6.4 Homeostasis is the Maintenance of a Stable Internal Environment Flashcards
What is homeostasis?
Internal environment is maintained within set limits around an optimum
Why is it important that core temperature remains stable?
- Maintain stable rate of enzyme controlled reactions and prevent damage to membranes
- Temp too low = enzyme and substrate molecules have insufficient kinetic energy
- Temp to high = enzymes denature
Why is it important that blood ph remains stable?
- Maintain stable rate of enzyme controlled reactions and optimum conditions for other proteins
- Acidic pH = H+ ions interact with H bonds and ionic bonds in tertiary structure of enzymes - shape of active site changes so no ES complexes form
Why is it important that blood glucose concentration remains stable?
- Maintain constant blood water potential: prevent osmotic lysis
- Maintain constant concentration of respiratory substrate: organism maintains constant level of activity regardless of environmental conditions
Define negative feedback
Self-regulatory mechanisms return internal environment to optimum when there is fluctuation
Define positive feedback
A fluctuation triggers changes that result in an even greater deviation from the normal level
Outline the general stages involved in negative feedback
- Receptors detect deviation
- Coordinator
- Corrective mechanism by effector
- Receptors detect that conditions have returned to normal
Suggest why separate negative feedback mechanisms control fluctuations in different directions
Provides more control, especially in case of ‘overcorrection’, which would lead to a deviation in the opposite direction from the original one
Suggest why coordinators analyse inputs from several receptors before sending an impulse to effectors
- Receptors may send conflicting information
- Optimum response may require multiple types of effector
Why is there a time lag between hormone production and response by effector?
It takes time to:
- produce hormone
- transport hormone in the blood
- cause required change to target protein
Name the factors that affect blood glucose concentration
- Amount of carbohydrate digested from diet
- Rate of glycogenolysis
- Rate of gluconeogenesis
Define glycogenesis
Liver converts glucose into glycogen
Define glycogenolysis
Liver hydrolyses glycogen into glucose which can diffuse into blood
Define gluconeogenesis
Liver converts glycerol and amino acids into glucose
Outline the role of glucagon when blood glucose concentration decreases
- Alpha cells in Islets of Langerhans in pancreas detect decrease and secrete glucagon into bloodstream
- Glucagon binds to surface receptors on liver cells and activates enzymes for glycogenolysis and gluconeogenesis
- Glucose diffuses from liver into bloodstream
Outline the role of adrenaline when blood glucose concentration decreases
- Adrenal glands produce adrenaline. Binds to surface receptors on liver cells and activate enzymes for glycogenolysis
- Glucose diffuses from liver into bloodstream
Outline what happens when blood glucose concentration increases
- Beta cells in Islets of Langerhans in pancreas detect increase and secrete insulin into bloodstream
- Insulin binds to surface receptors on target cells to:
a) increase cellular glucose uptake
b) activate enzymes for glycogenesis
c) stimulate adipose tissue to synthesise fat
Describe how insulin leads to a decrease in blood glucose concentration
- Increases permeability of cells to glucose
- Increase glucose concentration gradient
- Triggers inhibition of enzymes for glycogenolysis
How does insulin increase permeability of cells to glucose?
- Increases number of glucose carrier proteins
- Triggers conformational change which opens glucose carrier proteins
How does insulin increase the glucose concentration gradient?
- Activates enzymes for glycogenesis in liver and muscles
- Stimulates fat synthesis in adipose tissue
Use secondary messenger model to explain how glucagon and adrenaline work
- Hormone receptor complex forms
- Conformational change to receptor activates G-protein
- Activates adenylate cyclase, which converts ATP to cyclic AMP (cAMP)
- cAMP activates protein kinase A pathway
- Results in glycogenolysis
Explain the causes of Type 1 diabetes and how it can be controlled
- Body cannot produce insulin e.g. due to autoimmune response which attacks Beta cells in Islets of Langerhan
- Treat by injecting insulin
Explain the causes of Type 2 diabetes and how it can be controlled
- Glycoprotein receptors are damaged or become less responsive to insulin
- Strong positive correlation with poor diet/obesity
- Treat by controlling diet and exercise regime
Name some signs and symptoms of diabetes
- High blood glucose concentration
- Glucose in urine
- Sudden weight loss
- Blurred vision
- Excessive urination
Suggest how a student could produce a desired concentration of glucose solution from a stock solution
- Volume of stock solution: required concentration x final volume needed/ concentration of stock solution
- Volume of distilled water = final volume needed - volume of stock solution
Outline how colorimetry could be used to identify the glucose concentration in a sample
- Benedict’s test on solutions of known glucose concentration. Use colorimeter to record absorbance
- Plot calibration curve: absorbance (y axis), glucose concentration (x axis)
- Benedict’s test on unknown sample. Use calibration curve to read glucose concentration at its absorbance value
Define osmoregulation
Control of blood water potential via homeostatic mechanisms
Describe the gross structure of a mammalian kidney
- Fibrous capsule: protects kidney
- Cortex: outer region consists of Bowman’s capsules, convoluted tubules, blood vessels
- Medulla: inner region consists of collecting ducts, loops of Henle, blood vessels
- Renal pelvis: cavity collects urine into ureter
- Ureter: tube carries urine to bladder
- Renal artery: supplies kidneys with oxygenated blood
- Renal vein: returns deoxygenated blood from kidney to heart
Describe the structure of a nephron
- Bowman’s capsule at the start of a nephron: cup-shaped, surrounds glomerulus, inner layer of podocytes
- Proximal convoluted tubule (PCT): series of loops surrounded by capillaries, walls made of epithelial cells with microvilli
- Loop of Henle: hairpin loop extends from cortex into medulla
- Distal convoluted tubule (DCT): similar to PCT but fewer capillaries
- Collecting duct: DCT from several nephrons empty into collecting duct, which leads into pelvis of kidney
Describe the blood vessels associated with a nephron
- Wide afferent arteriole from renal artery enters renal capsule and forms glomerulus: branched knot of capillaries which combine to form narrow efferent arteriole
- Efferent arteriole branches to form capillary network that surrounds tubules
Explain how glomerular filtrate is formed
- Ultrafiltration in Bowman’s capsule
- High hydrostatic pressure in glomerulus forces small molecules out of capillary fenestrations against osmotic gradient
- Basement membrane acts as a filter.
- Blood cells and large molecules e.g. proteins remain in capillary
How are cells of the Bowman’s capsule adapted for ultrafiltration?
- Fenestrations between epithelial cells of capillaries
- Fluid can pass between and under folded membrane of podocytes
State what happens during selective reabsorption and where it occurs
- Useful molecules from glomerular filtrate eg glucose are reabsorbed into the blood
- Occurs in PCT
Outline the transport processes involved in selective reabsorption
How are cells in the PCT adapted for selective reabsorption?
- Microvilli: large SA for co-transporter proteins
- Many mitochondria: ATP for active transport of glucose into intercellular spaces
- Folded basal membrane: large SA
What happens in the loop of Henle?
- Active transport of Na+ and Cl- out of ascending limb
- Water potential of interstitial fluid decreases
- Osmosis of water out of descending limb
- Water potential of filtrate decreases going down descending limb: lowest in medullary region, highest at the top of ascending limb
Explain the role of DCT
- Reabsorption of water via osmosis
- Reabsorption of ions via active transport
Permeability of walls is determined by action of hormones
Explain the role of the collecting duct
Reabsorption of water from filtrate into interstitial fluid via osmosis through aquaporins
Explain why it is important to maintain a Na+ gradient
- Countercurrent multiplier: filtrate in collecting ducts is always beside an area of interstitial fluid that has a lower WP
- Maintains WP gradient for maximum reabsorption of water
What might cause blood WP to change?
- Level of water intake
- Level of ion intake in diet
- Level of ions used in metabolic processes or excreted
- Sweating
Explain the role of the hypothalamus in osmoregulation
- Osmosis of water out of osmoreceptors in hypothalamus causes them to shrink
- This triggers hypothalamus to produce more antidiuretic hormone
Explain the role of the posterior pituitary gland in osmoegulation
Store and secretes the ADH produced by the hypothalamus
Explain the role of ADH in osmoregulation
- Makes cells lining collecting duct more permeable to water:
- Binds to receptor –> activates phsophorylase –> vesicles with aquaporins on membrane fuse with cell-surface membrane - Makes cell lining collecting duct more permeable to urea:
- WP in interstitial fluid decreases
- More water reabsorbed = more concentrated urine
