Cellular Transport Mechanisms

Cellular Transport

• Importance: Obtaining nutrients for energy and metabolism, excreting waste, maintaining pH and ionic balance, generating ionic gradients for nervous and muscle activity.

Types of Transport

• Passive Transport: Movement down the concentration gradient, no energy expenditure.
• Active Transport: Movement against the concentration gradient, requires energy expenditure.

Passive Transport Examples

• Simple Diffusion: Movement of molecules or ions down their concentration gradient (high to low), across the lipid bilayer. Examples: CO2, O2, ether, lipid-soluble Vitamin D.
  • Factors affecting speed: concentration gradient, temperature, molecule diameter, solution density.
• Facilitated Diffusion: Movement of molecules across the membrane with the help of carrier or channel proteins.
  • Channels: Water-filled pores with gating mechanisms.
  • Carriers: Bind to substances, undergo conformational changes during transport. Carriers are specific to substances.
  • Examples: Ions of Na, K, Ca, Cl, molecules of amino acids, glucose.

Osmosis

• Definition: Diffusion of water across a selectively permeable membrane.
• Aquaporins: Channels used by water for transport.
• Osmotic Pressure: Pressure required to stop water movement into a solution.
• Osmotic Potential: Tendency of water to move into a medium by osmosis.
• Water Potential: Potential energy of water, symbol Ψ (Psi), measured in kilopascals.
• Higher solute concentration = Lower Water Potential.
• Tonicity: Relative concentration of two fluids.
  • Isotonic Solutions: Equal solute concentration.
  • Hypertonic Solutions: Higher solute concentration than the other solution, high osmotic pressure, low water potential.
  • Hypotonic Solutions: Lower solute concentration than the other solution, low osmotic pressure, high water potential.

Osmosis Examples

• Blood plasma is isotonic to RBCs.
• Plant cells are hypertonic to the growing medium.
• Marine Amoeba are isotonic to seawater.
• Freshwater Amoeba/Paramecium are hypertonic. They use a contractile vacuole to expel excess water.
• Hemolysis: Bursting of cells in a hypotonic solution.
• Crenation: Shrinking of cells in a hypertonic solution.
• Plasmolysis: Shrinking of protoplasm from the cell wall in plant cells placed in a hypertonic medium.

Active Transport

• Definition: Movement across a membrane against a concentration gradient.
• Reason: Cells need higher concentrations of certain substances than their surroundings.
• Examples: K ions are more concentrated inside cells but are still actively transported.
• Process: Requires metabolic energy, depends on membrane protein carriers.
• Ex: Transport of ions of Na, Ca, K, glucose, and amino acids.

Active Transport Proteins

• Uniporters: Transport one type of solute across a membrane.
• Cotransporters: Transport two ions or molecules simultaneously.
  • Symporters: Transport two solutes in the same direction (e.g., glucose and Na ions).
  • Antiporters: Transport two molecules in opposite directions (e.g., K and Na ions).

Example of Active Transport

• Sodium-Potassium Pump (Na+ - K+ Pump): Transports Na and K ions against their concentration gradients.

This quiz explores the critical concepts of cellular transport, including passive and active transport mechanisms essential for maintaining cell homeostasis. It highlights types of transport, such as simple diffusion and facilitated diffusion, along with their importance and factors influencing their efficiency.