Osmosis
Osmosis is the movement of water across a membrane, such as the membrane around a cell, from a weaker solution to a more concentrated one. It happens because cell membranes let water through much more easily than they let through many dissolved substances (solutes). Membranes like this are called semipermeable.[1][1][2] Osmosis is a kind of passive transport, which means the cell does not have to spend energy on it.[3]
Diffusion
Osmosis is closely related to diffusion. Diffusion is a process that happens by itself, in which a substance moves from a region of high concentration to a region of low concentration. In the end, the difference in concentration disappears.[4] It happens because of the random movement of molecules. There is a net movement from high to low concentration simply because there are more molecules in the more concentrated region.[4] As one biology textbook puts it, "Diffusion expends no energy."[5] Each substance diffuses down its own concentration gradient, whatever other substances are doing.[5]
Small molecules without an electric charge, such as oxygen and carbon dioxide, can diffuse straight through the cell membrane. Larger or charged molecules cross much less easily.[6][1] Some substances cross with the help of transport proteins in the membrane. This is called facilitated diffusion, and it also needs no energy.[7][4] Diffusion only works well over short distances, which is one reason why cells are small or flattened.[8]
Hypotonic, hypertonic and isotonic solutions
Scientists compare the solution outside a cell with the cytoplasm inside it:
- A hypotonic solution has a lower solute concentration than the cell, so water enters the cell.
- A hypertonic solution has a higher solute concentration than the cell, so water leaves the cell.
- An isotonic solution has the same concentration, so there is no net movement of water.[9][1]
A cell placed in a hypotonic solution gains water by osmosis and swells, while a cell in a hypertonic solution loses water and shrinks.[1][1] Many cells can recover from small changes within minutes, by losing or gaining ions until the concentrations inside and outside match.[10]
Osmosis in animal cells
Animal cells have no cell wall. If a red blood cell is put into a hypotonic solution, it takes up water and swells, and it may burst.[11][12] In a hypertonic solution it loses water and shrinks.[12] Some single-celled organisms, such as paramecia and amoebas, also lack cell walls. They use contractile vacuoles to pump out extra water so that they do not burst.[12]
Osmosis in plant cells
Plant cells are usually more concentrated than the fluid around them, so water tends to enter them.[10] This builds up a pressure, called turgor pressure, which pushes against the cell wall.[10][12] Turgor pressure supports plants that are not woody, and the soft parts of trees, such as leaves.[10] If a plant cell is placed in a hypertonic solution, it loses water, and the membrane pulls away from the cell wall. This is called plasmolysis. It makes plants lose their support and wilt.[10][10][12] Cell walls also protect the cells of fungi and bacteria from bursting.[12]
Water channels
Water can also pass through the membrane by way of small proteins called aquaporins.[10] Each aquaporin channel lets through only water, about a billion molecules a second, in single file.[13] Aquaporins are especially common in cells where moving water is important, such as those of plant roots and the kidney.[13]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Karp, Gerald; Iwasa, Janet; Marshall, Wallace (2018). Karp's Cell Biology (Global ed.). Singapore: John Wiley & Sons. p. 338. ISBN 978-1-119-45417-5.
{{cite book}}:|access-date=requires|url=(help) - ↑ Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 47. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
- ↑ Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 205. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
- ↑ 4.0 4.1 4.2 Karp, Gerald; Iwasa, Janet; Marshall, Wallace (2018). Karp's Cell Biology (Global ed.). Singapore: John Wiley & Sons. pp. 336–337. ISBN 978-1-119-45417-5.
{{cite book}}:|access-date=requires|url=(help) - ↑ 5.0 5.1 Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 206. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
- ↑ Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 46. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
- ↑ Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 48. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
- ↑ Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 207. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
- ↑ Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 210. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
- ↑ 10.0 10.1 10.2 10.3 10.4 10.5 10.6 Karp, Gerald; Iwasa, Janet; Marshall, Wallace (2018). Karp's Cell Biology (Global ed.). Singapore: John Wiley & Sons. p. 339. ISBN 978-1-119-45417-5.
{{cite book}}:|access-date=requires|url=(help) - ↑ Karp, Gerald; Iwasa, Janet; Marshall, Wallace (2018). Karp's Cell Biology (Global ed.). Singapore: John Wiley & Sons. pp. 334–335. ISBN 978-1-119-45417-5.
{{cite book}}:|access-date=requires|url=(help) - ↑ 12.0 12.1 12.2 12.3 12.4 12.5 Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 211. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
- ↑ 13.0 13.1 Karp, Gerald; Iwasa, Janet; Marshall, Wallace (2018). Karp's Cell Biology (Global ed.). Singapore: John Wiley & Sons. p. 340. ISBN 978-1-119-45417-5.
{{cite book}}:|access-date=requires|url=(help)