Edexcel A Level Biology B Topic 9: Control Systems Exam Questions

About Edexcel A Level Biology B Topic 9: Control Systems

Topic 9 of Edexcel A Level Biology B covers how organisms detect and respond to change. You learn how hormones act in mammals, including how peptide and steroid hormones affect target cells, and how plant growth regulators such as auxins and gibberellins, and the pigment phytochrome, control plant responses. The mammalian nervous system section covers neurones, resting and action potentials, synaptic transmission and how light is detected by rods and cones. You then study control of heart rate, the principles of homeostasis and negative feedback, osmoregulation by the kidney and ADH, and temperature regulation in ectotherms and endotherms. Data on action potentials and ECGs often appear.

What you need to know

• Chemical control in mammals Hormones are secreted by endocrine glands into the blood. Peptide hormones bind to receptors on the cell surface and act through second messengers; steroid hormones pass through the membrane and act on gene transcription.
• Chemical control in plants Auxins cause cell elongation and phototropism; gibberellins promote stem elongation and seed germination. Phytochrome converts between Pr and Pfr in red and far-red light and controls flowering.
• Nervous transmission The resting potential is maintained by the sodium-potassium pump. An action potential involves Na+ influx (depolarisation) and K+ efflux (repolarisation). At synapses, neurotransmitters cross the cleft and bind postsynaptic receptors.
• Detection of light Rods contain rhodopsin and give high sensitivity in dim light; cones give colour vision and high acuity. Light bleaches rhodopsin, hyperpolarising rod cells.
• Control of heart rate The SAN sets the rhythm; the cardiovascular centre in the medulla adjusts it using sympathetic and parasympathetic nerves in response to chemoreceptors and baroreceptors.
• Homeostasis Negative feedback keeps conditions near a set point. ADH increases water reabsorption in the collecting ducts. Endotherms regulate body temperature using physiological responses, while ectotherms rely mainly on behaviour.

Common mistakes in Topic 9: Control Systems

• Saying peptide hormones enter cells to act on DNA; they bind to cell surface receptors.
• Stating that sodium is actively pumped in during an action potential; Na+ diffuses in through voltage-gated channels.
• Confusing rods and cones: rods are more sensitive in dim light, cones give colour vision.
• Describing ADH as reducing water reabsorption; it increases water reabsorption.
• Saying ectotherms cannot control body temperature; they regulate it mainly through behaviour.

Try three Topic 9: Control Systems exam-style questions