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This 9 page Class Notes was uploaded by Kezia Notetaker on Wednesday August 31, 2016. The Class Notes belongs to bio1134 at Mississippi State University taught by Martha Barton in Fall 2016. Since its upload, it has received 67 views. For similar materials see General Biology I in Biology at Mississippi State University.
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Date Created: 08/31/16
CHAPTER 5: MEMBRANE STRUCTURE, SYNTHESIS, AND TRANSPORT Plasma membrane: the membrane that separates the internal content of cell from its external environment. FUNCTIONS OF CELLULAR MEMBRANE: Cell signaling Production of energy intermediates Protein sorting Selective uptake and export of ions and molecules Cell compartmentalization/ grouping Cell and nuclear division Adhesion of cells to each other & to the extracellular matrix Anchoring of the cytoskeleton MEMBRANE STRUCTURE Membrane’s framework is the phospholipid bilayer (2 layers of phospholipid). Phospholipid is the structural component of the membrane which is considered as amphipathic (has hydrophobic & hydrophilic region) and is selectively permeable. Membranes also contain protein, glycol protein, glycolipid. The half part of a phospholipid bilayer (1 layer) is called leaflet. Membrane is a mosaic of lipid, protein, and carbohydrate molecules Membrane Bound Protein Transmembrane proteins; Region(s) are physically embedded in the hydrophobic portion of the phospholipid. Lipid anchored protein; An amino acid of the protein is covalently attached to the lipid. (lipid tails are inserted to the hydrophobic portion of the membrane do the protein will attached firmly) Peripheral membrane protein; non covalently bond either to integral membrane protein that projects out of the membrane, or the polar head groups of phospholipid. Picture: FLUIDITY OF MEMBRANES Categorized as semifluid because lipids and proteins can move laterally within the membrane. Most lipids can rotate and move laterally within the leaflet. Flip-flop of lipids (from 1 leaflet to another) requires action of enzyme flippase. Energy is also required (ATP). Factors that effects fluidity: Length of phospholipid tails Shorter tails are less likely to interact so the membrane becomes more fluid. Double bonds in phospholipid tails Double bonds create a kink in a lipid tail, making it more difficult for the neighboring tails to interact so the bilayers become more fluid. Cholesterol Tends to be stabilized membranes that effects depend on the temperature. Not all integral membrane can move 10-70% of membrane proteins maybe restricted in their movement. Integral membrane proteins may be bound to components of the cytoskeleton, which restricts the proteins from moving laterally. Membrane proteins may be also attached to molecules that are outside the cell, such as the interconnected network of proteins that forms the extracellular matrix. STHESIS OF MEMBRANE COMPONENTS IN EUCARYOTIC CELLS Synthesis of lipids The cytosol and endomembrane system work together to synthesize lipid. Fatty acids building blocks are made via enzymes in cytosol or taken into cells from food. The process occurs at cytosolic leaflet in the smooth ER. Transfer of lipids to other membranes Lipids in ER membrane can diffuse laterally to nuclear envelope. Transported via vesicles to golgi, lysosomes, vacuoles, or plasma membrane. Lipid exchange protein: extract lipid from one membrane for the insertion of another. Synthesis of transmembrane proteins most are directed to the ER membrane first. Next, it can be transferred from the ER to other membranes of the cell via vesicles. Picture Glycosylation Process of covalently attaching a carbohydrate to a protein or lipid Glycoprotein : carbohydrate on protein Glycolipid : carbohydrate on lipid Can serve as a recognition signal for other cellular proteins; play a role in cell surface recognition. Helps protect protein from damage. N-link glycosylation: Carbohydrate attached to nitrogen atom of asparagine side chain. O-link glycosylation : Carbohydrates attached to oxygen atom of serine or threonine side chain (occurs only in golgi). OVERVIEW OF TRANSPORT MEMBRANES Plasma membrane is selectively permeable which means it allows only certain passage of ions (permeable to small hydrophobic molecules). This structure ensures that: Essential molecules enter; Metabolic intermediates remain; Waste products exit. Passive transport: Requires no energy Molecules move from high to low concentration. 3 Types: Simple diffusion: solute moves across membrane without help. Facilitated diffusion: solute move across membrane by help from a membrane protein. Osmosis: the movement of water. Active transport Molecules are transported from low to high concentration; energy is needed. Example: Na+K+ ATPase sets up the Na+ and K+ gradients required for transmission of nerve impulses. Important note: Hypertonic: solute (substance) concentration is higher that the solvent (water) concentration. Hypotonic: solute concentration is lower that the solvent. Isotonic: equal solute and solvent concentration. TRANSPORT PROTEINS Transport proteins are transmembrane proteins that provide a passageway for the movement of ions and hydrophilic molecules across membranes 2 classes based on types of movement: Channel Form an open passageway for direct diffusion of molecules or ions across the membrane (mostly gated). Ex: aquaporis Transporter Conformational change transports solute across membrane principal pathway for uptake of organic molecules (sugar, amino acid, nucleotides). Types: Uniporter: transfer single ion or molecule Symporter: 2 or more molecules/ ions transported in same direction. Antiporter: 2 or more ions/molecules transported in opposite direction. EXOCYTOSIS & ENDOCYTOSIS Exocytosis Material inside the cell is packaged into vesicles and exported out of the cell. Endocytosis Plasma membrane folds inward to form vesicle and bring substances into the cell.
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