_CELL BIOLOGY: Cytoskeleton Structure & Function

Cytoskeleton

• Microtubules: semi-rigid tubular structures composed of polymerized tubulin heterodimers, dynamic with steady addition and dissociation of tubulin.
• Microtubules are important for maintaining cell shape and as tracks for transport of vesicles and organelles by motor proteins kinesin and dynein.

Microfilaments

• Short, flexible, highly dynamic filaments of actin subunits.
• Changes in length and interactions with binding proteins regulate cytoplasmic viscosity and movement.
• Myosins are motor proteins that bind and move along actin filaments, carrying vesicles or producing cytoplasmic movement.

Intermediate Filaments

• Most stable cytoskeletal component, conferring strong mechanical stability to cells.
• Composed of various protein subunits in different cells, including vimentin, nuclear lamins, neurofilament proteins, and keratins.
• Average diameter of 8-10 nm, between that of actin filaments and microtubules.

Inclusions

• Not metabolically active, primarily storage sites, such as lipid droplets, glycogen granules, pigment granules, or residual bodies (also called lipofuscin).

Mitochondria

• Double membrane-enclosed organelles with arrays of enzymes specialized for aerobic respiration and production of ATP.
• Synthesize most ATP during aerobic cellular respiration by digestion of fuel molecules in the presence of oxygen.
• Mitochondrial diseases are a group of genetic conditions that affect how mitochondria produce energy.

Cytoplasmic Organelles

• Metabolically active structures inside the cell membrane, such as mitochondria, ribosomes, and proteasomes.
• Positioned in the cytoplasm by movements along the polymers of the cytoskeleton, which also determines a cell's shape and motility.

Ribosomes

• Macromolecular machines that assemble polypeptides from amino acids on molecules of transfer RNA (tRNA) in a sequence specified by mRNA.
• Functional ribosome has two subunits of different sizes bound to a strand of mRNA.
• Engage in protein synthesis, with bound ribosomes producing proteins that are secreted or incorporated into the plasma membrane.

Endoplasmic Reticulum

• Convoluted membranous network that extends from the surface of the nucleus throughout most of the cytoplasm.
• Encloses a series of intercommunicating channels called cisternae (reservoirs).

Endoplasmic Reticulum

• ER is a major site for vital cellular activities, including biosynthesis of proteins and lipids.
• Responsible for various cellular activities.

Golgi Apparatus

• Completes posttranslational modifications of proteins produced in the RER.
• Packages and addresses proteins to their proper destinations.
• Consists of many smooth membranous saccules, containing enzymes and proteins being processed.
• Named after histologist Camillo Golgi who discovered it in 1898.

Lysosomes

• Sites of intracellular digestion and turnover of cellular components.
• Spherical-shaped membrane-bound organelles formed from the Golgi apparatus.
• Contain about 40 different hydrolytic enzymes.
• Particularly abundant in cells with great phagocytic activity (e.g., macrophages, neutrophils).
• Digest microbes or materials, including ingested by the cell, worn out cellular components, or the entire cell.

Mitochondria

• Double membrane-enclosed organelles with arrays of enzymes specialized for aerobic respiration and production of ATP.
• Synthesize most ATP during aerobic cellular respiration by digestion of fuel molecules (e.g., glucose) in the presence of oxygen.
• Mitochondrial diseases are a group of genetic conditions that affect how mitochondria in the cells produce energy.
• Can affect almost any part of the body, including cells of the brain, nerves, muscles, kidney, heart, liver, eyes, pancreas, etc.

Cytoskeleton

• A complex array of microtubules, microfilaments (also called actin filaments), and intermediate filaments.
• Determines the shapes of cells, plays an important role in the movements of organelles and cytoplasmic vesicles, and allows the movement of entire cells.
• Microtubules and actin microfilaments can be clearly distinguished by TEM.
• Immunocytochemistry using antibodies against their subunit proteins can demonstrate arrays of microfilaments and microtubules.
• Actin filaments are most concentrated at the cell periphery.
• Microtubules are oriented in arrays that generally extend from the centrosome area near the nucleus into the most peripheral.

Microtubules

• Semi-rigid tubular structures with walls composed of polymerized tubulin heterodimers.
• Structure is often very dynamic, with steady addition and dissociation of tubulin.
• Important in maintaining cell shape and as tracks for transport of vesicles and organelles by the motor proteins kinesin and dynein.
• Diameter of 25 nm.

Microfilaments

• Short, flexible, highly dynamic filaments of actin subunits.
• Changes in length and interactions with binding proteins regulate cytoplasmic viscosity and movement.
• Myosins are motor proteins that bind and move along actin filaments, carrying vesicles or producing cytoplasmic movement.
• Diameter of 5-7 nm.

Intermediate Filaments

• The most stable cytoskeletal component, conferring strong mechanical stability to cells.
• Composed of various protein subunits in different cells, including vimentin, nuclear lamins, neurofilament proteins, and keratins.
• Display an average diameter of 8-10 nm, between that of actin filaments and microtubules.
• Serve to provide mechanical strength or stability to cells.

Inclusions

• Not metabolically active and are primarily storage sites.
• Examples include lipid droplets, glycogen granules, pigment granules, or residual bodies (also called lipofuscin).