Mass Transport Flashcards
What type of protein is haemoglobin
Globular protein
Describe the different structure of a haemoglobin molecule
Primary: specific sequence of amino acids in each of the polypeptide chains
Secondary: hydrogen bonds form and it coils into an alpha helix
Tertiary: polypeptide chain folds into a precise that gives O2 binding properties
Quaternary: each polypeptide is associated with a haem group containing a ferrous ion (Fe2+) that can bind to 1 O2
How many oxygens can each haemoglobin carry
4
What must haemoglobin be able to do to be efficient at transporting oxygen
- must readily associate with O2 at gas exchange surfaces
- must readily dissociate with O2 at respiring tissues where it’s released and needed for respiration
How does haemoglobin achieve its efficiency
Because its shape changes in the presence of certain substances, like CO2
- in the presence of CO2 the new shape of the haemoglobin molecule binds more loosely to oxygen
- as a result haemoglobin dissociates its oxygen
Why do some haemoglobin have a high affinity for oxygen
In the presence of certain substances haemoglobin has a high affinity for oxygen as its shape changes to give it properties that associate with oxygen
What suggestions have scientists come up with to explain differences in affinity amongst haemoglobin
Have a slightly different amino acid sequence which produces different tertiary and quaternary structures so different shapes with different O2 binding properties
What is the definition for loading/associatinf
The process through which haemoglobin binds with oxygen
Definition for unloading/dissociating
The process through which haemoglobin loses its oxygen
What is partial pressure
the pressure exerted by oxygen within a mixture of gases
How does partial pressure of oxygen change up a mountain
The partial pressure of O2 decreases because as amplitude increases oxygen makes up less of the gases in the atmosphere and so causes a smaller % of the pressure -> air is much thinner and less oxygen in each m3 of air
What is an oxygen dissociation curve
The graph of the relationship between the saturation of haemoglobin with oxygen and the partial pressure of oxygen
Explain the shape of the oxygen dissociation curve
- The shape of the haemoglobin makes it hard for the first oxygen to bind to one of the sites in its 4 polypeptides because they are close together -> so at low oxygen concentrations, little oxygen binds to haemgoblin and so the gradient is shallow initially
- However, the binding of the first oxygen molecule changes the quaternary structure of the haemoglobin causing it to change shape -> this change makes it easier for other polypeptides to bind to an oxygen
- Therefore it takes a smaller increase in partial pressure of oxygen to bind the 2nd oxygen than the 1st -> this is called positive cooperativity
- The gradient of the curve therefore steepens
- However, after the binding of the 3rd molecule -> with the majority of the binding sites occupied it’s less likely that a single oxygen will find an empty binding site
- So the gradient of the curve reduces and the graph flattens off
The further to the left the curve…
The greater the affinity of haemoglobin for oxygen
The further to the right the curve…
The lower the affinity of haemoglobin for oxygen
More CO2…
Lower affinity for oxygen
Less CO2…
High affinity for O2
Relationship with pH of CO2 and oxygen
The higher the rate of respiration -> the more carbon dioxide the tissues produce -> the lower the pH -> the greater the haemoglobin shape change -> the more readily oxygen is unloaded -> the more oxygen is available for respiration
Why do large organisms have a transport system
They have a small SA:V ratio so needs can’t be met by body surface alone
Features of transport systems
- suitable medium to carry materials (usually water based as water readily dissolves substances, can be moved easily and can be a gas)
- a form of mass transport
- a closed system of tubular vessels
- a mechanism for moving the transport medium within vessel -> requires a pressure difference (muscle contraction eg)
What type of circulatory system do mammals have and why
Closed, double circulatory
- because when blood is passed through the lungs it’s pressure is reduced + if it were to go immediately to the rest of the body its low pressure would make circulation very slow
- so blood is returned to the heart to increase pressure which makes circulation quicker for mammals who have high body temp and metabolic rate
Advs of closed circulatory system
- maintained pressure
- speed
- pressure can be altered
- distribution of blood around body can be controlled
How does the heart maintain a unidirectional flow of blood
- valves which prevent backflow of blood and maintain blood pressure
- muscle contracts to increase pressure in the atrium and ventricles so blood moves to where there’s lower pressure towards to arteries
What is the name of the main valves in the heart
Atrioventricular valves (AV Valves)
Function of the circulatory system
- to transport blood around the body
- O2 and glucose for respiration
- carry away waste products like urea
Aorta
Connected to the left ventricle and carries oxygenated to all parts of the body except the lungs
Vena cava
Connected to the right atrium and brings deoxygenated blood back from the body tissues
Pulmonary artery
Connected to the right ventricle and carries deoxygenated blood to the lungs
Pulmonary vein
Connected to the left atrium and brings oxygenated blood from the lungs
What blood vessels supply the heart muscle with oxygen for respiration
The coronary arteries
What is diastole
Relaxation of the heart
What is systole
Contraction of the heart
What do the ventricular walls do after contraction
They relax and their walls recoil reducing pressure within the ventricle
Structure of valves
- made up of flaps that are cusp shaped
- when pressure is greater in the convex side of the cusps they move sort to let blood pass
- when pressure is greater in the concave side blood collects within the cusps and pushes them shut to prevent passage of blood
How to calculate cardiac output
Heart rate x stroke volume
What is cardiac output
The volume of blood pumped by one ventricle of the heart in 1 minute
Describe ventricular pressure
- It is low at first but increases as the ventricles fill with blood as the atria contract - the AV valves close as ventricular pressure is higher than atrial pressure
- The AV valves close and pressure rises as ventricle walls contract
- As pressure rises that of the aorta blood is forced into the aorta past the semilunar valves
- Pressure falls as the ventricles empty and the walls relax
Why is atrial pressure always relatively low
Because the thin walls of the atrium can’t create much force
Why are systole and diastole important in the cardiac cycle
Because they decrease and increase the volumes of the atria and ventricles so that a pressure gradient is created
What do arteries do
Carry blood away from heart into arterioles
What do arterioles do
Smaller than arteries and control blood flow from arteries to capillaries
What do capillaries do
Tiny vessels that links arterioles back to veins
What do veins do
Carry blood from capillaries back to the heart
What layers do arteries, arterioles and veins have
- tough fibrous outer layer -> resists pressure changes from both within and outside
- muscle layer -> can contract to control flow of blood
- elastic layer -> help to maintain blood pressure by stretching and recoiling
- endothelium layer -> smooth to reduce friction and thin to allow diffusion
- lumen -> not a layer but allows blood to flow
Which blood vessel carries out exchange
Capillaries
Artery structure related to function
- thicker muscle layer -> to constrict + dilate to control volume of blood passing through
- thick elastic layer -> for high pressure
- no valves -> no backflow due to constant high pressure
Thickness of arterioles muscle layer compared to arteries and why
Thicker as the contraction of this muscle layer allows constriction of the lumen -> restricts flow and so controls its movement into capillaries
Do vein have valves and why
Yes as there would be backflow due to low pressure
Why do veins have thin walls
Low pressure
Capillary structure related to function
- numerous and branched -> large SA for exchange
- narrow lumen -> RBCs squeezed flat against side reduces diffusion pathway
What is hydrostatic pressure
Pressure caused by the blood as it’s pumped through arteries, arterioles and capillaries
Where is hydrostatic pressure the greatest and what this means
At the arteriole end of the capillary
-> its greater than WP difference and so tissue fluid is forced out
What is tissue fluid called in the blood
Plasma
What 2 forces oppose outward hydrostatic pressure
- Hydrostatic pressure of tissue fluid outside capillaries
- The lower water potential of the blood due to plasma proteins -> causes water to move back into blood
HOWEVER, the combined effect creates an overall pressure that pushes tissue fluid out of capillaries at the arterial end
What is ultrafiltration
When the pressure is only enough to force small molecules out of capillaries leaving cells and proteins in the blood as they are too big
How does tissue fluid return to the circulatory system
- The loss of the tissue fluid from capillaries reduces hydrostatic pressure in them
- As a result, when the blood reaches the venous end it’s hydrostatic pressure is lower than that of the tissue fluid outside
- So tissue fluid is forced back into the capillaries
- In addition the plasma has lost water and still contains proteins and so has a low water potential than outside
- As a result, water leaves the tissue by osmosis down a water potential gradient
Where is the remainder of tissue fluid carried
Via the lymphatic system
How are the contents of the lymphatic system moved
By:
1. Hydrostatic pressure of the tissue fluid that has left capillaries
2. Contraction of body muscles
Features of xylem
- one way flow
- water and dissolved ions
- no end walls between cells
- thick walls with lignin to withstand cohesion pressure
Features of phloem
- water and dissolved sugars
- sieve plates with pores
- companion cells
- bidirectional flow
Why do xylems have dead cells
To prevent water flowing in and out by osmosis
Why do phloems have living cells
To respire to produce ATP for energy
What role do stomata play in transpiration
When they open the rate of gas exchange and transpiration is higher
When they close the rate decreases
Explain the role of cohesion in transpiration
- Water evaporates from mesophyll cell due to heat from the sun leading to transpiration
- Water molecules form hydrogen bonds between one another and so stick together -> this is cohesion
- Water forms a continuous column across the mesophyll cells + down the xylem
- As water evaporates from the cells in the leaf to air spaces beneath stomata, more water molecules are drawn up as a result of this cohesion
- A column of water is therefore pulled up the xylem as a result of transpiration -> this is called the transpiration pull
- Transpiration pull puts the xylem under tension as it creates negative pressure within the xylem -> hence the name cohesion-tension theory
Movement of water across the cells of a leaf
Mesophyll cells lose water to air spaces by evaporation due to heat from the sun
-> these cells now have a lower WP and so water enters by osmosis from neighbouring cells
-> the loss of water from these cells lowers their WP and so they takes water from their neighbouring cells
-> in this way a water potential gradient is created that pulls water from the xylem across the leaf mesophyll and finally into the atmosphere
Evidence to support cohesion tension theory
- change in diameter of tree trunks according to rate of transpiration -> during the day there’s more tension in xylem due to more transpiration so trunk shrinks
- if a xylem is broken and air enters the tree can’t draw up water due to the continuous column of water being broken and so water molecules can’t stick together
- when xylem vessel is broken water doesn’t leak out instead air is drawn in due to it being under tension
What are sources and sinks
Sources - sites of production
Sinks - places where it needed or stored
Mechanism of translocation
- Facilitated diffusion of sucrose from photosynthesising cell to companion cell as there’s a higher conc in source
- Cotransport of sucrose with H+ ions
- H+ ions are actively transported from companion cells into cell walls where they can diffuse through carrier proteins into sieve plates
- Water moves from xylem to phloem by osmosis increasing hydrostatic pressure
- The respiring cells use up sucrose or store excess as starch -> sucrose is constantly being transported into these cells by active transport (depending on conc) from sieve tubes lowering their water potential
- Due to this low WP in respiring cells, water moves in via osmosis decreasing hydrostatic pressure of the sieve tubes
- So there’s a high HP in sieve tube elements, near the source, and a low HP in sieve tube elements near sink (creates a hydrostatic gradient)
Evidence supporting mass flow (translocation)
- there’s pressure in sieve tubes as sap releases when they’re cut
- conc of sucrose is higher in leaves (source) than roots (sink)
- downward flow in the phloem occurs in daylight but stops at night
- increases in sucrose levels in the leaf are followed by similar increases in the phloem shortly after
- metabolic poisons/lack of oxygen inhibit translocation of sucrose in the phloem
- companion cells posses many mitochondria and readily produce ATP
Evidence against mass flow (translocation)
- function of sieve plates is unclear as they would hinder mass flow
- not all solutes move at same speed -> they should if movement is via mass flow
- sucrose is delivered to all regions at the same rate -> should go quicker to ones with lowest sucrose conc as mass flow suggests
2 methods of investigating transport in plants
- ringing experiments
- tracer experiments
What ringing experiments
- a section of the outer layers (bark and phloem) is removed around circumference of a woody stem while still attached to rest of plant
- after a period of time, the region above the missing tissue swells due to accumulation of sugars
- and non-photosynthetic tissues below the removed tissue die while those above continue to grow
What do ringing experiments suggest
Is that phloem rather than xylem is the tissue responsible for translocation of sugars in plants -> as the ring of tissue removed did not include the xylem
-> if it were the tissue responsible you wouldn’t expected sugars to accumulate and tissues below to die
What a tracer experiments
- radioactive isotopes are used to trace movement of substances in plants
- isotopes can be incorporated into sugars produced in photosynthesis and can be traced as they move using autoradiography
- thin cross sections can be taken from the plant stem and put on X-ray film
- the film becomes blackened where it’s been exposed to radiation
- the blackened regions are found to correspond to where phloem tissue is in the stem
Evidence that translocation of organic molecules occurs in phloem
- when phloem is cut, a solution of organic molecules flows out
- plants provided with radioactive CO2 can be shown to have radioactively labelled carbon in phloem after time
- aphids mouthparts can feed on sap from sieve plates -> these show variation in sucrose contents of leaves mirrored by identical changes later in the phloem
- the removal of a ring of phloem around the circumference of a stem leads to the accumulation of sugars above the ring and their disappearance below it
What can we investigate using a potometer
Rate of transpiration under different:
- humidities
- wind speeds
- light intensities -> how wide stomata open -> increased evaporation
- temperatures
