Membrane Transport Processes
- Diffusion: the migration of molecules or ions as a result of their own random movements, from a region of higher concentration to a region of lower concentration.
- Osmosis: – is the movement of water through a semipermeable membrane movement of water (at constant temp. and pressure) is from the solution with lower concentration of solutes to the solution of higher concentration of solutes (or from the more pure water to less pure water)
- Tonicity: Tonicity refers to the relative concentration of solute on either side of a membrane.
In an isotonic solution, the concentration of solute is the same on both sides of the membrane (inside the cell and outside). A cell placed in an isotonic solution neither gains or loses water. Most cells in the body are in an isotonic solution.
A hypotonic solution is one that has less solute (more water). Cells in hypotonic solution tend to gain water.
A hypertonic solution is one that has a high solute concentration. Cells in a hypertonic solution will lose water.
4. Facilitated Transport/ Diffusion:
– the transport of molecules across a cellular membrane thru specific protein channels / carrier molecule (facilitating pathway) from a region of high conc. to a region of low conc.
– process is driven by conc. differences and does not require energy.
5. Active Transport:
– the pumping of molecules or ions across a cellular membrane through a carrier protein
– from a region of lower concentration to one of higher concentration.
– therefore against the “current” or concentration gradient.
– such a process requires energy.
In cells, some molecules must be moved against their concentration gradient to increase their concentration inside or outside the cell. This process requires the input of energy and is known as active transport. As with facilitated diffusion, special transporters in the membrane are used to move the molecules across the membrane. The plasma membrane is not the only cellular membrane that requires active transport. All organelles surrounded by membranes must concentrate some molecules against their concentration gradients.
Active Transport appears to be of two general types, (i) primary active transport and (ii) secondary active transport.
(i) Primary active transport is directly related with chemical energy (ATP) or electric energy (electron flow). Exampless of primary active transport are Na+, K+ translocating ATPase in mammals and proton translocating ATPase of bacteria.
Types of Active Transporters
There are three types of active transporters in cells: (1) Coupled transporters link the “downhill” transport of one molecule to the “uphill” transport of a different molecule; (2) ATP -driven pumps use the energy stored in adenosine triphosphate (ATP) to move molecules across membranes; (3) Light-driven pumps use the energy from photons of light to move molecules across membranes. Light driven pumps are found mainly in certain types of bacterial cells.
Most of the energy expended by a cell in active transport is used to pump ions out of the cell across the plasma membrane. Because ions have an electrical charge, they do not easily cross membranes. This phenomenon allows large ion concentration differences to be built up across a membrane. Highly selective transporters are present in membranes that pump certain ions up their concentration gradients, but ignore other ions.
The NA + -K + Pump. One of the best understood active transport systems is the sodium-potassium pump, or NA + -K + pump. This carrier protein is a coupled transporter that moves sodium ions out of the cell while simultaneously moving potassium ions into the cell. Because of the pump, the sodium ion concentration inside the cell is about ten to thirty times lower than the concentration of sodium ions in the fluid surrounding the cell. The concentration of potassium ions inside the cell is almost exactly the opposite, with a ten-to thirtyfold higher concentration of potassium ions inside the cell than outside.
Because the cell is pumping sodium from a region of lower concentration (inside) to a region of higher concentration (outside), the NA + -K + pump must use energy to carry out its pumping activity, and this energy is supplied by ATP. For this reason, the NA + -K + pump is also considered an enzyme . It belongs to a class of enzymes known as ATPases that use the energy stored in ATP to carry out another action. Other membrane transporters use the energy from ATP to pump ions like calcium, amino acids , and other electrically charged molecules either into or out of the cell.
Ions carry a positive or negative electrical charge so that these gradients have two components: a concentration gradient and a voltage or electrical gradient. For instance, sodium ions are positively charged. The higher concentration of sodium ions outside of the cell than inside means that outside of the cell will have a positive charge and the inside of the cell will have a negative charge. This potential difference, or voltage, across the membrane can be used as an energy source to move other charged molecules. Positively charged molecules will be attracted towards the inside of the cell and negatively charged molecules will be attracted to the outside of the cell. It is, in fact, this electrical potential that causes positively charged potassium ions to enter the cell through the Na-K pump, even though they are moving up their concentration gradient.
The potential energy of the gradient can be used to produce ATP or to transport other molecules across membranes. One of the most important uses of the NA + gradient is to power the transport of glucose into the cell. The NA + -glucose cotransporter moves sodium down its concentration gradient, and glucose up its gradient, as both move into the cell.
(b) Secondary active transport- It depends upon chemiosmotic energy (membrane potential or and/or ion gradients). Examples of secondary active transport are the glucose transport system of the intestinal epithelium of mammals and the lactose permease system in E.coli.
The free surface of the intestinal epithelium has numerous microvilli which are formed by projections of the brush border membrane. Primary active transport results in the Na+ being pumped in the cell. The electrochemical sodium ion gradient can be then utilized for secondary active transport of glucose in to the cell against the concentration gradient. Thus there is glucose-Na+ co-transport catalyzed by a glucose carrier. Such sodium-dependent transport has been observed for various amino acids and sugars in different vertebrates and for amino acids in bacteria.
The sodium pump maintains a higher concentration of Na+ outside the cell than on the inner side. This results in a tendency for Na+ to enter the cell. This is done in the form of a carrier-sugar or carrier-amino acid complex.