Membrane Transport
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Membrane transport involves the collection of various mechanisms to regulate the passage or movement of small molecules and solutes across the cell membrane. The mechanism by which molecules are transported into or out of the cell depends on whether the molecule is being absorbed by a macromolecule such as proteoglycan, protein or peptide, or is being excreted as a waste product.
The four main types or mechanisms of passive transport are filtration, facilitated diffusion, simple diffusion, and osmosis. Transport is sensed by the influx of a solute into the cell by any one or all of these mechanisms, depending on the molecule, then this will cause a number of signals to be made, which will start or stop a transport cycle. Transport is extremely important in maintaining homeostasis within living organisms. One example would be the transport of glucose, which is required for all living cells to survive and perform their functions. Insulin, which is produced by beta cells in an inactive state and stored in vesicles called granules located within its cell membrane.
Membrane transport also involves passive mechanisms where molecules are carried in one direction across the membrane without using any energy to move them along; these include diffusion and osmosis. Additionally, there are active transport processes that use energy to move those same molecules from place to place across the membrane; these include facilitated diffusion and carrier-mediated transport.
For active transport processes the particles pass from a lower concentration to a higher one across a cell membrane. For facilitated transport, the molecules already have sufficient surface charge to move through the membrane and are then actively transported. This is a very efficient mechanism as it uses relatively little energy.
Membrane transport can also be described as a process of entry into the cell followed by a process of exit from the cell, however, this does not completely describe membrane transport. The distinction between active and passive transport is made here mainly for clarity; in practice all three types occur simultaneously in all cells and systems in which molecules must cross membranes, excluding extremely small molecules. Transport occurs both ways across most membranes (inside to outside or outside to inside).
Exocytosis and endocytosis are the main bulk transport processes that are used in eukaryotes. Exocytosis is a process by which signals from either outside or inside of the cell initiate exocytic vesicles, which transport molecules out of the cell. In contrast, endocytosis is used by cells to absorb extracellular molecules into vesicles that form within the cells and then travel through the cell’s membrane. This process closely resembles exocytosis in that it requires an extracellular signal for it to occur as well as a lipid bilayer with receptors on its surface for targeting molecules within a specific range.
The major similarities between passive transport process and active transport process is that both passive transport process and active transport process happen in cells. Passive transport process usually occurs in the lumen of cells, while active transport process usually involves moving solutes against their concentration gradient across a cell membrane. In passive transport, the energy needed for movement is provided by osmotic pressure (difference in solute concentration) or energy released by ATP molecules. Active Transport requires a high amount of energy whereas passive transports requires little to no energy. There is a concentration gradient required for active transport whereas there are no concentrations necessary for passive transportation.
The major differences between passive transport process and active transport process are that unlike passive transport, active transport is a specific type of cellular movement in which substrate or solute molecules move against their concentration gradient. This is done with the help of proteins embedded in a biological membrane. In contrast, many substances can go through a membrane by diffusion as long as their net concentration (concentration on both sides) is equal to or higher than that at equilibrium outside the cell (allowing water to pass through).
Reference
Stillwell, W. (2016). Membrane transport—Chap. 19. An introduction to biological membranes: Composition, structure and function, 423-451.
