MembranesMembranes

Introduction Introduction Properties attributed to living organisms

(movement, growth, reproduction &metabolism etc) depend on membranes

All membranes – same general structure (lipid & protein mols)

Currently accepted concept of membrane:“FLUID MOSAIC MODEL”

(membrane is a bimolecular lipid layer)The proteins, most of which float within

the lipid bilayer- determine’s the membrane biological functions

Membrane structureMembrane structurediff type of cell has its own

function – unique membrane structure

Proportion & type of lipid and protein varies among cells.

Membrane lipidsMembrane lipidsAmphiphatic mol

suspend in water:- Hydrophobic – buried in

water- Hydrophilic – exposed to

waterPhospholipid form into

bimolecular layers when sufficiently concentrated (basis of cell membrane)

Function of membrane Function of membrane lipids:lipids:1) Membrane fluidity Phospholipids in the plasma

membrane can move within the bilayer

Lateral diffusion - movement of lipids & proteins in membrane, rapid & spontaneously process

Transverse diffusion – movement of lipids & proteins from one side of lipid bilayer to the other, rare.

Membrane fluidity is determined by the % of unsaturated FAs in its phospholipid molecules.

High conc of unsaturated chain- more fluid membrane

2) Selective permeabilityallow certain molecules or ions to pass

through it by diffusionThe hydrophobic nature of the membrane

makes it impenetrable to the transport of ionic and polar substances.

Membrane proteins regulate the movements of ionic and polar substances by binding to the protein carrier or by providing a channel.

Nonpolar substances diffuse through lipid bilayer down their concentration gradient

3) Self-sealing capability

When lipid bilayer disrupted, immediately & spontaneously reseal

Because a break in a lipid bilayer exposes the hydrophobic hydrocarbon chains to water.

In living cells, certain protein component of membrane, cystoskeleton, calcium ion also assist in membrane resealing.

Lipid bilayer

When hydrophobic tails of lipid bilayer exposed to polar water mol,

lipid form hydrophilic edges consisting of polar head groups

As membrane edges draw closer to each other

They fuse and reform the bilayer

4) AsymmetryBiological membranes are asymmetricLipid composition of each half of a

bilayer is diff.Because each side of membrane is

exposed to diff. environment. Eg. the human red blood cell

membrane possesses more phosphatidylcholine and sphingomyelin on its outside surface.

Membrane proteinsMembrane proteinsprotein molecule that is attached

to, or associated with the membrane

Most membranes require proteins to carry out their functions

Classified according to their structural relationship to membrane :-

1)Integral proteins2)Peripheral proteins

Integral proteins - are embedded in and/or extend through

the membrane.- Can be extracted by disrupting membrane

with organic solvents/detergents- Ion channel, proton pump

Peripheral proteins - are bound to membranes primarily

through interactions with integral proteins (hydrophobic, electrostatic, non covalent)

- Can be released from membrane by gentle methods (pH change)

- Hormone, enzyme

Membrane functionsMembrane functionsMembranes are involved in:

1)Transport of molecules and ions into and out of cells and organelles

2)Binding of hormones and other biomolecules

1) Membrane transport1) Membrane transport

mechanisms that regulate the passage of solutes such as ions and small molecules through membranes

Ions & mols constantly move across cell plasma membranes & organelles

movements of most solutes through the membrane are mediated by membrane transport proteins

types of membrane transport are passive transport and active transport.

a) Passive transport

Diffusion of solute through membrane

No need of energy Concentration gradient represents

the potential energy 3 types 1) simple diffusion 2)

facilitated diffusion 3) osmosis

1) simple diffusion molecules move through a membrane

down its concentration gradient ([H] to [L])

There is net movement of solute until an equilibrium is reached

Higher concentration gradient = faster the rate of solute diffusion

Diffusion of gas – proportional to concentration gradient

Diffusion of organic mols – depend of molecular weight & lipid solubility

2) facilitated diffusionTransport of large/charged mols from [H]

to [L] through special channels or carriersChannels = tunnel-like transmembrane

proteinEach type is designed for transport

specific soluteEg. Aquaporins- are channel proteins

specific to water molecules, water molecules are small enough to pass thru lipid bilayers, rate of movements is slow- polar.

Ion channels: open and close in response to an electrical/chemical stimulus

Carriers – specific solute bind to the carrier on one side of membrane and cause a conformational change in the carrier to shuttle them across membrane

The solute is then translocated across the membrane and released.

3) osmosisPassive transport of water across

a membraneAbility of water to move to pass

through a semi permeable membrane from a solution of lower solute concentration (dilute) to a solution of higher solute concentration (concentrated).

Similarities between Simple Diffusion and Facilitated Diffusion

1) Down the concentration gradient (From high concentration to low concentration)2) No energy is required

Differences

b) Active transport

Energy is required to transport molecules against a concentration gradient

Energy derived from ATP hydrolysis, or other energy sources is required to move the mols against concentration gradient

2 types – primary active transport & secondary active transport

1) primary active transport Energy provide directly by ATP hydrolysis Transmembrane ATP-hydrolyzing enzyme

use energy from ATP hydrolysis to drive the transport of ion/mols

eg Na+-K+ pump – primary transporter Na+ and K+ gradients for maintain cell vol

and membrane potential Typically, K+ conc is low outside an animal

cell and high inside the cell Na+ conc is high outside an animal cell and

low inside the cell. The Na+-K+ pump maintains these conc

gradients using the energy of 1 ATP to pump 3 Na+ out and 2 K+ in.

2) secondary active transport

Concentration gradient by primary active transport harness to move substances across membrane

Eg Na+ gradient created by Na+-K+ pump is used in kidney tubule cells and intestinal cells to transport D-glucose

Bulk transportBulk transportExocytosis- Large molecules such as

polysaccharides and proteins cross the membrane via vesicles.

- During exocytosis, a transport vesicle budded from the Golgi apparatus is moved by cyoskeleton to the plasma membrane.

- When the 2 membranes in contact, the bilayers fuse and spill contents to the outside.

Endocytosis- During endocytosis, a cell brings in

macromolecules by forming new vesicles from the plasma membrane.

- Endocytosis is a reversal of exocytosis- but diff protein involved in these processes.

- A small area of plasma membrane sinks inward to form a pocket.

- As the pockets deepens, it pinches in to form a vesicle containing the material outside the cell.

3 types of endocytosis3 types of endocytosisPhagocytosisPinocytosisRecepto-mediated endocytosisPhagocytosis (cellular eating)- The cell engulfs a particle by

extending pseudopodia around it and package it in a large vacuole.

- The content of the vacuole are digested when the vacuole fuses with lysosome.

Pinocytosis (cellular drinking)- A cell creates a vesicle around a

droplet of extracellular fluid. - All included solutes are taken into

the cell- nonspecific process.

Receptor-mediated Receptor-mediated endocytosisendocytosisSpecific. Only allow certain

substances.This process is triggered when

extracellular substances/ligands bind to to receptor on the membrane surface.

The receptor proteins are clustered at the coated pits.

Binding of ligands to receptors triggers the formation of a vesicle by the coated pit, bringing the bound substances into the cell.

EXP 6: Extraction of lipidsEXP 6: Extraction of lipidsResults:A6 = 62.3848g

A3A4 = 63.6418gA1A2 = 62.9922gA7A8 = 63.3789

MID-TERM TEST 1 = Friday, 9/11/2012, 4-5 pm @ DKG 2 & 3 Introduction to biochemistry – lipids.