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Histology Lecture 2 Cell Structure CELL CELLS ARE THE STRUCTURAL AND FUNCTIONAL UNITS OF LIFE. PROKARYOTES AND EUKARYOTES:There are two basic cell types. Prokaryotic cells typically are small,singlecalled organisms(eg,bacteria)that lack a nuclear envelope,histones and membranous organelles. EUKARYOTIC CELLS are larger than prokaryotes and have a nuclear envelope,histones and membrane-limited organelles All cells require energy captured from their environment. The typical molecule involved in storing this energy is ATP. Energy from sun, chemicals, eating other cells. Cells must make DNA, RNA and proteins Cells use sugars, nucleotides, amino acids, lipids and ATP to make components General features of metabolism similar in all cells, details can vary Prokaryotes Eukaryotes Cell components The living substances of the cell is known as Protoplasm Protoplasm has two compartments: Cytoplasm and Karyoplasm Cytoplasm extending from the plasma membrane to the nuclear envelope Karyoplasm forming the contents of the nucleus Volumes of Intracellular Compartments (liver cell) Cytosol Mitochondria RER Smooth ER & Golgi Nucleus Peroxisomes Lysosomes Endosomes %age of Total Cell Volume 54 22 9 6 6 1 1 1 DIFFERENTIATION The process that The cells synthesize specific proteins, change their shape, And become very Efficient in specialized Functions. Eukaryotic cell structure Cell Membrane The cell membrane forms a selectively permeable barrier between the cytoplasm and the external milieu. Plasma membrane studied at the EM level Functions Selective Barrier Facilitated transport Exocytosis Endocytosis Phagocytosis Pinocytosis Attachment (via junctions Cell Membrane Molecular Composition *phospholipids, Cell membrane is based on a lipid bilayer contains glycolipids and cholesterol *It is composed of a phospholipid bilayer and associated integral and peripheral proteins *Membrane proteins carry out most of the specialized functions of cell membrane by participating in membrane transport mechanisms, by forming attachments to cytoskeletal and extracellular proteins and by constituting the receptors of the plasma membrane *Glycoclayx composed usually of carbohydrate chains, coats the cell surface which are not in contact with cytosol Amounts of Membrane Types in 2 Cells Plasma membrane RER SER Golgi Mitochondria OM IM Nucleus IM Lysosome, perox., endosome %age of Total Membrane Liver Pancreas 2 5 35 60 16 <1 7 10 7 32 0.2 1.2 4 17 0.7 UNK Table 10-1 % Lipid Composition of Different Cell Membranes Chol Liver PM 17 RBC PM 23 E. coli memb 0 PE 7 18 70 PS 4 7 trace PC 24 17 0 SM 19 18 0 GL 7 3 0 Lipid rafts (~70 nm dia) are typically rich in sphingolipids & cholesterol. The thicker membrane in the raft can accommodate some of the larger transmembrane proteins, so they tend to accumulate in LR. Asymmetrical distribution of PL in membrane of rbc. PC (r) & SM (b) PS (-) & PE Cholesterol is not shown, but is ~equal in the layers. Animal cells use PL asymmetry to distinguish between live and dead cells. Before a cell undergoes apoptosis the PS is facing the cytoplasm. At death, the PS translocates to the outer leaflet. This signal activates adjacent macrophages that cell is dying. PS translocation is by 2 mechanisms: PL translocator that normally moves PS from the outer leaflet to the inner leaflet is inactivated. The “scramblase” that normally moves PL between leaflets nonspecifically, is activated. Membrane protein content can vary widely from just 25% in myelin membranes to 75% in mitochondria and chloroplast membranes. In most cells the membrane is about 50% protein. Bcz proteins have much higher MW than PLs, the ratio is about 50 PL to 1 protein. Like glycolipids, membrane proteins often have carbohydrate chains on their exposed regions contributing to the glycocalyx. Single-pass and multi-pass transmembrane proteins Membrane Proteins are associated with the membrane in various ways Integral membrane proteins Peripheral membrane proteins Glycophorin 1 million per cell Function unknown Spectrin is a peripheral membrane protein on the inner surface of the rbc plasma membrane, MW 220-240 Kd The majority of transmembrane proteins are glycosylated Detergent micelle in water Plasma membrane is less than 5% of typical eukaryotic cell total membrane. Plasma membrane studied at the EM level Functions Selective Barrier Facilitated transport Exocytosis Endocytosis Phagocytosis Pinocytosis Attachment (via junctions 2 main classes of membrane transport proteins: carriers and channels Conformational change leading to passive transport Comparison of Active and Passive Transport Electrical gradient Chemical gradient 3 Ways of Powering Active Transport Amino acid sequence of carrier proteins mediating passive transport are similar to those of carriers involved in active transport suggesting an evolutionary relationship between these families of proteins. Transport of material in or out of a cell takes place by the processes of ENDOCYTOSIS & EXOCYTOSIS The invaginated cell membrane fuses To form an ENDOCYTOTIC VESICLE or ENDOSOME Endocytosis divided into two categories: Ù Phagocytosis and Pinocytosis Pinocytosis is used when cell take up small molecules and fluid. the two main vesicles involved in transport of substances into cells are derived from surface membrane invaginations called coated pits and caveoli phagocytosis Phagocytosis is the process of engulfing large particulate matter such as microorganisms PINOCYTOSIS CAVEOLI are invaginations Braced by the Protein CAVEOLI Pinocytosis and receptor mediated endocytosis Caveolae in plasma membrane of fibroblast (no protein coat); caveolin is integral membrane protein Coated Pits and Vesicles The coated pits and vesicles function in receptor mediated endocytosis and in the pathway for cell secretion There are at least two types of coated vesicles: clathrin coated and nonclathrin coated Clathrin coated vesicles are associated with transport vesicles which function in: 1- The regulated secretory pathway; 2- In transport of lysosomal proteins 3- In receptor mediated endocytosis ×Polypeptides of clathrin form a basket-like structure which coats the cytoplasmic surface of the membrane ×Nonclathrin coated vesicles are either those which move between the endoplasmic reticulum and the Golgi complex, those which are a part of the constitutive secretory pathway, or those which move between the Golgi cisternae