GASEOUS EXCHANGE

Structure of a hemoglobin

Hemoglobin is a conjugated protein with quaternary structure. A molecule of hemoglobin consists of 4 polypeptide chains which are 2 alpha chains and 2 beta chains. Each of these polypeptide chains is associated with heme group with an iron(II) ion at its centre. Each iron ion can bind with one oxygen molecule. Each hemoglobin binds up to 4 molecules of oxygen to form oxyhemoglobin which is Hb(O₂)₄.

Hemoglobin have high affinity for oxygen when the partial pressure of oxygen is high in the lungs. Hemoglobin releases oxygen easily, when the partial pressure of oxygen is low in the respiring tissues. Hemoglobin carries carbon dioxide from respiring tissues to the lungs, where partial pressure of oxygen is low. Carbon dioxide binds to the amino group of hemoglobin to form carbaminohemoglobin.

Human respiratory system

Explain the oxygen dissociation curve for adult hemoglobin and how it is affected by the Bohr shift.

The dissociation curve for hemoglobin is a sigmoid-shaped curve. The saturation of hemoglobin with oxygen varies with partial pressure of oxygen. Initial uptake of one oxygen molecule by an iron atom of a hemoglobin increases affinity of oxygen by three other subunits.

Low partial pressure of oxygen corresponds to the situation in the respiring tissue. When partial pressure of oxygen is low, oxygen is released. High partial pressure of oxygen corresponds to the situation in the lungs. When partial pressure of oxygen is high, oxygen is taken up by hemoglobin.

Bohr effect occurs when there is lower pH or increase in the partial pressure of carbon dioxide which shifts the oxygen dissociation curve to the right. Oxygen more readily released to the respiring tissue by hemoglobin.

Opening of stoma:

During the day, photosynthesis occurs in the chloroplast in the guard cells. The concentration of carbon dioxide decreases and this cause the pH increases. Phosphorylase enzyme converts starch into glucose-1-phosphate in the guard cells.

The water potential in the guard cells decreases. So water molecules from the subsidiary cells diffuse into the guard cells through osmosis. The turgor pressure of guard cells increases. The guard cells become turgid. The uneven thickness of the wall of the guard cells cause the guard cells to bow outward when turgid. As a result, the stoma opens.

Closing of stoma:

When the light is absence, carbon dioxide released from the respiration is accumulated in the intercellular spaces because photosynthesis is not occur. The pH in the guard cells decreases. A decrease in pH stimulate the formation of starch from soluble sugars. Due to the conversion of sugar to starch, the water potential in the guard cells increases. Water molecules diffuse out from the guard cells to the subsidiary cells. The turgor pressure of guard cells decreases. The guard cells become flaccid. The uneven thickness of the guard cells cause the guard cells to bow inward when flaccid. As a result, the stoma closes.

Role of potassium ions in the mechanism of stomatal opening and closing.