State that protein carriers move molecules or ions across a membrane during active transport.

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3.3 Active Transport

Definition: Active transport is the movement of molecules or ions **against** their concentration (or electro‑chemical) gradient, from an area of low concentration to an area of high concentration. Because the movement is unfavourable, it requires an input of energy.

Energy source – link to respiration

  • Energy is supplied by the hydrolysis of adenosine‑triphosphate (ATP):
    ATP → ADP + Pi + energy
  • ATP is produced in the mitochondria during aerobic respiration, so the energy for active transport ultimately derives from cellular respiration.

Role of protein carriers

  • Only **carrier proteins embedded in the cell membrane** can move substances against a gradient.
  • Simple diffusion and facilitated‑diffusion proteins can only allow movement **down** a gradient.

Why active transport matters (core points)

  • Maintains ion concentrations that are essential for osmotic balance, membrane potentials and pH regulation.
  • Creates electro‑chemical gradients that drive secondary active transport, allowing the cell to import nutrients without using ATP directly.

Core examples

  • Na⁺/K⁺‑ATPase (Na⁺/K⁺ pump) – moves 3 Na⁺ out and 2 K⁺ in per ATP hydrolysed.
    • Also underlies ion re‑absorption in kidney tubules and nutrient uptake in intestinal epithelium.
  • Root‑hair H⁺‑ATPase (plants) – exports H⁺ using ATP, generating a strong H⁺ gradient that powers secondary transport of mineral ions (e.g., NO₃⁻ symport, K⁺ antiport).

Types of active transport

Type Mechanism Typical example
Primary active transport Direct use of ATP by the transporter to change its conformation. Na⁺/K⁺‑ATPase (Na⁺/K⁺ pump)
Supplementary detail – secondary active transport
Secondary active transport Uses the energy stored in an ion gradient created by a primary transporter. The gradient drives co‑transport of another substance (symport = same direction, antiport = opposite direction). Glucose/Na⁺ symport in intestinal epithelial cells

General steps of a carrier‑mediated active transporter (optional deeper insight)

  1. Binding: Substrate binds to a specific site on the carrier protein on one side of the membrane.
  2. Conformational change: ATP hydrolysis (or the energy from an ion gradient) induces a shape change, exposing the binding site to the opposite side.
  3. Release: Substrate is released into the new environment where its concentration is higher.
  4. Reset: The carrier returns to its original conformation, ready for another cycle.

Illustration (suggested diagram)

Transporter protein spanning the membrane with substrate‑binding sites on both sides. The diagram should show the four steps above, highlighting the ATP‑driven conformational change.

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