biochem 002 a tour of the cell part 2 (microscopy, ribosomes, prokaryote, eukaryote, nucleus, ER, go

in many cells, microtubules grow out of a centrosome near the nucleus. The centrosome is a "microtubule-organizing center." In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring

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• 1. a core of microtubules sheathed by the plasma membrane
• 2. A basal body that anchors the cilium or flagellum
• 3. A motor protein called dynein, which drives the bending movements of a cilium or flagellum

• 1. the dynein arms alternately grab, move, and release the outer microtubules
• 2. Protein cross-links limit sliding
• 3. Forces exerted by dynein arms cause doublets to curve, bending the cilium or flagellum

the structural role of microfilaments is to bear tension, resisting pulling forces within the cell. They form a 3-D network called the cortex just inside the plasma membrane to help support the cell's shape. Also, bundles of microfilaments make up the core of microvilli of intestinal cells

they are built as a twisted double chain of actin subunits

Microfilaments that function in cellular motility contain the protein myosin in addition to actin.

In muscle cells, thousands of actin filaments are arranged parallel to one another. Thicker filaments composed of myosin interdigitate the thinner actin fibers

Localized contraction brought about by actin and myosin also drives amoeboid movement. Pseudopodia extend and contract through the reversible assembly and contraction of actin subunits into microfilaments.

cytoplasmic streaming is a circular flow of cytoplams within cells. This streaming speeds distributon of materials within the cell. In plants actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming

intermediate filaments range in diameter from 8-12 nanometers, larger than microfilaments but smaller than microtubules. They support cell shape and fix organelles in place. Intermediate filaments are more permanent cytoskeleton fixtures than the other two classes

• -cell walls of plants 
• -the extracellular matrix of animal cells
• -intercellular junctions

the cell wall protects the plant cell, maintains it shape, and prevents excessive uptake of water

Plant cell walls are made of cellulose fibers embedded in other polysaccharides and proteins

• primary cell wall: relatively thin and flexible
• middle lamella: thin layer between primary walls of adjacent cells

secondary cell wall: added between the plasma membrane and the primary cell wall

plasmodesmata are channels between adjacent plant cells. Through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell

The ECM is made up of glycoproteins such as collagen, proteoglycans, and fibronectin. 

ECM proteins bind to receptor proteins in the plasma membrane called integrins

• Functions of the ECM:
• -Support
• -Adhesion
• -Movement
• -Regulation

• -plasmodesmata
• -tight junctions
• -desmosomes
• - gap junctions

tight junction, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid.

fasten cells together into strong sheets

provide cytoplasmic channels between adjacent cells