Cell Transport Mechanisms: Passive Transport

Cell Transport Mechanisms

  • Passive: No energy (ATP) required.
    • Diffusion
    • Facilitated Diffusion
  • Active: Energy (ATP) required.
    • Protein channels
    • Co-transport
    • Bulk transport

Plasma Membrane

  • Boundary separating living cell from surroundings.
  • Exhibits selective permeability, regulating cell composition.

Diffusion (Passive Transport)

  • Movement from high to low concentration; no energy required.
  • Brownian movement: inherent vibration of molecules causing collisions and spread.
  • Dynamic equilibrium: molecules cross membrane equally in both directions.

Facilitated Diffusion (Passive Transport)

  • Materials diffuse across plasma membrane with help of membrane proteins.
  • For polar molecules/ions repelled by hydrophobic parts of cell membrane.

Facilitated Diffusion - Transport Proteins

  • Channel proteins: corridors for specific molecules/ions.
    • Aquaporins: facilitate diffusion of water.
    • Ion channels: transport ions.
    • Gated channels: open/close in response to stimulus.
  • Carrier proteins: bind with substance and aid diffusion through membrane, change shape to translocate solute.

Passive Transport Recap

  • No energy required; molecules move down concentration gradient.
  • Facilitated diffusion uses transport proteins.
  • Water uses aquaporins for faster transport.

Osmosis

  • Diffusion of water across a semipermeable membrane down its concentration gradient.

Tonicity and Osmolarity

  • Tonicity: how extracellular solution changes cell volume via osmosis.
  • Osmolarity: total solute concentration.
    • Low osmolarity: more water relative to solute.
    • High osmolarity: less water relative to solute.

Tonicity Solutions

  • Isotonic: Equal solute concentration inside and outside the cell; no net water movement.
  • Hypertonic: Higher solute concentration outside; cell loses water.
  • Hypotonic: Lower solute concentration outside; cell gains water.

Water Balance in Cells Without Walls

  • Osmoregulation: control of solute concentrations and water balance.
  • Hypertonic environment: protists like Paramecium use contractile vacuoles to pump out water.

Animal Cells

  • Hypotonic: cell lyses (bursts).
  • Isotonic: no change in cell size.
  • Hypertonic: cell shrinks (crenates).

Water Balance in Cells with Walls

  • Hypotonic: cell becomes turgid (firm).
  • Isotonic: cell becomes flaccid (limp).
  • Hypertonic: cell undergoes plasmolysis (membrane pulls away from wall).