edge of life separates living cell from its surroundings 8nm thick means 8000 membranes equal the...
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MEMBRANE STRUCTURE
AND FUNCTION
PLANT AND ANIMAL CELL
PLANT AND ANIMAL CELL
PLASMA MEMBRANE
Edge of life Separates living cell from its surroundings 8nm 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. 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)
1950s 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
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 ice 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 and 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 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 lipids 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 PHOTOSYNTHESISPHOTOSYSTEM 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 O2 and expulsion of CO2 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, CO2 and O2 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 (3billion 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 TRANSPORTDIFFUSION
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: 180L 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 mV)
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+/Ca2+ 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
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