Everything from the 5 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
The endocrine system is made of ductless glands that secrete hormones into the blood; only target cells with the matching receptor respond. Examples: ADH from the posterior pituitary acts on collecting duct cells; insulin (beta cells) and glucagon (alpha cells) from the islets of Langerhans in the pancreas act on liver and other cells. The nervous system sends electrical impulses along neurones to specific effectors: it is faster, more precise and shorter-lived. A sensory neurone carries impulses from a receptor to the CNS; an intermediate (relay) neurone connects it to a motor neurone, which carries impulses to an effector. Sensory receptor cells are transducers: a stimulus changes the permeability of their membrane, producing a receptor potential; if this is big enough, an action potential starts in the sensory neurone. In a taste bud, Na+ from salt enters a chemoreceptor cell, depolarising it, so Ca2+ channels open and neurotransmitter is released onto a sensory neurone.
Do not say hormones travel only to their target organ. Hormones are carried in the blood to every tissue; only target cells respond, because only they have receptors with a shape complementary to the hormone.
A receptor potential is graded: its size depends on the stimulus strength. An action potential is all-or-nothing. Only when the receptor potential pushes the sensory neurone past threshold does an action potential start.
At rest the inside of an axon is about -70 mV relative to the outside. The Na+/K+ pump uses ATP to move 3 Na+ out for every 2 K+ in, and the membrane is more permeable to K+, which leaks out; the membrane is relatively impermeable to Na+, so the inside stays negative. A stimulus that reaches threshold opens voltage-gated Na+ channels: Na+ floods in (depolarisation, to about +40 mV). Then Na+ channels close and voltage-gated K+ channels open: K+ diffuses out (repolarisation), overshooting slightly (hyperpolarisation). During the refractory period the Na+ channels cannot reopen while the resting potential is restored. Local circuits depolarise the next region of membrane. In a myelinated neurone this happens only at the nodes of Ranvier, so the impulse jumps (saltatory conduction), much faster. Because action potentials are all the same size, a stronger stimulus is coded as a higher frequency, and the refractory period sets the maximum frequency.
During repolarisation K+ moves out by diffusion through channels, not by the pump. The Na+/K+ pump keeps the concentration gradients in place, but the fast changes in an action potential are all caused by ions diffusing through voltage-gated channels.
A stronger stimulus does not make action potentials bigger or faster. It increases their frequency (and may stimulate more neurones). Refer to the refractory period when explaining the upper limit.
A cholinergic synapse uses acetylcholine (ACh) as its neurotransmitter. The presynaptic knob contains many mitochondria and vesicles of ACh; it is separated from the postsynaptic membrane by a synaptic cleft about 20 nm wide. When an action potential arrives, voltage-gated Ca2+ channels open and Ca2+ diffuses into the knob. This makes vesicles move to and fuse with the presynaptic membrane, releasing ACh by exocytosis. ACh diffuses across the cleft and binds to receptor proteins on the postsynaptic membrane, opening Na+ channels; Na+ diffuses in and depolarises the membrane, and if threshold is reached an action potential is set up. Acetylcholinesterase in the cleft hydrolyses ACh to choline and ethanoic acid, which are taken back into the knob and recombined using ATP. Transmission is one-way because only the presynaptic side has vesicles and only the postsynaptic side has receptors.
Calcium ions do not cross the cleft and do not bind to the postsynaptic receptors. Ca2+ acts inside the presynaptic knob, triggering vesicle fusion. It is ACh that diffuses across the cleft.
Choline is recycled, but it is reabsorbed into the presynaptic knob, not the postsynaptic neurone. The postsynaptic neurone does not store or release ACh at this synapse.
A striated muscle fibre is a long, multinucleate cell packed with myofibrils, each made of repeating sarcomeres from one Z line to the next. Thin actin filaments attach to the Z lines; thick myosin filaments are held at the M line. The A band is the length of the myosin (dark), the I band has actin only (light), and the H zone has myosin only. At the neuromuscular junction ACh depolarises the sarcolemma; the action potential spreads down the T-tubules and makes the sarcoplasmic reticulum release Ca2+. Ca2+ binds to troponin, which moves tropomyosin off the myosin-binding sites on actin. Myosin heads bind, forming cross-bridges, and tilt (the power stroke), pulling actin towards the M line. ATP binds to detach each head and is hydrolysed to re-cock it. The filaments do not change length: the I band and H zone shorten, the A band stays the same.
The filaments themselves do not shorten. Only the overlap changes. If a question asks which band stays the same length, the answer is the A band, because it equals the length of the myosin filaments.
ATP is needed to detach the myosin head as well as to re-cock it. The power stroke itself happens when ADP and Pi are released. This is why muscles lock in rigor mortis after death: with no ATP, the heads cannot let go of actin.
The Venus fly trap has modified leaves with two lobes, each carrying sensitive trigger hairs. Bending a hair produces a receptor potential; if two hairs are touched within about 20 seconds, action potentials spread across the lobes. These trigger rapid movement of ions and water, changing the turgor of cells in the lobes, so the lobes flip from convex to concave and the trap snaps shut in a fraction of a second. Auxin, made in shoot tips, stimulates elongation growth: it binds to a receptor and stimulates proton pumps to pump H+ into the cell wall; the acidic wall activates expansins, which loosen the links between cellulose microfibrils, so water entering by osmosis stretches the wall and the cell elongates. In a germinating barley grain, the embryo releases gibberellin, which travels to the aleurone layer and switches on the gene for amylase; amylase hydrolyses starch in the endosperm to maltose, providing sugars for the embryo's respiration and growth.
The Venus fly trap has no muscles and no nerves. Its rapid movement comes from action potentials and changes in turgor and cell wall properties, not from contraction.
Auxin does not make the cell divide here, and it is the cell wall that becomes acidic, not the cytoplasm. The key steps are protons pumped into the wall, acidic wall, loosening, then water uptake and elongation.