Cells and organisation · GCSE Biology
Osmosis and transport
Diffusion, osmosis and active transport: concentration gradients, partially permeable membranes and the potato practical.
Diffusion is any particle down a gradient. Osmosis is water only, through a partially permeable membrane. Active transport is against the gradient and costs energy from respiration.
The important bits
What you need to know
- 1
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. It does not require energy from respiration.
- 2
Osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
- 3
A partially permeable membrane lets small molecules such as water through but restricts larger solute molecules.
- 4
Active transport moves substances from a lower to a higher concentration, against the gradient, using energy from respiration. Root hair cells take in mineral ions this way.
- 5
The rate of diffusion increases with a steeper concentration gradient, a higher temperature, a larger surface area and a shorter diffusion path.
- 6
A large surface area to volume ratio speeds exchange. Small organisms and specialised exchange surfaces (alveoli, villi, root hairs) rely on this.
- 7
In the potato osmosis practical, cylinders in dilute solution gain mass as water enters; cylinders in concentrated solution lose mass as water leaves. No mass change means the solutions are isotonic.
Go deeper
Three processes, three exam traps
Students mix the names because all three involve membranes and concentrations. Diffusion is the general process for gases and solutes: oxygen into blood, carbon dioxide out of a leaf, digested food into villi. Osmosis is reserved for water, and the membrane must be mentioned. The water moves from the more dilute solution (higher water concentration) to the more concentrated solution (lower water concentration). Active transport is the odd one out because it needs energy. That is why root hair cells are packed with mitochondria: nitrate ions are often more concentrated inside the cell than in the soil water, so they cannot arrive by diffusion alone.
Go deeper
What the potato data actually show
Cut equal cylinders, blot them, record start mass, leave them in a range of sucrose concentrations, blot again and record end mass. Percentage change in mass = (change ÷ start mass) × 100. Plot percentage change against concentration. Where the line crosses zero, the sucrose matches the potato’s cell contents: no net water movement. Points above zero mean water entered (outside was more dilute). Points below zero mean water left (outside was more concentrated). Repeat readings and control the temperature, surface area and time. The blotting step matters: leftover solution on the surface looks like a mass gain.
See the idea in action
A potato cylinder has a start mass of 4.00 g and an end mass of 3.64 g in 1.0 mol/dm³ sucrose. Change = −0.36 g. Percentage change = (−0.36 ÷ 4.00) × 100 = −9%. Water left the cells by osmosis because the sucrose solution was more concentrated than the cell sap. On a graph this point sits below the x-axis.
Exam technique
Turn knowledge into marks
If the question says water, write osmosis and name the partially permeable membrane. If it says mineral ions into a root, write active transport and mention energy from respiration. “Particles move from high to low” is diffusion, not osmosis, unless the particle is water.
Common mistakes
Do not give these marks away
- 01
Describing osmosis as “water from high to low concentration” without saying high water concentration, or omitting the membrane.
- 02
Claiming active transport does not need energy, or putting it down a concentration gradient.
- 03
Using raw mass change instead of percentage change when potato cylinders start at different masses.
Which process moves mineral ions from a dilute soil solution into a more concentrated root hair cell?
AOsmosis
BDiffusion
CActive transport
DTranspiration
Show the answer
Active transport. The ions move against the concentration gradient, so the cell must use energy from respiration. Diffusion and osmosis only move substances down a gradient.
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 concentrated solution through a partially permeable membrane.
How is active transport different from diffusion?
Diffusion is down a concentration gradient and does not need energy. Active transport is against the gradient and uses energy from respiration.