Cell Physiology, the Plasma Membrane, and Membrane Potential

What are the main components of the cytoplasm?

Cytosol (water with proteins, salts, sugars, and solutes), cytoplasmic protein fibers (actin, intermediate filaments like keratin and neurofilaments, microtubules), and organelles.

Name the major cytoplasmic protein fibers and one function for each.

Actin (microfilaments): support cell shape and movement

• Intermediate filaments (keratin, neurofilaments): structural support

• Microtubules: cell shape, transport, centrioles, cilia, flagella

What are the main organelles in a cell and their basic functions?

Mitochondria: ATP production, has unique DNA

• Ribosomes: protein synthesis

• ER (Rough): protein assembly/modification

• ER (Smooth): lipid/fatty acid/steroid synthesis

• Golgi apparatus: protein modification, sorting, packaging

• Lysosomes: degrade bacteria and old organelles using acid hydrolases

• Peroxisomes: degrade long-chain fatty acids, free radicals; generate H₂O₂

What is the functional difference between lysosomes and peroxisomes?

Lysosomes: degrade biological macromolecules using acid hydrolases

• Peroxisomes: degrade toxins, long-chain fatty acids, and free radicals using catalases and oxidases

Define a receptor and explain its clinical significance.

A receptor is a protein that binds specific molecules (ligands) to trigger cellular responses. Clinically, receptors are targets for drugs and are important in signaling pathways.

What types of molecules can diffuse through the cell membrane without assistance?

Lipophilic molecules, some inorganic ions, and water.

Define the following terms: osmosis, osmolality, osmolarity, osmotic pressure.

Osmosis: movement of water across a semipermeable membrane

• Osmolality: solute concentration per kg of solvent

• Osmolarity: solute concentration per liter of solution

• Osmotic pressure: pressure needed to stop water movement across a membrane

Define tonicity, isotonic, hypertonic, hypotonic, hemolysis, and crenation.

Tonicity: effect of a solution on cell volume

• Isotonic: no net water movement; cell size unchanged

• Hypertonic: water leaves cell; cell shrinks (crenation)

• Hypotonic: water enters cell; cell swells (may cause hemolysis)

What are the main types of intercellular junctions and their proteins?

Gap junctions: connexin – allow communication

• Tight junctions: occludin – prevent leakage

• Adherens junctions: catenin – mechanical attachment

• Desmosomes: keratin – strong adhesion

Explain the Na⁺/K⁺ pump.

Uses ATP to pump 3 Na⁺ out and 2 K⁺ into the cell, maintaining resting membrane potential and ion gradients essential for cell function.