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Mechanisms of Transport Through the Cell Membrane

Membrane Permeability

Cell membrane is made up of lipids

Allows passage of only lipid-soluble substances e.g. CO2, O2, steroid hormones, etc.

Does not allow passage of water-soluble substances e.g. sodium ions, chloride ions, etc.

Transport proteins on the cell membrane make holes in the membrane

Allow passage of water-soluble substances.

Types of Transport Mechanisms


  • Passive process.

  • Driven by electrochemical gradient i.e. substance moves down the electrochemical gradient.

  • e.g. glucose concentration is higher outside the cell than inside → glucose diffuses from outside to inside down this concentration gradient.

  • Electronegativity inside the cell attracts positively charged ions and repels negatively charged ions.

Active Transport

  • Substance is moved against electrochemical gradient.

  • Energy is required.

  • Depending on source of energy there are two types of active transport: Primary and Secondary.

Primary Active Transport
  • Energy is obtained directly from ATP.

  • e.g. Na-K-ATPase pump

    • Moves Na from inside to outside the cell (against electrochemical gradient).

    • Moves K from outside to inside the cell (against electrochemical gradient).

    • Uses ATP for this.

Secondary Active Transport
  • Energy is obtained from downhill movement of another solute.

e.g. Na-Glucse Cotransporter

Allows downhill movement of Na

Uses energy from this movement of Na to move glucose uphill

Here transport of glucose is secondary active transport.


  • Movement of water from low osmolarity to high osmolarity.

  • Passive process.


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