Secondary active transport

Secondary Active Transport (biochemistry)

1. Secondary Active Transport Maryam Fida (o-1827)

2. Secondary active transport  is a form of  active transport  across a biological membrane in which a transporter protein couples the movement of an ion (typically Na +  or H + ) down its electrochemical gradient to the uphill movement of another molecule or ion against a concentration/electrochemical gradient

3. Na+ ions are pumped out of the cell by Na + K + ATPase to lower it’s concentration in the cell Special transporters acting as symporters bring about by facilitated diffusion a simultaneous diffusion of certain solutes along with Na + into the cells e.g. movement of Na + into the cell interior along its chemical and concentration gradient in association with a glucose molecule aided by a symporter SGLT-2 (Sodium Dependent Glucose Transporter 2)

4. Since this movement into the cells is dependent on the primary active transport property of Na + K + ATPase and their movement is secondary to the movement of Na + ions; it is called secondary active transport

5. Transport of small substances (ions or molecules): Can be classified as follows:

6. Uniport system: This system involves the transport of a single solute molecule through the membrane e.g.: Glucose transporters in various cells 2. Co-transport system: D-Glucose, D- Galactose and L-amino acids are transported into the cells by Na + - dependent co-transport system. Na + is not allowed to accumulate in the cells and it is pumped out by “sodium pump” Symport system : It is a co-transport system in which the transporter carries the two solutes in the same direction across the membrane (ii) Antiport system : It is a type of co-transport system in which two solutes or ions are transported simultaneously in opposite directions Example: Chloride and bicarbonate ion exchange in lungs & in red blood cells

7. Exocytosis and Endocytosis These processes bring about movement of large molecules and particulate material across the cell membranes

8. Exocytosis Extrusion / Secretion of cytoplasmic proteins that occur in secretory granules or vesicles The membrane of the secretory vesicle fuses to the cell membrane Followed by breakdown of area of fusion which results in outpouring of contents of the vesicle to the exterior of the cell without any break in the cell membrane Exocytosis needs energy The process induces a local and transient change in Ca++ concentration which triggers exocytosis

9. Types of macromolecules released by exocytosis They fall into 3 categories : They can attach to the cell surface and become peripheral proteins, e.g. antigens (ii) They can become part of extracellular matrix, e.g. collagen and glycosaminoglycans (GAGs) (iii) Hormones like insulin, parathormone (PTH) and catecholamines are all packaged in granules, processed within cells to be released upon appropriate stimuli

10. Endocytosis This process involves a continuous ingestion by the cells of parts of their own membrane along with material present on the outside

11. Factors required for Endocytosis: Energy: Usually derived from ATP hydrolysis. Ca ++ Contractile element in the cell-probably the microfilament system.

12. 1. Phagocytosis: Phagocytosis (Greek word- Phagein -to eat) is the engulfment of large particles like viruses, bacteria, cells, or debris by macrophages and granulocytes They extend pseudopodia and surround the particles to form phagosomes which later fuse with lysosomes to form Phagolysosomes in which the particles are digested

13. Pinocytosis: It is a property of all cells and leads to the cellular uptake of fluid and fluid contents

14. Fluid - Phase endocytosis (non-selective) Random and non-selective Uptake is proportionate to its concentration in the extracellular fluid Membrane invaginates internally to form a vesicle Portion of cell membrane that forms vesicle regenerates Vesicle becomes bound with primary lysosome Vesicle + primary lysosome = Secondary lysosome Hydrolytic enzymes in lysosomes cause lysis of macromolecules into amino acids, sugars and nucleotides which are released in cytoplasm for metabolic use

15. Receptor – mediated endocytosis (Absorptive Endocytosis) By coated vesicles and endosomes It is called absorptive and is selective because the process begins with the binding of the substance to be ingested with its specific receptor

16. Near the periphery of the cell’s interior, another structure called endosome (also called receptosome ) , is found The internalised coated vesicles fuse with the endosomes and discharge their macromolecules into the interior of the endosomes Example: The low density lipoproteins (LDL) molecule bound to receptors are internalized by means of coated pits