Transport

Effect of haemoglobin having a high affinity for oxygen

Associates with oxygen easily, so takes it up easily, but releases it less easily

Explanation for oxygen dissociation curve (includes how oxygen binds to haemoglobin)

• Gradient of curve is shallow initially as it is difficult for the first oxygen molecule to bind to haemoglobin, so at low oxygen concentrations, little oxygen binds to haemoglobin

• After the first oxygen molecule has been bound to the haemoglobin, it is easier to bind the second and third - positive cooperativity - since the quaternary structure of the haemoglobin molecule changes and therefore it changes shape. The gradient is steeper as it takes a smaller increase in partial pressure to bind the second and third oxygen molecules

• The gradient becomes shallower again since the probability of a fourth oxygen molecule binding to the haemoglobin is lower, as the majority of binding sites are occupied.

A species with haemoglobin with a higher affinity for oxygen will have an oxygen dissociation curve shifted to the…

left

Why does haemoglobin have a lower affinity for oxygen in the respiring tissues than the lungs

There is a higher concentration of carbon dioxide in the tissues, since it is produced as a waste product from aerobic respiration - this lowers the pH of the respiring tissues, which changes the bonding (e.g. ionic) in the tertiary structure and also quaternary structure of the haemoglobin, causing it to change shape

Why does foetal haemoglobin have a higher affinity for oxygen than adult haemoglobin

So that the foetus can obtain oxygen from its mother (via the placenta). It associates with oxygen at a low partial pressure of oxygen, when the maternal haemoglobin is dissociating with oxygen.

Why does foetal haemoglobin need to be (gradually) replaced by adult haemoglobin once the baby is born

So that oxygen can be easily unloaded at the respiring tissues, since the baby, once it is born, is more metabolically active than a foetus

Apoplast pathway

Water moves along the cell walls and intermembrane spaces

Symplast pathway (and full process)

Water moves through the cytoplasm of adjacent cells, moving from cell to cell via plasmodesmata. Process:

• Water evaporates from mesophyll cells into the air spaces, due to heat from the sun

• Those mesophyll cells have a lower water potential so water enters by osmosis from neighbouring cells

• That loss of water causes the neighbouring cells to have a lower water potential so the process repeats

Process of transpiration (cohesion-tension theory)

• Water evaporates from mesophyll cells, then diffuses out of the stomata (transpiration), due to heat from the sun

• Water molecules form hydrogen bonds with each other, so stick together, leading to cohesion

• Water forms a continuous unbroken column across the mesophyll cells and down the xylem

• As more water molecules evaporate, more water molecules are drawn up behind (due to this cohesion)

• Column of water is pulled up the xylem - this is the transpiration pull

• Transpiration puts the xylem under tension - there is a negative pressure within the xylem which pulls up more water from the roots

evidence to support the cohesion-tension theory

• Changes in diameter of tree trunks: in the day, when transpiration is happening at a higher rate, there is more tension, so more negative pressure in the xylem, so the walls of the xylem are pulled inwards and the trunk shrinks in diameter

• If a xylem vessel is broken and air enters, the tree can no longer draw up water, since the continuous column of water is broken so the water molecules can no longer stick together

• When a xylem vessel is broken, water does not leak out, therefore the xylem is under tension, not under pressure

How does sucrose enter the sieve tube elements

• Sucrose diffuses into the companion cells from the photosynthesising cells at the source

• Companion cells actively transport H+ ions into the space between cell walls using ATP

• H+ ions diffuse into the sieve tube elements down their concentration gradients, transporting sucrose molecules with them - this co-transport occurs using co-transport proteins

How does sucrose move through sieve tube elements

• Sucrose has been actively transported into the sieve tube elements at the source

• Making the water potential of the sieve tube elements lower than that of the xylem

• Water enters the sieve tube elements by osmosis, creating a high hydrostatic pressure within them

• Sucrose is actively transported into cells at the sink, lowering these cells’ water potential so water also moves from the sieve tube elements into the sink cells

• This means that the hydrostatic pressure in the sieve tube elements is lower at the sink than at the source

• The sucrose solution therefore moves by mass flow down this hydrostatic gradient from source to sink

Evidence to support the mass flow hypothesis

• There is pressure within the sieve tubes - sap leaks out when they are cut

• Increase in sucrose concentration in the leaves is followed by increase in sucrose concentration in the phloem shortly after

• Downwards flow in phloem occurs in the light but stops when the leaves are shaded

• Metabolic poisons / lack of oxygen stops translocation of sucrose

• Companion cells contain many mitochondria and readily produce ATP

Evidence against the mass flow hypothesis

• Function of sieve plates unclear - seems that they would hinder translocation

• Not all solutes move at the same speed - they should do if mass flow was correct

• Sucrose is delivered at the same rate to all regions, not faster to those with the lowest concentration

Ringing and conclusions drawn from it

• A section of the outer part of the trunk of a tree is removed, which contains the phloem but not the xylem

• After some time the region of the trunk directly above the removed part swells, and contains a solution containing sugars

• Above the trunk the tree lives, below it dies

• This shows that the phloem is responsible for transporting sucrose, not the xylem

Radioactive tracing

Plants exposed to CO2 with 14C are then found to have sugars with 14C in their phloem shortly after

What happens during diastole

• Atria are relaxed and fill with blood from the vena cava and pulmonary vein

• Pressure in atria grows as they fill with blood, and eventually exceeds the pressure in the ventricles, which are also relaxed - this means that the ventricle walls have recoiled and therefore the volume in the ventricles is greater, so the pressure is lower than the aorta and pulmonary artery, so the semi-lunar valves are closed

• The atrio-ventricular valves open, as pressure is higher in the atria than the ventricles, so blood enters the ventricles, aided by gravity

Atrial systole

Atria walls contract, forcing remaining blood into the ventricles, which remain relaxed

Ventricular systole

• After a short delay, which allows the ventricles to fill with blood, ventricles contract together

• Pressure increases in ventricles, becomes higher than in atria, so atrio-ventricular valves close

• Pressure increases until it is higher than just of aorta and pulmonary artery, so semi-lunar valves open

• Blood is forced through the semi-lunar valves into the aorta and pulmonary artery

What are valves in the veins called

Pocket valves

Artery structure and function

• Smooth muscle withstands high blood pressure

• Elastic tissue stretches and recoils to maintain blood pressure

• Smooth endothelium reduces friction

• Protein coat prevents artery wall splitting

• Narrow lumen to maintain high blood pressure

• Folded inner lining (endothelium) - can stretch to allow increased blood flow

Structure and function of arterioles

• Thick muscle layer (relatively thicker than arteries) - smooth muscle contracts and relaxers to change lumen diameter, to control blood flow into capillaries - vasoconstriction and vasodilation

• Less elastic tissues than arteries as pressure is lower

Structure and function of veins

• Pocket valves to prevent backflow of blood

• Wide lumen to allow large volume of blood to flow through

• Thin walls with fewer smooth muscle and elastic fibres as pressure is low

Structure of capillaries

• Thin walls - one cell thick - for short diffusion distance

• Numerous and highly branched - large surface area

• Narrow diameter, so permeate tissues, so no cell is far from a capillary so short diffusion pathway

• Lumen so narrow that RBCs are squeezed flat against the capillary wall - even shorter diffusion distance

• Spaces between the endothelial cells that allow white blood cells to escape into the tissue fluid to combat infections

What does tissue fluid contain

Glucose, amino acids, fatty acids, ions in solution, oxygen, carbon dioxide, (urea?)

Formation of tissue fluid and its return

• High hydrostatic pressure in the arterial end of the capillary forces small molecules out of the capillary, by ultrafiltration

• The loss of tissue fluid inside the capillary lowers the hydrostatic pressure

• This means the hydrostatic pressure at the venous end is lower than in the tissues around it, so tissue fluid is forced back into the capillary

• Water potential is also lower in the venous end than the tissue, as the plasma has lost water and still contains proteins, so water moves into the venous end by osmosis

• The tissue fluid now contains less oxygen, glucose, amino acids and fatty acids, but contains more carbon dioxide (and urea?)

• Remainder of the tissue fluid that does not return to the capillary enters the lymphatic system, moved by hydrostatic pressure and contraction of body muscles, and reenters the bloodstream via two ducts that join veins close to the heart