TRANSPORTMECHANISMBy: Mrs. Kalaivani Sathish

INTRODUCTION• Transport of various substances across a

plasma membrane may occur by several mechanisms, which includes, diffusion (passive diffusion), facilitated diffusion and active transport.

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DIFFUSION• Diffusion of a substance may occur

either passively (called passive diffusion) or may required some carrier protein, which facilitates diffusion (Called Facilitated diffusion)

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PASSIVE DIFFUSION• Diffusion or passive diffusion refers to a

process in which a solute moves down an electro chemical gradient from one side of the membrane to the other.

• It is an energy independent process, which takes place by the dissipating its free energy stored in the gradient.

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OSMOSIS• Water moves readily through a semi

permeable membrane from a region of lower solute concentration to a region of higher solute concentration. This process is called as osmosis.

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OSMOSIS

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A hypertonic solution is a particular type of solution that has a greater concentration of solutes on the outside of a cell when compared with the inside of a cell.

A hypotonic solution is any solution that has a lower osmotic pressure than another solution.

When the cell is placed into a hypotonic solution it rapidly gains water by osmosis and swells, converselywhen the cell is placed in the hypertonic solution, it losses water by osmosis and shrinks.

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Once the internal solute concentration becomes equal to the external solute concentration, internal and external fluids are said to be isotonic or iso osmotic.

At this stage no movement of water into or out of the cells occur.

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Facilitated Diffusion• Large polar molecules such as glucose and amino acids,

cannot diffuse across the phospholipid bilayer. Also ions such as Na+ or Cl- cannot pass.

• These molecules pass through protein channels instead. Diffusion through these channels is called FACILITATED DIFFUSION.

• Movement of molecules is still PASSIVE just like ordinary diffusion, the only difference is, the molecules go through a protein channel instead of passing between the phospholipids.

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Active Transport• To move substances against a concentration

or electrochemical gradient, a cell must use energy. Active transport mechanisms do just this, expending energy (often in the form of ATP) to maintain the right concentrations of ions and molecules in living cells

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2 Types of Active Transport Mechanism

• Primary Active Transport – these are found in all living organisms and require an energy source such as ATP for the translocation of the cations.

• These transporters since utilize ATP are also referred to as ATPases.

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• Secondary Active Transport – they utilize a secondary source of energy such as trans – membrane electro chemical gradient of either Na+ or H + to translocate small organic molecules e.g. Glucose and Amino Acids etc.

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