Transport in Cells

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

The word 'net' matters: particles are always moving in random directions, but more end up where there were fewer to start with. Over time, this evens out the concentration (the system reaches equilibrium).

Diffusion happens with:

• Gases — oxygen into red blood cells, carbon dioxide out into the alveoli.
• Dissolved substances — glucose into cells, urea out of cells, mineral ions across membranes.

Diffusion does NOT need energy from respiration. It's a passive process driven by the random movement of particles. This contrasts with active transport, which moves substances AGAINST a gradient and DOES need respiration energy.

Examples in the human body:

• Oxygen diffuses from alveoli (high O₂) into blood (low O₂).
• Carbon dioxide diffuses from blood into alveoli.
• Glucose diffuses from gut into bloodstream.
• Urea diffuses from cells into blood (taken to kidneys for removal).

Four factors all increase the rate of diffusion:

Factor	How it increases rate
Concentration gradient (steeper)	Bigger difference between two regions → more particles moving down the gradient per second
Temperature (higher)	Particles have more kinetic energy → move faster → more collisions and faster diffusion
Surface area (larger)	More area for particles to cross at once
Distance (shorter)	Less to travel → faster equilibrium reached

AQA tip. The exam likes the phrase 'short diffusion DISTANCE' (or 'thin membrane'). Both score the mark.

Biological adaptations of exchange surfaces:

Effective exchange surfaces (e.g. alveoli in lungs, villi in intestine, gills in fish) share four key features that maximise diffusion:

• Large surface area (folded or in many tiny units).
• Thin walls — short diffusion distance.
• Good blood supply — maintains steep concentration gradient by removing diffused substances.
• Ventilation / flow — keeps a fresh supply on one side (e.g. breathing in fresh oxygen, water flowing past gills).

As an organism (or a cell) gets bigger, its volume grows faster than its surface area. The ratio of surface area to volume decreases.

For a cube of side length $L$:

• Surface area = $6L^2$
• Volume = $L^3$
• Ratio SA:V = $6/L$

Worked example.

Cube side	SA ($6L^2$)	Volume ($L^3$)	SA:V
1 cm	6 cm²	1 cm³	6:1
2 cm	24 cm²	8 cm³	3:1
4 cm	96 cm²	64 cm³	1.5:1

What this means biologically:

• A bacterium (tiny, high SA:V) can absorb everything it needs by diffusion alone.
• A whale (huge, low SA:V) cannot — diffusion across its skin would never supply enough oxygen. Hence the need for lungs (with vast surface area) and a circulatory system.

Single cells have a high SA:V; multicellular organisms need specialised exchange surfaces to overcome the limitation.

Osmosis is the net movement of water molecules from a region of higher water concentration to a region of lower water concentration, across a partially permeable membrane.

Equivalent phrasings (all accepted by AQA):

• 'From a dilute solution to a concentrated solution.'
• 'Down a water-concentration gradient.'
• 'From low solute concentration to high solute concentration.'

Partially permeable means the membrane lets water through but blocks larger solute molecules (like sugar or salt). The cell membrane is naturally partially permeable.

Why water moves. Water molecules are small and bounce through the membrane in both directions. But on the more dilute side, MORE water molecules are present to bounce — so the NET movement is from dilute to concentrated.

Effect on cells:

• Animal cells in dilute solutions: water rushes in, cell swells, may burst (no cell wall).
• Animal cells in concentrated solutions: water leaves, cell shrinks ('crenates').
• Plant cells in dilute solutions: water enters vacuole, cell becomes turgid (firm) — useful for support.
• Plant cells in concentrated solutions: water leaves vacuole, cell becomes flaccid, then plasmolysed (membrane pulls away from wall).

Aim: Investigate the effect of a range of sugar (or salt) concentrations on the mass of potato chips.

Method:

1. Cut several identical potato chips (same length and width). Pat dry.
2. Weigh each chip and record initial mass.
3. Place each chip in a separate test tube containing a different sugar concentration (e.g. 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol/dm³).
4. Leave for the same time (e.g. 30 min).
5. Remove chips, pat dry, weigh again and record final mass.
6. Calculate percentage change in mass for each chip:

$\% \text{ change in mass} = \dfrac{\text{final mass} - \text{initial mass}}{\text{initial mass}} \times 100$

7. Plot a graph of % change in mass (y-axis) vs sugar concentration (x-axis). Draw a smooth line of best fit.

Interpreting results:

• At low sugar concentration: water moves INTO chip (cell sap is concentrated relative to outside). Mass INCREASES → +ve % change.
• At high sugar concentration: water moves OUT of chip. Mass DECREASES → -ve % change.
• The point where the line crosses ZERO is the isotonic point — sugar concentration outside equals water concentration inside cell sap, so no net movement.

Why use percentage change? Because chips start with different masses. % change normalises the data so all chips can be compared fairly.

Why dry the chips before weighing? Surface water inflates the measured mass. Pat dry consistently to ensure fair comparison.

Sources of error:

• Different initial chip sizes/shapes → SA:V varies.
• Surface water not blotted off consistently.
• Different temperatures (affect osmosis rate).
• Time variations between samples.

Active transport is the movement of particles from a region of lower concentration to a region of higher concentration — that is, AGAINST a concentration gradient.

This needs energy because particles naturally want to move down a gradient (diffusion); to push them the other way requires work.

The energy comes from respiration (specifically, ATP made in mitochondria). That's why cells specialised for active transport contain LOTS of mitochondria.

How it works (high level): Carrier proteins in the cell membrane pick up the substance on one side and 'pump' it through to the other side. ATP fuels the conformational change in the protein. (You don't need this molecular detail for AQA — just the energy + against-gradient idea.)

Key contrasts:

Process	Direction	Energy needed?
Diffusion	Down the gradient (high → low)	No (passive)
Osmosis	Water from dilute → concentrated	No (passive)
Active transport	UP the gradient (low → high)	YES (from respiration)

Example 1: Root-hair cells absorbing mineral ions.

• Plants need mineral ions (nitrate, magnesium, etc.) from soil to make proteins, chlorophyll, DNA.
• In soil, ion concentrations are LOWER than inside the root-hair cell.
• Diffusion would actually move ions OUT of the cell — useless for the plant.
• So the cell pumps ions IN against the gradient by active transport, using energy from respiration.
• Root-hair cells have many mitochondria for this purpose.

Example 2: Glucose absorption in the small intestine.

• After a meal, glucose concentration in the gut is HIGHER than in the blood → diffusion handles most absorption.
• BUT as digestion ends, glucose in the gut runs low and the body wants to absorb every last bit.
• At this stage, gut glucose is LOWER than blood glucose → diffusion would move glucose backwards into the gut.
• Active transport pumps glucose from gut to blood against the gradient — so none is wasted.
• Cells lining the small intestine have many mitochondria for this.

Other (not assessed but useful) examples: kidney tubules reabsorbing glucose; sodium ions in nerve cells.