MEMBRANE STRUCTURE AND FUNCTION PLANT AND ANIMAL CELL
MEMBRANE STRUCTURE AND FUNCTION
PLANT AND ANIMAL CELL
PLANT AND ANIMAL CELL
PLASMA MEMBRANE � Edge of life � Separates living cell from its surroundings � 8 nm thick means 8000 membranes equal the thickness of thin page � Controls traffic into and out of the cell � Selectively permeable (make fundamentals of life) � Membranes form earlier in evolution of life � They enclose the solution different from its surroundings. � Membranes are vital because they separate the cell from the outside world. They also separate compartments inside the cell to protect important processes and events.
COMPOSITION OF MEMBRANE � Made up of lipids and proteins � Abundant lipids are phospholipids � Phospholipid is an amphipathic molecule having hydrophilic head and hydrophobic tail � Head consists of choline, phosphate and glycerol � Membrane proteins are also amphipathic having hydrophilic and hydrophobic region
UNSATURATED PHOSPHOLIPID MOLECULE
UNSATURATED PHOSPHOLIPID MOLECULE
FLUID MOSAIC MODEL
HISTORY � 1915: membranes isolated from RBCs were chemically analyzed, found to consist of lipids and proteins � 1925: Two Dutch scientists suggested membranes are bilayer of phospholipids because they could exist as stable boundary between two aqueous compartments � 1930 -40: Danielli and Davson studied triglyceride lipid bilayer over a water surface with the polar heads facing outward. They always formed droplets (oil in water) and the surface tension was much higher than that of cells. However, by the addition of proteins, the surface tension was reduced and the membranes flattened out.
RED BLOOD CELLS
� Surface of phospholipid bilayer adheres less strongly to water than the surface of biological membranes. � 1935: Davson and Danielli suggested that membranes are coated on both sides with hydrophilic proteins (Sandwich model) � 1950 s EM studies supports sandwich model � Two problems with sandwich model: � Membranes with different functions differ in structure and chemical composition � Proteins dissolve in cytosol but membrane proteins are not very soluble in water because they are amphipathic � If amphipathic proteins are layered on the surface of membrane their hydrophobic part would be in aqueous surroundings
SANDWICH MODEL
PROPERTIES OF PHOSPHOLIPID
PHOSPHOLIPID BILAYER
PHOSPHOLIPID BILAYER
SANDWICH MODEL
PLASMA MEMBRANES OF TWO CELLS SEPARATED BY INTERCELLULAR SPACE Four Unit membranes , Two unit membranes form plasma membrane of each cell
BACTERIAL CELL MEMBRANE
FLUID MOSAIC MODEL � 1972: S. J. Singer and G. Nicolson proposed that membranes proteins reside in the phospholipid bilayer with their hydrophilic regions protruding � This molecular arrangement maximize the contact of hydrophilic regions of proteins and phospholipids with water in cytosol and extracellular fluid � Membrane is a mosaic of protein molecule bobbing in a fluid bilayer of phospholipids � Confirmed by the freeze fracture split studies of membrane image EM
MEMBRANE STRUCTURE
FREEZE FRACTURE TECHNIQUE � A technique used to look at membranes that reveal the pattern of integral membrane proteins. � General outline of technique: 1. Cells are quickly frozen in liquid nitrogen (19°C), which immobilizes cell components instantly. 2. Block of frozen cells is fractured. This fracture is irregular and occurs along lines of weakness like the plasma membrane or surfaces of organelles. 3. Surface is removed by a vacuum (freeze etching) 4. A thin layer of carbon is evaporated vertically onto the surface to produce a carbon replica. 5. Surface is shadowed with a platinum vapor. 6. Organic material is digested away by acid, leaving a replica 7. Carbon-metal replica is put on a grid and examined by a transmission electron microscope.
FREEZE FRACTURE SCHEME
IMAGE OF MEMBRANE BY FREEZE FRACTURE
FREEZE FRACTURE DRAWING OF MEMBRANE
EM MICROGRAPHS OF MEMBRANE
FUNCTIONS OF MEMBRANES � What is the main function of the cell membrane? � Diverse functions in the different regions and organelles of a cell. However, at EM level, they share a common structure � The cell membrane regulates what enters and leaves the cell and also provides protection and support. � The Lipid Bilayer gives cell membranes a flexible structure that forms a barrier between the cell and its surroundings. � Cell Membrane protein molecules embedded in the lipid bilayer, some of which have carbohydrate molecules attached to them.
FLUID MOSAIC MODEL
FLUID MOSAIC MODEL
FLUID MOSAIC MODEL
INTEGRAL PROTEIN MOLECULE
FLUIDITY OF MEMBRANES � Membranes are not static sheets of molecules � Membranes molecules have hydrophobic interactions which are much weaker than covalent bonds � Lipids and proteins can shift laterally � Lipids can shift in flip-flop manner from one lipid layer to another is rare because hydrophilic part of the molecule has to pass through the hydrophobic region. � Lateral movement of phospholipid is rapid. � Adjacent phospholilipids switch positions about 107 times/sec: 2μm /sec
UNSATURATED AND SATURATED PHOSPHOLIPIDS AND CHOLESTEROL MOLECULE
FLUIDITY OF MEMBRANES Membranes remain fluid as temp decreases until the phospholipid become closely packed � Solidification temperature depends on the types of lipids it is made off � Unsaturated hydrocarbons have kinks at double bond are more fluid � Saturated hydrocarbons have no double bonds and tightly packed are less fluid and more viscous � Steroid cholesterol is wedged shaped between phospholipid of animal cell at 37°C makes the membrane less fluid by restraining the movement of phospholipid. It hinders the close packing of phospholipid therefore lowers the temperature of membrane solidification (fluidity buffer) �
UNSATURATED AND SATURATED PHOSPHOLIPIDS AND CHOLESTEROL MOLECULE
UNSATURATED AND SATURATED PHOSPHOLIPIDS AND CHOLESTEROL MOLECULE
EFFECT OF TEMPERATURE � Membranes work better when fluid. � Solidification: changes permeability and inactivate enzymatic proteins � Too fluid membranes can not support the protein function � Extreme environments pose a challenge for life and leads to evolutionary adaptation � Fishes in extreme cold environment have more unsaturated phospholipids � Bacteria at hot springs (90°C) have unusual phospholipids concentration � In Winter wheat % of unsaturated phospholipids increases in autumn
FUNCTIONS OF MEMBRANE PROTEINS � Proteins are mosaic part of membranes � Diverse proteins exist: 50 kinds of proteins in RBCs � Proteins determine most of the functions of membranes � Different types of cells contain different sets of proteins � Two types: �Integral proteins: in the hydrophobic interior and span the membrane �Peripheral protein: not embedded in lipid bilayer loosely bound on the surface of membrane
SOME ARE IMMOBILE DUE TO THEIR ATTACHMENTS WITH CYTOSKELETON OR EXTRACELLULAR MATRIX FLUID MOSAIC MODEL
PROTEINS ARE LARGER AND MOVE MORE SLOWLY
FUNCTIONS OF MEMBRANE PROTEINS � Transport Proteins: Spans around the membrane and provide hydrophilic channel across the membrane. Others shuttle a substance from one side to the other by changing shape (carrier protein), some may hydrolyze ATP as energy source to actively pump substances across the membrane
TRANSPORT PROTEIN
FUNCTIONS OF MEMBRANE PROTEINS �Enzymatic activity: A protein in the membrane may be an enzyme with its active site exposed to substances in adjacent solution. Several enzymes are arranged in series to carry out the various steps in metabolic pathways.
PROTEINS OF PHOTOSYNTHESIS PHOTOSYSTEM I
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