturation with oxygen
90
80
70
60
At any oxygen partial pressure, oxyhaemoglobin
releases more oxygen when the carbon dioxide is
of a higher concentration. So when more carbon
dioxide is present, haemoglobin is less oxygen
saturated, causing the oxyhaemoglobin
dissociation curve shift downwards and to the
right – the Bohr shift.
50
40
30
20
10
0
0
2
10
6
8
4
Partial pressure of oxygen (kPa)
12
The Bohr effect results in oxygen being more
readily released where more carbon dioxide is
produced from respiration: what muscles need for
14 respiration to continue.
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Questions on Units 1.4 – 1.9 on Transport in Animals
1
(a) The diagram below shows a simplified version of the mammalian circulatory system.
Lungs
A
C
B
D
Rest of
body
The system is described as a double circulatory system.
(i)
Explain why the mammalian circulatory system is a double circulatory system.
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
(2 marks)
(ii) Identify two advantages of having a double circulatory system.
1 ………………………………………………………………………………………….
……………………………………………………………………………………………
2 ………………………………………………………………………………………….
……………………………………………………………………………………………
(2 marks)
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(iii) In the diagram, identify the four chambers of the mammalian heart, A, B, C and D.
Letter on diagram
Chamber of mammalian heart
A
B
C
D
(2 marks)
(b) The diagram below shows some of the internal features of the mammalian heart.
Z
V
Y
W
X
The table below lists some functions of features from the mammalian heart.
Function
Letter
contracts to force blood into the ventricle
E
wall of muscle separating the two ventricles
F
forces blood out into arteries
G
open and close to let blood into the ventricles/prevent backflow into atria
H
open and close to let blood out of the heart/prevent backflow into ventricles
I
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Complete the table below by
 identifying each of the features listed, using an appropriate letter from the diagram above
 matching a function to each feature, using an appropriate letter from the table above.
One has been done for you.
Feature
Letter in diagram
Letter in table
septum
X
F
ventricle
atrioventricular valves
atrium
semilunar valves
(8 marks)
(c) The graph below shows the pressure of blood at each circuit of the mammalian double circulatory
system.
Q
Blood pressure
P
J
(i)
K
L
M
N
O
From the diagram, identify P and Q by naming the circulatory circuit of each:
P ………………………………………………………………………………………….
Q ………………………………………………………………………………………….
(2 marks)
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(ii) Explain the pressure changes over the course of each circuit.
Refer to the letters J to O throughout your answer.
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
(6 marks)
Total: 22 marks
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2
The diagram below shows some blood vessels under the microscope.
Artery
Vein
W
X
lumen
Y
Z
(a) Identify W to Z.
W ………………………………………………………………………………………………
X ……………………………………………………………………………………………….
Y ……………………………………………………………………………………………….
Z ……………………………………………………………………………………………….
(4 marks)
(b) Explain the significance of each of the following:
(i)
A vein has a much wider lumen than an artery
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
(3 marks)
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(ii) There is more of tissues W, X and Z present in an artery’s structure than in a vein
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
(4 marks)
(iii) The artery shown is circular, whereas the vein is a squashed oval-shape
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
(2 marks)
The diagram shows a blood capillary.
4.3
-3.3
1.6
-3.3
1.1
-1.3
1.1
-1.3
Hydrostatic pressure
Effect of water potential
(c) Add to the diagram:
(i)
Two arrows to show the net movement of fluid into and out of the capillary at each end of
the blood vessel.
(1 mark)
(ii) Labels on each end of the capillary for either “arteriole end” or “venule end”.
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(1 mark)
(d) Calculate each of the following.
(i)
The net inflow of fluid from the diagram:
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
Answer: ……………………………………… kPa
(2 marks)
(ii) The net outflow of fluid from the diagram:
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
Answer: ……………………………………… kPa
(2 marks)
(e) Explain the effects of water potential pressure and hydrostatic pressure on the movement of fluid
into and out of the capillary.
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
(5 marks)
Total: 24 marks
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3
(a) Complete the following passage, information of the cardiac cycle.
When all four of the chambers are relaxed, the stage of the cycle is called ………………………
During atrial ………………… , the atria contract and blood is pushed through the
………………………………. valves into the ventricles below. The valves then close to prevent
the backflow of blood into the atria. At ventricular systole, the ventricles contract to push the
blood into the ………………… which go away from the heart. The …………………………
valves which the blood passes through at this stage are forced shut by the pressurised blood in the
arteries. This prevents the backflow of blood into the ventricles. When the blood has completed its
full circuit of the body, it flows through the ………………… and back into the heart.
(6 marks)
(b) The diagram shows the internal features of the mammalian heart.
The ventricular septum and Purkyne tissue have been labelled.
ventricular septum
Purkyne tissue
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(i)
Sort the following stages and descriptions into the correct order.
A The AVN conducts the electrical wave
B The wave cannot spread through a disc of non-conducting tissue at the base of the atria
C The electrical wave spreads down the conducting Purkyne tissue
D An area of the wall in the right atrium called the SAN generates a wave of excitation
E This causes the atria to contract and pushes blood through to the ventricles below
F The wave spreads across the ventricles, from the apex upwards
G The wave spreads through the atria
H This causes the ventricles to contract and push blood through to the arteries
Record your answers in the boxes below, starting from left to right.
D
(7 marks)
(ii) As the wave of excitation generated by the sinoatrial node (SAN) spreads down the Purkyne
tissue, a delay between the contraction of the atria and ventricles is caused.
Explain why this delay is important.
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
(3 marks)
(iii) What would be the result if the Purkyne tissue stopped working properly?
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
(2 marks)
Total: 18 marks
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(a) The graph below shows the sigmoid curve for the oxygen dissociation curve for both maternal
haemoglobin and foetal haemoglobin.
100
90
Percentage saturation with oxygen
4
A
80
B
70
60
50
40
30
20
10
0
0
2
10
6
8
4
Partial pressure of oxygen (kPa)
12
14
Identify A and B as maternal or foetal haemoglobin.
A ……………………………………………………………………………………………….
B ……………………………………………………………………………………………….
(2 marks)
(b) Explain the importance of the shift from the maternal haemoglobin curve to the foetal
haemoglobin curve shown in the graph.
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
(3 marks)
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(c) One method of transporting carbon dioxide around the body is as hydrogen carbonate ions.
Explain how carbon dioxide can be transported as a hydrogen carbonate ion (HCO3-).
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
(7 marks)
(d) Identify and briefly explain two other ways, apart from as hydrogen carbonate ions, in which
carbon dioxide can be carried.
1 ……………………………………………………………………………………………….
…………………………………………………………………………………………………
…………………………………………………………………………………………………
2 ……………………………………………………………………………………………….
…………………………………………………………………………………………………
…………………………………………………………………………………………………
(4 marks)
Total: 16 marks
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The structure and function of xylem and phloem transport tissues in plants
Simply put, every living organism needs to take in substances from its environment, and dispose of waste substances
back into its environment. The transport system in a plant which does this moves substances around the plant in special
tissue called vascular tissue. There is xylem tissue which transports water and soluble minerals upwards, and there is
phloem tissue which transports sugars upwards and downwards. Phloem and xylem tissues are found together in
vascular bundles. These bundles may often contain some other tissue types to give the plant extra support.
ARRANGEMENT OF TRANSPORT TISSUES IN PLANTS
Vascular bundles are arranged very differently in the different parts of a
plant. In the root, the xylem (stained green-blue in the photo) are arranged
in some sort of star or cross-shape. The phloem tissue are arranged in
smaller groups in between each spike of the xylem cross-shape.
Around the bundle is a special layer of cells called the endodermis. This has
a key role in getting water to the xylem vessels. Inside the endodermis is a
layer of meristem cells called the pericycle.
The vascular bundles are found near the outer edge in
the stem. The xylem is found towards the inside of
each vascular bundle (see the diagram), and the
phloem towards the outside. In between the xylem and
the phloem vessels there is a layer of cambium. This is
a layer of meristem cells which can divide to produce
more phloem or xylem.
phloem
cambium
xylem
The diagram to the right (top) shows a vascular bundle
in close detail. The below diagram shows a transverse
section of a stem, where you can see the entire outer ring
of vascular bundles.
medulla
cortex
xylem
In a leaf, however, the vascular bundles form the midrib
and veins of the leaf. Usually, the pattern tends to be veins
branching away from the midrib, forming a network which
gradually gets smaller and thinner with distance from the
midrib. Within each vein, the xylem can be seen on top of
the phloem.
small vein
xylem
central midrib
phloem
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separate vascular
bundles
phloem
XYLEM
Xylem tissue consists of the xylem vessels to transport water and
dissolved minerals; fibres to help support the plant and living
parenchyma cells. The xylem vessels are the main feature. These
are long cells with thick walls impregnated with lignin (see 1.14
Tissues and Organs). The lignin makes the cells waterproof, and this
causes the cells to die, so their contents and end walls decompose,
leaving a hollow tube of dead cells. The lignin strengthens the tube
and prevents the vessel from collapsing.
Because of its adaptations, xylem tissue can carry water and minerals from roots to the very top of the plant:



It is made from dead cells end-to-end to form a continuous hollow column
The tubes are narrow so the water column does not break easily and capillary action can be effective
Lignin deposited in the walls in spiral, annular (circles) or reticulate (broken rings) patterns allows xylem to
stretch as the plant grows and enables the stem or branch to bend
PHLOEM
The function of phloem is to transport sugars from one part of the
plant to another. This can be upwards or downwards. Phloem tissue
consists of two types of cell: sieve tube elements and companion
cells.
The sieve tube elements are not true cells as they contain very little
cytoplasm and no nucleus. They are lined up end-to-end to form a
tube, which transports the sugars (usually sucrose). The sucrose is
dissolved in water to form sap. Unlike xylem, this tube contains
cross-walls at intervals, perforated by many pores to allow the sap
to flow. Hence the cross-walls are called sieve plates and the tubes
sieve tubes.
Between the sieve tubes are small cells, each with a large nucleus, and dense cytoplasm. These are companion cells.
Because of their many active processes, they have large numbers of mitochondria to produce the required amount of
ATP. These cells carry out the metabolic processes using the ATP energy, such as loading the sucrose in the tubes.
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The movement of water between cells and the effects of water potential on plant cells
The term water potential (ψ) refers to the measure of the tendency of water molecules to diffuse from one place to
another. For a more detailed introduction into water potential and to read about its effect on animal and plant cells, see
1.8 Movement Across Cell Membranes.
Water molecules always move from a region of high water potential (less negative) to an area of lower water potential
(more negative). Pure water has a water potential of zero – this means that the water potential cannot get any higher,
because the scale for water potential (measured in kilopascals – kPa) is negative. Because pure water has such a high
water potential, it has a very high tendency to lose water to its surroundings, as they will have a lower water potential.
In a plant cell, the cytoplasm contains salts and sugars
(solutes) that will reduce its water potential. This is
because having the solutes lowers the number of “free”
water molecules in the cytoplasm. Therefore, all plant
cells always have a negative water potential.
If you place a plant cell in pure water, it will take up
water molecules by osmosis because the water potential
of the cell is lower than that of the water surrounding it.
However, it will not continue to absorb water until it
bursts, because it has a strong cellulose cell wall. Once
the cell has taken as much water as it is going to, it is
turgid. The water inside starts to exert pressure on the
cell wall, called pressure potential (also ψ). As this
pressure potential builds up, the intake of water slows
down.
When a plant cell is places in a solution with a very low
water potential, for example a concentrated salt
solution, it will lose water via osmosis. This is because the
solution has a much lower water potential than the cell,
so the water molecules move out of the cell. The
cytoplasm and vacuole begin to shrink and shrivel.
Eventually, the cytoplasm stops pushing against the cell
wall and is called incipient plasmolysis. If the water loss
continues, the plasma membrane will lose contact with
the cell wall too. This is plasmolysis.
Cell cytoplasm
ψ = -500kPa
Water enters the cell
down the water
potential gradient
Pure water
ψ = 0kPa
▲ The water potential of the cell is negative, but the
pure water surrounding it is zero (the highest possible
water potential), so water molecules move into the cell
▼ The cell on the left has a higher water potential
because its value is less negative, so water molecules
move from it into the cell on the right which has a
lower water potential (it has a higher negative value)
ψ = -600kPa
ψ = -1400kPa
The table below shows relative water potentials and where they may be found:
Highest water potential 0kPa
Lower water potential -10kPa
Very low water potential -500kPa
Pure water
Dilute solution
Concentrated solution
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No solute dissolved
Small amount of solute dissolved
Large amount of solute dissolved
Water movement between plant cells and the uptake of water from the roots to the leaves
WATER MOVEMENT BETWEEN CELLS
Water molecules can move between adjacent cells. A water molecule moves from a cell with a higher water potential
(less negative) to one with a lower water potential (more negative). For example:
Ψ = -600 kPa
Ψ = -1100 kPa
There are three possible ways in which water molecules can travel between cells:
The vacuolar pathway (A) is one route water can take. Here, the water
molecules enter the cytoplasm through the plasma membrane, and travel
across the cytoplasm, and through the vacuole, back through some
cytoplasm, onto the next vacuole, etc
[cytoplasm – vacuole – cytoplasm]
The symplast pathway (B) is a second possible route. The water travels
only through adjoining cells by their cytoplasm (not vacuoles), via
plasmodesmata (strands of cytoplasm which connect two different cells’
cytoplasm)
[cytoplasm – cytoplasm – cytoplasm]
The apoplast pathway (C) is the other route. The cellulose cell walls have
many water-filled spaces between them which water can travel across. In
this pathway, the water molecules do not cross any plasma membranes,
meaning that dissolved mineral ions and salts can be carried with it
[cell wall – cell wall – cell wall]
A
B
C
Water moving by the apoplast pathway is forced to travel through the
cytoplasm of the endodermis, because the endodermis cells have a
waterproof layer in their cell walls called suberin. This is organised in
bands called Casparian strips.
UPTAKE OF WATER THROUGH THE ROOTS
Water enters root hair cells by osmosis. The water potential of root hair cells is lower than the surrounding water. The
water can then move across the root via osmosis, down the concentration gradient, to the endodermis.
The endodermal cells pump minerals into the xylem using active transport. This lowers the water potential in the xylem
and causes water to follow by osmosis.
The water is then pulled up the xylem by the transpirational pull generated by the water leaving the xylem and then
transpiring from the leaves.
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4 – Osmosis moves the water
across the leaf cells
5 – Evaporation of water
from cell surfaces
3- Water transported
up the xylem due to
transpirational pull
1 - Water osmotically
transported (and minerals
by active transport) into
the root hair cells and
through adjacent cells
6 – Diffusion of the
water vapour out of the
leaf (transpiration)
2 - Water enters the xylem to
replace lost water (water potential
lowered as other water molecules
leave the xylem)
THE CASPARIAN STRIP
The endodermis is a layer of cells which surrounds the xylem. Many of these cells have a waterproof layer in their cell
walls which appears in bands. This band is called the Casparian strip. This blocks the apoplast pathway, because it
means that the water cannot travel along cell walls, so water is forced to follow the symplast pathway.
The purpose of the Casparian strip is to ensure that the water carrying the salts and dissolved mineral ions has to travel
through the plasma membranes to the cellular cytoplasm. To allow this, there are transport proteins (see 1.8
Movement Across Cell Membranes) in the membranes. Certain substances can be actively transported into the xylem
from adjacent cells, including nitrate. This decreases the water potential of the xylem, meaning there is a steep water
potential gradient from the cells surrounding the xylem to the xylem itself, therefore forcing water to move from those
cells into the xylem. This is another result of the Casparian strip blocking the apoplast pathway.
MOVEMENT OF WATER UP THE STEM
There are three processes which aid the movement of water up the stem:
1
Root pressure – the action of the endodermis moving minerals into the xylem by active transport drives water into
the xylem (by osmosis), which in turn pushes water up the xylem as new water enters the vessel
2
Transpirational pull – the loss of water from the leaves must be replaced by water coming up the xylem. Water
molecules are attracted to each other by the forces of cohesion. These forces are strong enough to hold the
molecules together in a long chain or column. As the molecules at the top of the column are lost, the whole chain
is pulled up as one lot, creating the transpirational stream.
3
Capillary action – the same forces also attract the water molecules to the sides of the xylem – this is called
adhesion. Because the xylem are very narrow, these forces can pull the water molecules up the sides of the vessel
LEAVING THE LEAF
Most of the water which leaves the leaf exits through stomata. These are tiny pores in the epidermis. A tiny amount
also leaves through the waxy cuticle. Water evaporates from the cells immediately below the guard cells, lowering the
water potential in those cells, causing water from neighbouring cells to enter them by osmosis.
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The loss of water vapour from (mainly) the leaves of a plant
Transpiration is an inevitable process of photosynthesis. This is because during photosynthesis, the stomata are opened
in the leaves for gaseous exchange to happen – this allows water to be lost through the leaves.
Water enters the leaves through xylem vessels (see 2.12 Water Uptake and Movement Up the Stem) and passes to the
mesophyll cells by osmosis. Water evaporates from the surface of the mesophyll layer cells, forming water vapour. The
spongy mesophyll cells have large air spaces in between them where this vapour can collect, making the water vapour
potential rise. Once the water vapour potential inside a leaf is higher than outside the leaf, the water molecules will
diffuse (not osmosis – but diffusion) out of the leaf.
mesophyll cells
xylem
So we can summarise transpiration as involving three simple processes:



osmosis from the xylem vessels to the mesophyll cells (
)
evaporation from the surface of the mesophyll cells into intercellular air spaces (
diffusion of water vapour through the stomata (
)
)
As water leaves the xylem in the leaf, it must be replaced from below. Water moves up the xylem from the roots to
replace the water which has been lost. This constant movement of water due to the loss and replacement of water via
transpiration is called the transpiration stream.
The rate of transpiration can be measured using a potometer. This device measures the water uptake in a plant using a
cut shoot, so it is not an exact measure of the transpiration rate – but because approximately 99 per cent of water taken
up is lost by transpiration, it does give a reasonable estimate of water loss.
FACTORS AFFECTING TRANSPIRATION RATE
There are a large number of both plant (internal) and environmental (external) factors which affect the rate of
transpiration, as shown in the table:
Plant Factors
Environmental Factors
Number of leaves (and area of leaves)
Light intensity
Density and distribution of stomata
Temperature
Thickness of waxy cuticle
Wind speed
Specialised adaptations
Humidity
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Key:
Blue means that as this factor increases,
transpiration rate also increases
Pink means that as this factor increases,
transpiration rate decreases
When the rate of transpiration is too high, i.e. more than the rate of water uptake, the plant cells will lose their turgidity
and non-woody plants will wilt and die. The leaves of woody plants will also wilt and die.
XEROPHYTES
There are a number of plants which are particularly well-adapted to living in certain conditions. These plants are known
as xerophytes. Their adaptations help to reduce water loss from their leaves:









they have smaller leaves, often even like needles, reducing surface area so less water is lost through transpiration
they have a very thick waxy cuticle (for example, holly leaves)
densely packed spongy mesophyll reduces the air spaces, so transpiration happens at a slower rate
some are able to close the stomata when water availability is low
many have hairs on the surface of their leaves to trap a layer of air close to the surface: this air becomes saturated
with moisture and will reduce the water potential gradient from inside the leaf to outside, therefore reducing the
rate of transpiration
rolling the leaves so that the lower epidermis is not exposed to the atmosphere can trap air that becomes saturated
pits containing stomata at their base also trap air that can become saturated with water vapour, reducing the water
vapour potential gradient and therefore reducing water loss
cacti and other succulents have specialised water-storing facilities
there may be either very long roots to increase water uptake or very short roots to exploit overnight moisture
The image below shows a cross-section of marram grass. This plant is specialised for living in sand dunes, where
conditions are particularly harsh. Any water which is available drains away quickly, the sand is often salty, and the
leaves are often exposed to very windy conditions. Some of the xerophytic features above are shown below:
leaf rolled up to trap air inside
thick waxy cuticle to reduce water evaporation from surface
trapped air inside with a high water vapour potential
hairs on lower surface reduce movement of air
stomata in pits to trap air with moisture close to the stomata
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Moving sugars around the phloem tubes of a plant
The process of translocation is the movement of assimilates (sugars and other chemicals made by plant cells) in the
phloem tissue. The sugars are transported around phloem tubes in the form of sucrose. A source releases sucrose into
the phloem, and a sink removes sucrose from the phloem.




leaf
(source)
phloem
xylem
sucrose
molecule
bud
(sink)
 The leaf is described as a source because it produces sucrose to be transported. The first diagram shows two
processes happening: the sucrose molecules are moving from the leaf into the phloem by active transport (this is
because they are moving from a low-to-high concentration, as even though the leaf produces sucrose, phloem is
filled with it); and as the sugar content of the sap (phloem contents) increases, its water potential decreases, so
water moves into the phloem from the xylem by osmosis
 As water moves into the phloem, the turgor pressure of the phloem increases, and this pushes the contents of the
phloem vessel down the sieve elements
 The contents of the phloem move down to the sink (in this case, a bud) where the sucrose is needed, and there is a
low concentration of sucrose molecules on the sink’s side of the phloem tube
 The sucrose molecules move into the sink by diffusion (it would have to be facilitated diffusion, because the
molecules are very big and are polar – charged – so would need specific transport proteins). This will increase the
water potential of the phloem, and therefore it is usually the case that water molecules will return to the xylem
Generally speaking, because source cells produce sucrose, they are photosynthetic. Similarly, sink cells cannot produce
sucrose, and therefore they are not synthetic. They need the sucrose to convert into glucose for respiration.
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HOW SUCROSE IS LOADED INTO THE PHLOEM
For more detail on the structure and function of phloem vessels, see 2.10 Xylem and Phloem. The diagram below shows
how sucrose molecules are loaded into phloem.
Sieve element of
sieve tube
Companion
cell
Neighbouring cell
(e.g. spongy mesophyll)

Hydrogen ion
pump protein in
membrane
+
H
+
H
sucrose
Co-transporter
protein in
membrane

sucrose

sucrose
Sieve plate between
neighbouring elements
1 Hydrogen ions are
actively pumped out
of the companion cell,
via a transport protein
in the membrane
2 The ions diffuse
back into the
companion cell via
co-transport proteins
(carrying sucrose with
them) – this is
facilitated diffusion
3 Sucrose moves from
a high concentration
in the companion cell
to low concentration
in the sieve tube
element by diffusion
(mainly through the
plasmodesmata)
Co-transport is a process of transport of molecules where two substances are attached together, so they can diffuse at
the same time. Sucrose is a good example of a molecule which co-transports alongside the hydrogen ions as above. It
uses transport proteins, because the two molecules together are too large for simple diffusion, and are both polar,
therefore it is a form of facilitated diffusion. It uses no energy so is a passive process.
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Questions on Units 1.10 – 1.14 on Transport in Plants
1
Diagram A shows a transverse section through the stem of a dicotyledonous plant.
Diagram B shows the outline of a transverse section through the root of the same plant.
C
Diagram A
Stem
Diagram B
Root
(a) Complete diagram B to show distribution of the transport vessels phloem and xylem.
(2 marks)
(b) The diagram below shows an enlarged version of structure C.
X
Y
Z
Identify the structures X, Y and Z below and state their function.
(i)
structure X; ……………………………………………………………………………….
function; ………………………………………………………………………………….
(i)
structure Y; ……………………………………………………………………………….
function; ………………………………………………………………………………….
(i)
structure Z; ……………………………………………………………………………….
function; ………………………………………………………………………………….
(6 marks)
Total: 8 marks
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2
Marram grass is a special type of plant which has specialised features designed to reduce the rate of
transpiration because of its harsh climate.
(a) Define the term transpiration.
…………………………………………………………………………………………………
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(2 marks)
The diagram below shows a transverse section of some marram grass.
(b) What is the name given to types of plant designed to reduce transpiration rate?
…………………………………………………………………………………………………
(1 mark)
(c) Outline three adaptations marram grass has which help it to reduce the transpiration rate, shown
the diagram.
You may annotate the diagram above as part of your answer.
…………………………………………………………………………………………………
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(6 marks)
Total: 9 marks
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3
Below is a diagram which shows four plant cells.
Ψ = -600 kPa
Ψ = -800 kPa
Ψ = -850 kPa
Ψ = -1150 kPa
(a) On the diagram;
(i)
using an arrow for each, label the vacuolar, symplast and apoplast pathways;
(3 marks)
(ii) using an arrow, show the direction of the movement of water molecules through the cells.
(1 mark)
(b) Explain the movement in water through the cells.
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
…………………………………………………………………………………………………
(3 marks)
(c) Complete the following paragraph by filling in each gap with the correct word or phrase.
Water from the root cells moves into the xylem vessel by …………………… as the ……………
…………………… of the xylem decrease when water moves out of the top. The xylem are
composed of …………… cells. At the top of the xylem tube, water molecules move out into the
leaves, where there is a …………… water potential than in the xylem. The movement of water
molecules up the xylem (forced by the removal of water from the top) is called the
………………………… …………… Water moves around the cells in the leaf by osmosis, and
………………………… from the surface of the spongy mesophyll cell (this process is called
transpiration). The water then moves out of the leaf by …………………………
(7 marks)
Total: 14 marks
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4
The diagram shows part of a phloem tube (sieve element), a companion cell and a neighbouring cell.
Sieve element of
sieve tube
Companion
cell
Neighbouring cell
(e.g. spongy mesophyll)
Pump protein in
membrane
Transport protein
in membrane
Sieve plate between
neighbouring elements
The diagram shows the features required for exchange of sucrose into the phloem vessel.
(a) Explain how sucrose molecules are loaded into the phloem from the neighbouring cell shown.
You may refer to and annotate the diagram in your answer.
…………………………………………………………………………………………………
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(7 marks)
(b) Explain the following terms;
(i)
sucrose source
.……………………………………………………………………………………………
.……………………………………………………………………………………………
(1 mark)
(ii) sucrose sink
.……………………………………………………………………………………………
.……………………………………………………………………………………………
(1 mark)
(c) Which, a source or sink, is photosynthetic? .…………………………………………… (1 mark)
Total: 10 marks
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