What you'll learn
- How substances move across cell membranes by diffusion, osmosis and active transport.
- How to decide which transport process is happening from the direction of movement.
- How to describe the potato osmosis core practical.
- How to calculate percentage gain or loss of mass in osmosis.
Start point: cells have boundaries
Every cell is surrounded by a cell membrane. This controls what enters and leaves the cell. Some substances, such as oxygen, carbon dioxide, water and mineral ions, need to move across this membrane.
A particle means a tiny piece of matter, such as an atom, molecule or ion. In cells, substances are often dissolved in water.
Solution, solvent and solute
A solution is a mixture where a substance is dissolved. The solvent is the liquid doing the dissolving, usually water in biology. The solute is the dissolved substance, such as sucrose or mineral ions.
Concentration gradient
Concentration means how much of a substance there is in a given volume. A concentration gradient is the difference in concentration between two areas.
Cells use three main transport processes: diffusion, osmosis and active transport. They are easiest to compare side by side.
The big comparison
Diffusion and osmosis move substances down a concentration gradient and do not require energy from respiration. Active transport moves substances against a concentration gradient and does require energy.
Diffusion
Diffusion happens because particles are always moving randomly. If there are more particles on one side than the other, more particles will randomly move away from the crowded side than back towards it. This creates a net movement.
Diffusion
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient.
Diffusion can happen in gases and liquids. In cells, small substances such as oxygen and carbon dioxide can diffuse across cell membranes.
For example:
- Oxygen diffuses into cells for respiration.
- Carbon dioxide diffuses out of cells after respiration.
- Dissolved substances may diffuse into or out of cells if the membrane allows them through.
What affects the rate of diffusion?
Diffusion is faster when:
- the concentration gradient is steeper
- the temperature is higher, because particles have more kinetic energy
- the surface area is larger
- the diffusion distance is shorter
Predicting oxygen diffusion
A respiring muscle cell has a lower oxygen concentration than the blood around it. Which way will oxygen move?
- Compare the concentrations: oxygen concentration is higher in the blood and lower inside the muscle cell.
- Apply the diffusion rule: particles move from higher concentration to lower concentration.
- Oxygen therefore diffuses from the blood into the muscle cell, down its concentration gradient.
Thinking diffusion only goes one way
Individual particles move randomly in both directions. The important idea is net movement: overall, more particles move from high concentration to low concentration.
Osmosis
Osmosis is a special type of diffusion involving water molecules only.
A partially permeable membrane lets some particles pass through but not others. Cell membranes are partially permeable: water can pass through, but many larger dissolved substances cannot.
Osmosis
Osmosis is the net movement of water molecules through a partially permeable membrane from a dilute solution to a more concentrated solution.
You may also see osmosis described as water moving from a high water concentration to a low water concentration. These two descriptions mean the same thing:
- A dilute solution has lots of water compared with solute.
- A concentrated solution has less water compared with solute.
Osmosis in cells
If a cell is placed in a dilute solution, water moves into the cell by osmosis. If it is placed in a concentrated solution, water moves out of the cell by osmosis.
Plant cells have a cell wall, so they can become firm when water enters. Animal cells do not have a cell wall, so too much water entering can make them burst.
Predicting mass change in a potato cylinder
A potato cylinder is placed in a concentrated sucrose solution. What happens to its mass?
- The sucrose solution outside the potato cells is concentrated, so it has a lower water concentration than the inside of the potato cells.
- Water moves by osmosis from the higher water concentration inside the potato cells to the lower water concentration outside.
- The potato cylinder loses water, so its mass decreases.
Osmosis wording
For GCSE answers, make sure you say water moves through a partially permeable membrane. Do not just write “the solution moves”.
Active transport
Sometimes cells need to take in substances even when there is already a higher concentration inside the cell. This cannot happen by diffusion, because diffusion only gives net movement down a concentration gradient.
Active transport
Active transport is the movement of substances from a lower concentration to a higher concentration, against a concentration gradient, using energy from respiration.
Active transport uses carrier proteins in the cell membrane. A carrier protein is a membrane protein that helps move a specific substance across the membrane.
Examples include:
- root hair cells absorbing mineral ions from the soil
- cells in the small intestine absorbing glucose from digested food
Diagrams may show the energy as ATP, which is the cell’s usable energy-transfer molecule.
Choosing active transport
A root hair cell absorbs nitrate ions from soil even though the nitrate ion concentration is higher inside the root hair cell than in the soil. Which process is being used?
- The ions are moving from a lower concentration in the soil to a higher concentration inside the root hair cell.
- Movement from low to high concentration is against the concentration gradient, so it cannot be diffusion.
- The process must be active transport, and it requires energy from respiration.
Calling active transport faster diffusion
Active transport is not just “fast diffusion”. It is a different process because it moves substances against the concentration gradient and needs energy.
Core Practical: Investigating osmosis in potatoes
In this practical, potato cylinders are used because potato cells have partially permeable membranes. You place potato pieces in different sucrose solution concentrations and measure how their mass changes.
Method
- Cut potato cylinders to the same length and diameter.
- Gently blot each cylinder dry with paper towel.
- Measure and record the initial mass of each cylinder using a balance.
- Place the cylinders into different concentrations of sucrose solution, such as 0.0, 0.2, 0.4, 0.6 and 0.8 mol/dm³.
- Keep control variables the same: volume of solution, time left in solution and temperature.
- Remove the cylinders, gently blot them dry again, then measure the final mass.
- Calculate the percentage change in mass and plot a graph against sucrose concentration.
Variables in the potato osmosis practical
- Independent variable: concentration of sucrose solution.
- Dependent variable: change in mass of the potato cylinder.
- Control variables: potato size, volume of solution, time, temperature and potato type.
Forgetting to blot the potato
Surface water adds extra mass that did not come from osmosis. Blot gently before both weighings so the mass change is due to water moving into or out of the potato cells.
Interpreting the results
In dilute solutions, the potato usually gains mass because water enters by osmosis. In concentrated sucrose solutions, the potato usually loses mass because water leaves by osmosis.
Isotonic point
The isotonic point is where there is no overall change in mass because there is no net movement of water into or out of the potato cells.
On a graph, the isotonic point is where the line crosses 0 percentage change in mass.
Calculating percentage gain or loss of mass
Use this formula:
percentage change in mass=final mass−initial massinitial mass×100\text{percentage change in mass} = \frac{\text{final mass} - \text{initial mass}}{\text{initial mass}} \times 100percentage change in mass=initial massfinal mass−initial mass×100A positive answer means a percentage gain in mass. A negative answer means a percentage loss in mass.
Calculating percentage change in mass
A potato cylinder has an initial mass of 2.50 g and a final mass of 2.20 g. Calculate the percentage change in mass.
-
Find the change in mass using final mass minus initial mass.
change in mass=2.20−2.50=−0.30 g\text{change in mass} = 2.20 - 2.50 = -0.30\,\text{g}change in mass=2.20−2.50=−0.30g -
Divide by the initial mass and multiply by 100.
−0.302.50×100=−12%\frac{-0.30}{2.50} \times 100 = -12\%2.50−0.30×100=−12% -
Interpret the sign: the mass decreased, so this is a 12% loss in mass.
Why percentage change is better
Potato cylinders may not all start with exactly the same mass. Percentage change makes the results easier to compare fairly.
In the exam
- For diffusion, osmosis and active transport questions, always compare the concentrations on the two sides of the membrane first.
- For osmosis answers, include the words water, partially permeable membrane and the correct direction of movement.
- For the potato practical, name the independent variable, dependent variable and at least two control variables.
- In percentage change calculations, use the initial mass as the denominator, not the final mass.
Check yourself
- How is active transport different from diffusion?
- Why does a potato cylinder gain mass in a dilute solution?
- What does it mean if the percentage change in mass is negative?
