Section 6 - Organisms respond to changes in their environment: 16. Homeostasis

Question 1

What is Homeostasis

Answer 1

The maintenance of an organisms internal environment within restricted limits.
(Involves maintaining the chemical make-up, volume and other features of the tissue fluid to allow for the normal functioning of cells)

Question 2

Why is homeostasis important

Answer 2

• Proteins (eg. enzymes) are sensitive to pH and temperature changes, and may be denatured.
• Changes in water potential of tissue fluid may cause cells to shrink, expand or burst
• Maintaining a constant internal environment despite external changes allows organisms to have a wider geographical range (Increasing the chance of finding food and shelter, aiding survival)

Question 3

What are the control mechanisms of self-regulating system that carries out homeostasis

Answer 3

• Optimum point: Point at which system operates best
  → Monitored by…
• Receptor: Detects and variation from the optimum point (stimulus)
  → Informs the…
• Coordinator: Coordinates information from the receptor
  → Transmits instructions to…
• Effector: Brings about changes to return system to optimum point (muscle or gland)
  → Will result in…
• Feedback mechanism: Process by which the internal conditions return to their normal state

Question 4

What are endotherms

Answer 4

Organisms that derive their heat from metabolic activities that take place inside their bodies

Question 5

How do endotherms regulate body temperature

Answer 5

• Vasoconstriction/vasodilation: Changing the diameter of the arterioles alters the volume of blood close to the skin, to alter the loss of heat
• Shivering: Involuntary muscle movements increasing metabolic activity to release heat
• Raising hair: If the hair is raised by erector muscles, a layer of air is trapped against the skin, acting as an insulator to trap heat
• Altering metabolic rates: Metabolic activity releases heat, so hormones alter the rate, depending on the heat required
• Sweating: Evaporation of sweat is an endothermic process, absorbing energy from the body, reducing the temperature
• Behavioural mechanisms: Sheltering from wind, basking in sun, huddling together, etc.

Question 6

What are ectotherms

Answer 6

Organisms that obtain a proportion of their heat from external sources (environment)

Question 7

How do ectotherms regulate body temperature

Answer 7

• Exposing themselves to the sun, to increase their body temperature
• Taking shealter, preventing themselves from overheating by remaining in the shaded
• Gaining warmth from the ground

Question 8

What is a negative feedback mechanism

Answer 8

When the change produced by the control system returns the conditions back to the optimum point, leading to the reversal in the stimulus detected, turning off the system
(There are separate feedback mechanisms to regulate variations from the mean in each, giving a greater degree of homeostatic control)
eg. Regulation of blood glucose conc.

Question 9

What is a positive feedback mechanism

Answer 9

When deviations from the optimum conditions result in a response that leads to further deviation from normal levels.
Occurs in some systems, but can happen if there is a breakdown of a control system (as a result of certain diseases)
eg. Generator potential, leading to the formation of an action potential

Question 10

What are hormones

Answer 10

Chemicals produced by endocrine glands and secreted directly into the blood, to be carried to target cells with complementary receptors That lead to a response
- Effective in very low concentration
- Widespread and long-lasting effect

Question 11

What is the second messenger model of enzyme action
(eg. Glycogenolysis)

Answer 11

• Adrenaline (1st messenger) binds to a transmembrane protein receptor on the cell-surface membrane of a liver cell
• This causes the protein to change shape on the inside of the membrane
• This leads to the activation of the enzyme ‘Adenyl Cyclase’, which converts ATP into ‘cyclic AMP’ (cAMP)
• The cAMP (2nd messenger) binds to and activates the enzyme ‘Protein Kinase’
• This enzyme then catalyses the conversion of Glycogen into glucose, releasing it into the blood for use in respiration

Question 12

What hormones are involved in the regulation of blood glucose

Answer 12

• Insulin: Decreases blood glucose concentration
• Glucagon: Increases blood glucose concentration
  (+ Adrenaline: Increases blood glucose concentration)

Question 13

What is the role of the pancreas in the regulation of blood glucose

Answer 13

• Produces enzymes for digestion (Protease, amylase, lipase) and hormones for regulating blood glucose
• Organ is mainly made up of cells that produce digestive enzymes, there is a small group of hormone-producing cells, called the ‘Islet of Langerhans’
• ‘Islet of Langerhans’ is made up of:
  
  • α-cells (larger and produce glucagon)
  • β-cells (smaller and produce insulin)

Question 14

What is the role of the liver in the regulation of blood glucose

Answer 14

The liver is where the hormones from the pancreases have an effect that alters the blood glucose concentration.
There are three main processes associated with the regulation of blood glucose, which occur in the liver:
- Glycogenesis
- Glycogenolysis
- Gluconeogenesis

Question 15

What is Glycogenesis

Answer 15

The conversion of glucose into glycogen
∴ Can be stored in the liver, lowering blood glucose concentration

Question 16

What is Glycogenolysis

Answer 16

The breakdown if glycogen into glucose
∴ Releases glucose from the liver, increasing blood glucose concentration

Question 17

What is Gluconeogenesis

Answer 17

The production for glucose from sources other than carbohydrates
∴ If the glycogen store runs out, the liver will produce glucose from other sources such as glycerol and amino acids, increasing blood glucose concentration

Question 18

What are the main factors that influence blood glucose concentration

Answer 18

Blood glucose comes from 3 sources:
- Food (absorbed following the hydrolysis of carbohydrates)
- Glycogenolysis (Hydrolysis of glycogen)
- Gluconeogenesis (Produced from other sources)

∴ Fluctuations in blood glucose concentration are caused by:
- Sporadic eating and varied diets
- Different respiratory rates depending on activity level
- etc.
(These fluctuations are stabilised at an optimum level by hormones)

Question 19

Where is Insulin produced and when is it released

Answer 19

ꞵ-cells in the ‘Islets of Langerhans’ of the pancreas, have receptors that detect an increase in blood glucose concentration, and respond by releasing Insulin into the blood.

Question 20

What is the effect of an increased insulin concentration in the blood

Answer 20

‘Insulin’ attaches to complementary glycoprotein receptors on the cell-surface membrane of almost all body cells (not red blood cells), resulting in:
- Change in the tertiary structure of carrier proteins, opening them to allow glucose to enter the cell by facilitated diffusion
- Increase in the number of carrier proteins, by causing vesicles (containing transmembrane proteins) to fuse with the cell-surface membrane
- Activation enzymes that convert glucose into glycogen and fat (for storage)

∴ Insulin causes blood glucose concentration to be lowered

Question 21

How is insulin part of a negative feedback loop

Answer 21

Insulin is released when blood glucose conc. is too high, and the hormone causes it to be reduced back to the optimum level, creasing it’s own production.

Question 22

Where is glucagon produced and when is it released

Answer 22

α-cells in the ‘Islets of Langerhans’ of the pancreas, have receptors that detect a decrease in blood glucose concentration, and respond by releasing glucagon into the blood.

Question 23

What is the effect of an increased glucagon concentration in the blood

Answer 23

‘Glucagon’ attaches to complementary receptors on the cell-surface membrane liver cells, resulting in:
- Protein receptors opening as the hormone attaches, allowing glucose to enter the blood
- Activation of enzymes that catalyse glycogenolysis (Glycogen → Glucose)
- Activation of enzymes involved in gluconeogenesis (formation of glucose from amino acids, glycerol, etc)

∴ Glucagon causes blood glucose concentration to be increased

Question 24

How is glucagon part of a negative feedback loop

Answer 24

Glucagon is released when blood glucose conc. is too low, and the hormone causes it to be increased back to the optimum level, creasing it’s own production.

Question 25

Where is adrenaline produced

Answer 25

Adrenal glands (just above the kidneys)

Question 26

What is the role of adrenaline in regulating blood glucose concentration

Answer 26

• Attaches to protein receptors on the cell-surface membrane of target cells (1st messenger), leading to the the conversion of glycogen to glucose through the ‘second messenger model of hormone action’
• Inactivates the enzymes that catalyse glycogenesis (Glucose → Glycogen)

∴ Adrenaline causes blood glucose concentration to be increased

Question 27

How does the use of both insulin and glucagon allow for a self-regulating control of blood glucose concentration

Answer 27

The two hormones act antagonistically to give a self-regulating system:
- Insulin lowers glucose conc.
- Glucagon increases glucose conc.
- Negative feedback determines the quantity of each produced to control fluctuations about an optimum value

Question 28

What is Diabetes

Answer 28

Disease in which a person is unable to metabolise carbohydrates (glucose), so blood glucose can’t be controlled

Question 29

What are the two types of ‘sugar diabetes’ (diabetes mellitus)

Answer 29

• Type 1: Insulin dependent
• Type 2: Insulin independent

Question 30

What is Type 1 Diabetes

Answer 30

Insulin dependent:
- The body is unable to produce insulin
- Normally begins in childhood and develops quickly
- May be a result of an autoimmune response, where the body’s immune system targets it’s own cells (β-cells)

Question 31

What is Type 2 Diabetes

Answer 31

Insulin independent
- Normally due to the glycoprotein receptors on body cells being lost, or they lose their responsiveness to insulin
- However, may be due to inadequate supply of insulin from pancreas
- Usually occur in later life, developing gradually
- Caused by obesity and poor diet, leading to the cells becoming insensitive to insulin

Question 32

What are the main signs and symptoms of diabetes

Answer 32

• High blood glucose conc.
• Glucose in urine
• Excessive need to urinate
• Genital itching or frequent episodes of thrush
• Weight loss
• Blurred vision
• Tiredness
• Increased thirst and hunger

Question 33

How is type 1 diabetes managed and controlled

Answer 33

Insulin injections (2-4 times daily)
- This provides a substitute for insulin that can’t be produced
- Dose must match the glucose intake to maintain blood glucose conc.
∴ Blood glucose conc. monitored using biosensors
- Insulin must be injected, as is a protein, and being take orally would lead to it being digested
- If managed in this way, people can live normal lives

Question 34

How is type 2 diabetes managed and controlled

Answer 34

Regulating the intake of carbohydrates in the diet to match the level of exercise
- May be supplemented by insulin injections, or by drugs to stimulate insulin production

Question 35

What is osmoregulation

Answer 35

The homeostatic control of the water potential of the blood, maintaining an optimum conc. of water and dissolved salts (carried out by the Nephrons, in the kidneys)

Question 36

What are the main structural features of a Mammalian kidney

Answer 36

• Fibrous capsule: Outer membrane that protects the kidneys
• Cortex: Light coloured outer region made up of the Bowman’s capsule, convoluted tubules and blood vessels
• Medulla: Dark coloured inner region made up of the loops of Henle, collecting duct and blood vessels
• Renal pelvis: Funnel shaped cavity that collects urine into the ureter
• Ureter: Tube that carries urine to the bladder
• Renal artery: Supplies the kidney with blood from the heart
• Renal vein: Returns the blood to the heart

Question 37

What is a Nephron

Answer 37

Functional unit of the kidney that carries out osmoregulation, made up of a narrow tube up to 14mm long.

Question 38

What are the main structural features of a nephron

Answer 38

• Bowman’s (renal) capsule: Closed end at the start of the nephron
  
  • Cup shaped, surrounded by capillaries (glomerulus)
  • Inner layer made of specialised cells called podocytes
• Proximal convoluted tubule: Series of loops before the loop of Henle
  
  • Surrounded by capillaries
  • Inner wall made of epithelial cells, containing microvilli
• Loop of Henle: Long hairpin loop
  
  • Surrounded by capillaries
  • Extends from the cortex into the medulla (and back)
• Distal convoluted tubule: Series of loops after the loop of Henle
  
  • Fewer capillaries than proximal convoluted tubule
  • Inner wall made of epithelial cells
• Collecting duct: Tube connecting the distal convoluted tubule to the pelvis of the kidney
  
  • Connected to a number of different nephrons

Question 39

What are the main blood vessels associated with a nephron

Answer 39

• Afferent arteriole: Vessel that arises from the renal artery and enters the Bowman’s capsule, supplying the nephron with blood
• Glomerulus: Multi-branched know of capillaries from which fluid is forced out of the blood and into the Bowman’s capsule
• Efferent arteriole: Vessel that leaves the Bowman’s capsule, with smaller diameter than the afferent arteriole, resulting in an increased blood pressure within the glomerulus (causing ultrafiltration)
• Blood capillaries: Connected network of capillaries surrounding the loop of Henle and convoluted tubules, allowing salts, water and glucose to be reabsorbed (merge into venules and then the Renal vein)

Question 40

What are the stages of osmoregulation that occur in a nephron (in the kidneys)

Answer 40

1) Formation of a Glomerular filtrate by ultrafiltration in the Bowman’s capsule
2) Reabsorption of glucose and water by the proximal convoluted tubule
3) maintenance of a gradient of sodium ions in the medulla, by the loop of Henle
4) Reabsorption of water by the distal convoluted tubule and collecting duct

Question 41

What is the role of the Bowman’s capsule in the Nephron

Answer 41

Ultrafiltration of the blood, resulting in the formation of a glomerular filtrate

Question 42

What is the process that leads to the formation of a Glomerular filtrate by ultrafiltration in the Bowman’s capsule

Answer 42

• Blood enters the kidney through the renal artery, which branches into millions of arterioles
• The Afferent arterioles enter the Bowman’s capsule and divide into a complex knot of capillaries (glomerulus)
• The glomerular capillaries later merge into the efferent arteriole
• The efferent arteriole has a smaller diameter than the afferent arteriole, so there is a build up of hydrostatic pressure in the glomerulus
• This causes water, glucose and mineral ions to be forced out of the capillaries to form the ‘glomerular filtrate’ (Blood cells and proteins are too large, so remain in the blood)

Question 43

What resists the movement of the filtrate out of the glomerulus

Answer 43

• Connective tissue and endothelial cells of the blood capillaries
• Epithelial cells of the Bowman’s capsule
• Hydrostatic pressure of the fluid in the Bowman’s capsule space
• Low water potential of the blood in the glomerulus

Question 44

What are the main adaptations in the Nephron that allow the filtrate to leave the glomerulus, despite the resistance against this

Answer 44

• The efferent arteriole has a smaller diameter than the afferent arteriole, so there is a build up of hydrostatic pressure in the glomerulus, forcing the filtrate out of the blood
• The inner layer of the Bowman’s capsule is made up of specialised cells called ‘Podocytes’
  
  • Highly branched with spaces between them
  • Allows the filtrate to pass between cell, rather than through them, reducing resistance
• The endothelium of the glomerular capillaries also have spaces between to allow the filtrate to pass

Question 45

What is the role of the Proximal convoluted tubule in the Nephron

Answer 45

The reabsorption of glucose and water from the filtrate, back into the blood (nearly 85% of filtrate is reabsorbed)

Question 46

What is the process of the reabsorption of glucose and water by the Proximal convoluted tubule

Answer 46

• Na+ is actively transported out of the epithelial cells lining the proximal convoluted tubule and into the blood
• ∴ Na+ diffuses down it’s conc. gradient from the Lumen of the proximal convoluted tubule into these epithelial cells
• By co-transport, the Na+ carries specific molecules such as glucose into the epithelial cells during this facilitated diffusion
• This glucose then diffuses down it’s conc. gradient into the blood
• ∴ Valuable molecules (glucose, amino acids, water, etc) can be reabsorbed into the blood

Question 47

What are the main adaptations of the Proximal convoluted tubule that allows substances to be reabsorbed

Answer 47

• Microvilli on the epithelial cells provide a large surface area for absorption from the lumen
• Infoldings at the base of cells also provide a large surface area for absorption into the blood
• High density of mitochondria in these cells provide ATP for the active transport of Na+

Question 48

What is the structure and function of the loop of Henle in the Nephron

Answer 48

Hairpin-shaped tubule that extends into the medulla of the kidney
- Allows water to be reabsorbed from the collecting duct, by maintaining a water potential
- Descending limb: Narrow with thin walls, permeable to water
- Ascending limb: Wider, with thick walls, impermeable to water

Question 49

What is the process that occurs in the loop of Henle, so a gradient of sodium ions (and water potential) is maintained and water can be reabsorbed

Answer 49

1) Na+ is actively transported out of the ascending limb, creating a low water potential in the region between the two limbs (Interstitial region)
2) The walls of the ascending limb are impermeable to water, so water instead passes out of the filtrate from the descending limb by osmosis, and moves into the blood
3) Na+ moves down it’s conc. gradient into the descending limb, while water is progressively lost
∴ Water potential of the filtrate is lowered further down the loop (lowest at the bottom)
4) At the base of the ascending loop, Na+ is actively transported out of the filtrate as it moves up (step 1), progressively increasing it’s water potential
5) This movement of ions means that the interstitial space between to ascending limb and the collecting duct has a high water potential at the top, decreasing further into the medulla
6) The collecting duct is permeable to water, so water passes out of it into the interstitial space, and into the blood vessels
7) The water potential of the filtrate is lowered as it moves down the collecting duct, but due to the opposite movement in the ascending limb and the movement of Na+ (step 5), the water potential of the interstitial space also decreases further down
∴ A water potential gradient is maintained down the collecting duct, so water can continue to be reabsorbed

Question 50

What is the role of the distal convoluted tubule in the Nephron

Answer 50

Makes final adjustments to the water and salts that are reabsorbed (controlling blood pH), as the permeability of the walls is altered by the presence of various hormones

Question 51

What is a counter-current multiplier

Answer 51

When two liquids flow past each other in opposite directions, resulting in an exchange of substances that is greater that if they flowed in the same direction

Question 52

How does the loop of Henle and the collecting duct act as a counter-current multiplier

Answer 52

Due to the opposing motion of the filtrate in the ascending loop of Henle and the collecting duct, the filtrate in the collecting duct always meets interstitial fluid with a lower water potential
- ∴ Although small, a water potential gradient is maintained all the way down the collecting duct
- ∴ There is a steady flow of water out of the filtrate and into the blood, so ~80% is reabsorbed

Question 53

What is the role of hormones in osmoregulation

Answer 53

Homeostatic control of osmoregulation is achieved by the Hormone ‘ADH’ (anti-diuretic hormone) that acts on the distal convoluted tubule and the collecting duct

Question 54

What is the effect of ADH on the cell-surface membrane of cells in the walls of the distal convoluted tubule and collecting duct

Answer 54

ADH increases the permeability to water, so more can be reabsorbed
- Specific receptors on the cell-surface membrane bind to ADH, leading to the activation of the enzyme ‘phosphorylase’
- This enzyme causes vesicles in the cell to fuse with the cell-surface membrane
- These vesicles are made up of plasma membranes, containing numerous water protein channels, called ‘aquaporins’
∴ Cell becomes more permeable to water as a result of ADH
(so more can be reabsorbed into the blood)

Question 55

What can cause a fall in water potential of the blood

Answer 55

• Too little water being consumed
• Too much sweating
• Large amounts of ions being taken in (eg. sodium chloride)

Question 56

What is the hormonal response to a fall in blood water potential

Answer 56

• Osmoreceptors in the hypothalamus detect a fall in blood water potential
  
  • Lower blood water potential causes water to move out of the receptor cells, so they shrink
  • Results in nerve impulses being sent to the pituitary gland
• The pituitary gland secretes ADH into the blood
• ADH travels to the Kidney, where it increases the permeability of the cell-surface membrane of cells lining the distal convoluted tubule and collecting duct
• This increased permeability allows Urea to leave the tubule, lowering the water potential of the surrounding fluid
• This means water then leaves the collecting duct and is reabsorbed into the blood
• As the reabsorbed water came from the blood originally, this process alone can’t increase the blood water potential, it just prevents it getting lower
• ∴ To increase blood water potential, the osmoreceptors send nerve impulses to the brain to induce thirst
• Osmoreceptor then detect the increase in water potential, so cause less ADH to be released, resulting in the nephron’s permeability to water returning to normal
  (= negative feedback)

Question 57

What can cause a rise in the water potential of the blood

Answer 57

• Large vol. of water being consumed
• Salts being used up in metabolism and not replaced by diet

Question 58

What is the hormonal response to a rise in blood water potential

Answer 58

• Osmoreceptors in the hypothalamus detect an increase in blood water potential
  
  • Higher blood water potential causes water to move into the receptor cells, so they swell
  • Results in nerve impulses being sent to the pituitary gland
• The pituitary gland secretes less ADH into the blood
• This leads to a decrease in the permeability to water (and urea) of the distal convoluted tubule and collecting duct in the kidneys
• ∴ Less water is reabsorbed, so more dilute urine is produced and blood water potential falls
• Osmoreceptor then detect the decrease in water potential, so cause more ADH to be released, resulting in the nephron’s permeability to water returning to normal
  (= negative feedback)