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Conditions for life

Energy source
Liquid water
Chemical building blocks
Stable environment
where these conditions are met, living organisms can use energy to perform the complex set of chemical transformations (metabolic activities) within their cells that sustains their living state


Energy source

trapped and utilised by an organism for metabolic processes that maintain its living state


Liquid water

allow biochemical reactions to occur, and to dissolve chemicals and transport them both within cells and to and from cells


Chemical building blocks

available for use by an organism in cellular repair, growth and reproduction (C, O, N, H)


Stable environment

Stable environment must exist within the range of tolerance of an organism (pressure, temperature, light intensity, pH, salinity)


The cell theory

- All organisms are made up of cells and the products of cells
- all cells are derived from pre-existing cells
- cells contain genetic information (genes) that is used as instructions for growth, functioning and development.
- cell is the structural, organisational and functional unit of all living things in which the chemical reactions of life take place.


Size of cells

Cells are typically too small to see with the naked eye and microscopes are required to help view them.


SA:V ratio

As cells increase in size through an increase in cytoplasm, both their surface areas and volumes increase but not at the same rate. The internal volumes of cells expand at a greater rate than the areas of their plasma membrane. SA:V decreases as size increases
Sphere will have the smallest SA:V ratio


When a cell is too large

At some point in growing cells, SA:V becomes so small that the surface area is too small to supply its raw materials to its volume.
When this happens, the cell must divide into smaller cells with favourable SA:V ratios or evolve different shapes and elaborate folding to ensure efficiency of diffusion.



Cells that have no membrane-bound nucleus and no membrane-bound organelles



Cells that have a distinct nucleus and membrane-bound organelles


compare P and E

P: small (1-2um)
single, circular molecule of dna
chemically complex cell wall
ribosomes and cell membrane present
cytoskeleton absent
E: large (10-100um)
multiple, linear DNA molecules
larger ribosomes
cell wall only absent in animals
chemically simple cell wall in all others
cytoskeleton present