Biology Topic
Osmosis
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Study focus - 3 things
- Osmosis is the movement of water through a partially permeable membrane
- Water moves from a dilute solution to a more concentrated solution
- Osmosis affects whether plant and animal cells swell, shrink, or stay normal
Section 1What osmosis is
Osmosis is a special type of diffusion involving water. Water moves through a partially permeable membrane from where there is more water to where there is less water. In exam language, that means from a dilute solution to a more concentrated solution.
Section 2Dilute and concentrated solutions
A dilute solution has more water and less dissolved substance. A concentrated solution has less water and more dissolved substance. Water moves towards the side with more dissolved substance.
Section 3Osmosis in plant cells
Plant cells become firm when water enters by osmosis. This firmness is called turgor and helps plants stand upright. If plant cells lose too much water, they become flaccid and the plant may wilt.
Section 4Osmosis in animal cells
Animal cells do not have a cell wall, so too much water can make them burst. If they lose water, they shrink. This is why cells need the right water balance around them.
Examples
A piece of potato is placed in pure water and becomes firmer. What happened?
Water moved into the potato cells by osmosis because the pure water outside was more dilute than the cell sap inside.
A plant wilts after being left in salty soil. Why?
The salty soil is more concentrated than the plant cells, so water moves out of the cells by osmosis and the cells lose turgor.
Now let's water it
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