Haemoglobin Structure and Function

Haemoglobin

• 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.

Circulatory System of a Mammal

• 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.

Cardiac Cycle

• 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.

Phloem Transport

• 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 and Against Mass Flow

• 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

• 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.