5.1.1(a)-(b) - Communication systems and cell signalling
Multicellular organisms only work because their cells do not behave as isolated units. In this lesson you will learn why animals and plants need communication systems, and how cell signalling lets cells coordinate activity over short and long distances. The focus is deliberately broad: detailed feedback loops, neurones, hormones and second messengers come in later lessons.
Why Communication Is Needed
A unicellular organism can often respond directly to a change in its surroundings because the whole organism is one cell. A multicellular organism is different: most cells are buried inside tissues, many cells are specialised, and different organs may need to change their activity at the same time.
That creates two linked problems.
First, cells need information about changes in the external environment, such as light, temperature, water availability or a threat. Second, cells need information about changes in the internal environment, such as the concentration of glucose, carbon dioxide, water or ions around body cells.
Communication System
A communication system is a coordinated way of detecting information, transmitting a signal and causing appropriate cells, tissues or organs to change their activity.
During exercise, for example, muscle cells use more oxygen and produce more carbon dioxide. Coordinated increases in ventilation and blood flow help supply the active muscles and remove their waste; a response confined to one organ would not meet the demand.
Animals and plants both need communication. An animal may coordinate muscles, glands, blood flow and breathing. A plant may coordinate roots, shoots, leaves and stomata. Plants do not have nerves, but their cells still communicate using chemical signals.
Internal And External Changes
A useful way to understand the need for communication is to ask where a change begins and which part of the organism must respond.
| Change type | What changes? | Why communication is needed |
|---|---|---|
| External | Conditions outside the organism, such as light intensity, temperature, water availability or a threat | Not every cell is in direct contact with the outside; information must be passed to the cells or organs that respond |
| Internal | Conditions inside the organism, such as the composition of tissue fluid around cells | Many cells depend on a stable internal environment, so changes must be detected and coordinated responses must be produced |
The response may be local, involving nearby cells only, or it may involve several organs. For example, if water availability changes around a plant root, shoot tissues may also need to alter water loss. If an animal detects danger, several organs may need to change activity together.
The need for communication comes from multicellularity: cells are specialised, separated from many changes, and arranged into organs that must act together.
The word coordinate is important. It means the activities of different cells, tissues or organs are adjusted so they fit together as one response. A single organ changing by itself may not be enough for the whole organism to respond effectively.
How Cell Signals Work
Cell signalling is communication between cells. One cell produces or presents a signal; another cell receives that signal and changes its activity. The signal may be a chemical molecule released from a cell, or it may involve direct contact between adjacent cell surfaces.
Target Cell
A target cell is a cell that can respond to a particular signal because it has the correct receptor or recognition system for that signal.
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