What types of transport are seen across cell membranes?
Answer
There are two ways in which materials move across a cell membrane: passive and active diffusion. Passive diffusion requires no energy. It is observed in simple diffusion, carrier-mediated or facilitated diffusion, and osmosis. Filtration is also a type of passive transport. Active diffusion requires energy to diffuse. Passive diffusion involves materials moving down their concentration gradient from an area of high concentration to one of a lower concentration. For example, if you put salt in water you will observe that the salt will move from an area of high salt concentration to one of a lower salt concentration. The salt would spread out until it is evenly distributed. Carrier or channel-mediated facilitated diffusion is an act where lipophobic molecules (i.e. glucose, amino acids, and ions) bind to protein carriers and move through water-filled channels. Osmosis is a type of passive diffusion where water moves from a low solute concentration to a high solute concentration. This is in a situation where the membrane does not allow the solute to pass through. Filtration refers to the process by which blood is filtered to remove waste products and excess substances while retaining essential molecules and ions. The glomerulus and Bowman's capsule are the main parts involved in filtration within the kidneys. The process of filtration in the kidneys is driven by the pressure gradient between the blood in the glomerulus and the fluid in the Bowman's capsule. The pressure gradient forces water, ions, and small molecules to pass through the filtration membrane, while preventing larger molecules such as proteins and blood cells from passing through.
Active diffusion works so that material moves against its concentration gradient from an area of low concentration to one of high concentration. Active transport is important because it helps maintain cellular and bodily homeostasis. Defects in active transport lead to a variety of diseases, including cystic fibrosis and cholera.
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