Chapter 11 Study Guide Jyoti
Chapter 11 Study Guide Jyoti BIO 2600
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This 8 page Study Guide was uploaded by Markiesha Notetaker on Thursday April 7, 2016. The Study Guide belongs to BIO 2600 at Wayne State University taught by Dr. Jyoti Nautiyal in Winter 2016. Since its upload, it has received 211 views. For similar materials see Intr To Cell Biology in Biology at Wayne State University.
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Date Created: 04/07/16
Chapter 11 Study Guide Membrane Structure The plasma membrane consists of a two-ply sheet of lipid molecules about 5nm thick into which proteins are inserted. Types of proteins in the membrane: 1. Receptor proteins- receive signals from environment 2. Transport proteins- enable import and export of small molecules The simplest bacteria only has a single plasma membrane. Eukaryotic cells contain internal membranes including: Endoplasmic reticulum Nucleus Peroxisome Lysosome Transport vesicle Golgi apparatus Mitochondria Lipid Bilayer All cell membranes are composed of lipids and proteins and share a common structure. The lipids have a hydrophilic head and a hydrophobic tail. The most abundant in cell membranes are phospholipids. They have a phosphate head linked to a pair of hydrophobic tails. These tails are termed fatty acids only when they have a COO- group at one that attaches to glycerol. There is movement in the bilayer. It is able to bend and some phospholipid molecules can sometimes “flip-flop”. Meaning one can go from one half of the bilayer to the other half. Some molecules continuously exchange places with neighbors in the same layer. Fluidity Fluidity of the bilayer depends on the hydrocarbon tails. 1. Length of tail a. Chains are short= more fluidity because they don’t interact that much b. Usually between 14-24 carbon atoms (18-20 being the usual). 2. Number of double bonds they contain a. Unsaturated tails (with one or more carbon double bonds) make it harder to pack tightly so these make the membrane more fluid. b. Saturated tails- these are straight and make it easy to pack tightly so these make the membrane more rigid. Fluidity of the bilayer depends on temperature. Higher temperature- Tails are longer with less double bonds Fluidity of the bilayer depends on amount of cholesterol. Cholesterol is a sterol. It constitutes about 20% of the lipids in the plasma membrane. It is short and rigid and fills in the spaces left by unsaturated hydrocarbon tails. It stiffens the bilayer. It is not present in bacteria membranes, only eukaryotic membranes. Membrane Assembly Begins in the ER New phospholipids are manufactured by enzymes bound to the cytosolic (inside) surface of the endoplasmic reticulum. 1. The enzymes deposit the phospholipids into the cytosolic side of the bilayer. 2. Scramblases move random phospholipids from one half of the bilayer and put them in the other. 3. Final bilayer has equal amounts of phospholipids. Cell Membranes are Asymmetrical In the Golgi apparatus membrane, there are enzymes called flippases that remove specific phospholipids from the exterior space and flip them into the layer that faces the cytosol. This maintains the asymmetric arrangement of phospholipids. 1. Golgi membrane receives new phospholipids from ER 2. Flippase catalyzes transfer of specific phospholipids to specific sides Inside phospholipids- inositol, ethnolamine, serine Outside phospholipids- choline, sphingomyelin, carbohydrates (glycosylation), glycolipids Membrane Proteins These proteins allow for most membrane functions. A cell membrane consists of: 50% total mass due to proteins (because they are bigger) 50X more lipids (because they are smaller) They can be associated with the membrane in different ways including: 1. Transmembrane Proteins o Extends through the bilayer with parts on either side o Has hydrophobic and hydrophilic regions. 2. Monolayer a-helices a. Located almost entirely in cytosol and associated with the bilayer by an a-helix 3. Lipid-linked a. Entirely outside the bilayer and attached only by lipid groups covalently attached to the membrane. 4. Protein-attached a. Some are attached to the bilayer solely by attaching to other membrane proteins. To detach proteins that are directly attached to the bilayer (integral proteins) detergent is used. To detach ones that are indirectly bound (peripheral proteins) only gentle extraction is needed. The parts of transmembrane proteins that run through the hydrophobic interior of the bilayer are composed of amino acids with hydrophobic side chains. The polypeptide backbone though, is hydrophilic due to the peptide bonds between the amino acids. For this reason, transmembrane regions are mostly in a a-helix. Hydrophobic amino acid side chains on outside of helix (next to hydrophobic tails) Polypeptide backbone (polar) on inside of helix Beta sheets also can be rolled into a barrel shape and form a porin (large pore). Detergents can be used to purify proteins To study a specific protein, you must separate it by solubilizing the membrane with agents that destroy the lipid bilayer. The most commonly used is detergents. Amphipathic and only has one hydrophobic tail The hydrophobic ends of detergent interact with membrane spanning hydrophobic regions of proteins and hydrophobic tails of phospholipids This disrupts the bilayer. The other part of the detergent is hydrophilic and attaches to the protein and brings the protein into the solution with it. Plasma Membrane and Cell Cortex Plants, yeasts, and bacteria all have a rigid cell wall that encases the plasma membrane. The plasma membrane of animal cells is stabilized by the cell cortex that is attached to the underside of the membrane. Example) Red blood cells have a cortex. These cells are small and flattened Their cortex is a long thin flexible rod about 100 nm in length; provides shape and support for plasma membrane The spectrin meshwork is connected to the membrane through attachments to transmembrane proteins o Abnormalities in the spectrin meshwork can lead to anemia (fewer red blood cells and they are spherical and fragile) Example) Animal cells Cortex is rich in actin and myosin Restricting movement of membrane proteins Proteins can be localized to certain areas in the membrane creating membrane domains. 1. Tethered to areas inside of the cell (ex. Cortical skeleton) 2. Tethered to areas outside the cell (ex. Extracellular matrix) 3. To proteins on other cells (ex. Cell adhesion molecules) 4. Diffusion barriers (ex. Junctional complexes) a. Makes it so that certain proteins do not move to other areas. Depends solely on functions. Carbohydrate layer Not only do lipids have sugars attached to them, but proteins do to. Proteins with short sugar chains (oligosaccharides) on them are called glycoproteins. These proteins are only located on the outside of the plasma membrane and forms a coat of carbohydrates called a glycocalyx. This plays an important part in cell-cell recognition and adhesion. The carbohydrate layer serves as a distinctive clothing. It is recognized by other cell types that interact with it. Specific oligosaccharides in the layer are recognized by lectins. Example) White blood cells The carbohydrate on a white blood cell is recognized by a lectin on an infected blood vessel. The recognition causes for the carbohydrate on the white blood cell, called the neutrophil, to adhere to the blood vessel wall then migrate from the bloodstream into the infected tissue to destroy bacteria. Extra Notes from lecture Blood Typing
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