Active Transport and its Significance in Biological Systems
Active transport in biological systems is a physical process in which molecules and ions move from a region of lower concentration to a region of higher concentration. This means that they move against a concentration gradient unlike in diffusion which is an equally physical process, where molecules or ions move from a region of higher concentration to a region of lower concentration.
A vivid example of the significance of active transport can be seen in seaweeds which take up iodine in such a vigorous manner that it is more than two million times as concentrated inside the cells as in the surrounding water environment. Therefore any movement against a concentration gradient is called active transport.
How Active Transport Works in Biological Systems
Active transport also occur in biological systems that is actively producing energy by respiration. Physical conditions such as oxygen concentration and temperature influence the rate of active transport and there are further evidences from biochemical studies to show that active transport is linked with energy production. Anything that inhibits the formation or hydrolysis of ATP(Adenosine triphosphate), or inhibits it from being used also stops active transport from taking place or proceeding.
Cyanide is a good example of a compound that prevents active transport because it prevents ATP from being synthesized. The involvement of active transport in energy production can also be explained by the fact that certain cells that are known to undertake active transport on a very large scale have an unusually large amount of mitochondria, which is the energy storehouse of the cell.
The explanation of active transport can be found in several hypothesis but it is generally assumed by biologists that there are certain carriers in the plasma membrane which can attach themselves to the molecules or ions at the outer surface of the membrane and then convey them to the inner surface where they are deposited in the cytoplasm. The carrier then returns to the outer surface and repeats this process which definitely involves the resupply of energy from ATP.
There is another school of thought that suggest that the carrier molecule is a protein which transverses the entire membrane. The explanation goes further to explain that the substance to be transported becomes attached to the carrier on one side of the membrane such that the configuration of the carrier molecule then changes in such a way that the substance is moved through it to the other side of the membrane. This hypothesis is consistent with the view by biologists on how proteins are arranged in the plasma membrane.
Active transport gives further explanation that cells are selectively permeable, taking in or expelling certain ions while excluding some others. Movement of ions across the cell membrane is at different rates because it can be seen that most cells absorb potassium ion more readily compared to other cations. Interestingly, research on the uptake of ions by the roots of plants have shown that certain ions which show identical charges and similar properties compete with each other for uptake by the root and the one which is taken up is the ion that is most common or abundant in the plant surroundings. What this means is the ions are transported by one and the same carrier and that this carrier cannot distinguish between the ions. It must be noted that once a substance has been actively transported into the cell, it cannot leak out again down the concentration gradient and so the carrier acts as a one way valve.
In further explaining the significance of active transport, we must also understand that Active Transport controls what goes in and out of the cell. Many animal and plant cells take in potassium ions but expel sodium ions. These two ions are being moved in opposite directions within one and the same cell membrane.
In investigating the passage of some radioactive ions across the cell membrane of certain algal cells, it was discovered that the concentration of both potassium and sodium ions on the two sides of the membrane are interdependent, suggesting that the same carrier is used for transporting both ions.
As we have stated at the beginning, Active transport is a process that involves the movement of substances into and out of cells using energy. Cells use a large amount of energy in the form of ATP to maintain the right concentrations of ions and molecules.
Ions are moved down their electrochemical gradients by pumps such as the sodium-potassium pump and molecules through cotransporters or exchangers. When one molecule moves down its concentration gradient while another goes up, this is called a symporter or symport.
What Exactly is Active Transport in a Strict Molecular Sense?
Active transport is the process by which cells move molecules and ions against their concentration gradient. It requires specialised carrier proteins and energy in the form of adenosine triphosphate (ATP). ATP is produced through cellular respiration. The hydrolysis of ATP is needed to change the conformational shape of carrier proteins, making them highly selective for specific molecules or ions. There are four main types of active transport mechanisms: diffusion, facilitated diffusion, exocytosis and endocytosis.
In “standard” active transport, the movement of a molecule or ion down its concentration gradient is coupled to the movement of another molecule or ion up its concentration gradient. This is known as cotransport or, more specifically, symport. The transport proteins involved are called symporters or antiporters. If the two molecules move in the same direction they are transported by a symporter, and if they move in opposite directions, then they are transported by an antiporter.
The most common type of active transport is the sodium-potassium pump, which uses a molecule of ATP to pump sodium ions out of the cell and potassium ions into it, maintaining an electrochemical gradient essential for cellular functions. Other sources of energy for active transport include redox energy (as in the mitochondrial electron transport chain) and photon energy.
For example, the Na+-K+ ATPase in the cytoplasm of a liver cell will bind sodium ions to its channel until a molecule of ATP binds to it and causes the channel to change shape, “spitting” the sodium ions out onto the other side of the membrane.
Active transport can also involve bulk transport, by which a cell moves large molecules into or out of it. This can be accomplished by endocytosis, where the cell wraps vesicles around the substance to be transported, and exocytosis, where vesicles are’spit out’ of the membrane.
Finally, there is also passive transport, which involves the simple movement of substances down their concentration gradients. This can be done by osmosis, diffusion or filtration. Dysregulation of passive transport can lead to conditions such as cystic fibrosis, caused by a malfunctioning chloride channel, and diabetes, which is associated with defects in glucose transport into the cells.
Primary Active Transport in Biological Systems
In active transport, substances move across a membrane from a region of low concentration to a higher concentration. The movement is against a concentration gradient, and therefore requires energy. The energy can come from a chemical energy source like ATP or a physical energy source such as the mechanical movement of the molecules. In biological systems, the movement is usually facilitated by proteins called transporters. The transporters are complex machines that utilize a large amount of cellular energy to drive the process forward.
The most well-known example of active transport in biological systems is the movement of sodium and potassium ions across the intestinal epithelium. This is driven by a protein in the cell called Na+/K+-ATPase. The protein uses a complex mechanism to convert ATP into electrical energy that is then used to drive ions down the concentration gradient.
Another example of active transport in biological systems is the uptake of glucose in a plant cell. This is also driven by the same type of ATP-dependent ion pump. This type of active transport is also known as facilitated diffusion.
Other examples of active transport in biological systems include the synthesis of amino acids in the cytoplasm and the exocytosis of neurotransmitters from nerve cells. The synthesis of amino acids in the cytoplasm is powered by a similar ion pump to that of sodium and potassium ions in the intestinal epithelium. The exocytosis of neurotransmitters is a more complicated process that involves the fusion of the cell membrane with vesicles containing the chemicals to be released.
A third type of active transport is coupled transport. This is when the movement of one type of molecule is facilitated by the downhill movement of another molecule or ion. In the most common form of coupled transport, this is referred to as symport and occurs with protons and other ions. If the two compounds move in opposite directions across a membrane, this is referred to as antiport. Examples of coupled transport in biology include the symport of calcium and magnesium, the Na+/Ca2+ exchanger in kidney tubules and the SGLT1 sodium-glucose exchanger in intestinal epithelium.
Secondary Active Transport in Biological Systems
For living cells to survive, they must be able to bring in the materials necessary for cell function. This includes essential ions and larger molecules. These substances exist inside the cell in concentrations that are lower than those outside of the cell, so they need to be moved up their electrochemical gradients. This requires energy, which is provided by the mechanism known as active transport.
Active transport in biological systems involves moving molecules across the membrane from areas of low concentration to areas of high concentration. This happens by facilitated diffusion (mediated by channels and carriers), and it consumes cellular energy, usually in the form of ATP. This is different from passive transport, which uses kinetic energy or other energy sources.
During active transport, the driving ion or molecule is first transported up its concentration gradient by a primary pump. For example, the sodium-potassium pump in human cells moves three Na+ ions out for every two K+ ions that enter the cell, creating a charge imbalance across the membrane. This creates the electrical gradient that is needed for secondary active transport.
A second ion or molecule is then coupled to the driving ion through a carrier protein. This allows the driven molecule to move down its concentration gradient, using the energy that the driving ion used to drive up its concentration gradient. The resulting process is called cotransport, or secondary active transport. The coupling of the driving ion and the driven molecule is achieved by using specialized membrane proteins known as transporters. These proteins are either symporters, which allow substances to travel together in the same direction, or antiporters, which cause the substances to travel in opposite directions.
Secondary active transport is essential for cellular respiration, photosynthesis, and many other important processes. It involves the movement of substances, such as sugars, most amino acids, and many inorganic ions, from regions of low concentration to regions of high concentration. This requires a significant amount of energy, which is supplied by the energy-dependent process of ATP hydrolysis or by an external source such as the proton motive force generated by the mitochondrial electron transport chain.
Endocytosis
Cells must be able to transport substances both in and out of the cell. This is why the cell has bulk transport mechanisms such as endocytosis and exocytosis. Endocytosis shifts materials that cannot diffuse across the cell membrane into a vesicle derived from the cell’s plasma membrane, while exocytosis shifts materials out of a vesicle derived to the extracellular space. These two functions allow for an equal flow of nutrients in and waste out of the cell to maintain homeostasis.
Endocytosis can occur via four different pathways: caveolae, macropinocytosis, receptor-mediated endocytosis and phagocytosis. Receptor-mediated endocytosis binds specific molecules to receptors on the plasma membrane. When the molecule is absorbed by the membrane, the receptors signal for invagination and clathrins will form a vesicle around the molecule. The vesicle will then fuse with early endosomes, which sort the molecule and prepare it to enter the cell.
Caveolae are non-clathrin coated buds on the surface of the plasma membrane and are formed by the protein caveolin. These structures serve as “collection pits” and gather specific molecules for a variety of metabolic pathways. Macrophage pinocytosis is similar to phagocytosis in that it involves the uptake of large particles from outside the cell that are then engulfed by the plasma membrane. Unlike phagocytosis, pinocytosis does not require receptors to be triggered, but instead depends on the accumulation of a certain concentration of particles at the cell surface.
Both clathrin-dependent and -independent endocytosis are mechanosensitive processes, meaning that they can be stimulated or inhibited by mechanical stimuli such as membrane tension and rigidity. These mechanosensitive endocytic pathways can also be used by the cell to respond rapidly to changing environmental conditions and mechanohomeostasis.
Cells can also release molecules into the extracellular space via regulated exocytosis, which requires that the cell be polarized to activate the process. Regulated exocytosis is a key process in cell communication and signaling, as well as in cell size regulation. In contrast to regulated exocytosis, constitutive exocytosis does not require the presence of external signals and relies on the formation and fusion of internal vesicles with the plasma membrane.
