6-4 Homeostasis is the maintenance of a stable internal environment

Question 1

What is Homeostasis?

Answer 1

• Homeostasis serves to ensure that a constant internal environment consisting of factors such as temperature, water potential, pH and blood glucose level is maintained, despite changes in the external environment of the organism.

Question 2

What is negative feedback?

Answer 2

• Counteracts any change in internal conditions.

Question 3

What element must be present in order for negative feedback to work?

Answer 3

• Sensory receptors to detect changes.

Question 4

What three processes are used to regulate blood glucose in the liver?

Answer 4

• Glycogenesis
  o Making glycogen from glucose removed from the blood.
• Glycogenolysis
  o Breaking down stored glycogen into glucose, which can be released into the blood.
• Gluconeogenesis
  o Synthesis of glucose from other molecules such as amino acids.

Question 5

Describe the response to high blood glucose concentration?

Answer 5

• Rise in glucose concentration is detected by beta cells that are found in the islets of Langerhans in the pancreas.
• Insulin is secreted by beta cells, thus inhibiting the action of alpha cells.
• Insulin travels in the blood to target cells known as hepatocytes in the liver, fat and muscle cells.
• Binding of insulin to the receptors on the plasma membrane of these cells causes adenyl cyclase to convert ATP into cAMP.
• cAMP activates certain enzyme controlled reactions in the cells to stimulate the opening of glucose channels in the cell surface membrane, thus causing more glucose to enter the cell, which is then converted to glycogen or fats and subsequently used for respiration.

Question 6

Describe the response to low blood glucose concentration?

Answer 6

• Alpha cells in the islets of Langerhans in the pancreas detect a fall in blood glucose and secrete the hormone glucagon.
• Glucagon secretion inhibits beta cell action.
• Glucagon stimulation hepatocytes to convert glycogen into glucose.
• Glucose diffuses out of hepatocytes into the blood.
• Cells use fatty acids and amino acids for respiration instead.

Question 7

Describe the process of raising blood glucose levels using secondary messenger adrenaline?

Answer 7

• Adrenaline fuses to a receptor on the cell surface membrane of liver cells and causes the receptor to change shape on the inside of the membrane.
• The changing of the shape on the inside of the membrane activates the enzyme adenyl cyclase which converts ATP to cAMP, this acts as a second messenger.
• The cAMP then changes shape and activates protein kinase enzyme which catalyses the conversion of glycogen into glucose.

Question 8

What is type 1 diabetes?

Answer 8

• Type 1 is insulin dependent diabetes and occurs early in life and results in loss of insulin production.
• In some cases, the immune system destroys beta cells in the pancreas.
• As a result, people with type 1 diabetes have to control their blood sugar level by self-injecting insulin, dose matched to diet and exercise.

Question 9

What is type 2 diabetes?

Answer 9

• Type 2 diabetes is not insulin dependent and often appears later in life.
• It can be caused by decreased insulin production or by glycoprotein receptors on target cells becoming unresponsive to insulin.
• This is often caused by obesity and diet.
• This can be controlled by diet manipulation and exercise.

Question 10

Describe the structure of the kidney?

Answer 10

• AN outer fibrous capsule that protects the kidney
• A layer called the cortex made up of the Bowman’s capsules, convoluted tubules and blood vessels.
• Layer called the medulla made up of loops of Henle’s, collecting ducts and blood vessels.
• The renal pelvis, which collects urine into the ureter.

Question 11

Describe the 4-step process by which water is reabsorbed by the kidneys?

Answer 11

• Ultrafiltration
  o The blood enters the kidney via the renal artery which is under pressure from the heart.
  o This then divides into the afferent arteriole and then a complex network of capillaries called the glomerulus.
  o Water and all soluble plasma components are forced out of the glomerulus, however large proteins are too big to fit.
  o The pressure to do this is aided by the efferent arteriole leaving the glomerulus being narrower than the afferent arteriole entering.
• Selective Reabsorption
  o All glucose in the glomerular filtrate must be reabsorbed into the blood, however waste products like urea don’t need to be.
  o Glucose is reabsorbed in the process of co-transport from the epithelial cells of the proximal convoluted tubule to blood capillaries.
  o It is carried out by actively transporting sodium ions from the epithelial cells to the blood, creating a low concentration of sodium ions in the epithelial cells.
  o Sodium ions therefore consequently move in from the lumen of the proximal convoluted tubule by facilitated diffusion bringing in glucose.
  o The glucose then diffuses into blood capillaries.
• Loop of Henle
  o The loop of Henle acts as a counter-current multiplier.
  o It works to reabsorb water by a multi-step process.
  o To begin with sedum ions are actively transported out of the ascending limb using ATP.
  o This therefore creates a lower water potential between the two limbs (called the interstitial space).
  o The ascending limb is impermeable to water and therefore this mean that water only moves out of the descending limb by osmosis into the area of low water potential.
  o The water then enters the blood capillaries in this region by osmosis.
  o At the hairpin of the loop the water potential is at its lowest, where sodium ions are naturally diffusing out.
• Distal Convoluted Tubule and the Collecting Duct
  o Water natural moves out of the distal convoluted tubule and collecting duct by osmosis.
  o The collecting duct runs parallel to the loop of Henle and therefore as you move down into the medulla ion concentration increases.

Question 12

How can the permeability of the collecting duct be altered by hormones?

Answer 12

• Osmoreceptors in the hypothalamus in the brain detect changes in blood water potential.
• When this falls the receptor shrinks, therefore causing the hormone called ADH to be released.
• This passes to the posterior pituitary gland from where it is secreted into the blood.
• When it arrives at the kidney it binds to receptors on the surface of the collecting duct and activates the enzyme phosphorylase.
• This causes vesicles containing aquaporins to be embedded into the cell surface membrane.
• This increases water permeability as well as urea permeability.
• As a result, urea leaves the collecting duct, causing water to leave and be reabsorbed in the blood.