Cellular Biology & Homeostasis - Cell Membrane (VP 2024) PDF

Cellular Biology & Homeostasis CELL MEMBRANE PART 1 VP 2024 Clara Camargo, DVM LEARNING OBJECTIVES 1. Understand the functions and properties of biological membranes 2. List the components of cell membranes 3. List the types of cell membrane proteins and their roles 4. Understand the composition and...

Cellular Biology & Homeostasis CELL MEMBRANE PART 1 VP 2024 Clara Camargo, DVM LEARNING OBJECTIVES 1. Understand the functions and properties of biological membranes 2. List the components of cell membranes 3. List the types of cell membrane proteins and their roles 4. Understand the composition and structure of membrane lipids and the lipid bilayer 5. List some cell membrane visualization techniques 6. List the major cell membrane phospholipids, understand their structure and function CELL MEMBRANE No biological membranes = No life! regulates the movement of material into and out of the cell facilitates electrical signaling between cells defines the boundaries of organelles and separates complex chemical reactions  multiple differing functions Cell membranes: “Good fences make good neighbors” Overview of cell membrane functions https://www.youtube.com/watch?v=URUJD5NEXC8 CELL MEMBRANE Plasma membrane, cytoplasmic membrane Define the external boundaries of a cell Are flexible to allow for growth and movement, self-sealing and selectively permeable to polar solutes Are central to biological energy conservation and cell-to-cell communication Regulate the molecular traffic across boundary Can break and re-seal for fusion or fission In eukaryotic cells: compartmentalization to segregate processes and compartments Serve as attachment surface for cytoskeleton and extracellular structures THE EUKARYOTE CELL: A “Membranous” Unit Cellular compartmentalization: Functional separation within the cell 1 Cytoplasm 5 4 1. Plasma membrane 3 2. Nuclear membrane (inner and outer) Nucleus 3. ER membrane 4. Golgi apparatus membrane 2 5. Mitochondrial membrane (inner and outer) 5 Lysosomes 6. Peroxisomes and Lysosomes Endoplasmic reticulum 6 Mitochondria Plasma membrane 1 CELL MEMBRANE - COMPARTMENTALIZATION 1. Separation of antiparallel processes i.e. anabolic and catabolic processes can be held in separated compartments avoiding competition for the substrate or interference in the reactions Example: Fatty acid synthesis in cytosol and fatty acid oxidation in mitochondria 2. Separation of similar reactions serving different purposes i.e. similar reactions for different purposes and must therefore be held in independent compartments Example: fatty acid oxidation in mitochondria for energy production in peroxisomes for heat production 3. Coordination of different reactions which are involved in the same pathway (energy efficiency) Example: TCA and electron transport chain are the central point of energy metabolism in cells and are located in the mitochondria CELL MEMBRANE - COMPONENTS Lipids (phospholipids, sterol, glycolipids) about 50% of the mass of most animal cell membranes Proteins (transmembrane, peripheral) Carbohydrates (glycan groups) Water Divalent cations (Ca2+, Mg2+) Chemistry Libre Texts CELL MEMBRANE - PROTEINS Proteins that are part of or interact with biological membranes Physiology, Linda Constanzo 6th Ed. Most membrane proteins are transmembrane and mediate many functions such as  transport  catalysis of reactions (enzymes) Some transmembrane proteins serve as structural links connecting the cytoskeleton through the lipid bilayer to either the extracellular matrix or to an adjacent cell Others serve as receptors to detect and transduce signals Numerous different proteins are necessary for proper cell function and interaction  30% of human genome‘s proteins are membrane proteins Membrane proteins are targets of over 50% of all modern medicinal drugs CELL MEMBRANE - Lipids The lipid bilayer is a relatively impermeable barrier to most water-soluble (polar) molecules Lipid molecules make up about 50% of the mass of most animal cell membranes. Three main lipids:  Phospholipids (phosphoglycerides and sphingolipids)  Sterols (cholesterol in eukaryotic cells)  glycolipids All lipid molecules in cell membranes are amphipathic:  one hydrophilic (polar end)  one hydrophobic (nonpolar end)  Phospholipids are the most abundant membrane lipids CELL MEMBRANE - LIPID BILAYER Universal basis for membrane structure  The bilayer structure results from special properties of lipid molecules that cause their spontaneous assemblage into bilayers.  Common general structure: very thin film (average 5nm thick) of lipid and protein molecules, held together mainly by noncovalent interactions  Easily seen by electron microscopy. Specialized techniques are necessary (x-ray diffraction, freezefracture) to study details of its organization. Fluid mosaic model of cell membranes https://www.youtube.com/watch?v=LKN5sq5dtW4 Fluid, dynamic structures with most their molecules able to move in the plane of the membrane  FLUID MOSAIC MODEL CELL MEMBRANE – Thin layer A red blood cell’s membrane Membrane thickness will influence rate of diffusion Electron micrograph Lehninger- Principles of Biochemistry Viewed in cross-section: All cell membranes share a characteristic trilaminar appearance: plasma membrane appears as three-layer structure, 5-8 nm thick Trilaminar image consists of two electron-dense layers separated by a less dense central region. An electron micrograph of an E. coli cell highlighting the width of the cell inner and outer membranes and the cell wall. Zoom in: a schematic of the lipid bilayer. The red circle denotes the hydrophilic head consisting of a polar phosphoglycerol group and the pink lines represent the hydrocarbon chains forming a tight hydrophobic barrier excluding water as well as polar or charged compounds. Source: A. Briegel et al. Proc. Nat. Acad. Sci., 106:17181, 2009. CELL MEMBRANE – Prokaryotes vs Eukaryotes Prokaryotic plasma membranes are often composed: Most eukaryotic cells are more varied  one main type of phospholipid composed:  contain NO sterol (cholesterol) - with some rare  mixtures of different phospholipids exceptions  Their mechanical stability is steadied by an overlying cell wall  large amounts of sterols (cholesterol) stability and fluidity of membranes  phytosterol  fungal sterol  phytosterol Lehninger- Principles of Biochemistry CELL MEMBRANE – PROPERTIES summary Separates the cellular interior from the exterior Exchanges information with the environment in a controlled manner Membranes are extremely dynamic Membranes get information from the cell about metabolic status Membranes are selective barriers, allow selective transport of molecules into and out of the cell Membranes contain enzymes, transporters, receptors and other proteins https://vimeo.com/31411881 CELL MEMBRANE Membrane Fusion and Fission  Central to many cellular processes involving organelles and the plasma membrane  Allows for a combination of compartmentalization and transport of material  Both processes involve membrane reorganization without loss of continuity CELL MEMBRANE Membrane Fusion: 2 separate lipid bilayers merge to become 1  Transport vesicles from the ER fusing with Golgi membranes​, fusion of virus membrane Membrane Fission: Involves splitting of a membrane into 2 parts  Formation of vesicles by ER/Golgi apparatus to transport lipids and proteins to other organelles and to cell membrane CELL MEMBRANE – Micelles and bilayers Shape and amphipathic nature of lipid molecules:  causes their formation into bilayers spontaneously in aqueous environments RECAP: Hydrophilic molecules: dissolve readily in water due to their charged groups, or uncharged polar groups, which form either favourable electrostatic interactions, or hydrogen bonds, with water molecules Hydrophobic molecules are insoluble in water due to most or all of their atoms being uncharged and nonpolar and therefore unable to form energetically favourable interactions with water molecules Lipid molecules assemble with their hydrophobic tails in the interior and hydrophilic heads outside to water  Can do this in two ways: 1. Spherical micelles 2. Bimolecular sheets- bilayers Micelles, liposomes and bilayers Lipids spontaneously form micelles or bilayers in an aqueous environment Cone-shaped amphipathic molecules (fatty acids) form micelles, Cylinder-shaped molecules (phospholipids) form bilayers or liposomes CELL MEMBRANE The spontaneous closure of a phospholipid bilayer to form a sealed compartment is energetically favorable. Why? Closed structure is stable as it avoids the exposure of the hydrophobic hydrocarbon tails to water (which would be energetically unfavourable). This provides the bilayer‘s self-healing property: all free edges are avoided by closing in on itself. Vesicles and liposomes VESICLES are intra or extra cellular structures consisting of liquid/cytoplasm enclosed by a lipid bilayer  Exocytosis  Lysosomes  Endocytosis  Transport (ER to Golgi) It is separated from the cytosol → the inside of the vesicle can be different from the cytosolic environment  metabolism  transport  temporary storage of food and enzymes  chemical reaction chambers LIPOSOMES are vesicles made artificially in laboratory CELL MEMBRANE Techniques for visualizing cells: Electron microscopy (EM) 1. Scanning electron microscope (SEM): directly produces an image of the three-dimensional structure of the surface of a specimen 2. Transmission electron microscope (TEM): a beam of electrons is transmitted through a specimen to form an image, capturing fine detail 3. Freeze-fracture and freeze-etch electron microscopy: provide views of surfaces inside the cell VIDEO: https://en.wikipedia.org/wiki/Transmission_electron_microscopy THE FREEZE-FRACTURE TECHNIQUE: A special method to study the cell membrane Views of the internal organization of membranes are possible, expanding our understanding of the cell membrane It physically breaks apart (fracturing) a frozen biological sample Structural detail exposed by the fracture plane is then coated with a metal layer and visualized by transmission electron microscopy CELL MEMBRANE - PHOSPHOLIPIDS Polar head group and two hydrophobic hydrocarbon tails (fatty acids) Tails can differ in length (normally: 14-24 C) One FA usually contains one or more cis-double bonds (unsaturated), while other does not (saturated) Double bond creates small kink in tail Differences in length and saturation of fatty acid tails affect the ability of phospholipid molecules to pack together, and so guarantee the membrane fluidity CELL MEMBRANE - PHOSPHOLIPIDS The parts of a typical phospholipid molecule symbol schematic formula Space-filling model CELL MEMBRANE - PHOSPHOLIPIDS Four major phospholipids predominate in the plasma membrane of many mammalian cells: (make up more than half lipid mass in most membranes) 1. 2. 3. 4. Phosphatidylcholine Phosphatidylethanolamine Phosphatidylserine Sphingomyelin The bilayer is asymmetric with unequal distribution of  phospholipids  other lipids Phosphatidylinositol are present only in small  membrane proteins between the inner and the outer quantities, but are very important functionally layers (e.g. cell signaling) CELL MEMBRANE Four major phospholipids found in mammalian plasma membranes (A-C: phosphoglycerides; D: sphingolipid) At physiological ph:  Phosphatidylserine carries a net negative charge  while other phospholipids are electrically neutral (carrying one positive and one negative charge)