Flashcards · H420 · Topic 5

Communication, homeostasis and energy flashcards

35 cards on Topic 5, subtopics 5.1.1 to 5.2.2. Pick a subtopic or study them all.

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5.1.1 Communication and homeostasis

Negative vs positive feedback: Negative feedback: the response reverses the change, returning the factor towards the set point (homeostatic). Positive feedback: the response increases the change, as with oxytocin and uterine contractions during birth.
Responses of an endotherm that is too hot: Detected by the hypothalamus and skin receptors. Skin arterioles dilate (vasodilation), sweat glands secrete sweat that evaporates, hairs lie flat, no shivering, and metabolic rate is lowered.
Cell signalling between adjacent and distant cells: Adjacent cells: neurones release neurotransmitters across synapses. Distant cells: endocrine glands release hormones into the blood, which act on target cells with complementary receptors.
Ectotherm vs endotherm: Ectotherm: relies on external heat sources and mainly behavioural responses, such as basking (a lizard). Endotherm: generates heat from metabolism and uses physiological and behavioural responses to keep its core temperature fairly constant (a mammal).

5.1.2 Excretion as an example of homeostatic control

Role of the loop of Henle: The ascending limb pumps ions out but is impermeable to water, so the medulla has a very low water potential; water leaves the descending limb. This gradient lets the collecting duct reabsorb water. Longer loops allow more concentrated urine.
Glomerular filtration rate (GFR): The volume of filtrate formed per minute by all nephrons. A falling GFR shows kidney failure, with urea and creatinine rising in the blood.
How does a pregnancy test strip detect hCG?: Mobile monoclonal antibodies to hCG on coloured beads bind hCG in urine. Immobilised antibodies to hCG at the test line trap the complexes, giving a coloured line; a control line shows the strip works. Two lines: hCG present.
Roles of the liver in excretion: Excess amino acids are deaminated, giving ammonia, which the ornithine cycle converts to less toxic urea for excretion by the kidneys. The liver also detoxifies substances such as alcohol, using enzymes such as alcohol dehydrogenase and catalase.
Ultrafiltration and selective reabsorption in the nephron: Ultrafiltration: high blood pressure in the glomerulus forces small molecules (water, glucose, urea, ions) through the capillary endothelium, basement membrane and podocytes into the Bowman capsule. Selective reabsorption: in the proximal convoluted tubule, glucose, amino acids, ions and much water return to the blood.
How does ADH regulate water reabsorption?: Osmoreceptors in the hypothalamus detect a low blood water potential and the posterior pituitary releases more ADH. ADH makes the collecting duct walls more permeable to water, so more water is reabsorbed and a small volume of concentrated urine is made.

5.1.3 Neuronal communication

Generator potential vs action potential: Generator potential: graded and local; a bigger stimulus gives a bigger potential. Action potential: all-or-nothing and travels along the neurone; starts only when the generator potential reaches threshold.
Stages of an action potential: Threshold (about −55 mV): many voltage-gated Na⁺ channels open. Depolarisation: Na⁺ floods in, up to about +40 mV. Repolarisation: Na⁺ channels close, voltage-gated K⁺ channels open, K⁺ leaves. Hyperpolarisation: K⁺ channels slow to close.
Spatial vs temporal summation: Spatial: several presynaptic neurones release transmitter together to reach threshold. Temporal: repeated impulses from one presynaptic neurone add up to reach threshold.
Resting potential: value and how it is maintained: About −70 mV, inside negative to outside. The sodium–potassium pump moves 3 Na⁺ out for every 2 K⁺ in, using ATP, and the membrane is more permeable to K⁺, which diffuses back out.
Saltatory conduction in myelinated neurones: The myelin sheath made by Schwann cells insulates the axon, so depolarisation happens only at the nodes of Ranvier. The action potential jumps from node to node, so it travels much faster than along a non-myelinated axon.

5.1.4 Hormonal communication

How does a beta cell secrete insulin when blood glucose rises?: Glucose enters by facilitated diffusion; respiration makes more ATP; ATP closes K⁺ channels; the membrane depolarises; voltage-gated Ca²⁺ channels open and Ca²⁺ enters; insulin vesicles fuse with the membrane (exocytosis).
Type 1 vs Type 2 diabetes: Type 1: autoimmune destruction of beta cells, so little or no insulin; treated with insulin. Type 2: target cells respond poorly to insulin; linked to obesity and inactivity; treated with diet, exercise and drugs such as metformin.
Endocrine vs exocrine glands: Endocrine glands secrete hormones directly into the blood, with no ducts, such as the adrenal glands. Exocrine glands secrete into ducts, such as the pancreas releasing digestive enzymes into the pancreatic duct.
Adrenal cortex vs adrenal medulla: Cortex: secretes steroid hormones such as cortisol and aldosterone. Medulla: secretes adrenaline and noradrenaline, which prepare the body for fight or flight.
Roles of insulin and glucagon: Insulin (from beta cells in the islets of Langerhans) lowers blood glucose: cells take up more glucose and the liver converts glucose to glycogen. Glucagon (from alpha cells) raises it: glycogenolysis and gluconeogenesis in the liver.

5.1.5 Plant and animal responses

Sympathetic vs parasympathetic control of heart rate: Sympathetic neurones release noradrenaline and speed up the heart (via the SAN). Parasympathetic neurones release acetylcholine and slow it down.
Effects of auxin on shoots and roots, and of gibberellin and ABA in seeds: High auxin concentration stimulates shoot elongation but inhibits root elongation. Gibberellin promotes germination; abscisic acid maintains dormancy.
Organisation of the mammalian nervous system: Central nervous system: brain and spinal cord. Peripheral nervous system: sensory and motor neurones. The motor system has a somatic (voluntary) part and an autonomic (involuntary) part, which has sympathetic and parasympathetic divisions.
Knee-jerk reflex vs blinking reflex: Both are rapid, involuntary responses. Knee-jerk: a spinal reflex that helps keep balance. Blinking: a cranial reflex, coordinated in the brain stem, that protects the cornea from damage.
Skeletal, cardiac and involuntary muscle: Skeletal: striated, voluntary, contracts quickly and tires quickly. Cardiac: striated, branched cells, myogenic and involuntary. Involuntary (smooth): not striated, spindle-shaped cells, in the walls of the gut and blood vessels.
Sliding filament model in outline: Ca²⁺ binds troponin, moving tropomyosin off the binding sites on actin. Myosin heads bind actin, forming cross-bridges, and tilt (the power stroke), pulling actin towards the centre of the sarcomere. ATP detaches each head and its hydrolysis re-cocks it.

5.2.1 Photosynthesis

Photosynthetic pigments: which, where and how they are separated: Chlorophyll a, chlorophyll b and carotenoids, in the thylakoid membranes, grouped in photosystems. Each absorbs different wavelengths. They are separated by thin layer chromatography and identified by Rf values.
Light-dependent stage: what happens and what it makes: In the thylakoids: photolysis of water gives H⁺, electrons and oxygen; excited electrons pass along the electron transport chain; ATP is made by photophosphorylation (chemiosmosis) and NADP is reduced. Cyclic photophosphorylation makes ATP only.
Calvin cycle in three steps: In the stroma: CO₂ combines with RuBP, catalysed by RuBisCO, giving two GP; GP is reduced to triose phosphate (TP) using ATP and reduced NADP; most TP regenerates RuBP using ATP, and the rest is used to make glucose, amino acids and lipids.
Limiting factors of photosynthesis, and the effect of low light on the Calvin cycle: Light intensity, carbon dioxide concentration and temperature; the factor in shortest supply limits the rate. In low light less ATP and reduced NADP are made, so GP is not reduced: GP builds up while TP and RuBP fall.

5.2.2 Respiration

Glycolysis: gross and net ATP: Glucose is phosphorylated, split into two triose phosphate and oxidised to two pyruvate. 4 ATP are made and 2 used, so the net gain is 2 ATP, with 2 reduced NAD.
Chemiosmosis in mitochondria vs chloroplasts: Mitochondria: H⁺ pumped into the intermembrane space; electrons from reduced NAD and FAD; final acceptor oxygen. Chloroplasts: H⁺ pumped into the thylakoid space; electrons from light-excited chlorophyll; final acceptor NADP (non-cyclic).
Link reaction and Krebs cycle: products per glucose: Link reaction (matrix): 2 pyruvate → 2 acetyl CoA, 2 CO₂ and 2 reduced NAD. Krebs cycle (two turns): 4 CO₂, 6 reduced NAD, 2 reduced FAD and 2 ATP by substrate-level phosphorylation.
Anaerobic respiration in mammals vs yeast: Mammals: pyruvate is reduced to lactate by lactate dehydrogenase. Yeast: pyruvate is decarboxylated to ethanal, which is reduced to ethanol. Both regenerate NAD so glycolysis continues, giving 2 ATP per glucose.
Respiratory quotient (RQ): RQ = CO₂ produced ÷ O₂ taken in. Typical values: carbohydrate 1.0, protein about 0.9, lipid about 0.7. Measured with a respirometer.