Topic 3: Mass Transport Flashcards
Explain the adaptations of a red blood cell
Bioconcave shape:
- large SA:V for O2 diffusion + all haemoglobin molecules close to cell-surface membrane (short diffusion pathway)
- Cell flexible to bend + squeeze through narrow capillaries (sides touching = short diffusion pathway)
- Spherical cells would mean haemoglobin too far from membrane to load O2 in time available
- Contains only haemoglobin to increase O2-carrying capacity
What is a haemoglobin molecule?
Proteins with a quaternary structure that are efficient at loading O2 under one set of conditions but unloading it under other conditions
1 haemoglobin can carry 4 O2 molecules
Describe the levels of structure of a haemoglobin molecule
- Primary level: 2 alpha, 2 beta polypeptide chains
- Secondary level: each polypeptide chain coiled into a helix
- Tertiary level: each polypeptide chain folded into a precise shape
- Quaternary level: all 4 chains linked, making an almost spherical molecule. Each associated with a haem group, containing a ferrous Fe2+ ion, each can combine with 1 O2 molecule.
What is loading?
What is dissociating?
- Loading/associating: haemoglobin binds with O2 in the lungs
- Unloading/dissociating: haemoglobin releases its oxygen in the tissues
Describe affinity, and how it is used by haemoglobin
Affinity = an attraction between molecules resulting in the formation of a new molecule
Haemoglobins with a high oxygen affinity take up oxygen more easily, but find it harder to release it, and vice versa
Describe the role of haemoglobin, and how it achieves this
To transport oxygen from the lungs to respiring tissues
It changes its affinity for oxygen under different conditions (high affinity at lungs, low at tissues) to load/unload efficiently. Because its shape changes in the presence of CO2 (binds more loosely to O2), so it unloads
Describe how the oxygen affinity of haemoglobin changes based on its location in the body
At gas exchange surfaces, oxygen concentration is high and CO2 concentration is low, so oxygen affinity is high and haemoglobin loads.
This is reversed at respiring tissues
Why are there different types of haemoglobin?
Different species have different DNA, so produce different amino acid sequences, so haemoglobin molecules have a different shape and different oxygen affinity
What is partial pressure?
The pressure that would be exerted by one of the gases in a mixture if it occupied the same volume on its own
Describe the oxygen dissociation curve of haemoglobin
Very slow increase of saturation at low partial pressures, then a very steep increase, which flattens off at very high partial pressures.
% saturation of haemoglobin with oxygen on the y axis
partial pressure of oxygen on the x axis
Explain the oxygen dissociation curve of haemoglobin
- 4 polypeptide subunits are closely united = difficult for first O2 to bind = little O2 binds at low partial pressures = initial shallow gradient
- 1 bound O2 changes shape of haemoglobin so O2 molecules bind more easily to other subunits
- So smaller pO2 increase needed to bind more O2 (positive cooperativity) = gradient of curve steepens
- After 3 O2 molecules are bound, it’s harder for the 4th because it’s less likely to find an empty site = curve flattens off
What is positive cooperativity?
The binding of one oxygen molecule to haemoglobin makes it easier for the next oxygen to bind
What does the position of an oxygen dissociation curve mean?
The further to the left that the curve is, the greater the oxygen affinity, and vice versa
What is the Bohr effect?
Greater CO2 concentration means haemoglobin releases O2 more readily (oxygen-dissociation curve shifted to right), explaining why it has a high affinity at gas exchange surfaces + a low affinity at respiring tissues.
This is because CO2 is acidic, lowering the pH, so haemoglobin changes shape (lower oxygen affinity)
Describe the oxygen affinity of haemoglobins in animals living in low oxygen partial pressures. Give examples
Will have haemoglobin with a high oxygen affinity, so the oxygen dissociation curve is shifted to the left. This is because they need to be able to load oxygen more easily
e.g llama, lugworm
Describe the oxygen affinity of haemoglobins in animals with high metabolic rates. Give examples
Have haemoglobin with a low oxygen affinity, so the oxygen dissociation curve is shifted to the right. This is because they need to be able to unload oxygen at respiring tissues more easily
e.g small mammals, birds
What organisms need mass transport systems and why?
Active organisms with a low SA:V need a mass transport system because diffusion is too slow to efficiently meet all the needs of body cells
Describe some common features of mass transport systems
- Water: a medium in which to carry materials (water readily dissolves substances and can be moved around easily)
- Closed system: tubular vessels contain transport medium + forms a branching network to distribute it to all parts of an organism
- Mechanism for moving transport medium: requires a pressure difference between two parts of the system
- Unidirectional flow: valves used to prevent backflow = more efficient
- Controlling rate of flow: controls how much of each substance tissues get
How is mass transport achieved in animals and plants?
- Animals: use muscular contraction of body muscles/ a specialised pumping organ
- Plants: rely on natural passive processes, e.g water evaporation
Describe the double circulatory system in mammals
- Have a closed, double circulatory system where blood is confined to vessels and passes through the heart twice for each complete circuit
- Because when blood passes through lungs its pressure is reduced, it is returned to heart to boost pressure before being circulated to the rest of the body. Otherwise, its low pressure would make circulation very slow
What are the blood vessels going to and from the kidneys, and the liver called?
- Kidneys: renal artery/vein
- Liver: hepaptic artery/vein
- The vein going from the stomach/intestines to the liver is the hepaptic portal vein
Describe the structure of the human heart
- 2 separate pumps, separated by a septum (muscular wall). Left deals with oxygenated blood from lungs and vice versa, must keep them separated
- Need 2 pumps because blood drops pressure in capillaries in lungs, so blood is returned to heart to increase pressure.
- Each pump made from an atrium (thin walled + elastic to stretch to collect blood) and a ventricle (thick muscular walls to pump blood)
- Left ventricle = thicker muscle to contract enough pressure to pump to rest of body
- Atrioventricular valves prevent backflow into atria when ventricles contract. Left = bicupsid, right = tricupsid
Describe the vessels connecting to the heart
- Aorta: carries oxygenated blood from left ventricle to body
- Vena cava: brings deoxygenated blood from body to right atrium
- Pulmonary artery: carries deoxygenated blood from right ventricle to lungs
- Pulmonary vein: carries oxygenated blood from lungs to left atrium
- Heart muscle is supplied by coronary arteries, branching off the aorta shortly after leaving the heart.
What happens if the coronary arteries become blocked?
An area of the heart muscle is deprived of oxygen, leading to myocardial infarction (heart attack)
Explain some risk factors for cardiovascular disease
- Smoking: CO binds to haemoglobin = reduced O2 capacity = raised blood pressure + heart is oxygen deprived. Nicotine stimulates adrenaline = raised heart rate and b.p
- High blood pressure: heart has to work harder to pump, arteries more likely to develop aneurysm + burst, arteries thicken to resist pressure = restricted blood flow
- Diet: high salt levels = raised blood pressure. High saturated fat levels = increased LDL + blood cholesterol
- Blood cholesterol: HDLs remove cholesterol to liver = protect arteries, LDLs transport cholesterol to artery walls = atheroma development
What are the three phases of the cardiac cycle?
- Diastole
- Atrial systole
- Ventricular systole
Describe the diastole phase of the cardiac cycle
Relaxation of the heart muscle.
Atria fill so pressure increases - when it exceeds ventricular pressure, atrioventricular valves open - the passage of blood is aided by gravity
- Relaxation of ventricular walls = they recoil and reduce pressure = semi-lunar valves closed
Describe the atrial systole phase of the cardiac cycle
Contraction of atrial walls force remaining blood into ventricles. Ventricular walls remain relaxed
Describe the ventricular systole phase of the cardiac cycle
After a short delay, ventricular walls contract, increasing blood pressure, forcing the atrioventricular valves shut.
Ventricular pressure rises, opening semi-lunar valves, so blood is pumped into arteries
What is the role of valves and how do they achieve this?
Maintain unidirectional blood flow by preventing backflow.
They open when the difference in blood pressure on either side of the valve favours the movement of blood in the right direction. Otherwise, they stay closed
Describe the three types of valves
- Atrioventricular valves: close when ventricles contract to prevent backflow into the atria
- Semi-lunar valves: prevent backflow of blood from arteries into ventricles (when elastic walls recoil, increasing pressure in arteries)
- Pocket valves: in veins, ensure that when skeletal muscles contract, blood flows towards the heart, not away
Explain the structure of valves
Made of flaps of tough, flexible, fibrous tissue.
Cusp shaped: when pressure is greater on the convex side, flaps move apart to let blood flow through. Otherwise, blood collects in the ‘bowl’, pushing them together, preventing passage
Define cardiac output
The volume of blood pumped by one ventricle of the heart in one minute (dm^3/min)
Define stroke volume
The volume of blood pumped out at each beat of the heart (dm^3)
Give the equation for cardiac output
Cardiac output = heart rate x stroke volume
When do the atrioventricular valves close and why?
When the pressure in ventricles increases above the pressure in atria.
Prevents backflow of blood into the atria
When do the semi-lunar valves open and why?
When the pressure in ventricles increase above the pressure in arteries.
Blood moves from the ventricles to the arteries
When do the semi-lunar valves close and why?
When the pressure in arteries increases above the pressure in ventricles.
Prevents the backflow of blood into the ventricles
When do the atrioventricular valves open and why?
Pressure in atrium increases above the pressure in the ventricles.
Blood moves from the atria to the ventricles
What is the basic structure of all blood vessels, moving from the outside to the centre
- Tough fibrous outer layer: resists pressure changes from outside + within
- Muscle layer: can contract to control blood flow
- Elastic layer: maintains blood pressure by stretching and recoiling
- Endothelium: smooth to reduce friction and thin to allow diffusion
- Lumen: central cavity through which blood flows
What are the different types of blood vessel?
- Arteries
- Arterioles
- Veins
- Capillaries
What is the function of an artery?
Carry blood away from the heart into the arterioles
Describe and explain the structure of an artery
- Thick muscle layer compared to veins: smaller arteries can be constricted/dilated to control volume of blood in lumen
- Thick elastic layer compared to veins: b.p kept high to reach extremities, stretched in systole + recoils in diastole to maintain high b.p and smooth pressure changes
- Thick wall: resists bursting under pressure
- Aortic semi-lunar valve: blood constantly at high pressure so prevents backflow
What is the function of an arteriole?
Smaller arteries that control blood flow from arteries to capillaries
Describe and explain the structure of an arteriole
- Thick muscle layer compared to arteries: allows constriction of lumen to control movement into capillaries
- Thinner elastic layer than arteries: blood pressure is lower so not needed
What is the function of a vein?
Carries blood from capillaries back to the heart
Describe and explain the structure of a vein
- Thinner muscle layer than arteries: carry blood away from tissues so constriction + dilation not needed to control flow
- Thinner elastic layer than arteries: low b.p won’t make them burst, b.p is too low for recoil
- Thin wall: no need for thick wall as pressure too low to burst, lets them be flattened easily to control flow
- Valves: prevents backflow from low b.p. When body muscles contract, veins compress, increasing b.p. Valves ensure pressure only directs blood to the heart
What is the function of a capillary?
Tiny vessels that link arterioles to veins, where exchange occurs
Describe and explain the structure of a capillary
- Walls one cell thick: short diffusion pathway for exchange
- Many of them: large surface area for exchange
- Narrow diameter + lumen: permeates tissues so no cell is far from a capillary. Red blood cells squeezed flat against walls = short diffusion pathway.
- Spaces between endothelial cells: allow white blood cells to escape to deal with infections within tissues
Explain the importance of elastic recoil in the circulatory system
There is a pressure surge every time the left ventricle contracts, so the aorta wall bulges and elastic tissue stretches.
During diastole, pressure falls and elastic tissue recoils, maintaining blood pressure
What is tissue fluid?
A watery liquid containing glucose, amino acids, fatty acids, ions and oxygen, made from blood plasma.
It is the means by which materials are exchanged between blood + cells (supplies the cells and receives CO2 and waste materials).
It bathes all cells, so is the immediate environment of all cells
Describe how tissue fluid is formed
- Pumping by the heart creates a hydrostatic pressure at the arterial end of capillaries, causing tissue fluid to move out blood plasma
- This is opposed by:
- Hydrostatic pressure of tissue fluid outside capillaries resists outward movement
- lower water potential of blood due to plasma proteins remaining in capillaries, so water moves back in
Tissue fluid is still removed via ultrafiltration (large structures remain in the capillary)
Describe the return of tissue fluid to the circulatory system via the capillaries
- Loss of tissue fluid from capillaries reduces hydrostatic pressure
- So venous end of capillary has a lower hydrostatic pressure than the outside tissue fluid
- Tissue fluid is forced back into the capillaries down the pressure gradient
- Plasma has lost water + still contains proteins, so it has a lower water potential than tissue fluid
- Water leaves the tissue by osmosis down the water potential gradient
Describe the return of tissue fluid to the circulatory system via the lymphatic system
Lymphatic system drains back into the veins close to the heart.
Contents are moved by the hydrostatic pressure of tissue fluid that has left the capillaries, and the contraction of body muscles that squeeze lymph vessels.
Valves ensure it moves towards the heart.
What is transpiration?
The evaporation of water vapour from plants
Describe the structure of xylem
- On the inside of phloem in the vascular bundle
- Made from parenchyma cells resembling a series of tubes
- Tubes are kept open and strengthened by rings of lignin
- Carry water and dissolved ions
In the cohesion-tension theory, describe how water moves across the leaf
If stomata are open, water diffuses out air spaces into environment down water potential gradient.
Water lost from mesophyll cells by evaporation from cell walls. This is replaced by water from xylem either via cell walls or cytoplasm.
Cytoplasmic route:
- Mesophyll cells have low water potential due to evaporation
- Water moves in from neighbouring cells via osmosis, decreasing water potential
- This continues through the leaf and pulls water up from the xylem (transpiration pull)
In the cohesion-tension theory, describe how water moves up the xylem
Water molecules form hydrogen bonds with each other (cohesion).
This forms a continuous unbroken column along mesophyll cells and xylem.
Transpiration of water draws the molecules behind it up the xylem.
Transpiration pull puts xylem under tension (negative pressure), so it is called the cohesion-tension theory
Give the evidence supporting the cohesion-tension theory
- Change of tree trunk diameter due to rate of transpiration (during day, transpiration increases so more tension, pulls xylem walls in, decreasing diameter and vice versa)
- If a xylem vessel breaks and air enters, tree can’t draw up water (continuous column of water is broken, so there is no cohesion between water molecules)
- If a xylem vessel is broken, water doesn’t leak out as it would if it were under pressure. Instead air is drawn in, so it is under tension
Describe where the energy behind the movement of water up xylem comes from
Transpiration pull is passive so needs no metabolic energy.
Xylem vessels are dead so can’t actively move water. Vessels have no end walls for an unbroken water column.
Transpiration is driven by energy from heat from the sun
Describe how to use a potometer
- Cut a leafy shoot underwater (no water on leaves)
- Potometer is filled with water with no air bubbles
- Leafy shoot is fitted to the potometer underwater with a rubber tube, and potometer 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 a number of times = transpiration rate
- To reset it, open the tap on the reservoir and push down on the syringe until the bubble is at the start
What is translocation?
The process by which organic molecules and some inorganic ions are transported in a plant
Give some examples of substances that are moved by translocation
- sucrose
- amino acids
- K+
- Cl-
- Mg2+
What direction does translocation move in and why?
Works in both directions because sugars move from sources (sites of production) to sinks (where they will be used directly or stored for future use), both of which can be anywhere in the plant
What cells are phloem tubes made from?
Sieve tube elements lined end-to-end, which are all associated with companion cells
Describe the structure of sieve tube elements
- Not proper cells - have no nucleus/ribosomes and very little cytoplasm
- Lined up end-to-end forming a tube through which sugars are transported
- Tube has very thin sieve plates (cross walls with pores to flow through)
Describe the structure of companion cells
- Small cells between sieve tubes
- Have a large nucleus and lots of mitochondria (ATP to load sugar into the sieve tubes)
- Have small vacuoles
- Cytoplasm is linked to sieve tube elements by plasmodesmata (pores in the cell wall)
Describe how sucrose enters the sieve tube elements in the mass flow theory
- ATP hydrolysis lets H+ ions be actively transported out companion cells via proton pump
- Lowers H+ ion concentration inside the companion cell, setting up a concentration gradient
- H+ ions co-transport sucrose from the source cell to companion cell via co-transport protein
- Concentration of sucrose in companion cell increases above sieve tube element
- Sucrose molecules actively transported through carrier protein into sieve tube element
Describe how sucrose molecules move through the phloem in the mass flow theory
- Sucrose in sieve tubes lowers the water potential of fluid in the phloem
- Water moves in from the xylem via osmosis down the water potential gradient, increasing hydrostatic pressure
- Fluid moves from area of high to low hydrostatic pressure
- Sucrose is actively transported into sink cells for use in respiration / starch for storage
Describe the evidence supporting the mass flow hypothesis
- Pressure within sieve tubes shown by sap being released when cut
- Concentration of sucrose higher in leaves (source) than roots (sink)
- Downward flow in phloem occurs in daylight + ceases at night/in shade
- Increased sucrose levels in leaf = similar increase in phloem a bit later
- Metabolic poisons / a lack of oxygen inhibits translocation of sucrose in the phloem
- Companion cells possess many mitochondria + readily produce ATP
Describe the evidence that questions the mass flow hypothesis
- Function of sieve plates is unclear as they should hinder mass flow (suggested they have a structural function)
- Not all solutes move at the same speed, they should if the movement is by mass flow
- Sucrose is delivered at roughly the same rate to all regions, not the quickest to areas with the lowest sucrose concentration as mass flow would suggest
What are the 3 experiments used to investigate mass transport in plants?
- Ringing experiments
- Tracer experiments
- Using aphids
Describe ringing experiments to investigate transport in plants
- A section of outer protective + phloem layers removed around the circumference of a woody stem
- After time, the region of stem immediately above the missing tissue swells
- Samples of liquid in the swollen region are rich in organic substances
- Non-photosynthetic regions below the stem die while those above the ring continue growing
- Shows phloem transports sugars because xylem weren’t cut
Describe tracer experiments to investigate mass transport in plants
- Plant is grown around radioactive CO2, containing the 14C isotope, which is incorporated into sugars produced by photosynthesis
- Radioactive sugars are traced as they move through the plant using autoradiography
- Thin cross sections of stem appear blackened if exposed to radiation under an x ray
- Blackened regions correspond to where phloem is in the stem
Describe the use of aphids to investigate mass transport in plants
- Their needle-like mouthparts penetrate the phloem and can extract contents of sieve tubes
- These contents show daily variations in sucrose content of leaves that are mirrored a bit later by identical changes in the sucrose content of phloem
