IGCSE Biology: Movement Into and Out of Cells — Revision Notes

Condensed for the final weeks. For the full explanation, use the Movement Into and Out of Cells study guide.

Three processes at a glance

Process	Direction	Energy needed?	Tier
Diffusion	High → low concentration	No	Core
Osmosis	High → low water potential	No	Core (basic) / Supplement (full definition)
Active transport	Low → high concentration	Yes (from respiration)	Core (basic) / Supplement (detail)

Diffusion — all Core, no Supplement-only content

Net movement of particles from higher to lower concentration, down a concentration gradient, from random kinetic movement. Factors affecting rate: surface area, temperature, concentration gradient, distance. Because 3.1 has no Supplement-only material, it is one of the highest-value topics to secure completely regardless of tier.

Osmosis — the Core/Supplement trap

Core needs six things: the role of water as a solvent in organisms, with reference to digestion, excretion and transport; that water diffuses through partially permeable membranes by osmosis; that water moves into and out of cells by osmosis through the cell membrane; investigating osmosis using materials such as dialysis tubing; investigating and describing the effects on plant tissues of immersing them in solutions of different concentrations; and that plants are supported by the pressure of water inside the cells pressing outwards on the cell wall.

Supplement adds: describing osmosis as the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane; explaining the effects on plant cells of immersing them in solutions of different concentrations using the terms turgid, turgor pressure, plasmolysis and flaccid; and explaining the importance of water potential and osmosis in the uptake and loss of water by organisms.

Core candidates who half-learn the Supplement definition often lose marks trying to use “water potential” language they don’t actually need — know which level you’re being examined at.

Active transport — the one that needs energy

Moves particles against a concentration gradient, using energy from respiration — the opposite direction to diffusion, and the key distinguishing feature from both passive processes.

Worked example: identifying the process

An unfamiliar scenario describes ions being absorbed by root hairs into a region where ion concentration is already higher than in the soil.

Direction:     ions move from LOW concentration (soil) to HIGH
               concentration (root hair cell) -- against the gradient
Process:       must be active transport, since diffusion and osmosis
               only move substances DOWN a gradient
Energy:        confirm by checking respiration is available in root
               hair cells to supply the energy this requires

Being able to state direction and energy requirement from memory, then apply both to an unfamiliar scenario, is worth more in practice than reciting definitions alone.

Osmosis in plant cells: the four key terms

For Supplement candidates, apply turgid/turgor pressure/plasmolysis/flaccid precisely: a cell placed in a dilute solution gains water by osmosis and becomes turgid (firm, pressing outward against the cell wall, generating turgor pressure); a cell placed in a concentrated solution loses water and becomes flaccid, and if enough water is lost the cell membrane pulls away from the cell wall entirely — a state called plasmolysis. This vocabulary is precise and examiners expect exact terms, not approximate descriptions like “the cell shrank.”

Linking to the practical investigations

For the dialysis-tubing osmosis investigation, be ready to describe the method (filling tubing with a sugar solution, immersing it in water, measuring mass change) and explain the result using the same water-potential language covered above. For the diffusion factors practical, be ready to describe how you would vary and measure the effect of surface area, temperature, concentration gradient or distance on the rate of diffusion — for example, using agar blocks of different sizes with a coloured indicator to measure how quickly a substance diffuses to the centre.

Exam traps

• Using Supplement-only osmosis vocabulary (water potential) when the question only requires the Core statement.
• Confusing active transport’s direction with diffusion’s — active transport is the only one of the three that moves particles against a gradient.
• Forgetting osmosis is a special case of diffusion restricted to water across a partially permeable membrane, not a fully separate process.
• For practicals, not being ready to describe both the dialysis-tubing osmosis investigation and how to test factors affecting diffusion rate.

Active transport: the Supplement detail

Supplement candidates must also explain active transport’s importance for ion uptake by root hairs — plant roots actively absorb mineral ions from soil even when soil ion concentration is lower than inside the root hair cell, which is only possible because active transport moves particles against the gradient using respiration energy. Also know that protein carriers embedded in the membrane are what physically move molecules or ions across during active transport — a specific mechanism diffusion and osmosis do not require, since they rely on random particle movement alone rather than a carrier mechanism.

Building one comparison table from all three sections

Rather than revising diffusion, osmosis and active transport as three separate sets of notes, build a single table (as shown above) early in your revision and add to it as you cover each sub-topic — this makes exam-style “identify the process” questions faster to answer, since direction and energy requirement become an automatic lookup rather than something to reconstruct from memory each time.

Self-test

1. Which of the three processes requires energy, and where does that energy come from?
2. State the Core-only definition of osmosis (not the Supplement one).
3. Name the four factors affecting diffusion rate that the syllabus specifies.
4. Why is diffusion a reliable topic to secure fully on either tier?
5. What is the key vocabulary Supplement students must apply to plant-cell osmosis scenarios?

Answers: 1. Active transport; energy comes from respiration. 2. Water diffuses through partially permeable membranes by osmosis, moving into and out of cells through the cell membrane. 3. Surface area, temperature, concentration gradient, and distance. 4. Because diffusion has no Supplement-only content — the same statements apply at both Core and Extended tier. 5. Turgid, turgor pressure, plasmolysis and flaccid.