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14.114-15 · Homeostasis and Control

Homeostasis in Mammals — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 11 parts Lesson 1 of 31 6 quizzes

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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14.1 Homeostasis in Mammals — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain negative feedback, urea formation, kidney and nephron function, osmoregulation, blood glucose control and glucose testing.Homeostasis — regulation of the internal conditions of a cel…

  • Factors controlled
  • Opposite mechanisms
  • Nervous system
  • Endocrine system
  • Thermoregulation
  • Deamination

Part 1 of 9Homeostasis and negative feedback

Homeostasis and negative feedback
Homeostasis
  • Homeostasis regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.
Factors controlled
  • Factors controlled core body temperature, metabolic waste, blood pH, blood glucose concentration, blood water potential and respiratory gases.
Negative feedback
  • Negative feedback a receptor detects a stimulus, a coordination system passes the information and an effector reverses the change.
Opposite mechanisms
  • Opposite mechanisms separate responses for a rise and a fall give more precise control.
Nervous system
  • Nervous system fast, short-lived responses by impulses along neurones.
Endocrine system
  • Endocrine system slower, longer-lasting responses by hormones in the blood.
Thermoregulation
  • Thermoregulation if too hot: vasodilation, sweating, lower metabolic rate;
    • if too cold: vasoconstriction, shivering, more thyroxine and adrenaline.

Part 2 of 9Urea production

Excretion
  • Excretion removal of metabolic waste; CO2 and urea are made in the greatest quantities.
Source
  • Source urea is made in the liver from excess amino acids, which cannot be stored.
Deamination
  • Deamination removal of the amino group, which forms ammonia;
    • the keto acid is respired, converted to glucose, or stored as glycogen or fat.
Ammonia
  • Ammonia very soluble and toxic, disrupting blood pH, respiration and cell signalling.
Urea formation
  • Urea formation ammonia combines with CO2; urea is less soluble and less toxic.
2NH3 + CO2 → CO(NH2)2 + H2O.

Part 3 of 9Kidney structure

1
Functions
  • Functions osmoregulation of blood water content and excretion of urea and excess salts.
2
Blood supply
  • Blood supply renal artery brings blood with urea and salts; renal vein takes blood away.
3
Urine pathway
  • Urine pathway kidney, ureter, bladder (stores urine), urethra (releases urine).
4
Fibrous capsule
  • Fibrous capsule tough outer layer.
5
Cortex
  • Cortex contains glomeruli, Bowman's capsules and convoluted tubules.
6
Medulla
  • Medulla contains loops of Henle and collecting ducts.
7
Renal pelvis
  • Renal pelvis where the ureter joins the kidney.
8
Nephron
  • Nephron each kidney has thousands of nephrons, the functional units that form urine.

Part 4 of 9Ultrafiltration

1
Arterioles from the renal artery form the glomerulus inside Bowman's capsule.
2
The afferent arteriole is wider than the efferent, so pressure in the glomerular capillaries is high.
3
Small molecules are forced out, forming glomerular filtrate.
4
Filtrate crosses the capillary endothelium (with holes), the basement membrane and the podocytes of the capsule epithelium.
Filtrate
  • Filtrate amino acids, water, glucose, urea and inorganic ions.
Retained
  • Retained blood cells, platelets and large proteins stay in the blood.
Basement membrane
  • Basement membrane a collagen and glycoprotein mesh that acts as the filter.
Water potential
  • Water potential the pressure gradient outweighs the solute gradient, so water moves into the capsule.

Part 5 of 9Selective reabsorption in the proximal convoluted tubule

1Adaptations
  • Adaptations microvilli, many co-transporter proteins, many mitochondria and tightly packed cells.
2Sodium-potassium pumps
  • Sodium-potassium pumps in the basal membrane move Na+ out into the blood.
3Co-transport
  • Na+ enters through co-transporter proteins carrying glucose or amino acids.
4Diffusion
  • Diffusion solutes then diffuse into the blood through the basal membrane.
5All glucose
  • All glucose is reabsorbed, so none should be in urine.
6Water
  • Water follows by osmosis as solutes enter the blood.
7Urea
  • Urea a significant amount diffuses back into the blood.
8Loop of Henle and collecting duct
  • Loop of Henle and collecting duct reabsorb salts by diffusion, then water by osmosis.

Part 6 of 9Osmoregulation and ADH

1
Osmoreceptors in the hypothalamus detect a fall in blood water potential.
2
Impulses stimulate the posterior pituitary to release ADH.
3
ADH travels in the blood to the kidneys.
4
ADH binds to receptors on collecting duct cells, activating a signalling cascade that phosphorylates aquaporins.
5
Vesicles containing aquaporins fuse with the luminal membrane, making it more permeable to water.
6
Water leaves the collecting duct by osmosis into the medulla.
7
A small volume of concentrated urine is produced.
  • High water potential
    • no ADH is released, aquaporins leave the membrane and a large volume of dilute urine is produced.

Part 7 of 9Control of blood glucose

1Hormones
  • Hormones insulin and glucagon from the islets of Langerhans in the pancreas.
2Fall in glucose
  • α cells secrete glucagon, which binds liver receptors and activates a G protein and adenylyl cyclase.
3Second messenger
  • Second messenger adenylyl cyclase makes cAMP from ATP, which activates protein kinase A and a cascade ending in glycogen phosphorylase.
4Glycogenolysis
  • Glycogenolysis glycogen is broken down to glucose, and the cascade amplifies the signal.
5Rise in glucose
  • Rise in glucose glucose enters β cells by facilitated diffusion and is respired, making ATP.
6
ATP closes K+ channels, the membrane depolarises, Ca2+ channels open and insulin is secreted by exocytosis.
7Insulin
  • Insulin binds receptors on muscle, fat and liver cells, adds GLUT proteins to the membrane and activates glucokinase and glycogen synthase.
8Liver cells
  • Liver cells are the only effectors of glucagon; muscle cells have no glucagon receptors.

Part 8 of 9Test strips and biosensors

Renal threshold
  • Renal threshold above this blood glucose level, glucose appears in urine, which suggests diabetes.
Test strip
  • Test strip immobilised glucose oxidase and peroxidase on a pad.
  • Glucose oxidase makes gluconic acid and hydrogen peroxide.
  • Peroxidase uses hydrogen peroxide to form a brown compound.
  • Colour is compared with a chart; darker means more glucose.
  • It shows only whether the threshold was exceeded, not the current blood level.
Biosensor
  • Biosensor glucose oxidase immobilised on a recognition layer under a partially permeable membrane.
  • Hydrogen peroxide is oxidised at an electrode.
  • Current is proportional to blood glucose and is amplified for a digital reading in seconds.
Specificity
  • Specificity strips give a positive result for glucose only, not fructose, sucrose or lactose.

Part 9 of 9More from the specification

More from the specification
Selective Reabsorption in the Proximal Convoluted Tubule
  • The process of selective reabsorption occurs predominantly along the proximal convoluted tubule,
  • where all glucose, amino acids,
  • and vital mineral ions are actively transported back into the blood plasma of surrounding peritubular capillaries.
Osmotic Gradient and Water Reabsorption
  • Active transport of solutes from the filtrate into interstitial fluid decreases local water potential,
  • which causes water to be reabsorbed passively by osmosis through aquaporin channels.
Homeostatic Osmoregulation and ADH
  • When blood plasma solute concentration increases, osmoreceptors in the hypothalamus trigger the release of antidiuretic hormone (ADH) from the posterior pituitary,
  • increasing collecting duct permeability and reducing urine volume.
Ultrafiltration Across the Renal Corpuscle
  • High hydrostatic blood pressure in the glomerular capillaries forces small molecules and water through the fenestrated endothelium and basement membrane into the nephron lumen to form glomerular filtrate.
Reabsorption
  • Return of useful substances from the filtrate to the blood.
Plasma
  • The liquid part of blood.
Cause
  • A factor that directly produces an effect through a known mechanism.
Selective reabsorption
  • Return of useful substances from the filtrate to the blood.
Target cells
  • Cells that respond to a particular hormone.

Key terms

TermMeaning
HomeostasisRegulation of internal conditions to maintain optimum function.
DeaminationRemoval of the amino group from an amino acid.
UltrafiltrationFiltration of blood under pressure in the glomerulus.
Selective reabsorptionReturn of useful substances from filtrate to blood.
OsmoregulationControl of the water potential of body fluids.
ADHHormone increasing collecting duct permeability to water.
GlycogenolysisBreakdown of glycogen to glucose.
BiosensorDevice using an immobilised enzyme to measure a substance electrically.

Avoid these mistakes

1
Saying negative feedback is harmful — it reverses a change.
2
Saying homeostasis keeps conditions constant — it keeps them within limits around a set point.
3
Saying muscle cells respond to glucagon — only liver cells have glucagon receptors.
4
Saying urine strips show current blood glucose — they only show it exceeded the renal threshold.

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