Cell Biology Exam 2 PDF Chapter 4 review study guide
Cell Biology Exam 2 PDF Chapter 4 review study guide BIOL 225
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This 6 page Study Guide was uploaded by MelLem on Thursday March 31, 2016. The Study Guide belongs to BIOL 225 at Simmons College taught by Dr. Lopilato in Fall 2016. Since its upload, it has received 58 views. For similar materials see Cell Biology in Biology at Simmons College.
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Date Created: 03/31/16
Exam 2 Review Cell Biology Chapter 4 : The Structure and function of the plasma membrane 4.1 Overview of Membrane Functions 1. Compartmentalization • The plasma membrane encloses the contents of the entire cell. • The nuclear and cytoplasmic membranes enclose diverse intracellular spaces • Membrane compartmentalization allows specialized activities to proceed without external interference and enables cellular activities to be regulated independently of one another 2. Scaffold for Biochemical Activities • Because of their construction, membrane provide the cell with extensive frame work in which can be ordered for effective interaction. 3. Providing a selectively permeable barrier • Membranes prevent the unrestricted exchange of molecules from one side to the other. • Membranes provide the means of communication between the compartments they separate • Gated “Bridges” that promote the movement of select elements in and out of the enclosed space. 4. Transporting Solutes • Contains the machinery for physically transporting substances from one side of the membrane to the other. • *Genrally from a region where the solute is present at a low concentration to an area where the solute is at a higher concentration (NOT DIFFUSION) • Diffusion – going from the higher area of concentration to an area of lower concentration. • The plasma membrane is able to transport ions, thereby able to establish ionic gradients. (Which is important for nerve and muscle cells). 5. Responding to External Stimuli • Signal Transduction – Response of a cell to external conditions (external Stimuli) • Receptors – membranes posses receptors – they combine with specific molecules (ligands) or respond to other types of stimuli like light and mechanical tension. 6. Intercellular Interactions • Plasma membrane allows cells to recognize and signal one another to adhere (when appropriate) and exchange materials and information. • Proteins within the plasma membrane may also facilitate the interaction between extracellular materials and the intracellular cytoskeleton. 7. Energy Transduction • Energy transduction – the process by which one type of energy is converted to another • The most fundamental energy transduction occurs during photosynthesis – when – energy in the sunlight is absorbed by membrane bound pigments, converted to energy and stored as carbohydrates • In eukaryotes, the machinery for this energy is housed in the membrane of the chloroplast and mitochondria. 4.3 The Chemical Composition of Membranes • Membranes are lipid-‐protein assemblies in which the components are held together in a thin sheet by non-‐covalent bonds. • The lipid bilayer serves as a structural backbone and to provide a barrier that prevents random movements of water soluble molecules over the membrane (in and out of the cell) Membrane Lipids • Contains a wide diversity of lipids – all are amphiapathic • Amphiapathi – contain both hydrophobic and hydrophilic regions. • There are three main types of membrane proteins o Phosphoglycerides o Sphingolipids o Cholesterol • Phosphoglycerides o Most membrane lipids have phosphate groups and are deemed phospholipids. o Since many have a glycerol backbone – theyre phosphoglycerides • Diglycerides o Phosphoglycerides – only have two of the hydroxyl groups of the glycerol that are esterifies to fatty acids. The third is esterified to a hydrophilic phosphate group. • Sphingolipids o Less abundant class of membrane lipids o All have two hydrophobic hydrocarbon chains at one end and a hydrophilic region at the other. • Cholesterol o Sterol (cholesterol) in certain animal cells that may constitute up to 50% of the lipid molecules in the plasma membrane. o Plant cells contain “cholesterol like” sterols The nature and important of the lipid bilayer • Membrane lipids also provide the precursors for highly active chemical messengers that regulate cellular function. The asymmetry of membrane lipids • All of the glycolipids of the plasma membrane are in the outer leaflet o They serve as receptors for extracellilar glands • Membrane Carbohydrates o The plasma membrane of eukaryotes also has carbohydrates present depending on the cell type, it can be 2-‐10% carbohydrate content in the membrane. o Most (90%0 of the carbohydrates in the membrane are covalently linked to proteins to form glycoproteins. o The remainder of the carbohydrates are linked to lipids to form glycolipids. o All of the carbohydrates of the plasma membrane face outward into the extracellular space. o The carbohydrates of the glycoproteins are present as short, branches hydrophilic oligiosaccharides (typically having less than 15 sugars) o Carbohydrate projections play an important role in mediating the interactions of a cell with its environment 4.4 The Structures And Functions of Membrane Proteins • A membrane may contain hundreds of different proteins • Each membrane protein has a defined orientation relative to the cytoplasm – this asymmetry is referred as “sidedness” 3 different classes distinguished by their intimacy with the lipid bilayer: • Integral Proteins o Penetrates the lipid bilayer o Are known as transmembrane proteins – they pass entirely through the lipid bilayer – have domains that protrude from both the extracellular and cytoplasmic sides of the membranes. • Peripheral Proteins o Located entirely outside of the lipid bilayer, on either the extracellular or the cytoplasmic side. (associated with the membrane surface by noncovalent bonds) • Lipid Anchored Proteins o Located outside the lipid bilayer on either the extracellular side or the cytoplasmic side o Covalently linked to a lipid molecule that is situated within the bilayer. • Integral Membrane Proteins o Function as receptors that bind specific substances at the membrane surface, as channels or transporters that are involved in the movements of ions and solutes across the membrane. o They are also amphiopathic • Freeze Fracture Analysis o The concept that proteins penetrate through membranes rather than remaining external to the bilayer was derived from the results of freeze fracture analysis o Cracks the lipid bilayer, leaves a “pit” in the bilayer where the proteins were. (Figure 4.15a represents this method of analysis) • Peripheral Membrane Proteins o Associated with the membrane by weak electrostatic bonds o These proteins provide mechanical support for the membrane and function as an anchor for integral membrane proteins. o Other peripheral proteins on the internal plasma membrane surface function as enzymes, specialized coats, or factors that transmit transmembrane signals. • Lipid-‐ Anchored Membrane Proteins o Numerous proteins present on the external face of the plasma membrane are bound to the membrane by a small, complex oligiosaccharided linked to a molecule of phosphatidylinositol. o Those containing this type of glycosyl-‐phosphatidylinositol linkages are called GPI-‐Anchored proteins. 4.7 The movement of Substances across Cell Membranes • The lipid bilayer of the membrane is ideally suited to prevent the loss of charged and polar solutes from the cell. o But – some special provisions must be made to allow the movement of nutrients, ions, waste products, and other compounds across the membrane. Two types of movement • Substances may move through the membrane passively by diffusion, OR actively by an d energy-‐couples transport process • Both movemenets lead to the net-‐flux of a particular ion or compound. • Net Flux – indicates that the movement of the substance into the cell and out of the cell is NOT balances, but exceeds the other. • Substances may move across the membrane by: o Simple Diffusion – Through the lipid bilayer o Simple Diffusion -‐ Through an aqueous protein lined channel (diffusion that is facilitated by a protein transporter) o Active Transport – Requires an energy driven protein pump that is capable of moving substances against a concentration gradient The energetics of solute movement • Diffusion -‐ The spontaneous process in which a substance moves from a region of higher concentration to a region of lowere concentration Diffusion of substances through membranes • Substances must be in higher concentration on one region and lower in the other. • Membrane must be permeable to the solute. • Must satisfy 1/both: o Solute can pass directly through bilayer -‐or-‐ o Solute can traverse through an aqueous pore that spans the membrane • Partition Coefficient – The ration of its solubility in a non polar solvent. • It is evident that the greater the lipid solubility, the faster the penetration. • Another factor determining the rate of penetration of a compound through a membrane is size. • IF two compounds have similar partition coefficients, the one with the smaller size will penetrate the membrane faster than that of the larger molecule. • Very small uncharged molecules penetrate very rapidly through the cellular membrane. • Membranes are highly permeable to small inorganic molecules such as O2, CO2, NO, and H2O, they are thought to slip between adjacent phospholipids. • Larger polar molecules, sugar, a.a’s and phosphoryared intermediates exhibit poor membrane permeability. • Materials that must enter from the blood stream, but exhibit poor permeability must enter through special mechanisms AND NOT by simple diffusion. The diffusion of Water through membranes • Membranes are said to be semi-‐permeable. • Water moves readily through a semi permeable membrane from a region of lower solute concentration to an area of higher solute concentration. • This process is known as osmosis -‐ the diffusion of water. • The area with a higher solute concentration is said to be hypertonic or hyperosmotic. • The area with a lower solute concentration is said to be hypotonic or hypoosmotic. (When a cell is palced into a hypotonic solution, the cell gains water by osmosis and swells (lysis)) • The cells volume is controlled by the difference between the solute concentration inside the cell and that in the extracellular medium. • Small integral proteins called aquaporins allow the passive movement of water from one side of the plasma membrane tot the other. The diffusion of ions through membranes • The lipid bilayer is highly impermeable to charged substances including ins such as Na+, K+. Ca+2 and Cl-‐ • The rapid movement (conductance) of these ions across membranes play a critical role in a large amount of cellular activities. • Ion Channels -‐ openings in the membrane that are permeable to certain (specific) ions. • Most ions channels are highly selective in allowing only one particular type of ion to pass through the pore. • Most ion channels that have been identified can exist in open or closed conformation or said to be gated. • The opening/closing of the gates are subject to complex physiological regulation.
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