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A-Level Biology: Homeostasis
Terms in this set (51)
What is homeostasis?
The maintenance of a constant internal environment
Why is it important to maintain a core body temperature?
- If the body temperature is too high then enzymes may denature - the enzyme vibrates too much which breaks the H bonds causing a change in 3D structure
- If temperature is too low then enzyme activity is reduced
Why is it important to maintain a constant pH?
- If blood pH is too high or too low enzymes become denatured
- The hydrogen bonds are broken so the 3D structure is changed, causing the active site to change shape
- Metabolic reactions are slowed.
What is it important to maintain a constant concentration of glucose in the blood?
- Too high = water potential of blood is reduced, water diffuses out of the cell and cells shrivel up and die
- Too low = cell can't carry out normal activities because there isn't enough glucose for respiration to provide energy
What is negative feedback?
Receptors detect when a level is far from the optimum level and the information is passed to an effector via the nervous or hormonal system.
The effector responds to counteract the change, bringing the level back to normal.
When does the negative feedback mechanism stop working?
If the change of a level is too big then the effectors may not be able to counteract the change
What is positive feedback?
Effects respond to further increase the level away from the normal level
Useful to rapidly activate something
Examples of positive feedback
Widening of the cervix during birth
Which factors must be controlled in the blood?
What is a hormone?
Chemicals that travel in the blood plasma towards target cells
What hormones control glucose concentration?
Where are insulin and glucagon secreted from?
Islets of Langerhans in the pancreas
Which cells secrete glucagon and which cells secrete insulin?
Glucagon = alpha cells
Insulin = beta cells
Which factors influence blood glucose concentration?
Eating food containing carbohydrates
Exercising as more glucose is used in respiration to release energy
What is glycogenolysis?
Glycogen is broken down into glucose
What is glycogenesis?
Glucose is converted into glycogen
What is gluconeogenesis?
Glucose is formed from glycerol and amino acids
What effect does insulin have?
Lowers blood glucose concentration
How does insulin lower blood glucose concentration?
1) Binds to specific receptors on liver and muscle cells
2) Increases the permeability of muscle-cell membranes to glucose by creating more channel proteins
- vesicles containing glucose transporters move towards the membrane and fuse with it when insulin binds, forming more channel proteins
3) Activates enzymes in liver and muscle cells that convert glucose into glycogen
4) The cells can store glycogen as an energy source in the cytoplasm
5) Also increases the rate of respiration of glucose, especially in muscle cells
What effect does glucagon have on blood glucose concentration?
Raises blood glucose concentration
How does glucagon increase blood glucose concentration? (second messenger model)
1) Binds to its specific receptors on the liver cell
2) Activates an enzyme called adenylate cyclase
3) This converts ATP into cyclic AMP (cAMP), the second messenger
4) cAMP activates the enzyme protein kinase A
5) Protein kinase A activates a chain of reactions which break glycogen into glucose
How does adrenaline increase glucose blood concentration?
Uses the same second messenger model as glucagon
Also inhibits glycogenesis and activates glycogenolysis, and activates glucagon secretion
More glucose is made available to the muscles for respiration
What is the negative feedback mechanism for the detection of low blood glucose?
1) Blood glucose falls
2) Detected by alpha cells in islets of Langerhans
3) Glucagon secreted
4) Detected by target cells (hepatocytes and muscle cells)
5) Glucose formed by glycogenolysis and decreased respiration
What is the negative feedback mechanism for the detection of high blood glucose?
1) Blood glucose rises
2) Detected by beta cells in islets of Langerhans
3) Insulin secreted
4) Detected by target cells (hepatocytes)
5) Glucose removed from blood by glycogenesis and increased respiration
What is Type 1 diabetes?
1) The immune system attacks the beta cells in the islets of Langerhans
2) They can't produce insulin
What is hyperglycaemia?
When blood glucose level rises and stays high after eating
The kidneys can't reabsorb all the glucose so some is excreted in urine
How is Type 1 diabetes treated?
- Regular insulin injections throughout the day or insulin pumps which deliver a constant supply if insulin
What is hypoglycaemia?
A big drop in blood glucose levels
Can be dangerous so insulin therapy has to be controlled
What is Type 2 diabetes?
1) Beta cells don't produce enough insulin
2) Or body cells don't respond to insulin
3) The insulin receptors don't work
4) Blood glucose concentration is higher than normal
How is Type 2 diabetes treated?
Regular exercise and a balanced diet
Glucose-lowering medication is taken if needed
Eventually insulin injections may be needed
How do you reduce the risk of getting type 2 diabetes?
Eat a balanced diet, low in fat, sugar and salt
How can you determine the concentration of a glucose solution?
1) Colorimetry of known samples
2) Plot a calibration curve of absorbance against concentration
3) Test unknown solution
4) Compare to the graph
What is the function of the kidneys?
1) Excrete waste products
2) Regulate the water potential of blood
4) Selective reabsorption
What is the structure of a nephron?
What is the glomerulus?
A bundle of capillaries looped inside a hollow ball called a Bowman's capsule
What is the afferent arteriole?
Takes blood into the glomerulus
What is the efferent arteriole?
Takes filtered blood away from the glomerulus
Why is the efferent arteriole smaller in diameter?
Allows the blood in the glomerulus to be under high pressure
What is ultrafiltration?
Substances are filtered out of the blood and passed into tubules that surround the capillaries
How is blood filtered through the nephron?
1) Blood from the renal artery enters small arterioles in the cortex of the kidney
2) Each arteriole splits into a glomerulus
3) The high pressure from the efferent arteriole forces liquid and small molecules in the blood out of the capillaries and into the Bowman's capsule.
4) The substances that enter the Bowman's capsule are called the glomerular filtrate - this passes along the rest of the nephron and useful substances are reabsorbed
5) Large molecules like proteins and blood cells stay in the blood as they are too large
6) The final filtrate flows through the collecting duct and passes out of the kidney along the ureter
What is selective reabsorption?
1) Useful substances leave the tubules of the nephrons and enter the capillary network that wrapped around them
2) The epithelium of the wall of the proximal convoluted tubule (PCT) has microvilli for the reabsorption of useful materials from the glomerular filtrate
3) Useful solutes like glucose are reabsorbed along the PCT by active transport and facilitated diffusion
4) Water enters the blood via osmosis because the water potential of blood is lower than that of the filtrate
5) Water is reabsorbed from the PCT, loop of Henle, distal convoluted tubule (DCT) and the collecting duct.
6) The filtrate which remains is urine which passes along the ureter to the bladder
Where does selective reabsorptions take place?
As glomerular filtrate flows along the proximal convoluted tube (PCT), through the loop of Henle and along the distal convoluted tube (DCT)
What is urine made up of?
Water and dissolved salts
Hormones and excess vitamins
What is osmoregulation?
The maintenance of water potential of the blood
What happens if the water potential of blood is too high?
Less water is reabsorbed via osmosis into the blood from the tubules of the nephrons
This means that urine is more dilute
What happens if the water potential of blood is too low?
More water is reabsorbed by osmosis into the blood from the tubules of the nephron
Urine is more concentrated
Where is the loop of Henle located?
The medulla (inner layer of kidneys)
How does the loop of Henle maintain a sodium ion gradient?
1) At the top of the ascending limb, sodium ions are pumped out into the medulla using active transport
2) The ascending limb is impermeable to water, so water stays in the tubule, creating a low water potential in the medulla
3) This causes water to move out of the descending limb into the medulla, so the filtrate is more concentrated
4) The water in the medulla is reabsorbed back into the blood through the capillary network
5) Near the bottom of the ascending limb, sodium ions diffuse out of the medulla, further lowering the water potential
6) Water moves out of the DCT via osmosis and is reabsorbed into the blood
7) The first three stages increase the ion concentration in the medulla, thus lowering the water potential
8) This causes water to move out of the collecting duct and be reabsorbed into the blood by the capillary network
How is water reabsorption controlled by hormones?
1) When the water potential of blood decreases, water will move out of the cells by osmosis
2) This causes the cell to decrease in volume, which then sends signals to the posterior pituitary gland
3) This releases the hormone ADH into the blood
4) More water is reabsorbed from the tubules into the medulla and into the blood via osmosis
Which cells monitor the water potential of blood?
Osmoreceptors in the hypothalamus
What does the hormone ADH do?
Causes the walls of the DCT and collecting duct to be more permeable to water when the water potential of blood decreases
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