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).