Topic 6: CELLS: STRUCTURE & FUNCTIONAL COMPONENTS (lecture 8)OBJECTIVES:1. Be able to differentiate prokaryotic vs. eukaryotic cells.2. Know the classes of major chemical components in the cytosol.3. Know the general structures, properties and general function of the majororganelles- nucleus, ER (both rough and smooth), Golgi apparatus, lysosomes andperxisomes.4. Be able to compare and contrast the structure and functions of mitochondria vs.chloroplasts.5. Be able to differentiate between the various types of cell junctions.Cell- the ultimate functional unit of life (self-replicating, energy-transducing etc.);however, viruses/viroids and prions (potentially infectious proteins) push this definitionfurther (more later…)Fig. 7.1- cells vary in terms of their size; from microscopic to macroscopic (large).Prokaryotic vs. eukaryotic cells(1) prokaryotic (fig. 7.4) - no distinct nucleus; genetic material is not present ina nucleus but rather is aggregated in a nucleoid; lack membrane-enclosedinternal structures (organelles); characteristically have a capsule (which maybe the target of antibiotics). BACTERIA.(2) eukaryotic - distinct nucleus and membrane-bound organelles; yeast,animal and plant cells. Typical animal cell (fig. 7.7)- Typical pant cell (fig. 7.8)- unique structures are chloroplast, presence of rigid cell wall, central vacuole surrounded by a tonoblast, plasma membrane is penetrated with plasmadesmata.Cytoplasm - the material within the area bounded by the plasma membrane excludingthe nucleus; includes organelles and the semi-fluid material referred to as cytosol.Cytosol is actually a term which refers to an experimental product of cell isolation. Ifyou gently break open cells and then centrifuge the material, the cytosol is the semi-fluid material that does not sediment. Cytosol consists mostly of the following:(1) water(2) inorganic ions- mostly K+, Cl-; some Na+, PO42- and Mg2+; trace amounts of Ca2+(3) low molecular weight organic compounds- glucose, free amino acids, nucleotides(4) macromolecules- proteins (enzymes), complex carbohydrates(Note: the cytosol differs radically in chemical composition form the fluid that bathesthe cells, the so-called extracellular or interstitial fluid; we’ll talk about this later)1There is considerable controversy as to the exact nature of the cytoplasm. Thetraditional view is that it is simply a random mixture of the above components whichpassively diffuse throughout the intracellular space. Others view the cytoplasm as astructured and organized space in which the above components may associate withvarious regions of the cell. In other words, distinct compartments may exist even in theabsence of membrane barriers. Scientific opinion is beginning to move towards thislatter point of view.Membrane-bound compartments- OrganellesNucleus- contains the bulk of genes (note: mitochondria and chloroplasts have theirown genome!)fig. 7.9- nuclear envelope; double membrane perforated by pores; inner region issupported by the nuclear lamina which consists of a dense network of protein fibers.chromatin- DNA/protein complexeschromosomes- condensed chromatin structures prior to cell divisionnucleolus- organizing centerEndoplasmic reticulum (ER)- extensive double membrane system spread throughoutthe cell; according to your text it represents 50% of total membrane in the cell. Thisstructure is extensively involved in protein synthesis and packaging as well as otherprocesses including membrane biosynthesis, detoxification and storing of certainmaterials. There are two basic types of ER- (1) smooth ER and (2) rough ER (see fig.7.11).(1) smooth ER- diverse array of functions-biosynthesis of various lipids including steroids, fatty acids & phospholipids-conversion of glucose-6-phosphate to glucose ( G6Pase; glucose-6 phophatase)-detoxification of foreign substances; converts them into more polar and easilyexcretable compounds by action of enzymes known as mixed functionoxidases-sarcoplasmic reticulum (SR) in muscle; stores and releases calcium (more later)(2) rough ER- studded with ribosomes (ribosomes = macromolecular complexes consisting of RNA and protein; site of translation of genetic message intoprotein); primary site of synthesis of proteins bound for the cell and those thatwill be secreted to the outside (usually are glycoproteins and are wrapped inmembrane vesicles). Membrane fragments are also made here.Golgi Apparatus (fig. 7.12)- packages vesicles of protein for transport to the exterior.Two sides to the structure: (1) cis side or face (receives materials from the ER;reassembles them into laminar type structure) and (2) trans side or face ( new vesiclespinch off form Golgi for transport elsewhere). Chemical constituents of membranes arealtered during residence in this structure.2Lysosomes (figs. 7.13 & 7.14)- membrane bound structures that contain degradativeenzymes that hydrolyze the major classes of organic compounds; engulf and digestmaterials acquired from the outside by phagocytosis and internal fragments(autophagy). Formed by the Golgi apparatus.Vacuoles- membrane bound structures that have a diverse array of functions; particlesengulfed during phagocytosis become food vacuoles; plants have central vacuoles (fig.7.15) which are used to create storage sites for materials such as citric acid.Peroxisomes- specialized organelles which have enzyme catalyzed reactions thatultimately produce hydrogen peroxide (H2O2) as a product. H2O2 is highly reactive butthere is an enzyme (peroxidase) that detoxifies H2O2.Mitochondria and Chloroplasts- extremely specialized organelles; very likely derivedin an evolutionary sense from early prokaryotic symbionts of other larger prokaryoticcells (Endosymbiontic theory of origin of eukaryotic cells);(1) both mitochondria and chloroplasts have their own genetic info and proteinsynthetic machinery.(2) however, a large fraction of proteins in mitochondria and chloroplasts are codedfor by genes in the nucleus; they are syhthesized on ribosomes in the cytoplasm.Mitochondria (fig. 7.17)- 1- 10 m long (except for giant mitochondria from insect flightmuscle cells); two double membrane systems- outer double membrane penetrated by pores which are very
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