We start with the cell, because so much of what happens to us when we get sick, and how we get healthy again, can be explained by what happens on a cellular level. The cell is extremely complex and I will only touch on the basics in these posts, but at least we can have a rudimentary understanding.
We have discussed cell membranes (May 24), as well as cell organelles (May 31). Before we look at the nucleus of the cell, I would like to do a short post on some of the mechanisms for molecular movement across the cell membrane. Transport across the cell membrane is important to understand, because a lot of the newer research seems to focus on this aspect.Living cells constantly interact with the external environment, like tissue or blood. In order to do that, materials must move through the plasma membrane, taking in some substances and secreting or excreting others. There are several methods by which movements can occur: diffusion, osmosis, facilitated diffusion, active transport, filtration, endocytosis and exocytosis. We will look at each of these briefly.
Diffusion: Movement of molecules from an area of greater concentration to an area of lesser concentration. An example would be the diffusion of oxygen from the lungs into capillaries.
Osmosis: The diffusion of water. The kidney tubules use osmosis to reabsorb water.
Facilitated diffusion: Diffusion assisted by a carrier protein. The carrier protein moves molecules across cell membranes. The intake of glucose by most cells happens by facilitated diffusion.
Active transport: Movement of molecules from an area of low concentration to an area of high concentration assisted by a carrier protein. Active transport requires ATP (Adenosine triphosphate- a specialized nucleotide that traps and releases biologically useful energy). An example of a process that uses active transport would be the absorption of amino acids and glucose from food by the cells of the small intestine.
Filtration: Movement of water and dissolved substances from an area of higher pressure to an area of lower pressure. This would be used in the first step in the formation of urine.
Endocytosis: The membrane engulfs something and draws it into the cell in membrane bound vesicles. An example of this would be white blood cells engulfing bacteria.
Exotysosis: Membrane bound vesicle fuses with the cell membrane, releasing it's contents outside the cell. An example of this would be the release of neurotransmitters by nerve cells or the cells of kidney tubules reabsorbing.
How does this affect you?
In 1985 Michael Brown and Joseph Goldstein won the Nobel prize in medicine for receptor-mediated endocytosis. They were investigating the internalization of cholesterol by cells in the bloodstream. For more information on their findings, read the discussion of endocytosis on the biology reference website.
