MDCAT Biology · Biology · Cell structure and function. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The fluid mosaic model (Singer and Nicolson, 1972) describes the plasma membrane as a double layer of phospholipids with proteins scattered in it. The water-loving heads face outwards and the water-hating tails face inwards. It is fluid because the molecules move sideways, and a mosaic because of the pattern of proteins.
The membrane is selectively permeable. Small non-polar molecules such as oxygen and carbon dioxide pass straight through the lipid. Ions and larger polar molecules need channel or carrier proteins.
Movement down a concentration gradient is passive and needs no ATP. Movement against the gradient is active and needs ATP and a carrier protein.
Saying water moves from a 'high concentration' to a 'low concentration' without saying of what. Water moves towards the more concentrated solution, which has the lower (more negative) water potential.
Answer: Water moves by osmosis from cell X to cell Y.
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