MEMBRANE STRUCTURE AND FUNCTION PLANT AND ANIMAL CELL

MEMBRANE STRUCTURE AND FUNCTION

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

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. However, they always formed droplets (oil in water) and the surface tension was much higher than that of cells. However, by the addition 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

PROPERTIES OF PHOSPHOLIPID

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

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

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 fore 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

FUNCTIONS OF MEMBRANE PROTEINS �Signal transduction: Membrane protein has binding site of a specific shape that it fits the shape of chemical messenger i. e. hormone. This binding cause a change in the confirmation of protein to allow it to relay the message inside cell by binding the cytoplasmic protein. (receptor proteins)

RECEPTOR PROTEINS BLOCK HIV ENTRY INTO CELLS

FUNCTIONS OF MEMBRANE PROTEINS � Cell–cell recognition: Glycoproteins of one cell membrane are recognized by membrane proteins of the other cell. It is short lived bindings ●Cell-cell recognition is by carbohydrates on the extra cellular surface of the membrane ● Membrane carbohydrates are short, branched chains of less than 15 sugar units ● Covalently bond to lipids (glycolipids), and proteins (glycoproteins) ● Vary from species to species, individuals of same species, organ to organ, one cell type to other cell of same species. ● They distinguish one cell from other cell Example: four blood groups A, B, AB and O are. due to variation in the carbohydrate of glycoproteins on the surface of RBCs

FUNCTIONS OF MEMBRANE PROTEINS � Intercellular joining: Membrane proteins of adjacent cells may hook together by various junctions i. e. gap junctions. Long lasting binding � Attachment to cytoskeleton and extracellular matrix (ECM): Microfilaments and other elements of cytoskeleton noncovalently bound to membrane proteins, maintain the cell shape and stabilize location of certain membrane proteins. ECM coordinates extra and intracellular changes

ROLE OF MEMBRANE CARBOHYDRATES IN CELL-CELL RECOGNITION � � � Cell-cell recognition is by carbohydrates on the extra cellular surface of the membrane Membrane carbohydrates are short, branched chains of less than 15 sugar units Covalently bond to lipids (glycolipids), and proteins (glycoproteins) Vary from species to species, individuals of same species, organ to organ, one cell type to other cell type of same species. They distinguish one cell from other cell Example: four blood groups A, B, AB and O are. due to variation in the carbohydrate of glycoproteins on the surface of RBCs

SYNTHESIS AND SIDEDNESS OF MEMBRANES � Membrane has distinct inside and outside faces � Two lipid layers differ in specific lipid composition � Protein has directional orientation in the membrane � This sidedness is determined during the synthesis of membrane by ER and Golgi apparatus

SYNTHESIS OF MEMBRANE COMPONENTS

Membranes regulate transport across the cellular boundaries to make their existence � Control the steady traffic of small molecules and ions in both directions � Chemical exchange between the muscle cell and extracellular fluid � Sugars, AAs and other nutrients enter the cell and metabolic products leave it through plasma membrane � Intake of O 2 and expulsion of CO 2 � Regulate concentrations of inorganic ions Na+, K+, Ca+ and Cl- by shuttling them � Membranes are selectively permeable and regulate the traffic of substances �

PERMEABILITY OF LIPID BILAYER � Nonpolar molecules i. e. , hydrocarbons, CO 2 and O 2 are hydrophobic can dissolve in the lipid bilayer of membrane and cross easily � Hydrophilic ions and polar molecules are impeded by the hydrophobic interior � Polar molecules i-e, water, glucose, other sugars pass slowly through lipid bilayer � Charged atoms and molecules cross the membranes even more slowly

TRANSPORT PROTEINS � Proteins of the membranes play key roles in regulating transport � Channel proteins have hydrophilic channel that is used as tunnel by certain ions and molecules � Aquaporins (channel proteins) facilitate the movement of water molecule across (3 billion molecules/sec) � Carrier proteins change the shape according to specific molecule for shuttling only that molecule � i. e. , a selective glucose transporter increase the transport 50, 000 times more across membrane

TRANSPORT PROTEINS

Selective permeability of the membrane is determined by both lipids and proteins

PASSIVE TRANSPORT DIFFUSION � Molecules have thermal energy due to their constant motion that results in diffusion � By diffusion the molecules spread evenly in available space � Movement of dye molecule through the synthetic membrane toward water is by diffusion � Any substance will diffuse down its concentration gradient or from higher concentration towards lower concentration � Diffusion is spontaneous process needing no input of energy

PASSIVE DIFFUSION

DIFFUSION THROUGH PERMEABLE MEMBRANE

DIFFUSION � Uptake of oxygen by the cell. Dissolved oxygen diffuses into the cell through plasma membrane as long as it is consumed by cellular respiration � Diffusion of a substance across the biological membrane is called a passive transport � Potential energy drives the diffusion � Selectivity of the biological membranes control the rate of diffusion of various molecules depends on the need of cell

OSMOSIS � The diffusion of free water across a selectively permeable membrane is called osmosis � Two sugar solutions of variable concentrations are on either side of the selectively permeable membrane. The pores of the membrane are so small that only water molecule can pass not the sugar molecule � Water will move from higher concentration towards lower concentration

OSMOSIS VIA SELECTIVELY PERMEABLE MEMBRANE

WATER BALANCE OF THE CELLS WITHOUT WALLS Tonicity is the ability of a surrounding solution to cause a cell to gain or loose water � Tonicity depends on the concentrations of solutes in a solution that can not cross the membrane relative to the inside of cell � If there is higher concentration of solutes in surrounding solution water will leave the cell or vice versa � Isotonic solution: no gain or loss of water and cell will remain same � Hypertonic solution: more solutes out side, cell will loose water, shrivel and die. i. e. higher salinity of a lake cause the death of animals � Hypotonic solution: less solutes out side, water will enter the cell faster than it leaves, the cell will swell and burst like an over filled balloon �

WATER BALANCE OF CELL WITHOUT WALLS

� Sea water is isotonic to marine animals. � Terrestrial animal cells live in isotonic environment. � Organisms that lack cell walls have adaptations for osmoregulation. i. e. unicellular protist Paramecium caudatum lives in hypotonic pond water have plasma membrane less permeable to water. It slowly uptake the water but does not burst due to having a contractile vacuole which force water out of the cell as fast as it enters by osmosis.

PARAMECIUM CAUDATUM CONTRACTILE VACUOLE

WATER BALANCE OF CELLS WITH WALLS � The cell of plants, fungi and prokaryotes are surrounded by walls which help I maintain the cell’s water balance � In hypotonic solution protoplasm swells and exert pressure on the wall in response the wall exerts pressure called turgor pressure and cell become turgid � In Isotonic solution no water enters the cell and cell become flaccid � In hypertonic solution the plasma lemma pulls away from wall and cell become plasmolysed and become dead

WATER BALANCE OF PLANT AND ANIMAL CELLS

FACILITATED DIFFUSION � Many polar molecules and ions impeded by the lipid bilayer diffuse passively by membrane proteins that span the membranes via facilitated diffusion � Channel proteins provide corridors to allow specific molecules or ions to cross the membrane � Aquaporin proteins are in high number in certain kidney cells to reclaim water from urine before excretion (Absence: 180 L urine/day have to drink equal volume of water

FACILITATED DIFFUSION

AQUAPORINS

� Ion channel proteins: trans port ions � Gated channel proteins open and close in response to stimulus (electrical) allowing K+ to leave the cell � Other gated channel proteins open and close when specific substance other than the one to be transported binds to the protein � Both gated channel proteins are important in function of nervous system � Carrier proteins bind and release the substance by conformational change (Glucose transporters)

ION CHANNEL PROTEIN

MECHANICALLY GATED CHANNEL PROTEINS

CHEMICALLY AND MECHANICALLY GATED CHANNEL PROTEINS

SIGNAL BINDING CHANNEL PROTEINS

PASSIVE TRANSPORT

ACTIVE TRANSPORT � Active transport is to pump solutes across the membrane against its concentration gradient � Carrier proteins perform active transport � Maintains the internal concentration of solutes of the cell i. e. animal cell has higher conc. K+ and lower conc. of Na + � Plasma Membrane maintains steep gradient by pumping Na+ out of the cell and K+ in cell � ATP supplies the energy for active transport by transferring its terminal phosphate group � Example is Sodium Potassium pump: 3 Na+ leave the cell and 2 K+ enters the cell

ION PUMPS Cells have voltages across the plasma membranes � Voltage is electrical potential energy � Cytoplasmic side is negatively charge relative to extracellular side due to an unequal distribution of ions and cations on both sides � Voltage across the membrane is called membrane potential (ranges -50 to -200 m. V) � Membrane potential affects the traffic of all charged substances across the membranes � Inside of cell is negative compared with out side the membrane potential favors the passive transport of cations into the cell and anions out of the cell � Chemical force (ion’s conc. gradient) and electrical force (membrane potential) act together is called electrochemical gradient � Ions diffusion is down to its electrochemical gradient �

� i. e. [Na+] inside the resting nerve is much lower than out side it. Upon cell’s stimulation channel opens that facilitate Na + diffusion. Na+ fall down their electrochemical gradient driven by [Na+] and by the attraction of these ions to negative side (inside) of membrane � Na+/K+ pump translocate 3 Na + outside and 2 K + inside the cell in one crank of cycle and stores energy as voltage � Transport protein generates voltage across a membrane is called electrogenic pump i. e. Na/K pump in animal cell and proton pump in plants, fungi and bacteria that pumps H +

SODIUM POTASSIUM PUMP

PROTON PUMP � Proton pump transfers H + out side the plasma membrane in extracellular solution � Generates voltage across the membranes � Electrogenic pumps store energy that can be tapped for cellular work � Proton gradient is used for ATP synthesis during cellular respiration

PROTON PUMP

PROTON PUMP GENERATE CELLULAR ATP

PROTON PUMP INHIBITORS USED TO CONTROL ACIDITY IN STOMACH

COTRANSPORT �A single ATP-powered pump that transport a specific solute can indirectly drive active transport of other solutes is called cotransport � A cotransporter protein separate from H+ pump drives the active transport of amino acids, sugars, several other nutrients into cell. i. e. sucrose-H+ transporter � Sucrose-H+ cotransporter load sucrose produced by photosynthesis into leaf veins � During diarrhea cotransporter of colon reabsorb Na from waste to maintain constant level in the body but diarrhea expels waste so rapidly that reabsorption is not possible and Na level falls. � For treatment salt and glucose is given which is picked up by intestinal cotransporter and pass in the blood.

COTRANSPORT

ANTIPORTER PROTEINS � An antiporter (counter-transporter) is an integral membrane protein involved in 2° active transport of two or more different molecules or ions across a phospholipid membrane � In 2° active transport, one species of solute moves along its electrochemical gradient, allowing a different species to move against its own electrochemical gradient � Example, the Na+/Ca 2+ exchanger removes cytoplasmic calcium, exchanges one calcium ion for three sodium ions.

ANTIPORTER

TYPES OF TRANSPORTS ACROSS THE MEMBRANES

EXOCYTOSIS � Exocytosis is the secretion of biological molecules by fusion of vesicles with plasma membranes � Golgi vesicles carrying substances move to the plasma membrane and fuses its membrane with PM and content of the vesicle spills outside the cell � i. e. cells in pancrease secretes insulin extracellularly by exocytosis. Neurons release neurotransmitters by exocytosis that signals other neurons or muscle cells. � i. e. Plant cell wall synthesis is by exocytosis

EXOCYTOSIS

EXOCYTOSIS AND ENDOCYTOSIS

ENDOCYTOSIS � In take of biological molecules and particles by forming vesicles from plasma membrane is called endocytosis � A small area of plasma membrane sinks inward to form a pocket which deepens and pinches in forming vesicles containing extracellular material � Three types of endocytosis: 1. Phagocytosis (cellular eating) 2. Pinocytosis (cellular drinking) 3. Receptor mediated endocytosis

� Phagocytosis: a cell engulfs a particle wrapping pseudopodia around it and packaging like a food vacuole. Particle will be digested after lysosomal fusion with it. � Pinocytosis: a cell gulps droplets of extracellular fluid into tiny vesicles. � Receptor-mediated endocytosis: a cell aquire bulk quantities of specific substances embedded in the membrane are proteins with specific receptor site exposed to exterior fluid from where ligands bind. The receptor proteins cluster in region of membrane called coated pits which are lined on exterior by coat proteins. The ingested material is liberated out and receptors recycled to plasma membrane by same vesicle

ENDOCYTOSIS (PHAGOCYTOSIS) AND EXOCYTOSIS IN BACTERIAL CELL

TRANSPORTER PROTEINS