Cellular Transport

Moving substances across a cell membrane

Cells need substances from their surroundings and must remove waste products. Their cell membranes control what can enter and leave. Three processes move substances across these membranes: diffusion, osmosis and active transport.

To understand the direction of movement, first consider concentration: the amount of a substance in a given volume. A solution containing more sugar in the same volume has a higher sugar concentration. A difference in concentration between two regions is a concentration gradient.

Diffusion: random movement produces a net direction

Particles in liquids and gases are constantly moving randomly. Diffusion is their spreading out and mixing, resulting in net movement from higher to lower concentration, down a concentration gradient. Some substances can cross cell membranes this way.

Imagine more oxygen molecules outside a cell than inside it. Molecules move in both directions, but more enter than leave. This overall difference is the net movement. As the concentrations become equal, there is no net movement, although individual molecules continue moving.

Diffusion is passive: the cell does not need to supply energy from respiration. For example, oxygen diffuses from the air in the lungs into the blood, while carbon dioxide diffuses from the blood into the lungs to be breathed out.

What makes diffusion faster?

Three factors affect the rate of diffusion across a membrane. When comparing them, assume the other conditions stay the same.

• A greater difference in concentration increases the rate. A steeper concentration gradient produces a greater difference between the numbers of particles moving in each direction.
• A higher temperature increases the rate. Particles have more kinetic energy and move faster.
• A larger membrane surface area increases the rate. More particles can cross the membrane at the same time.

Surface area concerns how much membrane is available for exchange, rather than how fast an individual particle moves.

Osmosis: water moving through a partially permeable membrane

A solute is a dissolved substance, such as salt or sugar. A dilute solution has relatively little solute and a higher concentration of water. A more concentrated solution has more solute and a lower concentration of water.

Osmosis is the net movement of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane. Water therefore moves down its own concentration gradient.

Cell membranes are partially permeable: they allow small molecules such as water through but prevent larger molecules from passing. In an osmosis model, the membrane allows water to cross while retaining the dissolved solute. Like diffusion, osmosis does not require energy from respiration.

If the solution around plant tissue is more dilute than the solution inside its cells, water enters by osmosis and the tissue gains mass. If the surrounding solution is more concentrated, water leaves and the tissue loses mass. When the solutions are balanced, water still crosses both ways, but there is no net movement and no mass change due to osmosis.

Active transport: moving against the gradient

Diffusion cannot produce net movement of a substance from lower to higher concentration. Cells can achieve this using active transport, which requires energy transferred from respiration.

Plant root hair cells absorb mineral ions from very dilute soil solutions. The mineral ion concentration can already be higher inside the cells than in the soil. Active transport allows further uptake against the concentration gradient, supplying minerals needed for healthy growth. This is different from the uptake of water, which occurs by osmosis.

Active transport also allows sugar molecules to be absorbed from the human gut into the blood when the sugar concentration in the gut is lower than that in the blood. Absorption can therefore continue when diffusion alone would not move sugar into the blood.

The key distinction is direction relative to the gradient: diffusion moves particles down it, osmosis moves water down its water concentration gradient, and active transport moves a dissolved substance against it.

Required practical: changing the mass of plant tissue

The investigation tests how a range of salt or sugar solution concentrations affects the mass of plant tissue. Potato cylinders are suitable because they can be cut to similar dimensions and weighed before and after soaking.

You need potato tissue, a cork borer, a cutting tile and knife or scalpel, a ruler, a balance, labelled containers, a measuring cylinder, paper towels, a timer and a range of known salt or sugar solution concentrations. Use one solute throughout the investigation and include water as the zero-concentration solution.

1. Prepare potato cylinders of the same diameter and length, preferably from the same potato. Remove any skin.
2. Gently blot each cylinder and record its initial mass in grams.
3. Measure equal volumes of the different solutions into labelled containers. Fully immerse each cylinder in its assigned solution.
4. Leave all cylinders for the same measured time at the same temperature.
5. At the end of that time, remove the cylinders and gently blot them in the same way to remove surface liquid. Record their final masses.
6. Use several cylinders at each concentration so that you can calculate a mean percentage change and compare the spread of results.

Use cutting tools on a cutting tile and cut away from fingers under teacher supervision. Do not eat the tissue or taste the solutions, and wipe up spills promptly.

The independent variable is solution concentration. The dependent variable is the change in tissue mass, usually expressed as a percentage. Keep tissue dimensions and source, solution volume, soaking time and temperature constant. These controls make it more likely that differences in mass change are caused by concentration.

The measurement period ends after the chosen soaking time; it does not require a visible colour change or complete cessation of osmosis. Mass measurements reveal whether there has been a net gain or loss of water. Blotting is important because liquid clinging to a cylinder would otherwise be weighed with the tissue.

Calculating and comparing mass changes

First calculate the change in mass:

$$\text{change in mass} = \text{final mass} - \text{initial mass}$$

Then express it relative to the starting mass:

$$\text{percentage change in mass} = \frac{\text{final mass} - \text{initial mass}}{\text{initial mass}} \times 100$$

For example, suppose a potato cylinder initially has a mass of 2.00 g and has a final mass of 2.20 g after soaking. Its percentage change is . This positive value indicates a net gain of water. A negative percentage change indicates a net loss.

Percentage change allows fairer comparisons between samples with slightly different initial masses. Calculate it for each cylinder before finding the mean at each concentration: add the percentage changes and divide by the number of cylinders.

Repeats also show how variable the results are. Record the minimum and maximum percentage changes at each concentration. A wide range suggests more uncertainty in the mean than a narrow range. Investigate any unusual result rather than automatically discarding it. Consistent blotting, equal dimensions and precise mass measurements help reduce uncertainty.

Drawing and interpreting an osmosis graph

Put solution concentration on the horizontal axis and mean percentage change in mass on the vertical axis. Label both axes with units, use evenly spaced scales and include positive and negative percentage changes. Plot the points and draw a suitable line or curve of best fit through the overall trend, rather than forcing it through every point. You can show the spread of repeats with vertical range bars from the minimum to the maximum result.

Suppose a set of potato samples produces the pattern below.

Data for Sugar concentration and change in potato mass

Points above zero represent mass gain: water entered the tissue by osmosis. Points below zero represent mass loss: water left the tissue. The concentration where the best-fit line crosses zero estimates the solution concentration at which there is no net water movement into or out of the tissue. In this constructed example, that concentration is 0.40 mol/dm³.

Estimate a crossing between measured points by reading from the line to the concentration axis. If all results are above zero or all are below zero, the tested range has not located the crossing: further concentrations would be needed. Greater scatter makes the estimated crossing less certain.

Transport processes