Comprehensive study guide for Biology A Level, made by an Oxford Biochemistry student with all 9s at GCSE and 3 A*s at A Level! Information arranged by spec point. Concise notes written using past papers, multiple textbooks, class notes and more.
a. The need for communication systems in multicellular organisms
Survival and activity:
- Cell metabolism relies on enzymes. These enzymes require specific conditions to work:
o A suitable temperature, a suitable pH, an aqueous environment (water potential) that
keeps the substrates and products in solution, a lack of toxins and excess inhibitors.
- Without these conditions, the cells will become inactive and die.
- In multicellular organisms, cells are specialised and rely upon one another, as they must be able
to communicate in order to coordinate their activities.
- There is a need for communication systems in multicellular organisms to:
o Deal with threats from changing external environments.
o Deal with threats from changing internal environments.
o Coordinate the activities of different organs.
Threats from changing external environments:
- The external environment changes, which may place stress on the living organism.
o E.g. a cooler environment will cause greater heat loss.
o The environment can include water, air, soil, etc.
- It the organism is to remain active and survive, it must modify its behaviour or physiology in
response to changes in the environment to reduce the stress.
o The environmental change is a stimulus and the way in which the organism changes its
behaviour or physiology is its response.
- Slow changes in the environment elicit a gradual response – e.g. the artic fox has a much thicker
white coat in winter and a thinner grey/brown coat in summer.
- There can also be rapid changes in the environment – e.g. the appearance of a predator.
Threats from changing internal environments:
- Most multicellular organisms have a range of tissues and organs.
- Many of the cells and tissues are not exposed to the external environment – they are protected
by epithelial tissues and organs such as skin or bark. In many animals, the internal cells and tissues
are bathed in tissue fluid.
o This is the environment of the cells.
- As cells undergo metabolic reactions, they use up substrates and create new products, some of
which are unwanted or toxic, or can act as inhibitors (end-product inhibition).
o These substances move out of the cells into the tissue fluid. The activities of the cells
therefore alter their own environment.
o E.g. if CO2 builds up in the tissue fluid outside the cells, it will alter the pH of the tissue
fluid and could disrupt the action of proteins like enzymes.
- The accumulation of excess waste or toxins in this internal environment must act as a stimulus to
cause removal of these waste products so that the cell can survive.
o E.g. the reduced pH of the blood stimulates greater breathing activity that expels the CO 2.
- The build-up of waste products in the tissue fluid may also act directly on the cells, which respond
by reducing their activities so that less waste is produced.
o However, this response may not be beneficial for the whole organism.
Maintaining the internal environment:
, - The internal environment of the cell is maintained through the excretion of waste, homeostasis,
cell signalling and negative feedback.
- E.g. the composition of the tissue fluid is maintained by the blood.
o Any wastes or toxins accumulating in the tissue fluid are likely to enter the blood. To
prevent their accumulation in the body, they must be removed by excretion.
- The concentrations of substances in the blood must be monitored closely.
o This ensures that the body removes enough of the waste products to maintain good
health, whilst supplying the cells in the body with the substrates they need.
Coordinating the activities of different organs:
- A multicellular organism is more efficient that a unicellular organism as its cells are differentiated.
Its cells are specialised to perform particular functions.
- Different organs have different functions and may be far from other organs that they need to
work with. A good communication system ensures that these different parts of the body work
together effectively.
- A good communication system will:
o Cover the whole body.
o Enable cells to communicate with each other.
o Enable specific communication.
o Enable rapid communication.
o Send conserved messages.
o Enable both short-term and long-term responses.
b. The communication between cells by cell signalling
Cell signalling – the way in which cells communicate with each other
- One cell will release a chemical that is detected by another cell. The second cell will respond to
the signal released by the first cell.
- The two major systems of communication that work by cell signalling are the neuronal system and
the hormonal system.
- Neuronal system – neurones in networks signal to each other across synpases.
o Cells involved: neurones (nerve cells).
o The signal travels across synapse junctions.
o The effects are rapid and directed (localised).
- Hormonal (endocrine) system – cells in an endocrine organ release a hormone into the blood.
Only target cells respond.
o Cells involved: cells in endocrine organs, target cells.
o The signal travels through the blood to the target cells. Target cells have receptors that
are specific and complementary to the ligand/hormone.
o The effects are more long-term.
To include signalling between adjacent cells and signalling between distant cells.
c. The principles of homeostasis
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