Cell Transport

Active Transport

• Protein channels/carrier proteins move molecules against a concentration gradient.
• This requires energy, unlike passive transport.

Molecules Moving Across the Phospholipid Membrane

• Examples: Oxygen ($O<em>2$), glucose ($C</em>6H<em>{12}O</em>6$), carbon dioxide ($CO_2$).

Factors Affecting Substance Movement Across the Plasma Membrane

• Size of the molecule.
• Concentration gradient.
• Chemical composition (charge: positive, negative, or polar).

Passive Transport

• Does not require additional energy input from the cell.
• Molecules move along the concentration gradient.
• Movement is from an area of high concentration to an area of low concentration.

Hydrophobic Molecules

• Definition: Does not mix with water (water-fearing).
• Examples: Oxygen ($O<em>2$), Carbon Dioxide ($CO</em>2$).

Osmosis

• Definition: The diffusion of water molecules through a selectively permeable membrane.

Diffusion

• Movement of molecules from an area of high concentration to an area of low concentration.
• Continues until equilibrium is reached (equal concentrations on each side of the membrane).
• For a molecule to move via diffusion:
  • Must be small enough to fit between phospholipids.
  • Must be hydrophobic to move through the hydrophobic tails of the bilayer.
• Molecules move at equal rates once equilibrium is reached.

Facilitated Diffusion

• Involves protein channels and carrier proteins.
• These proteins help move substances across the plasma membrane without additional energy.
• Carrier Proteins:
  • Bind to specific molecules (e.g., amino acids, glucose).
  • Undergo a shape change to transport the molecule across the membrane.
• Channel Proteins:
  • Control the diffusion of ions.
  • Ions bind to specific channels, causing them to open and allow passage.
• Integral proteins are needed for large or charged molecules to pass through the membrane.

Osmosis Explained

• Definition: The passive movement of water molecules across a selectively permeable membrane from an area of low solute concentration to an area of high solute concentration.

Tonicity and Water Movement

• Isotonic Environment:
  • No net movement of water molecules.
  • Concentration of solutes is equal inside and outside the cell.
• Hypotonic Environment:
  • Net movement of water molecules into the cell.
  • The solution outside the cell has a lower solute concentration.
• Hypertonic Environment:
  • Water moves out of the cell.
  • The solution outside the cell has a higher solute concentration.
• If the concentration of water molecules inside a cell is lower than the concentration outside, more water will diffuse into the cell than leave the cell until equilibrium is reached.

Bulk Transport

• Endocytosis: Moving substances into a cell.
  • The cell engulfs extracellular material by folding its plasma membrane around it, creating a vesicle.
  • Types:
    • Phagocytosis: Engulfing solid materials (e.g., bacteria).
    • Pinocytosis: Engulfing liquids.
• Exocytosis: Moving substances out of a cell.
  • A membrane-bound vesicle moves to the plasma membrane, fuses with it, and releases its contents outside the cell.
  • Important for:
    • Removing waste materials.
    • Secreting cellular products (e.g., hormones, digestive enzymes).