4.2 Movement into and out of Cells

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe and explain simple diffusion, facilitated diffusion, osmosis, active transport, endocytosis, and exocytosis.
  2. Investigate simple diffusion and osmosis using plant tissue, Visking (dialysis) tubing, and agar.
  3. Show that surface area to volume (SA:V) ratios decrease as size increases, by calculating SA and volume of simple 3D shapes.
  4. Investigate how changing the SA:V ratio affects the rate of diffusion using agar blocks of different sizes.
  5. Investigate the effects of placing plant tissues in solutions of different water potentials, and use the results to estimate the water potential of the tissue.
  6. Explain water movement between cells and solutions using water potential, and explain the different effects on plant cells and animal cells.

Section 1: Transport Processes — An Overview

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Substances need to move into and out of cells constantly. Cells need to take in nutrients and oxygen, and remove waste products. There are several different ways this can happen, and they fall into two main categories:

  • Passive transport — does not require energy (ATP). Substances move down their concentration gradient (from high to low concentration) using only the natural movement of molecules.
  • Active transport — does require energy (ATP). Substances are moved against their concentration gradient (from low to high concentration).

The main processes are:

ProcessEnergy needed?Direction of movement
Simple diffusionNoHigh → low concentration
Facilitated diffusionNoHigh → low concentration
OsmosisNoHigh → low water potential
Active transportYes (ATP)Low → high concentration
EndocytosisYes (ATP)Into the cell (bulk)
ExocytosisYes (ATP)Out of the cell (bulk)

Section 2: Simple Diffusion

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What Is It?

Simple diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration, directly through the phospholipid bilayer (the fatty layer that makes up the cell membrane). This movement happens because all molecules have kinetic energy — they are always randomly moving around. Over time, this random movement causes molecules to spread out evenly.

The movement is described as net movement, meaning more molecules move from high to low than in the opposite direction. Eventually the concentration on both sides equalises — this is called equilibrium.

What Molecules Can Cross This Way?

Only certain molecules can pass directly through the phospholipid bilayer by simple diffusion. They must be:

  • Small in size
  • Non-polar (not electrically charged) — this is important because the bilayer's core is hydrophobic (water-hating), so charged molecules get blocked

Examples include:

  • Oxygen (O₂)
  • Carbon dioxide (CO₂)
  • Lipid-soluble vitamins (A, D, E, K)
  • Steroid hormones
  • Water molecules (even though water is slightly polar, its tiny size allows it to squeeze through)

Factors Affecting the Rate of Simple Diffusion

The rate of diffusion is affected by the following factors:

  • Concentration gradient — the bigger the difference in concentration between two sides, the faster diffusion occurs. More molecules on one side means more random movement across.
  • Surface area — the larger the surface area through which diffusion happens, the more molecules can cross at once. This directly increases the rate.
  • Temperature — higher temperature gives molecules more kinetic energy, so they move faster and diffuse more quickly.
  • Molecular size (molecular mass) — larger, heavier molecules move more slowly, so they diffuse at a lower rate.
  • Membrane thickness — a thicker membrane means molecules have a greater distance to cross, slowing diffusion.

These relationships are summarised by Fick's Law:

Rate of diffusion ∝ (Concentration gradient × Surface area) ÷ (Membrane thickness × Molecular mass)

In plain English: diffusion is faster when there's a large difference in concentration and a big surface area, and it's slower when the membrane is thick or the molecules are large.

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