_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).

Learn about the structure, function, and importance of microtubules and microfilaments in cells, including their composition, dynamics, and roles in cell shape and transport.