The centrosome is a microtubule organizing center e In animal cells the

The centrosome is a “microtubule-organizing center” e. In animal cells, the centrosome has a pair of centrioles , each with nine triplets of microtubules arranged in a ring f. Microtubules control the beating of cilia and flagella , locomotor appendages of some cells g. Cilia and flagella differ in their beating patterns h. Cilia and flagella share a common structure i. A core of microtubules sheathed by the plasma membrane ii. A basal body that anchors the cilium or flagellum iii. A motor protein called dynein , which drives the bending movements of a cilium or flagellum i. How dynein “walking” moves flagella and cilia i. Dynein arms alternately grab, move, and release the outer microtubules ii. Protein cross-links limit sliding 5

iii. Forces exerted by dynein arms cause doublets to curve, bending the cilium or flagellum D. Microfilaments (Actin Filaments) a. Microfilaments are solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits b. The structural role of microfilaments is to bear tension, resisting pulling forces within the cell c. They form a 3-D network called the cortex just inside the plasma membrane to help support the cell’s shape d. Bundles of microfilaments make up the core of microvilli of intestinal cells e. Microfilaments that function in cellular motility contain the protein myosin in addition to actin f. In muscle cells, thousands of actin filaments are arranged parallel to one another g. Thicker filaments composed of myosin interdigitate with the thinner actin fibers h. Localized contraction brought about by actin and myosin also drives amoeboid movement i. Pseudopodia (cellular extensions) extend and contract through the reversible assembly and contraction of actin subunits into microfilaments j. Cytoplasmic streaming is a circular flow of cytoplasm within cells k. This streaming speeds distribution of materials within the cell l. In plant cells, actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming E. Intermediate Filaments a. Intermediate filaments range in diameter from 8–12 nanometers, larger than microfilaments but smaller than microtubules b. They support cell shape and fix organelles in place c. Intermediate filaments are more permanent cytoskeleton fixtures than the other two classes Concept 6.7: Extracellular components and connections between cells help coordinate cellular activities Most cells synthesize and secrete materials that are external to the plasma membrane These extracellular structures include a. Cell walls of plants b. The extracellular matrix (ECM) of animal cells c. Intercellular junctions A. Cell Walls of Plants a. The cell wall is an extracellular structure that distinguishes plant cells from animal cells b. Prokaryotes, fungi, and some protists also have cell walls 6

c. The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water (“Listen to the corn grow”) d. Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein e. Plant cell walls may have multiple layers i. Primary cell wall : relatively thin and flexible ii. Middle lamella : thin layer between primary walls of adjacent cells iii.