Cell Membrane Transport

Membrane Transport

To utilize molecules and energy derived from food, they must be transported across cell membranes.

Importance of Nutrient Transport

Nutrient transport is essential for all cells. Intestinal cells are used to illustrate the basic principles of membrane transport due to their specialized function in absorbing nutrients.

Plasma Membrane Permeability

The plasma membrane is selectively permeable, allowing some molecules to pass through while restricting others.

Simple Diffusion

  • Lipids can move freely across the lipid bilayer.
  • Lipid molecules move down their concentration gradient (from high to low concentration).
  • Simple diffusion is a form of passive transport.
  • Passive transport does not require energy from the cell.
  • Depends on:
    • Concentration gradient
    • Size
    • Charge of the molecule or ion

Facilitated Diffusion

  • Most molecules cannot cross the lipid bilayer directly.
  • Molecules like fructose move into intestinal cells via facilitated diffusion using transport proteins.
  • Fructose moves down its concentration gradient.
  • Facilitated diffusion acts only on specific molecules.
  • Transport proteins can become saturated, reducing the rate of diffusion.
  • It is a form of passive transport, not requiring cellular energy.

Osmosis and Aquaporins

  • The majority of water molecules cross the plasma membrane by facilitated diffusion, also known as osmosis.
  • Osmosis occurs through specific protein channels called aquaporins.

Active Transport

  • Some ions, like sodium and potassium, are transported against their concentration gradients.
  • The sodium-potassium pump actively transports these ions from low to high concentration.
  • This process requires energy from the cell and is termed active transport.
  • Energy from ATP is used to move three sodium ions out of the cell and two potassium ions into the cell.
  • 3Na+ out,2K+ in3Na^+ \text{ out}, 2K^+ \text{ in}
Primary Active Transport
  • The sodium-potassium pump directly uses ATP, so it's categorized as primary active transport.
Secondary Active Transport
  • Secondary active transport also moves chemicals against their concentration gradients but does not directly use ATP.
  • It uses the downhill gradient of one ion or molecule to drive the uphill movement of another.
  • Example: Sodium and glucose move into the cell via secondary active transport (cotransport or symport).
  • Sodium ions move down their concentration gradient, which is maintained by the sodium-potassium pump.
  • Glucose moves against its concentration gradient because its intracellular concentration can exceed the extracellular concentration.

Vesicular Transport

  • Cells transport larger materials, like proteins, between intracellular and extracellular fluids using vesicles.
Exocytosis
  • Vesicles fuse with the plasma membrane and release their contents outside the cell.
Endocytosis
  • The plasma membrane pinches inward, forming a vesicle containing material from outside the cell.

Summary

Cells utilize various processes, including passive transport (simple and facilitated diffusion, osmosis), active transport (primary and secondary), exocytosis, and endocytosis, to move materials across their membranes.