TRANSPORTATION ACROSS CELL MEMBRANE

Dr.S.Harshad

• Cell volume and composition of ICF maintained irrespective of changes in volume and concentration in Extracellular environment by transfer of substances across cell membrane

• Broadly classified into1. Direct passage without involvement of carrier

molecules2. Passage thru channels or carrier proteins

ClassificationPassive Transports

Simple diffusionFacilitated diffusionOsmosisFiltration , Bulk Flow & Solvent drag

Active Transports

Primary active transport Secondary active transport

Vesicular Transports

ExocytosisEndocytosisTranscytosis

Passive transports

• Features:1. Transport is downhill2. Does not require energySimple diffusion:• Random movement of molecules.• Molecules move from the area of higher

concentration to area of lower concentration.• Occurs until diffusion equilibrium is achieved.

• Factors that determine the rate of diffusiona) Properties of the substanceb) Properties of the membrane

Properties of substance

• Concentration and electrical gradients of substance• Permeability of substance thru membraneI. Lipid solubilityII. Molecular sizeIII. Charge on the moleculeIV. TemperatureV. Distribution of channels in membraneVI. Pressure gradient

Properties of membrane• Surface area• Thickness of membrane• FICK’S LAW OF DIFFUSION:Net rate of diffusion (J)= -DA . Ci-Co TD- diffusion constantA- surface areaT- thickness of membrane Ci- concentration of substance inside the cellCo- Concentration of substance outside the cell.

Facilitated diffusion

• aka carrier mediated diffusion• Features:1. Faster rate of transport2. Saturation kinetics3. Competitive inhibition4. Specificity

Osmosis

• Process of movement of solvent from low solute concentration to high solute concentration through semipermeable membrane.

Filtration, Bulk Flow & Solvent Drag

• Filtration is defined as the process by which fluid is forced through a membrane because of the difference in pressures on two sides.

• When filtration results in movement of greater quantity of water- bulk flow.

• During bulk flow, it carries dissolved particles along with it- solvent drag.

Active transports• Active transport is the transport of

substances from a region of lower concentration to higher concentration using energy, usually in the form

of ATP.

• Types1. Primary active transport2. Secondary active transport

Primary active transport

• Operated by ion pumps• Solute is transported against its

electrochemical gradient with the use of energy.

• Energy is derived from ATP

Pumps involved in active transport

1.Sodium-potassium pump• Found in many cells

2.Calcium pump• Found in membrane of Sarcoplasmic reticulum 3.Potassium hydrogen pump• Found in Gastrointestine cell membrane

Secondary active transport

• Transfer of one solute against its concentration gradient by using the energy generated by gradient of another solute.

• The transport may be o In the same direction (symport)o In the opposite direction (antiport)

Vesicular Transport

• The transport process that occurs by either fusion of vesicle or formation of vesicle.

Types:Endocytosis Exocytosis

Endocytosis

• Substance is taken into the cell by means of vesicle formation.

• The size of the vesicle varies from 0.1um to 2um

• Types:1. Phagocytosis2. Receptor mediated endocytosis3. Pinocytosis

Phagocytosis• aka Cell Eating• Ingestion of large particles or microorganisms• Vesicles vary from 1-2 um in diameter• Neutrophils, monocytes, macrophages are

phagocytic cells.

Receptor mediated endocytosis

Pinocytosis

• aka Cell Drinking• Fluid phase endocytosis• Substances in extracellular fluid are

internalized to the cell.

Exocytosis

• Export from the cell• Vesicles move to the cell surface, fuse with cell

membrane and release the content.

Transcytosis/ Cytopemisis

• Vesicular transport limited within the cell• Vesicles are formed within the cell and

transported into the cytoplasm.

Thank you…