bio 202 unit 2

the cytoskeleton

• a network of fibers extending throughout the cytoplasm

• it organizes the cell’s structures and activities, anchoring many organelles

• it is composed of 3 types of molecular strucutres

  • microtubules

  • microfilaments

  • intermediate filaments

• provides structrual support to the cell, allows cells to maintain shape

• it interacts with motor proteings to produce cell motility

• intracellular transport: inside the cell, vesicles and other organelles can use motor proteins to travel along track

microtubules

are the thickest of the 3 components of the cytoskeleton

• are hollow, tube-shaped rods made from tubulin proteins

  • each tubulin unit is a dimer composed of one α-tubulin and
    one β-tubulin polypeptide.

    • α- and β-tubulin units create a strong cylindrical structure

  • tubulin dimers assemble together to form the wall of the microtubule

• about 25 nm in diameter and about 200 nm to 25 microns long

• are constructed of dimers of tubulin

  • functions:

    • shaping the cell

    • guiding movement of organelles

    • separating chromosomes during cell division

    • resist compression

microfilaments

also called actin filaments, are the thinnest component

• solid rods about 7 nm in diameter, built as a twist double chain of actin subunits

• a network helps support the cell’s shape

• they form a cortex just inside the plasma membrane to help support the cell’s shape

• bundles make up the core of microvilli of intestinal cells

• ones that function in cellular motility contain the protein myosin in addition to actin

  • cells move along the surface by extending pseudopodia (cellular extensions) and moving toward them

  • cytoplasmic streaming (in plant cells) is a circular flow of cytoplasm within cells, driven by actin-myosin interactions

intermediate filaments

• built from fibrous proteins rather than globular ones

• range in diameter from 8 to 12 nm, larger than microfilaments bu smaller than microtubules

• they support cell shape and fix organelles in place

• are more permanent cytoskeleton fixtures than the other 2 classes

• the nuclear lamina is made up of these filaments

  • gives the nucleus strength, stability, and elasticity to withstand mechanical stress

• microtubules and microfilaments built from globular protein

centrosomes and centrioles

• in animal cells, microtubules grow out from a centrosome near the nucleus

• the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring

• other eukaryotic cells many organize micro-tubules in the absence of centrosomes

• plants do not have centrioles

cilia and flagella

• microtubules control the beating of flagella and cilia, microtubule-containing extensions that project from some cells

• many unicellular protists are propelled through water by cilia or flagella

• motile cilia are found in large numbers on a cell surface, whereas flagella are limited to one or a few per cell

structure of cilia and flagella

• contain a core of microtubules arranged in a 9 + 2 pattern: nine outer microtubule doublets surround 2 central microtubules

• radial spokes and cross-linking proteins help organize/stabilize microtubules

• dynein is the motor protein

  • uses ATP to generate sliding between neighboring microtubule doublets, which is converted into the bending motion of cilia and flagella

cell junctions

neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact

cell junctions in animal cells

• 3 types of cell junctions are common in epithelial tissues

  • tight junctions: membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid

  • desmosomes (anchoring junctions) fasten cells together into strong sheets

  • gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells

tight junctions in animal cells

• membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid

  • seal neighboring cells together and are connected to actin filaments

desmosomes (anchoring junctions) in animal cells

• fasten cells together into strong sheets

  • strong anchoring junctions that connect cells to intermediate filaments

gap junctions (communicating junctions) in animal cells

• provide cytoplasmic channels between adjacent cells

• protein channels that connect neighboring cells; allow passage of ions between adjacent cells

plasmodesmata in plant cells

• channels connecting adjacent plant cells that pass through the cell wall and create direct connections between neighboring cells

• allow water, ions, sugars, signaling molecules, and some proteins/RNAs to move from one cell to another without crossing the plasma membrane each time

• help coordinate cell-to-cell communication and transport, allowing groups of plant cells to function as an interconnected system

• the endoplasmic reticulum extends through the plasmodesma as a narrow central tube called the desmotubule, creating continuity between the ER of adjacent plant cells