OCR Specification focus:
‘Animals and plants must respond to internal and external change and coordinate the activities of different organs in multicellular bodies.’
Organisms must detect and respond to both internal and external environmental changes to maintain optimal conditions for survival. Effective communication systems enable coordination across specialised cells and organs, ensuring homeostasis and adaptive behaviour in complex multicellular life.
The Need for Communication Systems
Maintaining Coordination in Multicellular Organisms
In multicellular organisms, individual cells become specialised, carrying out distinct roles such as muscle contraction, secretion, or nerve transmission. While this specialisation increases efficiency, it creates a dependency on coordination between different cell types and organs to sustain life. Without communication systems, organs would act independently, leading to physiological imbalance and system failure.
Example: Muscle cells require oxygen supplied by the respiratory system and nutrients from the digestive system. Coordination ensures both systems work in synchrony to meet demand during exercise.
Coordination: The process by which different organs and systems in the body work together to maintain equilibrium and respond effectively to stimuli.
Internal and External Changes
Organisms constantly face changes in their internal and external environments. Internal changes arise from metabolic activities, while external changes stem from variations in the surroundings.
Internal changes include fluctuating blood glucose, carbon dioxide, and pH levels.
External changes include temperature, light intensity, and availability of water or nutrients.
A communication system must allow cells to sense these changes and respond appropriately through physiological adjustments or behavioural actions.
Essential Features of a Good Communication System
To maintain homeostasis and efficient coordination, a biological communication system must:
Cover the whole organism — ensuring all organs receive information.
Enable rapid communication for time-sensitive responses.
Produce specific responses appropriate to the stimulus.
Allow both short- and long-term effects — such as rapid nerve impulses or prolonged hormonal action.
Enable negative feedback mechanisms to restore equilibrium.
A well-designed communication network must therefore be reliable, specific, and adaptable.
Types of Communication in Living Organisms
1. Cell Signalling
All forms of communication depend on cell signalling — the process by which one cell produces a signal that is detected and acted upon by another.
A labelled schematic of paracrine, autocrine, and juxtacrine signalling. It highlights how signals act on nearby, self, or contact-dependent target cells to coordinate local responses. This image includes only fundamental categories; any pathway detail is intentionally omitted to match the syllabus focus. Source.
Cell signalling: The process through which cells communicate by producing, transmitting, and receiving chemical or electrical signals that elicit a specific response in target cells.
Two major types of cell signalling are recognised:
Neuronal signalling — rapid, electrical impulses transmitted via neurones.
Hormonal signalling — slower, chemical signals (hormones) released into the bloodstream.
2. Electrical (Neuronal) Communication
The nervous system provides a fast, targeted communication network.
Electrical impulses travel along neurones to specific effectors such as muscles or glands.
Transmission occurs within milliseconds, allowing instantaneous responses.
Coordination of reflexes and voluntary actions depends on these pathways.
Neuronal communication ensures short-term, precise, and reversible control of body functions such as muscle contraction or pupil dilation.
3. Chemical (Hormonal) Communication
The endocrine system communicates using hormones, which are chemical messengers secreted by endocrine glands into the blood.
Hormones travel to target cells that possess specific receptors.
Effects are typically slower in onset but longer-lasting than neuronal responses.
Examples include:
Adrenaline, coordinating the ‘fight or flight’ response.
Insulin, regulating blood glucose concentration.
Chemical communication provides system-wide coordination, influencing metabolism, growth, and reproduction.
Coordination Between Systems
Integrating Nervous and Hormonal Control
Although distinct, nervous and hormonal systems are highly interconnected. The hypothalamus acts as a key interface, monitoring internal conditions and controlling hormone secretion via the pituitary gland.
Example: In temperature regulation, the hypothalamus detects blood temperature changes and initiates both neuronal (sweating, shivering) and hormonal (thyroxine release) responses.
This integration ensures precise and efficient homeostatic control.
Homeostasis and the Role of Communication
Homeostasis — the maintenance of a stable internal environment — depends on continuous communication between receptors, effectors, and coordinators.
Homeostasis: The regulation of internal conditions within narrow limits to maintain a stable internal environment despite external changes.
Components of a Homeostatic System
Receptors: Detect changes in the internal or external environment (stimuli).
Coordination centres: Process information and decide the appropriate response.
Effectors: Carry out the response to restore conditions to the set point.
Communication links these components to enable negative feedback.
A diagram of a negative feedback loop maintaining body temperature via hypothalamic detection and effector responses (e.g., vasodilation/vasoconstriction). It exemplifies how receptor–coordinator–effector communication restores conditions toward a set point—precisely the principle required by the specification. Extra detail specific to thermoregulation is included only as a worked context for the general feedback model. Source.
Negative and Positive Feedback in Communication
Negative feedback occurs when a change triggers a corrective mechanism that restores the system towards its set point.
Example: If body temperature rises, mechanisms such as vasodilation and sweating act to cool the body.
Positive feedback, in contrast, amplifies changes and drives processes to completion, such as during labour contractions.
Both rely on accurate and efficient communication between receptors and effectors.
Plant Communication Systems
Coordination in Plants
Although lacking a nervous system, plants possess sophisticated communication mechanisms that coordinate growth, development, and responses to stimuli.
Hormonal control: Plant hormones like auxins, gibberellins, and abscisic acid regulate tropisms and stress responses.
Electrical signalling: Plants can transmit electrical impulses through specialised tissues in response to touch or damage.
Chemical signalling: Volatile organic compounds act as signals between leaves or even between different plants, warning of herbivory or stress.
These systems allow plants to respond adaptively to light, gravity, and predation, maintaining internal stability and survival in changing environments.
Summary of the Need for Communication
Multicellularity necessitates coordination.
Cell signalling provides the foundation for all communication.
Nervous and hormonal systems deliver fast and slow responses, respectively.
Homeostasis depends on continuous feedback and integration.
Both animals and plants rely on communication networks to sense, coordinate, and respond to change.
FAQ
Single-celled organisms are independent, performing all life processes within one cell. They only require signalling for environmental responses, not intercellular coordination.
Multicellular organisms, however, consist of specialised cells that depend on one another. Communication systems are therefore essential to coordinate activities across organs and maintain internal balance.
Specificity arises from receptor–ligand interactions. Target cells possess receptors that bind only to particular signal molecules (e.g. hormones or neurotransmitters).
If a cell lacks the receptor, it cannot respond to that signal.
This mechanism prevents inappropriate responses elsewhere in the body.
This precise matching ensures efficient and controlled coordination of physiological processes.
Failure of communication disrupts coordination and homeostasis. Consequences depend on the affected pathway:
If nervous signals fail, muscles may not contract or organs may not receive stimuli.
If hormonal control fails, long-term processes like growth or metabolism can become unbalanced.
Ultimately, failure to maintain stable internal conditions could result in cell damage or even organ failure.
Plants use chemical and electrical signalling rather than nerves.
Hormones (e.g. auxins, gibberellins, abscisic acid) regulate growth and responses.
Electrical impulses can pass through plasmodesmata or specialised tissues to trigger movement or closure reactions.
Chemical messengers travel via the xylem, phloem, or diffusion between cells.
These methods allow plants to coordinate growth, respond to stimuli, and adapt to environmental stress.
Negative feedback restores systems to their set point after a deviation. When a change occurs, receptors detect it, effectors act to counter it, and the original state is re-established.
For example:
A rise in body temperature triggers sweating and vasodilation to cool the body.
A fall in temperature triggers shivering and vasoconstriction to conserve heat.
By opposing rather than reinforcing change, negative feedback maintains stability within narrow physiological limits.
Practice Questions
Question 1 (2 marks)
Explain why multicellular organisms require communication systems.
Mark Scheme:
1 mark for recognising that different cells and organs in multicellular organisms are specialised and perform distinct functions.
1 mark for explaining that communication systems are required to coordinate activities between these specialised parts to maintain homeostasis or respond effectively to changes.
Question 2 (5 marks)
Describe the main features of an effective communication system in animals and explain why each feature is important for maintaining homeostasis.
Mark Scheme:
Award up to 5 marks for any of the following valid points, each clearly explained:
(1 mark) Communication must cover the whole organism – ensures that information can reach all parts of the body.
(1 mark) System must allow for rapid transmission of information – enables quick responses to internal or external changes.
(1 mark) Messages must be specific to the target cells or organs – ensures that only the correct effectors respond, preventing inappropriate actions.
(1 mark) Must allow for short-term and long-term responses – rapid control via nerves and longer-lasting control via hormones.
(1 mark) Must enable negative feedback mechanisms – allows conditions to be restored to their normal set point after deviation.
(5 marks total: award 1 mark for each correctly described and justified feature.)
