Mode of Material Transport Across Plasma Membrane
The plasma membrane acts as a semipermeable barrier between the cell and the extracellular environment. This permeability must be highly selective if it is to ensure that essential molecules such as glucose, amino acids and lipids can readily enter the cell, that these molecules and metabolic intermediates remain in the cell, and that waste compounds leave the cell. In short, the selective permeability of the plasma membrane allows the cell to maintain a constant internal environment. In consequence, in all types of cells there exists a difference in ionic concentration with the extracellular medium. Similarly, the organelles within the cell often have a different internal environment from that of the surrounding cytosol and organelle membranes maintain this difference. For example, in lysosomes the concentration of proteins is 100 to 1000 times that of the cytosol. This gradient is maintained solely by the lysosomal membrane. Transport across the membrane may be passive or active. It may occur via the phospholipid bilayer or by the help of specific integral membrane proteins, called permeases or transport proteins.
A Passive transport. It is a type of diffusion in which an ion or molecule crossing a membrane moves down its electrochemical or concentration gradient. No metabolic energy is consumed in passive transport. Passive transport is of following three types:
1. Osmosis. The plasma membrane is permeable to water molecules. The to and fro movement of water molecules through the plasma membrane occurs due to the differences in the concentration of the solute on its either sides. The process by which the water molecules pass through a membrane from a region of higher water concentration to the region of lower water concentration is known as osmosis. The process in which the water molecules enter into the cell is known as endosmosis, while the reverse process which involves the exist of the water molecules from the cell is known as exosmosis. In plant cells due to excessive exosmosis the cytoplasm along with the plasma membrane shrinks away from the cell wall. This process in known as plasmolysis.
A cell contains varity of solutes in it, for instance, the mammalian erythrocytes contain the ions of potassium, calcium, phosphate, dissolved haemoglobin and many other substances. If the erythrocyte is placed in a 0.9% solution of sodium chloride, then it neither shrinks nor swells. In such case, because the intra-cellular and extra-cellular fluids contain same concentration and no osmosis takes place. This type of extra-cellular solution or fluid is known as isotonic solution or fluid. If the concentration of NaCl solution is increased above 0.95 then the erythrocytes are shrinked due to excessive exosmosis. The solutions which have higher concentrations of solutes than the intracellular fluids are known as hypertonic solutions. Further, if the concentration of NaCl solution decreases below 0.9% the erythrocytes will swell up due to endosmosis. The extra-cellular solution having less concentration of the solutes than the cytoplasm are known as hypotonic solutions.
Due o endosmosis or exosmosis the water molecules come in or go out of the cell. The amount of the water inside the cell causes a pressure known as hydrostatic pressure. The plasma membrane maintains a balance between the osmotic pressure of the intra-cellular and inter-cellular fluids.
2 2. Simple diffusion. In simple diffusion, transport across the membrane takes place unaided, i.e., molecules of gases such as oxygen and carbon dioxide and small molecules enter the cell by crossing the plasma membrane without the help of any permease. During simple diffusion, a small molecule in aqueous solution dissolves into the phospholipid bilayer, crosses it and then dissolves into the aqueous solution on the opposite side. There is little specificity to the process. The relative rate of diffusion of the molecule across the phospholipid bilayer will be proportional to the concentration gradient across the membrane.
33. Facilitated diffusion. This is a special type of passive transport, in which ions or molecules cross the membrane rapidly because specific permeases in the membrane facilitate their crossing. Like the simple diffusion, facilitated diffusion does not require the metabolic energy and it occurs only in the direction of a concentration gradient. Facilited diffusion is characterized by the following special feature:
TThe rate of transport of the molecules across the membrane is far greater than would be expected from a simple diffusion.
T The process is specific; each facilited diffusion protein transports only a single species of ion or molecules.
TThere is a miximum rate of transport, i.e., when the concentration gradient of molecules across the membrane is low, an increase in concentration gradient results in a corresponding increase in the rate of transport. Currently, it is believed that transport proteins form the channeles through the membrane that permit certain ions or molecules to pass across the latter.
EExample of Facilited Diffusion
I Ionic transport through charged pores. Nerveconduction is propagated along the axonal membrane by action potential which regulates opening and closing of two main types of ion channeles(i.e., channel proteins with water filled pores): Na⁺ channels and K⁺ channels. At the point of stimulation there is a sudden and several hundred fold increase in permeability to Na⁺, which reaches its peak in 0.1 millisecond. At the end of the period, the membrane again becomes essentially impermeable to Na⁺, but the K⁺ permeability increases and this ion leaks out of the cell, repolarising the nerve fibre. In other words, during the rising phase of the spike, Na⁺ enter through the Na ⁺ channels, and in decending phase K⁺ is extruded through the K⁺ channels.
S. Such ion channeles also occur in other types of cells such as muscle, sperm and unfertilized ovum. They are not coupled to an energy source (ATP), so the transport they meiate is always passive (''down hill''), allowing specific ions mainly Na⁺, K⁺, Ca²⁺, and Cl⁻ to diffuse down their electrochemical gradient across the lipid bilayer has two functional elements: (1) a selective filter which determines the kind of ion that will be transported;(2) a gate which by opening and closing the channel, regulates the ion flow. In both Na⁺, and K⁺ channels, the gating mechanism is electrically driven and is controlled by the membrane potential, without the need of other energy source. In the resting condition both Na⁺, K⁺, Ca²⁺ , Cl⁻, and K⁺ channels are closed. With depolarisation, the channel is opened and during repolarisation, it closes again Na⁺, and K⁺, channel opens.
C. Calcium ion channels(Ca⁺ - channels) occur in axonal membranes and other membranes for the entra Carbonic material Protein material nce of Ca+ ions in the cell. Ca⁺ ions have a fundamental role in many cellular activities such as axocytosis, secretion, cell mobility, cell growth, fertilization and cell division. In the neuronal membrane, there are a number of Ca⁺ channels that are driven by the membrane potential and are essential in the release of neurotransmitters (acetylcholine).
2. D- hexose permease of erythrocytes. The plasma membrane of mammalian erythrocytes and other body cells, contains specific channel proteins for the facilitated diffusion of glucose into the cells. They are called glucose transporter, glucose permease or D-hexose permease. After the glucose is transported into the erythrocytes, it is rapidly phosphorylated to form glucose-6-phosphate. Once phosphorylated, the glucose no longer leaves the cell; moreover, the concentration of the simple glucose in the cell is lowered. As a result, the concentration gradient of glucose across the membrane is increased, allowing the facilitated diffusion to continue to import glucose. Since no cellular membrane contains any permease for facilitated diffusion of phosphorylated compounds, so a cell can retain any type of molecule by phosphorylating them, e.g., ATP and phosphorylated nuclosides are never released from the cells containing a normal intact plasma membrane. However, permeases for ATP and ADP do exist in a mitochondrial membrane to allow these molcules to move across it.
The D- hexose permease of the erythrocytes is an integral and transmembrane protein of 45,000 daltons M.W. It accounts for 2 per cent of erythrocyte membrane protein. D- hexose permease, most probably operates in the following way: the binding of glucose to a site on the exterior surface of the permease triggers a conformational change in the polypeptide. This change somehow generates a pore in the protein that allows the bound glucose to pass through the membrane.
3. Anion exchange permease of erythrocyte. Band 3 polypeptide of plasma membrane of the erythrocytes and other cells is an ion exchange permease protein which catalyzes an one-for-one exchange of anions such as chloride (Cl⁻) and bicarbonate (HCO₃ ⁻) across the membrane (called chloride shift; erythrocyte has 100,000 times more permeability of Cl- than other cells). The rapid flux of anions in the erythrocyte facilitates the transport in the blood of CO₂ from the tissues to the lungs. Waste CO₂ that is released from cell into the capillary blood, diffuses across the membrane of erythrocyte. In its gaseous from, CO₂ dissolves poorly in aqueous solutions such as blood plasma, but inside the erythrocyte the potent enzyme carbinic anhydrase coverts it into a bicarbonate anion:
carbonic
CO₂ + H₂O ←……………………..→ H⁺
+ HCO₃
anhydrase
This process ocurs while the haemoglobin in the erythrocyte is releaseing its oxygen into the blood plasma. The removal of oxygen from haemoglobin induces a change in its conformation that enables a globin histidine (amino acid) side chain to bind to the proton produced by carbonic anhydrase enzyme. The bicarbonate anion formed by carbonic anhydrase is transpoted out of the erythrocyte in exchange for a chloride (Cl⁻) anion:
HCO₃ + Cl⁻ ←……………..→ HCO₃⁻ + Cl⁻
(in) (out) (out) (in)
As the total volume of the blood plasma is about twice that of the total erythrocyte cytoplasm, this exchange triples the amount of bicarbonate that can be carried by blood as a whole. Without the presence of an anion exchange protein (i.e., band 3 protein), bicarbonate anions generated by carbonic anhydrase would remain within the erythrocyte and blood would be unable to transport all of the CO₂ produced by tissue. The entire exchange process is completed within 50 millisecond (ms) during which time
5X10⁹ HCO₃⁻ ions are exported from the cell.
The process is reversed in the lungs:
HCO₃ ⁻ diffuses into the erythrocyte in exchange for a Cl⁻ . Oxygen binding to haemoglobin causes release of the proton from haemoglobin. The CO₂ diffuses out of the erythrocyte and is eventually expelled in breathing. The exact mechanism of anion transport by the band 3 protein is still unknown.
B. Active transport. Active transport uses specific transported proteins, called pumps, which use metabolic energy (ATP) to move ions or molecules against their concentration gradient. For example, in both vertebrates and invertebrates, the concentration of sodium ion is about 10 to 20 times higher in the blood than within the cell. The concentration of the potassium ion is the reverse, generally 20 to 40 times higher inside the cell. Such a low sodium concentration inside the cell is maintained by the sodium-potassium pump. There are different types of pumps for the different types of ions or molecules such as calcium pump, proton pump,etc.
Eaxmple of Active Transport
1. Na⁺- K⁺ - ATPase. It is an ion pump or cation exchange pump which is driven by energy of one ATP molecule to export three Na⁺ ions outside the cell in exchange of the import of two K⁺ ions inside the cell. Electrical organs of eels are found to be very rich in this enzyme or pump. Na+ - K⁺ - ATPase is a transmembrane protein which is a dimer having two subunits: one smaller unit which is a glycoprotein of 50,000 daltons M.W. The larger subunit of Na⁺ - K⁺ATPase performs the actual function of cation transport. It has three sites on its extracytoplasmic surface: two sites for K⁺ ions and one site for the inhibitor ouabain. On its cytosolic side, the larger subunit contains three sites for three Na⁺ ions and also has one catalytic site for a ATP molecules. It is believed that the hydrolysis of one ATP molecule somehow drives conformational changes in the Na⁺ - K⁺ - ATPase that allows the pump to transport three Na⁺ ions out and two K⁺ions inside the cell.
2. Calcium ATPase. Calcium pump or Ca⁺ - ATPase pumps Ca⁺ - ions out of the cytosol, maintaining a low concentration of it inside the cytosol. In some types of cells such as erythrocytes, the calcium pumps are located in the plasma membrane and function to transport Ca²⁺ ions out of the cell. In contrast, in muscle cells Ca²⁺ - ion pumps are located in the membrane of ER or sarcoplasmic reticulum. The Ca²⁺ - ATPase Ca²⁺ transports from the cytosol to the interior of the sarcoplasmic reticulum for causing the relaxation of the muscle cells. Release of Ca²⁺ ions from the sarcoplasmic reticulum into the cytosol of muscle cells causes contraction of the muscle cells. Sarcoplasmic reticulum tends to concentrate and store Ca²⁺ions by the help of following two types of reservoir proteins:(1) Calsequestrin (44,000 daltons M.W.; highly acidic protein) which tends to bind up to 43 Ca²⁺ions with it. (2) High affinity Ca²⁺ - bing protein which binds Ca²⁺ ions and also reduces the concentration of free Ca²⁺ions inside the sarcoplasmic reticulum vesicles and decreases the amount of energy needed to pump Ca²⁺ions into it from the cytosol.
A calcium pump is a 100,000 M.W., polypeptide, forming 80 percent of integral membrane protein of sarcoplasmic reticulum. In it hydrolysis of one ATP molecule transports two Ca²⁺ ions in the counter- transport of one Mg²⁺ ion.
Na⁺, K⁺, Ca²⁺ , Cl⁻
, HCO₃ ⁻, CO₂ , N⁺, 5X10⁹ HCO₃⁻
CO₂ + H₂O ˎ ͟͞ ˋ H⁺ + HCO₃
HCO₃ + Cl⁻ ˎ ͟͞ ˋ
HCO₃⁻ +
Cl⁻
- Important Link
- Power of self skills growth
- Health care is the successful in life journey
- Top professional Cources
- Top courses in low cost
- Best professional Cources in Ranchi Jharkhand
- Hospital management cources
- Sanitary Health Inspector Course in jharkhand
- High demand in govt. of sanitary health inspector cources
- Hospital management cources
- Theolog course with govt recognised college