Osmosis
Osmosis is the diffusion of water from a dilute
solution to a more concentrated solution through a
partially permeable membrane The membrane lets water molecules through but not the larger solute
molecules
Water moves towards the side with more dissolved solute . In an
animal cell in pure water, water enters and the cell may burst; in a
concentrated solution, water leaves and the cell shrinks. Plant cells are
protected by the cell wall: they become turgid (firm) in water and
flaccid (soft) in concentrated solution
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Osmosis Osmosis is the movement of WATER across a partially permeable membrane, from a dilute solution to a concentrated one — down the water's concentration gradient. partially permeable membrane dilute solution lots of water concentrated solution less water, more solute net movement of water water molecule — small enough to cross the membrane solute molecule — too big to cross vacuole turgid in a dilute solution water moves IN, the vacuole swells and pushes the membrane hard against the cell wall — the cell is firm, and that is what holds the plant up water vacuole flaccid in a concentrated solution water moves OUT, the vacuole shrinks and the membrane pulls away from the wall — the cell is limp, and the plant wilts water In an animal cell there is no cell wall, so taking in too much water bursts the cell (lysis) and losing too much makes it shrivel (crenation) — which is why animals must control their water balance so carefully. Two diagrams of a partially permeable membrane between a dilute and a concentrated solution, with water molecules moving across towards the concentrated side, and beside it a plant cell turgid and flaccid
Define osmosis in terms of solutions : from a dilute to a
concentrated solution, or from a high water concentration to a low
water concentration. "Water moves from high to low concentration" with no
subject is marked wrong: high concentration of what ?
Only water moves in osmosis. The salt, the sugar and "the solution"
do not move through the membrane
No mention of "water potential", "diffusion gradient" or "balancing": those
are not the GCSE definition and they cost marks when they replace it
The rest of this topic is in the full Exam Guide The complete AQA GCSE Biology guide — every topic, exam checklist and common mistake — is coming soon. Until then, the first topic in every unit is free to read.
In the full topic
• Osmosis• Required Practical 3: osmosis in plant tissue• Active transportRead the free biology topics →