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?
Maintains stable rate of enzyme-controlled reactions & prevent damage to membranes
Temperature too low= enzyme & substrate molecules have insufficient kinetic energy.
Temperature too high= enzymes denature
Why is it important that blood pH remains stable?
Maintains stable rate of enzyme- controlled reactions (& optimum conditions for other proteins)
Acidic pH= H+ ions interact with H-bonds & 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/ crenation of cells
- Maintain constant concentration of respiratory substrate: organism maintains constant level of activity regardless of environmental conditions
Define negative and positive feedback
Negative feedback: self-regulatory mechanisms return internal environment to optimum when there is a fluctuation
Positive feedback: a fluctuation triggers changes that result in an even greater deviation from the normal level
Outline 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 an effector?
It takes time to:
- produce hormone
- transport hormone in the blood
- cause required change to the target protein
Name factors that affect blood glucose concentration
- amount of carbohydrate digested from diet
- rate of glycogenolysis
- rate of gluconeogensis
Define glycogenesis
liver converts glucose into the storage polymer glycogen
Define glycogenolysis
liver hydrolyses glycogen into glucose which can diffuse into blood
define gluconeogenesis
liver converts glycerol & amino acids into glucose
Outline role of glucagon when blood glucose concentration decreases
- alpha cells in Islets of Langerhans in pancreas detect decrease & secrete glucagon into bloodstream
- Glucagon binds to surface receptors on liver cells & activates enzymes for glycogenolysis & gluconeogensis
- glucose diffuses from liver into bloodstream
Outline role of adrenaline when blood glucose concentration decreases
- Adrenal glands produce adrenaline. It binds to surface receptors on liver cells & activates 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 & secrete insulin into bloodstream
- Insulin binds to surface receptors on target cells to:
a) increase cellular glucose uptake
b) activate enzymes for glycogenesis (liver & muscles)
c) stimulate adipose tissue to synthesise fat
Describe how insulin leads to a decrease on blood glucose concentration
- Increases permeability of cells to glucose
- Increases 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 glucose concentration gradient?
- Activates enzymes for glycogenesis in liver & 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 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 of Islets of Langerhans
Treat by injecting insulin
Explain causes of Type 2 diabetes and how it can be controlled
Glycoprotein receptors are damages 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
- Polyuria
- Polyphagia
- Polydipsia
- Blurred vision
- Sudden weight loss
Suggest how a student could produce a desired concentration of glucose solution from a stock solution
Volume of stock solution= required conc x final volume needed/ conc of stock solution
Volume of distilled water= final volume needed - volume of stock solution
Outline how colorimetry could be used to identify 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 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, loop of Henle and 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 structure of a nephron
Bowman’s capsule at start of 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: harpin 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 blood vessels associated with a nephron
Wide afferent arteriole from renal artery enters renal capsule & 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 (urea, water, glucose, ions) out of capillary fenestration AGAINST osmotic gradient
Basement membrane acts as filter. Blood cells & large molecules e.g. proteins remain in capillary
How are cells of Bowman’s capsule adapted for ultrafiltration?
- Fenestrations between epithelial cells of capillaries
- Fluid can pass between & under folded membrane of podocytes
State what happens during selective reabsorption and where it occurs
Useful molecules from glomerular filtrate e.g. glucose are reabsorbed into blood
Occurs in proximal convoluted tubule
How are cells in proximal convoluted tubule adapted for selective reabsorption?
Microvilli- large surface area for co-transporter proteins
Many mitochondria- ATP for active transport of glucose into intercellular spaces
Folded basal membrane- large surface area
What happens in loop of Henle?
- Active transport of Na+ and Cl- out of ascending limb.
- Water potential of interstital fluid decreases
- Osmosis of water out of descending limb (ascending limb is impermeable to water)
- Water potential of filtrate decreases going down descending limb: lowest in medullary region, highest at top of ascending limb
Explain role of distal convoluted tubule
Reabsorption:
a) of water via osmosis
b) of ions via active transport
permeability of walls is determined by action of hormones
Explain role of collecting duct
Reabsorption of water from filtrate into interstitial fluid via osmosis through aquaporins
Explain why it is important to maintain an Na+ gradient
Countercurrent multiplier: filtrate in collecting ducts is always beside an area of interstitial fluid that has a lower water potential
Maintains water potential gradient for maximum reabsorption of water
What might cause blood water potential to change?
- level of water intake
- level of ion intake in diet
- level of ion used in metabolic processes or excreted
- sweating
Explain the role of the hypothalamus in osmoregulation
- Osmosis out of osmoreceptors in hypothalamus causes them to shrink
- This triggers hypothalamus to produce more antidiuretic hormone (ADH)
Explain the role of the posterior pituitary gland in osmoregulation
Stores and secretes the ADH produced by hypothalamus
Explain role of ADH in osmoregulation
- Makes cells lining collecting duct more permeable to water:
- Binds to receptor -> activates phosphorylase -> vesicles with aquaporins on membrane fuse with cell-surface membrane - Makes cells lining collecting duct more permeable to urea:
- water potential in interstitial fluid decreases
- more water reabsorbed= more concentrated urine
