cellular transport

Dr. Saidunnisa, M.DChairperson and ProfessorBiochemistry

Learning ObjectivesLearning Objectives At the end of the session the

student shall be able to:1. Explain the structure of the

plasma membrane.2. Explain various transport

mechanisms across plasma membrane.

3. Interpret altered transport mechanisms, in disorders.

About Cell MembranesAbout Cell Membranes

1.All cells have a cell membrane

2.Functions: a.Controls what enters

and exits the cell to maintain an internal balance called homeostasis

b.Provides protection and support for the cell

TEM picture of a real cell membrane.

Plasma membrane structurePlasma membrane structure

Membrane is referred to as fluid mosaic.

Proteins are integral proteins which span the cell membrane.

Peripheral proteins which are attached to the membrane surface through electrostatic bonds to lipids or integral proteins.

Is lipid bilayer.

It consists of a mosaic of proteins and lipid molecules that move laterally in the plane of the membrane.

Plasma membrane structurePlasma membrane structure

Many of the proteins and lipids on the external leaflet contain covalently bound carbohydrate chains and therefore are glycoprotein's and glycolipids.

This layer of carbohydrate on the outer surface of the cell is called Glycocalyx.

Plasma membranePlasma membrane

About Cell Membranes (continued)About Cell Membranes (continued)

3.Structure of cell membrane

Lipid Bilayer -2 layers of phospholipidsa.Phosphate head is polar

(water loving)b.Fatty acid tails non-

polar (water fearing)c.Proteins embedded in

membrane

Phospholipid

Lipid BilayerCholesterol is interspersed between the phospholipids, which maintains membrane fluidity.

Proteins in plasma Proteins in plasma membranemembrane1. Integral proteins: are

transmembrane proteins with hydrophobic amino acids side chains interact with hydrophobic portions of the lipids.

Hydrophilic amino acids protrude into the aqueous medium.

Many of these proteins function as either channels or transporters for the movement of the compounds across the membrane.

Proteins in the RBC Proteins in the RBC membranemembrane

Example: Prominent integral proteins in the RBC membrane are:

Band 3: Channel for transport of HCO3 into the RBC in exchange for CL-

Helps to carry the HCo3 to the lungs where it is expired as CO2.

Glycophorins, which carry negative charge that repels other cells, prevents agglutination (clumping of RBC ) thereby maintaining free flow of blood.

Peripheral ProteinsPeripheral Proteins: : Bound through weak electrostatic

interactions to integral proteins. Example: Spectrin group of proteins.

Present on the cytosolic side of the cell provides mechanical support for the membrane.

In RBC band-3 is connected to Spectrin filaments via the protein ankyrin.

In skeletal muscle Dystrophin protein is a member of Spectrin family any mutations to its gene causes muscular dystrophies.

Cellular Transport!Cellular Transport!

The process by which molecules are moved from one side of the cell membrane to the other

Two Types◦Passive◦Active

Transport of molecules across Transport of molecules across the Plasma membranethe Plasma membrane Cells require transport systems; to permit entry of small polar

compounds (Glucose) Need to concentrate compounds

inside the cell (K+). Expel other compounds (Ca+2, and

Na+).

Types of transport Types of transport mechanismsmechanisms

Passive Transport Passive Transport

Doesn’t require energyMove molecules from high to low concentration◦Like coasting down hill on a bike

Types ofTypes of Passive TransportPassive Transport

Simple diffusion Facilitated diffusion, & Gated channels

Simple diffusionSimple diffusionGases such as O2

and CO2 and lipid-soluble substances (steroid hormones) can cross the Plasma Membrane by simple diffusion.

Energy not required.

Movement is from high concentration to lower concentration.

Simple diffusionSimple diffusionLipid insoluble

substances like water diffuse through membranes through ion channels or pores.

Example: renal tubular cells contain large pores called aquaporins, which permit high rate of water flow from a region of high water concentration to low water concentration.

Nephrogenic diabetes insipidus have impaired function of these aquaporins.

Facilitated diffusionFacilitated diffusion Solute moves along the

concentration gradient. No energy is needed. Important distinguishing feature is

it occurs through mediation of carrier or transport proteins which undergoes conformational change to deliver the molecule to the other side of the membrane.

Mechanism of Facilitative Mechanism of Facilitative DiffusionDiffusion

Ping-pong Model:Carrier protein exists

in two conformations.In Pong conformation,

it is exposed to the side with high solute concentration.

In Ping conformation, it is exposed to the side with low solute concentration and it is released.

Facilitative DiffusionFacilitative Diffusion Hormones regulate this

diffusion. Examples:1. Insulin increases glucose

transport in skeletal muscle and adipose tissue.

2. Insulin increases the amino acid transport in liver.

Gated channels Gated channels In these the transmembrane

proteins either open or close in response to a stimulus.

Example: CFTR (cystic fibrosis transmembrane conductance regulator) is a CL- channel found on cell membranes, mutation in this protein results in cystic fibrosis. It is regulated through Phosphorylation.

Types of Types of Active TransportActive Transport1. Na+, K+ ATPase (Sodium-

Potassium Pump)2. Endocytosis3. ExocytosisImportant for:

◦ nerve impulses◦muscle control

Active transportActive transportOccurs against the concentration

gradient.Requires energy (ATP).It is a carrier mediated process.Example: Na+, K+ ATPase pump.

This is responsible for the maintenance of high K+ and low Na+ concentrations in the cell.

Active transport Active transport (Na(Na++,K,K++ATPase)ATPase)

This is brought about by an integral plasma membrane protein called Na+, K+ ATPase.

which pumps 3Na+ ions from inside the cell to outside and brings 2K+ ions from the outside to the inside .

with concomitant hydrolysis of intracellular ATP.

Uniport: involves the movement of a single molecule through the membrane. Example: transport of glucose to the RBC

Symport: simultaneous transport of two different molecules in the same direction. Example: transport of Na+ and Glucose to the intestinal mucosal cell from the gut.

Antiport: Simultaneous transport of two different molecules in the opposite direction. Example Exchange of Cl- for HCO3

- in the RBC.

Active transportActive transport

Active Transport:Active Transport:

Endocytosis and Endocytosis and ExocytosisExocytosisAllow for passage of food and

waste particles that are too big to pass through the plasma membrane and protein channels

EndocytosisEndocytosis

Membrane folds in and forms a vesicle Contents are digested by cellular enzymes

(endo- means inside)

Exocytosis Exocytosis

1. Substance packaged into vesicle 2. Vesicle fuses with plasma membrane3. Substance released from cell

(exo- means outside)

Inside Cell Outside Cell

Case-1Case-1Mr. Allan took Michael with him to a

picnic at the shore, where they ate steamed crabs.

Later that night, Michael experienced episodes of vomiting and watery diarrhea, and Mr. Allan rushed him to the hospital emergency room.

Michael hands and feet were cold, he appeared severely dehydrated, and he was going into hypovolemic shock.

He was diagnosed with cholera, caused by the bacteria Vibrio cholera.

V.CholeraeV.CholeraeV.Cholerae epidemics are associated

with unsanitary conditions affecting the dinking water supply and are rare in united states.

However these bacteria grow well under alkaline conditions found in sea water and attach to chitin in shell fish.

Sporadic cases occur in south east U.S.

Cholera toxin binds to the Glycocalyx of gycolipid called GMI-Ganglioside of intestinal mucosal cells.

Cholera toxin absorbed into the intestinal mucosal cells, it is processed and complexed with Arf (ADP-ribosylation factor) activates Phosphorylation of the regulatory domain of CFTR by protein kinase A.

Thus the channel opens and CL-

and H2O flow from the cell into the intestinal lumen.

Cholera toxin also increases the efflux of sodium resulting in diarrhea, vomiting and dehydration.

TreatmentTreatmentDehydration is treated with an oral

rehydration solution containing Na+ K+ and Glucose.

Glucose facilitates Na+ movement through Symport mechanism.

Along with the transport of Na+ and glucose the water loss from the cells is prevented leading to rehydration.

Learning Check Learning Check

1. Explain various transport mechanisms across plasma membrane.

2. What are the Biochemical changes involved in cholera?

3. What is the biochemical basis for treatment of cholera?

Case-2Case-2A new born girl is delivered after

her mother had an uncomplicated 9-month pregnancy. The family is concerned because their 10-year old son has been diagnosed with cystic fibrosis. They request that their pediatrician order a sodium chloride sweat test to determine if their new born daughter has the disease. The disease is due to a defect in which type of proteins.

Cystic fibrosis is a an inherited disease of the secretory glands, that make mucus and sweat.

Caused by a mutation in the gene cystic fibrosis transmembrane conductance regulator (CFTR).

increased amounts of sodium and chloride in their sweat

Excess mucus production, frequent chest infections and coughing/shortness of breath

Diagnosis Diagnosis The most commonly used form of

testing is the sweat test.CF can also be diagnosed by

identification of mutations in the CFTR gene.