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Transport Through Cell Membrane:
Cell membrane is made up of a liquid bilayer which is crossed by proteins. Having roles to create some structure or force in the cell membrane, through which molecules from intra or extra cellular space can enter or leave the cell. Cell membrane is a liposoluble structure; it’s made up of fat. Some proteins function as channels through which molecule can enter or leave the cell. The transport is performed by two types of mechanisms, namely passive and active. We talk about passive transport and active transport. We are talking about the transport of macromolecules, and micromolecules.
Transport of macromolecules:
This process is performed by exocytosis , and endocytosis. Both mechanisms are performed by
mean of some vesicles. Exocytosis needs the transport of protein from inside to outside of cell.
The vesicle fuse with cell membrane and it’s contents are released outside. This process is used by
using energy, it’s an active process. The movement of this process needs calcium.
Endocytosis, is the process by which different macromolecule are introduced inside the cell by use of vesicles. There are two types of endocytosis: pinocytosis and phagocytosis. Pinocytosis: molecules of fluid are introduced inside of the cell, the molecules are not visible under the microscope, it occurs at the level of all cells. Phagocytosis: some micro‐organisms, bacteria for example, are introduced into the cell. The bacteria make contact with the cell; some pseudopods are formed from the cell surrounding the bacteria to create the vesicle.
Passive transport:
Different molecules are transported inside or outside the cell in the direction of an electric or chemical gradient, from a higher concentration to a lower concentration. DIFFUSION : can be SIMPLE OR FACILITATED. The molecules of a solution are in a continuous movement if the temperature is not the absolute 0. The movement is named Brownian movement. Because of this continuous movement, the molecules knock each other, repel each other and change their direction. In the compartment where the concentration of solution is higher, these knocking is higher so there is a tendancy of the molecules to move in the space or another compartment where their concentration is lower. This movement is named diffusion. Diffusion depends on several factors, the concentration gradient , the greater the gradient, the bigger the diffusion, another factor is temperature , the higher the temperature, the bigger the speed of diffusion, another factor is the diffusion coefficient , namely the amount of substance diffusing across a unit area through a unit
concentration gradient during a unit of time. All molecules don t have the same diffusion coefficient gradient, for CO2 is 20X bigger than O2. The bigger is the area of diffusion, bigger the diffusion, the bigger the molecular weight, the lower is the diffusion. Diffusion can be performed in both direction, from inside to outside or vice‐versa. Cell membrane is the structure by which molecules get entrance or leave, and it’s liposoluble. For a lot of substance it’s difficult to cross, so there is a diffusion for
liposoluble substances : they cross the cell membrane trough intermolecular spaces of the
bilayer, in this way, oxygen, carbodioxyde, nitrogen, alcohol cross the cell membrane because they are liposobluble. There are no structures from which they cross the cell membrane.
For hydosoluble molecules there are structures at the cell membrane by which the molecules
enters or leave the cell, there have been described channels for water and ions, they use some tunnels, channels, they are integral protein which cross the whole bilayer. The channels for water are named aquaporins, they are some tetramers, they have four subunits, and in the middle is the channel from where water passes. 2003 noble price for chemistry for finding this channel, even if they were first described by a Romanian teacher of cluj. They are found at the level of all cell membranes.
Ions pass also trough the cell membrane, through some channels , their diffusion depends on the
electric charge of the channel, if an ion is positive the inner surface of the channels must be
negatively charged to attract the ion. The diffusion of ion depends on several factors, electric
charge of the channel, size of the ion , the hydration energy of the ion , the electrical
gradient across the cell membrane. Usually ions bind some water molecules, the hydration energy
depends on the number of molecules of water they bind. Properties of the ion transport through the channels : Selective permeability (= specificity of the channel for the ion), and the fact that the channels are opened or closed by gates. It has been described several channels, for instance the channel for sodium, it has a diameter of 0,3/0,5 nm, the inner surface of the channel is negatively charged and through this channel only sodium can pass. Potassium channels they have a size 0,3/0,3nm, but it s not negatively charged, nevertheless potassium enters this channel because it binds less molecules of water and hydrated potassium is smaller than hydrated sodium. Chloride channel is a dimer, but it has been also described some channels that are pentamers (= it has been described at the level of acetyl‐choline receptors, a neuromediator released at the level of synapses). Channels are of several types, namely voltage gated channels, ligand gated channels, and mechanical gated channels. Voltage gated channels , are channels which are opened by changing the membrane potential beyond a certain threshold value. This type of channels are found in many cell membranes, and they are involved in generation and conduction of action potential along the nerves axons. For instance sodium and potassium channels, are voltage gated channels. Cell membrane has a certain potential, a difference of charge between the inside and the outside, they are positively charged outside and negatively charged inside. The channels are open only when the potential reaches a certain value, for example pass from ‐ 90 to ‐70, depends on the value of the cell membrane to be opened. Ligand gated channels : channel which are opened only by binding of a specific substance which is named ligand or agonist; and by binding of this substance or the agonist, the channel suffers
Active transport:
Transport of substance against their electrical or chemical gradient, happens with energy expenditure. The molecules are transported in the compartment where their concentration is higher. There is primary and secondary active transport. Primary active transport: The energy in case of primary active transport is supplied directly by hydrolyzation of ATP to ADP. Because in this process ATP is hydrolyzed, the structure by which the transport occurs is also called ATP‐ase, for example Na/K ATP‐ase of Na/K pump. It has been described several types of pumps for sodium, potassium, calcium, chloride, hydrogen ions. The best known pump is sodium/ potassium ATP‐ase. Which is found in all cell membranes which has the role to pump 3 ions of sodium outside of the cell and introduce 2 ions of potassium inside of the cell. It s an integral protein existing at the cell membrane. This pump is made of 2 subunits, alpha(α ) 100 000 Da 1,65 10^‐24and Beta(β)55 000Da. Αlpha is the subunit from which the transport of ion is performed, at the level of the alpha subunit, intra‐cellulary there are several sites, 3 for binding sodium, one site for binding ATP, and one more named site of phosphorylation (by hydrolation of ATP the phosphate released will phosphorylate and aspartate of the alpha subunit). On the outer side of the pump, there are two sites for binding two potassium, and one for ouabaïne (oubain) an inhibitor of the pump. Three ions of sodium are bound in the inner surface with ATP, alpha subunit has also a catalytic activity, it hydrolyse ATP, the phosphate is realease, this phosphate will phosphorylate an aspartate of the unit, and by this the pump will suffer and conformational change, the sodium is transported outside. After that the pump is dephosphorylated, this makes another conformational change, the two potassium can be then introduced inside. But because more positive charges are transported outside, this pump is also named electrogenic pump because it increase the potential difference between inside and outside of the cell? For calcium an ATP‐ase is described, they are found at the level of cell membrane, but also at the level of certain organells mitochondria and sarcoplasmic reticulum. Another pump, hydrogen/ potassium pump, this pump s found at the level of gastric glands, it has the role to pump hydrogens ions in gastric juice. Hydogen ATP‐ase : found In the membranes of some organells, golgi opparators, lysosomes, and mitochondria. By this pump hydrogen ions are pump inside of the organells, in order to maintain the cytoplasma at a certain pH. Secondary active transport: The solutes are transported against their concentration gradient, and the energy is supplied by the concentration gradient of sodium ion generated by sodium/potassium ATPase, it pumps sodium outside of the cell. Together with sodium another substance is transported, the second substance can be transported in the same direction with sodium, the mechanism is then named co transport, or symport. In this way glucose and amino acids are absorbed at the level of the digestive tract, they are also reabsorbed in kidney at the level of tubular cells. Glucose and amino acids, and phosphates are transported with sodium in intestinal mucosa and renal tubule. Sodium and chloride are transported together in distal tubule of kidneys. Countertransport, or ANTIPORT, in this case the energy supply is also produced by sodium, but the second substance is transported in the opposite side of sodium, so outside of the cell. This countertransport, or antiport mechanism is
described for sodium and hydrogen, it takes place at the level of proximal tubule of nephron. Another countertransport is described for sodium and calcium, it is found in almost every cell, but 3 sodium ions are introduced while 1 ion of calcium is transported in opposite direction.
