Transport of Substances through Cell Membranes
Where it comes to the transport of substances through cell membranes, the mechanisms involved may be either active or inactive. Simple absorption, osmosis, and assisted transport are examples of passive processes. Active mechanisms, on the other hand, include the active transport of particles and molecules.
Osmosis
Osmosis is a mechanism in which water travels from a more dilute solution to a more condensed solution through a semi-permeable membrane. It is important to understand that osmosis involves the flow of water over a possible gradient. Similarly, the partially permeable membrane encourages certain molecules to get through while stopping others from doing so. Therefore, the availability of some substances in the solutes makes the cell membrane allow passage of some substances. Osmosis is important in the human body as it assists in the osmoregulation process that maintains the concentration of the cell cytoplasm. Osmosis differs from simple diffusion, it requires a partially permeable membrane unlike diffusion is a membrane-free process. For active transport, the process requires energy produced by ATP molecules, but osmosis only requires a concentration difference.
Diffusion
Diffusion entails the movement of particles such as molecules, atoms, or ions from a highly concentrated area to a lowly-concentrated region. The process occurs to create uniformity in the concentration of substances. Typical examples of diffusion include the exchange of gases in the alveoli, during respiration, and when photosynthesis is taking place. A high diffusion rate requires a large surface area, a big concertation difference, and a relatively shorter distance (Fisher, Williams & Lineback, 2011). Diffusion being a passive mode of a cell process means that it does not require any energy which makes it different from active transport. On the other hand, diffusion differs from facilitated transport as it rarely depends on temperature while the latter is largely dependent on temperature.
Facilitated Transport
Facilitated transport refers to the process through which molecules move along a concentration gradient through a membrane with the assistance of a carrier protein. Important to note is that the carriers have a specific shape and therefore allowing particular molecules to move across the membrane. On the other hand, selection of molecules can be through the criteria of size and the charge of molecules. The movement of molecules relies on the charge of molecules which keeps the concentration gradient of molecules high and thus ensuring there is a steady movement (Fisher, Williams & Lineback, 2011). The presence of channel proteins provides a conducive environment for the movement of molecules across the membranes. Facilitated transport is different from active transport as active entails a movement that is against the concentration gradient. Contrasting osmosis to facilitated transport reveals that osmosis only allows the passing of water molecules across a semi-permeable membrane while facilitated transport allows particles of a given size and shape.
Active Transport
Regarding active transport, the process involves the movement of molecules from a lowly-concentrated region to a highly-concentrated region. Therefore, the molecules move against the concentration gradient with the help of energy obtained from an ATP molecule. Important to note is that the transmembrane protein molecules have hydrophilic heads that make it possible for them to move through the cell membrane (Fisher, Williams & Lineback, 2011). The interaction of transport protein molecules with the passenger molecules enables them to move against the concentration gradient and to be specific amino acids and ions play a significant role in this process. The primary difference between active transport, diffusion, and osmosis is that only active transport relies on energy for the cell process to take place.
Reference
Fisher, K., Williams, K., & Lineback, J. (2011). Osmosis and Diffusion Conceptual Assessment. Cell Biology Education, 10(4), 418-429. http:\/\/dx.doi.org\/10.1187\/cbe.11-04-0038
