# Control and Coordination

> CIE A-Level Biology · CIE 9700 A-Level
> Source: https://www.owlsprep.com/study/cie-9700-u16-overview/
> Weight: n/a

This unit explores how multicellular organisms coordinate internal functions and respond to external stimuli, covering fast nervous communication and slower hormonal control in animals and plants.

**Prerequisites:** Cell membrane structure and transport across membranes; Multicellular organism tissue and organ organisation

## Learning objectives

- Distinguish between the key characteristics of nervous and hormonal control mechanisms in animals
- Explain how nerve impulses are generated, propagated and transmitted between neurones across synapses
- Describe the structure and function of the mammalian eye and photoreceptor cells
- Outline how plant growth regulators coordinate plant growth and responses to environmental stimuli

## Unit at a Glance

This unit follows a clear learning arc, starting from the cellular basis of nervous communication in animals, building up to whole system responses, then covering hormonal control in both animals and plants. We begin with the fundamental unit of the nervous system (the neurone), then learn how signals travel along and between neurones. We apply this knowledge to the mammalian eye, then compare nervous control to the slower, long-range hormonal system, before finishing with coordination in plants that lack a nervous system entirely.

This unit includes the following sub-topics:
- [Neurone structure](https://www.owlsprep.com/study/cie-9700-u16-neurone-structure/) — Covers the structure and function of motor, sensory and relay neurones in mammalian nervous systems.
- [Nerve impulse propagation](https://www.owlsprep.com/study/cie-9700-u16-nerve-impulse-propagation/) — Explains resting potential, action potential generation and propagation along myelinated and unmyelinated axons.
- [Synaptic transmission](https://www.owlsprep.com/study/cie-9700-u16-synaptic-transmission/) — Describes how signals are transmitted across cholinergic synapses and how chemicals/drugs affect this process.
- [Reflex arcs](https://www.owlsprep.com/study/cie-9700-u16-reflex-arcs/) — Outlines the structure and function of reflex arcs and their role in rapid protective responses.
- [Eye structure and photoreception](https://www.owlsprep.com/study/cie-9700-u16-eye-structure-and-photoreception/) — Covers mammalian eye structure, rod/cone cell function, and light detection by visual pigments.
- [Hormonal communication](https://www.owlsprep.com/study/cie-9700-u16-hormonal-communication/) — Compares hormonal and nervous control, and covers mechanism of steroid and non-steroid hormone action.
- [Plant growth regulators](https://www.owlsprep.com/study/cie-9700-u16-plant-growth-regulators/) — Explains the role of auxins and other regulators in plant tropisms and growth coordination.

## Common pitfalls

- **Wrong:** Confusing the characteristics of nervous and hormonal control
  - Why it fails: Exam questions frequently ask for direct comparisons, so mixing these up loses easy marks
  - Correct: Remember: nervous = fast, specific, short-lived effect; hormonal = slow, widespread, long-lived effect
- **Wrong:** Reversing sodium and potassium ion movement during action potentials
  - Why it fails: This is one of the most frequently exam errors on this topic
  - Correct: Depolarization = $Na^+$ ions flow into the neurone; Repolarization = $K^+$ ions flow out of the neurone

## Cheatsheet

| Key Concept | Unit-Level Summary |
| --- | --- |
| Resting potential | ~-70 mV, maintained by $Na^+/K^+$ pump: 3 $Na^+$ out, 2 $K^+$ in |
| Action potential | All-or-nothing event: depolarization to ~+40 mV followed by repolarization |
| Saltatory conduction | Impulse jumps between nodes of Ranvier in myelinated axons, speeds transmission |
| Synaptic transmission | Calcium influx triggers neurotransmitter release into the synaptic cleft |
| Rod vs cone cells | Rods: high sensitivity, low acuity, non-colour; Cones: low sensitivity, high acuity, colour |
| Nervous vs hormonal | Nervous: electrical/chemical signaling; Hormonal: only chemical signaling via blood |
| Auxin function | Promotes cell elongation in shoots, causes phototropism via differential growth |

## What's next

Begin your study of this unit with the first sub-topic on neurone structure, which lays the critical cellular foundation for all topics on nervous communication that follow. After you complete all sub-topics in this unit on control and coordination, you will move on to the next unit on homeostasis, which builds on the coordination concepts you learn here to explore how organisms maintain stable internal conditions.

- [Neurone structure](https://www.owlsprep.com/study/cie-9700-u16-neurone-structure/)
- [Nerve impulse propagation](https://www.owlsprep.com/study/cie-9700-u16-nerve-impulse-propagation/)
- [Synaptic transmission](https://www.owlsprep.com/study/cie-9700-u16-synaptic-transmission/)

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From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-9700-u16-overview/