Cells and organisation · GCSE Biology
Osmosis
Define osmosis for GCSE Biology: water from a dilute to a more concentrated solution through a partially permeable membrane, turgor, plasmolysis and the potato-chip graph.
Osmosis is water only, through a partially permeable membrane, from dilute (high water concentration) to concentrated (low water concentration). No energy from respiration.
The important bits
What you need to know
- 1
Osmosis is the net movement of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
- 2
A dilute solution has a high concentration of water molecules; a concentrated solution has a lower concentration of water (more solute). Water moves down its own concentration gradient.
- 3
A partially permeable membrane lets water through but restricts many solute molecules. Cell membranes and visking tubing behave this way.
- 4
Osmosis does not require energy from respiration. It is a type of diffusion, reserved for water.
- 5
Animal cells in pure water gain water, swell and may burst (lysis). In concentrated solution they lose water and shrivel (crenation).
- 6
Plant cells in dilute solution become turgid: the cellulose wall stops bursting. In concentrated solution they become flaccid, then plasmolysed as the membrane pulls away from the wall.
- 7
Root hair cells take in water from soil by osmosis because cell sap is more concentrated than soil water. Turgor helps support non-woody plant tissues.
- 8
Percentage change in mass = (change in mass ÷ start mass) × 100. Use it to compare potato chips that did not start at identical masses.
Quotations worth analysing
Short evidence. Real method.
“Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.”
Three pieces must be present: water, dilute to concentrated, partially permeable membrane. Missing any one drops the definition mark.
“Water moves from a high water concentration to a low water concentration.”
If you write “high to low concentration” without saying water, the examiner may think you mean the solute. Specify water.
“percentage change = (change in mass / original mass) × 100”
A negative percentage change means the chip lost water. The intercept on a graph is the isotonic concentration.
Go deeper
Why is “high to low” a dangerous shortcut for osmosis?
Students say “water moves from high to low concentration” and mean the sucrose, which is the wrong direction. Water moves from the dilute side (lots of water, little solute) to the concentrated side (less water, lots of solute). Safer phrases: “from a dilute solution to a concentrated solution” or “from high water potential to low water potential” on Triple papers that use water potential. The membrane is not optional in the definition. Diffusion of perfume in air is not osmosis. In a cell, the membrane is the barrier; in the practical, the potato cell membranes (and walls) control the water movement that changes the chip’s mass. Always name water and the membrane in the first sentence of an osmosis answer.
Go deeper
What happens inside a plant cell as the outside solution changes?
In distilled water, water enters, the vacuole swells, the cytoplasm is pressed against the cellulose wall, the cell is turgid, and the tissue feels firm. In a weak sucrose solution matching the cell sap, there is no net water movement (isotonic). In strong sucrose, water leaves, the cell is flaccid, then the membrane peels off the wall (plasmolysis) and the tissue is floppy. Animal cells have no wall, so the same water entry can burst them. That is why a red-blood-cell diagram in distilled water shows lysis. Support in leaves and herbaceous stems depends on turgor; wilting is mass loss of water by osmosis and transpiration. Exam diagrams of plasmolysis want the membrane drawn away from the wall with the gap labelled.
Go deeper
How do I read an osmosis mass-change graph?
Plot percentage change in mass on the y-axis against sucrose concentration on the x-axis. Points above zero: the chip gained water because the outside was more dilute than the cells. Points below zero: water left. Where the line crosses zero, external sucrose matches the average concentration of the potato cell contents — no net osmosis. That intercept is the result many six-mark practicals ask you to estimate. A steeper slope means a bigger water-potential gap. Anomalous points often come from poor blotting (surface sucrose adds fake mass) or chips of different surface area. Repeat, calculate a mean, and draw a line of best fit rather than joining dots.
See the idea in action
A potato chip has start mass 5.00 g and end mass 4.40 g in 1.0 mol/dm³ sucrose. Change = −0.60 g. Percentage change = (−0.60 ÷ 5.00) × 100 = −12%. Water left the cells by osmosis because the sucrose solution was more concentrated than the cell sap (lower water concentration outside). On a graph this point sits below the x-axis. If another chip in 0.2 mol/dm³ sucrose gained 6%, the isotonic concentration lies between 0.2 and 1.0 mol/dm³, where the line would cross zero.
Exam technique
Turn knowledge into marks
Write water, dilute to concentrated, and partially permeable membrane in the definition. For cells, name turgid, flaccid or plasmolysed. For data, use percentage change, not raw grams, if start masses differ.
Common mistakes
Do not give these marks away
- 01
Describing osmosis as solute moving, or omitting the partially permeable membrane.
- 02
Saying water moves from concentrated to dilute solution.
- 03
Using raw mass change instead of percentage change when chips start at different masses.
A plant cell is placed in a concentrated sucrose solution. What happens?
AWater enters by osmosis and the cell bursts
BWater leaves by osmosis and the cell may become plasmolysed
CSucrose enters by osmosis and the cell becomes turgid
DThere is no water movement because osmosis needs energy
Show the answer
Water leaves by osmosis and the cell may become plasmolysed. The outside solution is more concentrated (lower water concentration), so water leaves through the partially permeable membrane. The membrane may pull away from the wall. Osmosis does not need energy.
Quick questions
If this is the bit you searched
What is osmosis in GCSE Biology?
The net movement of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
Does osmosis require energy?
No. It is passive. Active transport is the process that moves substances against a gradient using energy from respiration.
What is plasmolysis?
When a plant cell loses so much water by osmosis that the cell membrane pulls away from the cellulose wall. The cell is no longer turgid.
Why do potato chips change mass in different sucrose solutions?
Water enters or leaves by osmosis depending on whether the sucrose is more dilute or more concentrated than the cell sap. Mass gain means water in; mass loss means water out.