Homeostasis Flashcards
Cells function most efficiently if
they are kept in near optimum conditions
What are the conditions maintained in mammals
-core temperature
-metabolic waste (eg. carbon dioxide and urea)
-blood pH
-blood glucose concentration
-blood water potential
-concentration of the respiratory gases (carbon dioxide and oxygen) in the blood
Homeostasis
The regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes
Why is homeostasis important
it ensures the maintenance of optimal conditions for enzyme action and cell function
Negative feedback control loops involve
-A receptor (or sensor) – to detect a stimulus that is involved with a condition / physiological factor
-A coordination system (nervous system and endocrine system) – to transfer information between different parts of the body
-An effector (muscles and glands) – to carry out a response
Negative feedback loop
1) receptor detects a stimulus that is involved with the condition/physiological factor
2) receptor sends information through the nervous system to central control in the brain or spinal cord
3) central control instructs an effector to carry out an action
4) the factor (stimulus) is continuously monitored by receptors so that it fluctuates around a set point or ideal value
The outcome of a negative feedback loop
-The factor / stimulus is continuously monitored
-If there is an increase in the factor, the body responds to make the factor decrease
-If there is a decrease in the factor, the body responds to make the factor increase
Internal stimuli
are factors located inside the body that are detected and cause a response
External stimuli
-includes touch and pain, vision, smell, taste, sound, and balance (equilibrium).
-these sensory stimuli are activated by external changes
receptors
detects a stimulus that is involved with the condition/physiological factor
The two different coordination systems that homeostasis relies on in mammals
-Nervous system – information is transmitted as electrical impulses that travel along neurones
-Endocrine system – information is transmitted as chemical messengers called hormones that travel in the blood
nervous system is usually required for
fast, but short-lived responses
the endocrine system is result in
slower, but longer-lasting responses (although this is not always the case and some hormones can act very quickly)
effector
a tissue or organ that carries out an action in response to a stimulus; muscles and glands are effectors
corrective action
a response or series of responses that return a physiological factor to the set point so maintaining a constant environment for the cells within the body
set point
the ideal value of a physiological factor that the body controls in homeostasis
excretion
The removal of the waste products produced by the many metabolic reactions that occur within the body
Urea is produced in the
liver
Urea is produced from
excess ammino acids
deamination
the removal of amino group from each amino acid to access the useful energy still present in the protein
process of deamination
-The amino group (-NH2) of an amino acid is removed, together with an extra hydrogen atom
-These combine to form ammonia (NH3)
-The remaining keto acid may enter the Krebs cycle to be respired, be converted to glucose, or converted to glycogen/fat for storage
How is ammonia really damaging
-Ammonia is a very soluble and highly toxic compound
-It dissolves in the blood to form alkaline ammonium hydroxide, disrupting blood pH
-It can impact the reactions of cell metabolism such as respiration
-It interferes with cell signalling processes
How and why is ammonia converted to urea
Ammonia is combined with carbon dioxide to form urea
2NH3 + CO2 = CO(NH2)2 + H2O
Urea is less soluble and less toxic than ammonia
The two functions of kidney
-As an osmoregulatory organ - they regulate the water content of the blood (vital for maintaining blood pressure)
-As an excretory organ - they excrete the toxic waste products of metabolism (such as urea) and substances in excess of requirements (such as salts)
The function of renal artery
carries oxygenated blood (containing urea and salts) to the kidneys
The function of renal vein
carries deoxygenated blood (that has had urea and excess salts removed) away from the kidneys
The function of the ureter
carries urine from a kidney to the bladder
the function of the bladder
stores urine (temporarily)
the function of the urethra
releases the urine outside of the body
what is the fibrous capsule
a fairly tough outer layer that surrounds the kidney
Three main areas beneath the fibrous capsule
-The cortex (contains the glomerulus, as well as the Bowman’s capsule, proximal convoluted tubule, and distal convoluted tubule of the nephrons)
-The medulla (contains the loop of Henle and collecting duct of the nephrons)
-The renal pelvis (where the ureter joins the kidney)
nephrons
-thousands of tiny tubes that each kidney contains
-the structural and functional unit of the kidney composed of Bowman’s capsule and a tubule divided into three regions: proximal convoluted tubule, loop of Henle and distal convoluted tubule
Bowman’s capsule
the cup-shaped part of a nephron that surrounds a glomerulus and collects filtrate from the blood
glomerulus
-a group of capillaries within the ‘cup’ of a Bowman’s capsule in the cortex of the kidney
-Each glomerulus is supplied with blood by an afferent arteriole (which carries blood from the renal artery)
-The capillaries of the glomerulus rejoin to form an efferent arteriole
proximal convoluted tubule
part of the nephron that leads from Bowman’s capsule to the loop of Henle
loop of Henle
the part of the nephron between the proximal and distal convoluted tubules
distal convoluted tubule
part of the nephron that leads from the loop of Henle to the collecting duct
collecting duct
tube in the medulla of the kidney that carries urine from the distal convoluted tubules of many nephrons to the renal pelvis
The two stages involved in the formation of urine in nephron
- Ultrafiltration
- Selective reabsorption
Where ultrafiltration occurs
Bowman’s capsule
Where selective reabsorption occurs
proximal convoluted tubule
What is ultrafiltered in the glomerulus
Small molecules (including ammino acids, water, glucose, urea and inorganic ions) are filtered out of the blood capillaries of the glomerulus and into the Bowman’s capsule to form filtrate known as glomerular filtrate
What is selective reabsorption?
Useful molecules are taken back (reabsorbed) from the filtrate and returned to the blood as the filtrate flows along the nephron
Ultrafiltration process
1)The capillaries get narrower as they get further into the glomerulus which increases the pressure on the blood moving through them (which is already at high pressure because it is coming directly from the renal artery which is connected to the aorta)
2)This eventually causes the smaller molecules being carried in the blood to be forced out of the capillaries and into the Bowman’s capsule, where they form what is known as the filtrate
3)The blood in the glomerular capillaries is separated from the lumen of the Bowman’s capsule by two cell layers with a basement membrane in between them:
-The first cell layer is the endothelium of the capillary – each capillary endothelial cell is perforated by thousands of tiny membrane-lined circular holes
-The next layer is the basement membrane – which is made up of a network of collagen and glycoproteins
-The second cell layer is the epithelium of the Bowman’s capsule – these epithelial cells have many tiny finger-like projections with gaps in between them and are known as podocytes
4)As blood passes through the glomerular capillaries, the holes in the capillary endothelial cells and the gaps between the podocytes allow substances dissolved in the blood plasma to pass into the Bowman’s capsule
5)The fluid that filters through from the blood into the Bowman’s capsule is known as the glomerular filtrate
6)The main substances that pass out of the capillaries and form the glomerular filtrate are: amino acids, water, glucose, urea, and inorganic ions (mainly Na+, K+, and Cl-)
7)Red and white blood cells and platelets remain in the blood as they are too large to pass through the holes in the capillary endothelial cells
8)The basement membrane acts as a filter as it stops large protein molecules from getting through
How ultrafiltration occurs
-Ultrafiltration occurs due to the differences in water potential between the plasma in the glomerular capillaries and the filtrate in the Bowman’s capsule
-Water potential is increased by high pressure and decreased by the presence of solutes
-Overall, the effect of the pressure gradient outweighs the effect of the solute gradient
-Therefore, the water potential of the blood plasma in the glomerulus is higher than the water potential of the filtrate in the Bowman’s capsule
-This means that as blood flows through the glomerulus, there is an overall movement of water down the water potential gradient from the blood into the Bowman’s capsule
How pressure affects water potential in the glomerulus and Bowman’s capsule and the subsequent water movement
-As the afferent arteriole is wider than the efferent arteriole, the blood pressure is relatively high in the glomerular capillaries
-This raises the water potential of the blood plasma in the glomerular capillaries above the water potential of the filtrate in the Bowman’s capsule
-Water moves down the water potential gradient, from the blood plasma in the glomerular capillaries into the Bowman’s capsule
How solute concentration affects water potential in the glomerulus and Bowman’s capsule and subsequent movement of water
-Whilst the basement membrane allows most solutes within the blood plasma to filter into the Bowman’s capsule, plasma protein molecules are too big to get through and stay in the blood
-As a result, the solute concentration in the blood plasma in the glomerular capillaries is still higher than that in the filtrate in the Bowman’s capsule
-This makes the water potential of the blood plasma lower than that of the filtrate in the Bowman’s capsule
-Water moves down the water potential gradient from the Bowman’s capsule into the blood plasma in the glomerular capillaries
Why selective reabsorption needs to occur
Because many of the substances that end up in the glomerular filtrate actually need to be kept by the body
the lining of the proximal convoluted tubule is composed of
a single layer of epithelial cells
Water and salts are reabsorbed via
the Loop of Henle and collecting duct
How the proximal convoluted tubule is adapted for its function
-Microvilli
-Co-transporter proteins
-A high number of mitochondria
-Tightly packed cells
How the many microvilli present in the luminal membrane aids in reabsorption
Increases surface area for reabsorption
How the many co-transporter proteins in the luminal membrane aids in reabsorption
Each type of co-transporter protein transports a specific solute (eg. glucose or a particular ammino acid) across the luminal membrane
How the amount of mitochondria aids in reabsorption
They provide energy for sodium-potassium (Na+ - K+) pump proteins in the basal membranes of the cells
How the tightly packed cells aids in reabsorption
This means that no fluid can pass between the cells (all substances reabsorbed must pass through the cells)
How the selective reabsorption of solutes occurs
1)Blood capillaries are located very close to the outer surface of the proximal convoluted tubule
–As the blood in these capillaries comes straight from the glomerulus, it has very little plasma and has lost much of its water, inorganic ions, and other small solutes
2)The basal membranes (of the proximal convoluted tubule epithelial cells) are the sections of the cell membrane that are closest to the blood capillaries
3)Sodium-potassium pumps in these basal membranes move sodium ions out of the epithelial cells and into the blood, where they are carried away
4)This lowers the concentration of sodium ions inside the epithelial cells, causing sodium ions in the filtrate to diffuse down their concentration gradient through the luminal membranes (of the epithelial cells)
5)These sodium ions do not diffuse freely through the luminal membranes – they must pass through co-transporter proteins in the membrane
6)There are several types of these co-transporter proteins – each type transports a sodium ion and another solute from the filtrate (eg. glucose or a particular amino acid)
7)Once inside the epithelial cells these solutes diffuse down their concentration gradients, passing through transport proteins in the basal membranes (of the epithelial cells) into the blood
Molecules reabsorbed from the proximal convoluted tubule during selective reabsorption
1)All glucose in the glomerular filtrate is reabsorbed into the blood – this means no glucose should be present in the urine
2)Amino acids, vitamins, and inorganic ions are reabsorbed
–The movement of all these solutes from the proximal convoluted tubule into the capillaries increases the water potential of the filtrate and decreases the water potential of the blood in the capillaries
–This creates a steep water potential gradient and causes water to move into the blood by osmosis
3)A significant amount of urea is reabsorbed too because the concentration of urea in the filtrate is higher than in the capillaries, causing urea to diffuse from the filtrate back into the blood
Reabsorption of water and salts
–As the filtrate drips through the Loop of Henle necessary salts are reabsorbed back into the blood by diffusion
–As salts are reabsorbed back into the blood, water follows by osmosis
–Water is also reabsorbed from the collecting duct in different amounts depending on how much water the body needs at that time
osmoregulation
The control of the water potential of body fluids
Specialised sensory neurones that monitor the water potential of the blood
osmoreceptors
where are osmoreceptors located
in the hypothalamus of the brain
What happens when the osmoreceptors sense a decrease in the water potential of blood
–Nerve impulses are sent along these sensory neurones to the posterior pituitary gland (another part of the brain just below the hypothalamus)
–These nerve impulses stimulate the posterior pituitary gland to release antidiuretic hormone (ADH)
–ADH molecules enter the blood and travel throughout the body
–ADH causes the kidneys to reabsorb more water thus reducing the loss of water from urine
How is water reabsorbed from filtrate in the nephron
through osmosis
The effect of ADH on the kidneys
1) ADH causes the luminal membranes of the collecting duct cells to become more permeable to water
2) ADH does this by causing an increase in the number of aquaporins (water-permeable channels) in the luminal membranes of the collecting duct cells. This occurs in the following way:
–Collecting duct cells contain vesicles, the membranes of which contain many aquaporins
–ADH molecules bind to receptor proteins, activating a signalling cascade that leads to the phosphorylation of the aquaporin molecules
–This activates the aquaporins, causing the vesicles to fuse with the luminal membranes of the collecting duct cells
–This increases the permeability of the membrane to water
3) As the filtrate in the nephron travels along the collecting duct, water molecules move from the collecting duct (high water potential), through the aquaporins, and into the tissue fluid and blood plasma in the medulla (low water potential)
4) As the filtrate in the collecting duct loses water it becomes more concentrated
5) As a result, a small volume of concentrated urine is produced. This flows from the kidneys, through the ureters, and into the bladder
What happens when the water potential of the blood is too high
The exact opposite of what normally happens:
–Osmoreceptors in the hypothalamus are not stimulated
–No nerve impulses are sent to the posterior pituitary gland
–No ADH released
–Aquaporins are moved out of the luminal membranes of the collecting duct cells
–Collecting duct cells are no longer permeable to water
–The filtrate flows along the collecting duct but loses no water and is very dilute
–A large volume of dilute urine is produced
–This flows from the kidneys, through the ureters, and into the bladder
Why shouldn’t the concentration of glucose in the blood decreases below a certain level
cells may not have enough glucose for respiration and may not be able to function normally
why shouldn’t the concentration of glucose in the blood increases above a certain level
it disrupts the normal function of cells, potentially causing major problems (water diffusing out of cells)
Blood glucose concentration is controlled by
two hormones secreted by endocrine tissue in the pancreas
endocrine tissue is made up of
groups of cells known as the islets of Langerhans
the two cell types that the islets of Langerhans contain
α cells that secrete the hormone glucagon
β cells that secrete the hormone insulin
α and β cells act as
the receptors and initiate the response for controlling blood glucose concentration
How do α cells respond when there is a decrease in blood glucose concentration?
they secrete glucagon
How do β cells respond when there is a decrease in blood glucose concentration?
they stop the secretion of insulin
decrease in blood insulin concentration reduces
the use of glucose by liver and muscle cells
The control of blood glucose concentration by glucagon
1) Glucagon binds to receptors in the cell surface membranes of liver cells
2) This binding causes a conformational change in the receptor protein that activates a G protein
3) This activated G protein activates the enzyme adenylyl cyclase
4) Active adenylyl cyclase catalyses the conversion of ATP to the second messenger, cyclic AMP (cAMP)
5) cAMP binds to protein kinase A enzymes, activating them
6) Active protein kinase A enzymes activate phosphorylase kinase enzymes by adding phosphate groups to them
7) Active phosphorylase kinase enzymes activate glycogen phosphorylase enzymes
8) Active glycogen phosphorylase enzymes catalyse the breakdown of glycogen into glucose. This process is known as glycogenolysis
9) The enzyme cascade described above amplifies the original signal from glucagon and results in the releasing of extra glucose by the liver to increase the blood glucose concentration back to a normal level
Increase in blood glucose concentration
1) When the blood glucose concentration increases above the normal range it is detected by the β cells in the pancreas
2) When the concentration of glucose is high, glucose molecules enter the β cells by facilitated diffusion
3) The cells respire this glucose and produce ATP
4) High concentrations of ATP causes the potassium channels in the β cells to close, producing a change in the membrane potential
5) This change in the membrane potential causes the voltage-gated calcium channels to open
6) In response to the influx of calcium ions, the β cells secrete the hormone insulin
7) Insulin-containing vesicles move towards the cell-surface membrane where they release insulin into the capillaries
8) Once in the bloodstream, insulin circulates around the body stimulating the uptake of glucose by muscles cells, fat cells and the liver
target cells of insulin
all of the cells that have specific insulin receptors on their cell surface membranes so;
-muscle cells
-fat storage cells
-adipose tissue
-liver cells
Actions of insulin
1) Insulin binds to specific receptors on the membranes of its target cells
2) The binding of insulin to receptors on target cells stimulates the cells to add more glucose transporter proteins to their cell surface membranes, increasing the permeability of the cells to glucose, these glucose transporter proteins are known as GLUT proteins
3) When blood glucose levels are low GLUT proteins are stored inside the cell in the membranes of vesicles, but when insulin binds to the surface receptors the vesicles move to the cell surface membrane and fuse with it, adding GLUT proteins to the membrane
4) The rate of facilitated diffusion of glucose into the target cells increases as a result of the increase in GLUT proteins
-Insulin causes the activation of an enzyme known as glucokinase. Glucokinase phosphorylates glucose, trapping it inside cells
-Insulin causes the activation of another enzyme; glycogen synthase. Glycogen synthase converts glucose into glycogen in a process known as glycogenesis
Negative Feedback Control of Blood Glucose
-α and β cells in the pancreas act as the receptors
-They release the hormones glucagon (secreted by α cells) and insulin (secreted by β cells)
-Liver cells act as the effectors in response to glucagon and liver, muscle, and fat cells act as the effectors in response to insulin
The presence of glucose in urine is an indicator
that a person may have diabetes
If blood glucose concentration increases above a value known as the renal threshold,
not all of the glucose from the filtrate in the proximal convoluted tubule is reabsorbed and some will be left in the urine
The reason people with diabetes have glucose in their urine
People with diabetes cannot control their blood glucose concentration so that it remains within normal, safe limits. Glucose concentration, therefore, can be above the renal threshold and not get reabsorbed in the proximal convoluted tubule
What are test strips
they are strips that can be used to test urine for the presence and concentration of glucose
The two enzymes which are immobilised on a small pad at one end of the test strip
glucose oxidase
peroxidase
Measuring urine glucose concentration
The pad on a test strip is immersed in the urine sample for a short time
If glucose is present:
–Glucose oxidase catalyses a reaction in which glucose is oxidised to form gluconic acid and hydrogen peroxide
–Peroxidase then catalyses a reaction between the hydrogen peroxide and a colourless chemical (chromogen) in the pad to form a brown compound and water
–The colour of the pad is compared to a colour chart – different colours represent different concentrations of glucose (the higher the concentration of glucose present, the darker the colour)
–Urine tests only show whether or not the blood glucose concentration was above the renal threshold whilst urine was collecting in the bladder – they do not indicate the current blood glucose concentration
What is a biosensor used for
can be used by people with diabetes to show their current blood glucose concentration
Measuring blood glucose concentration
1) A biosensor uses glucose oxidase (but no peroxidase) immobilised on a recognition layer
2) Covering the recognition layer is a partially permeable membrane that only allows small molecules from the blood to reach the immobilised enzymes
3)When a small sample of blood is tested, glucose oxidase catalyses a reaction in which any glucose in the blood sample is oxidised to form gluconic acid and hydrogen peroxide
4) The hydrogen peroxide produced is oxidised at an electrode that detects electron transfers
5)The electron flow is proportional to the glucose concentration of the blood sample. The biosensor amplifies the current, which is then read by a processor to produce a digital reading for blood glucose concentration. This process is complete within a matter of seconds
Environmental stimuli causing stomata to open
–increasing light intensity
–low carbon dioxide concentrations in the air spaces within the leaf
Environmental stimuli causing stomata to close
–darkness
–high carbon dioxide concentrations in the air spaces within the leaf
–low humidity
–high temperature
–water stress – when the supply of water from the roots is less and/or there are high rates of transpiration
Advantage of stomata opening during the day
Leaves gain carbondioxide for photosynthesis
Advantage of stomata closing during the day
water is retained inside the leaf, which is important in times of water stress
Disadvantage of stomata opening during the day
leaves lose large amounts of water by transpiration
Disadvantage of stomata closing during the day
supply of carbon dioxide decreases so the rate of photosynthesis decreases
Stomata open and close in a
daily rhythm
What happens to stomata when they are kept in constant darkness
the daily rhythm of opening and closing of the stomata continues
Opening of stomata during the day causes
–maintains the inward diffusion of carbon dioxide and the outward diffusion of oxygen
–allows the outward diffusion of water vapour in transpiration
Closing of stomata at night when photosynthesis cannot occur
–reduces the rate of transpiration
–conserves water
The features that guard cells have
–Thick cell walls facing the air outside the leaf and the stoma
–Thin cell walls facing adjacent epidermal cells
–Cellulose microfibrils arranged in bands around the cell
–Cell walls have no plasmodesmata
–Cell surface membrane is often folded and contains many channels and carrier proteins
–Cytoplasm has a high density of chloroplasts and mitochondria
–Chloroplasts have thylakoids but with few grana (unlike those in mesophyll cell chloroplasts)
–Mitochondria have many cristae
–Several small vacuoles rather than one large vacuole
When are guard cells open
when they gain water and become turgid
How do guard cells gain water
through osmosis
Mechanism to open stomata
1)In response to light, ATP-powered proton pumps in the guard cell surface membranes actively transport hydrogen (H+) ions out of the guard cell
2)This leaves the inside of the guard cells negatively charged compared to the outside, causing channel proteins in the guard cell surface membranes to open, allowing potassium (K+) ions to move down the electrical gradient and enter the guard cells
3) The potassium (K+) ions also diffuse into the guard cells down a concentration gradient. The combination of the electrical gradient and concentration gradient is known as an electrochemical gradient
4) The influx of potassium (K+) ions increases the solute concentration inside the guard cells, lowering the water potential inside the cells
5) Water now enters the guard cells by osmosis through aquaporins in the guard cell surface membranes. Most of the water enters the vacuoles, causing them to increase in size
6) This increases the turgor pressure of the guard cells, causing the stoma to open
7) The bands of cellulose microfibrils only allow the guard cells to increase in length (not diameter)
8)The thin outer walls of the guard cells bend more easily than the thick inner walls. This causes the guard cells to become curved, opening up the stoma
Mechanism to close stomata
1)When certain environmental stimuli are detected (that lead to the closing of the stomata), the proton pumps in the guard cell surface membranes stop actively transporting hydrogen (H+) ions out of the guard cell
2)The potassium (K+) ions leave the guard cells
3)The water potential gradient is now reversed and water leaves the guard cells by osmosis
4)This causes the guard cells to become flaccid, closing the stoma
What gets produced during times of water stress and why
the hormone abscisic acid (ABA) is produced to stimulate the closing of their stomata
Abscisic Acid & Stomatal Closure
1)Guard cells have ABA receptors on their cell surface membranes. ABA binds with these receptors, inhibiting the proton pumps and therefore stopping the active transport of hydrogen (H+) ions out of the guard cells
2)ABA also causes calcium (Ca2+) ions to move into the cytoplasm of the guard cells through the cell surface membranes
3)The calcium ions act as second messengers; they cause channel proteins to open that allow negatively charged ions to leave the guard cells
4)This stimulates the opening of further channel proteins that allow potassium (K+) ions to leave the guard cells
5)The calcium ions also stimulate the closing of channel proteins that allow potassium (K+) ions to enter the guard cells
6)This loss of ions increases the water potential of the guard cells. Water leaves the guard cells by osmosis
7)The guard cells become flaccid, causing the stomata to close
