Active Transport and Bulk Transport


Active Transport 

It is movement of materials across the membrane against  their chemical  connection or electrochemical gradient. This transport requires energy which is provided by ATP. Active transport occurs in case if both ions and nonelectrolytes, e.g., salt intake by plant cells, ions, glucose and phenolphthalein in case of renal tubules, sodium and potassium in case of nerve cells, etc. Various  evidences  supporting  active transport are: 

(i) Absorption is reduced  or stopped  with the decrease  in oxygen  content of the surrounding environment. 

(ii) Inhibited by metabolic inhibitors like cyanides and by substances similar  to solutes.

(iii) Absorption of different substances is selective. 

(iv) Accumulation of salt and other substances against  their concentration gradient. 

(v) Decrease in temperature  decreases  absorption .

(vi) It is more rapid than diffusion. 

(vii) It shows saturation kinetics,that is, the rate of transport  increases with increase in solute concentration till a maximum is achieved and beyond this value the rate of membrane transport  does not increase indicating that it takes place through the agency  of special organic molecules called carrier molecules, carrier particles or carrier proteins. There is a special carrier molecule for each solute particle ( ion or molecule) which has its  binding  site on two surfaces  of the membrane. The solute particle ( or substrate) combines with the carrier to form carrier solute complex. The carrier  undergoes a conformational change in the bound state which transport the solute to the other side of the membrane where the solute is released. Energy  is used in bringing about the conformation change in  the carrier. 

Many animal cells operate a sodium _ potassium exchange pump at their plasma membrane with the help of enzyme ATP_ ase which also functions as a carrier  molecule. The  enzyme  hydrolyses ATPA to release energy which is used in bringing  about  conformational changes in the carrier. For every  ATP  molecule hydrolysed, three Na+ ions are pumped outwardly  and two K+ ions are pumped inwardly.A similar proton pump operates in chloroplasts, mitochondria and bacteria. Na+ _ K+ exchange pump performs the following  functions:

(i) Maintains a positive  potential  on the outer side of the membrane and relatively electro_ negative  potential  on the inner side.

(ii) It creates a resting potential  in the nerve cells.

(iii) The pump maintains water balance of living cells.

(iv) It helps in urine formation. 

(v) It takes part in excretion of salt as in marine animals. Sea gulls and penguins which drink sea water excrete excess salt through nasal glands. The nasal salt glands have sodium _ potassium pump in the  plasma membranes of their cells which pump out sodium actively and chlorine passively. Nasal secretion of these birds possesses 1.5__  3.0 times more NaCI concentration than the one present in the blood.  

(vi) The unsecreted and unmetabolised excess Na+ ions present  in the extra_ cellular fluid have a tendency  to pass back into the cells. Other substances combine with sodium ions and pass inwardly along with them, e.g., glucose,  amino acids in intestine. The phenomenon is called secondary  active transport as compared to Na+ _ K+ exchange  pump which is called primary  active transport. 

Other important pumps include Calcium pump ( RBCs, musc), K+ pump, Cl- pump, K+ _H+ exchange  pump. The last one occurs in guard cells. 

Active transport is a means of 

(i) Absorption of most nutrients from the intestine. 

(ii) Reabsorption of useful material from uriniferous tubules.

(iii) Absorption  of nutrients by cells rapidly and selectively. 

(iv)  Maintenance of resting potential  in nerve cells.

(v) Maintaining  water and ionic balance  between  cells and extracellular fluid.

(vi) Excretion by salt glands.

( vii) Absorbing substances against  concentration gradient. 

Bulk Transport 

It occurs by two methods,  pinocytosis and Phagocytosis which involve the enclosure of the material under transport, in the vesicles of the membrane. The vesicles are also called carrier  vesicles. The inward transport  of carrier  vesicles is called endocytosis while the outward transport  of substances  by means of carrier  vesicles is known as exocytosis. It is common  in secretory  and excretory  cells.

(i) Pinocytosis: Pinocytosis is the bulk transport of fluid matter and substances dissolved  in it across the cell membrane by forming minute detachable  vesicles. It is also called cell drinking. Solute intake may be selective or nonselective. Selective solute intake occurs  through specific  pits having receptor  sites. The plasma membrane invaginates as soon as solute or ligand  particles form complexes  with receptor sites. The invaginates deepens and gets pinched  off as a vesicle called pinosome which migrates towards the interior where it liberates the materials either in the  cytoplasm or a vacuole. Pinocytosis is quite common  in the cell lining  the blood capillaries.  Macromolecules enter cells only through  pinocytosis. 

(ii) Phagocytosis:It is also called cell eating, Phagocytosis is the transport of solid matter like food, foreign  particles, pathogens,  etc. across  the membrane by forming detachable vesicles. These vesicles are called phagosomes which  are formed by imagination  of plasma membrane in the region of solid particles,  followed by rapid evagination on the periphery, formation of a vesicle and pinching off the latter into the interior as phagosome. It fuses with a lysosome to produce a digestive vacuole. 

     The undigested parts are usually  the thrown out of the cell in the process of exocytosis called ephagy or cell vomiting. Phagocytosis by some white blood corpuscles is an important defence mechanism of the animal body. Spleen and Liver destroy old erythrocytes through Phagocytosis. 

Role of Membranes in Cellular Movement:

Membrane take part in cellular locomotion  by two methods:

(1) Pseudopodial Movement:Pseudopodia are temporary   blunt outgrowths formed by three developments_ sol_ gel changes,  cytoplasmic streaming and extension of plasma membrane. Pseudopodia are being  regularly  produced and withdrawn with new pseudopodia forming in the advancing  region while pre_ existing pseudopodia being withdraw from the receding  end. Pseudopodia are used as feet in moving  over a solid substratum and surface of water or liquid. Since they are not permanent  or true structures,  they are also called fake feet, e.g., Amoeba,macrophages, white blood corpuscles. 

(2): Undulations:They are small protrusions, projections or rufflings  of the membrane  which pass out like a regular wave in the area of contact with a solid substratum. New undulations develop in the advancing region while the older ones are withdrawn  or modified  in the receding  ones. Formation  of undulations does not require  cytoplasmic streaming. They are formed  due to regular ruffing  of the plasma membrane,  e.g., fibroblasts. 

Stomach               Pancreas

RNA                    Nucleic Acids


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