EFB325 Cell Physiology

In the final portion of the course, we will focus much more on features of multicellular organisms; how the shape and movement of the cells is controlled, how cells are connected to each other, how organisms develop including the normal death of cells, how cells communicate with each other over long distances, and finally how cancer occurs when some or many of those processes are disrupted by mutations.

Cytoskeleton

Cytoskeleton is a scaffolding of proteinaceous fibers within the cell

• composed of three type of fibers: intermediate filaments, microtubules, and microfilaments

Intermediate filaments

• polymer of sets of many different types of cell specific, fibrous rod (not globular) proteins

for example: keratin in epithelial cells (there are even different types of keratin in different epithelial cell types); vimentin in connective tissues and muscle cells; neurofilaments in nerve cells; nuclear lamins in all cells

• two proteins coil together, then coils associate together to form filament
• length is not dynamic, usually very stable: nearly all of the intermediate filament protein is bound into assembled filaments
• except when nuclear lamina is broken down and reassembled during cell division

intermediate filaments involved in:

• maintaining cell shape-tension bearing (act like ropes)
• example-keratins in epithelial cells, keep the cells intact when they are stretched

Microtubules

• composed of the globular proteins alpha-tubulin and beta-tubulin
• those two associate together, then assemble into filaments, which assemble into a hollow tube

Microtubules can rapidly and dynamically assemble and disassemble

• about half of the tubulin protein in a cell is in the form of monomers, the rest is bound into microtubules
• microtubules are usually assembled and originate from the centrosomes - specifically from rings of gamma-tubulin, but can also be anchored to basal bodies in the case of cilia and flagella
• microtubules have a polarity, because one end (the plus end) can grow and shrink much faster than the other end (the minus end)
• the stability of the plus end is regulated by the binding of GTP: when GTP is bound, the microtubule grows; when the GTP is hydrolyzed to GDP, the subunits disassemble and the microtubule shrinks=dynamic instability
• the plus end can also be stabilized when it binds to a capping protein, which is usually anchored to some part of the cell; thus forming a more stable structure within the cell

Motor proteins associated with microtubules can use energy from ATP hydrolysis to move along the microtubules

• the kinesins and dyneins have two globular "heads" that bind to the microtubule and by changing shape (using energy from ATP) can "walk" along the microtubule
• kinesins move toward the plus end, dyneins move toward the minus end
• these motor proteins can carry molecules and organelles along a microtubule
• in the case of flagella and cilia, the motor proteins are bound between two doublet microtubules and when they move along one of the microtubules, it causes bending of the cilia/flagella

microtubules are involved in:

• chromosome movement during cell division
• movement of organelles
• cilia and flagellar movement - flagella have a special arrangement of 9 doublet microtubules in a ring around 2 singlet microtubules in the center

Actin microfilaments

• composed of the globular protein actin
• intertwined chains of actin polymers that have a plus end and minus end, like microtubules
• actin microfilaments can dynamically grow and shrink, much like microtubules
• subunits that bind ATP are more stable and filaments grow; when ATP is hydrolyzed to ADP, the filaments become unstable and shrink
• half of the actin protein in a cell is in the form of monomers, the rest is bound into filaments

Actin microfilaments can form a meshwork (linked by other proteins) just inside the plasma membrane=cell cortex

• dynamic growing and shrinking of the actin microfilaments can drive cell crawling
• microfilaments grow at the plus end, which is associated with the plasma membrane

Motor proteins (myosins) associated with actin microfilaments can function in contraction

• myosins have a globular "head" that can bind to the microfilament and using energy from ATP hydrolysis can "walk" along toward the plus end by unbinding, changing shape, rebinding, then changing back to the original shape

Skeletal muscle cells have extensive myofibrils that accomplish contraction

• myofibrils are made up of connected individual units called sarcomeres
• sarcomeres have actin filaments anchored at their plus ends to Z disks, the minus ends of the actin filaments overlap bundles of myosin-II (myosin filaments, like double-headed arrows) in the middle of the sarcomere
• when triggered, the myosin filaments use energy from ATP hydrolysis to "walk" along the actin microfilaments, pulling the sets of filaments toward each other and contracting the sarcomere
• when the signal to contract stops, the myosin filaments release the actin filaments, and they slide back and relax
• the trigger to contract comes from an action potential in a nerve that triggers Ca²⁺ channels in the sarcoplasmic reticulum (a special form of the ER in muscle cells) to open, releasing Ca²⁺, which stimulates contraction; Ca²⁺-ATPases pump the Ca²⁺ back into the sarcoplasmic reticulum upon relaxation

microfilaments involved in:

• muscle contraction
• amoeboid movement
• cytoplasmic streaming
• cell division

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