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Biology A-level > Homeostasis > Flashcards

Homeostasis Flashcards

A level AQA biology

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1
Q

What is homeostasis?

A

The maintenance of a stable internal environment

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2
Q

What 3 things affect the internal environment?

A
  1. Temperature
  2. pH
  3. Glucose
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3
Q

Describe negative feedback

A
  1. Receptors detect when a level is too high or too low, and the information’s communicated via the nervous system or the hormonal system to effectors
  2. The effectors respond to counteract the change
  3. Negative feedback mechanism
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4
Q

Describe positive feedback

A
  1. The effectors respond to further increase the level away from normal
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5
Q

Why can positive feedback be useful?

A

Useful to rapidly activate something e.g a blood clot after an injury

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6
Q

When can positive feedback occur?

A

When a homeostatic system breaks down

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7
Q

What is the normal blood glucose concentration?

A

90 mg per 100cm3 of blood

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8
Q

When does blood glucose concentration rise and fall?

A

Rises - eating food containing carbohydrates

Falls - after exercise (more glucose used in respiration to release energy)

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9
Q

What monitors blood glucose concentration?

A

Cells in the pancreas

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10
Q

What 2 hormones control blood glucose concentration?

A
  1. Insulin

2. Glucagon

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11
Q

Where are insulin and glucagon secreted from?

A

islets of Langerhans (cells in the pancreas)

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12
Q

What does insulin do? Describe the process

A

Lowers blood glucose concentration when it’s too high

  1. Insulin binds to specific receptors on the cell membrane of liver cells and muscle cells
  2. It increases the permeability of muscle-cell membranes to glucose, so the cells take up more glucose. This involves increasing the number of channel proteins in the cell membranes
  3. Insulin also activates enzymes in liver and muscle cells that convert glucose into glycogen
  4. The cells are able to store glycogen in their cytoplasm, as an energy source
  5. Glycogenesis (glucose—> glycogen)
  6. Insulin also increases the rate of respiration of glucose, especially in muscle cells
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13
Q

What does glucagon do? Describe the process

A

Increases blood glucose concentration when it’s too low

  1. Glucagon binds to specific receptors on the cell membranes of liver cells
  2. Glucagon activates enzymes in liver cells that break down glycogen into glucose (glycogenolysis)
  3. Glucagon also activates enzymes that are involved in the formation of glucose from glycerol and amino acids
  4. The process of forming glucose from non-carbohydrates is called gluconeogenesis
  5. Glucagon decreases the rate of respiration of glucose in cells
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14
Q

Describe the negative feedback mechanism when blood glucose concentration is too high

A
  1. Pancreas detects blood glucose concentration is too high
  2. Beta cells secrete insulin and alpha cells stop secreting glucagon
  3. Insulin binds to receptors on liver and muscle cells
  4. Cells take up more glucose, glycogenesis is activated, cells respire more glucose
  5. Less glucose in blood
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15
Q

Describe the negative feedback mechanism when blood glucose concentration is too low

A
  1. Pancreas detects blood glucose concentration is too low
  2. Alpha cells secrete glucagon and bet cells stop secreting insulin
  3. Glucagon binds to receptors on liver cells
  4. Glycogenolysis and gluconeogenesis are activated, cells respire less glucose
  5. Cells release glucose into the blood
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16
Q

How does insulin make glucose transporters available for facilitated diffusion?

A

When insulin levels are low, GLUT4 (glucose transporter) is stored in vesicles in cytoplasm of cells. When insulin binds to receptors on the cell-surface membrane, it triggers the movement of GLUT4 to the membrane. Glucose can then be transported into the cell through the GLUT4 protein, by facilitated diffusion

17
Q

How does adrenaline increase the blood glucose concentration?

A

Adrenaline (hormone secreted from your adrenal glands found above kidneys) is secreted when there’s a low concentration of glucose in your blood, when you’re stressed or when you’re exercising. Adrenaline binds to receptors in the cell membrane of liver cells. It activates glycogenolysis and inhibits glycogenesis. It also activates glucagon secretion and inhibits insulin secretion, which increases glucose concentration. Adrenaline gets the body ready for action by making more glucose available for muscles to respire

18
Q

What is a second messenger?

A

A chemical signal

19
Q

How can adrenaline and glucagon activate glycogenolysis (glycogen —> glucose) inside a cell even though they bind to receptors on the outside of the cell?

A

The receptors for adrenaline and glucagon have specific tertiary structures that make them complementary in shape to their respective hormones. Adrenaline and glucagon bind to their receptors and activate adenylate cyclase, which converts ATP into a second messenger (chemical signal). The second messenger is called cAMP (cyclic AMP). cAMP activates protein kinase A, which activates a chain of reactions that break down glycogen into glucose (glycogenolysis)

20
Q

What is diabetes?

A

A condition where blood glucose concentration can’t be controlled properly

21
Q

Describe type 1 diabetes

A

Immune system attacks beta cells in islets of Langerhans so they can’t produce any insulin. Hyperglycaemia can occur. The kidneys can’t absorb all this glucose, so some of it is excreted in urine

22
Q

How is type 1 diabetes treated?

A

Insulin therapy

23
Q

Describe type 2 diabetes

A

Occurs when the beta cells don’t produce enough insulin or when the body’s cells don’t respond properly to insulin. So cells don’t take up enough glucose meaning blood glucose concentration is higher than normal

24
Q

How can you determine the concentration of glucose solution?

A

Colorimetry

  1. Quantitative Benedict’s reagent- blue colour lost but brick red precipitate not produced
  2. Colorimetry can be used to measure light absorbance of solution after the quantitative Benedict’s test has been carried out
  3. The higher the glucose concentration the paler the solution, decreasing absorbance of solution
25
Q

What do the kidneys do?

A

Excrete waste and regulate blood water potential

26
Q

What is ultrafiltration?

A

As the blood passes through capillaries in the cortex (outer layer) of the kidneys, substances are filtered out of the blood and into long tubules that surround the capillaries

27
Q

What is selective absorption?

A

Useful substances, such as glucose and the right amount of water, are reabsorbed back into the blood after ultrafiltration. The remainder unwanted substances pass along to the bladder and are excerpted as urine

28
Q

Describe ultrafiltration (10 steps)

A
  1. Blood from the renal artery enters smaller arterioles in the cortex of the kidney
  2. Each arteriole splits into a glomerulus ( a bundle of capillaries looped inside a hollow ball called a Bowman’s capsule)
  3. This is where ultrafiltration takes place
  4. The arteriole that takes blood into each glomerulus is called the afferent arteriole, and the arteriole that takes blood away from the glomerulus is called the efferent arteriole.
  5. The efferent arteriole is smaller in diameter than the afferent arteriole, so the blood in the glomerulus is under high pressure
  6. The high pressure forces liquid and small molecules in the blood out of the capillary and into the Bowman’s capsule
  7. The liquid and small molecules pass through 3 layers to get into the Bowman’s capsule and enter the nephron tubules - the capillary wall. a membrane (basement membrane) and the epithelium of the Bowman’s capsule
  8. Larger molecules like proteins and blood cells can’t pass through, so stay in the blood. The substances that enter the Bowman’s capsule are known as the glomerular filtrate
  9. The glomerular filtrate passes along the rest of the nephron and useful substances are reabsorbed along the way
  10. Finally, the filtrate flows through the collecting duct and passes out of the kidney along the ureter
29
Q

Describe selective reabsorption (6 steps)

A
  1. Selective reabsorption takes place as the glomerular filtrate flows along the proximal convoluted tubule (PCT) through the loop of Henle, and along the distal convoluted tubule (DCT)
  2. Useful substances leave the tubules of the nephrons and enter the capillary network that’s wrapped around them
  3. The epithelium of the wall of the PCR has microvilli to provide a large surface area for the reabsorption of useful materials from the glomerular filtrate (in the tubules) into the blood (in the capillaries)
  4. Useful solutes, like glucose, are reabsorbed along the PCT by active transport and facilitated diffusion
  5. Water enters the blood by osmosis because the water potential of the blood is lower than that of the filtrate. Water is reabsorbed from the PCT, loop of Henle, DCT and the collecting duct
  6. The filtrate that remains is urine, which passes along the ureter to the bladder
30
Q

Describe osmoregulation

A

If the water potential of the blood is too low (the body is dehydrated), more water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is concentrated, so less water is lost during excretion

If the water potential of the blood is too high (the body is too hydrated), less water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more dilute, so more water is lost during excretion

Water is reabsorbed into the blood along almost all of the nephron, but regulation of water potential mainly takes place in the loop of Henle, DCT and collecting duct. The volume of water reabsorbed by the DCT and collecting duct is controlled by hormones

31
Q

How does the loop of Henle maintain a Sodium Ion Gradient? (5 steps)

A

Loop of Henle - ascending and descending limb

  1. Near the top of the ascending limb, Na+ ions are pumped out into the medulla using active transport . The ascending limb is impermeable to water, so the water stays inside the tubule. This creates a low water potential in the medulla, because there’s a high concentration of ions
  2. Because there’s a low water potential in the medulla than in the descending limb, water moves out of the descending limb (permeable to water) into the medulla by osmosis. This makes the filtrate more concentrated (the ions can’t diffuse out - the descending limb isn’t permeable to them). The water in the medulla is reabsorbed into the blood through the capillary network
  3. Near the bottom of the ascending limb Na+ ions diffuse out into the medulla, further lowering the water potential in the medulla. The ascending limb is impermeable to water, so it stays in the tubule
  4. Water moves out of the distal convoluted tubule (DCT) by osmosis and is reabsorbed into the blood
  5. The first 3 stages massively increase the ion concentration in the medulla, which lowers the water potential. This causes water to move out of the collecting duct by osmosis. As before, the water in the medulla is reabsorbed into the blood through the capillary network
32
Q

What is the water potential of the blood monitored by?

A

Cells (osmoreceptors) in a part of the brain (hypothalamus)

33
Q

How is water reabsorption controlled?

A

When the water potential of the blood decreases, water will move out of the osmoreceptor cells by osmosis. This causes the cells to decrease in volume. This sends a signal to other cells in the hypothalamus, which send a signal to the posterior pituitary gland. This causes the posterior pituitary to release a hormone called antidiuretic hormone (ADH) into the blood. ADH makes the walls of the DCT and collecting duct more permeable to water. This means more water is reabsorbed from these tubules into the medulla and into the blood by osmosis. A small amount of concentrated urine is produced, which means less water is lost from the body

34
Q

How does ADH change the water content of the blood when it’s too low?

A

Blood ADH level rises when you’re dehydrated

  1. The water content of the body drops, so its water potential drops
  2. This is detected by osmoreceptors in the hypothalamus
  3. the posterior pituitary gland is stimulated to release more ADH into the blood
  4. More ADH means that the DCT and collecting duct become more permeable, so more water is reabsorbed into the blood by osmosis
  5. A small amount of highly concentrated urine is produced and less water is lost
35
Q

How does ADH change the water content of the blood when it’s too high?

A

Blood ADH level falls when you’re hydrated

  1. The water content of the blood rises, so its water potential rises
  2. This is detected by the osmoreceptors in the hypothalamus
  3. The posterior pituitary gland releases less ADH into the blood
  4. Less ADH means that the DCT and collecting duct become less permeable, so less water is reabsorbed into the blood by osmosis
  5. A large amount of dilute urine is produced and more water is lost

Biology A-level (14 decks)

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