Water is essential for the survival of all living beings. But, the input and output of water entering the body’s cells must be regulated. This regulation of water is known as Osmosis. Osmosis consists of a solvent and a solution, for example pure water and salt water, being separated by a semi- permeable membrane. This semi – permeable membrane allows water molecules to flow through freely from any direction, whereas the salt molecules will not be able to pass through the membrane as they are too large. Water will always move from the side with the highest water concentration to the side with the lowest water concentration, this is to equalise the concentration of constituents on both sides of the membrane. (Borg, 2017). Osmosis is known as a special type of diffusion. Osmosis is needed within animals as it keeps the water input and output within the body regulated, but osmosis itself also must be regulated. This is known as osmoregulation. Osmoregulation is responsible for monitoring the physiological processes to help maintain a concentrated balance of the body fluids inside and outside the cell. (Osmoregulation – an overview | ScienceDirect Topics, n.d.)

Water can enter the body in a variety of different ways for example; drinking, eating and living in an aquatic environment. As water can enter the body, it can also be lost from the body through; urine, faeces, vomit, lactation etc. Species have evolved over time to keep their cells alive through osmosis and osmoregulation. For example, marine fish are hypotonic to their environment. This means that their body fluids contain more water molecules and less solute molecules in comparison to their environment, which is sea water. (BioBook | Leaf: Why is osmosis so important in biology?, n.d.) If marine fish had not evolved and adapted to their environment, their cells would burst due to swelling after taking in too much water. Baldisserotto et al.,( 2007) states that as marine fish are hypotonic to their environment and lose a lot of water, they counteract this by engulfing sea water. 60-85% of this seawater is then absorbed by the intestine. The oesophagus of marine fish absorbs a lot of the salt, but it is almost impermeable to water. This makes the salt level of the water in the intestine much lower, resulting in more water and less salt being absorbed by the intestine and excess salt being excreted in any small amounts of urine that is produced. This is an adaptation that has been developed by marine fish to respond to water lose due to their environment.

On the other side of this example, there are also species that are hypertonic to their environment, such as fresh water fish. This means that the cells have a higher salt concentrations than the surrounding environment. (BioBook | Leaf: Why is osmosis so important in biology?, n.d.) Fresh water fish are opposite to marine fish so they take in water as their cells are constantly losing water. If the fish did not take in this extra water their cells would shrink up and die. (Garrey, 1916).

Terrestrial animals have numerous ways of conserving water though osmosis, such as waterproof skin, feathers fur etc. One of the more advanced adaptations for conserving water is the kidneys. The Loop of Henle resides within each nephron within the kidneys. It consists of a descending limb and an ascending limb. The descending limb is impermeable to solvents, but water can move around freely, this is where the water is absorbed back into the cells. Due to the absorption of water in the descending limb previously, it results in the filtrate in the ascending limb to become concentrated. Within the ascending limb, some solvents are reabsorbed but it is moderately impermeable to water. Any extra waste is then transported to the bladder and excreted as concentrated urine. The more dehydrated the animal is the more concentred the urine becomes. (Regulation of Water Balance, n.d.)

In summary, the evidence shown within this report reflects osmosis as a necessity for life within a whole organism. Species and cells have developed over time so that their bodies can remain hydrated in the most efficient way possible. Osmosis can be displayed differently throughout a variety of species, all with their own similar yet vastly different adaptations.



Baldisserotto, B., Mancera Romero, J. M. and Kapoor, B. G. (eds) (2007) Fish osmoregulation. Enfield, N.H: Science Publishers.

BioBook | Leaf: Why is osmosis so important in biology? (n.d.). [Online] [Accessed on 3rd December 2017] https://adapaproject.org/bbk_temp/tiki-index.php?page=Leaf%3A+Why+is+osmosis+so+important+in+biology%3F.

Borg, F. (2017) ‘What is Osmosis? Explanation and Understanding of a Physical Phenomenon.’

Garrey, W. E. (1916) ‘The Resistance of Fresh Water Fish to Changes of Osmotic and Chemical Conditions.’ American Journal of Physiology — Legacy Content, 39(3) pp. 313–329.

Osmoregulation – an overview | ScienceDirect Topics (n.d.). [Online] [Accessed on 2nd December 2017] https://www.sciencedirect.com/topics/neuroscience/osmoregulation.

Regulation of Water Balance (n.d.). [Online] [Accessed on 5th December 2017] https://courses.washington.edu/conj/bess/water/water.htm.

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