Plasma Membranes and Cytoplasmic Membrane Systems PDF

PLASMA MEMBRANES AND CYTOPLASMIC MEMBRANE SYSTEMS Bianca Louise Fuentes, MD Faculty, Department of Biology PLASMA MEMBRANE Also called cell membrane Only 5-10 nm wide Trilaminar appearance Semipermeable membrane (selective) 3 4 FLUID MOSAIC MODE...

PLASMA MEMBRANES AND CYTOPLASMIC MEMBRANE SYSTEMS Bianca Louise Fuentes, MD Faculty, Department of Biology PLASMA MEMBRANE Also called cell membrane Only 5-10 nm wide Trilaminar appearance Semipermeable membrane (selective) 3 4 FLUID MOSAIC MODEL By Singer and Nicholson, 1972 6 CHEMICAL COMPOSITION OF MEMBRANES Membranes are lipid–protein assemblies in which the components are held together in a thin sheet by noncovalent bonds. The lipid bilayer serves primarily as a structural backbone of the membrane and provides the barrier that prevents random movements of water-soluble materials into and out of the cell. The ratio of lipids to proteins in a membrane varies, depending on the type of cellular membrane (plasma vs. endoplasmic reticulum vs. Golgi), the type of organism (bacterium vs. plant vs. animal), and the type of cell (cartilage vs. muscle vs. liver). 7 8 CHEMICAL COMPOSITION OF MEMBRANES MEMBRANE LIPIDS Membranes are Amphipathic (contain both hydrophilic and hydrophobic regions) 3 main types: Phosphoglycerides Sphingolipids Cholesterol 9 CHEMICAL COMPOSITION OF MEMBRANES MEMBRANE LIPIDS Phosphoglycerides Most membrane lipids contain a phosphate group, which makes them phospholipids. Because most membrane phospholipids are built on a glycerol backbone, they are called phosphoglycerides. Membrane glycerides are diglycerides—only two of the hydroxyl groups of the glycerol are esterified to fatty acids; the third is esterified to a hydrophilic phosphate group. A membrane fatty acid may be fully saturated (i.e., lacking double bonds), monounsaturated (i.e., possessing one double bond), or polyunsaturated (i.e., possessing more than one double bond). Phosphoglycerides often contain one unsaturated and one saturated fatty acyl chain. 10 11 UNSATURATED Fatty Acids contain DOUBLE BONDS 12 CLINICAL CORRELATE FISH OIL two highly unsaturated fatty acids. EPA and DHA found at high concentrations in fish oil. EPA and DHA contain five and six double bonds, respectively and are incorporated into phosphatidylethanolamine (PE) and phosphatidylcholine (PC), most notably in the brain and retina. EPA and DHA are described as omega-3 fatty acids because their last double bond is situated three *EPA (eicosapentaenoic acid) carbons from the omega (CH3) end *DHA (docosahexaenoic acid) of the fatty acyl chain. 13 CHEMICAL COMPOSITION OF MEMBRANES MEMBRANE LIPIDS Cholesterol For membrane fluidity Sphingolipids Derivatives of sphingosine, an amino alcohol Sphingosine + fatty acid = ceramide Examples: sphingomyelin, glycolipid (cerebroside, ganglioside) 14 CHEMICAL COMPOSITION OF MEMBRANES MEMBRANE CARBOHYDRATES (CHO) Glycocalyx- sugar coat 2 types: CHO (oligosaccharides) + Proteins = Glycoproteins (90%) CHO + Lipids = Glycolipids (10%) Blood type A has an enzyme that adds an N-acetylgalactosamine to the end of the chain. A person with type B blood has an enzyme that adds galactose to the chain terminus.. People with AB blood type possess both enzymes. People with O blood type lack enzymes capable of attaching either terminal sugar. 15 16 CHEMICAL COMPOSITION OF MEMBRANES MEMBRANE PROTEINS Integral Proteins Transmembrane proteins They pass entirely through the lipid bilayer and thus have domains that protrude from both the extracellular and cytoplasmic sides of the membrane. Peripheral Proteins located entirely outside of the lipid bilayer, on either the cytoplasmic or the extracellular side, yet are associated with the surface of the membrane by noncovalent bonds. Lipid-anchored proteins located outside the lipid bilayer, on either the extracellular or the cytoplasmic surface, but are covalently linked to a lipid molecule that is situated within the bilayer. 17 FLUID MOSAIC MODEL By Singer and Nicholson, 1972 18 CELL TRANSPORT Passive Active High concentration --> Low conc Low concentration --> High conc Downhill Uphill With or Without carrier protein With Carrier proteins Normal Kinetic Motion of Water Additional energy source CELL TRANSPORT Passive Active Simple Diffusion Primary Active Facilitated Diffusion Secondary Active Simple Diffusion Passive | Downhill Two pathways: (1) interstices of lipid bilayer for lipid-soluble substances (2) through water channels Image source: Chapter 4, Guyton and Hall Physiology 14th edition Osmosis Net Diffusion of Water Image source: Chapter 4, Karp’s Cell and Molecular Biology, 9th edition Facilitated Diffusion Passive | Downhill Carrier-mediated diffusion Conformational change of carrier protein Image source: Chapter 4, Guyton and Hall Physiology 14th edition Primary Active Transport Active | Uphill Energy derived from ATP breakdown Examples: (1) Na-K ATPase pump (2) H-K ATPase pump Image source: Chapter 4, Guyton and Hall Physiology 14th edition Primary Active Transport Active | Uphill H-K ATPase pump in the stomach inhibited by Proton Pump Inhibitors (PPIs) Image Source: https://www.ritemed.com.ph/products/rm-omeprazole-40-mg-tab Secondary Active Transport Active | Uphill Energy derived from Sodium transport Examples: (1) Sodium-Glucose Co-transport (2) Sodium-Calcium Countertransport Image source: Chapter 4, Guyton and Hall Physiology 14th edition ENDOMEMBRANE SYSTEMS An extensive membrane network within the cytoplasm. Endoplasmic reticulum, Golgi complex, endosomes, lysosomes and vacuoles Transport vesicles move through the cytoplasm in a directed manner, often pulled by motor proteins that operate on tracks formed by microtubules and microfilaments of the cytoskeleton. When it reaches its destination, a vesicle fuses with the membrane of the acceptor compartment, which receives the vesicle’s soluble cargo as well as its membranous wrapper. Repeated cycles of budding and fusion shuttle a diverse array of materials along numerous pathways that traverse the cell. 27 ENDOMEMBRANE SYSTEMS Membrane contact sites the cell can build temporary tethers that connect two organelles in a dynamic and regulated way The best characterized are ER-mitochondria and ER-Golgi contact sites. 28 Schematic diagram illustrating the process of vesicle transport by which materials are transported from a donor compartment to a recipient compartment. Vesicles form by membrane budding, during which specific membrane proteins (green spheres) of the donor membrane are incorporated into the vesicle membrane and specific soluble proteins (purple spheres) in the donor compartment are bound to specific receptors. When the transport vesicle subsequently fuses with another membrane, the proteins of the vesicle membrane become part of the recipient membrane, and the soluble proteins become sequestered within the lumen of the recipient compartment. 29 A biosynthetic pathway can be discerned in which proteins are synthesized in the endoplasmic reticulum, modified during passage through the Golgi complex, and transported from the Golgi complex to various destinations, such as the plasma membrane, a lysosome, or the large vacuole of a plant cell. This route is also referred to as the secretory pathway, because many of the proteins synthesized in the endoplasmic reticulum (as well as complex polysaccharides synthesized in the Golgi complex are destined to be discharged (secreted or exocytosed) from the cell. 30 Secretory activities of cells can be divided into two types: constitutive and regulated. Constitutive secretion, materials are transported in secretory vesicles from their sites of synthesis and discharged into the extracellular space in a continual manner. Most cells engage in constitutive secretion, a process that contributes not only to the formation of the extracellular matrix, but also to the formation of the plasma membrane itself. 31 Secretory activities of cells can be divided into two types: constitutive and regulated. Regulated secretion, materials are stored as membrane-bound packages and discharged only in response to an appropriate stimulus. Regulated secretion occurs, for example, in endocrine cells that release hormones, in pancreatic acinar cells that release digestive enzymes, and in nerve cells that release neurotransmitters. In some of these cells, materials to be secreted are stored in large, densely packed, membrane-bound secretory granules Proteins, lipids, and complex polysaccharides are transported through the cell along the biosynthetic or secretory pathway. 32 33  Endocytic pathway operates in the opposite direction. By following the endocytic pathway, materials move from the outer surface of the cell to compartments, such as endosomes and lysosomes, located within the cytoplasm. 34 ENDOPLASMIC RETICULUM Evolved from invaginations of the plasma membrane. Inside ER membranes is called the luminal or cisternal space. Undergoes turnover and reorganization. Two compartments: Rough ER Smooth ER Has ribosomes No ribosomes Network of flattened sacs (cisternae) Highly curved and tubular Continuous with outer membrane of nuclear Forms an interconnecting system of pipelines envelope traversing the cytoplasm Rough-surfaced vesicles Smooth-surfaced vesicles 35 SMOOTH ER (SER) Synthesis of steroid hormones in the endocrine cells of the gonads and adrenal cortex. Detoxification in the liver of a wide variety of organic compounds like ethanol. Detoxification is carried out by the cytochrome P450 family. Sequesters calcium ions within the cytoplasm of cells (skeletal and cardiac muscle cells) 36 ROUGH ER (ER) Synthesis of secreted proteins, lysosomal proteins and integral membrane proteins. The RER is the starting point of the biosynthetic pathway: site of synthesis of proteins, carbohydrate chains, and phospholipids that journey through the membranous compartments of the cell. The addition of sugars (Glycosylation) to the Asparagine residues of proteins begins in the RER and continues in the Golgi complex. Enzyme responsible: Glycocyltransferases 37 GOLGI COMPLEX Flattened, dislike, membranous cisternae with dilated rims and associated vesicles and tubules. Functions as a processing plant, modifying the membrane components and cargo synthesized in the ER before it moves on to its target destination. Functionally distinct compartments: CGN- Cis or entry face closest to the ER – Sorting station TGN - Trans or exit face at the opposite end of the stack – Segregated into vesicles/ Delivery unit 38 39 CLASSES OF COATED VESICLES COPII-coated Moves materials from the ER “forward” to the Golgi vesicles complex COPI-coated Moves materials in a retrograde direction: 1. from vesicles Golgi complex to ER; 2. from TGN backward to CGN Clathrin-coated Move materials from TGN to endosomes, lysosomes vesicles and plant vacuoles; move materials from the plasma membrane to cytoplasmic compartments along the endocytic pathway 40 41 LYSOSOMES An animal cell’s digestive organelle Contains at least 50 hydrolytic enzymes produced in the rough ER. The enzymes are called acid hydrolases as they show optimal activity at acidic pH (Optimum pH is 4.6) Plays a role in cell turnover or regulated destruction of own organelles and their replacement. This is called AUTOPHAGY or Cell Suicide. Once the digestive process in the autophagolysososome has been completed, the organelle is termed residual body. 42 43 PLANT VACUOLE 90% in a plant cell Solutes and macromolecules are temporarily stored. May also store host of toxic compounds. Tonoplast, contains several active transport systems that pump ions into the vacuolar compartment to a concentration much higher than that in the cytoplasm or the extracellular fluid. Site of intracellular digestion and may have some acid hydrolases found in lysosomes. 44 45  Endocytic pathway operates in the opposite direction. By following the endocytic pathway, materials move from the outer surface of the cell to compartments, such as endosomes and lysosomes, located within the cytoplasm. 46 ENDOCYTIC PATHWAY Two categories: bulk-phase and receptor-mediated BULK-PHASE ENDOCYTOSIS RECEPTOR-MEDIATED ENDOCYTOSIS also known as pinocytosis clathrin-mediated endocytosis is the nonspecific uptake of extracellular uptake of specific extracellular fluids. Any molecules, large or small, macromolecules (ligands) following that happen to be present in the their binding to enclosed fluid also gain entry into the receptors on the external surface of the cell. plasma membrane. 47 48 ENDOCYTIC PATHWAY Following internalization, vesicle-bound materials are transported to a dynamic network of vesicles known as endosomes. The fluid in the lumen of endosomes is acidified by a H+-ATPase in the boundary membrane. Endosomes are divided into two classes: early endosomes, which are typically located near the peripheral region of the cell, and late endosomes, which are typically located closer to the nucleus. According to the prevailing model, early endosomes progressively mature into late endosomes. This transformation from an early to a late endosome is characterized by: a decrease in pH, an exchange of Rab proteins (e.g., between Rab5 and Rab7), and a major change in the internal morphology of the structures. 49 50 PHAGOCYTOSIS “cell-eating” Many single-celled protists, such as amoebas and ciliates, make their livelihood by trapping food particles and smaller organisms and enclosing them within folds of the plasma membrane. The folds fuse to produce a vacuole (or phagosome) that pinches off inwardly from the plasma membrane. The phagosome fuses with a lysosome, and the material is digested within the resulting phagolysosome. Mammals possess a variety of “professional” phagocytes, including macrophages and neutrophils, that wander through the blood and tissues phagocytizing invading organisms, damaged and dead cells, and debris. 51 52