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Transport
 The need for, and functioning of a transport system in multicellular
plants
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 Structure and functioning of the mammalian heart
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 In single- celled organism, this can happen quickly enough by diffusion alone
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 In a large organism, diffusion is no longer sufficient
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 Large organisms solve these difficulties in two ways:
 They have transport systems that carry substances by mass flow from one
part of the body to another, rather than relying solely on diffusion
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Define the term transpiration and explain that it is an inevitable
consequence of gas exchange in plants
Transpiration - the loss of water vapour from a plant to its environment, by diffusion
down a water potential gradient; most transpiration takes place through the stomata in the
leaves
...
The walls of the mesophyll cells are wet and some of this
water evaporates into the air spaces so that the air inside the leaf is usually saturated with
water vapour
...
If there is a water potential gradient between the air
inside the leaf and the air outside, then water vapour will diffuse out of the leaf down this
gradient
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Plants cannot therefore avoid losing water vapour by transpiration
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The amount of water vapour lost by transpiration from the leaves of a plant can be very
great
...
Thus water must move into
the leaves equally to replace this lost water
...
This
makes it very difficult to investigate directly how different factors such as humidity,
temperature, wind speed, or light intensity affect the rate of transpiration
...
As a very high
proportion of the water takes up by a stem is lost in transpiration, and as the rate at which
transpiration is happening directly affects the rate of water uptake, this measurement can
give a very good approximation of thee rate of transpiration
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Everything must be completely water-tight and alright, so that no leakage of water occurs,
and so that no leakage of water occurs, and so that no air bubbles break the continuous
water column
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It also helps to cut the end of the stem underwater
and with a slanting cut before placing it in the photometer, as air bubbles are less likely to
get trapped against it
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Photometer can be simpler than this one
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Water is therefore drawn along the capillary tubing
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The rates of water uptake will have a close relationship to the rates of transpiration under
different conditions
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Leaves of dicotyledonous
plants
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The
water potential in the soil is generally higher than in the air
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Transpiration maintains the water potential gradient
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The water then moves from the root hair cell to neighbouring cell by osmosis down a
water potential gradient
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Water also seeps into the cell wall of the root hair cell
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The water then seeps into
and along the cell walls of neighbouring cells
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In most plant roots, the apoplast pathway carries more water than the symplast
pathway
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Each cell has a ring of impermeable SUBERIN around it, forming
the CASPARIAN STRIP
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It
therefore travels through these cells by the symplast pathway
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Water moves up the xylem vessels by mass flow- that, is, in a similar way to water
flowing in a river
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There is a relatively low
hydrostatic pressure at the top of the column, produced by the loss of water by
transpiration
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In a leaf, water moves out of the xylem vessels through pits, and then across the leaf
by the apoplast and symplast pathways
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Outline the roles of nitrate ions and of magnesium ions in plants

Ions
Nitrate, NO3-

Some roles in plant
Nitrate is used to help
make amino acids;
organic bases, e
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adenine; proteins;
nucleotides, e
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ATP;
coenzymes, e
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NAD,
NADP; chlorophyll
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Some
enzymes have
magnesium ions at their
active sites, e
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ATPases
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A xerophyric is a plant that is adapted to live in an environment where water is in short
supply
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This reduces the amount of surface
area from which water vapour can diffuse
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This reduces the quantity of water that can diffuse
through the surface of the leaf into the air
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This produces a layer of high water potential around the stomata, reducing
the water potential gradient and therefore reducing the rate of diffusion of water
vapour from inside the leaf to outside
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Assimilates: substances such as sucrose that have been made within plant
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To create the pressure differences needed for mass flow in phloem, the

plant has to use energy
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The pressure difference is produced by active loading of sucrose into the sieve elements at
the place from which sucrose is to be transported
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This is usually a photosynthesising leaf
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Loading a high concentration of sucrose into a sieve element greatly decreases the water
potential in the sap inside it
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This causes a correspondingly high build up in
pressure
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A pressure difference is therefore created between the source and the sink
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At the sink, sucrose mat be
removed and used, causing the water to follow by osmosis, and this maintaining the
pressure gradient
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Thus, sap flows both upwards and downwards in phloem
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Explain the translocation of sucrose using the mass flow hypothesis;
Loading sucrose into phloem
In leaf mesophyll cells, photosynthesis in chloroplasts produces triose sugars, some of which
are converted into sucrose
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It may move by the symplast pathway, moving from cell to cell via plasmodesmata
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At the
moment little is known about how the sucrose crosses the cell surface membrane of the
mesophyll cell to enter the apoplast pathway
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Sucrose is loaded
into a companion cell or directly into sieve elements by active transport
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This creates a large excess of hydrogen ions in the apoplast outside the
companion cell
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The sucrose molecules are carried through the co-transporter molecule into the
companion cell, against the concentration gradient for sucrose
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Unloading sucrose from phloem

Unloading occurs into any tissue which requires sucrose
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Phloem unloading requires energy, and similar methods to those used for loading
are probably used
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One such enzyme is invertase, which
hydrolysis sucrose to glucose and fructose
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The blood that flows through them is pulsing and
at a high pressure
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The artery wall also contains variable amounts of smooth muscle
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These also contain smooth muscle in
their walls, which can contract and make the lumen (space inside) smaller
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Capillaries are tiny vessels with just enough space for red blood cells to squeeze through
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Capillaries deliver nutrients, hormones
and other requirements to body cells, and take away their waste products
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Veins carry low-pressure blood back to the heart
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The lumen is
larger than in arteries, reducing friction which would otherwise slow down blood
movement
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Blood is kept moving through many veins, for example those in the legs, by the
squeezing effect produced by contraction of the body muscles close to them, which are
used when walking
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These transport oxygen from lungs to respiring tissues
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This increases their surface area to volume ration,
allowing rapid diffusion of oxygen into and out of them
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They do
not contain nucleus or mitochondria
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These are which blood cells that engulf and digest unwanted cells, such as
damaged body cells or pathogens
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Lymphocytes
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Each lymphocyte is able to recognise one
particular pathogen and respond to it by secreting one particular type of antibody or by
attacking it
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Near the arteriole end of a
capillary, there is relatively high pressure inside the capillary, and plasma leaks out through
these gaps to fill the spaces between the body cells
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



Tissue fluid is therefore very similar to blood plasma
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The tissue fluid baths the body cells
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Some tissue fluid moves back into the capillaries, becoming part of the blood plasma once
more
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However, some
of the tissue fluid collects into blind-ending vessels called lymphatic vessels
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Lymphatic vessels have valves that allow fluid to flow into them and along them but not
back out again
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The lymph passes through lymphatic glands where white
blood cells accumulate
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Describe the role of haemoglobin in carrying oxygen and carbon
dioxide;
Haemoglobin (Hb) is a protein with quaternary structure
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Each haem
group contains an Fe2+ ion which is able to combine reversibly with oxygen, forming
oxyhaemoglobin
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Hb

+

4O2



HbO8

Describe and explain the significance of the dissociation curves of
adult oxyhaemoglobin at different carbon dioxide levels (the Bohr
effect);
Oxygen concentration can be measured as partial pressure, in kilopascals (kPa)
...
At high partial pressures of oxygen, all the haemoglobin will be combined
with oxygen and we say that it is 100% saturated with oxygen
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In
the
lungs, the partial pressure of oxygen may be around 12kPa
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In a respiring, the partial pressure of oxygen may be around 2kPa
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Therefore, when Hb from the lungs arrive at a respiring muscle it gives up more than 70% of
the oxygen it is carrying
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When this
happens, the haemoglobin combines with H+ ions and releases oxygen
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This is called Bohr effect
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Describe and explain the significance of the increase in the red blood
cell count of humans at high altitude;
Adaptation to high altitude:
At high altitude, the air is less dense and the partial pressure of oxygen is lower than at sea
level
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After some time at high altitude, the number of red blood cells in the blood increases
...
Therefore,
even though each Hb molecule carries less oxygen on average than at sea level, the fact that
there are more of them helps to supply the same amount of oxygen to respiring tissues
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When they return to low altitude, their extra red blood cells can supply oxygen to their
muscles at a greater rate than in an athlete who has not been to high altitude, giving them a
competitive advantage
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It is made of
interconnecting cells, whose cell surface membranes are very tightly joined together
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The large, arching blood vessel is the largest artery, the aorta, with branches leading
upwards towards the head, and the main flow doubling back towards to the rest of the
body
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This, too, branches
very quickly after leaving the heart, in to two arteries taking blood to the right and left
lungs
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The pulmonary veins bring blood back to the heart from
the left and right lungs
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These branch from the aorta, and deliver oxygenated blood to the walls of the heart itself
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The two chambers on
the left of the heart are completely separated from those on the right by a wall of muscle
called the septum
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The upper chamber on each side of the heart is called an atrium or sometimes auricle
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Blood from the
venae cavae flows into the right atrium, white blood from the pulmonary veins flows into
the left atrium
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Blood flows into the ventricles from the atria, and is
then squeezed out into the arteries
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The atria and ventricles have valves between them, which are known as the atrioventricular
valves
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Describe the mammalian circulatory system as a closed double
circulation;
Cardiovascular system is made up of a pump, the heart and a system of interconnecting
tubes, the blood vessels
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Systemic circulation
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It retunes to the right side of the heart
in the vena cava
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Pulmonary circulation
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The final part of the journey is along the
pulmonary veins, which return it to the left side of the heart
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The combination of pulmonary circulation and systematic circulation makes double
circulation system
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Contraction of the cardiac muscle in the walls of the
heart therefore causes the walls to squeeze inwards on the blood inside the heart
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The complete squeeze inwards on the blood
inside the heart
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The complete
sequence of one heart beat is called the cardiac cycle
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The rhythmic, coordinated contraction of the cardiac muscle
in different parts of the heart is coordinated through electrical impulses passing through the
cardiac muscle tissue
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This has an intrinsic rate of contraction a little higher than
that of the rest of the heart muscle
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This causes the
muscle to contract
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When the action potentials reach the atrioventricular node (AVN) in the septum,
they are delayed briefly
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This causes
the ventricle to contract slightly after the atria
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This is ventricle systole
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During this time, the heart muscles relax
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