28.

(a) Explain the effects of gibberellins on plants.

(b) Describe the mechanism of pulmonary gaseous exchange.

(c) Mention the site of secretion and function of the following hormones:

(i) Choleystokinin    (ii) Oxytocin

(iii) Insulin

(a) Gibberellins are chemically gibberellic acid. The effect of gibberellins on plant are :-

(i) The elongation of stem which is carried out by the increase in cell length.

(ii) Overcome the genetic dwarfism in certain plants. Such dwarfism is caused by the mutation of a single gene that is responsible for a block in metabolic pathway leading to the synthesis of gibberellins. When gibberellins are applied to dwarf plants, they elongate normally.

(iii) Breaking of dormancy when the buds of deciduous trees and evergreens growing in temperate regions usually become dormant in late summer or early fall.

(iv) Gibberellins can substitute the photoperiodic or temperature requirement for flowering in certain plants. Its application increase the number of male flowers and decreases the female flowers.

(v) Seed germination is carried by gibberellins.


(b) 

Pulmonary gas exchange takes place in the lungs between the alveoli and the blood. It is also referred to as ‘external respiration’ as it involves the respiratory processes that have contact with the external environment.

The process of pulmonary gas exchange removes CO₂ from the blood and replenishes the bloods O₂ supply.Gas exchange occurs down a pressure gradient, by the process of diffusion.
The air that we breathe consisting of a mixture of gases including O₂ and CO₂.These gases each have a pressure related to their concentration within the gas mixture. These individual pressures are termed partial pressures.

Differences in the partial pressures between the gases in the alveoli and blood create a pressure gradient across the respiratory membrane (membrane separating the alveoli and blood capillaries).

If the pressure on each side of the membrane were the same there would be no exchange of gas and no movement of O₂ and CO₂. Where the partial pressures of O2 and CO₂ are different gas exchange occurs.

Gases move from an area of high concentration (high pressure) to an area of low concentration (low pressure). As the freshly inspired air in the alveoli is high in O₂, the O₂ diffuses across the respiratory membrane into the blood where the concentration of O₂ is low.
The pulmonary artery contains venous (deoxygenated) blood which is carried to alveolar capillaries. The blood inside Capillaries has low oxygen tension but high CO₂ tension. Thus oxygen from alveolar air passes into pulmonary capillaries and CO₂ from Capillaries passes into alveolar air simply by the process of diffusion. When O₂ diffuses into blood in external respiration most of it enters RBC to unite with a respiratory pigment haemoglobin to form unstable compound called oxy haemoglobin. Haemoglobin is complex protein compound containing iron which has a great affinity for oxygen. Thus, oxygenated blood is transported to the body.

c) 

Hormone	Site of secretion	Function
Cholecyctokinin	Enteroendocrine cells in the duodenum	Responsible for stimulating the digestion of fat and protein
Oxytocin	Posterior pituitary gland	Initiates muscle contraction in the uterus during childbirth
Insulin	of the pancreas	Lowers the sugar level of the blood