What you need to know
- 2.5(i–ii)active transportMovement of molecules or ions against their concentration gradient through carrier proteins, using energy from ATP., ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi., carrier proteinA membrane protein that binds a specific molecule and changes shape to move it across the membrane, in facilitated diffusion or active transport., endocytosisTaking material into a cell by surrounding it with cell membrane to form a vesicle. It uses ATP. and exocytosisReleasing material from a cell when a vesicle fuses with the cell surface membrane. It uses ATP..
Sodium–glucose cotransport in the real-world example is explained as enrichment. It does not add a new required mechanism to the specification statements listed here.
By the end, you should be able to…
- Explain how active transport can maintain a concentration difference.
- Link ATP hydrolysisA reaction that breaks a covalent bond using a molecule of water. to protein conformational change.
- Distinguish endocytosis and exocytosis from transport through a pore.
Build your understanding
An energy source changes what is possible
active transportMovement of molecules or ions against their concentration gradient through carrier proteins, using energy from ATP. moves a substance against its concentration gradientThe difference in concentration of a substance between two regions., or for ions against the relevant electrochemical gradient, using energy. Specific carrier proteinA membrane protein that binds a specific molecule and changes shape to move it across the membrane, in facilitated diffusion or active transport. or pumps bind the transported substance and change shape. In directly ATP-driven pumps, ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. hydrolysisA reaction that breaks a covalent bond using a molecule of water. supplies energy for this cycle.
ATP is an immediate energy source: coupling its hydrolysis to the transport mechanism enables a process that would not occur by passive movement alone. Cells regenerate ATP through metabolism; saying “energy is created by ATP” is inaccurate.
Maintain a gradient despite leakage
An active transport mechanism can maintain a higher concentration on one side even while passive leakage acts in the opposite direction. If ATP supply is reduced, active pumping can slow; the existing gradient may then decline as passive movement continues.
The presence of a carrier does not by itself establish active transport. A carrier can also mediate facilitated diffusionPassive movement of molecules down their concentration gradient through channel or carrier proteins.. Look for evidence about gradient direction, energy dependence and other possible effects of the experimental treatment.
Move bulk material using vesicles
In endocytosisTaking material into a cell by surrounding it with cell membrane to form a vesicle. It uses ATP., part of the cell surface membraneThe partially permeable membrane around a cell that controls which substances enter and leave. encloses material and pinches off as a vesicle inside the cell. This can bring in particles or fluid that do not pass through a channel. In exocytosisReleasing material from a cell when a vesicle fuses with the cell surface membrane. It uses ATP., a vesicle fuses with the cell-surface membrane and releases its contents outside.
Vesicle transport requires energy and membrane rearrangement. It is not defined by movement up a soluteA substance that is dissolved in a solvent. gradient. Exocytosis adds vesicle membrane to the surface; endocytosis removes a portion from it. Membrane recycling helps balance these processes.
Follow the process
Direct active transport: the substance binds a specific carrier on one side of the membrane → ATP is hydrolysed → the carrier changes shape → the substance is released on the other side, against its concentration or electrochemical gradient. If ATP supply falls, pumping slows and passive leakage can reduce the gradient.
Vesicle transport is a branch, not the same line. Endocytosis: membrane encloses material → a vesicle pinches off inside the cell. Exocytosis: a vesicle fuses with the cell-surface membrane → contents leave. Neither is a solute being pushed through a pore. An answer can start with the outcome, a higher concentration inside, and then give the ATP and shape-change mechanism.
NOT: endocytosis is entry. Exocytosis is exit. Neither is a solute squeezed through a pump.
- ATP is hydrolysed. Energy is not created
- If ATP supply falls, pumping slows and leakage can reduce the gradient
- A carrier can also do facilitated diffusion: check the direction and the energy
- Exocytosis adds membrane; endocytosis removes it
Two traps this diagram blocks
- Do not reverse endocytosis and exocytosis. Endo means into the cytoplasm.
- Do not say the pump works because it is “the opposite of diffusion” without naming ATP and the shape change.
From the diagram alone
Question
Describe how a carrier protein and ATP can move an ion into a cell against its concentration gradient. [3]
Show a model answer
The ion binds a specific carrier on the outside. ATP is hydrolysed and the carrier changes shape. The ion is released inside the cell, where its concentration is already higher.
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: the ion is more concentrated inside the cell. Y: a pump uses ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. hydrolysisA reaction that breaks a covalent bond using a molecule of water. to change shape and release the ion on the inside. Z: the gradient is maintained even though some ions leak back. Incomplete: “It uses energy, so it is the opposite of diffusionThe net movement of molecules from a region of higher concentration to a region of lower concentration. It is passive..” Improved: “ATP hydrolysis changes the carrier’s shape so the ion is moved against its gradient.” The improvement names the carrier and the direction.
Keep vesicles separate from pores
For endocytosisTaking material into a cell by surrounding it with cell membrane to form a vesicle. It uses ATP. or exocytosisReleasing material from a cell when a vesicle fuses with the cell surface membrane. It uses ATP., describe membrane enclosure or fusion. Do not describe a large particle squeezing through a carrier protein.
Key explanations
- Pumping can maintain a gradient even while some substance leaks back passively. Less ATP means less pumping, so the gradient can fall.
- A carrier protein is not proof of active transportMovement of molecules or ions against their concentration gradient through carrier proteins, using energy from ATP.. Carriers also carry out facilitated diffusionPassive movement of molecules down their concentration gradient through channel or carrier proteins. down a gradient.
Avoid this misconception
The claim: Every energy-dependent transport process is simply diffusion in reverse.
The correction: Pumps can move solutes against gradients, but endocytosis and exocytosis move enclosed material by vesicles and membrane rearrangement.
How a cell keeps more of something on one side, using ATP
Left alone, ions spread out until both sides are equal. Cells keep them unequal on purpose: more sodium outside, more potassium inside. They do it with protein pumps that use energy from ATP. For big things, cells use a different trick: they wrap them in pieces of membrane called vesicles.
What the model shows
A piece of cell membrane with one pump protein, the ions on each side, and a small supply of ATP. A second view shows vesicles carrying material into and out of the cell.
What to look for
Where the ions are released compared with where there are already more of them. How many ions move for each ATP. What happens to the graph when the ATP runs out. In the vesicle view, how the membrane folds in or joins up.
What it tells us
Active transport moves substances against their concentration gradient. A carrier protein changes shape when ATP gives it a phosphate. The gradient only lasts while ATP keeps the pump going, because ions leak back. Vesicles move large things without them crossing through a protein.
Be careful what you claim
The ion numbers are tiny, the pump is slowed down so you can follow it, and the leak rates are made up. The one-ion pump is a simplified version; the real sodium–potassium pump moves 3 Na⁺ out and 2 K⁺ in per ATP.
Try it in the model: just watch the pump go round a few times and read the caption under it at each step.
Active transport and vesicles
A carrier protein that uses ATP to move ions against their gradient while some leak back, and vesicles that carry bulk material in and out. Simplified: the one-ion mode is a teaching model; the real Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in for each ATP.
What you see
- A membrane with outside the cell above and the cytoplasm below. One large carrier protein (the pump) goes round its cycle, with ATP, ADP and phosphate shown.
- Counts of Na⁺ and K⁺ on each side, an ATP store and counter, and a graph of the concentration differences over time. A few leak channels let ions drift back.
- A second view with vesicles: endocytosis and exocytosis at the cell-surface membrane.
How to use it
- Choose the simple one-ion pump or the real Na⁺/K⁺ pump. Turn the ATP supply on or off, and change the speed.
- In the vesicle view, press Endocytosis or Exocytosis and watch the membrane.
- Start again resets the counts to equal on both sides.
What it shows
- Active transport moves ions against their concentration gradient using energy from ATP hydrolysis, through a carrier that changes shape.
- The Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in per ATP.
- A gradient is maintained against passive leak only while ATP is available.
- Endocytosis and exocytosis move bulk material in vesicles: no carrier, no pore.
Limits of the model
- Simplified: the one-ion mode is a teaching model. The real Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in for each ATP, as in the second mode.
- Ion numbers are tiny and the cycle is slowed down a lot (real pumps cycle about 100 times a second). Leak rates are chosen so the effect is visible.
- Charge and membrane potential are not modelled here; see the action potential model (8A 3). Vesicle sizes, speeds and ATP costs are illustrative.
Guided tour · step 1 of 8
- Watch the carrier in the simulator go round its cycle a few times.
- Follow the caption under the membrane, and the counts in Readings and questions below the simulator.
Ions bind on one side, ATP phosphorylates the carrier, the carrier changes shape, and the ions are released on the other side.
Pump
The supply stands for respiration making ATP. With it off, the small ATP store runs down.
Readings and questions
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Why can a cell accumulate an ion against its gradient while ATP is available?
Show suggested answer
A specific pump couples an energy-releasing process such as ATP hydrolysis to conformational changes that transport the ion against its gradient.
Practice question
Retrieval and explanation · suggested answer
Describe how a secretory cell releases a protein packaged inside a vesicle.
Show suggested answer
The vesicle moves to and fuses with the cell-surface membrane. Its contents are released outside by exocytosis; the vesicle membrane becomes part of the surface membrane.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
An inhibitor reduces ATP production and solute uptake. Does this single observation prove that the solute crosses through an ATP-driven pump?
Show a hint
Consider indirect effects of ATP depletion.
Explore the reasoning
No. ATP depletion can affect several processes, including maintenance of ion gradients, vesicle movement and cell viability. The finding supports energy dependence, but further evidence about the transporter, gradient and cell condition is needed.
Recall this lesson
Lesson recall questions
Question 1 of 5: List four processes by which substances cross a cell-surface membrane.
Passive, active and bulk transport
Practice question
Retrieval and explanation · suggested answer
List four processes by which substances cross a cell-surface membrane.
Lesson: 2A 4: Active Transport
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Why rehydration research uses transport biology
2A · Active TransportOral rehydration salts contain a defined glucose–electrolyte mixture. Their use connects intestinal solute absorption with water movement; the formulation is not simply a claim that “more sugar is…Specification 2.5(i) · 2.5(ii) · 2.4Read & use the scienceClose article
Why this matters
Oral rehydration salts contain a defined glucose–electrolyte mixture. Their use connects intestinal solute absorption with water movement; the formulation is not simply a claim that “more sugar is better”. The WHO formulation illustrates why quantities and combinations matter. Sodium and glucose can enter intestinal cells together using a sodium gradient; an ATP-dependent pump helps maintain that gradient, and water follows changes in water potential. The cotransporter itself need not split ATP. This is an extension of the transport model, not a recipe for making a treatment at home. It shows how researchers combine several mechanisms to design a tested formulation rather than treating a single ingredient as a cure.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.5(i) understand what is meant by passive transport (diffusion, facilitated diffusion), active transport (including the role of ATP as an immediate source of energy), endocytosis and exocytosis
- 2.5(ii) understand the involvement of carrier and channel proteins in membrane transport
- 2.4 understand what is meant by osmosis in terms of the movement of free water molecules through a partially permeable membrane, down a water potential gradient
Use the science
Why can an indirectly energy-dependent uptake process slow when ATP production is inhibited?
Explore a possible response
The pump maintaining the driving ion gradient can slow, so the gradient becomes weaker and can no longer support the same uptake. The cotransporter need not hydrolyse ATP directly.
Explore the source
A medicine that blocks a glucose carrier
2A · Active TransportSome medicines lower blood glucose by changing what happens in the kidney rather than by supplying insulin. Empagliflozin inhibits a sodium–glucose cotransporter, reducing glucose reabsorption so more glucose…Specification 2.5(i) · 2.5(ii)Read & use the scienceClose article
Why this matters
Some medicines lower blood glucose by changing what happens in the kidney rather than by supplying insulin. Empagliflozin inhibits a sodium–glucose cotransporter, reducing glucose reabsorption so more glucose leaves in urine. A cotransporter couples the movement of two substances; this extends your knowledge of carrier proteins and energy-dependent transport. The glucose transporter does not have to split ATP itself for the process to depend indirectly on energy used to maintain an ion gradient. Blocking one transport route can therefore alter the movement of a different molecule. Detailed kidney anatomy and prescribing decisions are beyond this lesson.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.5(i) understand what is meant by passive transport (diffusion, facilitated diffusion), active transport (including the role of ATP as an immediate source of energy), endocytosis and exocytosis
- 2.5(ii) understand the involvement of carrier and channel proteins in membrane transport
Use the science
If a glucose carrier is inhibited, does that necessarily mean the phospholipid bilayer has become more permeable to glucose? Explain.
Explore a possible response
No. Carrier-mediated transfer can decrease while the lipid barrier remains unchanged. A change in transport rate does not by itself identify a change in bilayer permeability.