Diffusion & Osmosis in Cells

Plant and animal cells behave differently when exosmosis or endosmosis occurs, due to their structural differences:

1. Plant cells have a cell wall, which allows for only slight changes in size, and also prevents lysis of the cells.

2. Plant cells have large central vacuoles, which are not present in animal cells. Hence during osmosis, water moves between cell sap in central vacuole and the solution surrounding the cells. For animal cells, water moves between cytoplasm and the solution surrounding the cells.

Cell surface membranes affect the entry and exit of water and substances.
Factors that affect diffusion and osmosis are:

1. Surface area available: An increase in surface area leads to an increase in rate of diffusion and osmosis.

2. Permeability of membranes: Usually cell membranes are partially permeable. However, when membranes are exposed to organic solvents which dissolve the lipid bilayer (eg. ethanol or ether), or when cells are heated, the structure of the lipid bilayer is disrupted. This means that the membrane becomes fully permeable, which allows larger molecules inside the cell (eg. proteins) to diffuse out of the cell.

Points to Note for Structured Qn on Osmosis:

Turgid, Flaccid and Plasmolysis are terms used only for PLANT cells.

Instead, for a RBC, terms like haemolysis (bursting or lysis) and crenation (shrinkage) should be used.

Terms such as Exosmosis and Endosmosis should be used when referring to living cells only. The term Osmosis should be used when referring to experiments with non-living membranes such as visking tubings/dialysis tubings.

When referring to osmosis, use the term water potential, not water concentration as well as water potential gradient, not concentration gradient.

State specifically the shape of the RBC as being flattened and bi-concave.

If there is equal water potential in both cell and solution, state that the water potential gradient is zero resulting in no net movement of water molecules, thus no osmosis.

Use the template that is on the blog to help you phrase your answers!