Active Transport, Endocytosis and Exocytosis, and Cell Size

Active transport moves substances against their concentration gradient, from low concentration to high, which takes energy in the form of ATP because it works opposite to the direction molecules naturally tend to spread. Specific carrier proteins act as pumps to do this work; the sodium-potassium pump, which maintains very different sodium and potassium concentrations inside and outside nerve and muscle cells, is one of the most important examples in the entire human body, and it underlies how nerve signals travel (returned to in Unit 12). Bulk transport moves much larger amounts of material by a different strategy entirely: in endocytosis, the cell membrane wraps around material and pinches it off into an internal vesicle, bringing it into the cell -- phagocytosis ("cell eating") engulfs large particles, while pinocytosis ("cell drinking") takes in surrounding fluid. In exocytosis, a vesicle inside the cell moves to the membrane, fuses with it, and releases its contents outside the cell, the mechanism cells use to secrete hormones, digestive enzymes, or neurotransmitters.

Cell size itself is limited by a simple geometric fact: as a cell grows larger, its volume increases by the cube of its radius while its surface area only increases by the square of its radius, so volume grows much faster than surface area. Since a cell absorbs nutrients and expels waste only across its surface, but must support all of the volume inside, a cell that grows too large eventually cannot exchange materials fast enough to support itself. This surface-area-to-volume limit is a major reason cells divide instead of simply growing indefinitely (Unit 5), and why large organisms are built from enormous numbers of small cells rather than a smaller number of giant ones.