Secondary Active Transport (biochemistry)

1. Secondary Active Transport

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. 1. 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”
(i) 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

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

10. 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

12. Types of macromolecules released by exocytosis
• They fall into 3 categories:
(i) 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

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

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

17. 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

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

19. 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

20. 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

21. • 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