The Plasma membrane.docx

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Anatomy and Physiology
Jacqueline Carnegie

The Plasma membrane  The plasma membrane, also called the cell membrane separates the intracellular fluid within the cell and the extracellular fluids outside the cell. STRUCTURE OF PLASMA MEMBRANE – FLUID MOSAIC MODEL  Thin structure composed of a double layer of lipid molecules with protein molecules dispersed in it.  Called the Fluid mosaic model because membrane proteins float about in the membrane lipid bilayer. MEMBRANE LIPIDS  Lipid bilayer is composed largely of phospholipids, some glycolipids and cholesterol and areas called lipid rafts.  Phospholipid: has a phosphate group on two long chain lipids. The polar head is charged and hydrophilic while the nonpolar tail is hydrophobic and uncharged.  Glycolipids: lipids that are attached to sugar groups. Only found on the outer surface  Cholesterol: formed between hydrophilic tails, stabilize the membrane, decrease mobility of the phospholipids and the fluidity of the membrane. Too much cholesterol causes membranes to lose its flexibility.  Lipid Rafts : assemblies of saturated phospholipids MEMBRANE PROTEINS: allows to communicate with the environment  Integral Proteins: firmly inserted into the lipid bilayer. Most are transmembrane proteins that span the entire membrane. Some transmembrane proteins cluster together to form channels or pores. Others are involved in transport, some are receptors for hormones. All Integral proteins have a hydrophilic and hydrophobic regions.  Peripheral Proteins: not embedded in the lipid bilayer. Attached loosely to integral proteins. Include a network of filaments that help support the membrane from its cytoplasmic side. Some peripheral proteins are enzymes. Involved in attachment functions, shape changes. CYTOSKELETON: anchors to plasma membrane, can also interact with receptors GLYCOCALYX  Ensemble of carbohydrates attached to lipids and proteins on extracellular face.  Sugar coating on plasma membrane provide cell recognition ex: sperm recognizes an ovum  Glycocalyx changes when cell becomes cancerous – can even change repeatedly to avoid recognition by immune system minor damage to the plasma membrane usually not a problem because it reseals naturally Cell Junctions Three factors that act to bind cells together  Glycoproteins in the Glycocalyx act as adhesive  Wavy contours of the membranes of the adjacent cells act fit together  Special cell junctions form TIGHT JUNCTIONS − Fusion of series of integral protein molecules of adjacent plasma membranes to prevent passage of molecules Ex: epithelial cells lining digestive tract keep digestive enzymes and micro-organisms entering the bloodstream. (Importance: immune and toxin protection) DESMOSOMES − Anchoring junctions: cytoplasmic face of each plasma membrane is covered by plaque − Adjacent cells are held together by thin protein filaments that extend together from the plaque − Thicker keratin filaments extend from the cytoplasmic side to anchor the plaque on the cell’s opposite side (by this way desmosomes bind neighboring cells together as well as contribute to a continuous internal network) − This distributes tension throughout cellular sheet and reduces chances of tearing − Abundant in tissues subjected to great mechanical stress ex: skin and heart muscle GAP JUNCTIONS − Communicating junction between cells − Adjacent plasma membranes are very close and cells are connected by hollow cylinders called connexons (composed of transmembrane proteins) − Ions, simple sugars and other small molecules pass through these water filled channels from one cell to the next − Abundant in electically excitable tissues. Ex: heart and smooth muscle FUNCTIONS OF PLASMA MEMBRANE 1. effective barrier between the intracellular & extracellular fluids 2. selectively permeable 3. allows the cell to respond to changes in the extracellular fluid 4. site of cell-to-cell interaction and recognition Membrane Transport  Cells are bathed in an extracellular fluid called Interstitial fluid (barrier between cytoplasm)  Interstitial fluid is a filtrate of blood and contains: salts, sugar, amino acids, vitamins, hormones, metabolites, gases (2 and CO2), fatty acids, neurotransmitters, waste products.  Plasma membrane is selectively permeable  To maintain homeostasis and function normally, a cell must be extract needed nutrients , keep valuable cell proteins inside & discard wastes.  Transport across Plasma membrane can be active or passive PASSIVE TRANSPORT 1. DIFFUSION − Passive transport by diffusion saves cellular energy − Tendency of molecules or ions to move from an area with higher concentration to an area with lower concentration along their concentration gradient. − Greater the concentration, higher the collisions and faster the net movement/diffusion of particles − Driving force for diffusion is kinetic energy of the molecules − Rate of diffusion depends on : a) molecular size (smaller the faster) b) temperature (warmer the faster) c) gradient slope (higher, more potential) − Diffusion ex: movement of ions across cell membrane, neurotransmitters between synapse − The plasma membrane is a hydrophobic barrier for diffusion therefore a molecules should be; a) Lipid soluble b) Small enough to pass through membrane channels c) Assisted by carrier molecule Simple Diffusion  Non-polar lipid soluble substances diffuse directly through the lipid bilayer. Ex: 2 , C2 , fat soluble vitamins, urea, alcohol.  Molecule is moving down the concentration gradient. Facilitated Diffusion  Molecules unable to pass through the plasma membrane are transported by a) By binding to a protein carrier in the membrane b) Moves through water filled protein channels  Ex : Glucose, other sugars, some amino acids and ions.  Features of facilitated diffusion a) Specific b) Not ATP-requiring c) Limited by carrier saturation d) Movement down concentration gradient e) Can be inhibited by certain substances Carrier-mediated facilitated Channel-mediated facilitated diffusion diffusion  Lipid insoluble molecules (sugars and  Channels are transmembrane proteins amino acids) too large to pass through  Substances like ions or water is membrane pores/channels transported through aqueous channels  Carriers are transmembrane integral  Channels are selective due to pore proteins that are specific for each class size, charges of amino acid that line of molecules channels  Alterations in the carrier allows  Leakage channels are always open molecules to bind and the carrier  Opening of gated channels are protein move the binding site from one controlled by chemically or electrical face of the membrane to the other signals  Can be limited by the number of  Can be inhibited, show saturation and protein carriers present. When all usually specific carries are engaged it is said to be saturated. 2. OSMOSIS − The unassisted diffusion of water from an area of low to an area of high solute concentration through a selectively permeable membrane − Water moves through gaps between hydrophobic tails in the plasma membrane − Water also moves freely through water specific channels constructed by transmembrane proteins called aquaporins(AQPs) which allow single file diffusion of water molecules − AQPs are abundant in red blood cells involved in water balance such as kidney tubule cells. − Osmosis occurs whenever the water concentrations or solute concentrations differ in the two sides (as solute concentration increases, water concentration decreases) − If distilled water is present on both sides no net movement occurs − Osmolarity refers to the total concentration of all solute particles in a solution (mOsmol/L
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