Chapter 14: Homeostasis Flashcards
(89 cards)
1
Q
What is homeostasis?
A
Homeostasis is maintaining a relatively constant
environment for the cells within the body, despite
changes in external environment
2
Q
What features influence how well the cell functions and why?
A
o Temperature- low temperature, slow metabolic reactions.
High temp however denatures enzymes and proteins
o Water potential- low water potential would cause
water to move out of cells which slows/stops metabolic cell functions. High water potential however causes cells to swell and may burst
o Concentration of glucose- too less, no energy for cell to respire. Too high would affect osmotic balance and disturb metabolism of cells.
3
Q
What is a stimulus?
A
A stimuli is a change in physiological factors
4
Q
Give a brief overview of homeostatic control.
A
The receptor detects stimuli and sends information to the central control in the brain or the spinal cord. The sensory information is known as the input. The central control instructs an effector to carry out an action, which is called the output. these actions are sometimes called corrective actions as their effect is to correct the change.
5
Q
What is the set point?
A
It is an ideal value of a factor that the factor fluctuates around.
6
Q
What is negative feedback?
A
- Negative feedback minimises the difference between the actual value of the factor and the ideal value or the set point.
- When a factor is increased, an effector is stimulated that makes the factor decrease, and vice versa.
7
Q
What are the coordination systems that affect the homeostatic mechanisms in mammals and give a brief overview of them.
A
- The nervous system- information in the form of electrical impulses is transmitted along nerve cells.
- The endocrine system- it uses chemical messengers called hormones that travel in the blood in a form of long distance cell signalling.
8
Q
In which cells is most of the heat produced in mammals and what happens to the heat?
A
Much of the heat is produced in liver cells that have a huge requirement for energy. The heat they produce si absorbed by the blood flowing through the liver and distributed around the body.
9
Q
Where in the brain is the central control for body temperature and how?
A
Hypothalamus has thermoreceptor cells that receives a constant input of sensory information about the temperature of the blood and surroundings.
10
Q
What happens when the core temperature decreases? (5)
A
The hypothalamus sends impulses that activate the following responses:
- Vasoconstriction- muscles in the walls of arterioles that supply blood to the capillaries near the skin surface contract. This narrows the lumens of the arterioles and reduces the supply of blood to the capillaries so that less heat is lost.
- Shivering- the involuntary contraction of skeletal muscles generates heat which is absorbed by the blood and carried around the rest of the body.
- Raising body hairs- muscles at the base of hairs in the skin contract to increase the depth of fur so trapping air close to the skin.
- Decreasing the production of sweat- this reduces the loss of heat by evaporation from the skin surface.
- Increasing the secretion of adrenaline- this hormone from the adrenal gland increases the rate of heat production in the liver.
11
Q
What happens when the core temperature increases? (3)
A
The hypothalamus increases the loss of heat and decreases heat production:
- Vasodilation- the muscles in the arterioles in the skin relax, allowing more blood to flow through the capillaries so that heat is lost to the surroundings.
- Lowering body hairs- muscles attached to the hairs relax so they lie flat, reducing the depth of fur and reducing insulation.
- Increasing sweat production- Sweat glands increase the production of sweat which evaporates on the skin surface so removing heat from the body.
12
Q
Describe how the endocrine system plays a role in the regulation of body temperature when surrounding temperature increases or decreases.
A
When the environmental temperature decreases gradually, the hypothalamus releases a hormone which activates the anterior pituitary gland to release thyroid stimulating hormone(TSH). TSH stimulates the thyroid gland to secrete the hormone thyroxine into the blood. Thyroxine increases metabolic rate, which increases heat production especially in the liver. When temperatures start to increase again, the hypothalamus responds by reducing the release of TSH by the anterior pituitary gland so less thyroxine is released from the thyroid gland.
13
Q
What is the positive feedback loop?
A
• Is not used in keeping conditions constant as it increases effect when stimulus is increased
• This is useful in other areas such as transmission of
nerve impulses where the factor must be increased
14
Q
What is excretion?
A
It is the removal of unwanted products of metabolism.
15
Q
Where is urea produced and from what?
A
It is produced in the liver from excess amino acids.
16
Q
To where is the urea transported and how? Why is it transported there?
A
It is transported from the liver to the kidneys in solution in blood plasma. The kidneys remove the urea from the blood and excrete it, dissolved in water, as urine.
17
Q
What is deamination? How does it occur? Draw the deamination process.
A
Deamination is the removal of an amino group (NH2) from a molecule. This is done in the liver when there is an excess of protein, rather than wasting a useful energy source.
In the liver cells, the amino group(NH2) of an amino acid is removed, together with the hydrogen atom. These combine to produce ammonia. The keto acid that remains may enter the Krebs cycle and be respired or it may be converted to glucose or converted to glycogen or fat for storage.
18
Q
What happens after deamination?
A
Due to ammonia being very soluble and highly toxic, damage is prevented by converting ammonia immediately to urea which is less soluble and less toxic. This is done by combining ammonia with carbon dioxide to produce urea and water.
19
Q
What is the main nitrogenous excretory product of humans? What are the other two excretory products?
A
Urea is the main nitrogenous excretory product. Small quantities of other nep, namely creatinine and uric acid are also produced.
20
Q
Where is creatinine made and what from? What is its function in the body?
A
A substance called creatine is produced from certain amino acids in the liver. Much of this creatine is used in the muscles in the form of creatine phosphate, where it acts as an energy store. However, some is converted to creatinine and excreted.
21
Q
What is uric acid made from?
A
Uric acid is made from the breakdown of purines from nucleotides, not from amino acids.
22
Q
What are the artery and vein that receives and returns blood to the kidney called?
A
Renal artery and renal vein
23
Q
What is the name of the tube that carries urine from the kidney to the bladder?
A
Ureter
24
Q
What is the name of the tube that carries urine from the bladder to the outside of the body?
A
Urethra
25
Give a brief overview of the different sections of the kidney
The whole kidney is covered by a fairly tough capsule, beneath which lies the cortex. The central area is made up of the medulla. Where the ureter joins, there is an area called the pelvis.
26
What are the tubes in the kidneys called?
Nephrons
27
Describe the structure of a nephron
One end of the tube forms a cup shaped structure called a Bowman's capsule, which surrounds a tight network of capillaries called a glomerulus. The glomeruli and capsules of all nephrons are in the cortex of the kidney. From the capsule, the tube runs towards the medulla of the kidney first forming the proximal convoluted tubule, and then a long hairpin loop in the medulla known as the loop of Henle. The tubule then runs back up into the cortex, where it forms another twisted region called the distal convoluted tubule, before finally joining a collecting duct that leads down through the medulla and into the pelvis of the kidney.
28
What is the name of the arteriole that supplies blood to the glomerulus?
The afferent arteriole which forms a branch of renal artery supplies blood.
29
What do the capillaries of the glomerulus rejoin to form? What does this tube do after passing through the glomerulus?
They rejoin to form the efferent arteriole. The efferent arteriole leads off to form a network of capillaries running closely alongside the rest of the nephron. Blood from these capillaries flows into a branch of the renal vein.
30
What is the two stage process in which the kidney produces urine? Give a brief overview
The first stage, ultrafiltration, involves filtering small molecules, including urea, out of the blood and into the Bowman's capsule. From the Bowman's capsule the molecules flow along the nephron to the ureter.
The second stage, selective reabsorption, involves taking back any useful molecules from the fluid in the nephron as it flows along
31
By how many and what layers in the blood in the glomerular capillaries separated from the lumen of the Bowman's capsule?
The blood in the glomerular capillaries is separated from the lumen off the Bowman's capsule by two cell layers and a basement membrane.
32
Give a brief overview of the layers separating the blood in the glomerular capillaries separated from the lumen of the Bowman's capsule.
1. The first cell layer is the lining or endothelium of the capillaries which have gaps in it, but there are far more gaps than in other capillaries.
2. The basement membrane is made up of collagen and glycoproteins.
3. The second cell layer is formed from epithelial cells which make up the inner lining of the Bowman's capsule. These cells have many tiny finger like projections with gaps in between them and are called podocytes.
Both the cell layers have gaps which are quite large, making it easy for substances dissolved in the blood plasma to get through from the blood into the capsule. However, the basement membrane acts as a filter, stopping large protein molecules and red/white blood cells from getting through.
33
What is the glomerular filtration rate?
The rate at which the fluid filters from the blood in the glomerular capillaries into the Bowman's capsule is called the glomerular filtration rate.
34
What is the average glomerular filtration rate in humans for both kidneys?
125 cm^3/min
35
What is the effect of water potential on glomerular filtration rate and what influences it? What affects water potential?
The water potential between the plasma in the glomerular capillaries and the filtrate in the Bowman's capsule determines the rate at which fluid is filtered. In other words, the differences in water potential between the two influences the glomerular filtration rate.
Water potential is lowered by the presence of solutes and raised by the high pressures.
1. Inside the blood capillaries in the glomerulus, the blood pressure is relatively high, because the diameter of the afferent arteriole is wider than that of the efferent arteriole, causing a head of pressure inside the glomerulus. This tends to raise the water potential of the blood plasma above the water potential of the contents of the Bowman's capsule.
2. However, the concentration of solutes in the blood plasma is higher than that of the filtrate due to the presence of plasma proteins that are too big to fit through the gaps of the cell layers and the basement membrane. This difference in solute concentration tends to make the water potential in the blood capillaries lower than that of the filtrate in the Bowman's capsule.
Overall, the differences in pressure outweighs the effect of the solute concentration, making the water potential of the capillaries in the glomerulus higher than that of the filtrate in the Bowman's capsule.
36
Where does most of the reabsorption take place?
Most of it takes place in the proximal convoluted tubule.
37
What is the lining of the proximal convoluted tubule made of?
It is made of a single layer of cuboidal epithelial cells
38
How are the cells of the proximal convoluted tubule adapted for their function of selective reabsorption? (4)
■ microvilli to increase the surface area of the inner
surface facing the lumen
■ tight junctions that hold adjacent cells together so that fluid cannot pass between the cells (all substances that are reabsorbed must go through the cells)
■ many mitochondria to provide energy for sodium–
potassium (Na+–K+) pump proteins in the outer
membranes of the cells
■ co-transporter proteins in the membrane facing
the lumen.
39
Are the blood capillaries very close to the tubules?
Yes
40
Describe the contents of the capillaries after exiting the glomerulus
The blood in theses capillaries has come directly from the glomerulus, so it has much less plasma in it than usual and has lost much of its water and many of the ions and other small solutes.
41
What are the cells that are the closest to the capillaries?
The basal membranes of the cells lining the proximal convoluted tubule are those nearest to the capillaries.
42
What happens to the sodium while in the proximal convoluted tubule? *full
Sodium-potassium pumps in these membranes move sodium ions out of the cells. The sodium ions are carried away in the blood. This lowers the concentration of sodium ions inside the cell, so that they passively diffuse into it, down their concentration gradient, from the fluid into the lumen of the tubule. However, the sodium ions do not diffuse freely into the basement membrane cells and can only enter through special co-transporter proteins in the membrane. There are two kinds of co-transporter proteins, each of which transports something else, such as a glucose molecule or an amino acid at the same time as the sodium ion. The passive movement of sodium ions into the cell down their concentration gradient provides the energy to move the glucose molecules, even against a concentration gradient. This movement of glucose and of other solutes is known as secondary or indirect active transport, since the energy(as ATP) is used in the pumping of sodium ions, not in moving the solutes. Once inside the cell, glucose diffuses down its conc. gradient through a transport protein in the basal membrane into the blood.
43
How much of the glucose is reabsorbed in the proximal convoluted tubule?
All of the glucose in the glomerular filtrate is transported out of the proximal convoluted tubule and into the blood. No glucose is left in the filtrate, so no glucose is present in urine.
44
What happens to the amino acids, vitamins and many sodium and chloride ions in the proximal convoluted tubule?
They are reabsorbed back into the blood.
45
What is the effect of the selective reabsorption in the proximal convoluted tubule?
The removal of the solutes from the filtrate greatly increases its water potential. The movement of solutes into the cells and then into the blood decreases the water potential there, so a water potential gradient exists between the filtrate and the blood. Water then moves down this gradient from the tubule through the cells and into the blood.
There is also a great reduction in the volume of liquid remaining.
46
What happens to the reabsorbed solutes from the proximal convoluted tubule?
They are carried away, back into the circulation.
47
What happens to the urea in the proximal convoluted tubule and how?
Urea is a small molecule which passes easily through cell membranes. Its concentration in the filtrate is considerably higher than that of the blood in the capillaries, so it diffuses passively through the cells of the proximal convoluted tubule and into the blood. About half of the urea is reabsorbed in this way.
48
Are uric acid and creatinine reabsorbed in the proximal tubule?
No
49
What is actively secreted in the proximal convoluted tubules and by what?
Creatinine is actively secreted by the cells of the proximal convoluted tubule into its lumen.
50
What percentage of the volume of filtrate enters the loop of Henle?
64%
51
Where is the loop of Henle found in the kidney?
The hairpin loop runs deep into the medulla of the kidney before turning back towards the cortex again.
52
Which limb of The loop of Henle is permeable to water?
The descending limb is permeable to water, whereas the ascending limb is not.
53
What happens in the loop of Henle?
The cells that line the ascending limb of the loop actively transport sodium and chloride ions out of the fluid in the loop, into the tissue fluid. This decreases the water potential in the tissue fluid and increases the water potential of the fluid inside the ascending limb. As the fluid goes upwards in the ascending limb, it becomes more dilute and has a higher water potential.
The cells lining the descending limb are permeable
to water and also to sodium and chloride ions. As
the fluid flows down this loop, water from the filtrate
moves down a water potential gradient into the tissue
fluid by osmosis. At the same time, sodium and chloride
ions diffuse into the loop, down their concentration
gradient. So, by the time the fluid has reached the very
bottom of the hairpin, it contains much less water and
many more sodium and chloride ions than it did when
it entered from the proximal convoluted tubule. The
fluid becomes more concentrated towards the bottom
of the loop.
54
What is a counter-current multiplier?
Having the two limbs of the loop running side by side, with the fluid flowing down in one and up in the other, enables the maximum concentration of solutes to be built up both inside and outside the tube at the bottom of the loop. This mechanism is called a counter-current multiplier.
55
What cells are permeable to urea?
The cells lining the ascending limb and the cells lining the collecting duct
56
How do the two parts of the distal convoluted tubule function?
The first part functions like the ascending limb of the loop of Henle and the second part functions like the collecting duct.
57
What is osmoregulation?
It is the control of water potential of bodily fluids.
58
What is the water potential constantly monitored by?
By specialised sensory neurones called osmoregulators in the hypothalamus.
59
What is ADH? What is it made up of?
Antidiuretic hormone is a peptide hormone made up of nine amino acids.
60
What happens when there is a decrease in the water potential of the blood?
When the osmoreceptors detect a decrease below the set point, nerve impulses are sent to stimulate the posterior pituitary gland to release ADH. The target calls of ADH are the cells of the collecting duct in the kidney. ADH molecules bind to the cell surface membranes of the cells of the collecting duct, which in turn activate the enzymes inside the cells. This activation of enzymes causes vesicles containing the aquaporins to move towards the cell membrane and fuse with it, so increasing the permeability to water. So, as water flows through the collecting duct, water molecules move through the aquaporins out of the tubule and into the tissue fluid, down the water potential gradient(tissue fluid in medulla has lower water potential). The fluid in the collecting ducts loses water and becomes more concentrated. The secretion of ADH has caused the reabsorption of water. The volume of fluid flowing into the bladder will be smaller and more concentrated.
61
What happens when there is an increase in water potential of the blood?
When there is an increase in the water potential of the blood, the osmoreceptors in the hypothalamus are no longer stimulated and the neurones in the posterior pituitary gland stop secreting ADH. This causes the aquaporins to move out of the cell membranes of the collecting duct cells, back into the cytoplasm as part of the vesicles. This makes the collecting duct impermeable to water. The fluid flows down the collecting duct without losing any water, so a dilute urine collects in the pelvis.
62
Why do the collecting duct cells not respond immediately to a reduction in ADH secretion?
This is because it takes time for the ADH already in the blood to be broken down.
63
In what form is carbohydrate transported int he bloodstream?
Glucose
64
How much glucose is normally present in 100cm^3 of blood?
Between 80 and 120mg
65
What is the homeostatic control of blood glucose concentration done by?
By two hormones secreted by the endocrine tissue in the pancreas.
66
What is the group of cells that control the blood glucose concentration called?
Islets of langerhans
67
What are the two types of cells of the islets of langerhans and what do they secrete?
a-cells secrete glucagon
| b cells secrete insulin
68
What happens when there is an increase in glucose concentration in the blood?
As the blood flows through the pancreas, the a and b cells detect the increase in concentration. The a cells respond by stopping secretion of glucagon and the b cells respond by secreting insulin into the bloodstream, where it is carried to all the parts of the body.
Insulin is a cell signalling molecule. Due to being a protein, it cannot pass through membranes and therefore, binds with a receptor in the cell membrane and affects the cell indirectly through intracellular messengers. Insulin stimulates the cells with these receptors to increase the rate at which they absorb glucose from the blood, convert it to glycogen and use it in respiration. This results in a decrease in the conc of glucose in the blood. Glucose can only enter cells through transporter proteins known as GLUT. When insulin binds with the receptors on a cell, the vesicles with GLUT proteins are moved towards the cell membrane and fuse with it. GLUT proteins faciltate the movement of glucose into the cell.
Insulin also stimulates the activation of the enzyme glucokinase, which phosphorylates glucose. This traps glucose inside cells, because phosphorylated glucose cannot pass through the transporters in the cell membrane. Insulin also stimulates the activation of two other enzymes, phosphofructokinase and glycogen synthase, which together add glucose molecules to glycogen. This increases the size of the glycogen granules inside the cell.
69
What happens when there is a decrease in the blood glucose concentration?
A decrease in blood glucose conc is detected by the a and B cells in the pancreas. The a cells respond by secreting glucagon, while the B cells respond by stopping the secretion of insulin.
It reduces the uptake of glucose. Glucagon binds to different receptor molecules in the cell membranes of liver cells. The binding of glucagon to a receptor activates a G protein that in turn activates an enzyme within the membrane that catalyses the conversion of ATP to cyclic AMP, which is a second messenger. Cyclic AMP binds to kinase enzymes in the cytoplasm that activates other enzymes. Kinase enzymes activate enzymes by adding phosphate groups to them in a process known as phosphorylation. This enzyme cascade amplifies the original signal from glucagon. Glycogen phosphorylase is at the end of the enzyme cascade and when activated, catalyses the breakdown of glycogen to glucose. It does this by removing glucose units. This increases the conc of glucose inside the cell so that it diffuses out through the GLUT2 transporter proteins into the blood.
Glucose is also made from amino acids and lipids in a process known as gluconeogenesis.
70
Which other hormone increases the concentration of glucose in the blood?
The hormone adrenaline.
71
What are the two types of diabetes?
Type 1 diabetes, which is also known as insulin-dependent diabetes and type 2 diabetes which is also known as non-insulin-dependent diabetes
72
What is the difference between type 1 and 2?
In type 1, the pancreas is incapable of secreting sufficient insulin and this form of diabetes usually begins early in life.
In type 2, the pancreas does secrete insulin, but the liver and muscle cells do not respond properly to it. It begins relatively late in life.
73
How does glucose lead to a decrease in blood pH?
The uptake of glucose into cells is slow and the cells lack glucose and start to metabolise fats and proteins which can lead to a build up of keto acids which decrease the blood pH.
74
In which type of diabetes are regular injections of insulin given?
Type 1
75
What presence in the urine may indicate diabetes?
The presence of glucose and ketones
76
What is the renal threshold and what happens when the concentration increases above this value?
Above this value, not all of the glucose is reabsorbed from the filtrate in the proximal convoluted tubule and some will be present in the urine.
77
Why is the presence of proteins in the urine worrying?
This is because protein molecules are too large to be filtered, however some protein molecules may pass through which are then reabsorbed by endocytosis in the proximal tubule, broken down and the amino acids absorbed in the blood.
78
What immobilised enzymes do the dipsticks detecting glucose contain?
Glucose oxidase and peroxidase.
79
How does the dipstick testing for glucose in the urine work?
The glucose oxidase catalyses the oxidation of glucose into gluconolactone. Hydrogen peroxide is also produced. Peroxidase catalyses the reaction between hydrogen peroxide and a colourless chemical in the pad to form a brown compound. The more the glucose, the darker the colour.
80
What is a problem with the dipstick?
They do not indicate the current blood glucose concentration, but rather the time whiel urine was collecting in the bladder.
81
How does a biosensor work?
The biosensor uses a pad with glucose oxidase. A small sample of blood is placed on the pad which is inserted into the machine. The glucose oxidase catalyses the reaction to produce gluconolactone and at the same time a tiny electric current is generated. The more glucose there is, the greater the current and the greater the reading from the biosensor.
82
What do stomata open in response to?
1. increasing light intensity
| 2. Low carbon dioxide concentrations in the air spaces within the leaf.
83
What do stomata close in response to?
1. Darkness
2. High carbon dioxide concentrations in the air spaces in the leaf
3. Low humidity
4. High temperature
5. Water stress
84
What is each stomatal pore surrounded by?
Two guard cells
85
How do stomata pores open?
A decrease in water potential is needed before water can enter the cell by osmosis. This is done by the activities of the transporter proteins in the cell membrane. ATP powered proton pumps in the membrane actively transport hydrogen ions out of the guard cells. The decrease in the hydrogen ion concentration causes the channel proteins to open to allow potassium ions to enter as the K ions are attracted to the negative charge and move down an electrical gradient towards the negatively charged region. They also diffuse into the cells down a concentration gradient. Such a combined gradient is an electrochemical gradient. The extra potassium ions in the cell lowers the solute potential and hence the water potential of the cell, causing water to move in by osmosis through aquaporins in the membrane. This increases the turgor of the guard cells and the stoma opens.
86
Which wall of the pore is thicker?
The walls surrounding the stoma are thicker than the walls furthest from the pore.
87
How do stomata pores close?
When the hydrogen ion pump proteins stop and the potassium ions leave the guard cells and enter neighbouring cells, which allows the water potential gradient to be in the opposite direction, so water leaves the guard cells.
88
What is the stress hormone called and what is tis function? Where is it found?
It is called ABA-abscisic acid and it is produced in plants to stimulate stomatal closure. It is found in almost all cells that possess chloroplasts or amyloplasts.
89
How does ABA achieve its function?
It inhibits the proton pumps to prevent hydrogen from being pumped out. It also stimulates the movement of calcium ions into the cytoplasm through the cell surface membrane which acts as a second messenger to activate channel proteins to open to allow negatively charged ions to leave the guard cells. This in turn, stimulates the opening of channel proteins which allows the movement of potassium ions out of the cells. At the same time, the calcium ions also stimulate the closure of the channel proteins that allow potassium ions to enter. The loss of ions raises the water potential of the cells and water passes out by osmosis. This in turn causes the guard cells to become flaccid and the stomata close.
