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Transport Mechanism - Passive Transport , Active Transport, Bulk Transport

Transport Mechanism

 

Membrane transport refers to the transport of various substances such as ions and small molecules across through biological membranes which are proteins embedded lipid bilayers, membrane transport occur by several mechanisms that regulate the passage of solutes.

 

Cell Membrane

 

Cell membranes are selectively permeable. It allow selected, specific compound to enter or leave the cells.

It maintaining normal functioning of cell through biological membranes.


Read here, Fluid mosaic model of Bio membrane structure by Singer and Nicolson.


Bio membrane or Cell membrane plays an essential role in transport of -

 

1. Solutes

2. Ions

3. Small molecules

4. Movement of compounds

5. Sugar

6. Steroids

7. Amino acids

8. Fatty acids


Membrane Transport Function


Membrane Transport is essential because of key functions include -

1. It maintain PH and ionic concentration within cell.

2. It secretes useful substance or excretes toxic waste substances.

3. It exchange of material is necessary to maintain cell function.

4. It obtain nutrients supply for energy and raw material for cell.

5. It keeps balance of salts, nutrients, and protein within range that keep cells, tissue, and organism live.

6. It generates an ionic gradient; ion movement, essential for muscular and nervous activities.

7. Bio membrane protects cell's internal environment.


Types of Membrane Transport


There are different types of transport mechanism across bio membrane which are-


1. Passive Transport

2. Active Transport

3. Bulk Transport

 




1. Passive Transport

Passive transport is a type of membrane transport that does not require cell energy to move substances across plasma membranes.

The four main kinds of passive transport are diffusion and  osmosis.

 

A. Diffusion

Diffusion is passive transport, the type of diffusion are -

1. Passive diffusion

2. Filtration

3. Dialysis

4. Facilitated Diffusion


B. Osmosis


2. Active Transport

A. Sodium pump


3. Bulk Transport

Bulk membrane transports are -


A. Endocytosis

B. Exocytosis


1. Diffusion

Diffusion is the process where in molecules move from a region of higher concentration to region of lower concentration.


A. Passive Diffusion

Passive transport is type of membrane transport that doesn’t require energy to move substance across cell membranes.

Two key points -


1. Cell membrane passive role in transfer of substance means No use of cell energy.

2. It doesn’t require semi-permeable membrane.


“Passive diffusion is the process by which molecule diffuse from a region of higher concentration to region of lower concentration.”

Diffusion of substance occurs through channels or pore. It is most important mechanism for passage of drug through membrane.


Rate of Diffusion


Rate of Diffusion depends upon following factor –


1. Number of particle per unit volume

2. Density of medium

3. Temperature

4. Pressure

5. Distance

6. Water soluble, solutes and ions; K, Cl, HCO3 diffuse through tiny pore in plasma membrane by electrochemical gradient; ion movement across membrane.

7. Molecular weight of diffusing molecules.

8. Gases always diffuse from region of higher partial pressure to region of low partial pressure.

9. Plasma membrane is permeable to both O2, CO2 gases due to presence of pores or channels in it.

10. Diffusion helps in respiratory gas exchange; O2 and CO2 between tissue cells and environment.

11. Diffusion also involve in movement of solutes and ions from one cell to another cell and also from cells to extracellular fluids.


B. Filteration


Filteration is types of membrane transport.

Filtration is the process in which solid particles in liquid or gaseous fluid are removed by use of filter medium; semi permeable membrane under pressure that permits the fluid to pass through but hold the solid particles.

Two major points -


1. It Require semi permeable membrane

2. Filteration is diffusion; movement of molecules, substance through semi permeable membrane under pressure.

Basic Requirements for Filteration are -


1. Filter medium

2. Fluid with suspended particle; solids

3. Driving force; pressure

4. Mechanical device that hold filter, medium contains fluid and permit application of force.

Example – Ultrafilteration; separation process occurs in Malpighian tubules of kidney.

Ultrafilteration occurs when fluid passes across a semi permeable membrane; a membrane that allow some substance to pass through due to a driving pressure.

In glomerular capillaries due to High pressure, cell-free and protein free-blood to flow into Bowman's capsule through capillary endothelium and tubular endothelial. Blood is left only with corpuscles, plasma proteins and fats.

Filtered fluid is called glomerular filterate; it contains water urea, amino acids, sodium, and potassium ion, glucose.

Glomerular filtrate undergo tubular reabsorption and tubular secretion to formation of urine and finally excreted out.


C. Dialysis

Dialysis is a separation process procedure to removing waste products and excess fluid from the blood when the person’s both kidneys has lost their renal function or kidney stop working properly. It often involves diverting blood to a machine to be cleaned.

Type of Dialysis

Kidney dialysis is two main type-

1. Haemodialysis

2. Peritoneal dialysis


D. Facilitated Diffusion


Facilitated diffusion is selective process; membrane allows only selective molecules and ion to pass through it without consuming any cellular energy and prevent other molecules from passing through membrane. It requires a carrier molecule for transport across the membrane.

Carrier protein is integral proteins of membrane and is highly specific.

“Facilitated Diffusion refers to transport of solute through carrier protein molecules transport membrane without consuming any cellular energy. It is passive movement of molecule along down concentration gradient”.

Material move across membrane from the region of high concentration to low concentration without consuming any cellular energy, it is downhill movement.


Examples of facilitated diffusion-

 

1. Glucose Transporter (GLUT)

2. Cl/HCO3 Exchangers

3. ATP/ADP Translocase


2. Osmosis

For osmosis process semi-permeable membrane is necessary.

“Osmosis is special type of diffusion of solvent (water) molecule through a semi permeable membrane from region of higher solvent concentration to the region of lower solvent concentration.”

Osmotic depends upon the number of presence of solute particles but solute particle cannot pass through the membrane.

Note - Osmosis applicable only to solvent part of solution and it always operates in liquid medium. For osmosis semi permeable membrane is must.



Osmotic Pressure

Osmotic pressure or osmotic potential is minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semi permeable membrane.

Osmotic pressure is minimum pressure that would be requiring to stop water from diffusing through semi permeable membrane by osmosis.

 

Osmotic Pressure = Applied Pressure to stop Osmosis




Types of Osmosis


The three types of osmotic conditions include- 

1. Isotonic

2. Hypotonic

3. Hypertonic






 

1. Isotonic solution

Isotonic condition occur when concentration of extracellular fluids is same that of intracellular fluid. In this condition both fluids have same osmotic concentration or pressure.

Result No osmosis takes place, and these are called isotonic solution. The cell neither shrinks nor swells.

Two solutions having same osmotic pressure across semi permeable membrane is referred to as an isotonic solution.

Example of isotonic solution is –


RBC placed in 0.9% NaCl solution

RBC placed in 5% glucose solution


2. Hypotonic solution

If cell is placed in a hypotonic solution, the net flow of water into the cell, cell will gains water by Endosmosis. Water move into cell and gain volume.

When extracellular fluid has low osmotic concentration than the intracellular fluid; solute concentration outside the cell is lower than inside the cell, and the solutes cannot cross the membrane, then that solution is hypotonic to the cell.

Water moves from an area of higher water concentration to one of lower concentration and flow of water into the cell.

 

Hypotonic solution is used to give fluid intravenously to patient to treat or avoid dehydration.

Example, in hypotonic solution Red blood cell, fill up with water and swells then burst.

 

3. Hypertonic solution

When extracellular fluid has high osmotic concentration than the intracellular fluid; solute concentration outside the cell is higher than inside the cell, movement of water outside the cell. Cell will lose water by Exosmosis.

If cell placed in hypertonic solution, it will shrinks called plasmolysis; cell lose water.

For example - in hypertonic medium Red blood cell become shrinken.

Hypertonic solutions are used for the soaking wounds.

 

Check out here the difference between Osmosis and Diffusion


2. Active Transport


1. Active Transport is process of transporting large sized ions or molecules across a membrane, which is insoluble in phospholipids, with the help of carrier molecules from low concentration to high concentration using cellular ATP energy.

2. It requires semi-permeable membrane.

3. It uses cellular Adenosine Triphosphate (ATP) energy.

 

4. This transport mechanism work against concentration gradient and is more rapid than diffusion.

5. It is an uphill movement of substance; from a region of lower concentration to higher concentration.

6. Carrier protein binds to the solute such as  glucose,  calcium, amino acid and transports it to other side of membrane.

7. Actively transported substances through cell membrane are ions of sodium, potassium, chloride, calcium, iron, iodine, glucose and amino acid.

8. Active transport is known as primary active transport.

 

Role of Active Transport

Active membrane transport plays a very important role in –


1. Conduction of Nerve impulse

2. Muscle contraction

3. Urine formation in kidney

 




Sodium-Potassium Pump

 

Sodium-Potassium Pump also known as Na+/K+ pump or Na+/K+-ATPase.

 

Active Transport occurs on the basis of pumping action, one substance out of cell and other substance inwards cell with the help of cellular energy. It results in the accumulation of substances within cell in higher concentration.

Active channel sodium pump is primary active transport system; It is sodium-potassium exchange pump that works in plasma membrane of cells to adjust the concentration of sodium (Na+) and potassium (K+) ion inside and outside the cells.

Active Transport is energy- requiring process of pumping molecules and ions across membrane against a concentration gradient. It transfers Na+ and K+ ion against their concentration gradient.

Active Transport process help maintain Homeostasis.


How does the Sodium-Potassium Exchange Pump Works?


1. Sodium-potassium exchange pump operates with carrier protein transport membrane with the help of enzyme ATPase. Carrier protein is needed for Sodium-Potassium Exchange function.

2. 3 sodium (Na+) ions bind with protein pump inside cell, and 2 potassium (K+) ions are bind with protein pump outside cell. Carrier protein then get energy from ATP and change shape.

3.  It pumps the 3 Na+ ions out of cell and 2 K+ ions pumped into the cell.

4. Cell maintains high concentration of Na+ ions outside plasma membrane than inside it.

5. Electrical events in nerve fibres are controlled by differential permeability of plasma membrane to Na+ and K+ ions.

6. Cell membrane is more permeable to K+ ion than to Na+ ions entering it.

7. This leads to increase positive (+) charge on outside cell membrane and negative (-) charge on inside of cell membrane.

8. This condition makes resting nerve fibre "polarize"; neuron is not stimulated.

9. Entry of Na+ leads to depolarization of nerve membrane; Na+ ions flow into the cell.

Whole process of pumping out Na+ ion and pumping in K+ ion into nerve fibre called Na+ / K+ exchange pump.

Na+/K+ exchange pump also help to control the water contents. It involves counter current mechanisms to excrete hypertonic urine for conserving body water. Used to concentrate urine in kidney by nephrons.


Factor Affective Active Transport


1. Carrier protein

2. Formation of ATP

3. Temperature

4. Inhibitors and toxic substance


Significance of Active Transport


1. Maintain ionic concentration and osmotic pressure.

2. Enabling cells to accumulate molecules or ions against the concentration gradient.

3. Enable flow of stimulus, maintains differential potential inside and outside nerve membrane and muscle cells.

4. Controlling cell osmo-regulation, maintaining osmotic balance; salt and water balance.

5. Maintain high K+ salt concentration required for protein synthesis, glycolysis and physiological process.

6. Helps cells maintain cell homeostasis.

 

What are the differences between Active and Passive transport?


3. Bulk Transport


Bulk transport is a type of membrane transport, involves the transport of macromolecules and large amount of substance such as solid food particles, lipid droplets, across plasma membrane by utilising cell energy.

1. It transport macromolecules by vesicles formation.

2. Material is transported in vesicles of the membrane, called carrier vesicles.

3. It requires energy from cell ATP, Ca+ ions and cytoskeleton system support.

4. Macromolecules are transported like proteins, fat droplet, polysaccharides, and polynucleotide.


Types of Bulk Transport

Bulk transport involve two Process-

1. Endocytosis

2. Exocytosis

 




1. Endocytosis

Endocytosis inside cell, protein from RER to Golgi complex, substance brought into cell.


Three methods of endocytosis are -

A. Pinocytosis

B. Phagocytosis

C. Receptor-mediated endocytosis








A. Pinocytosis


1.  Liquid droplets are ingested into cell, also called drinking of cell.

2. Bulk transport of fluid matter and substances dissolved in it (sugars, ions, amino acids) across the cell membrane by forming detachable vesicles called pinosome.

3. In pinocytosis is a cellular process in which substances are brought inside a cell. Cell membrane folds and creates small pockets and captures the cellular fluid and dissolved substances like  ions sugars, amino acids

 

B. Phagocytosis


1. Also called eating of cells.

2. Phagocytosis is a cellular process for ingesting and eliminating solid matter, larger particles, including microorganisms, foreign substances, and apoptotic cells.

3. It transport of solid matter like food, foreign particles, pathogens (bacteria, viruses) across membrane by forming detachable vesicles called phagosome.

 

2. Exocytosis



 

1. Exocytosis process is reverse of endocytosis and called celvomiting or ephagy.

2. In Exocytosis process, excretion of the substances with the help of the cell membrane. Substances are transported outside of cell.

3. It is process of exudating (emitted through pores) secretary or waste products outside the cell through plasma membrane. Exudated products include proteins, hormones, enzymes, undigested matter.

 

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