Functional Organization of the Cell

Detergents

Amphipathic molecules that can dissolve phospholipid membranes because they mimic phospholipids but are more water soluble.

Sol state

High-temperature state of a bilayer where phospholipids diffuse rapidly, making the membrane fluid.

Gel state

Low-temperature state of a bilayer where phospholipids diffuse slowly, making the membrane rigid.

Cholesterol

Rigid steroid that binds fatty acid side chains, reduces fluidity, and makes membranes more rigid.

Cholesterol flipping/flexing

Cholesterol and phospholipids move laterally in the bilayer; occasional flip-flop between leaflets occurs, contributing to membrane dynamics.

Phospholipid bilayer impermeability

Impermeable to large molecules and charged ions; selectively permeable to small uncharged molecules like O2, CO2, and water.

Outward vs cytoplasmic leaflet

Cytoplasmic leaflet enriched with phosphatidylethanolamine and phosphatidylserine; outer leaflet mainly phosphatidylcholine.

Integral proteins

Embedded in the membrane; can serve as pores, channels, carriers, pumps, receptors, adhesion molecules, enzymes, or signaling components.

Integral proteins: pore

Allow passive flow of water or small molecules across the membrane.

Integral proteins: channel

Allow ions or molecules to pass selectively, often gated.

Integral proteins: carrier

Transport specific molecules by conformational change; may be passive or active.

Integral proteins: pump

Use ATP (primary active transport) or gradients (secondary active transport) to move molecules against gradients.

Integral proteins acting as enzymes

Many integral proteins catalyze reactions, e.g., ion pumps hydrolyze ATP.

Integral proteins and cell signaling

Receptors transmit signals across the membrane, activating intracellular enzymes or second messengers.

Integrins

Cell-matrix adhesion molecules linking cells to extracellular matrix components (fibronectin, laminin).

Cadherins

Ca²⁺-dependent adhesion molecules that hold epithelial cells together, important in adherens junctions.

N-CAM

Neural cell adhesion molecule; Ca²⁺-independent adhesion molecule of the immunoglobulin family.

Nucleus

Organelle that stores, replicates, and transcribes genetic material; contains nucleolus, chromatin, nuclear pores, and nuclear lamina.

Nuclear lamina

Protein skeleton under the nuclear envelope; mutations in lamins cause Progeria (premature aging syndrome).

🔴 Progeria

Genetic disorder caused by defective nuclear lamins; leads to premature aging.

Rough ER

Site of synthesis, folding, and post-translational modification of secretory/membrane proteins.

🔴 Protein tagging in rER

Misfolded/unassembled proteins are tagged with ubiquitin and degraded in proteasomes.

Smooth ER

Synthesizes lipids and serves as a major calcium storage site.

Golgi complex

Processing station for proteins: glycosylation, phosphorylation, sulfation, proteolytic cleavage; directs proteins to correct destinations.

Mitochondria

Powerhouse of the cell; site of ATP production, contains and replicates its own genome.

Lysosomes

Digestive organelles with degradative enzymes and proton pumps to maintain an acidic environment; perform autophagy.

Proteasomes

Large complexes that degrade ubiquitin-tagged misfolded proteins.

Cytoplasm

Cellular space containing cytoskeleton and organelles; maintains cell shape and support.

Thin filaments

Actin filaments (~7 nm); maintain cell shape, enable motility.

Intermediate filaments

Fibers (~10 nm); provide mechanical strength.

Thick filaments (microtubules)

Tubulin polymers (~25 nm); form mitotic spindles, transport tracks, and cilia/flagella.

Apical membranes

Surface of epithelial cells facing lumen/external environment.

Basolateral membranes

Surface facing extracellular fluid and blood; anchored to basement membrane.

Junctional complexes

Specialized cell-cell adhesion structures in epithelia (tight, adherens, gap, desmosomes).

Tight junctions

Seal between epithelial cells to block passage of molecules; held by claudins.

🔴 Claudins

Transmembrane proteins forming the backbone of tight junctions.

Adhering junctions

Belt-like adhesions just below tight junctions; held by cadherins.

Gap junctions

Connexon channels allowing diffusion of ions and small molecules between cells.

Desmosomes

Spot adhesions held by cadherins; anchor to intermediate filaments and provide mechanical strength.