Transport - transpiration and haemoglobin

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

• 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

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