Haemoglobin Structure and Function

Study Notes

Haemoglobin is a water-soluble globular protein composed of two beta polypeptide chains and two alpha helices.
Each haemoglobin molecule forms a complex containing a haem group and can carry four oxygen molecules.
The affinity of oxygen for haemoglobin varies depending on the partial pressure of oxygen, which is a measure of oxygen concentration.
In the lungs, oxygen binds to haemoglobin tightly due to high partial pressure, a process known as loading.
During respiration, oxygen is used up, and the partial pressure decreases, causing a decrease in the affinity of oxygen for haemoglobin, leading to unloading.
Dissociation curves illustrate the change in haemoglobin saturation as partial pressure changes.
Fetal haemoglobin has a higher affinity for oxygen compared to adult haemoglobin to compensate for low oxygen levels in the placenta.
The affinity of haemoglobin for oxygen is also affected by the partial pressure of carbon dioxide, with increased CO2 levels decreasing the affinity.

In large organisms, the surface area to volume ratio is not sufficient for diffusion alone to supply oxygen and other substances to cells.
A circulatory system is necessary, consisting of a suitable medium (blood), a means of moving the medium (heart), a mechanism to control flow (valves), and a closed system of vessels.
The heart is a double pump, with one pump oxygenating blood in the lungs and the other pumping oxygenated blood to the body.
The heart has four chambers: left and right atria, and left and right ventricles.
Valves (bicuspid and tricuspid) separate the atria and ventricles.
Four main vessels connect the heart: aorta, pulmonary artery, pulmonary vein, and vena cava.

The heart is myogenic, initiating its own contraction.
The sinoatrial node in the right atrium is the pacemaker, initiating a wave of electrical stimulation that causes the atria to contract.
The ventricles contract after the atria, due to the septum blocking the wave of excitation.
The cardiac cycle involves atrial contraction, atrial relaxation, ventricular contraction, and ventricular relaxation.

Translocation is an energy-requiring process that transports assimilates like sucrose from sources (leaves) to sinks (roots and meristem).
The process involves active loading of sucrose into companion cells, facilitated diffusion, and osmosis-driven mass flow in the phloem.
Water enters the sieve tube elements, increasing hydrostatic pressure, and then moves down the tube from high to low pressure areas.
Sucrose is eventually removed from the sieve tube elements by diffusion or active transport.

Evidence for mass flow: pressure in sieve tube elements, higher sucrose concentration in leaves than roots, and inhibition of translocation by metabolic poisons or lack of oxygen.
Evidence against mass flow: unclear function of sieve plates, non-uniform solute movement, and uniform sucrose delivery to all regions.

Ringing experiments involve removing the phloem and bark of a tree, leaving only the xylem in the centre.
The tissue above the missing ring swells with sucrose solution, while the tissue below dies, indicating that sucrose is transported in the phloem.