Membrane mediated transportation: Diffusion

This post is a sequel to my last post on the cell membrane where I discuss the general structure of a typical cell membrane. In the post, the basic components of the membrane and how they fitted into the fluid mosaic model were highlighted.

The phospholipid bilayer provides the basic component on which other smaller components were built. It restricts the entry and exit of polar molecules as well as ionic ones. Assisting the phospholipid bilayer in its duty are channel proteins and carrier proteins which are both involved in the selective transport of polar molecules and ions across the membrane. In addition, the proteins sometimes act as enzymes that serve to catalyse certain metabolic processes within the cell. They also act as receptor molecules for signalling between cells due to their specific shapes.

^{By LadyofHats Mariana Ruiz Villarreal - Own work. Image renamed from Image:Simple_difussion_in_cell_membrane.svg, Public Domain, Link}

In photosynthesis and respiration, the membrane proteins take part in energy transfer systems that exist in the membrane of chloroplasts as well as that of the mitochondria. Some membrane proteins are embedded with branching carbohydrate side chains with several possible shapes to enable each cell to have its identity. Such glycoproteins are generally referred to as antigens. They enable cells to recognize other cells and to behave in an organized way during tissue development.

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Membrane transportation

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Like I said in the previous post, the cell membranes act as a barrier to a lot of materials while being selectively permeable to some. Polar or water-soluble molecules such as glucose and amino acids are repelled by the non-polar hydrophobic lipids of the membrane. Consequently, the aqueous portion of the cell (the cytosol) is barred from escaping from the inner portion of the cell.

Despite all the stubbornness of the membrane, the transport of materials across it is still necessary for normal functioning of the cell. The cell needs to obtain nutrients, excrete waste materials, transport useful substance, generate ionic gradients which is quite essential for nervous and muscular activities as well as maintain suitable pH and ionic concentration within the cell for enzymatic activities. All these processes would not be possible without the transportation services provided by the cell membrane.

There are various mechanisms through which essential materials are transported across the membranes of various cells in the biological system. These include diffusion, osmosis, active transport and bulk transport. Diffusion and osmosis generally require no energy expenditure by the cell, hence they are referred to as passive transport. On the other hand, active transports and bulk transports both require the expenditure of certain magnitude of energy in form of Adenosine Triphosphate in order to take place within the system, hence they are generally referred to as active transport.

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Diffusion

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When smoke is released from the exhaust of a car, what usually happens? The smoke starts spreading and thinning out gradually until it can hardly be seen by ordinary eyes unless the exhaust keeps producing the smoke. Technically, the smoke moved from the source where it has the higher concentration to other regions that do not have smoke at all or lower concentration of it.

^{By © 2005 by Tomasz Sienicki- CC BY-SA 3.0, Link}

Another analogy of diffusion is like a bottle of perfume opened in an enclosed room. The molecules of the perfume would spread by diffusion until an equilibrium is reached when the perfume would have been evenly distributed throughout the enclosed room.

Diffusion is simply the movement of molecules or ions from a region of their high concentration to a region of their low concentration down a diffusion gradient.

Like I said earlier, diffusion processes within the cell do not require energy expenditure, they happen spontaneously and as such are generally termed passive. Diffusion across the cell membrane occurs by random motion of molecules which is due to kinetic energy of the molecule. Each type of molecule moves down its own diffusion gradient independently of other molecules. For example, oxygen diffuses from the cells of the lung into the blood while at the same time, carbon dioxide diffuses in out in completely opposite direction.

Diffusion across the cell membrane is affected by varieties of factors. The steepness of the diffusion gradient is one of the most important factors as far as the rate of diffusion is concerned. Diffusion of a substance between two points A and B is largely dependent on the difference in concentration between the two points. The larger the difference, the faster the rate of diffusion. It is advantageous for the cell to maintain steep diffusion gradients if rapid transportation of molecules is required and this can be achieved through different mechanisms depending on the type of cell. For example, breathing faster or speeding up the flow of blood through the lung will create a steep gradient for oxygen to diffuse into the cells of the lung and for carbon dioxide to diffuse out.

In addition to the diffusion gradient, the surface area of the cell membrane also play an important role in determining the rate at which molecules would diffuse in and out of the cell. The larger the surface area, the faster the rate of diffusion; all other factors being constant. For roughly spherical cells, the larger a cell is, the smaller its surface area in relation to its volume. This restricts the cell size. As such, a very large aerobic cell would not be able to obtain oxygen fast to satisfy its need if it relies solely on diffusion.

Also, the rate at which molecules diffuse through cell membranes is inversely proportional to distance; that is, the higher the distance, the longer it takes for diffusing molecule to reach its destination. Diffusion is therefore only effective over very short distances, a factor that also limits cell size. Cells rely on diffusion for internal transport of molecules, hence most are not larger than 50 microns in diameter with no part of the cell being more than 25 microns from the cell surface. For example, an amino acid molecule can travel a few micrometers in several seconds but would take several days to diffuse a few centimeters.

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Diffusion vs Facilitated Diffusion

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A lot of molecules (gases and liquids) do move across the cell membrane through diffusion. The respiratory gases such as oxygen and carbon dioxide diffuse rapidly through the membrane. In addition, water molecules, despite being polar are small enough to pass between the hydrophobic phospholipid molecules of the membrane without much interference. However, ions and larger polar molecules such as amino acids, sugars, fatty acids and glycerol are repelled by the hydrophobic region of the membrane and diffuse across it at a rather extremely slow pace. Hence, the movement of such molecule requires more than just diffusion.

^{By LadyofHats Mariana Ruiz Villarreal - Own work. Public Domain, Link}

Some ions and polar molecules can diffuse through special transport proteins known as channel proteins and carrier proteins. The proteins contain water-filled hydrophilic channels or pores whose shapes are specific for a particular ion or molecule. Several of these proteins can also combine to form channels through which diffusion can take place in either direction, in or out of the cell. These form of diffusion is called facilitated diffusion and the transport proteins that facilitate the passage of ions are called ion channels. They have the capacities to exist in open or closed state and as such, are referred to as being gated. They are quite important in the conduction of nerve impulses.

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Summary

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The cell membrane is quite important in regulating what comes-in and goes-out of the cell for normal functioning of cells, tissues, organs and the entire body systems of living organisms. A number of ways exist for molecules to be transported in and out of the membrane such as diffusion, osmosis, bulk and active transports.

Ordinary diffusion is quite important for the movement of certain molecules such as oxygen/carbon dioxide and water through the membrane. However, some relatively big molecules and ions even would diffuse, requires the involvement of carriers or special channels in order for them to diffuse in adequate quantities required for normal cell functioning. Such diffusion process is referred to as facilitated diffusion.

Thank you for reading.

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References

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khanacademy

dummies

biology discussion

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