14. Homeostasis Flashcards
What are some of the physiological factors controlled in homeostasis in mammals?
- Core body temperature
- Metabolic wastes- CO2 and urea
- Blood pH
- Blood glucose concentration
- Water potential of the blood
- The concentrations in the blood of the respiratory gases, oxygen and carbon dioxide
Name 4 features of tissue fluid which influence cell activities.
- Temperature - low temps slow down metabolic reactions; at high temps proteins and enzymes are denatured.
- Water potential - if it decreases water may move out of cells by osmosis. If it increases, water may enter the cell causing it to swell and maybe burst.
- Concentration of glucose in the blood- lack of it causes respiration to slow or stop which deprives the cell of an energy source. Too much of it can cause water to move out of the cell.
- pH - enzymes will function less efficiently and will be denatured at extreme values of pc.
What is homeostasis ?
The maintenance of a relatively constant internal environment for the cells within the body.
What is negative feedback ?
A process in which a change in some parameter brings about processes which return it towards normal
Explain the simple process of how urine is formed and excreted
Urea is produced in the liver from excess amino acids and is transported from the liver to the kidneys in solution in blood plasma. The kidneys remove urea from the blood and excrete it dissolved in water and this solution is called urine.
How is excess protein in the body resolved
By a process called deamination in the liver.
The amine group of an amino acid along with an extra hydrogen atom is removed - this forms ammonia.
How does the body deal with ammonia since it is so toxic? And what problems does it cause?
It is immediately turned to urea which is less soluble and less toxic.
Ammonia increases the pH in cytoplasms, it interferes with metabolic processes such as respiration and cell signaling in the brain
What are other nitrogenous excretory products produced by the body?
Creatinine and uric acid
Describe the structure of the kidney in terms of blood flow and urine flow.
Each kidney receives blood from a renal artery
Blood is returned via a renal vein
The ureter carries the urine from the kidney to the bladder
The urethra carries urine from the bladder to outside of the body
What are the two stages of urine making
Ultrafiltration- filtering small molecules, including urea out of the blood into the Bowman’s capsule to form filtrate from there it flows along the nephron towards the collecting duct.
Selective reabsorption- involves taking back any useful molecules from the filtrate as it flows along the nephron.
Describe the process of ultrafiltration
There is hydrostatic pressure within the glomerular capillaries. This filters the blood, forcing water, glucose, urea and ions into the Bowman’s capsule to form the glomerular filtrate. Proteins and cells are too big and are not filtered into the Bowman’s capsule so they stay in the blood.
Describe how the structure of Bowman’s capsule and the glomerular capillaries is suited to its function.
-The capillary endothelial wall is one cell thick, and there are gaps between cells to allow substances to pass through.
-The renal capsule has podocytes which have spaces in Bateman them to allow the filtrate to pass into the renal capsule.
-The connective tissue between the capillaries and renal capsule prevents larger molecules like proteins and red blood cells leaving the blood
Where does selective reabsorption occur?
In the proximal convoluted tubule.
How are the cells in the proximal convoluted tubule adapted for their function of reabsorption?
-Many microvilli on the surface facing the lumen of the nephron to increase the surface area for reabsorption of substances from filtrate in the lumen.
-Many co-transporter proteins in the luminal membrane
-Tight junctions that hold adjacent cells together so that fluid cannot pass between the cells.
-Many mitochondria to provide energy for sodium potassium pump proteins in the basal membranes of the cells
Describe the process of selective reabsorption in the proximal convoluted tubule
Sodium-potassium pumps move sodium ions out of the cells into the blood. This lowers the concentration of sodium ions inside the cells so that sodium ions in the filtrate diffuse down their concentration gradient through the luminal membranes only through special co-transporter proteins.
Describe the process of reabsorption in the loop of Henle and collecting duct.
There is a high concentration of sodium and chloride ions in the tissue fluid in the medulla. This tissue fluid enables a lot of water to be reabsorbed from the fluid in the collecting duct as it flows through the medulla. The filtrate continues through the distal convoluted tubule into the collecting duct after leaving the loop of Henle. Passing through again the regions where the solute concentration of the tissue fluid is very high and the w.p very low. Water can therefore move out of the collecting duct by osmosis until the water potential of urine is the same as the w.p of the tissue fluid in the medulla. This is all controlled by antidiuretic hormone (ADH)
Describe the process of reabsorption in the distal convoluted tubule and collecting duct.
Sodium ions are actively pumped from the fluid in the tubule into the tissue fluid where they pass into the blood. Potassium ions actively transported into the tubule. The rate at which these two ions are moved into and out of the fluid in the nephron helps to regulate the concentration of these ions in the blood.
What is osmoregulation?
The control of the water potential of body fluids involving the hypothalamus, posterior gland and the kidneys
What monitors the water potential of the blood and how?
Osmoreceptors- when these cells detect a decrease in the water potential of the blood below the set point, nerve impulses are sent along the neurons to where they end in the posterior pituitary gland. These impulses stimulate the release of antidiuretic hormone. The molecules of ADH enter the blood and are carried all over the body the effect of ADH is to reduce the loss of water in the urine by making the kidneys reabsorb as much water as possible.
Explain how the luminal membranes of the collecting ducts become more permeable to water than usual.
By increasing the number of the water-permeable channels known as aquaporins. ADH molecules bind to receptor proteins which stimulate the production of cyclic AMP (cAMP) which is a second messenger. Cyclic AMP activates a signaling cascade leading to the phosphorylation of the aquaporin molecules- the activation of these aquaporin molecules causes the vesicles to move towards the luminal membrane and fuse with it, so increasing the permeability of the membrane to water
Why do the water molecules move through the aquaporins out of the tubule and into the tissue fluid?
The tissue fluid in the medulla has a very low water potential and the fluid in the collecting ducts has a very high water potential.
How does ADH increase water reabsorption in the collecting duct?
- ADH binds to receptors in the cell surface membrane of the cells lining the collecting duct
- This activates a series of enzyme-controlled reactions ending with the production of an active phosphorylase enzyme.
- The phosphorylase causes vesicles surrounded by membrane containing water-permeable channels (aquaporins) to move to the cell surface membrane.
- The vesicles fuse with the cell surface membrane
- Water can now move freely through other aquaporins from the cells down the water potential gradient into the concentrated tissue fluid and blood plasma in the medulla of the kidney
What is the name of the group of cells in the pancreas which secretes hormones and what types are there?
Islet of Langerhans- contain a cells and B cells
a cells secrete glucagon
B cells secret insulin
How do the cells in the Islets of Langerhans respond to an increase in glucose concentration?
The a cells respond by stopping the secretion of glucagon
The B cells respond by secreting insulin into the blood plasma
How does insulin work in returning the blood glucose concentration back to normal?
It stimulates cells with receptors specific for insulin to increase the rate at which they absorb glucose from the blood and convert it into glycogen. Insulin also increases the use of glucose in respiration.
Name the different type of transporter proteins and where they are located
GLUT4- muscle cells
GLUT1- brain cells
GLUT2- liver cells
Which enzyme is activated insulin?
Insulin stimulates the activation of the enzyme glucokinase, which phosphorylates glucose. This traps glucose inside cells because phosphorylated glucose cannot pass through glucose transporters in the cell surface membranes.
Insulin also stimulates the activation of two enzymes, phosphofruktokinase and glycogen synthase which together catalyse the addition of glucose molecules to glycogen in a process known as glycogenesis.
What is the first reaction by the cells of the islets of Langerhans when a decrease in blood glucose concentration is detected?
The a cells respond by secreting glucagon while the B cells respond by stopping the secretion of insulin.
Explain what happens when there is a decrease in concentration of insulin in the blood.
The decrease reduces the rates of uptake and use of glucose by liver and muscle cells. Glucagon binds to different specific receptor molecules in the cell surface membrane of liver cells. The binding causes a conformational change in the receptor protein- that activates a G protein that in turn activates the enzyme adenylyl cyclase. Adenylyl cyclase catalyses the conversion of ATP to cyclic AMP which is a second messenger. Molecules of c-AMP bind to protein kinase A enzymes and activate them. Active protein kinase enzymes activate phosphorylase kinase enzymes by adding phosphate groups to them. This is an enzyme cascade
What is gluconeogenesis?
When glucagon stimulates the formation of glucose from amino acids, glycerol, pyruvate and lactate
Explain the process of using test strips to determine the glucose concentration in a sample
Test strips contain the enzymes glucose oxidase and peroxidase, these two enzymes are immobilized on a small pad at one end of the stick. The pad is covered by a cellulose membrane that only allows small molecules from the blood to reach the enzymes. The pad is immersed in urine for a brief time and glucose oxidase ONLY catalyses a chemical reaction in which glucose is oxidised into gluconic acid- if glucose is present. Peroxidase catalyses a reaction between hydrogen peroxide and a colorless chemical in the pad to form a brown compound. The resulting colour of the pad is matched against a colour chart which shows different concentrations of glucose.
Explain how a biosensor works in reading blood glucose concentration
It uses glucose oxidase immobilized on a recognition layer. Small molecules in the plasma pass through the membrane. Glucose molecules enter the active sites of the enzyme that catalyzes the reaction to produce gluconic acid and hydrogen peroxide. Hydrogen peroxide is oxidized at an electrode that detects electron transfers. The electron flow is proportional to the number of glucose molecules in the blood. The biosensor amplifies the current which is read by the meter which produces a digital reading for blood glucose concentration within seconds.
What are the weather conditions that cause stomata to open.
-increasing light intensity
-low carbon dioxide concentrations in the air spaces within the leaf
What are the weather conditions that cause stomata to close?
-darkness
-high carbon dioxide concentrations in the air spaces in the leaf
-low humidity
-high temperature
-water stress, when the supply of water from the roots is limited and there are high transpiration rates
What is the function of stomata in plants?
Stomata regulate gas exchange and water loss by opening and closing
How do proton pumps contribute to stomata opening?
ATP-powered proton pumps actively transport H+ ions out of guard cells, creating a negative charge inside the cells.
How does potassium ion movement affect guard cells?
K+ ions enter the guard cells due to the electrochemical gradient, lowering water potential and causing water to enter via osmosis
What is abiscisic acid?
An inhibitory plant growth regulator that causes closure of stomata in dry conditions
What is the role of Calcium ions in stomatal closure?
They act as second messengers to open ion channels, allowing K+ to leave guard cells.
What two gradients drive ion movement in guard cells?
Electrochemical and water potential gradients
What are the main differences between an open and closed stoma?
Open stomata have turgid guard cells with expanded vacuoles, while closed stomata have flaccid guard cells
How does abscisic acid (ABA) regulate stomatal closure?
ABA inhibits proton pumps, stimulates Ca2+ movement and triggers K+ efflux, leading to loss of turgor pressure
Describe the role of aquaporins in osmoregulation
- they are channel proteins
- more aquaporins increase cell surface membrane permeability to water of the collecting duct cells
- allow water to be reabsorbed
Describe and explain the action of ADH on the cells of the collecting duct when the water potential of the blood decreases
- ADH binds to cell surface membrane receptor Which activates a signaling cascade that leads to phosphorylation of the aquaporin molecules.
-this activates the aquaporins - vesicles move towards cell surface membrane
- ## this increases permeability of the membrane to water
State the role of the enzyme cascade
Amplifies the original signal from glucagon
State the function of glycogen phosphorylase
Catalyses the breakdown of glycogen to glucose
