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The key difference
Diffusion is the net movement of particles from high to low concentration. Osmosis is specifically the movement of water molecules through a partially permeable membrane from a dilute to a concentrated solution. Active transport moves substances against the concentration gradient using energy from respiration.
These three transport mechanisms explain how substances enter and leave cells. Examiners love to test whether you can distinguish between them: especially the subtle difference between diffusion and osmosis, and why active transport requires energy when the other two do not. This page gives you the definitive comparison.
Feature
Diffusion
Osmosis
Active Transport
Definition
Net movement of particles from a region of higher concentration to lower concentration
Net movement of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane
Movement of ions or molecules from lower concentration to higher concentration against the concentration gradient
Substance moved
Any molecule or ion (e.g. Oxygen, carbon dioxide, glucose)
Water molecules only
Specific ions and molecules (e.g. Mineral ions into root hair cells)
Direction relative to gradient
Down the concentration gradient (high → low)
Down the water potential gradient (high water potential → low water potential)
Against the concentration gradient (low → high)
Energy required?
No: passive process
No: passive process
Yes: energy from respiration (ATP)
Membrane required?
Not necessarily (can occur in air or solution)
Yes: partially permeable membrane required
Yes: requires carrier proteins in the membrane
Carrier proteins needed?
No
No (uses aquaporins or passes between phospholipids)
Yes: specific carrier proteins needed
Example in the body
Oxygen diffuses from alveoli into blood capillaries
Water moves into root hair cells from soil water by osmosis
Root hair cells absorb mineral ions from soil against the concentration gradient
Why Osmosis Is Not Just "Diffusion of Water"
A common simplification is to call osmosis "diffusion of water", but this loses marks. Osmosis specifically requires a partially permeable membrane and refers to the net movement of water from a dilute solution (higher water potential) to a more concentrated solution (lower water potential). If water moves without a membrane, it is just diffusion: not osmosis.
Why Active Transport Needs Energy
Active transport moves substances against their concentration gradient: from where they are less concentrated to where they are more concentrated. This is like pushing a ball uphill: it does not happen spontaneously. The energy comes from respiration (specifically ATP). This is why cells that carry out lots of active transport, such as root hair cells and cells lining the small intestine, contain many mitochondria.
Linking All Three in a Single Scenario
Consider a root hair cell absorbing substances from the soil:
Osmosis: water enters the root hair cell because the cell sap is more concentrated than the soil water (lower water potential inside the cell)
Active transport: mineral ions (e.g. Nitrate, magnesium) are absorbed against the concentration gradient, requiring energy from respiration
Diffusion: oxygen from the soil air diffuses into the root hair cell down the concentration gradient for use in respiration
Try a comparison question (3 marks)
Explain why a root hair cell needs to use active transport to absorb mineral ions from the soil, rather than relying on diffusion. [3]
Show the answer and marking guidance
The concentration of mineral ions is higher inside the root hair cell than in the soil solution
So mineral ions must move against the concentration gradient
Active transport requires energy from respiration / ATP to move substances against the gradient using carrier proteins
Original Cambridge-style practice written for this site.
Common comparison mistakes
When answering a compare question, write matched pairs rather than two separate descriptions.
Defining osmosis without mentioning a partially permeable membrane: this is a required part of the definition
Saying active transport needs "energy" without specifying it comes from respiration
Forgetting that diffusion can occur without a membrane (e.g. In air)
Confusing the direction: active transport goes from low to high concentration, not high to low