Molecular Bases of Cellular Motility and Molecular Biology of Cell Membranes

Vocabulary flashcards covering cellular motility systems (actin-myosin and microtubule-dynein), cilia and flagella structure, axoplasmic transport, and the molecular organization and lipid-protein composition of biological membranes.

Motor protein

A protein with enzymatic activity that hydrolyzes $ATP$ to release energy used for altering its tridimensional conformation, enabling it to step along a support to produce cellular movement.

Amoeboid locomotion

A movement mechanism driven by the actin-myosin system and sol-gel/gel-sol transitions of cytoplasm, observed in Amoeba proteus, neutrophil leukocytes, and fibroblasts.

Pseudopodia

Cylindrical cytoplasmic extensions produced through sol-gel transitions that direct the movement of Amoeba and neutrophil leukocytes toward sites of infection.

Lamellipodia

Flat cytoplasmic extensions emitted by fibroblasts to guide cell migration across surfaces during processes such as wound healing and collagen deposition.

Chemotaxis

Chemically directed cellular movement, exemplified by neutrophil leukocytes migrating toward peptides produced by bacteria.

Microvilli

Permanent, short cell extensions containing parallel actin microfilaments linked by fimbrin and villin, which increase the apical absorptive surface of enterocytes by approximately $20\text{ times}$.

Cytoplasmic streaming

Intracellular movements that displace organelles and mix the cytoplasm (such as mitochondrial movements and cyclosis in plant cells), influenced by light, $pH$, and temperature.

Axoplasmic transport

A specialized cytoplasmic transport process along neuronal axons classified into anterograde (perikaryon to axon terminal) and retrograde (axon terminal to perikaryon) directions.

Kinesin

A dimeric motor protein composed of $2$ heavy chains and $2$ light chains that uses $ATP$ to move cargo in $8\text{ nm}$ steps toward the $+$ end of microtubules in anterograde transport.

Dynein

A motor protein composed of $2-3$ heavy chains and multiple intermediate/light chains that moves cargo toward the $-$ end of microtubules in retrograde transport and powers ciliary/flagellar bending.

Axoneme

The internal microtubule core of cilia and flagella, consisting of $9$ peripheral doublets (each with a complete $13$-protofilament A subfiber and an incomplete $11$-protofilament B subfiber) surrounding $2$ central microtubules.

Nexin

An axonemal protein that interconnects adjacent peripheral microtubule doublets, restricting sliding and causing the axoneme to bend when dynein arms step along adjacent tubulins.

Immotile cilia syndrome

A condition (also called primary ciliary dyskinesia) caused by absent or deficient axonemal dynein, resulting in non-functional cilia and flagella, recurrent respiratory infections, and sterility.

Kartagener syndrome

The clinical association of immotile cilia syndrome with situs inversus, an anatomical anomaly in which internal organs are laterally transposed.

Centrosome

The microtubule-organizing center located adjacent to the nucleus in interphase animal cells, consisting of $2$ perpendicular centrioles surrounded by pericentriolar material containing pericentrin and $\gamma\text{-tubulin}$.

Basal body

A structure identical in organization to a centriole ($9$ microtubule triplets) that anchors the ciliary or flagellar axoneme and coordinates tubulin polymerization and motility.

Biological membranes

Two-dimensional, selectively permeable assemblies of proteins and lipids that compartmentalize cell interiors, provide barriers, and carry out metabolic and signal-transduction functions.

Phosphoglycerides

Amphiphilic membrane phospholipids consisting of glycerol esterified to two fatty acid chains ($12\text{ to }24$ carbon atoms) and a polar head group containing a phosphoric acid residue.

Sphingolipids

Membrane lipids built on sphingosine (an amino-alcohol with a long carbon chain) containing a polar head and two hydrophobic chains, with the fatty acid attached via an amide bond.

Cerebrosides

Simple glycolipids based on a sphingosine backbone whose polar head is a single monosaccharide residue, such as glucose or galactose.

Gangliosides

Complex sphingosine-based glycolipids that contain one or more sialic acid residues, conferring a negative electrical charge, and making up about $6\%$ of total lipids in neuronal plasmalemma.

Phase transition

The reversible temperature-dependent change of a lipid bilayer from a crystalline gel state to a fluid liquid-crystal state.

Flip-flop

The transversal diffusion movement of an intact phospholipid molecule from one leaflet of the lipid bilayer to the opposite leaflet.

Flippases

Membrane protein translocators that catalyze and facilitate the energy-dependent transversal movement (flip-flop) of phospholipids between bilayer leaflets.

Peripheral proteins

Extrinsic membrane proteins linked to the external surface of the lipid bilayer via electrostatic interactions with lipid polar heads or integral proteins, easily detached by altering ionic strength.

Integral proteins

Intrinsic membrane proteins possessing hydrophobic regions embedded directly within the lipid bilayer that can only be extracted by disrupting the membrane with detergents.

Raft domains

Transient thicker membrane microdomains enriched in cholesterol, sphingolipids, glycolipids, and GPI-anchored proteins that coordinate specialized membrane functions.