The main difference between diffusion and active transport is the direction of net movement relative to a gradient and whether an additional energy source is required. Diffusion is passive: particles move down a concentration or electrochemical gradient without cellular energy being supplied. Active transport uses membrane proteins and an energy source to move specific particles against a concentration or electrochemical gradient.
For IB Biology, stating only that diffusion moves particles from high to low concentration while active transport moves them from low to high concentration is a useful starting point, but a complete answer should also mention ATP, random molecular motion, membrane proteins, specificity, and why cells need both processes.
Diffusion vs active transport at a glance
| Feature | Diffusion | Active transport |
|---|---|---|
| Type of transport | Passive | Active |
| Direction of net movement | Down a concentration or electrochemical gradient | Against a concentration or electrochemical gradient |
| Typical concentration direction | Higher to lower concentration | Lower to higher concentration |
| Additional energy required | No | Yes |
| Direct use of ATP | No | Yes for primary active transport; indirect for secondary active transport |
| Membrane protein required | No for simple diffusion; yes for facilitated diffusion | Yes, usually a pump or cotransporter |
| Cause of movement | Random molecular motion and an existing gradient | Energy coupled to a conformational change or another ion gradient |
| Specificity | Simple diffusion depends mainly on particle size and chemical properties; facilitated diffusion is specific | Specific transport proteins move particular particles |
| Outcome | Tends to reduce a gradient | Can create or maintain a gradient |
| Example | Oxygen moving through a phospholipid bilayer | Sodium ions moved by the sodium-potassium pump |
This comparison is central to IB Biology cell membranes, particularly topic B2.1 Membranes and membrane transport. The broader relationships among membrane structure, permeability, proteins, and transport are covered in RevisionDojo’s IB Biology Cell Biology Explained exam-focused guide.
What is diffusion?
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration as a result of their random motion. It is described as passive because the cell does not supply ATP or another additional energy source to drive the movement.
Particles are moving randomly in all directions. When one region contains more particles than another, more particles are statistically likely to leave the high-concentration region than enter it. This produces a net movement down the concentration gradient even though individual particles continue moving in both directions.
Diffusion does not always involve a membrane. For example, a dye spreading through water is diffusion. In membrane questions, however, IB students must determine whether particles cross directly through the phospholipid bilayer or through a membrane protein.
Simple diffusion across cell membranes
In simple diffusion, particles move directly between phospholipids in the bilayer. Small non-polar molecules such as oxygen and carbon dioxide can cross this way because they dissolve in the membrane’s hydrophobic interior.
The current IB Biology course specifically uses oxygen and carbon dioxide as examples. Oxygen may diffuse into a respiring cell because respiration keeps its internal oxygen concentration relatively low, while carbon dioxide can diffuse out when it is being produced inside the cell.
Ions and most large polar molecules cannot cross the hydrophobic core freely. This selective barrier is explained further in RevisionDojo’s guide to phospholipids and selectively permeable membranes and its focused simple diffusion study notes.
Facilitated diffusion is still diffusion
Facilitated diffusion occurs when particles move down their gradient through specific channel or carrier proteins. The protein provides a hydrophilic route through the membrane, but it does not supply the energy that determines the direction of net movement.
This is why the presence of a membrane protein does not automatically mean active transport. A sodium ion passing down its electrochemical gradient through an open sodium channel is undergoing facilitated diffusion, not active transport. RevisionDojo’s channel protein notes provide targeted practice with this distinction.
What is active transport?
Active transport is the energy-dependent transfer of specific particles across a membrane against their concentration or electrochemical gradient. It is carried out by integral membrane proteins, commonly described as pumps or transporters.
For an uncharged particle, movement against the concentration gradient normally means movement from lower to higher concentration. For an ion, both concentration and electrical charge affect movement, so the more precise term is electrochemical gradient.
A pump typically works through the following sequence:
- A specific particle binds to the transport protein on one side of the membrane.
- Energy is supplied to the protein.
- The protein undergoes a conformational change, meaning its three-dimensional shape changes.
- The particle is exposed to the other side of the membrane and released.
- The protein returns to its original conformation so the cycle can repeat.
In the IB syllabus statement for pump proteins, students are expected to understand that pumps use energy from adenosine triphosphate (ATP) to transfer specific particles and can therefore move them against a concentration gradient. RevisionDojo’s active transport pump protein notes reinforce this mechanism.
Primary active transport
In primary active transport, the transport protein uses an energy source such as ATP hydrolysis directly. ATP is hydrolysed to ADP and inorganic phosphate, and the transfer of a phosphate group can change the pump’s conformation.
The sodium-potassium pump, or Na⁺/K⁺ ATPase, is an important example in animal cells. During each complete cycle, it uses one ATP to move three Na⁺ ions out of the cell and two K⁺ ions into the cell, with both ions being transported against their respective electrochemical gradients.
This maintains unequal ion distributions across the plasma membrane. Those gradients contribute to membrane potential, osmotic regulation, nerve function, and the energy supply for some forms of cotransport.
Secondary active transport
In secondary active transport, the transporter does not hydrolyse ATP directly. Instead, movement of one particle down its electrochemical gradient supplies the energy needed to move another particle against its gradient.
For example, a sodium-glucose cotransporter can couple Na⁺ movement down its electrochemical gradient to glucose uptake against the glucose concentration gradient. The sodium gradient was previously established by an ATP-driven pump, so the process ultimately depends on cellular energy even though the cotransporter itself does not use ATP directly.
This distinction is particularly relevant to higher-level study. It prevents the inaccurate claim that every active transporter must bind and hydrolyse ATP itself.
Why cells need both diffusion and active transport
Diffusion is efficient when a useful gradient already exists and the membrane is permeable to the particle. It allows gas exchange and rapid ion movement without requiring the cell to spend ATP on each transported particle.
However, diffusion alone would gradually reduce concentration differences. Cells often need to preserve internal conditions that are different from their surroundings, so they use active transport to create and maintain gradients.
| Cellular need | Relevant mechanism | Why it is suitable |
|---|---|---|
| Oxygen entering a respiring cell | Simple diffusion | Oxygen is small and non-polar, and respiration maintains a gradient |
| CO₂ leaving a cell | Simple diffusion | CO₂ can cross the bilayer down its gradient |
| Ions moving rapidly through an open channel | Facilitated diffusion | The channel allows passive movement down an electrochemical gradient |
| Maintaining high K⁺ and low Na⁺ concentrations inside an animal cell | Active transport | The sodium-potassium pump maintains non-equilibrium ion distributions |
| Absorbing a nutrient when its concentration is lower outside | Active transport or cotransport | Movement against the nutrient’s gradient requires an energy source |
| Restoring ion gradients after nerve activity | Active transport | Pumps counteract the gradient-reducing effects of ion diffusion |
Diffusion and active transport are therefore complementary rather than competing processes. Active transport establishes gradients, while diffusion can use the stored potential energy in those gradients to drive later movement.
Factors affecting diffusion and active transport
Factors affecting diffusion rate
A steeper concentration gradient generally produces faster net diffusion because the difference in particle concentration across the distance is greater. Other important factors include:
- Surface area: A larger membrane area provides more opportunities for particles to cross.
- Diffusion distance: A thinner barrier allows faster diffusion.
- Temperature: Higher temperature increases molecular kinetic energy and random motion, within biologically relevant limits.
- Particle size: Smaller particles generally diffuse more readily.
- Membrane properties: Lipid solubility, polarity, charge, and the availability of channel proteins affect permeability.
- Number of transport proteins: Facilitated diffusion may be limited by the number of available channels or carriers.
At equilibrium, particles do not stop moving. They continue crossing in both directions, but the rates are equal, so there is no net movement.
Factors affecting active transport rate
Active transport depends on functioning transport proteins and an adequate energy supply. Its rate may be influenced by:
- ATP availability for ATP-driven pumps
- Rate of cellular respiration
- Number and activity of pump proteins
- Availability of the transported particle
- Temperature and pH, because transport proteins have specific three-dimensional structures
- The size of the gradient opposing transport
- Inhibitors that interfere with respiration or pump function
Carrier-mediated transport can reach a maximum rate when all available transport proteins are occupied. This idea helps explain why increasing solute concentration does not always produce an unlimited increase in transport rate.
Common misconceptions in IB Biology
“All movement through proteins is active transport”
This is incorrect. Facilitated diffusion uses channel or carrier proteins but remains passive because particles move down their gradient without an additional energy source. Active transport uses a pump or coupled transporter to achieve energetically uphill movement.
“Diffusion stops at equilibrium”
Individual particles continue moving randomly at equilibrium. What stops is the net movement, because equal numbers cross in each direction per unit time.
“Active transport always moves substances into cells”
Active transport can move particles into or out of a cell. Direction depends on the orientation and function of the transporter. The sodium-potassium pump, for example, transports Na⁺ out and K⁺ in during the same cycle.
“Active transport always uses ATP directly”
This is accurate for ATP-driven primary active transport but not for all active transport. Secondary active transport uses energy stored in another particle’s electrochemical gradient, although ATP-dependent pumps usually established that gradient.
“Diffusion means particles only move from high to low concentration”
Particles move randomly in both directions. High-to-low describes the overall net movement, not the path followed by every individual particle.
“Osmosis and active transport are direct opposites”
Osmosis is the passive net movement of water across a selectively permeable membrane, whereas active transport concerns energy-dependent transport of specific solutes. RevisionDojo’s IB Biology osmosis exam guide explains the distinction among osmosis, diffusion, and active transport.
How to answer diffusion vs active transport exam questions
For a short comparison, identify at least two linked differences rather than listing unrelated facts. A precise response could state:
Diffusion is the passive net movement of particles down a concentration gradient due to random molecular motion, whereas active transport uses specific membrane proteins and an energy source to move particles against a concentration or electrochemical gradient.
For a longer question, structure the comparison around matched features:
- Direction: down the gradient versus against the gradient
- Energy: no additional cellular energy versus an energy input
- Protein involvement: optional for diffusion, essential for active transport
- Effect on gradients: diffusion reduces gradients, while active transport can establish or maintain them
- Example: oxygen diffusion versus the sodium-potassium pump
If the command term is explain, include a mechanism. For active transport, mention specific binding, ATP or coupled energy, a conformational change, and release on the opposite side. Writing only “active transport needs energy” is rarely sufficient for a multi-mark explanation.
Avoid saying that diffusion requires no energy at all in a physical sense. Diffusing particles possess kinetic energy, but the cell does not expend ATP to produce their net movement. In IB answers, “does not require an input of cellular energy” is especially precise.
Students can consolidate the complete syllabus context through RevisionDojo’s B2.1 membranes and membrane transport resources, then apply the distinction using the IB Biology membrane transport Questionbank. Flashcards are useful for definitions, while Jojo AI can help diagnose why a comparison answer is missing a required mechanistic link.
Conclusion
The essential distinction in diffusion vs active transport is that diffusion produces net movement down a concentration or electrochemical gradient without an additional cellular energy input, while active transport uses energy and specific membrane proteins to move particles against such a gradient. Simple diffusion occurs through the bilayer, facilitated diffusion occurs through proteins, and active transport depends on pumps or coupled transporters.
Strong IB answers use the terms net movement, gradient, ATP, transport protein, and conformational change accurately. RevisionDojo’s B2.1 Study Notes, Flashcards, Questionbank, and Jojo AI are useful for progressing from memorising these definitions to applying them in unfamiliar membrane-transport questions.
Sources and referenced URLs
- Official IB Biology specimen papers
- Official IB Questionbank
- NCBI Bookshelf: Principles of membrane transport
- NCBI Bookshelf: Transport of small molecules
- NCBI Bookshelf: Active transporters and ion gradients
- OpenStax Biology 2e: Active transport
- RevisionDojo: IB Biology Cell Biology Explained
- RevisionDojo: B2.1 membranes and membrane transport
- RevisionDojo: Simple diffusion study notes
- RevisionDojo: Channel proteins for facilitated diffusion
- RevisionDojo: Pump proteins for active transport
- RevisionDojo: Phospholipids and selectively permeable membranes
- RevisionDojo: What is osmosis?
- RevisionDojo: Membrane transport Questionbank
