3.2. Osmosis

Don't let the names trick you! Diffusion and osmosis operate by the exact same mechanism. 

It's just that diffusion refers to many types of molecules whereas osmosis is EXCLUSIVE to water diffusing through a partially permeable membrane.

Osmosis is the net movement of water molecules 

• from a region of higher water potential (dilute solution) 
• to a region of lower water potential (concentrated solution), 
• through a partially permeable membrane

In cells, water moves in/out of cells through the cell membrane (partially permeable)

Osmosis of Plant Cells

When animal and plant cells are placed in water (higher water potential):

• Cells take up water through their cell membranes by diffusion. Diffusion of water = osmosis.
• Plant cells become turgid (swollen), but do not burst because of their tough cell wall.
• Turgid cells side-by-side helps support the plant via turgor pressure (pressure of water inside the cells pressing outwards on the inelastic cell wall).
• Animal cells burst (because no cell wall)

When animal and plant cells are placed in concentrated sugar or salt solutions (lower water potential):

• Animal cells become flaccid (limp) --> cell becomes crenated in shape.
• Plant cells also become flaccid. 
• cell membranes pull away from the cell wall
• plasmolysis: when the cell membrane tears away from the cell wall; this can kill the cell

Importance of water potential (w.p.) and osmosis in 

A) Plants' water uptake

• When plants have enough water --> turgid cells --> turgor pressure --> plants stand upright
• When plants are thirsty --> flaccid cells --> no turgor pressure --> wilting plant
• Root hair cells absorb water from soil by osmosis for plant's usage. 
• W.p. in soil is higher than in root hair cells; water moves from soil --> root hairs

B) Animal cells and tissues

• fluid that cells sit in (such as tissue fluid and blood plasma) must be the same w.p. as inside the cells (otherwise, water will continue to enter cells, causing them to burst)
• In operations, surgeons must use saline solution (with similar w.p. to the body) to wash open wounds, to prevent cells from bursting in this process
• diarrhoea = loss of water faeces (a bacteria/virus may cause the intestines to lower in w.p., drawing more water into the gut)

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