Cell Membrane and Transport

Overview – Cell Membrane and Transport

The cell membrane is a dynamic lipid-protein barrier that regulates transport of substances into and out of the cell. Its structure and transport mechanisms are fundamental to physiology, pharmacology, and pathology. Understanding how materials cross this barrier—passively or actively—is crucial for interpreting cellular signalling, fluid balance, drug uptake, and immune responses.

Definition

The cell membrane (plasma membrane) is a semi-permeable phospholipid bilayer embedded with proteins. It separates the intracellular environment from the extracellular space and mediates communication, transport, and structural support.

Membrane Structure

Phospholipid Bilayer

  • Composed of diglycerides (phospholipids)
    • Hydrophilic head (glycerol + phosphate)
    • Hydrophobic tails (saturated and unsaturated fatty acids)
  • Molecules are held together by hydrophobic interactions
  • Allows flexibility and selective permeability
Lipid Bilayer
Lipid Bilayer
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Membrane Proteins

  • Peripheral Proteins
    • Associate with polar head groups
    • Easily removed by pH or ionic shifts
  • Integral Proteins
    • Span the bilayer
    • Must be solubilised with detergents to remove
    • Include transporters, receptors, and channels
cell membrane and transport - Imbedded membrane Proteins
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Transport Across Cell Membranes

Substances move across membranes via passive or active mechanisms.

Passive Transport

Requires no energy input. Substances move down their concentration or electrochemical gradient.

1. Simple Diffusion

  • Small, uncharged, lipid-soluble molecules
    • E.g. O₂, CO₂, ethanol, glycerol, steroids, fat-soluble vitamins
cell membrane and transport - Diffusion
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2. Facilitated Diffusion

  • Movement via membrane transport proteins
  • Two types:

Carrier Proteins

  • Bind specific solutes → undergo conformational change
  • High substrate affinity
  • Slower; inhibited by temperature
  • E.g. Glucose uniporters

Channel Proteins

  • Act as gated pores
  • Transport ions based on size and charge
  • No substrate binding
  • Not temperature-dependent
  • Rapid flow once opened
cell membrane and transport - Passive Transport Proteins
Passive Transport Proteins
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

3. Osmosis

  • Passive movement of water
  • Through the membrane or aquaporins
  • Moves from hypotonic → hypertonic to equalise solute concentration
  • Osmoregulation is vital for cellular integrity
Osmosis
Osmosis
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Active Transport

Requires energy (usually ATP). Substances move against their concentration gradient or are too large for diffusion.

Primary Active Transport

  • Direct use of ATP
  • E.g. Na⁺/K⁺-ATPase pump
    • 3 Na⁺ out, 2 K⁺ in
    • Maintains electrochemical gradient
    • Mechanism:
      1. Na⁺ binds to cytoplasmic side
      2. ATP phosphorylates protein
      3. Protein changes shape, releasing Na⁺
      4. K⁺ binds extracellularly
      5. Dephosphorylation restores original shape
      6. K⁺ released inside
      7. Cycle repeats
cell membrane and transport - The Sodium Potassium Pump
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Secondary Active Transport

  • Driven by ion gradient from primary transport
  • Symporters: Two solutes move in same direction
    • E.g. Na⁺-glucose symporter (glucose uptake using Na⁺ gradient)
  • Antiporters: Two solutes move in opposite directions
cell membrane and transport - Secondary Active Transporters
Unattributable

Vesicular Transport (Bulk Transport)

Used for macromolecules, fluids, and large particles

Exocytosis

  • Expels materials from cell
  • Vesicle fuses with plasma membrane
Active Transport Via Vesicles
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Endocytosis

  • Engulfs materials into vesicles
  • Three types:
    • Phagocytosis: Engulfs large particles (e.g. bacteria)
    • Pinocytosis: Engulfs fluid and small solutes
    • Receptor-mediated endocytosis: Specific ligand-receptor binding triggers internalisation
Active Transport Via Vesicles
Three Forms of Endocytosis Endocytosis is a form of active transport in which a cell envelopes extracellular materials using its cell membrane. (a) In phagocytosis, which is relatively nonselective, the cell takes in a large particle. (b) In pinocytosis, the cell takes in small particles in fluid. (c) In contrast, receptor-mediated endocytosis is quite selective. When external receptors bind a specific ligand, the cell responds by endocytosing the ligand.
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Summary – Cell Membrane and Transport

The cell membrane regulates material exchange through passive and active transport. From simple diffusion to complex receptor-mediated endocytosis, these processes underpin essential cellular function, drug transport, and fluid balance. For a broader context, see our Cell Biology & Biochemistry Overview page.

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