mass transport Flashcards
what is haemglobin
haemoglobin are a group of chemically similar molecules found in a wide variety of organisms
haemoglobin are protein molecules with a quaternary structure that has evolved to make them efficient at loading oxygen under one set of conditions and unloads oxygen under a different set of conditions
what is the structure of haemoglobin
like most proteins, haemoglobins have:
- primary structure
- secondary structure
- tertiary structure
- quaternary structure
what is the primary structure of haemoglobin
sequence of amino acids in the four polypeptide chains
what is the secondary structure of haemoglobin
in which each of these polypeptide chains coiled into a helix
what is the tertiary structure of haemoglobin
in which each of these polypeptide chains is folded into a precise shape - an important factor in its ability to carry oxygen
what is the quaternary structure of haemoglobin
in which ll four polypeptides are linked together to form an almost spherical molecule
Each polypeptide is associated with a haem group - which contains Fe2+ ion
Each a total of four o2 molecules that can be carried by a single haemoglobin molecule in humans
what is loading
the process by which haemoglobin binds with oxygen is called loading/ associating
In humans, this takes place in the lungs
what is unloading
the process by which haemoglobin releases its oxygen is called unloading or dissociating
what happens when haemoglobin has a high affinity for O2
takes up O2 easily and release it less easily
what happens when O2 has a low affinity for O2
haemoglobin with a low affinity for oxygen take oxygen less easily, but release it more easily
what is the role of haemoglobin
the role of haemoglobin is to transport oxygen
what is the most efficient way that haemoglobin can transport oxygen
to be efficient at transporting oxygen, haemoglobin must:
- readily associate with oxygen at the surface where gas exchange takes place
- readily dissociate from oxygen at the tissues requiring it
how does haemoglobin associate and dissociate at the same time
it changes its affinity (chemical attraction) for oxygen under different conditions
It achieves this because its shape changes in presence of certain substances, such as CO2
In the presence of CO2, the new shape of hemoglobin binds more loosely to oxygen
- as a result the hemoglobin releases its oxygen
different organisms have different haemoglobin - what does this mean
it means that they take up oxygen differently
why do different haemoglobins have different affinities fr oxygen
each species produces a haemoglobin with a slightly different amino acid sequence
the haemoglobin of each species therefore has a slightly different tertiary binding properties
depending on its structure haemoglobin molecules range from those that have a high affinity for to those that have a low affinity for oxygen
under what conditions causes haemoglobin to bind unevenly with oxygen
when haemoglobin is exposed to different partial pressures of oxygen it does not bind the oxygen evenly
what is the graph showing the relationship between the saturation of haemoglobin with oxygen ad the partial pressure called
the oxygen dissociation curve
explain the shape of the oxygen dissociation curve i
1) the shape of the haemoglobin molecule makes it difficult for the 1st oxygen molecule to bind to one of the sites on its four polypeptide subunits because they are closely united
Therefore at low oxygen concentrations, little oxygen binds to haemoglobin
-the gradient of the curve is shallow initially
2) however, the binding of this first oxygen molecule changes the quaternary structure of the haemoglobin molecule causing it to change
This change makes it easier for the other subunits to bind to an oxygen molecule
3) It, therefore takes a smaller increase in the partial pressure of oxygen to bind the second oxygen than it did to bind the first one
- know n as positive cooperativity because binding of the first molecule makes binding of the first molecule make the binding of the 2nd easier and so on - the gradient STEEPENS
4) Even though in theory, binding to the fourth oxygen molecule should be easier, in practice it is harder
This is simply due to probability with the majority the binding sites occupied, it is less likely that a single oxygen molecule will find an empty site to bind to
- the gradient of the curve reduces and the graph FLATTENS off
each species has a different haemoglobin shape what does this mean
each has different shapes and therefore different affinity for oxygen
when does the shape of haemoglobin change
the shape of any haemoglobin molecule can change under different conditions
this means that there are a large number of different oxygen dissociation curves
- they all have a roughly similar shape but differ in their position on the axes
what must be kept in mind when we are reading a dissociation curve
- the further to the LEFT the curve, the greater is the affinity of haemoglobin for oxygen (so it loads readily but unloads it less easily)
- the further to the RIGHT the curve, the lower is the affinity of haemoglobin for oxygen (so it loads less readily but unloads it more easily)
if the graph is to the left then…
increases in pH in tissue (low CO2)
Large animals with low metabolic rate e.g. elephant
myoglobin (pigment in muscles)
foetal HB
if the graph is to the right then…
decreases in CO2 (high CO2)
what does haemoglobin do in the presence of CO2
haemoglobin has a reduced affinity for O2 in the presence of CO2
the greater the concentration of CO2, the more readily the haemoglobin releases its oxygen - this is known as the Bohr effect and explains why the behaviour of haemoglobin changes in different regions of the body
how does the behaviour of haemoglobin changes at the gas - exchange surface (e.g. lungs)
the concentration of CO2 is low because it diffuse across the exchange surface and excreted from the organisms
the affinity of haemoglobin for O2 is increased, which couples with the high concentration of O2 in lungs, means that O2 is readily loaded by haemoglobin
The reduced CO2 concentration has shifted the O2 dissociation to the left
how does haemoglobin’s behaviour change at rapidly respiring tissue
in rapidly respiring tissues (e.g. muscles), the concentration of CO2 is high
The affinity of haemoglobin for O2 is reduced, which coupled with the law concentration of O2 in the muscles, means that O2 is readily unloaded from the haemoglobin into the muscle cells
The increased CO2 concentration has shifted the O2 dissociation curve to the right
what does the greater the concentration of CO2 do to haemoglobin
the greater the concentration of CO2 the more readily haemoglobin releases its oxygen
This is because dissolved CO2 is acidic and the low pH causes haemoglobin to change shape
how does haemoglobin load oxygen at exchange surfaces
- at the gas - exchange surface CO2 is constantly being removed
- pH is slightly raised due to the low concentration of CO2
- the higher pH changes the shape of haemoglobin into one that enables it to load oxygen readily
- his shape also increases the affinity of haemoglobin for oxygen, so it is not released while being transported in the blood to the tissues
how does haemoglobin unload oxygen at respiring cells
- in the tissues, CO2 is produced by respiring cells
- CO2 is acidic in solution, so the pH of the blood with is the tissue is lowered
- the lower the pH changes the shape of haemoglobin into one with a lower affinity for O2
- Haemoglobin releases the oxygen into the respiring tissue
what would the process of loading and unloading oxygen from haemoglobin described as
- it can be described as a flexible way of ensuing that there is always sufficient oxygen for respiring tissue
the more active a tissue…
the more active a tissue, the more oxygen is unloaded
This works as follows:
1. higher rate of respiration - more CO2 the tissues produce and therefore the lower the pH.
This leads to oxygen being readily unloaded leading to more O2 is available for respiration
how much of the haemoglobin saturated with oxygen
in humans,haemoglobin normally becomes saturated with oxygen as it passes through the lungs
in practice, not all haemoglobin molecules are loaded with ttheirr maximum FOUR oxygen molecules
As a consequence, the overall saturation of haemoglobin at atmospheric pressure is normally 97%
what happens when haemoglobin reaches a tissue with a low respiratory rate
only one of the oxygen molecules are normally released
blood returning to the lungs will therefore contain haemoglobin that is still 75% saturated with O2
what happens when haemoglobin reaches a tissue with a high respiring rate
if the tissue is very active, e.g. exercising muscle, then three O2 molecules will usually be uploaded from each haemoglobin molecule
how has species haemoglobin evolved to suit their needs
e.g. species that live in an environment with a lower partial pressure of O2 gas evolved haemoglobin that has a higher affinity for O2 than haemoglobin of animals that live where the partial pressure of O2 is higher
give an example of a species that lives in an environment where the partial pressure of O2 is low
LUNGWORM is an animal that lives on the seashore lungworm are not very active, spending almost all their life in a U - shaped burrow
oxygen diffuses into the lungworms’ blood from the water and it uses haemoglobin to transport oxygen to its tissue
when the tide goes out, the lungworm can no longer circulate a fresh supply of oxygenated water through its burrow
as a result, the water in the burrow contains progressively less oxygen as the lungworm uses it up
The lungworm has to extract as much oxygen as possible from the water in the burrow if it is to survive until the tide covers it again
how does the dissociation curve of the lungworm look like
the dissociation curve is shifted fat to the left of that of a human
This means that the haemoglobin of the lungworm is fully padded with oxygen even though it is little available in its environment
what is another example of an organism that lives in an environment with a low affinity
another example is the llama - an animal that lives at high altitude
at these altitudes, the atmospheric pressure is lowered and so the partial pressure is also lower
It is therefore difficult to load haemoglobin with O2
- LLamas also have a type of haemoglobin that has a higher affinity for O2 than human haemoglobin
how does the dissociation graph of llamas look like
it is shifted to the left
what happens to the exchange of material as organisms get bigger
all organisms exchange materials between themselves and the environment
small organisms this exchange takes place over the surface of the body
HOWEVER, with increasing size, the surface area to volume ratio decreases to a point where the needs of the organism cannot be met by the body surface alone
what do specialist exchange do
they absorb required nutrients and respiratory gases and remove excretory products
where are exchange surfaces located
specific regions of the organism
why do we need a transport system
a transport system is required to take materials to exchange surfaces and from exchange surfaces to cells
materials have to be transported between exchange surfaces and the environment
They also need to be transported between different parts of organisms
whether or not there is a specialised medium and a circulated pump depends on…
- the surface to area to volume ratio
- how active the organism is
the lower the surface area to volume ratio is, and the more active the organisms is, the greater is the need for a specialised system with a pump
what are the features of a transport system
- a suitable medium in which to carry materials e.g. blood
- a form of mass transport in which the transport medium is moved around in bulk over large distances - more rapid diffusion
- a closed system of tubular vessels that contains the transport medium and forms at branching network to distribute it to all parts of the organism
- a mechanism for moving the transport medium within vessels
This requires a pressure difference between one part an the other
what is usually the transport medium
it is normally a liquid based on water because water readily dissolves substances and can be moved around easily, but can be a gas such as an air breather in and out of the lungs
how are the features of a transport system achieved
in two main ways:
a) animals use muscular contraction either of the body muscles or of a specialised pumping organ, such as the heart
b) Plants rely on natural, passive processes such as the evaporation of water
e. g.
- a mechanism to maintain the mass flow movement in one direction, e.g. valves
- a means of controlling the flow of transport medium to suit the changing needs of different parts of the organisms
- a mechanism for the mass flow of water or gases, e.g. intercostal muscles and diagram during breathing in mammals
what is the circulatory system in mammals
mammals have a CLOSED CIRCULATORY SYSTEM in which blood is confined to vessels and passes twice through the heart
for each complete circuit of the body, its low pressure would make circulation very slow - blood is thereof returned to the heart to boost its pressure before being circulated t the rest of the tissue
what does the double pump system mean for the speed of substances being delivered to the body
As a result, substances are delivered to the rest of the body quickly, which is necessary as mammals have a high body temp and hence a high rate of metabolism
what is the heart
a muscular organ that lies in the thoracic cavity behind the sternum (breast bone)
It operates continuously and tireless throughout the life of a organism
what is the structure of the human heart
is really TWO SEPARATES PUMPS lying side by side
the pump on the left deals with OXYGENATED BLOOD
the pump on the right deals with DEOXYGENATED BLOOD
each pump has two chambers:
the ATRIUM
the VENTRICLE
what is the atrium
the atria RECIEVES blood from the VEINS
the atrium is thin-walled and elastic and stretches as it collects blood
what are the ventricles
the ventricles pump blodd AWAY form the heart
the ventricle has a much thicker muscular wall as it has to contract strongly to pump blood some distance, either to the lungs or the rest of the body
why not just pump the blood through the lungs to collect oxygen and then straight to the rest of the body before returning to the heart
if that was the case, the blood has to pass through tiny capillaries in the lungs in order to present a large surface area for the exchange of gases
in doing so, there is a very large drop in pressure and so the blood flow to the rest of the body would be very slow
how does the body maintain a high blood pressure
to increase the pressure of the blood, mammals have a system where blood is returns to the heart to increase pressure before it is distributed to the rest of the body
it is essential to keep the oxygenated blood in the pump on the left side separate from the deoxygenated blood in the pump on the right
where does the right ventricle pump to
the right ventricle pumps blood to the lungs and it has a thinner wall than the left ventricle
why does the left ventricle have a thick muscular was
it enables it to contract to create pressure to pump blood to the rest of the body
how much blood do the atria and ventricles pump
both atria contract together and both ventricles contract together
pumping the SAME VOLUME OF BLOOD
what are between each atrium and ventricle
valves
what do valves do
valves prevent the backflow of blood into the atria when the ventricles contract
what are the two main valves
the left atrioventricular (bicuspid) valve
the right atrioventricular (tricuspid) valve
heart
what are each chamber in the heart connected to
each of the four chambers of the heart is connected to large blood vessels that carry blood TOWARDS or AWAY from the heart
what are the vessels connected to the heart
the:
AORTA
VENA CAVA
PULMONARY ARTERY
PULMONARY VEIN
what is the aorta
the aorta is connected to the left ventricle and carries oxygenated blood to all parts of the body except the lungs
what is the vena cava
the vena cava is connecting to the right atrium and brings deoxygenated blood back from the tissues of the body (except the lungs)
what is the pulmonary artery
the pulmonary artery is connected to the right ventricle and carries deoxygenated blood to the lungs, where its oxygen is replenished and its carbon dioxide is removed
Unusually for an artery, it carries deoxygenated blood
what is the pulmonary vein
the pulmonary vein is connected ti the left atrium and brings oxygenated blood back from the lungs
unusually for a vein, it carries oxygenated blood
how do we supply the heart with oxygen
although oxygenated blood passes through the left side of the heart, the heart does not use this oxygen to meet its own great respiratory needs
Instead, the heart muscle is supplied by its own blood vessels called CORONARY ARTERIES, which branch off of the aorta shortly after it leaves the heart
what happens when the coronary arteries get blocked
blockage of these arteries, e.g. by a blood clot leads to myocardial infection, or heart attack because an area of the heart muscle is deprived of blood and therefore oxygen also
- the muscle cells are unable to respire aerobically
what are the different types of blood vessels
- arteries
- carries blood AWAY from the heart into arterioles
arterioles
-smaller arteries that control blood flow from arteries to capillaries
capillaries
-tiny vessels that link arterioles to veins
veins
-carry blood from capillaries BACK to the heart
what do all vlood vessesl have in common
tough fibrous outer layer
muscle layer
elastic layer
thin inner lining (endothelium)
lumen
what differs between each type of blood vessel is the relative proportions of each layer -this is due to the difference in the function of each type of vessels of performs
what is the function of the arteries
the function of arteries is to transport blood rapidly under high pressure from the heart to the tissues
how are arteries adapted for their role
the muscle layer is thick compared to veins.
- This means smaller arteries can be constricted and dilated in order to control the volume of blood passing through
the elastic layer is relatively thick compared to veins
- it is important that blood pressure in arteries is kept high if the blood is to reach the extremities of the body
The elastic wall is stretched at each beat of the heart (systole).
It then springs back when the heart releases (diastole)
This stretching and recoil action helped to maintain high pressure and smooth surges created by the beating of the heart
the overall thickness of the wall is great
this also resists the vessels bursting under pressure
there are no valves
except in the arteries leaving the heart because blood is under constant high pressure due to the heart pumping blood into the arteries. It, therefore, tends not to flow backwards
what is the function of arterioles
arterioles carry blood under lower pressure than arteries, from arteries to capillaries
They also control the flow of blood between the two
how does the structure of arterioles help them to carry out their function
- the muscle layer is relatively thicker than in arteries
- the contraction of this muscle layer allows constriction of the lumen of the arteriole. This restricts the flow of the bold and so controls its movement into the capillaries that supply the tissues with blood
- the elastic layer is relatively thinner than in arteries because blood pressure is lower
what is the function of veins
veins transport blood slowly, under low pressure, from the capillaries in tissue to the heart
how does the structure of the veins relate to the function
- the muscle layer is relatively thin compared to arteries because veins carry blood away from tissues and therefore their constriction and dilation cannot control the flow of blood to the tissue
- the elastic layer is relatively thin compared to arteries because the low pressure of the blood within the veins will not cause them to burst and pressure is too low to create a recoil action
- the overall thickness of the wall is small because there is no need for a thick wall as the pressure of blood within the veins will not cause them to burst and pressure is too low to create a recoil action
- there are valves at intervals throughout to ensure that bloods does not flow backwards, which it might otherwise due to the low pressure
When body muscles contract, veins are compressed pressurising the blood within them.
The valves ensure that this pressure directs the blood in one direction only: towards the heart
what is the function of the capillary
the function of the capillaries is to exchange metabolic materials such as O2, CO2 and glucose between the blood and the cells of the body
The flow of the blood in capillaries is much slower
This allows more time for the exchange of materials
how does the structure of capillaries relate to its function
- their walls consist of mostly the lining layer making them extremely thin, so the distance over which diffusion tales place is short
This allows rapid diffusion of materials between the blood and the cells - they are numerous and highly branched
this provides a large surface area for exchange - they have a narrow diameter and so permeate tissues which means that no cell is far from a capillary and there is a short diffusion pathway
- their lumen is so narrow that red blood cells are squeezed flat against the side of a capillary. This brings them even closer to which they supply oxygen. This again reduces the diffusion distance
there are spaces between the lining (endothelial) cells that allow white blood cells to escape in order to deal with infections within tissues
why do we need tissue fluid
although capillaries are small, they cannot serve every single cell directly
Therefore the final journey of metabolic materials is made in a liquid solution that bathes the tissues
-tissue fluid therefore a means by which materials from the tissues are exchanged between blood and cells
This liquid is called tissue fluid
what is tissue fluid
is a watery liquid that contains glucose, amino acids, fatty acids ions in solution and oxygen
tissue fluids supplied all of these substances to the tissues
after giving the tissues oxygen, glucose e.t.c what does tissue fluids receive
in return, they receive CO2 and waste materials from the tissues
what is tissue fluids formed from
tissue fluid is formed from blood plasma
what is blood plasma controlled by
it is controlled by various homeostatic systems
as a result, tissue fluid provides a mostly constant environment for the cells it surrounds
how is tissue fluid formed
- blood is pumped by the heart
passes along arteries, then the narrower arterioles and finally the narrower capillaries
pumping by the heart created a hydrostatic pressure at the arterial end of the capillaries
The hydrostatic pressure causes tissue fluid to move out of the blood plasma
the outward pressure is, however, opposed by two other forces:
- the lower water potential of the blood, due to the plasma protein that causes water to move back into the blood within the capillaries
the hydrostatic pressure of the tissue outside the capillaries which resists outward movement of liquid
what causes the tissue fluids to move out of the capillaries
the hydrostatic pressure is greater than the osmotic force so molecules are forced out of the arteriole end of the capillary
what are the two factors that oppose the outward pressure
the outward pressure is, however, opposed by two other forces:
- the lower water potential of the blood, due to the plasma protein that causes water to move back into the blood within the capillaries
the hydrostatic pressure of the tissue outside the capillaries which resists outward movement of liquid
what does the combined effect of all these factors create
the combined effect of all these factors is to create an overall pressure that pushed tissue fluid out of the capillaries at the arterial end
however, this pressure is only enough to force small molecules out of the capillaries
-therefore leaves cells and proteins in the blood because these are too large to cross the membranes
This type of filtration under pressure is called ultrafiltration
how does tissue fluid return to the circulatory system
once tissue fluid has exchanged metabolic materials with the cells it bathes ( as in cells live in tissue fluid) it is returned to the circulatory system
MOST TISSUE FLUID returns to the blood plasma directly via the capillaries
This return occurs as follows:
1. the loss of the tissue fluid from the capillaries reduces the hydrostatic pressure inside them
- as a result, by the time the blood has reacted to the venous end of the capillary network its hydrostatic pressure, is usually lower than that of the tissue fluid outside it
- therefore tissue fluid is forced back into the capillaries by the higher hydrostatic pressure outside them
- in addition the plasma has lost water and still contains proteins. It is therefore has a lower water potential than the tissue fluid
- as a result, water leaves the tissue by osmosis down a water gradient
what happens to the remainder of the tissue fluid
not all of the tissue fluid can return to the capillaries; the remainder is therefore carried back via the LYMPHATIC SYSTEM
how does the lymphatic system look
initially, they resemble capillaries (except that they have dead ends)
- they gradually merge into larger vessels that form a network through the body
where do the vessels drain their contents
these large vessels drain their contents back into the bloodstream via tow ducts that join to the heart (thoracic duct)
how are the contents of the lymphatic system moved
- it is moved by the pumping of the heart
- the hydrostatic pressure of the tissue that has left the capillaries
- contraction of body muscles that squeeze the fluid inside them away from the tissue in the direction of the heart
what is the cardiac cycle
the heart undergoes a sequence if events that is repeated in humans around 70 times each minuted when at rest
this is known as the cardiac cycle
what are the two phases to the beating of the heart
two phases to the heart beating:
contraction (systole)
relaxation (diastole)
where does contraction occur
contraction occurs separately in the ventricle and the atria and is therefore described in twode stages
when does relaxation take place
at the same time of constriction, relaxation takes place simultaneously in all chambers of the heart and is therefore treated as a single phase
describe what happens when the heart relaxes
- blood returns to the atria of the heart through the pulmonary bein (from the lungs) and the vena cava (from the body)
- atria become filled causing the pressure within them to rise
- when this pressure exceeds that in the ventricles, the atrioventricular valves open allowing the blood to pass into the ventricles - the muscular walls of both the atria and the ventricles are relaxed at this stage
- the relaxation of these ventricles causes them to recoil and reduces the pressure within the pulmonary artery closeventricle - this causes the pressure to be lower than the pressure within the aorta and the pulmonary artery and so the semi - lunar valves in the aorta and the pulmonary artery close accompanied by the characteristic “dub sound” if he heart beat
how do the atria contract
the contractions if the atria walls along with the recoil of the relaxed ventricle walls, forces the remaining blood into the ventricles from the atria
throughout stage the muscles of the ventricle walls remains relaxed
how do the ventricles contract
after a short delay to allow the ventricles to fill with blood, their walls contract simultaneously
This increases the blood pressure within them, forcing shut the atrioventricular valves and preventing backflow
the “lub” sound of those valves closing is a characteristic of the heart
atrioventricular valves close, the pressure in the ventricles rise further
Once pressure is greater than the pulmonary artery, blood is forced from the ventricles into these vessels
- ventricles have thick muscular walls mean they contract forcefully
This creates the high pressure necessary to pump blood around the body
why do the left ventricle have a thick wall and why does the right ventricle have thinner walls
thick wall of the left ventricle has to pump blood to the extremities of the body while the relatively thinner wall of the right ventricle has to pump to the lungs
what does pressure have to do with the flow of blood
blood is kept flowing onde direction through the heart and around the body by the pressure created by the heart
blood always will move from a region of higher pressure to one of lower pressure
why are valves important
there are situations within the circulatory system when the pressure difference results in the blood flowing in the opposite direction to which is desirable
in these cases, the valves close
what are the different valves in the hear
atrioventricular valves
semi - lunar valves
pocket valves
where are atrioventricular valves
- they are between the left atrium and ventricle and the right atrium and the ventricle
what is the job of the atrioventricular valves
- prevents backflow of the blood when contraction of the ventricles means that ventricular pressure exceeds partial pressure
closure of these valves ensure that, when the ventricles contract, blood within them moves to the aorta and pulmonary artery rather than back to the atria
where are the semi - lunar valves
in the aorta and the pulmonary artery
what do the semi - lunar valves do
these prevent the backflow of blood into the ventricles when the pressure in these vessels exceeds that in the ventricles
This arises when the elastic walls of the vessels recoil increasing the pressure within them and when the ventricle walls relax reducing the pressure within the ventricles
where are pocket valves
they are in veins and occur throughout the venous system
what do pocket valves do
these ensure that when the veins are squeezed e.g. when skeleton muscles contract, blood flow back towards the heart rather than away from it
how do valves open and close
when the pressure is GREATER on the CONCAVE side than the CONVEX side, blood collects within the “bowl” of the cusps
This pushes them together to form a tight fit that prevents the [assage of blood
when pressure is GREATER on the CONVEX side of these cusps rather than the CONCAVE side, they move apart to let blood pass between the cusps
how do these valves look
basically the same
- all made up of a number f flaps of tough, but flexible, fiborous tissue, which are cusp shaped
what is a closed circulatory system
- mammals have a close circulatory system
- means that the blood is confined to vessels an this allows the pressure within them to be maintained and regulated
what is cardiac output
it is the total volume of blood pumped by one ventricle of the heart in one minute
it is usually measured in dm3 min-1 and depends upon two factors
how do you work out the cardiac output
cardiac output =heart rate x stroke volume
how is water absorbed in plants
water is absorbed by the roots through extensions called root hair
what is the xylem
it us a thick, dead walled tubes that water is transported through
what is the main force that pulls water through the stem of a plant
evaporation of water from leaves - this process is called transpiration
the energy for this is supplied by the sun and the process is, therefore, a passive process
how does water move through the stomata
the humidity of the atmosphere is usually less than that of the air spaces next to the stomata
THEREFORE, the water potential gradient from the air spaces through the stomata to the air
Provided that the stomata are open, water vapour diffuses out of the air spaces into the surrounding air
water lost by diffusion from the air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells
how do plants control the rate of transportation
by changing the size of the stomatal pores, plats can control their rate of transpiration
how is water replaced after it was lost through transpiration
water is lost from mesophyll cells evaporation from their cell walls to the air spaces of the leaf
it is replaced by water reaching the mesophyll cells from their xylem either via cell walls/ via the cytoplasm
what is the cytoplasmic route of the water
in the case of the cytoplasmic route, the movement occurs because:
- mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun
- these cells now have a lower water potential and so water enters by osmosis from neighbouring cells
- the loss of water from these neighbouring cells lowers their water potential
- they, in turn, take in water from their neighbouring cells lower their water potential
In this way, a water potential gradient is established that pulls water from the xylem, across the leaf mesophyll, and finally out into the atmosphere
what is the main factor that is responsible for the movement of water up the xylem
main further that is responsible for the movement of water up the xylem, from the roots to the leaves is COHESION - TENSION
what is the movement of water up the stem
the movement of water up the stem occurs as follows:
- water evaporated from mesophyll cells due to heat from the sun leading to transportation
- water molecules form hydrogen bonds between one another and hence tend to stick together
- this is known as cohesion
- water forms a continuous,l unbroken column across the mesophyll cells and down the xylem
- as water evaporates from the mesophyll cells in the leaf into the air spaces beneath the stomata, more molecules of water are drawn up behind it as a result of this COHESION
- a column of water is, therefore, pulled up the xylem as a result of transpiration. This is called TRANSPIRATION PULL
- transpiration pull puts the xylem under tension, that’s a negative pressure within the xylem hence the name COHESION TENSION THEORY
- the force of transpirational pull can easily raise water up to 100m or more of the tallest trees
what is the evidence to support the cohesion - tension theory
-change in diameter of tree trunks according to the rate of transpiration
DURING THE DAY:
-wheen transpiring is at its greatest, there is more tension (more negative pressure) in the xylem
this pulls the wall of the xylem vessels inwards and causes the trunk to shrink in diameter
DURING THE NIGHT
- transpiration is at its lowest, there is less tension in the xylem and so the diameter of the trunk increases
what happens if the xylem of the tree breaks
If the xylem vessel is broken and air enters the tree it can no longer draw up water
This is because the continuous column of water is broken and so the water molecules can no longer stick together
when the xylem is broken, water does not leak out, as would be the case if it were under pressure.
Instead, the air is drawn in, which is consistent with it being under tension
why can’t the xylem actively move the water
xylem which the water passes are dead and so cannot actively move the water
Energy is nevertheless needed to drive the process of transpiration
This energy is in the form of heat that evaporates water from the leaves and it ultimately comes from the sun
how is the structure of the xylem essential for its function
xylem vessels have no end walls which means that the xylem forms a series of continuous unbroken tubes from root to leaves, which is essential to the COHESION TENSION THEORY
what is translocation
what is the process by which organic molecules and some mineral ions are transported from one part of a plant to another is called translocation
what is the tissue that transports biological molecules
in flowering plants, the tissue that transports biological molecules is called phloem
what is the phloem made up of
it is made up of sieve tube elements- long thin structures arranged end to end
end walls are perforated t form sieve plates associated with the sieve tube elements are cells called COMPANION CELLS
what happens once the plant produces sugars during photosynthesis
having produced sugars during photosynthesis, the plant transports them from the sites of production known as sources to the places they will be used directly or stored for future use - known as sinks
where are the sinks in a plant
hey can be anywhere in a plant, sometimes above and sometimes below the source
therefore, it follows that translocation of molecule in phloem can be in either direction
what are the organic molecules that are transported in translocation
organic molecules that can be transported include sucrose and amino acids
the phloem also transports inorganic molecules such as potassium, chloride, phosphate and magnesium ions
what is the mechanism of translocation
- current thinking favours the mass flow theory which can be divided into three phases:
1. TRANSFER OF SUCROSE INTO SIEVE ELEMENTS FROM PHOTOSYTHESISING TISSUE
2. MASS FLOW OF SUCROSE THROUGH SIVE TUBE ELEMENTS
3. TRANSFER OF SUCROSE FROM TH SIVE TUBE ELEMENTS INTO STORAGE OR OTHER SINK CELLS
what is the first phase of mass translocation
- TRANSFER OF SUCROSE INTO SIEVE ELEMENTS FROM PHOTOSYTHESISING TISSUE
- sucrose is manufactured from the products of photosynthesis in cells with chloroplasts
- the sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells
- hydrogen ions actively transported from companion cells into the spaces within cell walls using ATP
- these hydrogen ions then diffuse down a concentration gradient carrier proteins into the sieve tube elements
- sucrose molecules are transported along with the hydrogen ions in a process known as co - transport
The protein carried are therefore also known as co - transported
what is the second phase of mass flow the sieve tubes
- the sucrose produced by photosynthesising cells (source) is actively transported into the sieve tubes as described above
- This causes the sieve tubes to have a lower ( more negative) water potential
- as the xylem has a much higher (less negative) water potential, water moves from the xylem into the sieve tubes osmosis, creating a high hydrostatic pressure within them
- at the respiring cells (sinks), sucrose is either used up during respiring or converted to starch for storage
- these cells, therefore, have a low sucrose content and so sucrose is actively transported into them from the sieves lowering their potential
- due to this lowered water potential; water also moves into these respiring cells, from the sieve tubes, by osmosis
- the hydrostatic pressure of the sieve tubes in this region is therefore lowered
- as a result of water entering the sieve tube elements at the source and leaving at the sin, there is a high hydrostatic pressure and a low one at the sink
- there is therefore a mass flow of sucrose solution down this hydrostatic gradient in the sieve tubes
what is stage three of mass transport
the sucrose is actively transported by companion cells out of the sieve tubes and into the sink cells
what kind of process is mass flow
mass flwo is a passive process it occurs as a result of the active transport of sugars
Therefore the process as a whole is active which is why it is affected by, e.g. temperature and metabolic poisons
A model of this theory is below
cohesion and tension
- water evaporates from the leaves at the “top” of the xylem. This is a process called transpiration
2, this creates tension (suction), which pulls more water into the leaf
- water molecules are cohesive so when some are pulled into the leaf others follow. This means the whole column of water in the xylem, from the leaves down to the roots, moves upwards
- water then enters the stem through the roots
what are factors that affect the rate of transpiration
- light intensity
- temperature
- humidity
- wind
why does light intensity affect the rate of transpiration
the lighter it is the faster the rate of transpiration
this is because the stomata open when it gets lighter to let in CO2 for photosynthesis
when it is dark the stomata are usually close so there is little transpiration
why does temperature affect the rate of transpiration
the higher the temperature the faster the transportation rate
Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster
This increases the water potential gradient between the temperature making water diffuse out of the leaf faster
why does humidity affect the rate of transpiration
the lower the humidity the faster the rate of transpiration
The water potential gradient between the leaf and the air is increased which increases the are of transpiration
why does wind affect the rate of transpiration
the windier it is the faster the rate of transpiration
Lots of air movement blows away water molecules from around the stomata
This increases the water potential gradient which increases the rate of transcription
what is translocation
translocation is the movement of solutes to where they are needed in a plant
Solutes are sometimes called assimilate
It is an energy-requiring process that happens in the phloem
Translocation moves solutes from sources to sinks
The source is where assimilates are produced so they are at high concentration
The sink is where assimilates are used up so they are at a low concentration
what is the mass flow hypothesis
SOURCE
1. active transport is used to actively load the solutes (e.g. sucrose from photosynthesis) from companion cells into the sieve cells of tubes, so water enters the tubes by osmosis from the xylem and companion cells
- this creates a high pressure inside the sieve tubes at the source end of phloem
SINK
3. at the end, solutes have removed the phloem to be used up. This increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis. This lowers the pressure inside the sieve tubes
FLOW
4, the result is a pressure gradient from the source end to the sink end
- this gradient pushes solutes along the sieve tubes towards the sink
- when they reach the sink the solutes will be used (e.g. in respiration) or stored (e.g. as starch)
supporting evidence for downward flow
- if a ring of bark (which includes the phloem, not the xylem) is removed from a woody stem, a bulge forms above the ring
The fluid from the bulge has a higher concentration of sugars than the fluid below the ring
This is because the sugars can’t move past the area where the bark has been removed
- this is evidence that there can be a downward flow
supporting evidence for pressure gradient
pressure in the phloem can be investigated using aphids (they pierce the phloem then their bodies are removed leaving the mouthparts behind which allows the sap to flow out)
the sap flows out quicker nearer the leaves than further down the stem
evidence from radioactive tracers
Translocation of solutes can be modelled in an experiment using radioactive tracers
This can be done by supplying part of the plant with an organic substance that has a radioactive label, and then tracking its movement
the movement of these substances can be tracked using a technique called autoradiography
To reveal where the radioactive tracer has spread in a plant, the plant is killed (e.g. by freezing it with liquid nitrogen) and is placed onto photographic film
where the film turns black, the radioactive substance is present
- shows the translocation of substances from source to sink over time
e. g. the autoradiography of plants killed at different times shows an overall movement of solutes (e.g. products of photosynthesis from the leaves towards the roots)
