Lesson 4 of 5 / Membranes and transport
Pay energy to build a gradient
How can uptake continue when the inside already has more?
I can explain why active transport needs membrane proteins and an energy supply.
I can explain diffusion, osmosis, facilitated diffusion, active transport, endocytosis and exocytosis.Syllabus
Syllabus 8876, Core 1(j). Explain diffusion, osmosis, facilitated diffusion, active transport, endocytosis and exocytosis.
About 5 min
Make a guess. You are not marked.
A cell takes up ions slowly but against their gradient. Is this active transport?
Show the answer
Active means against the gradient.
Active transport couples energy to movement against a gradient. Speed is not part of the definition.
The key idea
Active transport couples an energy source to selective movement against a gradient, enabling cells to maintain unequal distributions.
Explore the idea
Can the pump maintain uphill movement?
ATP-driven conformational changes can move the ion against its gradient. The protein remains in the membrane and is reused.
Original simplified bilayer model. One highlighted crossing represents a mechanism, not a measured rate. Passive molecules move both ways; the gradient determines net movement.
Explanation
Net movement against a concentration or electrochemical gradient cannot be maintained by diffusion alone. Active transport couples that movement to an energy source. In a simple ATP-driven pump model, ATP hydrolysis powers changes in a carrier protein that move the bound substance.
The protein is selective because its binding sites interact with particular substances. A conformational change exposes the site to the other side, releases the substance and returns the carrier to its original state. The protein is reused; it is not transported through the membrane with the solute.
Maintaining ion gradients supports cell activities and uptake. If ATP production is severely reduced, an ATP-driven pump slows and the maintained gradient may dissipate through other routes. The effect is not instantaneous proof that every membrane process has stopped, because passive transport can still occur.
Step by step
- 1
State the uphill direction
Identify which side has the higher concentration or relevant electrochemical potential.
- 2
State energy coupling
ATP hydrolysis can drive changes in the transport protein.
- 3
State the consequence
Cells can accumulate or exclude substances beyond the distribution expected from passive movement.
Worked example
Uptake after ATP falls
A cell accumulates an ion against its gradient. After ATP production is inhibited, uptake falls but oxygen still enters. Explain the difference.
One way to explain it
The ion uptake depends on energy-coupled active transport, so reduced ATP can reduce pumping. Oxygen can continue entering by simple diffusion if its gradient persists; diffusion does not directly require ATP.
Why this answer works
- Classify each substance separately.
- Link the intervention to the energy-dependent mechanism.
Watch out for this
A student says: "Active transport means that a substance is moving quickly." What is wrong with this?
Show the answer
Active describes energy-coupled movement against a gradient, not its speed.
Check your understanding
Which extra feature is needed to explain sustained net uptake against a gradient?
Show the answer
Energy coupling to a suitable transport protein.
This can drive the uphill movement.
Quick recall
How can uptake continue when the inside already has more?