Cell Membrane Transport Mechanisms Quiz

Functional Systems of the Cell: Transport of Substances

• The lipid bilayer of the cell membrane acts as a barrier, preventing the movement of water and water-soluble substances between the extracellular and intracellular fluid compartments. It is not miscible with either fluid.

Cellular Transport

• Passive Transport (Small Molecules):
  • Simple Diffusion: Random movement of molecules through intermolecular spaces or with carrier proteins in the membrane, driven by concentration gradient.
  • Facilitated Diffusion: Movement of molecules across a membrane with the assistance of a carrier protein, utilizing a concentration gradient. This can be through channels (nonspecific transport) or carriers (specific).
• Active Transport (Small Molecules):
  • Movement of ions or substances against a concentration gradient, requiring energy in addition to kinetic energy (usually ATP). This involves carrier proteins.
• Large Molecules:
  • Exocytosis: Outbound transport of large molecules using vesicles.
  • Endocytosis: Inbound transport of large molecules using vesicles. Includes phagocytosis (ingestion of large particles like bacteria or tissue debris), and pinocytosis (ingestion of minute particles and fluids).

Diffusion

• Diffusion is the random movement of molecules.
• Types of diffusion through the cell membrane:
  • Simple Diffusion: Molecules move through the membrane without the help of carrier proteins.
  • Facilitated Diffusion: Molecules move through the membrane with the assistance/interaction of carrier proteins, which bind chemically to them.

Aqueous Diffusion

• Movement of molecules through the watery extracellular and intracellular spaces.
• Most capillaries have small water-filled pores that allow molecules up to the size of small proteins to diffuse between blood and the extravascular space.
• Governed by Fick's law.

Lipid Diffusion

• Movement of molecules through membranes and other lipid structures, occurring directly in the lipid bilayer if the molecule is lipid-soluble.
• Examples include oxygen, nitrogen, carbon dioxide, and alcohols.
• Governed by Fick's law.

Fick's Law of Diffusion

• Predicts the rate of movement of molecules across a barrier.
• Factors for calculating rate:
  • Concentration gradient (C1 - C2).
  • Permeability coefficient
  • Area
  • Thickness

Carrier-Mediated Diffusion

• Movement across the membrane facilitated by a macromolecule (carrier protein).
• Specific for certain chemical structures.
• Saturable process.
• Does not move against a concentration gradient (a type of passive transport).
• Example: Glucose uptake by cells.

Transport by Special Carriers

• Drugs may be transported by mechanisms that carry similar endogenous substances (e.g., amino acid carriers in the blood-brain barrier).
• Not governed by Fick's law.
• Capacity-limited.
• Probenecid: Selective inhibitor for uric acid transport, increasing its excretion (used in gout). Sometimes used with penicillin antibiotics to increase antibiotic blood levels and enhance their effectiveness to fight infections.

Endocytosis

• Involves the movement of large molecules into the cell.
• Pinocytosis: Ingestion of minute particles and fluids.
• Phagocytosis: Ingestion of large particles (bacteria, whole cells, or tissue debris).

Steps of Phagocytosis

• Attachment: Cell membrane receptors attach to surface ligands on particles.
• Evagination: Membrane surrounds the particle.
• Vesicle Formation: A phagocytic vesicle is formed.
• Contraction & Separation: Cytoplasmic fibrils contract, pinching off the vesicle from the membrane.
• The vesicle is now inside the cytoplasm

Lysosomes

• Digestive vesicles formed inside cells.
• Contain acid hydrolases that digest proteins, carbohydrates, and lipids.
• Breakdown products are released into the cytoplasm.
• Contain bactericidal agents (lysozyme, lysoferrin, and low pH) to destroy phagocytized bacteria..
• The remaining indigestible material forms a residual body that is eventually expelled from the cell via exocytosis

Mitochondria

• Site of cellular respiration.
• Breakdown of glucose, fatty acids, and amino acids with oxygen to produce ATP (adenosine triphosphate), a high-energy compound.
• Vital for supplying energy for cellular functions.

Uses of ATP for Cellular Function

• Transport: Active transport of substances through membranes.
• Synthesis: Chemical compound synthesis throughout the cell, such as protein synthesis.
• Mechanical work: Energy for muscle contraction.