Topic 9: Control Systems — Edexcel A-Level Biology B

Homeostasis keeps the internal environment within narrow limits around a set point, so conditions fluctuate slightly rather than staying fixed: this is dynamic equilibrium. Maintaining pH protects enzyme tertiary structure and metabolic rates; maintaining temperature keeps enzyme activity and membrane fluidity stable; maintaining water potential prevents cells shrinking or bursting and keeps tissue fluid balance. Negative feedback detects a deviation and reverses it, returning the value towards the set point, as in thermoregulation. Positive feedback amplifies a change away from the set point and is usually short-lived, as in the oxytocin-driven contractions of labour.

Endocrine glands are ductless tissues that secrete hormones into the blood, so a hormone travels in plasma to reach cells anywhere in the body. A target cell responds only if it carries the correct receptor, which gives specificity: the same blood-borne signal affects some tissues and not others. Two modes of action are required. In the first, adrenaline binds a receptor in the plasma membrane; the receptor triggers a second messenger inside the cell, and that messenger activates specific enzymes, amplifying the original signal. In the second, oestrogen is lipid-soluble, so it diffuses through the membrane, enters the cell and binds directly to transcription factors, altering gene transcription. Compare the two by asking where the receptor sits, whether a second messenger is used, and whether enzymes or transcription are affected.

Plants coordinate growth and development chemically rather than by a nervous system. Chemical control is brought about by plant growth substances, which are signalling molecules produced in one region and acting on cells elsewhere. Three named examples are required. Auxins, such as indoleacetic acid, are made in shoot and root tips and influence cell elongation, so they shape responses such as phototropism and gravitropism. Cytokinins promote cell division and delay leaf senescence, and they interact with auxins in controlling shoot and root branching. Gibberellins promote stem elongation, seed germination and fruit development. Learn each substance by its main site of production and its principal growth effect, then use that pattern to interpret an unfamiliar growth response.

The mammalian nervous system is organised into the central nervous system (brain and spinal cord) and the peripheral nervous system. The spinal cord has a central grey matter region surrounded by white matter, with a central canal. Brain regions have distinct locations and roles: the medulla oblongata at the brain stem controls breathing and heart rate; the cerebellum at the back of the brain controls balance and coordination; the cerebrum, the largest upper part, initiates movement; and the hypothalamus, located just above the pituitary gland, regulates temperature and osmoregulation. The peripheral nervous system divides into voluntary and autonomic systems. The autonomic system divides into sympathetic and parasympathetic divisions, acting antagonistically to maintain balanced control of internal organs.

The axon membrane maintains a resting potential of about −70 mV because Na⁺/K⁺ ATPase pumps three Na⁺ out for every two K⁺ in, while leak channels let K⁺ diffuse out and the membrane is less permeable to Na⁺. An action potential begins when a stimulus opens voltage-gated Na⁺ channels: Na⁺ enters, depolarising the membrane to about +40 mV. Na⁺ channels then inactivate and K⁺ channels open, so K⁺ leaves and the membrane repolarises and briefly hyperpolarises. This wave propagates along the axon. Myelinated axons transmit faster because myelin insulates the membrane and saltatory conduction jumps between nodes of Ranvier. At a synapse, an action potential opens Ca²⁺ channels, vesicles release acetylcholine or noradrenaline, and these bind receptors on the postsynaptic membrane. Excitatory postsynaptic potentials depolarise; inhibitory postsynaptic potentials hyperpolarise.

Drugs can alter nervous function by acting at synapses. Nicotine binds to acetylcholine receptors on the postsynaptic membrane and mimics acetylcholine, so it can trigger depolarisation and action potentials even without acetylcholine release. Lidocaine blocks voltage-gated Na⁺ ion channels in the axon membrane, preventing Na⁺ entry and stopping action potential formation, which blocks pain transmission. Cobra venom blocks acetylcholine receptors on the postsynaptic membrane, so acetylcholine cannot bind and the postsynaptic cell cannot be depolarised, causing paralysis. These examples show that drug effects depend on which part of synaptic transmission is targeted: receptor activation, ion channel blocking or receptor blocking.

The human retina is a layered sheet of nervous tissue at the back of the eye. Light passes through the vitreous humour and neural layers to reach the photoreceptors: rods and cones. Rods contain the pigment rhodopsin. When light is absorbed, the retinal component of rhodopsin changes shape, triggering a cascade that hyperpolarises the rod and reduces its release of inhibitory neurotransmitter. This disinhibition depolarises bipolar cells, which synapse with ganglion cells whose axons form the optic nerve and generate action potentials. Rods are numerous in the peripheral retina and function in dim light; cones are concentrated at the fovea and provide colour vision in bright light. This distribution allows vision across a wide range of light intensities.

Heart rate is controlled by the autonomic nervous system via the cardiac centre in the medulla oblongata. Baroreceptors in the aortic arch and carotid sinuses detect changes in blood pressure; chemoreceptors in the same regions detect changes in blood pH, carbon dioxide and oxygen. Sensory neurones carry this information to the cardiac centre, which coordinates the response. Sympathetic nerves release noradrenaline at the sinoatrial node (SAN), increasing heart rate. Parasympathetic nerves release acetylcholine at the SAN, decreasing heart rate. Additionally, the autonomic nervous system stimulates the adrenal medulla to release adrenaline into the blood, which also increases heart rate.

Gross kidney anatomy comprises the cortex, medulla, pelvis, renal vessels, and ureter. Microscopic nephrons include Bowman's capsule, glomerulus, proximal tubule, Loop of Henle, distal tubule, and collecting duct. Urea from liver deamination is removed by ultrafiltration in Bowman's capsule. The proximal tubule selectively reabsorbs glucose and ions. The Loop of Henle acts as a counter-current multiplier, concentrating the medulla to draw water. Hypothalamic osmoreceptors regulate pituitary ADH release to control collecting duct aquaporins. Kangaroo rats have elongated Loops of Henle to conserve water. Endotherms produce metabolic heat; ectotherms rely on surroundings. Endotherms regulate temperature behaviourally (basking, shade) and physiologically via thermoreceptors and hypothalamus controlling autonomic skin effectors (vasodilation, sweating).

Plant growth substances coordinate development. In Core Practical 14, germinating cereal grains release gibberellin, triggering amylase production. Amylase diffuses into starch agar, digesting it; iodine staining reveals a clear zone whose radius indicates amylase activity. Auxin promotes cell elongation and root growth, but suppresses lateral buds (apical dominance). Cytokinin and auxin interact antagonistically: applying cytokinin to a lateral bud overcomes auxin's suppression. Phytochrome controls flowering and photomorphogenesis via two interconvertible forms: Pr absorbs red light (converting to Pfr), and Pfr absorbs far-red light (converting to Pr). Pfr is the biologically active form; it enters the nucleus and binds to transcription factors, regulating the expression of light-responsive genes to trigger processes like flowering.