Cell Transport Mechanisms Quiz

Functional Systems of the Cell: Transport

• Cells require transport of substances across the cell membrane to maintain homeostasis and function.
• The lipid bilayer acts as a barrier, preventing the free movement of water and water-soluble substances between extracellular and intracellular fluids.
• Membrane transport occurs through passive and active mechanisms.

Passive Transport

• Diffusion: Random movement of molecules, either through membrane spaces or with carrier proteins. It's driven by the kinetic energy of molecules moving from high to low concentration.
• Types of Diffusion:
  • Simple diffusion: Movement through intermolecular spaces or directly across the lipid bilayer for lipid-soluble molecules.
  • Facilitated diffusion: Movement assisted by carrier proteins, essential for larger or insoluble molecules, still from high to low concentration gradient.
• Aqueous diffusion: Movement of molecules through the watery extracellular and intracellular spaces, common in capillaries. This is a passive process following Fick's Law.
• Lipid diffusion: Movement of molecules through membranes, driven by lipid solubility.
• Fick's law: Predicts the rate of movement of molecules across a barrier, influenced by concentration gradient, permeability coefficient, area, and thickness of the barrier.

Active Transport

• Active transport: Movement of ions or other substances against an energy gradient (often from low to high concentration) using carrier proteins and an energy source beyond kinetic energy. Requires ATP.
• Carrier-mediated diffusion: Substances move with a carrier protein, but does not move against the concentration gradient, utilizing no energy.

Transport by Special Carriers

• Drugs can be transported across barriers by specialized carrier mechanisms (e.g., amino acid carriers in the blood-brain barrier, weak acid carriers in the renal tubules).
• Carrier transport isn't governed by Fick's law and is capacity-limited.
• Selective inhibitors of these carriers (like probenecid) can be useful clinically (e.g., increasing uric acid excretion).

Endocytosis

• Cells take in large particles via endocytosis.
• Types of Endocytosis:
  • Pinocytosis: Ingestion of minute particles, creating vesicles of extracellular fluid.
  • Phagocytosis: Ingestion of large particles (bacteria, cells).
• Mechanism: Membrane engulfs particles (often via receptor binding), forming vesicles, transporting the molecules inside the cell.
• Very large or insoluble molecules need this mechanism.
• Opsonization: Bacteria attach to antibodies to assist in cell engulfment.

Phagocytosis Steps

• Cell membrane receptors attach to ligands on a particle.
• Membrane engulfs the particle.
• Cytoplasmic actin and other contractile fibrils surround the vesicle, pushing it inwards until the particle is fully inside.
• Contractile proteins pinch off the vesicle, creating another intracellular vesicle inside the cell.

Lysosomes

• Almost immediately after endocytosis, lysosomes fuse with the vesicle, releasing digestive enzymes within the particles.
• Digestive vesicle is formed with enzymes hydrolyzing proteins, carbohydrates, lipids, etc., producing basic molecules like amino acids, glucose,etc. that diffuse to the cytoplasm.
• Lysosomes contain bactericidal agents (lysozyme, lysoferrin, acid) to destroy engulfed bacteria, or other foreign matter, before causing cellular damage.
• Indigestible remains become residual bodies and are eliminated from the cell by exocytosis.

Mitochondria

• Mitochondria are organelles responsible for cellular respiration and energy production.
• Foodstuffs are oxidized with oxygen inside mitochondria, releasing energy used to make ATP (adenosine triphosphate), which is the main energy currency for cells.
• ATP is used for various cellular processes (active transport, synthesis, and mechanical work (muscle contraction)).

General Cell Concepts

• Cellular transport mechanisms are essential for maintaining homeostasis and normal cellular operation.

Test your knowledge on various cell transport mechanisms, including diffusion, active transport, pinocytosis, and phagocytosis. Delve into how substances like vitamin B12 and iron enter cells, and understand the roles of protein structures and macromolecules in these processes. Explore the differences between active and passive transport, and the significance of selective inhibitors.