Molecular and Cellular Bases Pt 2 Quiz

Ligand Gated Channels

• Acetylcholine binds to nicotinic receptors in skeletal muscle cells.
• Found in skeletal muscle cells and autonomic nervous system neurons.

Molecule Permeability

• Small hydrophobic molecules easily dissolve in lipid bilayers, showing high permeability.
• Small uncharged hydrophilic molecules diffuse slower than hydrophobic ones.
• Ions have low permeability through lipid bilayers regardless of size.

Osmosis

• Defined as the net movement of water based on concentration differences.
• Water crosses membranes via simple diffusion or aquaporins; humans have around 13 types of aquaporins.
• Osmotic pressure measures the pressure needed to halt osmosis.
• Isotonic solutions show no osmotic flow, appearing normal.
• Hypotonic solutions cause water influx, possibly leading to cell rupture.
• Hypertonic solutions result in water exiting cells, often making them shriveled.

Transport Mechanisms

• Active transport moves substances against concentration gradients, requiring energy (usually ATP).
• Passive transport includes simple diffusion (e.g., osmosis) and facilitated diffusion.

Types of Passive Transport

• Simple Diffusion: Movement of molecules through lipid bilayers without protein assistance, influenced by lipid solubility (e.g., oxygen, nitrogen, carbon dioxide).
• Facilitated Diffusion: Utilizes carrier proteins; rate depends on available carriers, cannot exceed Vmax.

Carrier Proteins and Channels

• Carrier proteins specifically bind and aid in the diffusion of molecules like sugars and nucleotides.
• Channels are integral membrane proteins allowing ion and small hydrophilic molecule transport via facilitated diffusion.
• Aquaporins enhance the rate of water movement across membranes.
• Ion channels facilitate ion flow, some being gated by voltage, ligands, or mechanical stimuli.

Ion Channel Types

• Voltage Gated Channels: Found in excitable cells (muscle, nerve, endocrine); essential for signal transmission.
• Ligand Gated Channels: Open in response to ligand binding.

Cell Transport

• Phospholipid bilayers are semi-permeable; lipid-soluble molecules pass easily while water-soluble molecules struggle.
• Membrane proteins facilitate alternative transport forms; they can act as channels or transport proteins.

Active vs Passive Transport

• Passive transport occurs down concentration gradients without energy, utilizing kinetic energy.
• Example of transcellular transport involves potassium moving via the sodium-potassium pump followed by passive diffusion.

Endocytosis and Exocytosis

• Both transport mechanisms use portions of the membrane to move impermeable molecules.
• Endocytosis includes:
  • Phagocytosis: Ingestion of large particles.
  • Pinocytosis: Ingestion of small particles.
  • Receptor Mediated Endocytosis: Specific uptake of cholesterol.
• Exocytosis: Vesicles fuse with the cell membrane to release contents; can involve consecutive secretion of proteins into the plasma membrane or extracellular matrix.