TRANSPORT ACROSS CELL MEMBRANE-II

بسم الله الرحمن الرحيم

Prepared byDr.Mohammed Sharique Ahmed Quadri

Assistant prof. PhysiologyAl Maarefa College

Objectives

• Describe how energy from ATP hydrolysis is used to transport ions against their electrochemical concentration differences

• Explain how energy from the Na+ and K+ electrochemical gradients across the plasma membrane can be used to drive the net “uphill” (against a gradient) movement of other solutes.

• Describe the characteristics of carrier-mediated transport, and distinguish between simple diffusion, facilitated diffusion, and active transport

• Describe the process of vesicular transport

Active Transport

Active transport• Moves a substance against its concentration

gradient

• Requires a carrier molecule

• Requires energy

Types of Active Transport

• Active Transport1. Protein Pumps• Primary active transport– Requires direct use of ATP

• Secondary active transport– Driven by an ion concentration gradient

established by a primary active transport system

2. Vesicular transport • Endocytosis• Exocytosis

Primary Active Transport

• Movement against concentration gradient • Hydrolysis of ATP directly required for the function

of the carriers.• Molecule or ion binds to “recognition site” on one

side of carrier protein.• Carrier protein undergoes conformational change.– Hinge-like motion releases transported molecules to

opposite side of membrane.

Primary Active Transport

Active Transport

Na+/K+ Pump

• Carrier protein has enzymes activity ( ATPase)

• Extrudes 3 Na+ and transports 2 K+ inward against concentration gradient.

Na+/K+ Pump

• Steep gradient created by this pump serves following functions:

• Provides energy for “coupled transport” of other molecules.

• Involvement in electrochemical impulses.

• Promotes osmotic flow.

Importance of Na+- K+ pump in intestinal epithelium

• High osmotic pressure created by movement of sodium causes water to move from intestinal lumen to interstitial space

• Protein and glucose r transported actively by cotransport with sodium

• Chloride passively follow the electrical gradient created by sodium

Secondary Active Transport

• Transport of two or more solutes are Coupled .• Energy needed for “uphill” movement

obtained from “downhill” transport of Na+.

• Hydrolysis of ATP by Na+/K+ pump required indirectly to maintain [Na+] gradient.

Secondary Active Transport

• Cotransport (symport):– Molecule or ion moving in the same direction as

Na+.

• Countertransport (antiport):– Molecule or ion moving in the opposite direction

of Na+.

cotransport

Counter transport– Molecule or ion moving

in the opposite direction – E.g. Na+-Ca2+ exchange– As with cotransport it

also uses Na gradient established by the Na+- K+ ATPase as an energy source

– Na+ moves downhill & Ca2+ moves uphill

Vesicular transport across membrane

• Exocytose

• Endocytosis

Types of Active Transport

• Endocytosis: taking bulky material into a cell • Uses energy• Cell membrane in-folds

around food particle• “cell eating”• forms food vacuole &

digests food• This is how white blood

cells eat bacteria!

Types of Active Transport

3. Exocytosis: Forces material out of cell in bulk• membrane surrounding the

material fuses with cell membrane

• Cell changes shape – requires energy

• EX: Hormones or wastes released from cell

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Vesicle-mediated transport Vesicles and vacuoles that fuse with the cell membrane may be utilized to release or

transport chemicals out of the cell or to allow them to enter a cell. Exocytosis is the term applied when transport is out of the cell.

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References

• Human physiology by Lauralee Sherwood, fifth edition

• Text book physiology by Guyton &Hall,11th edition

• Text book of physiology by Linda .s contanzo,third edition

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