Biological Membranes And Transport

• The plasma membrane is an envelop surrounding the cell.

• It separates and protect the cell from the external environment.

• Plasma membrane also provide a connecting system between the cell and its environment .

Detailed structure of the plasma membrane

• Chemical composition• The membranes are composed of lipids, protein

and carbohydrates.• The actual composition differ from tissue to

tissue.• Among the lipids, amphipathic lipids

(containing hydrophobic and hydrophilic groups) namely phospholipids, glycolipids and cholesterol are found in animal membranes.

• Many animal cell membranes have thick coating of complex polysaccharides referred to as glycocalyx.

Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembraneproteins

Filaments ofcytoskeleton

Peripheralprotein

Glycoprotein

Phospholipids

• Structure of membranes• Fluid mosaic model proposed by Singer and

Nicolson is a more recent and acceptable model for membrane structure.

• The biological membrane usually have a thickness of 5 – 8 nm.

• A membrane is essentially composed of a lipid bilayer.

• The hydrophobic (nonpolar) region of the lipids face each other at core of the bilayer while the hydrophilic (polar) region face outward.

More than lipids… • In 1972, S.J. Singer & G. Nicolson proposed

that membrane proteins are inserted into the phospholipids bilayer

It’s like a fluid…It’s like a mosaic…It’s the Fluid Mosaic Model!

Phospholipids

Fatty acid

Phosphate

• Fatty acid tails– hydrophobic

• Phosphate group head – hydrophilic

• Arranged as a bilayer

Aaaah, one of thosestructure–functionexamples

1. Extrinsic membrane proteins are loosely held to the surface of the membrane and they can be easily separated e.g. cytochrome c of mitochondria.

2. Intrinsic membrane proteins are tightly bound to the lipid bilayer and they can be separated only by the use of detergent or organic solvents e.g. hormone receptors.

Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembraneproteins

Filaments ofcytoskeleton

Peripheralprotein

Glycoprotein

Phospholipids

• Transport across the membranes• The biological membrane are relatively

impermeable.• The membrane, therefore forms a barrier for the

free passage of compounds across it.

1. Passive diffusion

2. Facilitated diffusion

3. Active transport

Passive transport• Passive transport is the diffusion of substances

across a biological membrane. • This occurs without the use of cellular energy.

2.Facilitated diffusion: This is somewhat comparable with diffusion solute moves along the concentration gradient (from higher to lower concentration) and no energy is required.

• But the most important distinguishing feature is that facilitated diffusion occurs through the mediation of carrier or transport protein.

• Specific carrier protein for the transport of glucose, galactose, leucine, phenylalanine etc. have been isolated and characterized.

• Mechanism of facilitated diffusion: A ping pong model is put forth to explain the occurrence of facilitated diffusion.

• According to this mechanism, a transport protein exists in two conformation, in the pong conformation it is exposed to the side with high solute concentration.

• This allow the binding of solute to specific sites on the carrier protein.

• The protein then undergoes a conformational change(ping state) to expose to the side with low solute concentration where the solute molecule is released.

• Insulin increases glucose transport in muscle and adipose tissue.

3.Active transport: Active transport occurs against a concentration gradient and this depend upon the supply of metabolic energy (ATP).

• Active transport is also carrier mediated process like facilitated diffusion.

• The most important primary Active transport systems are ion pumps.

Na+- K+ pump The cells have high intracellular K+

Concentration and low Na+ concentration. This is essentially needed for survival of cell. High cellular K+ is required for the optimal

glycolysis (pyruvate kinase is depend upon K+) and for protein biosynthesis.

Further Na+ and K+ gradients across plasma membrane are needed for the transmission of nerve impulses.

• Na+-K+ pump is responsible for the maintenance of high k+ and low Na+ concentration in the cells.

• This is brought about by an integral plasma membrane protein, namely the enzyme Na+-K+ ATPase.

• It consist of two α and two β subunits which may be represented as [αβ]2 .

• Na+-K+ ATPase pump 3Na+ ions from inside the cell to outside and bring 2k+ ions from outside to inside with a concomitant hydrolysis of intracellular ATP.

3Na+ (in)+ 2K+ ( out) +ATP 3Na+ (out)+ 2K+ (in)+ ADP+Pi

• Ouabain inhibit Na+-K+ ATPase pump.,• Ouabain is a steroid derivative extracted from

the seed of an African Shrub.• It is a poison used to tip the hunting arrows by

the tribal in Africa.

Transport summarysimplediffusion

facilitateddiffusion

activetransport

ATP

Transport system

1. Uniport system: This involves the movement of a single molecule through the membrane. E.g. transport of glucose to the erythrocytes.

2. Symport system: the simultaneous transport of two different molecules in the same direction. E.g. transport of Na+ and glucose to the intestinal mucosal cells from the gut.

3. Antiport system: The simultaneous transport of two different molecules in opposite direction e.g. exchange of Cl- and HCO-3 in the erythrocytes.

• Proton pump in stomach• This is an antiport transport system of gastric

parietal cells.• It is brought out by the enzyme H+ - K+ ATPase

to maintain highly acidic condition in the lumen of stomach.

• Proton pump antiport two cytoplasmic proton (2H+) and two extracellular potassium (2K+) ions for a molecule of ATP hydrolyzed.

• The chloride ions secreted by Cl- channel combine with proton to form gastric HCl.

• Passive transport of water-osmosis.• Osmosis is phenomenon of movement of water

from low osmotic pressure (dilute solution) to high osmotic pressure (concentrated solution) across biological membrane.

• The movement of water in body occurs through osmosis, and this process does not require energy.

• Certain medical and health complication are due to disturbance in osmosis e.g. edema, cholera, diarrhea, inflammation of tissue.

OSMOSIS

• Transport of macromolecules• The transport of macromolecule such as protein,

polysaccharides and polynucleotide across the membrane is equally important.

1. Endocytosis : Intake of macromolecules by the cells.

• It is estimated that approximately 2% of the exterior surface of plasma membrane possesses characteristic Coated-pits.

• The pits can be internalized to form coated vesicles which contain an unusual protein called Clathrin.

• The uptake of LDL molecules by the cells is a good example of endocytosis.

• Endocytosis occurs when the plasma membrane is pulled inwards and will form a “pocket” around a particular substance.

• The substance will become enclosed in the vesicle which is then pinched off and begins moving through the cytoplasm.

• Cells can bring in solids and liquids using this process.

2.Exocytosis: Release of macromolecules from the cells to outside.

• The release of macromolecules to the outside of the cells mostly occurs via the participation of Golgi apparatus.

• The macromolecules are transported to the plasma membrane in a vesicles and let out.

• The secretion of hormone e.g. Insulin usually occur by Exocytosis.

• Exocytosis is internal vesicles will fuse with the plasma membrane and the contents of the vesicle are released into the external environment of the cell.

• The cell can secrete substances they produced this way or excrete waste products.

Diffusion

Active transport

• The movement of a substance across a biological membrane against its concentration or electrochemical gradient with the help of energy input and specific transport proteins.

• This movement of particles occurs against the concentration gradient or from an area of low concentration to an area of high concentration with the use of ATP.

Active Transport

Pinocytosis

• A type of endocytosis in which the cell ingests extracellular fluid and its dissolved substances.

10. Use the sodium-potassium pump to explain how active transport can be used to move particles against a concentration gradient.

• The cell can hydrolyze ATP and use the energy released to move substances across the plasma membrane and against the concentration gradient. The energy is used to change the shape of membrane protein “pumps”.

• Each pump only transports particular substances.

10. Use the sodium-potassium pump to explain how active transport can be used to move particles against a concentration gradient.

• The particle will enter the pump on the side with a lower concentration and bind to a specific site for that type of particle.

• Energy from ATP is used to change the shape of the pump, the particle is released on the side of higher concentration, and the pump returns to its original shape.

9. Compare active and passive transport.

• Passive transport is the diffusion of particles across a biological membrane. This occurs without the use of cellular energy.

9. Compare active and passive transport.

• Active Transport is the movement of particles across biological membranes using energy from ATP. This movement of particles occurs against the concentration gradient or from an area of low concentration to an area of high concentration.

How about large molecules?• Moving large molecules into & out of cell– through vesicles & vacuoles– endocytosis• phagocytosis = “cellular eating”• pinocytosis = “cellular drinking”

– exocytosis

exocytosis

Transport summarysimplediffusion

facilitateddiffusion

activetransport

ATP

Membrane Proteins• Proteins determine membrane’s specific functions– cell membrane & organelle membranes each have unique

collections of proteins

• Membrane proteins:– peripheral proteins

• loosely bound to surface of membrane• cell surface identity marker (antigens)

– integral proteins • penetrate lipid bilayer, usually across whole membrane

• transmembrane protein

• transport proteins– channels, permeases (pumps)