Homeostasis Flashcards
1
Q
Homeostasis
A
- Maintenance of internal environment within restricted limits in organisms
- Involves trying to maintain chemical make-up, volume and other features of blood and tissue fluid within limits
- Also ensures that cells are in an environment that meets their requirements and allows them to function normally despite external changes
- This doesn’t mean their are no changes-continuous fluctuations brought by variations in external and internal conditions e.g. temperature, pH
- These changes occur around optimum point
- Homeostasis is the ability to return to that optimum point and maintain organisms in balanced equilibrium
2
Q
Internal environment
A
- Made up of tissue fluids that bathe each cell, supplying nutrients and removing wastes
- Maintaining feautures of fluid at optimum levels projects cells from changes in external environment
3
Q
Importance of homeostasis
A
- Important to maintain right core body temperature and blood pH
- This is because temperature and pH affect enzyme activity, and enzymes control the rate of metabolic reactions
- Also important to maintain right blood glucose concentration because cells need glucose for energy and blood glucose concentration affects the water potential of blood
4
Q
Effect of temperature on homeostasis
A
- Rate of metabolic reactions increases when the temperature’s increased
- More heat means more kinetic energy, so molecules move faster
- Makes the substrate molecules more likely to collide with the enzymes’ active sites
- Energy of these collisions also increases, which means each collision is more likely to result in a reaction
- If temperature gets too high, reaction essentially stops
- Rise in temperature makes enzyme’s molecules vibrate more
- If temperature goes above a certain level, this vibration breaks some of the hydrogen bonds that hold the enzyme in its 3D shape
- Active site changes shape and enzyme and substrate no longer fit (enzyme has denatured-no longer functions as a catalyst)
- If body temperature is too low, enzyme activity is reduced, slowing rate of metabolic reactions
- Highest rate happens at optimum temperature (37 C in humans)
5
Q
Effect of pH on homeostasis
A
- If blood pH is too high or low (highly alkaline or acidic) enzymes become denatured
- When an enzyme is denatured the reaction can still happen but too slow for body’s needs
- Ionic and hydrogen bonds that hold them in their 3D shape are broken, so shape of enzyme’s active site is changed and no longer works as a catalyst
- Highest rate of enzyme activity happens at their optimum pH (so metabolic reactions are fastest)
- Optimum usually around 7 but some enzymes work best at other pHs
6
Q
Logarithmic Scale
A
- pH calculated based on concentration of hydrogen ions in the environment
- Scale uses logarithm number instead of number itself
- Each value on scale is 10 times larger than the value before-so a solution of pH 3 contains 10 times more H+ ions than a solution of pH 4
- This is because the concentration of H+ can vary enormously and so it’s easier to compare values
- Converting values to logarithmic scale also makes it easier to plot both very small and large values on the same axis
- pH= -log10 [H+]
7
Q
Blood glucose concentration
A
Too high
- Water potential of blood is reduced to a point where water molecules diffuse out of cells into blood by osmosis
- High to low water potential across a partially permeable membrane
- Causes cells to shrivel up and die
- Maintenance of blood glucose concentration ensures constant water potential and glucose for respiration by cells
Too low
- Cells are unable to carry out normal activities because their isn’t enough glucose for respiration to provide energy
8
Q
Respiratory substrate
A
Substance that can be broken down during respiration to release energy (glucose)
9
Q
Negative Feedback
A
- When change produced by control system leads to a change in stimulus detected by receptor and turns system off
- Reversal of a change (high or low level) in the environment to return to the optimum position
- Receptor detects the change
- Communication systems (hormonal/nervous) inform the effectors
- The effector reacts to reverse the change to bring the level back to normal
- Only works within certain limits (if change is too big, effectors may not be able to counteract it)
- Normal level-level changes from normal-receptors detect change-communication(hormonal or nervous)-effectors respond
10
Q
Multiple negative feedback mechanisms
A
- Having more than one mechanism gives more control over changes in your internal environment than just having one negative feedback mechanism
- Also means you can actively increase or decrease a level so it returns to normal
- If you had one negative feedback mechanism, all you could do would be turn it on or off
- You’d only be able to actively change a level in one direction so it returns to normal
- Only one negative feedback mechanism means a slower response and less control
11
Q
Positive Feedback
A
- Response causes change to increase (amplifies change)
- Effectors respond to further increase level away from normal level
- Destabilizes the system
- Usually more harmful
- Does not lead to homeostasis
- Can be useful in certain situations (can rapidly activate processes in the body)
- Can also happen when a homeostatic system breaks down
- Normal level-level changes from normal-receptors detect change-communication(hormonal or nervous)-effectors respond
12
Q
Feedback mechanism
A
- Optimum point=point at which system operates best. This is monitored by a…
- Receptor=detects any deviation from optimum point and informs the…
- Coordinator=which coordinates information from receptors and sends instructions to the right…
- Effector=often muscle or gland, which brings about changes needed to return the system to the optimum point. This return to normality creates a…
- Feedback mechanism=by which a receptor responds to a stimulus created by the change to the system brought about by the effector
13
Q
Glucose concentration in the blood
A
- All cells need a constant supply of energy to work-so blood glucose concentration must be controlled
- Concentration of glucose is monitored by cells in the pancreas and rises after eating food containing carbohydrate
- Falls after exercise, as more glucose is used in respiration to release energy
14
Q
Hormonal control of blood glucose concentration
A
-
Hormonal system controls concentration using hormones insulin and glucagon
- Insulin and glucagon are chemical messengers that travel in the blood to their target cells (effectors)
- Both secreted by clusters of cells in the pancreas called the islets of Langerhans
- islets of Langerhans contain beta and alpha cells
- Beta cells secrete insulin into the blood and alpha cells secrete glucagon
- Insulin and glucagon act on effectors, which respond to restore the blood glucose concentration to the normal level
15
Q
Insulin
A
- (globular protein) Lowers blood glucose concentration when it’s too high
- Binds to specific receptors on the cell membranes of muscle cells and liver cells (hepatocytes)
- It increases the permeability of muscle-cell membranes to glucose, so cells take up more glucose
- This involves increasing the number of channel proteins in the cell membranes
- Tertiary structure of glucose transport carrier proteins changes, allowing more glucose into cells by facilitated diffusion
- Glucose transport channels activate enzymes converting glucose to glycogen and fat
- Insulin also activates enzymes in muscle and liver cells that convert glucose into glycogen
- The cells are able to store glycogen in their cytoplasm, as an energy source
- Process of forming glycogen from glucose is called glycogenesis
- Insulin also increases the rate of respiration of glucose, especially in muscle cells
16
Q
What happens when blood glucose concentration is lowered?
A
- Increased rate of absorption of glucose into cells (especially muscle)
- Increasing respiratory rate of cells that use up glucose
- Increased rate of glycogenesis
- Increased rate of coversion of glucose to fat
17
Q
Glucagon
A
- Raises blood glucose concentration when it’s too low
- Binds to specific receptors on cell membranes of liver cells and activates enzymes that break down glycogen into glucose
- Process of breaking down glycogen is called glycogenolysis
- Glucagon also activates enzymes involved in the formation of glucose from glycerol and amino acids
- Forming glucose from non-carbohydrates is called gluconeogenesis
- Glucagon decreases the rate of respiration of glucose in cells
18
Q
Hormones and the system
A
- Travel in the blood to their target cells, so responses produced by hormones are slower than those produced by nervous impulses
- Responses to hormones can occur all over the body if their target cells are widespread, unlike nervous impulses that are localised to one area
- Hormones are not broken down as quickly as neurotransmitters though, so their effects tend to last for longer
- Hormones are effective in low concentrations, widespread, longlasting, produced by glands, carried to target cells
19
Q
Negative Feedback
A
- Occurs when the stimulus causes the corrective measures to be turned off
- In doing so this tends to return the system to its original (optimum) level (prevents any overshoot)
- There are separate negative feedback mechanisms to regulate departures from the norm in each direction
20
Q
Rise in blood glucose concentration (negative feedback)
A
- When the pancreas detects blood glucose concentration is too high, the beta cells secrete insulin and the alpha cells stop secreting glucagon
- Insulin binds to receptors on liver and muscle cells (effectors)
- The liver and muscle cells respond to decrease the blood glucose concentration e.g. glycogenesis is activated
- Blood glucose concentration returns to normal
21
Q
Fall in blood glucose concentration (negative feedback)
A
- When the pancreas detects blood glucose concentration is too low, the alpha cells secrete glucagon and the beta cells stop secreting insulin
- Glucagon then binds to receptors on liver cells (effector)
- Liver cells respond to increase the blood glucose concentration e.g. glycogenolysis is activated
- Blood glucose concentration returns to normal
22
Q
Glucose transporters
A
- Channels proteins which allow glucose to be transported across a cell membrane
- Skeletal and cardiac muscle cells contain a glucose transporter called GLUT4
- When insulin levels are low, GLUT4 is stored in vesicles in the cytoplasm of cells, but 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
- Concentration of glucose is not constant but fluctuates around optimum point
23
Q
Adrenaline
A
- Hormone that’s secreted from your adrenal glands (found just above kidneys)
- Secreted when there’s a low concentration of glucose in your blood, when your stressed and when your exercising
- Adrenaline binds to receptors in the cell membrane of liver cells and does these things to increase blood glucose concentration:
- Activates glycogenolysis (caused by activation of enzymes)
- Inhibits glycogenesis
- Attaches to protein receptors on target cells
- 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
24
Q
Adrenaline (second messengers)
A
- Both adrenaline and glucagon can activate glycogenolysis inside a cell even though they binds to receptors on the outside of the cell
- Do this by the second messenger model- binding of hormone to cell receptors which changes it’s shape (on liver cell) activates an enzyme on the inside of the cell membrane, which then produces a chemical known as a second messenger
- Second messenger activates other enzymes in the cell to bring about a response
- Receptors for adrenaline and glucagon have specific tertiary structures that make them complementary in shape to their respective hormones
- To activate glycogenolysis, adrenaline and glucagon bind to their receptors and activate an enzyme called adenylate cyclase
- Activated adenylate cyclase converts ATP into a chemical called cyclic AMP (cAMP), which is a second messenger
- cAMP activates an enzyme called protein kinase A by changing it’s shape
- Protein kinase A activates a cascade (chain of reactions) that breaks down glycogen into glucose (glycogenolysis)
35
Q
Renal (Bowman’s) Capsule
A
- Cup-shaped and surrounds a mass of blood capillaries known as the glomerulus
- Inner layer of renal capsule is made up of specialised cells called podocytes
41
Q
Excretion of waste products
A
- Blood enters the kidney through the renal artery and then passes through capillaries in the cortex (outer layer) of the kidneys
- As blood passes through capillaries in the cortex, substances are filtered out of the blood and into long tubules that surround the capillaries (ultrafiltration)
- Useful substances, such as glucose and the right amount of water, are then reabsorbed back into the blood (selective reabsorption)
- Remaining unwanted substances pass along to the bladder and are excreted as urine
43
Q
Where does ultrafiltration take place?
A
- Blood from the renal artery enters small arterioles in the cortex of the kidney
- Each arteriole splits into a structure called a glomerulus-bundle of capillaries looped inside a hollow ball called a Bowman’s capsule
44
Q
Ultrafiltration
A
- The arteriole that takes blood into each glomerulus is called the afferent arteriole
- Arteriole that takes filtered blood away from the glomerulus is called the efferent arteriole
- Efferent is smaller in diameter than afferent, so blood in glomerulus is under high pressure
- High hydrostatic pressure forces liquid and small molecules in the blood out of the capillary and into the Bowman’s capsule
- Liquid and small molecules pass through 3 layers to get into the Bowman’s capsule and enter the nephron tubules
- Capillary endothelium, basement membrane and epithelium of Bowman’s capsule
- Larger molecules like proteins and blood cells can’t pass through so stay in the blood
- Substances that enter the Bowman’s capsule are known as glomerular filtrate (water, glucose and mineral ions squeezed out due to hydrostatic pressure)
- Glomerular filtrate passes along the rest of the nephron and useful substances are reabsorbed along the way
- Finally, filtrate flows through collecting duct and passes out of the kidney along the ureter
47
Q
Where does selective reabsorption occur?
A
- 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)
- Useful substances leave the tubules of the nephrons and enter the capillary network that’s wrapped around them
48
Q
Selective Reabsorption
A
- Epithelium of the wall of the PCT 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)
- Infoldings at their bases give a large surface area to transfer reabsorbed substances in blood capillaries and high density of mitochondria to provide ATP for active transport
- Useful solutes, like glucose, are reabsorbed along the PCT by active transport and facilitated diffusion
- 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
- The filtrate that remains is urine, which passes along the ureter to the bladder
54
Q
loop of Henle
A
- Located in the medulla (inner layer) of the kidneys
- Made up of 2 limbs-the descending limb and the ascending limb
- The limbs control the movement of sodium ions so that water can be reabsorbed by the blood
55
Q
Maintenance of a gradient of sodium ions by the loop of Henle
A
- Sodium ions are actively transported out of the ascending limb (near the top) using ATP provided by mitochondria
- This creates a low water potential (high ion concentration) in the region of the medulla between the 2 limbs (interstitial space)
- Ascending limb is impermeable to water, so water stays inside tubule
- Because there’s a lower water potential in the medulla than in the descending limb, water moves out of the descending limb (which is permeable to water) into the medulla by osmosis
- This makes the glomerular filtrate more concentrated (ions can’t diffuse out-descending limb isn’t permeable to them)
- Water in the medulla is reabsorbed into the blood through the capillary network
- Near the bottom of the ascending limb, Na+ ions diffuse out into the medulla, further lowering the water potential in the medulla (ascending limb is impermeable to water, so it stays in the tubule)
- Water moves out of the DCT by osmosis and is reabsorbed into the blood
- These stages increase the ion concentration in the medulla, which lowers the water potential
- This causes water to move out of the collecting duct by osmosis
- Does this through channel proteins specific to water (aquaporins)
- ADH can alter number of channels and control water loss
- By the time filtrate (now urine) leaves the collecting duct on it’s way to the bladder, it has lost most of it’s water and so has a lower water potential than the blood
- As before, the water in the medulla is reabsorbed into the blood through the capillary network
- The volume of water reabsorbed into the capillaries is controlled by changing the permeability of the DCT and the collecting duct
