Principles of Cell Membrane Function (Lectures 1 & 2)

Proteosome

degradation of intracellular proteins

Transmembrane Proteins

examples: ion channels/pres, carriers, active pumps move H20 Soluble substances, large polar molecules & ions alpha-helixes line up to facilitate movement of ions across membrane

Lysosome

intracellular degradation Contain powerful hydrolytic (digestive) enzymes Synthesized in the golgi complex as coated vesicles Debris can be brought into lysosome via endocytosis or if cell is damaged the lysosome releases its granular contents and digests the cell

Ligand Binding Receptors (Integral Membrane Proteins)

ligands bind to the extracellular domain ---> cause a conformational change ---> physiological response ex: GPCR ---> adregenic, muscarinc, angiotensin, vasopressin ex: second messengers (cAMP, IP3)

Membrane Permeability Depends on?

lipid:acqueous partition coefficient

Cytoskeleton Components

microtubules, microfilaments, intermediate filaments, microtrabecular lattice

Phospholipids in Inner Leaflet

phosphatidylserine, phosphatidylethanolamine phosphatidylinositol involved in signal transduction (PIP2) know chart organelle concentrations

Phospholipases

phospholipids are substrates for phospholipases Phospholipase C is effector for Gq/GPCR (cleaves PIP2 to IP3 and DAG activating PKC) PLA2 cleaves arachidonic acid in plasma membrane to prostaglandins + leukotrienes PI3 Kinase converts PIP2 to PIP3 PIP2 = Phosphatidylinositol 4,5 Biphosphate IP3 = Phosphatidylserine

Microfilaments

smallest element, made up of actin Function: 1. Important in contractile systems (muscles) 2. Cells moving away from each other during mitosis (cell division) 3. Responsible for ameboid movement (pseudopodia) 4. Stiffen microvilli structure (important in brush border)

Cholesterol

steroid ring binds to fatty acid chains and partially immobilizes them provides rigidity to plasma membrane hypercholesteremia's disrupt the ability of plasma membrane (phospholipids) to interact normally

Secretory Pathway

synthesis and processing of membrane and secreted proteins occurs in the RER RER > transport vesicles > cis-golgi > trans-golgi > secretory vesicles > plasma membrane Membrane proteins go through a sorting process and gather in the trans-golgi so carrier vesicles can insert them into their bilayers

Functions of Integral Membrane Proteins

1. Ligand-binding receptors 2. Ion Channels/Pores 3. Adhesion Molecules 4. Carrier Proteins 5. Active Pumps

Extracellular Ca2+ Concentration

1.1-1.4 (ionized) mmol/L

Intracellular Na+ Concentration

10-15 mEq/L

Intracellular Ca2+

10^-7 (super small) ionized mmol/L

Intracellular K+ Concentration

120-150 mEq/L

Extracellular Na Concentration

135-147 mEq/L

Sphingolipids

2nd major class of membrane lipids derived from sphingosine instead of glycerol interact with bilayer to prevent lipid molecules from diffusing away from membrane ex: spingomyelin & galactocerebrosides

Extracellular K+ Concentration

3.5-5 mEq/L

Extracellular Cl- Concentration

95-105 mEq/L

Vesicular Transport is regulated by:

Binding to small G-proteins (GTP/GDP binding proteins) that regulate assembly and delivery of contents SNARE proteins = 1. vesicular (vSNAREs) on vesicular membranes interact with 2. target (tSNAREs) on plasma membranes > exocytosis (neurotransmitters) Vesicles contain structural proteins or lipids in their membranes to direct specific membrane compartments (cell membranes)

Microtrabecular Lattice

Consists of a network of thin filaments running through the cytoplasm, anchoring to the PM suspend other cytoskeletal components (such as free ribosomes) may be involved in coordinating complex rxns in cytosol

Types of Secretion

Constitutive Secretion = occurs immediately after synthesis (leukocytes, hepatocytes, keratinocytes, fibroblasts) Regulated Secretion = substances stored in vesicles, exocytosis occurs in response to signals (neurons, endocrine cells, exocrine cells, and leukocytes)

Peroxisome

Detoxification of substances

Microtubule Stability

Dynamic instability is needed for normal function Provides tracks for movement of organelles Stability depends on rate of polymerization (addition of tubulin subunits) to rate of GTP hydrolysis to GDP by Beta subunit GTP cap stabilizes microtubule > if lost it becomes unstable and will depolymerize

Dyneins & Kinesins

Dyneins and Kinesins Use energy from ATP hydrolysis to move chromosomes, vesicles, and other organelles along microtubules Unidirectional, move cargo in one way Dyneins move things RETROGRADE (towards - end) Kinesins move things ANTEROGRADE (+ end)

Integral Membrane Proteins

Embedded in plasma membrane Attached by covalent bonds Types: Transmembrane - span lipid bilayer ECF/ICF multiple times - contain amphipathic alpha helices miscible with hydrophobic bilayer Embedded Lipid-anchored (bound to lipid by covalent bond)

Phospholipid Structure

Fatty Acids (FA tails) are nonpolar & hydrophobic Heads are Polar & Hydrophilic = amphiphatic, miscible in H20 or lipids forms a bilayer, that allows heads to dissolve in water and tails to repel water

Adhesion Molecules

Form physical contacts with the extracellular matrix that surround the cell for cell interactions examples: Cell- Matrix: integrins: allow communication between cell & its surrounding tissue Cell-Cell: - Cadherins: Ca2+ dependent (glycoprotein) - N-CAMs: Ca2+ independent, function in immune cell movement loss of cell adhesion molecules is observed in metastatic tumors

Smooth ER

Lack ribosomes (smooth appearance) Abundant in cells involved in lipid metabolism and synthesis (steroid hormones) SER in liver is a system known as the microsomal enzyme oxidizing system (MEOS) or cytochrome P450 Sarcoplasmic reticulum in muscles cells control levels of Ca2+ available to muscle for cell contraction

Microtubules

Largest cytoskeletal element, made of tubulin Function: 1. for cell structure and shape, especially neurons 2. transport secretory vesicles, especially neurons 3. Intracellular protein trafficking 4. sperm movement 5. cell division, during mitosis Both cilia and flagella are composed largely of microtubules, outgrowths of a basal body in the cell

Composition of Plasma Membrane

Lipids & Proteins are major constituents Lipids create hydrophobicity to prevent diffusion of many endogenous substances

Lipids of the Plasma Membrane

Lipids found primarily in glycerol based phospholipids glycerol is esterfied to fatty acids & phosphate = phosphodiester other lipids: glycolipid & cholesterol

Microtubule Structure

Long hollow, cyclindrical structure has protofilaments tubulin is a heterodimer = alpha and beta subunits Minus (-) End = closest to centrosome, bordered by alpha subunit Plus (+) End = extends from centrosome = beta subunit Alpha & Beta Subunits bind GTP and tubulin (faster on + side because only beta has GTPase Activity) examples: dynesin/kinesin

Tay-Sachs

Lysosomal storage disease Accumulation of gangliosides in neurons Causes seizure, blindness, degeneration of motor and mental performance

Actin

Most abundant protein F-Actin: filamentous, intact, attaches to cytoskeleton & proteins, (tips of microvilli) G-Actin: globular Interact with integrin receptors and form focal adhesion complexes for cell traction

Ion Channels/Pores (Integral Membrane Proteins)

Non-Gated - leaky channels are always open & ready for passive flow of ions down an electrochemical gradient Gated - regulated by various stimuli (chemical, voltage, mechanical) - gating determines when ions can move/whether It is open or closed

Peripheral Membrane Proteins

Not embedded in cell membrane, loosely attached via electrostatic interactions located on either side (just one side) Hydrophillic removed by salt solutions that disrupt ionic/hydrogen bonds example: Ankyrin "anchors" cytoskeleton in RBCS

Phospholipids in Outer Leaflet

Phosphatidylcholine, Sphingomyelin know chart organelle concentrations

Fluid-Mosaic Model of Membranes

Phospholipids & proteins "float" within the membrane & can diffuse "freely" over the membrane surface Free movement may be impeded by cytoskeletal structures in the cytoplasm Membranes are dynamic structures Membrane fluidity and permeability is influenced by cholesterol and degree of saturation of FA more double bounds (especially cis) = more fluid

Vesicular Transport

Products destined to remain inside the cell are in coated vesicles covered with clathrin (protein) > forms a lattice around the vesicle and associated proteins that later serve as recognition signals (receptor mediated endocytosis via clathrin coated pits) = Integral Proteins Products destined to leave the cell are in secretory vesicles > much larger than the coated vesicles and found in protein hormone producing cells > fuse with the plasma membrane and release contents via exocytosis

Chemical Nature of Phospholipids

Results in formation of a hydrophobic barrier to compartmentalize molecules to functionally distinct locations in the cell allows for creation of ion concentration gradients ex: proton H gradient for ETC, Na/K gradient needed for cell functions

Endoplasmic Reticulum

Rough ER (RER) = covered with ribosomes, site of protein synthesis 3 pathways for the insertion of membrane proteins depending on the organelle = mitochondrial, peroxisome, and plasma membrane/all other organelles mRNA has localization signal that determines if a secretory, membrane or cytosolic protein will be synthesized Secretory and membrane proteins are synthesized in the rough ER (RER) whereas cytosolic proteins are synthesized by free ribosomes in the cytosol All cells have RER (greater the secretory function the more abundant)

Cytosol

Semi-fluid (gelatinous) substance Contains cytoplasmic organelles Contains many enzymes that function in intermediary metabolism (sugars, fats, amino acids) Many "free" ribosomes to produce cytosolic proteins Cytosol also have inclusion bodies which fluctuates depending on the cell (glycogen storage vessels)

Intermediate Filaments

Stable (not reversibly assembled and disassembled) Neurofilaments are found in nerve cells and help stabilize projections from cell body Stabilize actin-myosin interaction in skeletal muscle Skin strength due partly to interlocking intermediate filaments (cells would rupture/blister without them) Proteins that make-up IF's are cell specific and used as cellular markers (vimentin is found in fibroblasts, cytokeratin in epithelial cells),

Golgi Apparatus

Stacked flattened membranous sacs (number of stacks is dependent on cell type)

Voltage Gated Ion Channels Characteristics

Voltage sensors = S4 Ion selective pore = between S5 & S6 Example = Na+ channels open/close (depolarize/hyperpolarize) alpha-subunits have 6 transmemebrane domains

Voltage Gated Ion Channels Examples

activated Na+ channels open and close as the membrane is depolarized or hyperpolarized transmembrane protein has voltage sensors and a selectivity filter example: Nicotinic Receptors when Na" channels open in response to ACh binding to alpha-subunits, Na+ moves down a concentration gradient

Cell/Plasma Membrane

cell envelope, separates cell from external environment serves as a barrier fluid within the cell is ICF, fluid outside is ECF (separates these compartments)

Glycocalyx (Peripheral Membrane Proteins)

consists of negatively charged glycosaminoglycans, giving the plasma membrane a fuzzy like coating Function: 1. repel negatively charged substances 2. attach to other cells 3. cell-cell recognition 4. play a major role in renal corpuscle (protein) where It is found on the lining of the capillary endothelium, negative charge repels large negatively charged macromolecules from leaking through the glomeruli