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Biology > 14. Homeostasis > Flashcards

14. Homeostasis Flashcards

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1
Q

What does ‘internal environment’ refer to?

A

All the conditions inside the body, where cells function. The immediate environment of a cell is the tissue fluid, so its composition affects cell function.

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2
Q

Which features of the tissue fluid affect cell function?

A

Temperature - too high can denature proteins, too low can slow metabolism.
Water potential - too high may cause cell to burst, too low can draw water out and slow metabolism.
Glucose concentration - too high draws water out, too low slows respiration.

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3
Q

Define ‘negative feedback’.

A

The process by which changes in an internal condition are kept within narrow limits of a set point - negative feedback minimises the distance.

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4
Q

Describe a negative feedback loop.

A
  • Receptor detects stimuli involved with changes in the physiological factor. It is constantly monitored so a steady input is obtained.
  • The input is sent through the nervous/endocrine system and to the central control, in the CNS.
  • The central control sends a message to an effector (muscle/gland) that carries out an output (corrective actions).
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5
Q

Distinguish between the nervous and endocrine systems.

A

Both are coordination systems; the nervous system transmits electrical impulses along neurones, and the endocrine system uses chemical messengers (hormones) which travel in the bloodstream as a form of long-distance cell signalling.

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6
Q

Where does thermal energy in endothermic homeotherms come from?

A

From respiration (mainly in the liver) - the blood absorbs this heat. Enables these animals to be active during the day or night.

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7
Q

How is thermoregulation initiated?

A

Thermoreceptor cells in the hypothalamus monitor blood temperature to keep it at a set point of 37°C. Skin receptors give an early warning for potential changes in core temperature.
The hypothalamus sends impulses to generate corrective responses.

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8
Q

Describe the body’s responses to high heat.

A

Vasodilation - muscles in the walls of arterioles supplying blood to capillaries near the skin surface RELAX.
Muscles at the base of hairs in the skin relax to lower the body hairs.
Sweat glands increase sweat production.

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9
Q

Describe the body’s responses to the cold.

A

Vasoconstriction - arteriole muscles contract.
Hair muscles contract to increase depth of fur and therefore insulation (air is a poor conductor of heat).
Skeletal muscles contract (shivering) to increase heat produced from respiration.
Adrenal glands secrete adrenaline, increasing the heat produced in the liver.

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10
Q

Describe behavioural changes in response to temperature.

A

Curling up/spreading out, finding a source of heat/cool, adding/removing clothing.

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11
Q

How is the anterior pituitary gland involved in thermoregulation?

A

The hypothalamus sends a hormone which stimulates the APG to release thyroid stimulating hormone. TSH stimulates the thyroid gland to release thyroxine into the blood, which increases the rate of metabolic reactions. When temperatures increase again, less hormone is sent to the APG, so less TSH is released.

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12
Q

Describe an example of positive feedback.

A

Breathing in air with high CO2 concentration -> increases blood CO2 concentration -> CO2 receptors stimulated -> breathing rate increases -> more CO2 inhaled.

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13
Q

Define ‘excretion’.

A

The removal of unwanted/toxic products of metabolism from the body.

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14
Q

How is CO2 excreted?

A

From aerobic respiration, CO2 enters the blood. The blood passes through the lungs in capillaries, where gas exchange occurs at the alveoli. CO2 is excreted in the air breathed out.

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15
Q

How does urea reach the bladder?

A

Transported from the liver to the kidneys via blood plasma. It is then converted into urine, moves through the ureter to the bladder and is excreted via the urethra.

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16
Q

How does the deamination of amino acids work?

A

The NH2 group plus an extra H atom are removed from the amino acid. These combine to form ammonia.
A keto acid is left over - this can enter the Krebs cycle or be converted to glucose or to glycogen/fat for storage.
The urea produced diffuses out of liver cells and into the blood plasma.

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17
Q

What is the equation for the formation of urea?

A

2NH3 + CO2 —> CO(NH2)2 + H2O

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18
Q

Why does ammonia need to be converted to urea?

A

NH3 is highly toxic and soluble - urea is less so. In aquatic animals, this conversion doesn’t need to happen as the ammonia can diffuse from the blood, out of the gills, and dissolve in the surrounding water.

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19
Q

Describe examples of other nitrogenous excretory products: creatine.

A

Creatine is formed in the liver from amino acids. It can be converted to creatine phosphate, to be used as an energy store in muscles, or it can be converted to creatinine which is excreted (actively secreted from PCT cells to its lumen).

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20
Q

Describe examples of other nitrogenous excretory products: uric acid.

A

Uric acid is formed from the breakdown of purines in nucleotides.

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21
Q

Outline the structure of the kidney.

A
  • Tough outer capsule
  • Cortex (PCT, DCT, collecting ducts)
  • Medulla (loop of Henle, collecting ducts)
  • Renal pelvis
  • Renal artery + vein
  • Ureter.
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22
Q

Outline the structure of the nephron.

A
  • Bowman’s capsule
  • PCT
  • Loop of Henle
  • DCT
  • Collecting ducts.
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23
Q

Outline the structure of the Bowman’s capsule.

A
  • Afferent + efferent arterioles + glomerulus
  • Capillary endothelium (many gaps)
  • Basement membrane (molecular filter, made of collagen and glycoproteins)
  • Bowman’s capsule epithelium (podocytes with finger-like pedicels which have gaps in between).
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24
Q

Which molecules are/aren’t moved into the Bowman’s capsule during ultrafiltration?

A

YES: Water, amino acids, glucose, nitrogenous excretory products, ions.
NO: large plasma proteins, blood cells, platelets.

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25
Q

Explain how glomerular filtration rate is affected by pressure.

A

The afferent arteriole is much wider than the efferent.
High ψp inside the glomerulus (plasma > filtrate).
Lower ψs in the plasma due to proteins means ψ decreases in filtrate.
Overall, ψp outweighs ψs, therefore ψplasma > ψfiltrate.
Water moves into the Bowman’s capsule.

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26
Q

Describe the cells of the proximal convoluted tubule.

A

Lined with one layer of cuboidal epithelial cells.

  • Microvilli on the surface facing the lumen
  • Tight junctions between cells to prevent fluid leakage
  • Many mitochondria to drive Na-K pumps in the outer membranes
  • Co-transporter proteins in the membrane facing the lumen.
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27
Q

How does selective reabsorption work in the PCT? (1)

A

Capillaries containing glomerular blood (with less plasma) are held near the basal membranes of the PCT cells.
Na-K pumps move Na+ into the blood, reducing the concentration inside the PCT cells.
Sodium then enters the PCT cells from the lumen via co-transporter proteins (diffusion).
This movement down a gradient provides the energy for glucose to move into the PCT cell via secondary active transport.

28
Q

How does selective reabsorption work in the PCT? (2)

A

Once inside the PCT cell, glucose moves down its concentration gradient, into the blood, via a transporter protein. Amino acids, vitamins and many Na+ and Cl- ions do the same.
Half of the urea is reabsorbed too, due to its small size and the filtrate urea concentration being higher than that of the PCT cells and capillaries. Uric acid and creatinine are not reabsorbed.

29
Q

How is water reabsorbed in the PCT?

A

Ψfiltrate is high and Ψblood is low, therefore water moves into the blood via osmosis.

30
Q

What is the purpose of selective reabsorption in the Loop of Henle and collecting ducts?

A

To create a high Na+/Cl- concentration in the tissue fluid of the medulla, enabling large amounts of water to be reabsorbed from the fluid in the collecting ducts.

31
Q

How does selective reabsorption work in the Loop of Henle? (1)

A

The descending limb is permeable to water and ions, whereas the ascending limb is only permeable to ions.
The A cells pump ions into the tissue fluid near the top, decreasing Ψtissue fluid and increasing ΨA.
The D cells lose water to TF going down, making the D fluid more concentrated at the bottom of the loop.
As concentrated fluid travels up A, ions move out down their concentration gradient.

32
Q

How does selective reabsorption work in the Loop of Henle? (2)

A

A cells and collecting duct cells are permeable to urea, so this diffuses out and into the medulla TF as well, increasing its concentration.
The fluid passes through the DCT and to the collecting ducts in the medulla. This is a region of low Ψ and high solute concentration, so more water moves into the tissue fluid via osmosis until Ψurine = ΨTF. This is controlled by ADH.

33
Q

What is a counter-current multiplier?

A

Fluid flowing down one limb and up the other creates a counter-current multiplier. This allows the maximum concentration of solutes to be built up inside and outside of the tube (at the base).

34
Q

How are animals in dry climates adapted to reduce dehydration?

A

They have thick medullae with long loops of Henle to increase urine concentration (eg. gerbils, kangaroo rats). The cells of the loop have many infolds, many Na-K pumps and many mitochondria with many cristae. This is for the purpose of moving as much Na+ into TF as possible.

35
Q

How does selective reabsorption work in the DCT?

A

The first part works like the ascending limb of the Loop of Henle; the second part works like the collecting duct.
DCT and collecting duct: Na+ pumped into TF, moves into blood down its concentration gradient. K+ is pumped into the tubule.
The rate of this movement can be varied to regulate the concentration of ions in the blood.

36
Q

Define ‘osmoregulation’.

A

The control of the water potential of body fluids. It is kept constant by the regulation of salt and water concentrations.

37
Q

Describe the negative feedback loop involved with osmoregulation.

A

Osmoreceptors in the hypothalamus swell/shrink in response to changes in water potential.
The hypothalamus sends impulses to the posterior pituitary gland to release/stop releasing ADH (made in the hypothalamus, stored in and secreted from the PPG).
ADH travels through/stops travelling through blood capillaries all over the body, to the kidneys, where it increases/decreases water being reabsorbed in the cells of the Loop of Henle and collecting ducts.

38
Q

How does the cell respond to ADH?

A

ADH binds to receptors on the CSM. This activates a series of enzyme-controlled reactions, ending with an activated phosphorylase enzyme.
Vesicles containing aquaporins in their membranes are stimulated to move through the cytoplasm and fuse with the CSM to increase its permeability to water.
From the tubule, water moves -> cells -> TF -> blood plasma. Collecting duct fluid decreases in Ψ and concentrated urine is produced.

39
Q

How does osmoregulation work when Ψblood is high?

A

Osmoreceptors are not stimulated, so less ADH is produced and released. Aquaporins in CSM of collecting duct cells are moved back into the vesicles, and the CSM is less permeable to water - less water is reabsorbed so dilute urine is produced.

40
Q

Is the response to a lack of ADH instantaneous?

A

The CD cells do not respond immediately - ADH takes time to break down but it only takes 10-15 minutes for aquaporins to be removed once this change is noticed.

41
Q

Describe the structure of glycogen.

A

Repeating alpha glucose units with 1,4 glycosidic bonds and 1,6 branches.

42
Q

What is the set point for blood glucose concentration?

A

80-120mg/100cm3

43
Q

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

A

The pancreas endocrine tissue is made up of groups of cells known as ‘islets of Langerhans’. These are split into alpha and beta cells - alpha secrete glucagon, beta secrete insulin.
These cells act as both the receptors and central control.

44
Q

Outline the negative feedback loop when blood glucose concentration increases.

A

Glucose is absorbed in the small intestine and travels through the bloodstream to the pancreas. Alpha and beta cells recognise this change - alpha stop secreting glucagon and beta start secreting insulin. Insulin travels through the blood plasma to its targets.

45
Q

Why does insulin not directly affect the cell?

A

It is a signalling molecule and cannot pass through the CSM, so it affects the cell indirectly by binding to a receptor. These receptors can be in liver cells, muscle cells or adipose tissue.

46
Q

What are examples of GLUT transporter proteins?

A

GLUT 1 - brain
GLUT 2 - liver
GLUT 4 - muscles

47
Q

How does insulin affect the cell?

A

It binds to a receptor in the CSM and stimulates vesicles containing GLUT 4 proteins to fuse with the CSM, increasing the uptake of glucose by the cell.

Insulin also activates the enzyme glucokinase. This enzyme phosphorylates glucose so that it cannot leave the cell through the GLUT 4 proteins.

Phosphofructokinase and glycogen synthase are also activated - these enzymes add glucose units to glycogen granules.

Respiration is also increased.

48
Q

How does glucagon affect the cell?

A

It binds to receptors in liver CSM, activating a G protein, which activates the enzyme adenylyl cyclase. This enzyme converts ATP -> cyclic AMP, which acts as a second messenger and activates kinase enzymes in a signalling cascade. Enzymes in the cascade are phosphorylated. Glycogen phosphorylase is produced, which hydrolyses 1,4 linkages in glycogen to release glucose units.
Glucose diffuses out of the cell through GLUT 2 proteins, increasing the concentration in the blood.
More glucose can be synthesised from amino acids/lipids in gluconeogenesis.

49
Q

Why does blood glucose concentration never remain constant?

A

There is a time delay between the stimulus and the action, resulting in oscillation about the set point.

50
Q

How is adrenaline involved in the regulation of blood glucose?

A

It binds to receptors on liver CSM to convert glycogen to glucose. It also stimulates the breakdown of glycogen stores in muscles during exercise (increased respiration).

51
Q

Describe Type 1 diabetes.

A

Aka insulin-dependent / juvenile-onset.
The pancreas cannot secrete sufficient insulin (due to a deficiency in the gene coding for insulin production OR an autoimmune attack on beta cells).

52
Q

How is Type 1 diabetes treated?

A

Insulin injections, blood samples, mini insulin pumps to deliver accurate amounts, or by maintaining a controlled diet.

53
Q

Describe Type 2 diabetes.

A

Aka non-insulin-dependent.
Liver and muscle cells do not respond well to insulin, so blood glucose concentration remains high. The PCT cannot reabsorb all the glucose in the blood so some remains in the filtrate and is passed in the urine, along with more water and salts.
Cell uptake of glucose is slow so fats and proteins are instead metabolised, leading to a buildup of keto acids in the blood.

This results in dehydration, hunger and low blood pH, all of which can cause coma. Between meals, blood glucose can drop rapidly due to lack of glycogen.

54
Q

How is Type 2 diabetes treated?

A

Maintaining a controlled diet, exercise, using insulin from genetically engineered cells.

55
Q

What does the presence of glucose/ketones in urine indicate?

A

Diabetes.

56
Q

What does the presence of proteins in urine indicate?

A 
Kidney problems eg. glomerular disease/infection (small proteins are not reabsorbed in the PCT via endocytosis).
Vigorous exercise.
High blood pressure.
Pregnancy.
High fever.
57
Q

How do dip sticks work?

A

Glucose oxidase and peroxidase are immobilised on a pad.
Glucose oxidase:
[ glucose -> gluconolactone + H2O2 ]
Peroxidase:
[ H2O2 + colourless chemical (chromogen) -> brown ]

The resulting colour can be matched against a colour chart. This method analyses the concentration when urine was collecting in the bladder.

58
Q

How do biosensors work?

A

A small blood sample is placed on a pad with glucose oxidase. Gluconolactone is produced, generating a small electric current which passes through an electrode. The current can then be amplified and read.

59
Q

How do daily rhythms of opening and closing work in stomata?

A

During the day, stomata open to let CO2 in and let O2 + H2O out. During the night, stomata close to prevent water loss from transpiration.
Stomata open in response to high light intensity and high CO2 concentration in the air spaces of the leaves.
They close in response to low humidity, high temperature, low CO2, the dark and water stress.

60
Q

What might cause water stress in plants?

A

Not enough water reaching the leaves from the roots, and high transpiration rates.

61
Q

How do stomata open?

A

The guard cells gain water via osmosis, triggered by a decrease in water potential in these cells.
ATP-powered proton pumps move H+ out of the cell, decreasing their concentration inside the cell. CSM channel proteins open and allow K+ in, down their electrochemical gradient.
Solute potential decreases as does water potential, so water enters the guard cells and they become turgid.

62
Q

How are guard cells shaped to allow opening/closing?

A

Their walls are thicker adjacent to the stomatal pore, allowing the outer walls to curve more. They have cellulose microfibrils arranged in hoops around the cells. This allows the cells to increase in length rather than diameter.

63
Q

How do stomata close?

A

They close when the proton pumps stop moving H+ out and when K+ leave the guard cells, entering neighbouring cells. Water potential inside the guard cells increases, so water moves out and these cells become flaccid.

64
Q

Why is transpiration necessary for the plant?

A

It is required for cooling and for maintaining a transpiration stream from roots to leaves (water and mineral ions need to be transported). Stomatal closure therefore only occurs when absolutely necessary (during water stress).

65
Q

Where is abscisic acid found?

A

It is found everywhere in ferns and flowering plants, and synthesised in chloroplasts and amyloplasts (large starch grains, no chlorophyll).

66
Q

What is abscisic acid?

A

A stress hormone, produced rapidly when temperatures are high or when water levels inside the plant are very low.

67
Q

How does abscisic acid affect the plant?

A

Binds to guard cell CSM, inhibiting H+ pumps and stimulating Ca2+ to travel through the CSM and tonoplast, into the cytoplasm.
Ca2+ is a second messenger; it opens channel proteins which allow anions to leave. In turn, K+ leave the cell (channels going out open, channels entering close).
Water potential increases inside the guard cells, allowing water to leave and the cells to become flaccid.

Biology (8 decks)

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