IB DP Biology- D3.3 Homeostasis - IB Style Questions For SL Paper 2 - New Syllabus
▶️ Answer/Explanation
(a)
Negative feedback is used in homeostasis to maintain the level of a variable within narrow limits around a set point or normal level. A change in the variable produces a response that opposes the original change; for example, an increase is followed by a decrease and a decrease is followed by an increase.
Positive feedback, in contrast, amplifies the original change rather than opposing it. It moves the variable further away from its initial level and can disrupt equilibrium. Positive feedback is generally used for a short period of time, rather than maintaining a variable at a constant level.
(b)
Blood glucose concentration is regulated by the pancreas, whose cells monitor changes in blood glucose concentration.
When blood glucose concentration increases above the normal level, the beta cells of the pancreas secrete insulin. Insulin is transported in the bloodstream to target cells. It stimulates body cells, including liver and muscle cells, to take up glucose from the blood.
Insulin also stimulates the conversion of glucose into glycogen for storage, particularly in liver and muscle cells. It also stimulates body cells to use glucose rather than fat as a substrate for cellular respiration. These processes reduce the blood glucose concentration back towards the normal level.
When blood glucose concentration falls below the normal level, the alpha cells of the pancreas secrete glucagon. Glucagon is transported through the bloodstream and stimulates the breakdown of stored glycogen into glucose, particularly in the liver. Glucose is then released into the blood, increasing the blood glucose concentration.
The opposing actions of insulin and glucagon form a negative feedback mechanism that keeps blood glucose concentration within narrow limits.
(c)
A decrease in blood pH can occur when there is an increase in the concentration of carbon dioxide in the blood. Carbon dioxide reacts with water to form carbonic acid, causing the pH of the blood to decrease.
The lower pH and increased carbon dioxide concentration are detected by chemoreceptors, including those associated with the brainstem and major arteries such as the carotid arteries and aorta.
The brainstem/medulla responds by increasing the rate and/or depth of ventilation. It sends impulses to the diaphragm and intercostal muscles, causing them to contract more frequently and/or more strongly.
The increased muscle contractions increase the ventilation rate. More carbon dioxide is therefore removed from the blood at the alveoli and exhaled. The carbon dioxide concentration in the blood decreases, causing the blood pH to increase towards its optimum level.
The heart may also contract more frequently or more strongly, increasing the rate of blood flow and helping transport carbon dioxide to the lungs for removal.