Lecture 9 - Plasma Membrane: Last Organelle PDF

LAST ORGANELLE: CELL MEMBRANE A DETAILED LOOK LESSON 9 (CHAPTER 5) BIO 131 Structure & Function of the Plasma Membrane ELECTRON MICROGRAPH (CELL MEMBRANE)  WHAT ARE MEMBRANES? Membranes cover the surface of every cell, and also surround most or...

LAST ORGANELLE: CELL MEMBRANE A DETAILED LOOK LESSON 9 (CHAPTER 5) BIO 131 Structure & Function of the Plasma Membrane ELECTRON MICROGRAPH (CELL MEMBRANE)  WHAT ARE MEMBRANES? Membranes cover the surface of every cell, and also surround most organelles within cells. They have a number of functions, such as: keeping all cellular components inside the cell allowing selected molecules to move in and out of the cell isolating organelles from the rest of the cytoplasm, allowing cellular processes to occur separately. a site for biochemical reactions allowing a cell to change shape. MEMBRANES: TIMELINE OF DISCOVERY The Davson–Danielli model was a model of the plasma membrane of a cell, proposed in 1935 by Hugh Davson and James Danielli. The model describes a phospholipid bilayer that lies between two layers of globular proteins. Sandwich Model Problems: varying chemical composition of membrane, hydrophobic protein parts PROBLEMS WITH THE DAVSON–DANIELLI MODEL It was unclear how the proteins in the model would permit the membrane to change shape without bonds being broken. Membrane proteins are largely hydrophobic and therefore should not be found where the model positioned them: in the aqueous cytoplasm and extracellular environment. The freeze-fracture method: revealed the structure of membrane’s interior Fluid Mosaic Model Mosaic: something made of small pieces  THE FLUID MOSAIC MODEL Proposed by E-face Jonathan Singer and Garth Nicolson in 1972. P-face protein This model suggested that proteins are found within, not outside, the phospholipid bilayer. FIRST MODEL VS. FLUID MOSAIC MODEL  THE FLUID MOSAIC MODEL PLASMA MEMBRANE What is it? What is it made of? What is its function?  Structure of the Cell Membrane Outside of cell Carbohydrate Proteins chains Lipid Bilayer Transport Protein Phospholipids Inside of cell (cytoplasm) Membrane Structure A membrane is a fluid mosaic of lipids, proteins and carbohydrate. The membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. → Amphipathic (hydrophobic & hydrophilic components) Hydrophilic Fluid Mosaic Polar heads Model of the love water & cell membrane dissolve. Hydrophobic fatty acid tails hide from water.  POLAR AND NONPOLAR BONDS There are two types of covalent bonds Nonpolar Covalent Bonds equal share of electrons Polar Covalent Bonds unequal share of electrons CHEMICAL FORMULA OF PHOSPHOLIPID  Fluid Mosaic Model - The plasma membrane is called the fluid mosaic model because - Phospholipids move within the membrane - Proteins in the membrane move among the phospholipids  HOW DOES THE MEMBRANE MOVE? ▪ A membrane is held in together by weak hydrophobic interactions. ▪Most membrane lipids and some proteins can drift laterally within the membrane ▪ Molecules rarely flip transversely (flip-flop) across the membrane, because hydrophilic parts would have to cross the membrane hydrophobic core. Fig. 8.4a  Lateral movement Flip-flop (~107 times per second) (~ once per month) Movement of phospholipids  Why is fluidity important?? ▪Membrane must be fluid to work properly. Solidification may result in permeability changes and enzyme deactivation. ▪Unsaturated hydrocarbon tails enhance membrane fluidity because kinks at the carbon-to-carbon double bonds hinder close packing of phospholipids.  Effects of Temperature on membranes As temperature cools: membranes switch from fluid to more solid state The temperature at which a membrane solidifies depends on the types of lipids What property of lipids would favor liquid versus solid state? Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids Membranes must be fluid to work properly; they are usually about as fluid as salad oil CHOLESTEROL ○ Cholesterol molecules embedded in the phospholipid bilayer controls the fluidity of the membrane (makes the membrane more or less fluid depending on the temperature). In animal membranes it resists changes by: limit fluidity at high temperatures hinder close packing at low temperatures.  Cholesterol Cholesterol within the animal cell membrane  MEMBRANE PROTEINS ▪ These proteins vary in both structure and function, and they occur in three spatial arrangements: The protein determine most of the membrane’s specific function. 1- Integral proteins – penetrate the hydrophobic core of bilayer - Often span the membrane 2- Peripheral proteins - Not embedded - Attached to extracellular or cytoplasmic surface 3. Lipid anchored proteins - Bound to lipid bilayer bound through lapidated amino acid residues  C A B Hydrophobic Hydrophilic interior ends  Some functions of membrane proteins Types of Membrane Proteins Receptor Proteins: Shaped in such a way that specific molecules bind to it. Allow a cell to respond to signals from other cells. Enzymatic Proteins: Catalyze specific reactions. Junction Proteins: Attach adjacent cells so that a tissue can fulfill a function. FUNCTIONS OF PERIPHERAL PROTEINS: To provide structural stability and shape to the plasma membrane  LIPID ANCHORED PROTEINS Proteins anchored in membranes through covalent bonding with membrane lipids Particular lipid groups anchor proteins to one of the surfaces of the plasma membrane The hydrophobic portion of the lipid is embedded in the membrane and the protein is bound covalently to the lipid Synthesized in the cytosol Carbohydrates Function: cell-cell recognition; developing organisms Glycolipids, glycoproteins (Antigens) Eg. blood transfusions are type-specific Plasma Membrane Components Carbohydrate Chains: ○ Contribute to cell’s “fingerprint”: allow cells to recognize one another. ○ Glycoproteins: Proteins with attached carbohydrate chains. ○ Glycolipids: Phospholipids with attached carbohydrate chains. Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual.  SUMMARY - FUNCTIONS OF THE DIFFERENT COMPONENTS Phospholipids – semi-permeability Proteins – peripheral and integral Glycolipids – information/signaling/binding Glycoproteins – information/signaling/binding Cholesterol – provide fluidity/flexibility (animal cells only) Construction of a Cell Membrane *Note, they use the pore protein = channel and channel= carrier!!