The big idea: Some substances cross the cell membrane without the cell spending any energy. This is passive transport.
The two simplest passive processes are simple diffusion and osmosis.
Both happen because particles move from where there are more of them to where there are fewer — they move down a gradient, all on their own.
Simple diffusion (first panel): small, non-polar molecules slip straight through the phospholipid bilayer, down the concentration gradient — no protein and no ATP needed.
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- Passive transport
- Movement of a substance across a membrane that does NOT require the cell to use energy (ATP). It happens down a gradient.
- Concentration gradient
- A difference in concentration between two regions — a higher concentration on one side of the membrane and a lower concentration on the other.
- Simple diffusion
- The net movement of small or non-polar particles down their concentration gradient, passing directly through the phospholipid bilayer.
- Osmosis
- The net movement of water molecules across a partially permeable membrane, from a region of higher water potential (more dilute) to a region of lower water potential (more concentrated).
- Partially permeable membrane
- A membrane that lets some substances through (such as water) but not others; the plasma membrane is partially permeable.
Net movement is the key word: Particles are always moving in both directions. Diffusion and osmosis describe the net (overall) movement — the direction in which more particles travel.
Net movement is from high concentration to low for diffusion, and from dilute to concentrated for water in osmosis. It stops being net once both sides are equal.
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Not everything can slip straight through the membrane. The bilayer has a hydrophobic (water-hating) core, so what gets through by simple diffusion depends on the molecule.
Small, non-polar molecules — such as oxygen (O₂) and carbon dioxide (CO₂) — and lipid-soluble molecules — such as steroid hormones — pass directly through the bilayer. Large or charged particles cannot, and need a protein instead (covered in 2.3.3).
| Type of molecule | Crosses the bilayer by simple diffusion? | Why |
|---|---|---|
| Small non-polar (O₂, CO₂) | Yes — easily | Small and not repelled by the hydrophobic core |
| Lipid-soluble / steroid (e.g. oestradiol) | Yes | Non-polar, so it dissolves into the hydrophobic core and passes through |
| Water (small but polar) | Slowly on its own; quickly via aquaporins | Polar, so it is slowed by the hydrophobic core; aquaporins give it a fast route |
| Large or charged (glucose, ions) | No | Too large, or repelled by the hydrophobic core — they need a protein |
Why non-polar molecules pass straight through: The middle of the bilayer is made of fatty acid tails, which are non-polar / hydrophobic.
A non-polar molecule (like a steroid hormone or O₂) is not repelled by this core — it dissolves into it and slips out the other side.
A polar or charged particle is repelled by the hydrophobic core, so it cannot use simple diffusion.
Osmosis — water moves toward the concentrated side: Water is small but polar, so it diffuses across the membrane slowly on its own. This special case of diffusion is called osmosis.
Water moves from where the solution is more dilute (more water, higher water potential) to where it is more concentrated (less water, lower water potential).
Many cells also have aquaporins — protein channels that let water cross much faster — but osmosis is still passive and still moves water down its gradient.
- Water potential
- A measure of how 'free' the water in a solution is to move. Pure water has the highest water potential; adding solute lowers it. Water moves from higher to lower water potential.
- Aquaporin
- A channel protein in the membrane that allows water molecules to cross quickly. It speeds up osmosis but does not change its direction, and uses no ATP.
Osmosis in an animal cell: in a hypotonic solution water enters and the cell swells (and may burst); in an isotonic solution there is no net movement; in a hypertonic solution water leaves and the cell shrinks.
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Rate of diffusion depends on the gradient: The steeper the concentration gradient, the faster the net diffusion.
If a cell is using up oxygen inside, and the outside oxygen concentration is raised, the difference across the membrane is larger — so oxygen diffuses in faster.
Temperature (faster-moving particles) and the surface area of the membrane also raise the rate; a thicker membrane lowers it. The size of the gradient is the one examiners ask about most.
Simple diffusion
- Moves small / non-polar molecules (O₂, CO₂, steroids)
- Straight through the phospholipid bilayer
- Down the concentration gradient (high → low)
- Passive — no ATP; faster when the gradient is steeper
Osmosis
- Moves water only
- Across a partially permeable membrane
- From dilute → concentrated (high → low water potential)
- Passive; aquaporins speed it up but use no ATP
A memory hook: Osmosis = the diffusion of water. Diffusion is for the dissolved particles; osmosis is the water itself moving the other way, toward the concentrated side, to even things out.
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How this is tested: Paper 1B data questions love membrane transport. A cell (often an egg cell) is placed in a solution and its mass is measured over time; you State the net direction of water movement from the trend (mass rising → water entering; mass falling → water leaving).
A short Explain then asks how aquaporins assist water movement.
On Paper 1A you might match a molecule to its route — a non-polar steroid hormone crosses via the phospholipid bilayer, not a protein.
On Paper 2 an Explain can ask how the external oxygen concentration affects the rate of oxygen diffusion into a cell.
IB-style question — read the net direction of water movement from data
A cell is placed in a salt solution. Over the next four minutes its mass steadily decreases. State the net direction of water movement across the plasma membrane, and explain what this tells you about the solution. [3]
How to score all three marks
- Read the trend. The cell's mass decreases, so it is losing water — water is moving out of the cell.
- State the net direction. The net movement of water is out of the cell, across the plasma membrane, by osmosis.
- Explain what it means. Water moves toward the more concentrated solution, so the outside solution must be more concentrated (lower water potential) than the cell — it is hypertonic to the cell. (Mark 1: water moves out. Mark 2: by osmosis / down the water-potential gradient. Mark 3: outside solution is more concentrated / hypertonic.)
Final answer
Net water movement is OUT of the cell (mass falls), by osmosis, because the outside solution is more concentrated (hypertonic / lower water potential) than the cell.
Why this scores full marks: It uses the data (mass falling → water leaving), names the process (osmosis) and explains the cause (the outside is more concentrated).
A common slip is stating the direction but never linking it to the water-potential difference — the 'explain' mark needs that cause.
| Feature | Simple diffusion | Osmosis |
|---|---|---|
| What moves | Any small or non-polar molecule (e.g. O₂, CO₂, a steroid hormone) | Water molecules only |
| Direction | Down the concentration gradient (high → low) | From higher water potential to lower (dilute → concentrated) |
| Across what | Straight through the phospholipid bilayer | Across a partially permeable membrane |
| Uses a protein? | No — molecules pass between the phospholipids | No (faster through aquaporin channels, but still passive) |
| Uses energy (ATP)? | No — it is passive | No — it is passive |