Chapter 5 Membrane Structure, Synthesis, and Transport PDF

Illustration by Smart-Servier Medical Art Chapter 5: Membrane Structure, Synthesis, and Transport  Table of contents 5. 5. 5.3 Fluidity of Synthesis of 1 Membrane Structure Membranes2...

Illustration by Smart-Servier Medical Art Chapter 5: Membrane Structure, Synthesis, and Transport  Table of contents 5. 5. 5.3 Fluidity of Synthesis of 1 Membrane Structure Membranes2 Membrane Components- Eukaryotic 4.4 5.5 5.6 Overview of Proteins that Carry Exocytosis and Membrane Transport Out Membrane Endocytosis Transport  1 5. Structure Membrane Illustration by Smart-Servier Medical Art  Illustration by Smart-Servier Medical Art Biological Membranes Plasma membrane (encloses the cytoplasm) and internal membranes that surround organelles are called biological membranes  Membrane Structure The framework of the membrane is the phospholipid bilayer Phospholipids are amphipathic molecules: Hydrophobic (water- fearing) region faces in, Hydrophilic (water-loving) region faces out Membranes also contains proteins and carbohydrates The two leaflets (halves of bilayer) are asymmetrical, with different amounts of each components  Fluid-Mosaic Model Membrane is considered a mosaic of lipid, protein and carbohydrate molecules Membrane resembles a fluid because lipids and proteins can move relative to each other within the membrane  Proteins Bound to Membranes Transmembrane proteins (integral membrane proteins) Have one or more regions physically embedded in the phospholipid bilayer; most transmembrane segments are alpha- helices Lipid-anchored proteins (integral membrane proteins) Lipid-molecule is covalently attached to an amino acid side chain within the protein; lipid tails inserted into the membrane Peripheral membrane proteins Noncovalently bound either to integral membrane proteins that project out from the membrane, or to polar head groups of phospholipids 5.2 Fluidity of Membranes Illustration by Smart-Servier Medical Art  Fluidity of Membranes Fluidity means that individual molecules remain in close association but can readily move within a membrane Membranes are semifluid Most lipids can rotate freely around their long axes and move laterally within the membrane leaflet But “flip-flop” of lipid from one leaflet to the opposite does not occur spontaneously Flippase requires ATP to transport lipids between leaflets https://www.youtube.com/watch?v=Qqsf_UJcfBc  Lipid Rafts Certain lipids associate strongly with each other to form lipid rafts A group of lipids floats together as a unit within the larger sea of lipids in the membrane Composition of lipid raft is different than the rest of membrane High concentration of cholesterol Unique set of membrane proteins  Factors Affecting Fluidity Length of phospholipid tails: Range from 14 to 24 carbons, 16 to 18 most common; shorter acyl tails are less likely to interact, which makes the membrane more fluid. Shorter tails allow transmembrane proteins to move more easily Presence of double bonds When a double bond is present lipid is said to be unsaturated; double bond creates a kink in the lipid tail, making it more difficult for neighboring tails to interact and making the bilayer more fluid Presence of cholesterol Cholesterol tends to stabilize membranes. Effects vary depending on temperature  Temperature and Fluidity Optimal level of fluidity essential for cell function, growth and division Too fluid= leaking Too solid= function of membrane proteins is inhibited Cells of many species adapt to temperature changes by altering the lipid composition of their membranes  Movement of Membrane Proteins Membrane proteins are larger than lipids but can still rotate and move laterally in the plane of a membrane; at a slower rate than lipids Flip-flop transmembrane proteins do not occur  Not All Integral Membrane Proteins Can Move Depending on the cell type, 10 to 70% of membrane proteins may be restricted in their movement Integral membrane proteins may be bound to components of the cytoskeleton, which restricts the proteins from moving laterally Membrane proteins may be also attached to molecules that are outside the cell, such as the interconnected network of proteins that forms the extracellular matrix  Illustration by Smart-Servier Medical Art 5.3 Synthesis of Membrane Components in Eukaryotic Cells  Synthesis of Membrane Components in Eukaryotic Cells Synthesis of Lipids In eukaryotes, the cytosol and endomembrane system work together to synthesize lipids Fatty acid building blocks are made via enzymes in cytosol or taken into cells from food ○ Building blocks are two fatty acids, one glycerol, one phosphate and a polar head group Process occurs at cytosolic leaflet of the smooth endoplasmic reticulum (ER)  Transfer of Lipids to Other Membranes Lipids in ER membrane can diffuse laterally to nuclear envelope Transport via vesicles to Golgi, lysosomes, vacuoles or plasma Lipid exchange proteins- extract lipid from one membrane for insertion in another  Synthesis of Transmembrane Proteins Except for proteins destined for semiautonomous organelles most transmembrane proteins are directed to the ER membrane first via an ER signal sequence If a polypeptide also contains a stretch of 20 mostly hydrophobic amino acids that form an alpha helix, this region will become a transmembrane segment From the ER, membrane proteins can be transferred via vesicles to other regions of the cell such as the Golgi, lysosomes, vacuoles, or the plasma membrane  Glycosylation Process of covalently attaching a carbohydrate to a protein or lipid Glycolipid- carbohydrate to lipid Glycoprotein- carbohydrate to protein Can serve as recognition signals for other cellular proteins Often play a role in cell surface recognition Helps protect proteins from damage  Glycosylation N-linked Glycosylation Attachment of carbohydrate to nitrogen atom of asparagine side chain Commonly occurs on membrane proteins that are transported to the cell surface O-linked Glycosylation Addition of sugars to oxygen atom of serine or threonine side chains Occurs only in Golgi In animals, important for the production of proteoglycans which are secreted from cells and help to organize the extracellular matrix  Clinical Correlation HbA1c, or hemoglobin A1c, is a blood test patients undergo to test for diabetes Hemoglobin, found in red blood cells, undergo irreversible glycosylation which lasts the entire life span of the red blood cell (120 days) HbA1c measures the percent of glycated hemoglobin, which gives an average blood sugar control over a 3-month period Normal: Below 5.7% Prediabetes: 5.7% to 6.4% Diabetes: 6.5% or higher of 5.4 Overview Transport Membrane Illustration by Smart-Servier Medical Art  Membrane Transport Membrane transport is the movement of ions and molecules across biological membranes The plasma membrane is selectively permeable Allows the passage of some ions and molecules but not others This structure ensures that: Essential molecules enter Metabolic intermediates remain Waste products exit  Ways to Move Across Passive Transport Membranes Requires no input of energy- down or with gradient Simple diffusion- substance moves across a membrane by passing directly through the phospholipid bilayer Facilitated diffusion- diffusion of a solute through a membrane with the aid of a transport protein Active Transport Moves a substance from an area of low concentration to one of high concentration with the aid of a membrane protein; requires input of energy  Phospholipid Bilayer Barrier Barrier to hydrophilic molecules and ions due to hydrophobic interior Ions and molecules are called solutes; they are dissolved in water which is a solvent Four factors affect the ability of solutes to pass through a phospholipid bilayer Size Polarity Charge Concentration  Relative Permeabilities High permeability occurs with gases and small uncharged molecules Moderate permeability occurs with water and urea Low permeability occurs with polar organic molecules Very low permeability occurs with ions, charged polar molecules, and large molecules  Clinical Correlation The cornea is one of the few organs that receives oxygen from the external environment, rather than blood vessels The cell membrane of corneal cells is permeable to oxygen. So, oxygen is able to pass directly through the cell membrane Contact lenses were designed to allow for the passage of oxygen to reach the cornea  Cells Maintain Gradients Living cells maintain a relatively constant internal environment different from their external environment Transmembrane gradient Concentration of a solute is high on one side of a membrane than the other Electrochemical gradient Both an electrical gradient and a chemical gradient Formation of a gradient requires input of energy  Osmosis Water diffuses through a membrane from an area with more water to an area with less water If the solutes cannot move, water movement can make the cell shrink or swell as water leaves or enters the cell  Solute Concentrations Across a Membrane Isotonic Hypertonic Hypotonic Equal solute Solute Solute concentrations on concentration is concentration is either side of the higher on one lower on one side membrane side of the of the membrane membrane  Osmosis in Animal Cells Animal cells must maintain a balance between extracellular and intracellular solute concentrations to maintain their size and shape Crenation- shrinking of a cell in a hypertonic solution Osmotic Lysis- swelling and bursting of a cell in a hypotonic solution  Osmosis in Plant Cells A cell wall prevents major changes in cell size If a cell takes up a small amount of water, the cell wall prevents osmotic lysis from occurring Plasmolysis- plasma membrane pulls away from the cell wall (when water exits the cell)  Osmosis in Freshwater Freshwater protists likeProtists Paramecium have to survive in a strongly hypotonic environment To prevent osmotic lysis, contractile vacuoles take up water and discharge it outside the cell Using vacuoles to remove excess water maintains a constant cell volume Out 5.5 Proteins That Carry Membrane Illustration by Smart-Servier Medical Art  Transport Proteins Transport proteins are transmembrane proteins that provide a passageway for the movement of ions and hydrophilic molecules across membranes Two classes based on type of movement Channels Transporters  Channels Form an open passageway for the facilitated diffusion of ions or molecules across the membrane Most are gated- open to allow the diffusion of solutes and close to prohibit diffusion Gated channels are controlled by the noncovalent binding of small molecules called ligands  Transporters Also known as carriers Conformational change transports solute across membrane Principal pathway for uptake of organic molecules, such as sugars, amino acids, and nucleotides  Transporter Types Uniporter Single molecule or ion Symporter or cotransporter Two or more ions or molecules transported in same direction Antiporter Two or more ions or molecules transported in opposite direction  Active Transport Movement of a solute across a membrane against its gradient from a region of low concentration to higher concentration Energetically unfavorable and requires the input of energy Primary active transport uses a pump Directly uses energy to transport solute Secondary active transport Uses a pre-existing gradient to drive transport  ATP-driven Ion Pumps Generate Electrochemical Gradients Na+/ K+ -ATPase Actively transports Na+ and K+ against their gradients using the energy from ATP hydrolysis 3Na+ are exported for every 2K+ imported into cell Antiporter- ions move in opposite directions Electrogenic pump- exports one net positive (+) charge Ion pumps play the primary role in the formation and maintenance of ion electrochemical gradients that drive many important cellular functions s and 5.6 Exocytosis Endocytosi Illustration by Smart-Servier Medical Art  Exocytosis and Endocytosis Used to transport large molecules and polysaccharides; involve packaging the transported substance into a membrane vesicle or vacuole Examples of Exocytosis and Endocytosis  Exocytosis Material inside the cell packaged into vesicles and excreted into the extracellular medium  Endocytosis Plasma membrane invaginates (folds inward) to form a vesicle that brings substances into the cell Three types of endocytosis: Receptor-mediated- receptor in the plasma membrane is specific for a given cargo; a vesicle forms to transport cardo into the cell Pinocytosis- membrane vesicles form from the plasma membrane to allow cells to internalize the extracellular fluid; “cell-drinking” Phagocytosis- an enormous membrane vesicle forms to engulf a large particle such as a bacterium  Clinical Correlation Most viruses enter cells via endocytosis They bind to an existing host cell receptor, and “trick” the host cell into bringing in virus Once the virus is inside, it can take over the host cell and cause damage