7: Mass transport Flashcards
What is haemoglobin?
Protein in red blood cells responsible for the transport of oxygen
What is the structure of haemoglobin?
Large quaternary protein 4 polypeptide chains - 2x α, 2x β
Each chain has a haem group - contains Fe2+
What gives red blood cells their red colour?
Iron ion present in haem groups in haemoglobin
How many molecules of O2 can a haemoglobin carry?
4
1 per haem group
What does oxygen affinity mean?
The tendency of the molecule to combine with oxygen
What is the reaction between haemoglobin and oxygen?
Hb + 4O2 -> Hb4O2
Haemoglobin + Oxygen -> Oxyhaemoglobin
What is partial pressure of O2 a measure of?
Oxygen concentration - (pO2) Higher the conc, higher the partial pressure
Where does oxygen enter haemoglobin?
Enters blood capillaries at the alveoli in the lungs
Why do multi-cellular organisms require a transport system?
Low SA:V means tissue is located too far from the exchange surface
What is the circulatory system?
Carries raw materials from specialised exchange organs to their body cells
What is the circulatory system made from?
Heart and blood vessels
What are the two circuits in the circulatory system?
Pulmonary - takes blood from heart to lungs and back
Systemic - takes blood around the body
What are the four different types of blood vessels?
Arteries Arterioles Capillaries Veins
What is the route of the pulmonary system in the circulatory system?
R atrium -> R ventricle -> Pulmonary artery -> lungs -> pulmonary vein -> L atrium
What vessels supply the heart with blood?
Coronary arteries
What does blood transport?
Respiratory gases
Products of digestion
Metabolic wastes
Hormones
What is the structure of haemoglobin from primary to Quaternary structure?
1° - sequence of amino acids in 4 polypeptide chains
2° - each polypeptide chain is coiled into a helix
3° - each polypeptide is folded into a precise shape (so it can carry oxygen)
4° - all four polypeptides linked to form a spherical molecule, each with a haem group
What is loading/associating?
Process whereby haemoglobin binds with oxygen
What is unloading/dissociating?
Process whereby haemoglobin releases its oxygen
How does oxygen bind to haemoglobin?
Each Fe2+ ion can combine with a single O2 molecule
Therefore 4 O2 molecules can bind to haemoglobin
Where does loading/associating occur in the human body?
Takes place in the lungs
Where does unloading/dissociating occur in the human body?
Takes place in human tissue
What makes haemoglobin efficient at transporting oxygen?
Readily associates with oxygen at surface of gas exchange
Readily dissociates with oxygen at tissue where needed
What affects whether oxygen associates/dissociates?
Substances such as carbon dioxide reduce the oxygen affinity of haemoglobin by changing its shape
What is the oxygen dissociation curve?
Graph of relationship between saturation of haemoglobin and the partial pressure of oxygen
Learn the shape of the oxygen dissociation curve
What causes the shape of the oxygen dissociation curve?
Haemoglobin shape means difficult for 1st O2 to bind to haem as they are close together in the molecule, explains shallow gradient at first
1st oxygen binding causes change in conformational shape of 4º structure, making it easier for O2 to bind to other haem groups so there is a steep gradient
After 3rd O2 binds even though its theoretically easier, it is more difficult as there is a lower probability an O2 will find the one free haem, causing a shallow gradient
What is positive cooperativity?
When the haemoglobin changes shape after the 1st O2 binds to make the binding of the further molecules easier
Why are there different types of haemoglobin?
Occurs due to slightly different amino acid sequence in different animals
Depending on structure, haemoglobin molecules have different oxygen binding affinity
Animals in more hypoxic environments would have evolved to have haemoglobin with a better oxygen binding affinity
What occurs if the oxygen affinity curve shifts to the left?
Greater affinity of oxygen
Therefore loads oxygen better and unloads less easily
What occurs if the oxygen affinity curve shits to the right?
The oxygen affinity of haemoglobin is reduced
Loads oxygen less readily but unloads more easily
What is the Bohr effect?
The reduced oxygen affinity of haemoglobin which occurs due to the increased concentration of carbon dioxide
How does the Bohr effect explain why haemoglobin works in the blood vessels in the lungs?
Low CO2 concentration as it diffuses to be excreted
Means O2 affinity of haemoglobin increases and high [O2] means O2 is more readily loaded by the haemoglobin
Oxygen dissociation curve shifts to the left
How does the Bohr effect explain why haemoglobin works near to respiring tissues?
High [CO2] as it is respiring
Means O2 affinity of haemoglobin decreases and low [O2] means O2 is more readily unloaded by the haemoglobin
Oxygen dissociation curve shifts to the right
Why does carbon dioxide cause the Bohr effect?
CO2 is acidic therefore lowers the pH which causes the haemoglobin to change shape
How does rate of respiration effect the rate of oxygen delivered?
The more the respiration the greater the CO2 produced by tissues
Lowers pH so changes shape of haemoglobin and shifts dissociation curve to the right
Oxygen is more readily unloaded to the tissue, so more oxygen available for respiration
How many O2 molecules are unloaded in which scenarios?
Usually only one O2 unloaded to normal tissue
Cells which are very active will usually cause 3 O2 to unload
What determines whether a specialised transport medium is needed with a pump?
Surface area to volume ratio of animal
How active an organism is
What are some common features of transport systems in complex organisms?
Suitable transport medium (e.g. blood)
Form of mass transport in which transport medium is moved over large distances (more rapid than diffusion)
Closed system of tubular vesseles that contain transport medium and forms a branching network to distrubte to all parts
Mechanism for moving transport medium within vessels, requires a pressure difference
Things that maintain mass flow in one direction (valves)
Menas of controlling flow rate based on needs
Mechanism for mass flow of water/gases
What makes a transport medium suitable?
Normally liquid based on water as water readily dissolves substaces and can be moved around easily
Can be a gas such as air breathed in and out of the lungs
How are pressure differences achieved in animals to transport the medium?
Muscular contraction either of body or specialised pumping organ (heart)
How are pressure differences achieved in plants to transport the medium?
Rely on passive processes such as evaporation of water
What circulatory systems are present in mammals?
Closed, double circulatory systems
Blood confined to vessels and passes twice through heart for each complete circuit of the body
Why does the blood go through the heart twice?
Pressure reduced after going through the lungs
Blood pressure boosted when tranported to the rest of tissue meaning that it gets to them quickly
Where is the heart found and what is it?
Thoracic cavity behind the sternum
Muscular organ that operates continuously
What is the structure of the heart?
Two seperate pumps with two chambers
What are the atria?
Thin-walled upper chamber which is elastic so stretches when it collects with blood
What are the ventricles?
Bottom chamber with a much thicker muscular wall to contract strongly to pump blood either to the lungs or the rest of the body
Which ventricles are responsible for pumping the blood to the lungs and to the rest of the body?
Left ventricle pumps blood to the rest of the body
Right ventricle pumps blood to the lungs
Why does the right ventricle have a thinner muscular wall than the left ventricle?
As right ventricle only pumps to the lungs so less pressure and contraction needed
What are the valves between the atria and ventricles called?
Left bicuspid valve
Right tricuspid valve
What is the aorta?
Vessel which is connected to the left ventricle
Carries oxygenated blood to all parts of the body (excpect lungs)
What is the vena cava?
Vessel connected to the right atria
Carries deoxygenated blood from the tissues of the body (except lungs) back to the heart
What is the pulmonary artery?
Vessel which connects right ventricle to the lungs
Carries deoxygenated blood to lungs where the O2 is replenished and CO2 removed
What is the pulmonary vein?
Vessel that connects the left atrium to the lungs
Brings oxygenated blood back from the lungs
How is the heart supplied with oxygen?
Coronary arteries branch off the aorta after it leaves the heart
Vessels are specifically just for heart muscle
What occurs if the coronary arteries are blocked?
Myocardial infarction - heart attack
Occurs as an area of the heart muscle is deprived of blood and oxygen which could cause cell death
What are the two phases to the beating of the heart?
Contraction (atrial then ventricular systole)
Relaxation (diastole)
Where does contraction occur?
Occurs separately in the ventricles and the atria
First artial systole, then ventricular systole
Where are the semi-lunar valves?
The valves between the ventricle and the pulmonary artery, also between the ventricle and the aorta
What occurs in diastole (relaxation)?
Blood enters atria and pressure builds
When pressure in atria is greater than ventricle, the atrioventricular valves open
Blood enters the ventricles, which are relaxed, as it has a lower pressure
Semi-lunar valves closed
Dub sound
What occurs in atrial systole?
Contraction of atrial walls and ventricles walls remained relaxed
Forces any remaining blood into the ventricles
What occurs in ventricular systole?
After a short delay after atrial systole, the ventricle walls contract simultaneously
Increases blood pressure which closes the atrioventricular valves
Once higher pressure than aorta and pulmonary artery it opens the semi-lunar valves and causes the blood to flow
What do the thick ventricle walls allow for?
Allows for forceful contraction so blood can go around the entire body
What are used to prevent the backward flow of blood?
Valves
How do valves work?
Open when the difference in blood pressure either side of them favours the movement of blood in the required direction
What is the use of the atrioventricular valves?
Prevents backflow of blood when contraction of the ventricles meaning ventricular pressure exceeds atrial pressure
Means blood flows through the aorta and pulmonary artery rather than the atria
What is the use of the semi-lunar valves?
Prevent backflow into ventricles when pressure in vessels exceeds that in the ventricles
Occurs when ventricles go back (recoil) during relaxation
Where are pocket valves found?
In the veins
What is the use of the pocket valves?
Ensure that when veins are squeezed the blood flows back towards the heart rather than away from it
What are valves made from?
Many flaps of tough, but flexible, fibrous tissue which are cusp-shaped (deep bowls)
How do valves work?
When pressure is greater on the convex side of these cusps (not concave) blood collects in the “bowl” of the cusps
This pushes them together to form a tight fit that prevents the passage of blood
What is the equation for cardiac output?
Cardiac output = heart rate x stroke volume
Learn the pressure and volume diagrams throughout the cardiac cycle
What are the four types of blood vessels and what do they do?
Arteries - carries blood away from the heart and 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 are the layers of the arteries arterioles and veins from outside inwards?
Tough fiborous outer layer
Muscle layer
Elastic layer
Endothelium (thin inner lining)
Lumen (central cavity)
What is the function of the tough fibrous outer layer in blood vessels?
Resists pressure changes from both within and outside
What is the function of the muscle layer in blood vessels?
Able to contract so can control the flow of blood
What is the function of the endothelium in blood vessels?
Smooth surface redicomg friction and thin to allow diffusion
What is the function of the lumen in blood vessels?
Cavity which allows the blood to flow through it
What is the structure of the arteries?
Thick muscle layer
Thick elastic layer
Thick wall
No valves
Why is the muscle layer thick in the arteries?
Means smaller arteries can be constricted and dilated to control th evolume of blood passing thorugh them
Why is the elastic layer thick in the arteries?
Blood pressure needs to be kept high to reach body’s extremities
Stretched in systole and relaxes in diastole maintaining high pressure and smoth pressure
Why is the wall thick in arteries?
Resists the vessel bursting under pressure
Why are there no valves in arteries?
Blood is under constant high pressure due to heart so it tends not to flow backwards
(however some found in arteries leaving the heart)
What is the structure of the arteriole and why?
Muscle layer is thicker than the arteries - allows constriction of lumen so restricts movements into capillaries next to tissue (vasocontriction)
Thin elastic layer - lower blood pressure
What is the structure of the veins?
Muscle layer is thin
Elastic layer is thin
Overall wall thickness is small
Valves are at intervals throughout
Why is the muscle and elastic and wall layer thin in veins?
Muscle layer - as carry blood away from tissues so contriction and dilation cannot control the flow of blood to the tissues
Elastic layer - low pressure of blood will not cause bursting and pressure too low to cause recoil action
Wall - no need for thick wall as pressure in veins is too low to create any risk of bursting, allows flattening to aid flow of blood within them
Why are there valves throughout the veins?
Ensure blood doesn’t flow backwards which could occur as there is low pressure
When body muscles contract, veins are compressed which pressurises blood within them. Valves ensure pressure firects blood in one direction to the heart
What is the structure of the capillaries and why?
Walls consist of mostly endothelium - thin so short diffusion pathway, rapid diffusion between blood and cells
Numerous and highly branched - large SA:V for exchange
Narrow diameter - permeates tissue meaning no cell is far from a capillary and a short diffusion pathway
Lumen is so narrow - RBCs squeezed flat against side of the capillary, reducing diffusion pathway as closer to cells
Spaces between endothelium cells - allow WBCs to escape to deal with infections within tissues
What is the tissue fluid?
Watery liquid that supplies substances to tissue and removes waste from them
It is the immediate environment of cells
Supplies: glucose, oxygen, ions in solution, amino acids, fattyt acids
Removes: CO2 etc.
What is tissue fluid made from?
Formed from blood plasma which is controlled by various homeostatic systems
Provides constant environment for cells it surrounds
What is hydrostatic pressure?
Pressure caused in arterial ends of capillaries by the heart
What does the hydrostatic pressure cause?
High pressure in arteriole capillaries causes tissue fluid to move out of blood plasma
Only enough to force small molecules out of the capillaries but proteins and cells stay in the blood
Why can blood cells and large proteins not leave the capillaries?
They are too big to fit through the gaps
How does plasma move to the interstitial space?
Low water potential due to proteins causes osmosis into blood
However high hydrostatic pressure has a greater effect, causing net movement out of the tissue fluid into the interstitial space
What happens at the venule end of the capillary?
Hydrostatic pressure is lower in the capillary than fluid around it
Tissue fluid is forced back into capillaries
Also plasma has less water and still has proteins so a lower water potential
Causes osmosis into the capillaries
What is filtration under pressure called?
Ultrafiltration
What happens to the tissue fluid which doesn’t diffuse back into the capillaries?
Returned via the lymphatic system which drains back into the bloodstream
Interstitial fluid which mostly contains carbon dioxide and waste materials
What does the lymphatic system do?
System of vessels that begin in the tissues which form a network
Drain their contents into the bloodstreams via two ducts that join veins close to the heart
What are the contents of the lymphatic system moved by?
Hydrostatic pressure of tissue fluid that has left the capillaries
Contraction of body muscles - squeeze lymph vessels
Valves - ensures fluid moves away from the tissue in the direction of the heart
What is the formation and return of tissue fluid process?
Blood plasma → tissue fluid by ultrafiltration
Tissue fluid → blood plasma by reabsoption
Tissue fluid → lymph by drainage
Lymph → blood plasma by returning via lymph vessels
How is water tansported through the plants?
Xylem vessels
What is the composition of xylem vessels?
Consist of dead hollow cells which have lignified walls
Wide lumen and they are linked end to end to create a long, hollow tube since the end cell walls have one or many perforations in them
Sidewalls have unlignified areas to allow lateral movement of water
How are xylems adapted to their function?
Lignin in cell wall is impermeable so waterproof
Wide lumen to allows a large amount of water to flow
What is the main force which pulls water through the xylem?
Transpiration - caused by evaporation of water from leaves
How does water move out of the plant?
Humidity in atmosphere less than air spaces next to stomata
Water potenital gradient from air spaces through stomata to the air
If the stomata is open, water vapour molecules diffuse out of them
How is water vapour replaced in air spaces in plants?
Water evaporating from cell walls of the surrounding mesophyll cells
How is the rate of transpiration changed in plants?
Changing the size of the stomatal pores
How is water lost from mesophyll cells?
Evaporation from mesophyll cell walls to air spaces of the leaf
How is the water in mesophyll cells replaced?
From the xylem either via cell walls or via the cytoplasm
What is the cytoplasmic route of water?
Mesophyll cells lose water to air spaces by evaporation due to heat from sun
Cells now have a lower water potential so water enters by osmosis from neighbouring cells
Loss of water from these neighbouring cells lowers their water potential
Water potential gradient is established that pulls water from the xylem
Define cohesion
Water molecules which stick together as they form hydrogen bonds with eachother
What is the theory of how water moves up the stem in the xylem?
Cohesion - tension theory
Explain what is involved in the cohesion-tension theory
Water evaporates from mesophyll cells leading to transpiration
Water molecules stick together and form a continuous, unbroken column across the mesophyll cells and down the xylem
Water evaporates from mesopyll cells, drawing up more water because of cohesion
A column of water is pulled up the xylem because of transpiration
Transpiration pull puts xylem under tenion, -ve pressure in xylem
What is transpiration pull?
A column of water being pulled up the xylem as a result of transpiration
What is the evidence for the cohesion-tension theory?
Change in diameter of tree trunks - when respiration is greatest (day) there is more tension and negative pressure in the xylem, pulls walls of xylem inwards so it shrinks in diameter
Xylem vessels - if broken and air enters it then the tree cannot draw up water because continuous column of water is broken, however water doesn’t leak out air is drawn in as it is under tension not under pressure
Is energy required to drive transpiration?
Transpiration requires energy to evaporate water
Transpiration pull is a passive process in dead cells
What is translocation?
Process by which organic molecules and mineral ions are transported from one part of a plant to another
What is the phloem?
The tissue that transports biological molecules
These molecules include: sucrose, amino acids, inorganic ions
What is the phloem made of?
Sieve tube elements - long thin structures arranged end to end
End walls are perforated to form sieve plates
A companion cell is bonded to one side of the sieve tube
What are sources and sinks in plants?
Sources - sites of production of sugars by photosynthesis
Sinks - Places where sugar can be used directly or stored for future use
Why does translocation have to work in all directions?
Sinks can be anywhere in the plant which can be above and below the source
What is the current theory of the mechanism of translocation?
Mass flow theory
What are the three steps of the mass flow theory?
Transfer of sucrose into sieve elements from photosynthesising tissue
Mass flow of sucrose through sieve tube elements
Transferof sucrose from the sieve tube elements into storage or sink cells
What occurs in the first step of the mass flow theory?
Sucrose is made in photosynthesising cells
Sucrose diffuses down conc gradient by facilitated diffusion into companion cells
H+ actively transported from companion cells to spaces within cell walls using ATP
H+ diffuse down a conc gradient through carrier proteins into sieve tube elements
Sucrose transported with H+ by co-transport via co-transport proteins
What occurs in the second step of the mass flow theory? (Mass flow of sucrose through sieve tube elements)
Sucrose producing from source, actively transported into sieve tubes
Reduces water potential of sieve tubes
Xylem has higher water potential than sieve tubes so osmosis occurs creating a high hydrostatic pressure within them
Sucrose used for energy or stored in sink, sucrose is actively transported into them from sieve tubes which lowers their water potential
Water moves into sink by osmosis from sieve tubes, lowering the hydrostatic pressure of the sieve tube in this reigon
High hydrostatic pressure at source, low at the sink meaning mass flow of sucrose solution down hydrostatic gradient
Is mass flow a passive process?
Mass flow is passive
However it is a result of active tranport - this is why it is affected by temp pressure and metabolic poisons
What is the experiment to show evidence for mass flow?
What is the evidence the mass flow hypothesis?
Pressure within sieve tubes (that’s why sap released when cut)
Conc of sucrose is higher in source than sink
Downward flow in phloem in daylight, this stops in the dark
Increases in sucrose in leaf followed by similar increase in sucrose in phloem
Metabolic poisons / lack of oxygen inhibits translation of sucrose
Companion cells possess many mitochondria and readily produce ATP
What is the evidence questioning the mass flow hypothesis?
Function of sieve plates is unclear, seem to hinder mass flow (might be structural to prevent bursting under pressure)
Not all solutes move at same speed - all should do if mass flow
Sucrose is delievered at more or less same rate to all regions, rather than going more quickly to the ones with lowest [sucrose] which it mass flow suggests
What is the third stage of the mass flow hypothesis? (Transfer of sucrose from the sieve tube elements into storage or other sink cells)
Sucrose is actively transported by companion cells out of the sieve tubes and into the sink cells
What is the funciton of bark?
Ouiter protective layer
What layers are found from out to in in plants?
Bark → Phloem → Xylem→ Pith
What is the procedure in ringing experiments?
Protective layer and phloem are removed around the circumference
Swelling occurs above the ring which has a high conc of sugar, this suggests sugars of the phloem accumulate above the ring
Death of tissue below the removed ring, suggests death is caused by interruption of flow of sugars to below region
What do ringing experiments suggest?
Phloem is responsible for translocating sugars in plants
If xylem was responsible for translocating sugars, you wouldn’t expect the tissue to die
What happens in tracer experiments?
Radioactive 14C used to make 14CO2 and plant uses this in sugar production in photosynthesis
X-ray film shows where the radioactive sugar is in the stem
Shown to be from where the phloem tissue is in the stem so it must be responsible for translocation
What is the evidence that translocation of organic molecules occurs in the phloem?
Organic molecules flows out when phloem cut
Plants with radioactive 14CO2 has radioactively labelled carbon in phloem shortly after
Removal of phloem ring around circumference leads to accumulation of sugars above the ring and disappearance below it
Aphids (insects that feed on plants) have mouthparts which penerate the phloem and extract what is in the sieve tube which is shown to be sucrose
