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Transcript
CHAPTER 7 A TOUR OF THE CELL Cytology: science/study of cells Light microscopy resolving power~ measure of clarity Electron microscopy TEM ~ electron beam to study cell ultrastructure SEM ~ electron beam to study cell surfaces Cell fractionation ~ cell separation; organelle study Ultracentrifuge ~ cell fractionation; 130,000rpm A cell is a living unit greater than the sum of its parts • While the cell has many structures that have specific functions, they must work together. Cell Types: Prokaryotic Nucleoid: DNA concentration No organelles with membranes Ribosomes:protein synthesis Plasma membrane: (all cells); semi-permeable Cytoplasm/cytosol(all cells) Cell types: Eukaryotic Nucleus:membrane enclosed organelle containing chromosomes Membrane bound organelles of specialized form and function Generally larger than prokaryotic cells Cell Size As cell size increases, the surface area to volume ratio decreases Rates of chemical exchange may then be inadequate for cell size Cell size, therefore, remains small Nucleus Genetic material… chromatin Chromosomes Nucleolus: rRNA; ribosome synthesis Double membrane with pores mRNA~ protein synthesis Ribosomes Protein manufacture Types: a) free cytosol;protein function in cell b) bound: endoplasmic reticulum; membranes, organelles and export The Endomembrane System Endoplasmic reticulum(ER) Continuous with nuclear envelope Smooth ER no ribosomes Synthesis of lipids Metabolism of carbohydrates Detoxification of drugs &poisons Rough ER With ribosomes Synthesis of secretory proteins (glycoproteins) Membrane production The Golgi apparatus ER products are modified, stored, and then shipped Cisternae: flattened membranous sacs Trans face(shipping) & cis face (receiving) Transport vesicles Lysosomes Sac of hydrolytic enzymes; digestion of macromolecules Phagocytosis Autophagy: recycle cell’s own organic material Tay-Sachs disease~ lipid digestions disorder Vacuoles Membrane-bound sacs(larger than vesicles) Food (phagocytosis) Contractile (pump excess water) Central (storage in plants) Tonoplast membrane Other Membranous Organelles 1. Mitochondria and chloroplasts are the main energy transformers of cells Both organelles have small quantities of DNA that direct the synthesis of the polypeptides produced by these internal ribosomes. • Mitochondria and chloroplasts grow and reproduce as semiautonomous organelles. 2. Peroxisomes generate and degrade H2O2 in performing various metabolic functions Mitochondria Site of cellular respiration have a smooth outer membrane and a highly folded inner membrane, the cristae inner membrane encloses the mitochondrial matrix, a fluid-filled space with DNA, ribosomes, and enzymes. chloroplasts found in plants and eukaryotic algae site of photosynthesis. Inside the innermost membrane is a fluidfilled space, the stroma, in which float membranous sacs, the thylakoids. Peroxisomes • generate and degrade H2O2 in performing various metabolic functions • bounded by a single membrane. • They form not from the endomembrane system, but by incorporation of proteins and lipids from the cytosol. The Cytoskeleton • Providing structural support to the cell, the cytoskeleton also functions in cell motility and regulation There are three main types of fibers in the cytoskeleton: microtubules, microfilaments, and intermediate filaments. Microtubules • the thickest fibers, are hollow rods about 25 microns in diameter. • They move chromosomes during cell division. • Another function is as tracks that guide motor proteins carrying organelles to their destination. cilia and flagella. • Microtubules are the central structural support • Cilia usually occur in large numbers on the cell surface. • There are usually just one or a few flagella per cell cilia and flagella. • A flagellum has an undulatory movement cilia and flagella. • Cilia move more like oars with alternating power and recovery strokes. cilia and flagella • have the same ultrastructure. Microfilaments • the thinnest class of the cytoskeletal fibers, are solid rods of the globular protein actin. designed to resist tension • form a three-dimensional network just inside the plasma membrane. Microfilaments • In muscle cells, thousands of actin filaments are arranged parallel to one another. • Thicker filaments, composed of a motor protein, myosin, interdigitate with the thinner actin fibers Microfilaments • In other cells, these actin-myosin aggregates are less organized but still cause localized contraction •Pseudopodia, cellular extensions, extend and contract through the reversible assembly and contraction of actin subunits into microfilaments. Microfilaments • In plant cells (and others), actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming. Intermediate filaments, • more permanent fixtures of the cytoskeleton than are the other two classes • reinforce cell shape • and fix organelle location. Cell Surfaces and Junctions 1. Plant cells are encased by cell walls 2. The extracellular matrix (ECM) of animal cells functions in support, adhesion, movement, and regulation 3. Intercellular junctions help integrate cells into higher levels of structure and function 4. The cell is a living unit greater than the sum of its parts Plant cells are encased by cell walls • The cell wall, found in prokaryotes, fungi, and some protists, has multiple functions. • In plants, the cell wall protects the cell, maintains its shape, and prevents excessive uptake of water. • It also supports the plant against the force of gravity. A mature cell wall consists of a primary cell wall, a middle lamella with sticky polysaccharides that holds cell together, and layers of secondary cell wall. The extracellular matrix (ECM) of animal cells functions in support, adhesion, movement, and regulation • In many cells, fibronectins in the ECM connect to integrins, intrinsic membrane proteins. . Intracellular junctions help integrate cells into higher levels of structure and function • Plant cells are perforated with plasmodesmata, channels allowing cysotol to pass between cells. Animal have 3 main types of intercellular links: tight junctions, desmosomes, and gap junctions • In tight junctions, membranes of adjacent cells are fused, forming continuous belts around cells. – This prevents leakage of extracellular fluid. Desmosomes (or anchoring junctions) fasten cells together into strong sheets, much like rivets. • Gap junctions (or communicating junctions) provide cytoplasmic channels between adjacent cells. Microtubules • In many cells, microtubules grow out from a centrosome near the nucleus. • In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring. • During cell division the centrioles replicate.