Biology · Cell biology. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Substances get into and out of cells in three ways. Diffusion is the spreading out of particles, giving a net movement from a higher concentration to a lower concentration. It needs no energy from the cell. Oxygen, carbon dioxide and urea move this way.
Osmosis is the diffusion of water only. Water moves from a dilute solution to a concentrated solution through a partially permeable membrane.
Active transport moves substances from a more dilute solution to a more concentrated solution, against the concentration gradient. This needs energy from respiration. Root hair cells take in mineral ions this way, and the gut absorbs sugar this way when its concentration in the gut is low.
In percentage change, students divide by the final mass. Always divide by the starting mass, and keep the sign: + for a gain, − for a loss.
Answer: +15.0%.
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