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A-level biology > Chapter 14: Homeostasis > Flashcards

Chapter 14: Homeostasis Flashcards

1
Q

What is homeostasis?

A

Maintenance of a relatively constant internal environment for the cells within the body.

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2
Q

What are some physiological factors controlled by homeostasis?

A
  1. Core body temperature
  2. Metabolic wastes (CO2 and urea)
  3. Blood pH
  4. Blood glucose concentration
  5. Blood water potential
  6. Concentration of respiratory gases e.g O2 and CO2 in blood
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3
Q

What is an internal environment?

A

All the conditions inside the body in which cells function e.g tissue fluid in cells is its immediate environment

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4
Q

Temperature and how it influences cell activities

A

Low temp- slow metabolic reactions
High temp- denaturation of enzymes

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5
Q

Water potential and how it influences cell activities

A

Low wp- water exits cells by osmosis, slower metabolic reactions
High wp- cells may burst

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6
Q

Concentration of glucose i blood and how it influences cell activities

A

Lack of glucose- slows down/stops respiration
Too much glucose- water exits cells by osmosis, slower metabolic reactions

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7
Q

pH and how it influences cell activities

A

Normal cytoplasm pH- 6.5 to 7.0
Outside this range- less efficient and denaturation of enzymes

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8
Q

Negative feedback

A

The mechanism of homeostasis whereby a change in some parameter brings about processes which return it towards normal.

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9
Q

How does negative feedback mechanism work?

A
  1. Factor rises/falls below set point.
  2. Receptor detects change in factor (stimulus).
  3. Effector receives information from receptor.
  4. Effectors act to increase/decrease factor (corrective action).
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10
Q

What is positive feedback?

A

A process in which a change in some parameter such as a physiological factor, brings about processes that move it further in the direction of the change.

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11
Q

Examples of positive feedback: childbirth

A

During childbirth, the body uses positive feedback to increase the strength and rate of uterine contractions.
When the baby pushes on the cervix, the body releases hormones that cause the uterus to contract.
The contractions cause pain, which stimulates the release of more oxytocin, a hormone that increases contractions.
This cycle continues until the baby is born and the pressure on the uterus decreases.

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12
Q

Examples of positive feedback: blood clotting

A

When a blood vessel is injured, the body uses positive feedback to stop the flow of blood:
Damaged tissue releases chemical signals that attract platelets to the injury site.
The platelets release more chemical signals, which attract more platelets.
This cycle continues until a clot forms that stops the bleeding.

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13
Q

Examples of positive feedback: lactation

A

When an infant suckles, it produces prolactin, which leads to milk production.
The more the infant suckles, the more prolactin is produced, which leads to more milk production.

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14
Q

Examples of positive feedback: CO2 in air

A

If a person breathes in air with a high concentration of CO2, concentration of CO2 in blood increases.
CO2 receptors increase rate of breathing.
Person breathes in faster, taking in more CO2.

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15
Q

What is excretion?

A

Removal of toxic/waste products of metabolism from the body.

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16
Q

Main excretory products: CO2

A

Carbon dioxide
is produced continuously by cells that are respiring
aerobically. The waste carbon dioxide is transported
from the respiring cells to the lungs, in the bloodstream.
Gas exchange occurs
within the lungs, and carbon dioxide diffuses from the blood into the alveoli; it is then excreted in the air we
breathe out .

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17
Q

Main excretory products: urea

A

Urea is produced in the liver.
Produced from excess
amino acids and is transported from the liver to the kidneys, in solution in blood plasma.
Kidneys remove urea from the blood and excrete it dissolved in water; the solution is called urine.

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18
Q

What is deamination?

A

The breakdown of excess amino acids in the liver, by the removal of the amine
group.
Ammonia and, eventually, urea are formed
from the amine group

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19
Q

Describe the process of deamination.

A
  1. In the liver cells, the amine group (–NH2) of an amino acid is removed, together with an extra hydrogen atom.
  2. These combine to produce ammonia (NH3).
  3. The keto acid that remains may enter the Krebs cycle and be respired or it may be converted to glucose or converted to glycogen or
    fat for storage.
  4. Several reactions, known as the urea cycle, are involved in combining ammonia and carbon dioxide to form
    urea.
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20
Q

Why is ammonia immediately converted to urea?

A

Ammonia is a very soluble and highly toxic compound.
Aquatic animals (e.g. fish that live in fresh
water): ammonia diffuses from the blood and dissolves in the water around the animal. Terrestrial animals e.g humans: ammonia increases the pH
in cytoplasm and it interferes with metabolic processes such as respiration and with cell signaling in the
brain.
Damage is prevented by immediately converting
ammonia to urea, which is less soluble and less toxic.

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21
Q

How much urea does the average adult human produce daily?

A

25-30g

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22
Q

What other nitrogenous wastes are produced, apart from urea?

A

Creatinine and uric acid

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23
Q

Creatinine

A

Made in the liver, from certain amino acids.
Much of this creatine is used in the muscles, in the form of creatine phosphate, where it acts as an energy store. However, some is converted to creatinine and excreted.

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24
Q

Uric acid

A

Made from the breakdown of purines from nucleotides, not from amino acids.

25
Structure of kidney: arteries and veins
Each kidney receives blood from a renal artery, and returns blood via a renal vein.
26
Structure of kidney: ureter
Narrow tube carrying urine from the kidney to the bladder
27
Structure of kidney: urethra
a single tube carrying urine from the bladder to the outside of the body.
28
Structure of kidney: capsule
Fairly tough and fibrous layer making up the outermost layer of the kidney.
29
Structure of kidney: cortex
Layer below capsule, contains glomerulus, Bowman's capsule, proximal convoluted tubule and distal convoluted tubule.
30
Structure of kidney: medulla
Contains loops of Henle and collecting duct.
31
Nephron
The structural and functional unit of the kidney composed of Bowman’s capsule and a tubule divided into three regions: proximal convoluted tubule, loop of Henle and distal convoluted tubule.
32
Bowman’s capsule
The cup-shaped part of a nephron that surrounds a glomerulus and collects filtrate from the blood
33
Glomerulus
A group of capillaries within the ‘cup’ of a Bowman’s capsule in the cortex of the kidney.
34
Proximal convoluted tubule
Part of the nephron that leads from Bowman’s capsule to the loop of Henle
35
Loop of Henle
The part of the nephron between the proximal and distal convoluted tubules
36
Distal convoluted tubule
Part of the nephron that leads from the loop of Henle to the collecting duct
37
Collecting duct
Tube in the medulla of the kidney that carries urine from the distal convoluted tubules of many nephrons to the renal pelvis
38
Afferent vs efferent arteriole
Glomerulus is supplied with blood that flows from a branch of the renal artery through an THICKER afferent arteriole. The capillaries of the glomerulus rejoin to form a THINNER efferent arteriole. Blood flows through the efferent arteriole into a network of capillaries running closely alongside the rest of the nephron and the collecting duct. Blood from these capillaries flows into venules that empty into a branch of the renal vein.
39
Kidney makes urine in what two stages?
1. Ultrafiltration 2. Selective reabsorption
40
What is ultrafiltration?
Filtration on a molecular scale, separating small molecules from larger molecules, such as proteins (e.g. the filtration that occurs as blood flows through capillaries, especially those in glomeruli in the kidney), out of the blood and into the Bowman's capsule to form glomerular filtrate.
41
Present and absent substances in glomerular filtrate
Present: soluble molecules, water, ions, urea and uric acid, creatinine, vitamins, glucose, salts, amino acids (Bonus mnemonic to remember: Some Wild Idiots Usually Create Very Gross Smelly Acidic pee) Absent: plasma proteins, RBC, WBC and platelets (Bonus mnemonic to remember: People Rarely Wear Pink)
42
Layers separating glomerulus from Bowman's capsule, and its adaptations
1) Endothelium of blood capillaries of the glomerulus * with many more gaps / fenestrations: Allow movement of substances from blood plasma easily into Bowman’s capsule lumen 2) Basement membrane * Mesh of collagen and glycoprotein fibres * Acts as main selective barrier/ filter: Prevents RBCs, WBCs & large plasma proteins (RMM > 68 000Da) from passing through 3) Epithelial cells of Bowman’s capsule (podocytes) * Inner lining of Bowman’s capsule * Wrap around capillaries of the glomerulus * Podocytes have many finger-like projections that forms gaps/ filtration slits between foot processes: Allow movement of substances from blood plasma easily into Bowman’s capsule lumen
43
Selective Reabsorption
movement of certain substances from the filtrate in nephrons back into the blood
44
Why is selective reabsorption important?
To reabsorb essential substances from filtrate / renal fluid back into blood
45
What substances are selectively reabsorbed?
Glucose, ions, amino acids, vitamins
46
Selective reabsorption at PCT steps
1. Active transport of Na+ ions from PCT cells into the blood in capillary 2. Concentration of Na+ ions in PCT cell decreases 3. Na+ diffuses from PCT lumen into PCT cells via facilitated diffusion via co-transporter carrier proteins 4. Na+ co-transported with glucose/amino acids / vitamins / Cl ions into cell 5. These diffuse into blood via transport protein via facilitated diffusion
47
Substances reabsorbed (or not) and percentages
Glucose is → ALL actively reabsorbed into blood → No glucose in urine * Amino acids, vitamins & Cl ions → Actively reabsorbed * Water →65% reabsorbed! → Passively reabsorbed * Urea → passively reabsorbed * Uric acid & creatinine → NOT reabsorbed * Creatinine → actively secreted/transported into lumen of PCT
48
Adaptations of PCT cells
a) Numerous microvilli (facing lumen) - large surface area for absorption b) Presence of different transport proteins in membranes (facing lumen) - i.e. cotransporters, pumps, aquaporins c) High infolding of basal membranes (facing blood capillaries) d) High density of mitochondria - provide energy in the form of ATP for active transport: to MAXIMIZE reabsorption efficiency e) Tight junctions holding adjacent cells together - separate proteins of front and basal membrane - so fluid cannot pass between cells, substance must pass through cells
49
Selective reabsorption at Loop of Henle steps
KEY POINTS TO REMEMBER: ASCENDING LIMB: PERMEABLE TO IONS, IMPERMEABLE TO WATER DESCENDING LIMB: PERMEABLE TO WATER, IMPERMEABLE TO IONS 1. Na+ and Cl- move out of ascending limb by active transport into medulla space 2. High concentrations of Na+ & Cl- ions in the medulla space →Renal fluid become more dilute and enters distal convoluted tubule 3. Water leaves the descending limb via osmosis and is reabsorbed 4. Urea, Na+ & Cl- ions in medulla space diffuse into descending limb →Fluid in the descending limb becomes very concentrated as it moves down the loop
50
Selective Reabsorption DCT steps
1st part of DCT = Similar to LOH (ascending limb) * Na+ & Cl- ions again actively transported into blood 2nd part of DCT = Similar to collecting duct * Water is reabsorbed into blood * Plus secretion of K+, H+ ions and urea into lumen from blood
51
Selective Reabsorption @ Collecting Duct steps
* Located at medulla * For osmoregulation →So water reabsorption can be switched on or off * Usually, tissue fluid of medulla has high concentrations of solutes * So water moves out of collecting duct * High reabsorption of water back into blood →Formation of urine
52
Which hormone controls rate of water reabsorption?
ADH (antidiuretic hormone)
53
Mechanism of ADH when water potential in blood is low
1. Low wp detected by osmoreceptors in the hypothalamus 2. Neurosecretory cells of the hypothalamus send nerve impulse to posterior pituitary glands 3. ADH hormone secreted from posterior pituitary and released into blood stream 4. ADH binds to receptors on plasma membrane of collecting duct and distal convoluted tubule 5. Receptor undergoes conformational change 6. Activates G-protein 7. This activates adenylyl cyclase 8. Catalyses production of cAMP from ATP (secondary messenger) 9. cAMP activates signalling cascade 10. Aquaporin molecules are phosphoylated 11. Vesicles move towards luminal membrane and fuse with it 12. Aquaporins increase membrane permeability of collecting duct 13. More water reabsorbed into blood stream 14. Smaller volume more concentrated urine produced
54
ADH mechanism when water potential is high
1. Osmoreceptors no longer stimulated 2. Neurons stop secreting ADH 3. Aquaporins move out of cell surface membrane of collecting duct, back into vesicles in the cytoplasm 4. Collecting duct is less permeable to water 5. Dilute urine and larger volume of urine produced 6. Water potential of blood decreases 7. Returns to set point
55
Endocrine glands
-Secretory cells -Releases secretions directly into blood capillaries in the glands -Secretions: Hormones - E.g. pituitary glands, thyroid, adrenal, ovary, testes, pancreas
56
Exocrine glands
-Secretory cells -Releases secretion into ducts/tubes (not blood capillaries) -Secretions: Not hormones -E.g. stomach, salivary glands, pancreas
57
Hormones and their characteristics
Secreted by endocrine glands * Hormones can be globular proteins OR steroids * E.g. Insulin – protein hormone Testosterone – steroid hormone 1. Small 2. Needed in small quantities 3. Secreted quickly after stimulus 4. Short life span (quickly broken down by enzymes/excreted via urine) 5. Bind only to specific receptors 6. Transported via blood stream to target cells
58
Insulin function steps
To reduce blood glucose concentration in blood 1. Detection of stimulus 2. Beta cells in islets of Langherham in pancreas secrete insulin 3. Insulin binds to receptors on the CSM of liver, skeletal muscle and adipose cells. 4.
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A-level biology (4 decks)

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