Introduction Transport System .
[1]. All the cells in the body must be supplied with essential substances like nutrients, water, electrolytes, etc. Cells also must get rid of many unwanted substances like waste materials, carbon dioxide, etc.
[2]. The cells achieve these by means of transport mechanisms across the cell membrane. Structure of the cell membrane is well suited for the transport of substances in and out of the cell.
[3]. Lipids and proteins of cell membrane play an important role in the transport of various substances between extracellular fluid (ECF) and intracellular fluid (ICF).
Basic mechanism of Transport .
Two types of basic mechanisms are involved in the transport of substances across the cell membrane:
1. Passive transport mechanism .
2. Active transport mechanism.
Passive transport .
 |
| simple diffusion through the cell membrane |
[1]. Passive transport is the transport of substances along the concentration gradient or electrical gradient or both (electrochemical gradient). It is also known as diffusion or downhill movement. It does not need energy.
[2]. Passive transport is like swimming in the direction of water flow in a river. Here, the substances move from region of higher concentration to the region of lower concentration.
[3]. Diffusion is of two types, namely simple diffusion and facilitated diffusion. Simple diffusion of substances occurs either through lipid layer or protein layer of the cell membrane.
[4]. Facilitated diffusion occurs with the help of the carrier proteins of the cell membrane. Thus, the diffusion can be discussed under three category :
1. Simple diffusion through lipid layer .
2. Simple diffusion through protein layer .
3. Facilitated or carrier-mediated diffusion.
Simple diffusion through lipid layer .
Lipid layer of the cell membrane is permeable only to lipid-soluble substances like oxygen, carbon dioxide and alcohol. The diffusion through the lipid layer is directly proportional to the solubility of the substances in lipids .
Simple diffusion through protein layer .
Protein layer of the cell membrane is permeable to water-soluble substances. Mainly, electrolytes diffuse through the protein layer.
Protein Channels or Ion Channels .
[1]. Throughout the central lipid layer of the cell membrane, there are some pores. Integral protein molecules of protein layer invaginate into these pores from either surface of the cell membrane. Thus, the pores present in the central lipid layer are entirely lined up by the integral protein molecules.
[2]. These pores are the hypothetical pores and form the channels for the diffusion of water, electrolytes and other substances, which cannot pass through the lipid layer. As the channels are lined by protein molecules, these are called protein channels for water-soluble substances.
Types of Protein Channels or Ion Channels .
Characteristic feature of the protein channels is the selective permeability. That is, each channel can permit only one type of ion to pass through it. Accordingly, the channels are named after the ions which diffuse through these channels such as sodium channels, potassium channels, etc.
Regulation of the Channels .
Some of the protein channels are continuously opened and most of the channels are always closed. Continuously opened channels are called ungated channels . Closed channels are called gated channels. These channels are opened only when required .
Gated Channels .
Gated channels are divided into three categories:
i. Voltage-gated channels .
ii. Ligand-gated channels .
iii. Mechanically gated channels.
i. Voltage-gated channels .
[1]. Voltage-gated channels are the channels which open whenever there is a change in the electrical potential. For example, in the neuromuscular junction, when action potential reaches axon terminal, the calcium channels are opened and calcium ions diffuse into the interior of the axon terminal from ECF.
[2]. Similarly, in the muscle during the excitation-contraction coupling, the action potential spreads through the transverse tubules of the sarcotubular system. When the action potential reaches the cisternae, large number of calcium ions diffuse from cisternae into sarcoplasm.
ii. Ligand-gated channels .
[1]. Ligand-gated channels are the type of channels which open in the presence of some hormonal substances. The hormonal substances are called ligands and the channels are called ligand-gated channels.
[2]. During the transmission of impulse through the neuromuscular junction, acetylcholine is released from the vesicles.
[3]. The acetylcholine moves through the presynaptic membrane (membrane of the axon terminal) and reaches the synaptic cleft. Then, the acetylcholine molecules cause opening of sodium channels in the postsynaptic membrane and sodium ions diffuse into the neuromuscular junction from ECF.
iii. Mechanically gated channels .
[1]. Mechanically gated channels are the channels which are opened by some mechanical factors. Examples are, channels present in the pressure receptors (Pacinian corpuscles) and the receptor cells (hair cells) of organ of Corti and vestibular apparatus.
[2]. When a Pacinian corpuscle is subjected to pressure, it is compressed resulting in deformation of its core fiber. This deformation causes opening of sodium channel and development of receptor potential
[3]. Sound waves cause the movement of cilia of hair cells in organ of Corti (cochlea), which is the receptor organ in the ear.
[4]. Movements of the cilia cause opening of potassium channels leading to the development of receptor potential . Similar mechanism prevails in hair cells of vestibular apparatus also .
Facilitated or carrier-mediated diffusion .
 |
| Stage 1. Glucose binds with carrier protein. Stage 2. Conformational change occurs in the carrier protein and glucose is released into ICF. |
[1]. Facilitated or carrier-mediated diffusion is the type of diffusion by which the water-soluble substances having larger molecules are transported through the cell membrane with the help of a carrier protein. By this process, the substances are transported across the cell membrane faster than the transport by simple diffusion.
[2]. Glucose and amino acids are transported by facilitated diffusion. Glucose or amino acid molecules cannot diffuse through the channels because the diameter of these molecules is larger than the diameter of the channels.
[3]. Molecule of these substances binds with carrier protein. Now, some conformational change occurs in the carrier protein. Due to this change, the molecule reaches the other side of the cell membrane .
Factors affecting rate of diffusion .
Rate of diffusion of substances through the cell membrane is affected by the following factors:
1. Permeability of the Cell Membrane .
Rate of diffusion is directly proportional to the permeability of cell membrane. Since the cell membrane is selectively permeable, only limited number of substances can diffuse through the membrane.
2. Temperature .
Rate of diffusion is directly proportional to the body temperature. Increase in temperature increases the rate of diffusion. This is because of the thermal motion of molecules during increased temperature.
3. Concentration Gradient or Electrical Gradient of the Substance across the Cell Membrane .
Rate of diffusion is directly proportional to the concentration gradient or electrical gradient of the diffusing substances across the cell membrane. However, facilitated diffusion has some limitation beyond certain level of concentration gradient.
4. Solubility of the Substance .
Diffusion rate is directly proportional to the solubility of substances, particularly the lipid-soluble substances. Since oxygen is highly soluble in lipids, it diffuses very rapidly through the lipid layer.
5. Thickness of the Cell Membrane .
Rate of diffusion is inversely proportional to the thickness of the cell membrane. If the cell membrane is thick, diffusion of the substances is very slow.
6. Size of the Molecules .
Rate of diffusion is inversely proportional to the size of the molecules. Thus, the substances with smaller molecules diffuse rapidly than the substances with larger molecules.
7. Size of the Ions .
[7a]. Generally, rate of diffusion is inversely proportional to the size of the ions. Smaller ions can pass through the membrane more easily than larger ions with the same charge. However, it is not applicable always. For instance, sodium ions are smaller in size than potassium ions.
[7b]. Still, sodium ions cannot pass through the membrane as easily as potassium ions because sodium ions have got the tendency to gather water molecules around them. This makes it difficult for sodium ions to diffuse through the membrane.
8. Charge of the Ions .
Rate of diffusion is inversely proportional to the charge of the ions. Greater the charge of the ions, lesser is the rate of diffusion. For example, diffusion of calcium (Ca++) ions is slower than the sodium (Na+) ions.
Special Types of Passive Transport .
In addition to diffusion, there are some special types of passive transport, viz.
1. Bulk flow.
2. Filtration .
3. Osmosis.
Bulk flow .
[1]. Bulk flow is the diffusion of large quantity of substances from a region of high pressure to the region of low pressure. It is due to the pressure gradient of the substance across the cell membrane. Best example for bulk flow is the exchange of gases across the respiratory membrane in lungs.
[2]. Partial pressure of oxygen is greater in the alveolar air than in the alveolar capillary blood. So, oxygen moves from alveolar air into the blood through the respiratory membrane.
[3]. Partial pressure of carbon dioxide is more in blood than in the alveoli. So, it moves from the blood into the alveoli through the respiratory membrane .
Filtration .
[1]. Movement of water and solutes from an area of high hydrostatic pressure to an area of low hydrostatic pressure is called filtration. Hydrostatic pressure is developed by the weight of the fluid.
[2]. Filtration process is seen at arterial end of the capillaries, where movement of fluid occurs along with dissolved substances from blood into the interstitial fluid . It also occurs in glomeruli of kidneys .
Osmosis .
[1]. Osmosis is the special type of diffusion. It is defined as the movement of water or any other solvent from an area of lower concentration to an area of higher concentration of a solute through a semipermeable membrane .
[2]. The semipermeable membrane permits the passage of only water or other solvents but not the solutes. Osmosis can occur whenever there is a difference in the solute concentration on either side of the membrane. Osmosis depends upon osmotic pressure.
Osmotic Pressure .
[1]. Osmotic pressure is the pressure created by the solutes in a fluid. During osmosis, when water or any other solvent moves from the area of lower concentration to the area of higher concentration, the solutes in the area of higher concentration get dissolved in the solvent.
[2]. This creates a pressure which is known as osmotic pressure. Normally, the osmotic pressure prevents further movement of water or other solvent during osmosis.
Reverse Osmotic Pressure .
Reverse osmosis is a process in which water or other solvent flows in reverse direction (from the area of higher concentration to the area of lower concentration of the solute), if an external pressure is applied on the area of higher concentration.
Colloidal Osmotic Pressure and Oncotic Pressure .
The osmotic pressure exerted by the colloidal substances in the body is called the colloidal osmotic pressure. And, the osmotic pressure exerted by the colloidal substances (proteins) of the plasma is known as oncotic pressure and it is about 25 mm Hg.
Types of Osmosis .
Osmosis across the cell membrane is of two types:
1. Endosmosis: Movement of water into the cell
2. Exosmosis: Movement of water out of the cell.
Active transport .
[1]. Active transport is the movement of substances against the chemical or electrical or electrochemical gradient. It is like swimming against the water tide in a river. It is also called uphill transport.
[2]. Active transport requires energy, which is obtained mainly by breakdown of high energy compounds like adenosine triphosphate (ATP).
Active Transport vs Facilitated Diffusion
Active transport mechanism is different from facilitated diffusion by two ways:
1. Carrier protein of active transport needs energy, whereas the carrier protein of facilitated diffusion does not need energy
2. In active transport, the substances are transported against the concentration or electrical or electrochemical gradient. In facilitated diffusion, the substances are transported along the concentration or electrical or electrochemical gradient.
Carrier Proteins of Active Transport .
Carrier proteins involved in active transport are of two types:
1. Uniport .
2. Symport or antiport.
1. Uniport .
Carrier protein that carries only one substance in a single direction is called uniport. It is also known as uniport pump.
2. Symport or Antiport .
[1]. Symport or antiport is the carrier protein that transports two substances at a time. Carrier protein that transports two different substances in the same direction is called symport or symport pump.
[2]. Carrier protein that transports two different substances in opposite directions is called antiport or antiport pump.
Mechanism of active Transport .
[1]. When a substance to be transported across the cell membrane comes near the cell, it combines with the carrier protein of the cell membrane and forms substance-protein complex. This complex moves towards the inner surface of the cell membrane.
[2]. Now, the substance is released from the carrier proteins. The same carrier protein moves back to the outer surface of the cell membrane to transport another molecule of the substance.
Substances transported by active transport .
Substances, which are transported actively, are in ionic form and non-ionic form. Substances in ionic form are sodium, potassium, calcium, hydrogen, chloride and iodide. Substances in non-ionic form are glucose, amino acids and urea.
Types of active transport .
Active transport is of two types:
1. Primary active transport .
2. Secondary active transport.
Primary active transport .
Primary active transport is the type of transport mechanism in which the energy is liberated directly from the breakdown of ATP. By this method, the substances like sodium, potassium, calcium, hydrogen and chloride are transported across the cell membrane.
Primary Active Transport of Sodium and Potassium .
[1]. Sodium and potassium ions are transported across the cell membrane by means of a common carrier protein called sodium-potassium (Na+-K+) pump. It is also called Na+-K+ ATPase pump or Na+-K+ ATPase.
[2]. This pump transports sodium from inside to outside the cell and potassium from outside to inside the cell. This pump is present in all the cells of the body. Na+-K+ pump is responsible for the distribution of sodium and potassium ions across the cell membrane and the development of resting membrane potential.
Mechanism of action of Na+-K+ pump .
 |
| Stage 1: Three Na+ from ICF and two K+ from ECF bind with ‘C’. Stage 2: Conformational change occurs in ‘C’ followed by release of Na+ into ECF and K+ into ICF . |
[1]. Three sodium ions from the cell get attached to the receptor sites of sodium ions on the inner surface of the carrier protein. Two potassium ions outside the cell bind to the receptor sites of potassium ions located on the outer surface of the carrier protein .
[2]. Binding of sodium and potassium ions to carrier protein activates the enzyme ATPase. ATPase causes breakdown of ATP into adenosine diphosphate (ADP) with the release of one high energy phosphate.
[3]. Now, the energy liberated causes some sort of conformational change in the molecule of the carrier protein. Because of this, the outer surface of the molecule (with potassium ions) now faces the inner side of the cell. And, the inner surface of the protein molecule (with sodium ions) faces the outer side of the cell .
[4]. Now, dissociation and release of the ions take place so that the sodium ions are released outside the cell (ECF) and the potassium ions are released inside the cell (ICF). Exact mechanisms involved in the dissociation and release of ions are not yet known.
Electrogenic activity of Na+-K+ pump .
[1]. Na+-K+ pump moves three sodium ions outside the cell and two potassium ions inside cell. Thus, when the pump works once, there is a net loss of one positively charged ion from the cell.
[2]. Continuous activity of the sodium-potassium pumps causes reduction in the number of positively charged ions inside the cell leading to increase in the negativity inside the cell. This is called the electrogenic activity of Na+-K+ pump.
Transport of Calcium Ions.
[1]. Calcium is actively transported from inside to outside the cell by calcium pump. Calcium pump is operated by a separate carrier protein. Energy is obtained from ATP by the catalytic activity of ATPase .
[2]. Calcium pumps are also present in some organelles of the cell such as sarcoplasmic reticulum in the muscle and the mitochondria of all the cells. These pumps move calcium into the organelles.
Transport of Hydrogen Ions .
[1]. Hydrogen ion is actively transported across the cell membrane by the carrier protein called hydrogen pump. It also obtains energy from ATP by the activity of ATPase.
[2]. The hydrogen pumps that are present in two important organs have some functional significance.
1. Stomach: Hydrogen pumps in parietal cells of the gastric glands are involved in the formation of hydrochloric acid
2. Kidney: Hydrogen pumps in epithelial cells of distal convoluted tubules and collecting ducts are involved in the secretion of hydrogen ions from blood into urine .
Secondary active transport .
[1]. Secondary active transport is the transport of a substance with sodium ion, by means of a common carrier protein.
[2]. When sodium is transported by a carrier protein, another substance is also transported by the same protein simultaneously, either in the same direction (of sodium movement) or in the opposite direction
[3]. Thus, the transport of sodium is coupled with transport of another substance.
[4]. Secondary active transport is of two types:
1. Cotransport .
2. Counter transport.
Sodium Cotransport .
[1]. Sodium cotransport is the process in which, along with sodium, another substance is transported by a carrier protein called symport. Energy for movement of sodium is obtained by breakdown of ATP.
[2]. The energy released by the movement of sodium is utilized for movement of another substance. Substances carried by sodium cotransport are glucose, amino acids, chloride, iodine, iron and urate.
Carrier protein for sodium Cotransport .
Carrier protein for the sodium cotransport has two receptor sites on the outer surface. Among the two sites, one is for binding of sodium and another site is for binding of other substance.
Sodium cotransport of glucose .
[1]. One sodium ion and one glucose molecule from the ECF bind with the respective receptor sites of carrier protein of the cell membrane. Now, the carrier protein is activated. It causes conformational changes in the carrier protein, so that sodium and glucose are released into the cell .
[2]. Sodium cotransport of glucose occurs during absorption of glucose from the intestine and reabsorption of glucose from the renal tubule.
Sodium cotransport of amino acids .
[1]. Carrier proteins for the transport of amino acids are different from the carrier proteins for the transport of glucose. For the transport of amino acids, there are five sets of carrier proteins in the cell membrane.
[2]. Each one carries different amino acids depending upon the molecular weight of the amino acids. Sodium cotransport of amino acids also occurs during the absorption of amino acids from the intestine and reabsorption from renal tubule.
Sodium Counter Transport .
Sodium counter transport is the process by which the substances are transported across the cell membrane in exchange for sodium ions by carrier protein called antiport. Various counter transport systems are:
1. Sodium-calcium counter transport.
In this, sodium and calcium ions move in opposite directions with the help of a carrier protein. This type of transport of sodium and calcium ions is present in all the cells .
2. Sodium-hydrogen counter transport .
In this system, the hydrogen ions are exchanged for sodium ions and this occurs in the renal tubular cells. The sodium ions move from tubular lumen into the tubular cells and the hydrogen ions move from tubular cell into the lumen
3. Other counter transport systems .
Other counter transport systems are sodium-magnesium counter transport, sodium-potassium counter transport, calcium-magnesium counter transport, calcium-potassium counter transport, chloride – bicarbonate counter transport and chloride-sulfate counter transport.
Special Types of active transport .
[1]. In addition to primary and secondary active transport systems, there are some special categories of active transport which are generally called the vesicular transport.
[2]. Special categories of active transport:
1. Endocytosis .
2. Exocytosis .
3. Transcytosis.
Endocytosis .
[1]. Endocytosis is defined as a transport mechanism by which the macromolecules enter the cell. Macromolecules (substances with larger molecules) cannot pass through the cell membrane either by active or by passive transport mechanism. Such substances are transported into the cell by endocytosis.
[2]. Endocytosis is of three types:
1. Pinocytosis .
2. Phagocytosis .
3. Receptor-mediated endocytosis.
1. Pinocytosis .
 |
| Process of pinocytosis |
[1]. Pinocytosis is a process by which macromolecules like bacteria and antigens are taken into the cells. It is otherwise called the cell drinking.
[2]. Mechanism of pinocytosis .
Pinocytosis involves following events:
i. Macromolecules (in the form of droplets of fluid) bind to the outer surface of the cell membrane
ii. Now, the cell membrane evaginates around the droplets
iii. Droplets are engulfed by the membrane
iv. Engulfed droplets are converted into vesicles and vacuoles, which are called endosomes .
v. Endosome travels into the interior of the cell
vi. Primary lysosome in the cytoplasm fuses with endosome and forms secondary lysosome
vii. Now, hydrolytic enzymes present in the secondary lysosome are activated resulting in digestion and degradation of the endosomal contents.
2. Phagocytosis .
[1]. Phagocytosis is the process by which particles larger than the macromolecules are engulfed into the cells. It is also called cell eating. Larger bacteria, larger antigens and other larger foreign bodies are taken inside the cell by means of phagocytosis.
[2]. Only few cells in the body like neutrophils, monocytes and the tissue macrophages show phagocytosis. Among these cells, the macrophages are the largest phagocytic cells.
[3]. Mechanism of phagocytosis
i. When bacteria or foreign body enters the body, first the phagocytic cell sends cytoplasmic extension (pseudopodium) around bacteria or foreign body .
ii. Then, these particles are engulfed and are converted into endosome like vacuole. Vacuole is very large and it is usually called the phagosome .
iii. Phagosome travels into the interior of cell .
iv. Primary lysosome fuses with this phagosome and forms secondary lysosome .
v. Hydrolytic enzymes present in the secondary lysosome are activated resulting in digestion and degradation of the phagosomal contents .
3. Receptor-mediated Endocytosis .
[1]. Receptor-mediated endocytosis is the transport of macromolecules with the help of a receptor protein. Surface of cell membrane has some pits which contain a receptor protein called clathrin. Together with a receptor protein (clathrin), each pit is called receptor-coated pit.
[2]. These receptor-coated pits are involved in the receptor mediated endocytosis .
[3]. Mechanism of receptor-mediated endocytosis
 |
| Mechanism of receptor-mediated endocytosis |
i. Receptor-mediated endocytosis is induced by substances like ligands
ii. Ligand molecules approach the cell and bind to receptors in the coated pits and form ligand receptor complex .
iii. Ligand-receptor complex gets aggregated in the coated pits. Then, the pit is detached from cell membrane and becomes the coated vesicle. This coated vesicle forms the endosome .
iv. Endosome travels into the interior of the cell. Primary lysosome in the cytoplasm fuses with endosome and forms secondary lysosome .
v. Now, the hydrolytic enzymes present in secondary lysosome are activated resulting in release of ligands into the cytoplasm .
vi. Receptor may move to a new pit of the cell membrane .
[4]. Receptor-mediated endocytosis play an important role in the transport of several types of macromolecules into the cells,
i. Hormones: Growth hormone, thyroid stimulating hormone, luteinizing hormone, prolactin, insulin, glucagon, calcitonin and catecholamines
ii. Lipids: Cholesterol and low-density lipoproteins (LDL)
iii. Growth factors (GF): Nerve GF, epidermal GF, platelet-derived GF, interferon
iv. Toxins and bacteria: Cholera toxin, diphtheria toxin, pseudomonas toxin, recin and concanavalin A
v. Viruses: Rous sarcoma virus, semliki forest virus, vesicular stomatitis virus and adenovirus
vi. Transport proteins: Transferrin and transcobalamine
vii. Antibodies: IgE, polymeric IgG and maternal IgG.
[5]. Some of the receptor-coated pits in cell membrane are coated with another protein called caveolin instead of clathrin. Caveolin-coated pits are concerned with the transport of vitamins into the cell.
Exocytosis .
Exocytosis is the process by which the substances are expelled from the cell. In this process, the substances are extruded from cell without passing through the cell membrane. This is the reverse of endocytosis.
Mechanism of Exocytosis .
Exocytosis is involved in the release of secretory substances from cells. Secretory substances of the cell are stored in the form of secretory vesicles in the cytoplasm. When required, the vesicles approach the cell membrane and get fused with the cell membrane. Later, the contents of the vesicles are released out of the cell .
Role of Calcium in Exocytosis .
Calcium ions play an important role during the release of some secretory substances such as neurotransmitters. The calcium ions enter the cell and cause exocytosis. However, the exact mechanism of exocytosis is not clear.
Transcytosis .
Transcytosis is a transport mechanism in which an extracellular macromolecule enters through one side of a cell, migrates across cytoplasm of the cell and exits through the other side.
Mechanism of Transcytosis .
[1]. Cell encloses the extracellular substance by invagination of the cell membrane to form a vesicle. Vesicle then moves across the cell and thrown out through opposite cell membrane by means of exocytosis.
[2]. Transcytosis involves the receptor-coated pits as in receptor-mediated endocytosis. Receptor protein coating the pits in this process is caveolin and not clathrin.
[3]. Transcytosis is also called vesicle trafficking or cytopempsis . Transcytosis plays an important role in selectively transporting the substances between two environments across the cells without any distinct change in the composition of these environments.
[4]. Example of this type of transport is the movement of proteins from capillary blood into interstitial fluid across the endothelial cells of the capillary. Many pathogens like human immunodeficiency virus (HIV) are also transported by this mechanism.
Molecular motors .
 |
| Kinesin and dynein motor molecules |
Molecular motors are the protein-based molecular machines that perform intracellular movements in response to specific stimuli.
Function of Molecular Motors .
1. Transport of synaptic vesicles containing neurotransmitters from the nerve cell body to synaptic terminal
2. Role in cell division (mitosis and meiosis) by pulling the chromosomes
3. Transport of viruses and toxins to the interior of the cell for its own detriment.
Types of Molecular Motors .
Molecular motors are classified into three super families:
1. Kinesin .
2. Dynein .
3. Myosin. .
1. Kinesin .
[1]. Kinesin transports substances by moving over the microtubules. Each kinesin molecule has two heads and a tail portion. One of the heads hydrolyses ATP to obtain energy. By utilizing this energy, the other head swings continuously causing movement of the whole kinesin molecule .
[2]. End portion of the tail carries the cargo (substances to be transported). Kinesin is responsible for anterograde transport (transport of substances towards the positive end of microtubule).
2. Dynein .
Dynein is almost similar to kinesin and transports substances by moving over the microtubules. But it is responsible for retrograde transport (transport of substances towards the negative end of microtubule).
3. Myosin .
Myosin transports substances by moving over micro filaments. Myosin are classified into 18 types according to the amino acid sequence. However, myosin II and V are functionally significant. Myosin II is involved in muscle contraction . Myosin V is involved in transport of vesicles.
Thanks for Visiting us
