Cells

Basic Cell Structure

• All cells are primarily composed of carbon, oxygen, nitrogen, hydrogen, with trace elements.
• Human cells have three main parts:
  • Plasma membrane: outer boundary; separates intracellular and extracellular fluids.
  • Cytoplasm: intracellular fluid with organelles; between nucleus and plasma membrane.
  • Nucleus: controls cellular activities.

Plasma Membrane

• Fluid lipid bilayer with dispersed proteins; phospholipids have polar heads (hydrophilic) and nonpolar tails (hydrophobic).
• Hydrophilic heads face surfaces; tails form the interior.
• Proteins make up about half the membrane; interact with both lipid heads and tails.
• Transmembrane proteins: transport channels, carriers, receptors.
• Peripheral proteins attach to integral proteins or lipids; cytoskeletal links help support and cell movement.
• Glycocalyx: carbohydrate-rich area on the exterior formed by glycoproteins and glycolipids.
• Cholesterol stabilizes membrane fluidity.
• Specializations:
  • Microvilli: increase surface area (absorption).
  • Junctions:
  • Tight junctions: impermeable seal between adjacent cells.
  • Desmosomes: strong anchoring junctions; resist mechanical stress.
  • Gap junctions: chemical communication between cells via connexons.
• CAMs (cell adhesion molecules) and receptors facilitate signaling, adhesion, and migration.

Membrane Transport Basics

• The plasma membrane is selectively permeable; substances move passively or actively.
• Passive transport (no energy): diffusion, facilitated diffusion, osmosis.
• Diffusion: high to low concentration; down a gradient. Requires:
  • Lipid solubility, small size, or carrier/channel assistance.
• Facilitated diffusion: carrier proteins or channel proteins aid entry.
• Osmosis: diffusion of water across a selectively permeable membrane.
• Osmolarity: total solute particles in solution; tonicity affects cell shape (isotonic, hypertonic, hypotonic).

Active Transport & Vesicular Transport

• Active transport uses ATP and carrier proteins to move substances against their gradient.
• Primary active transport: uses ATP directly (e.g., Na^+–K^+ pump).
  • For each cycle: $3 Na^+ ext{ out},\, 2 K^+ ext{ in}$
• Secondary active transport: use stored energy from Na^+ gradient to co-transport other solutes.
• Vesicular transport uses ATP/GTP for large particles:
  • Exocytosis: to the outside; hormones, neurotransmitters, mucus.
  • Endocytosis: into the cell; includes:
  • Phagocytosis (cell eating) – large particles; vesicle is a phagosome.
  • Pinocytosis (cell drinking) – extracellular fluid uptake; vesicle is an endosome.
  • Receptor-mediated endocytosis – specific uptake via receptors (e.g., enzymes, iron, cholesterol).
  • Clathrin-coated pits and uncoating regulate endocytosis.

Resting Membrane Potential

• Membrane potential: voltage across the membrane; cells are polarized at rest.
• Inside of the cell is negative relative to outside.
• Resting potential is largely set by the K^+ gradient and leakage channels.
• Na^+/K^+ pump maintains resting potential:
  • For each cycle: $3 Na^+ ext{ out},\ 2 K^+ ext{ in}$
• Ion movement follows electrochemical gradients; more Na^+ tends to enter, but the pump keeps concentrations and potential balanced.

Cell-Environment Interactions & Signaling

• CAMs provide adhesion and signals; roles in development, wound repair, immunity.
• Membrane receptors mediate signaling (chemical, electrical, and contact signaling):
  • Ligand binding triggers responses; different cells respond differently.
  • G-protein signaling with second messengers (e.g., cAMP, Ca^{2+}) modulates intracellular responses.
• Receptors can be catalytic enzymes, ion-channel linked, or associated with G proteins.

Cytoplasm & Organelles (Membranous vs Nonmembranous)

• Cytoplasm consists of:
  • Cytosol: fluid matrix.
  • Organelles: metabolic machinery (membranous or nonmembranous).
  • Inclusions: stored nutrients or other substances.
• Nonmembranous organelles: cytoskeleton, centrioles, ribosomes.
• Membranous organelles: mitochondria, peroxisomes, lysosomes, endoplasmic reticulum, Golgi apparatus.
• Mitochondria: powerhouse; ATP; contain own DNA/RNA; site of aerobic respiration.
• Ribosomes: protein synthesis; free ribosomes in cytosol; bound ribosomes on ER.
• Endoplasmic Reticulum (ER): network of tubules and cisternae; rough ER has ribosomes; smooth ER lacks ribosomes.
  • Rough ER: protein synthesis for secreted proteins and membranes; membrane factory.
  • Smooth ER: lipid metabolism, detox, glycogen breakdown, steroid synthesis.
• Golgi apparatus: modifies, concentrates, packages proteins/lipids; sorts and ships via vesicles; secretory vesicles to the plasma membrane.
• Lysosomes: digestive enzymes; cellular demolition crew; autolysis if damaged.
• Peroxisomes: detoxify harmful substances; contain oxidases and catalases; neutralize free radicals.
• Cyt