Tonicity and Osmoregulation
Introduced| English | Meaning |
|---|---|
| osmosis/ɒzˈməʊsɪs/ | The net movement of water molecules through a partially permeable membrane from higher to lower water potential. |
| hypertonic/ˌhaɪpəˈtɒnɪk/ | hypertonic |
| hypotonic/ˌhaɪpəʊˈtɒnɪk/ | hypotonic |
| isotonic/ˌaɪsəˈtɒnɪk/ | isotonic |
| osmoregulation/ˌɒzmɔːɡjʊˈleɪʃn/ | The control of water balance and dissolved solute concentrations in an organism. |
Water on the move
- Cells sit in watery surroundings, and water crosses the membrane constantly.
- Too much water in and a cell swells; too much out and it shrivels.
- Which way water moves depends on what is dissolved on each side.
- Getting this balance right is a matter of life and death for a cell.
Osmosis: water follows solute
- Osmosis 渗透 is the diffusion of water across a membrane.
- Water moves toward the side with more dissolved solute.
- The membrane lets water through but blocks most of the solute.
- So water flows to even out the concentrations.
How plant and animal cells respond to solutions of different water potential
Red onion cells in hypertonic 高渗的 solution: the membrane pulls away from the wall (plasmolysis)
Osmosis is the movement of…
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Osmosis is the diffusion of water across a membrane, from a dilute to a more concentrated solution.
Three kinds of surroundings
- Hypertonic: more solute outside — water leaves, the cell shrinks.
- Hypotonic 低渗的: less solute outside — water enters, the cell swells.
- Isotonic 等渗的: solute equal both sides — no net water movement.
- Tonicity simply describes the outside compared with the cell.
Osmosis and tonicity
Set the outside solute concentration and watch which way water moves — toward the more concentrated side.
A cell placed in a hypertonic solution (more solute outside) will…
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In a hypertonic solution water leaves the cell (toward the higher solute), so the cell shrinks.
In a hypotonic solution (less solute outside), a cell loses water and shrinks.
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False. With less solute outside, water moves into the cell toward the higher solute concentration, so the cell swells and an animal cell may burst. Cells shrink in a hypertonic solution.
When the solute is equal inside and out, with no net water movement, the solution is ____.
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In an isotonic solution the concentrations match, so there is no net osmosis.
Osmoregulation
- Osmoregulation 渗透调节 is how an organism controls its water balance.
- Your kidneys adjust how much water they keep or release.
- A freshwater fish constantly pumps out the water flooding in.
- Without this control, cells would swell or shrink out of action.
Select all true statements about tonicity.
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Osmosis moves water, not salt. The other three are correct.
Water moves toward the side with more solute — that can feel backwards. Think "water follows salt": put a cell in salty (hypertonic) water and it loses water and shrinks, because water flows out toward the saltier side.
A red blood cell in pure water:
- Pure water is hypotonic — far less solute than inside the cell.
- Water rushes in toward the higher solute inside.
- With no cell wall to stop it, the animal cell swells and bursts.
Osmosis moves water across a membrane toward the more concentrated side. A hypertonic outside shrinks a cell, a hypotonic outside swells it, and an isotonic outside leaves it unchanged. Osmoregulation is how organisms keep their water balance steady.
Write from a cell that shrinks
- An animal cell is placed in a solution with a higher solute concentration than its cytosol.
- The membrane lets water through more freely than the solute.
What happens to the cell's volume?
Water moves toward the higher solute concentration.
What happens to this animal cell?
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A hypertonic bath draws water out.