Mediated Transport



 Characteristics of Carrier Mediated Transport. 

Carrier mediated transport, whether active or passive, has three identifying features __


(1) Saturation Kinetics. 

The transport shows saturation Kinetics,i.e.,  with increase in the concentration of solute, the rate of transport increases still maximum is reached when no further increase occurs. This shows that the transport system has specific site where the molecules of the solute join for transport, and that there is a definite number of such sites in the cell membrane. When all these sites are occupied, the transport occurs at the maximum rate.In contrast to this, the rate of simple physical diffusion of  a solute across a membrane is directly proportional to its concentration. 



  ( ii) Specificity. 

The transport process shows specificity  for the material transported. The carrier protein molecule of a transport system has a binding site complementary to that of the molecule to be transported .Hence, the system cannot transport other kinds of molecules. 

(iii) Inhibition.

The carrier molecules of a transport system are inhibited  specifically by substances structurally related to the substrate like the enzymes.

     Thus, the working of the carrier molecules in the cell membrane resembles that of the enzymes. Unlike enzymes, however, the transport proteins usually do not catalyze chemical reactions. They catalyze  physical process, i.e., quicken the transport of molecules across a membrane. 


2. Internalization of Macromolecules, Fluid Droplets and Large Particles. 

Animal cells can also actively take in and turn out materials in masses much larger than in the higher to described processes. Such materials include macromolecules, liquid droplets and solid particles. Items of this size cannot cross the membranes by penetrating the lipid bilayer or which the help of membrane proteins. The process involving uptake of macromolecules and fluid droplets is known as  endocytosis, and that pertaining to the intake of particles is termed phagocytosis. The reverse process that expels materials in large masses is called exocytosis. Phagocytosis was earlier included in endocytosis, but is now recognized as a fundamentally different process.All the three processes ( endocytosis, phagocytosis and exocytosis) involve movement folding and fusion of cell membrane, which are dependent on its fluidity and mobility. They are active, energy_ requiring processes and mitochondria are usually oriented near the surface where the processes occur. Endocytosis and phagocytosis also require rapid synthesis of cell membrane. 

(i) Endocytosis. 

The term endocytosis refers to invagination of a small region of plasma membrane, forming an intracellular membrane_ bound vesicle. Endocytosis is of two types: pinocytosis and receptor _ mediated endocytosis. 




(b) Pinocytosis.

Pinocytosis is the nonspecific intake by  cell of a tiny droplet of extracellular fluid which cannot otherwise pass through the cell membrane. It is also called  cell drinking ( G.pinein= to drink, kytos= cell) .It was first observed by Edward in Amoeba.In this process, a small region of plasma membrane invaginates and a fluid droplet passes into the pocket so formed. The pocket deepens and finally nips off as a fluid__ filled vacuole, the pinocytotic vesicle, or pinosome.  Thus, any material dissolved or suspended in the extracellular fluid gets into the cell without any break  in the continuity of the plasma membrane. Pinocytosis is, thus, unspecific in material  uptake.The pinosome shifts into the interior of the cell.Here, it fuses with a lysosome for the digestion of the material taken. The digestion products diffuse into the surrounding cytoplasm. Not all Vacuoles seen in cells are formed in the above manner.The cells are also known to obtain by pinocytosis dissolved materials, such as ions, sugars and amino acids, which can enter by other mean too. The extracellular solutes, such as insulin and lipoproteins, are taken up by tissue cells by pinocytosis. 




   The main role of pinocytosis may be restoration of membrane dimensions by eliminating segments from it to neutralize addition made to it by exocytosis. 


(b)  Receptor _ mediated Endocytosis:

Receptor __ mediated endocytosis ( RME) is the specific uptake of macromolecules which otherwise cannot pass through the plasma membrane. In this process, a specific protein receptor on the membrane surface " recognizes" a particular extracellular macromolecule, called a ligand¹, and binds to it. The region of plasma membrane bearing the receptor _ ligand complex infolds and closes as a pinocytotic vesicle.The latter then fuses with a lysosomes for the digestion of macromolecule.

    The receptor proteins are often aggregated in certain regions of plasma membrane, which are linked on the cytoplasmic side by a fuzzy layer of protein. Such regions are called coatef pits.

       Endocytosis has been observed in many kinds of animal cells.In mammals, intestine, liver, kidney, blood and tumour cells regularly take up large molecules by this process. 
     

  Endocytosis is not shown by plant cells because of their rigid wall and internal turgor pressure. Endocytosis occurs in yeast cells.




Advantage of RME:

The receptor _ mediated endocytosis is advantageous because it internalises selected macromolecules, thereby reducing the needless ingestion of large volumes of extracellular fluid.

  Familial Hypercholesterolemia. Human cells take up cholesterol from the blood by RME for use in the synthesis of membranes and as a precursor for the synthesis of other steroids.If cholesterol receptor proteins are defective, cholesterol cannot enter  the cells, accumulate in the blood and causes early atherosclerosis,i.e., fat deposited on lining of blood vessels ( Brown and Goldstein).


(ii) Phagocytosis. 

Phagocytosis is the intake of solid particles by a cell. It is also called cell eating ( G.phagein= to eat,kytos= cell).It is beautifully seen in Amoeba.This organism ingests solid organic particles, which are too large to diffuse through its plasma membrane. Contact of a food particle with cell membrane induces the latter to put out tiny protoplasmic processes, the pseuopodia, around the food particle. The pseudopodia meet on the other side of the food particle and fuse.Amoeba, thus, acquires an internal vacuole, called food vacuole, or phagosome, containing the food particle in a droplet of water. Extension of pseudopodia around the particle being Phagocytosis is aided by actin_ containing microfilaments located just under the plasma membrane. Microfilaments play no role in endocytosis. Phagocytosis is the major feeding method in many unicellular organisms and simple ( lower) animals. 

    Certain leucocytes of blood take viruses, bacteria, injured and deal cells,fibres, dust particles and other waste matter by Phagocytosis. Such cells are called Phagocytes.The macrophages found in the connective tissue and liver sinusoids also carry on Phagocytosis. In human body, Phagocytes dispose off 100 billion women out red blood cells per day. The phagosome fuses with a lysosome for the digestion of its contents. The products of digestion diffuse into the surrounding cytoplasm. 

     Receptor __ mediated Phagocytosis:  Phagocytosis, for example of a bacterium, by a Phagocytosis is initiated by specific   protein  receptors present on the cell membrane. These receptors sequentially bind to complementary molecules ( ligand) on the bacteria's surface unit the pseudopodia envelope it fully.


        Resistance of Phagocytosis: Some strains of Streptococcus bacteria have around them a  carbohydrate capsule which inhibits binding and ingestion by phagocytes.These strains are likely to cause diseases. The strains without a capsule are easily ingested by Phagocytes before they can produce illness.The  bacteria  that cause tuberculosis and leprosy are easily engulfed by the Phagocytes but are not digested in them.

    Cellular Interdigitations:In certain compact tissues, finger_ like projections may grow one cell and fit into corresponding Pits in the adjacent cells. These projections are usually not permanent. They are slowly withdrawn and reformed, causing changes in the form of the cells. Some of the projections of one cell may at times pinch off a protrusion of a neighboring cell. The former cell, thus, acquires a food vacuole with a bit of cytoplasm of a nearby cell. This process is one of the methods of passing materials from cell to cell.


      Stimuli For Endocytosis  and Phagocytosis: The stimuli that cause the formation of endocytotic and phagocytotic vesicles are not known. However, chemical signals are involved in the cases of endocytosis that have been investigated. 

(iii) Exocytosis. 

In the cells of the glands, the secretion in a membrane _ bound secretory vesicle.The latter is budded from the Golgi apparatus and moved by cytoskeleton from the interior of the cell to the surface. Here the vesicle membrane, where the  lipid  molecules of the two bilayer rearrange themselves, fusing the two membranes. A passage is formed in the fused membrane and the vesicle discharges its contents outside the cell. The process is also called cellular vomiting or ephagy.The Vacuoles that turn out the materials are termed endocytotic vesicles. Undigested food left in the food Vacuoles is also eliminated from the cell by exocytosis. 
      
     The secretions discharged from gland cells by exocytosis include digestive  enzymes hormones, serum proteins, extracellular structural proteins, such as collagen and elastin.Release of neurotransmitters from neurons, delivery of wall  carbohydrates to the outside of the cell, and elimination of indigastilbe food particles also occur by exocytosis. 



Recycling of Cell Membrane. 

Endocytosis carries segments of cell membrane into the cytoplasm.The internalises membrane is replaced by the fusion of exocytotic vesicles with the cell membrane. The process of membrane recycling is called endocytic cycle.This cycle enable the cells to retain their surface area/ volume ration.


      Once the materials have been absorbed into a cell, intracellular distribution occurs mainly by diffusion and cyclosis. 

Regulation of Intracellular Contents:

A cell placed in solution which is not isotonic to it may adjust to the changed environment by varying its water content and becoming isotonic to its surrounding fluid.Some cells can absorb or expel water or certain solution and acquire an osmotic pressure that differs little from that of the surrounding medium. The protozoans living in hypotonic fresh water have evolved contractile Vacuoles which collect and pump out excess water from the cell.

          Plant inhabiting fresh water deal with the water that enters the cell by osmosis from the hypotonic environment in a different way.The plant cells lack contractile vacuole to pump out excess water, but their firm  Cellulose wall prevents their undue swelling. As water enters an internal pressure, called turgor pressureis generated which counterbalances the osmotic pressure and checks the entry of move water. Turgor pressure is characteristic of plant cells and gives support to the plant body.Leaves droop in the summer sun when the turgor pressure in their cell decreases due to the lack of water.


       Many sea organisms can selectively absorb and accumulate in their cells certain compounds from the sea water. Sea weeds accumulate iodine and lubricates accumulate vanadium so that the concentration of iodine or vanadium in the cells may be some 2,000,000 times of that in the sea water respectively. Only the living cells can move materials against concentration gradient as it needs energy, which a dead cell cannot provide. 



Difference Between Active Transport and Diffusion 

Active Transport 

1. Can move materials through a Biomembrane  against the concentration or electrochemical gradient. 


2. Needs carrier proteins to occur.

3. Uses energy of ATP or the movement of ions down a gradient across the membrane. 

4. Takes place in one direction only.

5. Is affected by oxygen supply, temperature and poisons.

6 Is a rapid process. 

7. Bring about selective uptake of materials. 

8. Stopped by  metabolic  inhibitors.

9. Leads to accumulation of materials in the cells.

10 . Is a vital process.


Diffusion

1. Can move materials across a biomembrane down the concentration or electrochemical gradient. 


2. Occurs without proteins. 


3. Does not use energy. 

4. Takes place in both directions. 

5. It's not affected by oxygen, temperature and poisons.

6. Is a slow process. 

7. Allows all transmissible molecules to pass through membranes. 

8. Not influenced by metabolic inhibitors.

9 Does not accumulate materials in the cells.

10. Is a physical process.


Differences Between Physical and Biological Model of Transport Across Biomembrane

 physical processes 

1. Do not require activity on the part of the cells.

2. Do not expend energy. 

3. Do not show specificity for the materials transported. 

4. Occur down the concentration gradient. 

5. Do not use protein molecules. 

6. Are slow processes.

7. Include diffusion, dialysis,  and filtration. 

Biological Processes 

1. Require activity on the part of the cells.

2. Often use energy, usually ATP.

3. Show specificity for the materials transported. 

4. Can occur down as well as against the concentration gradient. 

5. Often use protein molecules. 

6. Are rapid processes. 

7. Include transport by protein molecules and internalization of large masses.

Differences Between Engocytosis and Phagocytosis 

Engocytosis

1. It is the intake of extracellular fluid droplets and macromolecules. 

2. Cell membrane infolds to take up material. 

3. Endocytotic vesicle are only 0.1 ųm wide.


4. Microfilaments play no role in endocytosis. 

5. It is nutritive process.


Phagocytosis:

1. It is the intake of extracellular particles. 

2. Cell membrane grows around the particle as pseudopia.

3. Phagocytotic vesicles are 1 to 2 ųm. or more wide.

4. Microfilaments play an important role in Phagocytosis. 

5. It is a nutritive and defensive process. 

Role of Cell Membrane in Locomotion: 

The cell membrane plays a role in two types of cellular movements: pseudopodial
 and undulatory.

1.Pseudopodial Movement

This is shown by some organism, such as Amoeba, and by certain cells, such as white blood corpuscles and macrophages. It occurs by the formation of temporary finger_ like projection, the pseudopodia,  at the advancing end.The latter are formed by streaming of cytoplasm Forward and extension of the plasma membrane around the developing around the developing bulge. The existing pseudopodia are simultaneously withdrawn from the receding end.


 

  2. Undulatory Movement 

This is shown by some mammalian cells, such as fibroblasts. It occurs on a solid surface by wave_ like undulations of the plasma membrane in contact with the surface. The undulations temporarily attach to the surface and move as waves to propel the cell forward. 




Both the processes involve extension and folding of the plasma membrane which depend on its fluidity and flexibility. 


































































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