Why is ATP needed for active transport in IB Biology?

In IB Biology, ATP is needed because active transport moves substances against the concentration gradient, which is energetically unfavorable. Diffusion happens spontaneously because random motion makes “down the gradient” statistically likely. Pushing molecules from low concentration to high concentration requires an input of energy to change protein conformation and move the substance. ATP provides that energy directly in primary active transport. In secondary active transport, ATP is still involved, just earlier, because ATP-driven pumps create the gradient that powers cotransport. Saying “against the gradient, so ATP is required” is usually a safe scoring sentence.

Is facilitated diffusion passive or active in IB Biology?

Facilitated diffusion is passive transport in IB Biology because it does not use ATP. Even though it involves membrane proteins, the key criterion is direction: molecules still move down their concentration gradient. The proteins simply provide a pathway through the hydrophobic bilayer for polar molecules or ions. Another mark-friendly point is that facilitated diffusion can be selective, because specific channels transport specific ions. It can also be saturable, because there are limited numbers of transport proteins. Those details help you stand out in longer explanations.

How do I avoid mixing up osmosis and diffusion in IB Biology questions?

Start by identifying what is moving. In IB Biology, osmosis refers specifically to water molecules moving across a partially permeable membrane. Diffusion can involve any particles, including gases and solutes, and does not require a partially permeable membrane. Then identify the gradient: osmosis is driven by differences in solute concentration (or water potential), while diffusion is driven by the concentration gradient of the particle itself. Finally, remember common outcomes: hypotonic environments can cause cells to swell, while hypertonic environments can cause cells to shrink. If you can state those three steps clearly, you can usually rescue any transport explanation under time pressure.

IB Biology: Passive vs Active Transport Differences

IB Biology

RevisionDojo Team

•12/1/2025•5 min read

If you have ever stared at a membrane diagram and felt like the arrows were judging you, you are not alone. In IB Biology, membrane transport is one of those topics that seems simple until an exam question quietly asks, “Explain how direction, energy, and proteins differ.” Suddenly, diffusion and active transport blur together.

This post clears that up. You will walk away knowing the key differences between passive and active transport across cell membranes and how to phrase them the way IB markschemes like.

Quick checklist: passive vs active transport (IB Biology)

Use this as your fast exam filter in IB Biology:

Direction: passive transport goes down a concentration gradient; active transport goes against it.
Energy: passive transport uses no ATP; active transport requires energy (often ATP).
Proteins: passive transport may use channels/carriers; active transport uses pump proteins or cotransporters.
Outcome: passive transport trends toward equilibrium; active transport can build and maintain gradients.

For syllabus-aligned practice, start with B2.1 Membranes and membrane transport.

Passive transport in IB Biology: letting physics do the work

In IB Biology, passive transport is about particles moving because random motion plus a gradient creates a net flow. No ATP. No “push.” Just probability doing its thing.

Simple diffusion

Small, nonpolar molecules move straight through the phospholipid bilayer. Oxygen and carbon dioxide are classic examples, and the phrase to remember in IB Biology is “down the concentration gradient.”

Revision help: Simple diffusion across membranes notes.

Facilitated diffusion

Some substances cannot cross the hydrophobic core, so they use membrane proteins (channels or carriers). It is still passive transport because movement remains down the gradient.

If you want exam-style practice on protein-mediated transport, try the B2.1 Questionbank.

Osmosis

Osmosis is the passive movement of water across a selectively permeable membrane, often via aquaporins. In IB Biology, precision matters: it is water moving, not solute.

Deepen the definition and common mistakes with What is osmosis?.

Active transport in IB Biology: paying for direction

Active transport is what cells use when “downhill” is not good enough. In IB Biology, the headline is simple: substances move from low to high concentration, so energy must be supplied.

Primary active transport (ATP directly)

ATP powers pump proteins that change shape and move ions or molecules across the membrane. The sodium-potassium pump is a famous example in animal cells.

Support your explanations with Pump proteins for active transport notes.

Secondary active transport (energy indirectly)

Here, one gradient (often built by primary active transport) provides the “payment” to move another substance against its gradient. In IB Biology, this is where students often forget to mention that ATP is used earlier to create the gradient.

The difference that earns marks: equilibrium vs control

Passive transport slows as equilibrium approaches. Active transport keeps going as long as energy is available. That distinction shows up everywhere in IB Biology: nutrient uptake, nerve impulses, and maintaining stable internal conditions.

To connect transport to broader cell biology, browse IB Biology Cell Biology resources and the full IB Biology revision notes hub.

Final takeaway: master transport with RevisionDojo

In IB Biology, the key differences between passive and active transport across cell membranes come down to gradient direction, ATP use, and the transport proteins involved. Once you can say those clearly, diagrams stop feeling like riddles.

To lock it in, use RevisionDojo’s Study Notes, Flashcards, and Questionbank for B2.1, then test yourself with AI Chat explanations and Grading tools. When exams get close, the Predicted Papers, Mock Exams, and Tutors can turn shaky definitions into confident marks. Start here: B2.1 Membranes and membrane transport.

Quick checklist: passive vs active transport (IB Biology)