B7 Flashcards
what are haemoglobins
protein molecules with a quaternary structure that makes it efficient at loading oxygen in one set of conditions and unloading it in another
structure of haemoglobin
primary, secondary, tertiary, quaternary
- globular protein
- hydrophillic side chains face outwards
–> haemoglobin = soluble so good for transport - 2 pairs of polypeptides
- linked to form a spherical molecule
–> 2 pairs: 4 chains. 2 a-helices, 2 B-pleated sheets. - conjugated protein- w prosthetic groups (haem)
primary
sequence of amino acids in the 4 polypeptide chains
secondary
each chain = coiled due yo hydrogen bonding
tertiary
each chain = folded in precise shape– important in its ability to carry oxygem
quaternary
all 4 polypeptides = linked to form almost spherical molecule
- hydrogen, ionic, disulfide, hydrophobic
each polypeptide = associated with a haem group, which contain a ferrous Fe2+ ion
- each Fe2+ ion can combine with a single O2 molecule, so 4 O2 molecules can be carried per hb molecule in humans
loading/ associating
the process by which hb binds with O2
takes place in lungs
unloading/ dissociating
the process by which hb releases its O2
hb with high O2 affinity…
- load easily
- unload less easily
hb with low O2 affinity…
- unload easily
- load less easi;y
the role of haemoglobin
- how it is efficient
role: to transport O2
to be efficient:
- readily associate w O2 at gas exchange surface
- readily dissociate w O2 at tissues requiring it
hb changes its affinity for O2 under different conditions
- its shape changes in the presence of certain substances e.g. CO2
–> in the presence of CO2, the new hb shape binds more loosely to O2 so it releases its O2
gas exchage surface
- oxygen conc
- carbon dioxide conc
- affinity of hb for O2
- result
high
low
high
o2 = associated
respiring tissues
- oxygen conc
- carbon dioxide conc
- affinity of hb for O2
- result
low
high
low
o2 = dissociated
why are there different haemoglobins
+
why do different hb have different affinities for O2
- different organisms require hb to more/less readily associate with o2 at different partial pressures
- different haemoglobins have different shaped molecule
- each species produces hb with a slightly different amino acid sequence
- the hb therefore has slighty different tertiary/quaternary structures
–> hence different O2 binding properties
hb conditions
partial pressure of O2 =
pH =
O2 concentration
CO2 concentration
change in structure of hb –> change in affinity for oxygen
- what are the two ways this change in structure occurs
- different types of hb have diff tertiary structure
- the same hb can change its structure and so its own affinity for O2 in diff environmental conditions
what is the oxygen dissociation curve
explanation of the shape of oxygen dissociation curves
when hb is exposed to different partial pressures of O2, it does not bind to the O2 evenly
the graph of the relationship between the saturation of hb with oxygen and the partial pressure of O2
- ‘tense’ state
- the shape of the hb molecule makes it difficult for the 1st O2 molecule to bind to one of the sites of its 4 polypeptides as they are closely united
—> so at low concentrations, little O2 binds to hb
the gradient of the curve = initially shallow
2+3. conformational change of quaternary structure
- the binding of the 1st O2 molecule changes the quaternary structure of the hb
- this makes it easier for other subunits to bind to O2
—> it therefore takes a smaller increase in the partial pressure of O2 to bind the 2nd O2 compared to the 1st
—> known as POSITIVE COOPERATIVITY
- after the 3rd O2 molecule, binding 4th = harder
—> due to probability
majority of binding sites = occupied, so less likely that a single O2 molecule will find an empty site to bind to
—> graph flattens off
how does the CO2 concentration affect hb’s affinity for O2
hb has reduced affinity for O2 in the presence of CO2
the greater the concentration of CO2, the more readily the hb releases its O2
= THE BOHR EFFECT
why does the behaviour of hb change in different regions of the body
- gas exchange surface
- rapidly respiring tissues
gas exchange surface
- e.g. lungs
- low CO2 conc as it diffuses across the exchange surfaces and is excreted from the organism
- the affinity of hb for O2 is increased
–> coupled with a high O2 conc in the lungs
so O2 is readily loaded by hb
* the reduced CO2 conc shifts the oxygen dissociation curve to the left
rapidly respiring tissues
- e.g. muscles
- high CO2 conc
- affinity of hb for O2 is reduced
—> coupled with low conc of O2 in muscles
so O2 is readily unloaded by hb
* the increased CO2 conc shifts the oxygen dissociation curve to the right
why does greater CO2 conc lead to hb unloading O2
dissolved CO2 = acidic
the low pH causes hb’s tertiary structure to change
the process of loading, transporting, unloading O2
- at the gas exchange surface, CO2 = constantly being removed
- the pH is slightly raised due to low conc of CO2
- the higher pH changes the shape of hb into one that enables it to load O2 readily
- this shape also increases the affinity of hb for O2, so it is not released when being transported in blood to tissues
- in tissues, CO2 = produced by respiring cells
- CO2 = acidic in solution
- so the pH of the blood within tissues = lowered
- the lower pH changes the shape of the hb to one with a lower affinity for O2
- hb releases its O2 into respiring tissues
the process of loading, transporting, unloading O2 in more active, exercising tissues
the higher the rate of respiration, the more CO2 the tissues produce + LACTIC ACID PRODUCED + HIGHER TEMPERATURE, the lower the pH, the greater the hb changes shape, the more readily the O2 = unloaded, the mored O2 = available for respiration
why is hb’s overall saturation not 100%
what happens when hb reaches a tissue w a low respiratory rate
what happens when hb reaches an active tissue
- in humans, hb usually becomes saturated with O2 as it passes through the lungs
- in practice not all hb molecules are loaded with their maximum 4 O2 molecules
- as a consequence, the overall saturation of hb at atm = around 97%
- when this hb reaches a tissue with a low respiratory rate, only one of these molecules will normally be released
- the blood returning to the lungs will therefore contain hb that is still 75% saturated with O2
- when hb reaches a very active tissue, 3 O2 molecules will usually be unloaded from each hb
why do different species have different types of hb
types of hb have evolved within species as adaptations for different environmental conditions
e.g. a species of animal that lives in an environment with a lower pp of O2 have evolved hb that has a higher affinity for O2
lugworm and hb
- the lugworm is not very active, spending most of its time in a u-shaped burrow
- most of the time the lugworm is covered by sea water, which it circulates through its burrow
- O2 diffuses into the lugworm’s blood from the water and it uses hb to transport O2 to tissues
- when the tide goes out, the lugworm can no longer circulate a fresh supply of oxygenated water through its burrow
- as a result, the water in the burrow contains progressively less O2 as the lugworm uses it up
- the lugworm has to extract as much O2 as possible from the water in the burrow if it is to survive until the tide comes in again
- the O2 dissociation curve of lugworm hb is shifted far to the left than that of humans
–> hb of lugworm = fully loaded even if there is little O2 available
llama and hb
- lives at high altitudes
- at these altitudes, atm = lower so pp of O2 = lower
- it is therefore difficult to load hb with O2
- llamas have a type of O2 that has a higher affinity for O2 than humans- so O2 dissociation = shifted to left
why do large organisms have a transport system
- all organisms exchange materials between themselves and their environment
- in small organisms, this exchange can take place by diffusion over the body surface
- however, increasing in size, the surface area to volume ratio decreases, so the needs of the organism cannot be met by the body surface alone
- specialist exchange surfaces are needed to absorb nutrients and respiratory gases, and remove excretory products
- the exchange surfaces are located in specific regions of the organism
- a transport system is required to take materials from cells to exchange surfaces and vice versa
- materials also need to be transported between different parts of an organism
- as organisms have evolved to be more complex, their tissues and organs have become more specialised and dependent on each other
—> so transport systems = essential
whether or not there is a specialised transport medium, and whether or not is circulated by a pump is dependent on 2 factors:
the surface to volume ratio
the metabolic rate / how active the organism is
features of transport systems
- a suitable medium in which to carry materials
e.g. blood - normally a liquid based on water, because water readily dissolves substances that can be moved around easily, but can be a gas that is breathed out of the lungs
- a form of mass transport in which the transport medium is moved around in bulk over large distances
- more rapid than diffusion
- a closed system of tubular vessels that contains the transport medium
- and forms the 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 of the system and another
[achieved in two main ways:] - animals use muscular contraction either of the body muscles or of a specialised pumping organ, such as the heart
- plants rely on natural, passive processes such as the evaporation of water
- a mechanism to maintain the mass flow movement in one direction
e.g. valves - a means of controlling the flow of the transport medium to suit the changing needs of different parts of the organism
- a mechanism for the mass flow of water or gses
e.g. intercostal muscles + diaphragm
circulatory system in mammals
- why double circulatory?
- closed, double circulatory system
- blood is confined to vessels and passes twice through the heart for each complete circuit of the body
this is because, when blood is passed through the lungs, its pressure is reduced
- the blood has to pass through capillaries in the lungs in order to present a large surface area for gas exchange
- if it were to pass immediately to the rest of the body, its low pressure would make circulation very slow
- blood is returned to the heart to boost its pressure before being circulated to the rest of the tissue
- as a result, substances are delivered to the rest of the body quickly, which is necessary as mammals have high body temperatures and so a high metabolic rate
the vessels that make up the mammalian circulatory system = arteries, veins, capillaries
what are the two circulatory systems in the double circulatory system?
pulmonary circulation system (heart –> lung)
- oxygenates, removes O2 from the blood
systemic circulation system (heart –> rest of the body)
- oxygenated blood is pumped rapidly at increased pressure
what controls blood flow
heart rate
vasoconstriction/ vasodilation
valves
the structure of the heart
the heart is a muscular organ that lies in the thoracic cavity behind the sternum
left pump: oxygenated blood from lungs
right pump: deoxygenated blood from body
each pump has 2 chambers;
- atrium: thin walled and elastic and stretches as it collects blood
- ventricle: has much thicker muscular wall as it has to contract strongly to pump blood some distance, to lungs/body
although the two sides of the heart are in separate pumps, they pump in time with each other
both atria then both ventricles. pumping the same volume of blood
between each atrium + ventricle = valves that prevent backflow of blood into the atria when ventricles contract
- left atrioventricular - BICUSPID
- right atrioventricular - TRICUSPID
each of the 4 chambers of the heart = connected to blood vessels
- ventricles pump blood away from the heart into the arteries
- atria recieve blood from veins
why is the left ventricle wall thicker than the right?
the right ventricle pumps blood only to the lungs
the left ventricle has a thicker muscular wall that enables it to contract to create enough pressure to pump blood to the rest of teh body
pulmonary vessels
vessels connecting the heart to the lungs
aorta: connected to lv, carries oxygenated blood to body
vena cava: connect to ra, brings deoxy blood back from tissues
pulmonary artery: connected to rv, carries deoxy blood to lungs
pulmonary vein: connected to la, brings oxy blood back from lungs
supplying the heart muscle with O2
+ blockage of coronary arteries
- although oxy blood passes through the left side of the heart, it does not use this oxygen to meet its own respiratory needs
- the heart muscle = supplied with its blood vessels- coronary arteries, which branch off the aorta shortly after it leaves the heart
blockage of these arteries e.g. by blood clot, leads to a myocardial infarction, because an area of the heart muscle is deprived of blood and therefore oxygen
—> the muscle cells in this region are unable to respire aerobically so die
ATHEROMA: blockage of the coronary artery due to cholesterol/ fat/ calcium
cigarette smoking and a diet high in saturated fat increase the risk of myocardial infarction. explain how
- carbon monoxide combines with haemoglobin, causing less oxygen to be transported in the blood
- less O2 carried to myocardium muscle
- no O2 can reach the working cardiac muscle cells
- areas of the myocardium muscle + heart tissue die- as they are unable to respire aerobically
- causes myocardial infarctions
- eating a diet high in saturated fat causes deposits of fat/ cholesterol to be formed in coronary arteries
- this is atheroma formation- causing the constriction of coronary arteries
- this blocks blood flow to the heart muscle, so less O2 transported to the heart muscle.
journey of blood from vena cava –> aorta
VENA CAVA carries deoxygenated blood from the body–> right atrium
- superior- transports blood from head
- inferior- transports blood from rest of body
RIGHT ATRIUM collects deoxygenated blood and pumps it to RIGHT VENTRICLE
ATRIOVENTRICULAR VALVES: thin flaps of tissue in cup shape
- forced open by the pressure of blood moving into the ventricle
- when the ventricles contract, the valves fill with blood and remain closed
–> ensures blood does not flow back into atria
TENDINOUS CHORDS:
- string-like cords, attached to the walls of the ventricles, prevent valves turning inside out
RIGHT VENTRICLE: pumps deoxygenated blood to the lungs
through PULMONARY ARTERY
SEMILUNAR VALVES:
situated at the base of the major arteries prevent blood returning to heart
SEPTUM: separates the left and right sides of the heart
- ensures oxygenated and deoxygenated blood do not mix
PULMONARY VEINS:
carry oxygenated blood from lungs to LEFT ATRIUM
- la pumps blood –> LEFT VENTRICLE
left ventricle pumps blood to body via AORTA
ATRIOVENTRICULAR VALVES:
- prevent backflow of blood into the atria when the ventricles contract
the cardiac cycle: an overview
- the heart undergoes a sequence of events that is repeated in humans around 70 times per minute whilst at rest
- 2 phases to the beating of the heart: contraction (systole) and relaxation (diastole)
–> contraction occurs separately in ventricles and atria
–> for some time, relaxation takes place simultaneously in all chambers
- relaxation of the heart- diastole
blood returns to the ATRIA of the heart through the pulmonary vein (lungs) and vena cava (body)
- as the atria fill. the pressure in them rises
- when this pressure exceeds that of the ventricles. the ATRIOVENTRICULAR VALVES open, allowing the blood to pass into the VENTRICLES
- the passage of blood is aided by gravity
- the muscular walls of both the ATRIA and VENTRICLES are RELAXED at this stage
- the relaxation of ventricle walls causes them to recoil, reducing the pressure within the ventricle
- this causes the pressure to be lower than in the aorta and pulmonary artery, so the SEMI-LUNAR VALVES close, preventing backflow from arteries into ventricles down pressure gradient
- causes characteristic ‘dub’ sound of heartbeat
- contraction of atria (atrial systole)
- the contraction of the atrial walls, along with the recoil of the relaxed ventricle walls, forces the remaining blood into the ventricles from the atria
- throughout this stage, the muscle of the ventricle walls remains relaxed.
- contraction of the ventricles (ventricular systole)
- after a short delay to allow the ventricles to fill with blood, the ventricle walls contract simultaneously
- this increases the pressure within them, forcing shut the ATRIOVENTRICULAR VALVES and preventing backflow of blood into the atria
- the ‘lub’ sound of these valves closing is characteristic of the heartbeat
- with the AV valves closed, the pressure in the ventricles rises further
- one it exceeds that in the aorta in the pulmonary artery, blood is forced from the ventricles into these vessels
- the ventricles have thick muscular walls, which mean they contract forcefully
- this creates a high pressure necessary to pump blood around the body
- the thick wall of the LV has to pump blood to the extremities of the body.
valves in the control of blood flow
- blood is kept flowing in one direction through the heart and around the body by pressure created by the heart muscle
- blood will always flow from a region of higher pressure to one of lower pressure
–> however there are situations in the circulatory system when pressure differences would result in blood flowing in the wrong direction
–> this is when valves = used to prevent backflow of blood - valves are designed so they open whenever the difference in blood pressure either side favours movement of blood in the required direction
atrioventricular, semilunar, pocket
atrioventricular valves
between L/R A/V
- prevent backflow of blood when contraction of the ventricles means that ventricular pressure exceeds atrial pressure
- closure of these valves ensures that, when the ventricles contract, blood within them moves to the aorta and pulmonary artery rather than the atria
semilunar valves
in the aorta and pulmonary artery
- prevent backflow of blood into the ventricles when the pressure in these vessels exceeds that of the ventricles
- this arises when the elastic walls of the vessels recoil, increasing the pressure within them, and when the ventricle walls relax, reducing pressure in ventricles
pocket valves
in veins throughout venous system
- ensure that, when veins are squeezed i.e. when skeletal muscles contract, blood flows towards the heart
- made of flaps of tough, flexible fibrous tissue, cusp shaped
- when pressure = greater on convex side, open
- when pressure = greater on concave side, blood collects within dome of cusp
—> valves close
cardiac output
the volume of blood pumped by ONE VENTRICLE of the heart in one minute
measured in dm^3 min^-1
depends on:
- heart rate
- stroke volume - vol of blood pumped out each beat
appearance of electrocardiogram ECG
- healthy heart
- myocardial infarction
- fibrillation
- regular peaks + troughs- repeated in identical way
- less pronounced peaks, larger troughs, repeated in similar but not identical way
- heart muscle contracts in a disorganised way- irregular ECG
role of arteries
carry blood away from heart into arterioles
role of arterioles
smaller arteries that control blood flow from arteries –> capillaries
role of capillaries
tiny vessels that link arterioles to veins
role of veins
carry blood from capillaries back to heart
components of the basic layered structure of arteries, arterioles and veins
tough, fiborous outer layer: resist pressure changes from within + outside
muscle layer: can contract to control blood flow
elastic layer: helps to maintain blood pressure by stretching and recoiling
thin inner endothelium: smooth to reduce friction and thin to allow diffusion
lumen: central cavity through which blood flows
what differes between blood vessels is the RELATIVE PROPORTION of each layer
artery structure related to function
transports blood rapidly under high pressure from the heart to tissues
*thick muscle layer compared to veins
- smaller arteries can be constricted and dilated to control the volume of blood passing through them
*thick elastic layer compared to veins
- important that bp in arteries is kept high if blood is to reach extremities
- elastic wall is stretched at each beat of the heart (systole)
- it then springs back when the heart relaxes (diastole)
–> this stretch and recoil action helps to maintain high pressure and smooth pressure surges created by the beating of the heart.
*thick wall overall
- resists vessel bursting under pressure
*no valves
- EXCEPT semi-lunar valves in arteries leaving the heart
- because blood = under constant high pressure due to the heart pumping blood into arteries
- blood tends not to flow backwards
arteriole structure related to function
carry blood, under lower pressure than arteries, from arteries –> capillaries
*thick muscle layer compared to arteries
- the contraction of this muscle allows constriction of the lumen of the arteriole
- this restricts the flow of blood and so controls its movement into the capillaries that supply the tissues with blood
*thinner elastic layer compared to arteries
- bp = lower
vein structure related to function
transport blood slowly, under low pressure, from capillaries to tissues in the heart
*thin muscle layer compared to arteries
- veins carry blood away from tissues so their constriction/ dilation does not control blood flow to tissues
*thin elastic layer compared to arteries
- pressure is too low to create recoil or bursting
*overall thickness is small
- no need for a thick wall as the pressure is too low to risk bursting
- allows them to be flattened easily, aiding flow of blood
*valves at intervals throughout
- prevent backflow of blood
- when body muscles contract, veins are compressed, pressurising blood within them
- valves ensure this pressure directs the blood in one direction towards the heart
capillary structure related to function
to exchange metabolic materials e.g. oxygen, carbon dioxide, and glucose between blood + cells.
the flow of blood in the capillaries is much slower, allowing more time for exchange.
*walls consist mostly of lining layer
- extremely thin, so short diffusion distance
- allows for rapid diffusion
*numerous and highly branched
- increasing surface area for exchange
*narrow diameter
- so permeates tissues, so no cell is far away from a capillary, so short diffusion pathway..
*narrow lumen
- so RBC are squeezed flat against sides of capillary
–> bringing them even closer to the cells to which they supply oxygen
–> reducing diffusion distance
*spaces between endothelial cells
- allows WBC to escape in order to deal with infections within tissues
ALTHOUGH capillaries are small, they cannot serve every single cell directly
the final journey of metabolic materials is made in a liquid solution that bathes the tissues- tissue fluid
tissue fluid - composition +role
- a watery liquid that contains glucose, amino acids, fatty acids, ions, oxygen
- supplies all these substances to the tissues
- in return, receives CO2 and other waste materials from the tissues
- formed from blood plasma, composition of which is controlled by many homeostatic systems
–> providing a mostly constant immediate environment for cells
formation of tissue fluid
- blood pumped by the heart passes along arteries, then narrower arterioles and narrower capillaries
- pumping by the heart creates HYDROSTATIC PRESSURE at the arterial end of the capillaries
–> this causes tissue fluid to move out of the blood plasma
this outward pressure is, however, opposed by two forces:
- HYDROSTATIC PRESSURE OF THE TISSUE FLUID OUTSIDE CAPILLARIES
- resists outer movement of liquid (but net= out of capillaries) - THE LOWER WATER POTENTIAL OF THE BLOOD
- due to the plasma proteins, that causes water to move back into blood within capillaries
HOWEVER, the combined effect is to create an outward pressure that pushes tissue fluid our of the capillaries at the arterial end.
- this pressure is only enough to force small molecules out of the capillaries, leavgin all cells and proteins in the blood because they are too large to cross the membranes
- this type of filtration = ULTRAFILTRATION
return of tissue fluid to the circulatory system
- capillaries
- lymphatic system
once tissue fluid has exchanged metabolic materials with the cells it bathes, it is returned to the circulatory system
- most tissue fluid returns to the blood plasma directly via the capillaries:
- loss of tissue fluid from capillaries reduces the hydrostatic pressure inside them
- as a result, by the time the blood has reached the venous end of the capillary network, its hydrostatic pressure is usually lower than 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 but still contains proteins
- it therefore has a lower water potential than the tissue fluid
- as a result, water leaves the tissue fluid by osmosis down a water potential gradient
—> the tissue fluid has lost much of its oxygen and nutrients by diffusion into the cells that it bathed, but it has gained CO2 and waste materials in return
NOT ALL TISSUE FLUID CAN RETURN TO THE CAPILLARIES
- THE REMAINDER IS CARRIED BACK VIA THE LYMPHATIC SYSTEM
this is a system of vessels that begin in the tissues
- initially they resemble capillaries except they have dead ends, but they gradually merge into larger vessels that form a network throughout the body
- these larger vessels drain their contents back into the bloodstream via two ducts that join veins close to the heart
the contents of the lymphatic system are not moved by the pumping of the heart, instead:
- hydrostatic pressure of the tissue fluid thats left in the capillaries
- contraction of body muscles that squeeze the lymph vessels- valves in the lymph vessels ensure that the fluid inside them moves away from the tissues in the direction of the heart.
transport of water in the xylem- transpiration stream
MOVEMENT OF WATER OUT STOMATA
- The humidity of the atmosphere is usually less than that of the air spaces next to stomata
- As a result, there is a water potential gradient from the air spaces through the stomata to the air
- Provided the stomata are open, water vapour molecules diffuse 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
- By changing size of the stomatal pores, plants can control their rate of transpiration
MOVEMENT OF WATER ACROSS THE CELLS OF A LEAF
- Water is lost from mesophyll cells by evaporation from their cell walls to the air spaces of the leaf
- This is replaced by water reaching the mesophyll cells from the xylem, either via the cytoplasm or the cell walls
In the case of the cytoplasmic route, the water 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 cells lowers their water potential
- They, in turn, take in water from their neighbours by osmosis
So a water potential gradient is established that pulls water from the xylem, across the leaf mesophyll, and finally out into the atmosphere
MOVEMENT OF WATER UP STEM IN XYLEM
The main factor responsible is cohesion-tension
- Water evaporates from mesophyll cells due to heat from the sun leading to transpiration
- Water molecules form hydrogen bonds between one another and so stick together
o Cohesion - Water forms a continuous, 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
- The transpiration results in a column of water that is pulled up the xylem
o The transpiration pull - Transpiration pull puts the xylem under tension
o There is a negative pressure within the xylem
o Hence cohesion-tension theory
evidence to support cohesion-tension theory
*Change in diameter of tree trunks according to the rate of transpiration
o During the day when transpiration is greatest, there is more tension (negative pressure) in the xylem
o This pulls the walls of the xylem vessels inwards and causes the trunk to shrink in diameter
*If a xylem vessel is broken and air enters it, the tree can no longer draw up water
o The continuous column of water is broken, so water molecules no longer stick together
*When a xylem vessel is broken, water does not leak out, as would be the case if it was under pressure
o Instead, air is drawn in, consistent with it being under tension.
why is the transpiration pull passive
Transpiration pull is a passive process and therefore does not require metabolic energy to take place
- The xylem vessels are dead so cannot actively move the water
- Xylem vessels have no end walls
o Xylem forms a series of continuous, unbroken tubes from root to leaves, essential to the cohesion-tension theory of water flow up the stem - Energy is still needed to drive transpiration
o In the form of heat that evaporates water from leaves- from the sun
measurement of water uptake using a potometer
It is almost impossible to measure transpiration because it is difficult to condense and collect all the water vapour that leaves all parts of a plant
—> What we can easily measure is the amount of water taken up in a given time by a part of the plant like the leafy shoot
About 99% water taken up by a plant is lost during transpiration, which means that the rate of uptake is almost the same as the rate at which transpiration is occurring
—> We can then measure water uptake by the same shoot under different conditions
o Different humidities
o Different wind speeds
o Different temperatures
- In this way, we get a reasonably accurate measure of the effects of these conditions on the rate of transpiration.
The rate of water loss in a plant can be measured using a potometer
- A leafy shoot is cut under water. Care is taken not to get water on the leaves
- The potometer is filled completely with water, making sure there are no air bubbles
- Using a rubber tube, the leafy shoot is fitted to the potometer submerged in water
- The potometer is removed from under the water and all joints are sealed with waterproof jelly
- An air bubble is introduced to the capillary tube
- The distance moved by the air bubble in a given time is measured + mean calculated
- Using this mean value, the volume of water lost is calculated
- The volume of water lost against the time in minutes can be plotted on a graph
- Once the air bubble nears the junction of the reservoir tube and the capillary tube, the tap on the reservoir is opened and the syringe is pushed down until the bubble is pushed back to the start of scale on the capillary tube. Measurements then continue.
- The experiment can be repeated to compare the rates of water uptake under different conditions, or different species under the same conditions.
translocation
the process by which organic molecules and mineral ions are transported from one part of a plant to another
phloem structure
made of sieve tube elements, long thin structures arranged end to end
- end walls= perforated to form sieve plates
- associated with sieve tube elements = companion cells
sources and sinks
having produced sugars during photosynthesis, the plant transports them from the sites of production- SOURCES- to the place where they will be used directly or stored for future use- SINKS
as sinks can be anywhere in the plant. the translocation of molecules can be in any direction
what can phloem transport
organic molecules:
sucrose
amino acids
inorganic ions:
potassium
chloride
phosphate
magnesium
translocation
- the rate that material is transported through the phloem is too fast to be explained by diffusion
- MASS FLOW THEORY is a current theory to describe translocation.
*
- how sucrose transports from the source to the sieve tube element
- PHOTOSYNTHESIS occurring in the chloroplasts of leaves creates organic substances e.g. SUCROSE
- this creates a HIGH CONCENTRATION OF SUCROSE at the site of production, therefore sucrose diffuses down its concentration gradient into the companion cells via facilitated diffusion
- ACTIVE TRANSPORT OF H+ occurs from the companion cell into the spaces within the cell walls using energy
- this creates a concentration gradient and therefore the H+ move down their concentration gradient via carrier proteins into the sieve tube elements
- co-transport of sucrose with the H+ occurs via protein co-transporters to transport the sucrose into sieve tube elements.
*
- movement of sucrose within the phloem sieve tube element
- the increase of sucrose in the sieve tube element lowers the water potential
- water enters the sieve tube elements from the surrounding xylem vessels via osmosis
- the increase in water volume in the sieve tube element increases the hydrostatic pressure, causing liquid to be forced towards the sink
*
- transport of sucrose to the sink (respiring cells)
- sucrose is used in respiration at the sink, or stored as insoluble starch
- more sucrose is actively transported into the sink cell by companion cell, which causes the water potential to decrease
- this results in osmosis of water from the sieve tube element into the sink cell
- some water also returns to the xylem as the increased water potential of the sieve tube element as sucrose diffuses into the sink cell
- the removal of water decreases the volume in the sieve tube element and therefore the hydrostatic pressure decreases
- movement of soluble organic substances is due to the difference in hydrostatic pressure between the source and sink end of the sieve tube element
evidence supporting the mass flow hypothesis
evidence questioning the mass flow hypothesis
supporting
- there is pressure within the sieve tubes, as shown by sap being released when they are cut
- the concentration of sucrose is higher in the leaves (source) than in roots (sink)
- downward flow in the phloem occurs in daylight, but ceases when the leaves are shaded or at night
- increases in sucrose levels in the leaf are followed by similar increases in sucrose levels in the phloem a little later
- metabolic poisons and/or lack of oxygen inhibit translocation of sucrose in the phloem
- companion cells possess many mitochondria and readily produce ATP
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questioning
- the function of the sieve plates is unclear, as they would see, to hinder mass flow (may have structural function)
- not all solutes move at the same speed- they should do so if mvmt is by mass flow
- sucrose is delivered at more or less the same rate to all regions, rather than going more quickly to ones with the lowest sucrose concentration, which the mass flow theory would suggest
transport in xylem summary
- Water leaves from the air spaces in a plant transpiration
- This takes place mainly through pores called stomata in the epidermis of the leaf
- Water evaporates into the air spaces from mesophyll cells
- As a result, these cells have a lower water potential and so draw water by osmosis by neighbouring cells
- In this way, a water potential gradient is set up that draws water from the xylem
- Water is pulled up the xylem because water molecules stick together- cohesion
- During the night, the diameter of a tree trunk increases.
transport in phloem summary
- Transport of sucrose in plants occurs in the phloem, from places where it is produced, sources, to places where it is used up or stored, sinks
- One theory of how it is translocated is called the mass flow theory
- Initially the sucrose is transferred into sieve tube elements by the process of co-transport
- the sucrose is produced by photosynthesising cells that therefore have a low water potential due to this sucrose
- water therefore moves into them from nearby xylem tissue that has a higher water potential
- the opposite occurs in those cells (sinks) using up sucrose, and water therefore leaves them by the process of osmosis.
give the pathway a red blood cell takes when travelling in the human circulatory system from a kidney to the lungs
renal vein
vena cava –> right atrium
right ventricle –> pulmonary artery
tissue fluid is formed from blood at the arteriole end of a capillary bed
explain how water from tissue fluid is returned to the circulatory system
- plasma proteins remain in blood
- creates a water potential gradient
- reduces water potential of blood
- water moves into blood by osmosis
- also returns to blood by lymphatic system
how to improve quality of scientific drawings
ensure lines = continuous
add labels/ annotations/ title
add magnification scale bar
do not use shading/ hatching
2 precautions when clearing away after a dissection
carry/ wash sharp instruments by holding handle/ pointing away from body
disinfect instruments/ surfaces
disinfect hands
put organ in a separate bin to dispose
explain how an arteriole can reduce blood flow in capillaries
muscle contracts
constricts lumen
what blood vessel carries blood at the lowest pressure
vena cava
why is there a small increase in pressure and rate of blood flow in the aorta after blood has flowed through
elastic recoil of the aorta wall
smooths blood flow
describe the advantage of the Bohr effect during exercise
increases dissociation of oxygen
for aerobic respiration at the tissues/ muscles
an increase in the intensity of exercise produces an increase in the volume of CO2 produced
however, [a graph shows that] the pCO2 in air breathed out did not show a large increase during exercise
suggest one physiological change that would cause this result
explain how the physiological change would allow for the removal of the increase in the volume of CO2 produced
- increased breathing rate
same pCO2 per breath, but more breaths
- increased tidal volume
same pCO2 per breath, but inc volume of breath
describe and explain the effect of increasing CO2 conc on the dissociation of oxyhaemoglobin
increases oxygen dissociation
by decreasing blood pH
seal’s myoglobin dissociaton curve is much steeper than that of oxyhaemoglobin
- explain how the seal/s myoglobin dissociation curve shows the seal is adapted for diving
- much higher affinity for O2 than haemoglobin
- allows aerobic respiration when diving at lower pO2
– delays anaerobic respiration
explain how the left atrioventricular valve maintains unidirectional flow of blood
- when pressure in atrium > that in ventricle, valve opens
- when pressure in ventricle > that in atrium, valve closes
binding of one molecule of oxygen to haemoglobin makes it easier for a second oxygen molecule to bind
explain why.
- binding of first oxygen changes tertiary/ quaternary structure of haemoglobin
—> uncovers another binding site
explain the role of the heart in the formation of tissue fluid
contraction of the VENTRICLES produces high hydrostatic pressure
this forces water out capillaries
explain how a blockage in the lymphatic system could cause lymphodema
EXCESS tissue fluid cannot be reabsorbed so builds up
explain how changes in the shape of haemoglobin result in the s-shaped oxyhaemoglobin dissociation curve for HbA
- first O2 binds to Hb causing change in shape
- uncoveres other binding sites
- shape of Hb allows more O2 to bind more easily
why is HbA more useful in babies by the age of 6 months than HbFetal
HbA has lower affinity for O2 at low pp
so easier unloading of O2 for aerobic respiration
how is pressure generated inside the phloem in mass flow
sucrose is actively transported into phloem
reducing water potential in phloem
water moves into phloem by osmosis from xylem
why is phloem pressure reduced during the hottest part of the day
highest rate of transpiration
water lost through stomata
–> high tension in xylem
causes less water movement from xylem to phloem
why does water move up xylem
water evaporates from leaves –> transpiration
creates a water potential gradient
–> osmosis creates tension
hydrogen bonds between water molecules
–> cohesion maintains unbroken column of water
why may the median be used, instead of mean
presence of outliers
small sample size
precautions when using a scalpel safely
cut away from body
against a flat/hard/non-slip surface
describe the cohesion-tension theory of water transport in the xylem
water lost from leaf- transpiration
lowers water potential of mesophyll cells
water pulled up xylem
–> creating tension
water molecules cohere by hydrogen bonds
forming continuous water column
water molecules adhere to xylem walls
the rate of water movement through a shoot in a potometer may not be the same as the rate of water movement through the shoot of the whole plant
suggest why
plant has roots
describe the mass flow hypothesis for translocation
in source, sugars= actively transported into phloem
–>by companion cells
- lowers water potential of sieve tube element
–> water enters by osmosis
- increase in pressure causes mass movement towards sink
- sugars used for respiration / converted for storage
