|MadSci Network: Cell Biology|
Patrick: That is a good question. The main reason is that the amoeba has mechanisms regulate the water that would normally enter and leave via diffusion. In distilled water, cells would tend to swell since the water moves from high concentration (outside) to low concentration (inside). In cells, this process is offset by osmosis, or active transport. This is a mechanism that cells can use to push water or solutes against a concentration gradient, much like a reverse osmosis filter works in a house. Amoeba in particular possess a special organelle called a contractile vacuole which swells and contracts to release water into the cytoplasm as needed. This also helps them regulate the amount of water inside so that they don't 'pop' or shrink down too much. In the wild, this is an advantage for protists as they are likely to encounter many different types and purity of water and they need to be able to respond to changes in salinity or quality and not die. Red blood cells, on the other hand have two things going for/against them that prevent them from responding like this: #1, Red blood cells in humans are anuclear (frogs have them). This means that the cells do not transcribe a lot of genes into proteins in response to the environment, so they are not likely to adapt to conditions with new gene products, unlike an amoeba. #2, And probably the more important of the two. Throughout the body a red blood cell lives in an extremely stable environment. This includes temperature, pH, pressure, and salt and solute concentration. It is evolved to be isotonic with respect to the blood, and doesn't need mechanisms to cope with changes in H2O concentration. In fact, small changes in the balance of sodium and pH in your blood have serious and usually fatal consequences beause of the imbalance in the system. Changes in these properties affect the blood cells, nerve and muscle function and in particular they alter the dynamics by which red blood cells are able to change O2 for CO2 with the lungs, resulting in suffocation from inside. Hope this answers your question. The short answer is they don't need to because the world in which they live doesn't change much.
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