How does the direction of movement in active transport differ from facilitated diffusion?

Active transport moves substances against the concentration gradient (low to high), whereas facilitated diffusion moves them down the gradient (high to low). Active transport requires energy to overcome this gradient, while facilitated diffusion is a passive process.

Why can't channel proteins be used for active transport?

Channel proteins provide a fixed, water-filled pore that only allows substances to move down their concentration gradient. Active transport requires carrier proteins that can change shape to physically pump molecules against the gradient using ATP.

What is the difference between the energy requirements of osmosis and active transport?

Osmosis is a passive process that relies on the kinetic energy of water molecules moving down a water potential gradient. Active transport is an active process that requires metabolic energy released from the hydrolysis of ATP.

What is a common mistake regarding the role of mitochondria in active transport?

Students often forget that active transport is indirectly dependent on mitochondria. Since mitochondria produce the ATP required for the carrier proteins to change shape, any decrease in mitochondrial activity will slow down or stop active transport.

Why is it incorrect to say that active transport occurs until equilibrium is reached?

Active transport is designed to maintain a concentration difference, not eliminate it. It continues to pump substances to one side of the membrane to create or maintain a high concentration, moving away from equilibrium.

What happens to active transport if a cell is exposed to a respiratory poison like cyanide?

Active transport will stop because cyanide inhibits aerobic respiration, preventing the production of ATP. Without ATP, carrier proteins cannot undergo the conformational changes needed to move molecules against the gradient.

Define Active Transport.

Active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration and specific carrier proteins.

What is ATP and what is its role in membrane transport?

ATP (Adenosine Triphosphate) is the universal energy currency of the cell. In active transport, its hydrolysis provides the energy required for carrier proteins to change their shape and move solutes across the membrane.

What are carrier proteins in the context of active transport?

Carrier proteins are specific transmembrane proteins that possess binding sites for particular molecules. They use energy to switch between two conformational states, effectively pumping the solute across the membrane.

Why is active transport considered a 'selective' process?

It is selective because it relies on specific carrier proteins. Each protein has a binding site with a unique shape that only fits a particular molecule or ion, allowing the cell to control exactly which substances are pumped in or out.

Active Transport | OCR A-Level Biology

1. Definition & Core Concepts

Active Transport is the movement of molecules or ions across a cell membrane against a concentration gradient. Unlike passive processes, this movement does not happen spontaneously and requires the input of energy.
The process is mediated by carrier proteins, which are highly specific transmembrane proteins that bind to particular molecules or ions to facilitate their transfer across the lipid bilayer.
This mechanism allows cells to accumulate high concentrations of necessary substances, such as nutrients or ions, even when they are scarce in the external environment.

Diagram showing a carrier protein moving molecules from a low concentration area to a high concentration area using energy.

2. Underlying Principles

The primary energy source for active transport is Adenosine Triphosphate (ATP), which is produced during cellular respiration. The energy is released when ATP undergoes hydrolysis, breaking a phosphate bond to become ADP and inorganic phosphate.
The energy released from ATP hydrolysis is used to induce a conformational change (shape change) in the carrier protein. This physical movement shifts the bound molecule from one side of the membrane to the other.
Because the process relies on metabolic energy, the rate of active transport is directly influenced by factors that affect respiration, such as oxygen availability and temperature.

3. Methods & Mechanism

Step 1: Binding: The specific target molecule or ion binds to a receptor site on the carrier protein that is open to one side of the membrane.
Step 2: Phosphorylation/Energy Transfer: ATP binds to the protein and is hydrolyzed. The energy transfer causes the protein to change its three-dimensional shape.
Step 3: Release: The shape change closes the original binding site and opens the protein to the opposite side of the membrane, where the molecule is released.
Step 4: Recovery: The phosphate group is released from the protein, causing it to return to its original conformation, ready to repeat the cycle.

4. Key Distinctions

It is critical to distinguish active transport from passive transport mechanisms to understand how cells regulate their internal chemistry.

Feature	Facilitated Diffusion	Active Transport
Gradient	Down ( $High \to Low$ )	Against ( $Low \to High$ )
Energy	None (Passive)	ATP Required (Active)
Protein Type	Channel or Carrier	Carrier Only
Specificity	Highly Specific	Highly Specific

Unlike channel proteins which act as open pores, active transport only utilizes carrier proteins because the mechanism requires a controlled, energy-driven physical flip to move substances against a gradient.

5. Exam Strategy & Tips

Identify the Gradient: Always check the concentration levels on both sides of the membrane. If the movement is toward the higher concentration, it must be active transport.
Protein Specificity: Remember that carrier proteins are specific. If a question mentions a substance moving against a gradient, ensure you specify that a specific carrier protein is involved.
Metabolic Links: Look for clues involving mitochondria or oxygen. If a process stops when a respiratory inhibitor (like cyanide) is added, it is a strong indicator of active transport.
Terminology Precision: Do not use the term 'diffusion' when describing active transport. Diffusion is by definition passive; active transport is a distinct 'pumping' mechanism.

6. Common Pitfalls & Misconceptions

The 'Energy' Error: Students often think energy is needed to 'speed up' movement. In active transport, energy is required for the movement to happen at all because it opposes the natural direction of entropy.
Carrier vs. Channel: A common mistake is stating that channel proteins are used in active transport. Channel proteins cannot perform the conformational changes necessary to move substances against a gradient; they only allow downhill flow.
Equilibrium: Unlike diffusion, active transport does not aim for equilibrium. It aims to maintain a specific, often highly unequal, distribution of ions or molecules.

Active Transport

Summary

1. Definition & Core Concepts

2. Underlying Principles

3. Methods & Mechanism

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions

Active Transport

Summary

1. Definition & Core Concepts

2. Underlying Principles

3. Methods & Mechanism

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions