Movement of Substances In and Out of Cells

Why substances move

Cells are not sealed boxes. To stay alive they must take in useful substances such as oxygen, glucose and mineral ions, and get rid of waste such as carbon dioxide. All of this movement happens across the cell membrane, which controls what enters and leaves.

There are three processes you must know for spec 4BI1: diffusion, osmosis and active transport. The first two are passive (no energy needed); the third needs energy from respiration.

Diffusion

Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration, down a concentration gradient.

Particles in liquids and gases are always moving randomly. Where there are more particles, more of them happen to move outwards than inwards, so the overall ("net") flow is from high to low concentration. Diffusion does not need energy from respiration.

Key terms Concentration gradient — the difference in concentration of a substance between two regions.

Net movement — the overall movement once particles travelling in both directions are added together.

Examples of diffusion in living things:

Gas exchange in the lungs — oxygen ($O_2$) diffuses from the air in the alveoli (high $O_2$) into the blood (low $O_2$); carbon dioxide ($CO_2$) diffuses the other way, out of the blood into the alveoli.
Gas exchange in a leaf — $CO_2$ diffuses into the leaf for photosynthesis; $O_2$ diffuses out.
Glucose and ions can diffuse across membranes when a gradient exists.

Osmosis

Osmosis is the net movement of water molecules from a region of higher water concentration (a dilute solution, high water potential) to a region of lower water concentration (a more concentrated solution, lower water potential), through a partially permeable membrane.

A partially permeable membrane lets small water molecules through but not larger dissolved solute molecules. So only water moves. Osmosis is really just diffusion of water, but the term is reserved for water crossing a partially permeable membrane.

Watch out A common mistake is to say "water moves from low to high concentration." Always specify what is concentrated. Water moves from where the solution is dilute (lots of water) to where the solution is concentrated (little water).

Active transport

Active transport is the movement of particles against a concentration gradient — from a lower to a higher concentration — using energy released by respiration.

Because it works "uphill", it needs energy and carrier proteins in the membrane. Examples:

Root hair cells absorb mineral ions (such as nitrate) from the soil, even though the ions are already more concentrated inside the root than in the dilute soil water.
The small intestine absorbs glucose into the blood, even after much has already been taken up, so digestion is complete.

Exam tip If a question mentions movement happening against a gradient, or stops when a cell is poisoned / starved of oxygen, the answer is active transport — it relies on respiration for energy.

Comparing the three processes

Factors affecting the rate of movement

The rate of diffusion and osmosis increases with:

Concentration gradient — a steeper gradient (bigger difference) gives a faster rate.
Surface area — a larger surface area allows more particles to cross at once (e.g. villi, alveoli, root hairs are all folded or branched to increase area).
Distance — a shorter diffusion distance (thinner membrane) gives a faster rate.
Temperature — higher temperature gives particles more kinetic energy, so they move faster.

Osmosis in plant cells

Plant cells have a strong cellulose cell wall outside the membrane. What happens depends on the solution around the cell:

In a dilute solution (lots of water) water enters by osmosis. The cell swells and pushes against the wall, becoming firm or turgid. The wall stops it bursting. Turgid cells make a plant stand upright.
In a solution with the same concentration as the cell, there is no net movement.
In a concentrated solution water leaves by osmosis. The cell becomes soft or flaccid; the plant wilts.
If a lot of water leaves, the membrane and cytoplasm pull away from the wall — this is plasmolysis, and the cell is plasmolysed.

Key terms Turgid — a plant cell full of water, pushing firmly against its wall.

Flaccid — a plant cell that has lost water and is no longer firm.

Plasmolysis — when the membrane pulls away from the cell wall after major water loss.

Effect of osmosis on animal cells

Animal cells have no cell wall, so they are easily damaged by water movement:

In a dilute solution water enters by osmosis. The cell swells and may burst — this is lysis (in red blood cells, haemolysis).
In a concentrated solution water leaves and the cell shrinks and becomes crinkled — this is crenation.

This is why body fluids must be kept at a steady concentration.

Required practical: potato cylinders

You can investigate osmosis using potato:

1. Cut several potato cylinders of equal size; measure each starting mass (and length).
2. Place them in sugar (or salt) solutions of different concentrations, plus pure water.
3. Leave for a set time, dry gently and re-measure the mass.
4. Calculate the percentage change in mass for a fair comparison.

Worked example A cylinder starts at 4.0 g and ends at 4.6 g.

percentage change $= \dfrac{4.6 - 4.0}{4.0} \times 100 = +15\%$.

A gain in mass means water entered, so the solution was more dilute than the cell sap. The concentration where mass does not change equals the concentration inside the potato cells.

Required practical: Visking tubing

Visking tubing is an artificial partially permeable membrane. Filled with a starch and glucose mixture and placed in water, the small glucose molecules diffuse out (tested with Benedict's solution) while the large starch molecules stay in (tested with iodine). This models how the gut wall lets small digested molecules through but not large ones.

Exam tip Use control variables to score marks: same size of cylinder/tubing, same volume of solution, same temperature and time. Always state that you would repeat and average the results.