In this essay we will discuss about the scheme of cell membrane transport of molecules, with the help of suitable diagrams.
Compartmentalization is one of the main functions of cell membrane, transport nutrients, ions, and excretory substances from one side to the other. Generally, the permeation of small molecules across the membrane is quite different from engulfing molecules which are too large to penetrate the membrane (Fig. 4.21).
Transport of Small Molecules:
Based on the expenditure of energy membrane, transports are divided in to two- active and passive transports.
(a) Passive Transport:
Passive transport does not require an expenditure of metabolic energy, and materials flow down the concentration gradient. Examples of passive transport are diffusion, osmosis, and facilitated diffusion (Fig. 4.22).
(i) Simple Diffusion:
Diffusion is the movement of substances with the concentration gradient. As it does not need any helper molecules or energy, hence it is passive.
(ii) Osmosis:
Lipid membranes are semi-permeable; some substances pass through freely (water), while some do not (ions). Diffusion of water down its concentration gradient is called osmosis. Water moves from an area of higher solute concentration to an area of lower solute concentration, that is, toward the area where there is more solute, and thus less water. The area of less solute is called the hypotonic solution, while the area of more solute is called the hypertonic solution. If a semi-permeable membrane separates the hypotonic solution from the hypertonic solution, water will move across the membrane from the hypotonic to the hypertonic solution. No metabolic energy is involved (Fig. 4.23).
Consider two water solutions, one rich in ions (hyper tonic) and the other not (hypotonic), which are separated by a semi-permeable membrane. Water can move across the membrane in both directions, but because ions attract water and impede its random diffusion, water is retarded on the ion-rich side; therefore the rate from the ion-rich side is less than the rate of ions permeating the membrane from the other side.
The net movement of water toward the ion-rich solution builds up hydrostatic pressure, called osmotic pressure, which at some point will counteract the attraction of ions. The two sides will then be at equilibrium. Whenever two solutions are separated by a semi permeable membrane, net movement of water will be towards the solution more concentrated in substances that do not permeate the membrane.
(iii) Facilitated Diffusion:
Facilitated diffusion is the diffusion of a substance across a membrane, enhanced by transport protein in the membrane (Fig.4.24). The involvement of the transport protein makes facilitated diffusion a type of carrier-mediated transport, although it is passive. The transport membrane is specific to the substance being transported, that is, it only transports a particular substance. For example glucose, which is needed in large amounts by cells for energy, is one substance commonly transported into cells by facilitated diffusion.
The kinetics of facilitated (with a helper) transport is different from those of simple diffusion. In the latter, the rate of diffusion is proportional to the concentration of the diffusing molecules; the more of them, the more will diffuse across the membrane per unit time. In facilitated diffusion, however, the rate is limited by the availability of the helper molecules. Once all the helpers are saturated, the increasing concentration of diffusing molecules will only increase a waiting line for the helper and will not increase the rate of transport (Fig. 4.25).
Transport of Uncharged Molecules and Ions:
Gases like O2, N2, diffuse easily through membrane because they have no charge (partial or complete) to interact with water. Hydrophobic molecules (oils) have also no trouble permeating membrane. Ions do not penetrate because of charge and the solvation layer that would have to diffuse with them. Thus when considering transport of ions we must take into account their concentration gradient as well as the electrical gradient, the combined potential is called electrochemical potential. In most cells there is an unequal distribution of ions: Na+ 10 mM inside; 150 mM outside; K+ 150 mM inside; 5 mM outside. Electrochemical potential in the cell means that the Na and K ions experience different forces. The concentration gradient favors influx of Na and efflux of K, but a membrane potential of -70 mV potential favors influx of cations of any kind.
Ionophores are structures which work as transporters and dissipate ion gradients. Some form pores (channels) in the membrane through which ions can diffuse in or out of the cell, e.g., Gramicidin A is a peptide antibiotic with alternating D- and L-amino acids that forms a channel large enough for protons, Na+ and K+ ions to pass through, but is blocked by Ca2+. The trans-membrane channels that permit facilitated diffusion can be opened or closed.
They are said to be “gated”.
Some gated ion channels are:
(a) Ligand-Gated Ion Channels:
Many ion channels open or close in response to binding a small signaling molecule or “ligand“. Some ion channels are gated by extracellular ligands; some by intracellular ligands. In both cases, the ligand is not the substance that is transported when the channel opens.
External ligands bind to a site on the extra-cellular side of the channel. For example, Acetylcholine (ACh) the binding of the neurotransmitter acetylcholine at certain synapses opens channels that admit Na+ and initiate a nerve impulse or muscle contraction, while internal ligands bind to a site on the channel protein exposed to the cytosol. For example, “Second messengers”, like cyclic AMP (cAMP) and cyclic GMP (cGMP), regulate channels involved in the initiation of impulses in neurons responding to odors and light respectively.
(b) Mechanically-Gated Ion Channels:
These channels respond to mechanical impulses. For example, sound waves bending the cilia-like projections on the hair cells of the inner ear open up ion channels leading to the creation of nerve impulses that the brain interprets as sound.
(c) Voltage-Gated Ion Channels:
In neurons and muscle cells, some channels open or close in response to changes in the charge (measured in volts) across the plasma membrane. As an impulse passes down a neuron, the reduction in the voltage opens sodium channels in the adjacent portion of the membrane. This allows the influx of Na+ into the neuron and thus the continuation of the nerve impulse. Some 7000 sodium ions pass through each channel during the short period (about 1 millisecond) that it remains open.
Carriers: Permeases:
Proteins that act as carriers are too large to move across the membrane. They are transmembrane proteins with fixed topology. An example is the GLUT1 glucose carrier, in plasma membranes of various cells, including erythrocytes. GLUT1 is a large integral protein, predicted via hydro-pathy plots to include 12 trans-membrane α-helices.
Carrier proteins such as Glucose permease in erythrocytes are more complex than channels (Fig.4.26). The transported molecule (glucose) moves down its concentration gradient. Once inside the cell, the molecule is transformed into another, impermeant species, thus lowering the inside concentration and maintaining the concentration gradient.
Active Transport:
There are numerous situations in living organisms when molecules move across cell membranes from an area of lower concentration toward an area of higher concentration. In order to accomplish this, membranes have evolved elaborate schemes to pump the substance from the area of smaller concentration to a compartment with higher concentration. All these schemes cost the cell energy and thus are called active transport. If a molecule is to be transported from an area of low concentration to an area of high concentration, work must be done to overcome the influences of diffusion and osmosis. Since, in the normal state of a cell, large concentration differences in K+, Na+ and Ca2+ are maintained, it is evident that active transport mechanisms are at work (Fig. 4.27).
Many crucial processes in the life of cells depend upon active transport. Active transport mechanisms may draw their energy from the hydrolysis of ATP, the absorbance of light, the transport of electrons, or coupling with other processes that are moving particles down their concentration gradients. A vital active transport process that occurs in the electron transport process in the membranes of both mitochondria and chloroplasts is the transport of protons to produce a proton gradient. This proton gradient powers the phosphorylation of ATP associated with ATP synthase.
(a) Direct Active Transport:
The cytosol of animal cells contains a concentration of potassium ions (K+) as much as 20 times higher than that in the extra-cellular fluid. Conversely, the extracellular fluid contains a concentration of sodium ions (Na+) as much as 10 times greater than that within the cell. These concentration gradients are established by the active transport of both ions and in fact, the same transporter, called the Na+/K+ ATPase, does both jobs. It uses the energy from the hydrolysis of ATP to actively transport 3 Na+ ions out of the cell and for each 2 K+ ions pumped into the cell
(b) Na/K ATPase (Pump):
The process of moving sodium and potassium ions across the cell membrane is an active transport process involving the hydrolysis of ATP to provide the necessary energy. It involves an enzyme referred to as Na+/K+-ATPase. The function of Na/K ATPase is to set up the electrochemical gradient of the membrane. It does so by pumping Na out of the cell and pumping K into the cell (Fig. 4.28). The net effect is to create a chemical potential consisting of two concentration gradients (for Na and for K), as well as electrical potential because three positive charges are pumped out while two positive charges are pumped in. A negative potential inside the cell is thus created.
This process is responsible for maintaining the large excess of Na+ outside the cell and the large excess of K+ ions on the inside. It accomplishes the transport of three Na+ to the outside of the cell and the transport of two K+ ions to the inside. This unbalanced charge transfer contributes to the separation of charge across the membrane. The sodium-potassium pump is an important contributor to action potential produced by nerve cells. This pump is called a P-type ion pump because the ATP interactions phosphorylate the transport protein and causes a change in its conformation.
Biological Significance of Na/K Pump:
The crucial roles of the Na+/K+ ATPase are reflected in the fact that almost one-third of all the energy generated by the mitochondria in animal cells is used just to run this pump. It helps in establishing a net charge across the plasma membrane with the interior of the cell being negatively charged with respect to the exterior. This resting potential prepares nerve and muscle cells for the propagation of action potentials leading to nerve impulses and muscle contraction. The accumulation of sodium ions outside of the cell draws water out of the cell and thus enables it to maintain osmotic balance (otherwise it would swell and burst from the inward diffusion of water).
The gradient of sodium ions is harnessed to provide the energy to run several types of indirect pumps. H+ ATPase is another pump mechanism present in the body. The parietal cells of stomach use this pump to secrete gastric juice. These cells transport protons (H+) from a concentration of about 4 x 10-8 M within the cell to a concentration of about 0.15 M in the gastric juice (giving it a pH close to 1). Both of these pumps can be made to run backward. That is, if the pumped ions are allowed to diffuse back through 1 the membrane complex, ATP can be synthesized from ADP and inorganic phosphate.
Indirect Active Transport:
Indirect active transport uses the downhill flow of an ion to pump some other molecule or ion against its gradient. The driving ion is usually sodium (Na+) with its gradient established by the Na+/K+ ATPase. ATP is not directly involved, but it sets up the electrochemical gradient used to propel the driver. It can be of three types- uniport, symport or antiport (Fig. 4.29). In uniport single solute moves from one side of the membrane to the other.
(a) Symport:
In this type of indirect active transport, the driving ion (Na+) and the pumped molecule pass through the membrane pump in the same direction, i.e., is the passenger and the driver are transported in the same direction. Examples of symport transporters include; The Na+/glucose transporter- This trans-membrane protein allows sodium ions and glucose to enter the cell together. The sodium ions flow down their concentration gradient, while the glucose molecules are pumped. Later the sodium is pumped back out of the cell by the Na+/K+ ATPase. The Na+/glucose transporter is used to actively transport glucose out of the intestine and also out of the kidney tubules and back into the blood (Fig. 4.30).
(b) Anti-Port Pumps:
In anti-port pumps, the driving ion (again, usually sodium) diffuses through the pump in one direction providing the energy for the active transport of some other molecule or ion in the opposite direction. Examples include Mg2+ ions are pumped out of cells by a sodium-driven anti-port pump; Ca-Na anti-port takes place in cardiac muscle and sucrose-H anti-port in plant vacuoles. The Na+ / K+ ATPase is also an anti-port pump using the energy of ATP to pump Na+ out of the cell, while K+ in. This sodium/proton anti-port pump enables the plant to sequester sodium ions in its vacuole. Transgenic tomato plants that over express this sodium/proton anti-port pump are able to thrive in saline soils, too salty for conventional tomatoes.
Some Inherited Ion-Channel Diseases:
A growing number of human diseases have been discovered to be caused by inherited mutations in genes encoding channels.
(a) Chloride-channel diseases e.g.- Cystic fibrosis; inherited tendency to kidney stones (caused by a different kind of chloride channel than the one involved in cystic fibrosis)
(b) Potassium-channel diseases e.g.- some inherited life-threatening defects in the heartbeat and a rare inherited tendency to epileptic seizures in the newborn.
(c) Sodium-channel diseases e.g.- inherited tendency to certain types of muscle spasms.
Liddle’s syndrome- Inadequate sodium transport out of the kidneys, because of a mutant sodium channel, leads to elevated osmotic pressure of the blood and resulting hypertension (high blood pressure).
Transport of Large Molecules:
Membranes transport molecules too big to permeate the membrane by engulfing the substance and forming internal vesicles. Uptake of substances by such a mechanism is called endocytosis and the secretion is called exocytosis.
(a) Exocytosis:
In exocytosis, the transport vesicle fuses with the plasma membrane, making the inside of the vesicle as continuous with the outside of the cell. Exocytosis is used in secretion of protein hormones (insulin), serum proteins and extracellular matrix (collagen).
(b) Endocytosis:
It is a process whereby cells absorb material (molecules such as proteins) from the outside by engulfing it with their cell membrane. It is used by all cells of the body because most substances important to them are large polar molecules, and thus cannot pass through the hydrophobic plasma membrane. The function of endocytosis is the opposite of exocytosis.
The absorption of material from the outside environment of the cell is commonly divided into two processes:
i. phagocytosis and
ii. pinocytosis.
Receptor-Mediated Endocytosis:
Endocytosis is a more specific active event where the cytoplasm membrane folds inward to form coated pits. These inward budding vesicles bud to form cytoplasmic vesicles.
There are three types of endocytosis namely:
i. macropinocytosis,
ii. clathrin-mediated endocytosis, and
iii. caveolar endocytosis.
i. Macropinocytosis is the invagination of the cell membrane to form a pocket which then pinches off into the cell to form a vesicle filled with extracellular fluid (and molecules within it). The filling of the pocket occurs in a non-specific manner. The vesicle then travels into the cytosol and fuses with other vesicles such as endosomes and lysosomes.
ii. Clathrin-mediated endocytosis is the specific uptake of large extracellular molecules such as proteins, membrane localized receptors and ion-channels. These receptors are associated with the cytosolic protein clathrin which initiates the formation of a vesicle by forming a crystalline coat on the inner surface of the cell’s membrane (Fig. 4.31).
(Figure 4.31) Mechanism of clathrin-dependent endocytosis:
Clathrin and cargo molecules are assembled into clathrin-coated pits on the plasma membrane together with an adaptor complex called AP-2 that links clathrin with transmembrane receptors, concluding in the formation of mature clathrin-coated vesicles (CCVs). CCVs are then actively uncoated and transported to early/sorting endosomes.
iii. Caveolae consist of the protein caveolin-1 with a bi-layer enriched in cholesterol and glycolsphingolipids. Caveolae are flask shaped pits in the membrane that resemble the shape of a cave (hence the name caveolae). Uptake of extra-cellular molecules is also believed to be specifically mediated via receptors in caveolae.
(c) Phagocytosis:
Phagocytosis (literally, cell-eating) is the process by which cells ingest large objects, such as cells which have undergone apoptosis, bacteria, or viruses. The membrane folds around the object, and the object is sealed off into a large vacuole known as a phagosome. Removal of foreign materials or dead cells by immune cells is a form of endocytosis. For example, phagocytes are macrophages that line blood channels of liver (spleen) and eat up aging RBCs; monocytes penetrate inflamed tissue and remove the invading bacteria. Amoebas also use this process.
(d) Pinocytosis:
Pinocytosis (literally, cell-drinking) is the process of taking up liquid from the surrounding environment. It is a non-specific uptake of extra-cellular solution tiny pockets are formed along the membrane, filled with liquid, and then pinched off. This process is concerned with the uptake of solutes and single molecules such as proteins.