The biology of equilibrium – 1

Animal adaptations for homeostasis

Vijayalakshmi Nandakumar

You are looking at underwater footage of beautiful thriving sea life and wondering what it would be like to live in a glorious sea house. Or perhaps you went scuba diving and wished you could swim about free and unencumbered like all marine life, without the gear needed to stay underwater.

Staying underwater for extended periods without protective gear can be a challenge for many reasons, one of them being the salinity of sea water. It is common knowledge that excess salt can cause dehydration in tissues, and this is what makes it difficult to persist underwater for extended periods. Do you ever wonder how marine animals have adapted to living in water that is 99 per cent saline while still maintaining the fluid balance in their bodies to survive?

The big picture
At the very core of the process of adaptation is a central goal shared by all biological systems – homeostasis. The concept of homeostasis – that living things maintain a constant internal environment – was first suggested in the 19^th century by French physiologist Claude Bernard, who stated that “all the vital mechanisms, varied as they are, have only one object: that of preserving constant the conditions of life”.[1]

In other words, homeostasis is any self-regulating process by which biological systems tend to maintain stability while adjusting to conditions that are optimal for survival. If homeostasis is successful, life continues; if unsuccessful, disaster or death ensues. The stability attained is actually a dynamic equilibrium, in which continuous change occurs yet relatively uniform conditions prevail.[2]

Several key factors determine the stability of the internal environment. For example, body temperature, glucose levels, blood pressure, protection from infection, breathing patterns, waste management, light entry and not least of all, fluid volumes and ion concentration.[3]

In this article, we discuss animal adaptation as it relates to the regulation of fluid volume in biological systems.

Diving into the details
A living cell is a delicately balanced biochemical factory where metabolic activities are carried out. Inter-cellular fluid is the medium in which the cells are bathed. Through this medium the cell is able to access the respiratory gases and nutrients and at the same time excrete their metabolic wastes which are then collected by the blood. This represents the internal environment of the cells.

Maintaining stability in this internal environment in conjunction with changes to the outside environment requires the regulation of ion content and fluid volume inside the cell. The goal is to maintain a stable tonicity (concentration of solutes/ions per fluid volume).

Osmoregulation is the process of regulating the tonicity in cells. Osmoregulation uses the principle of osmosis to regulate the fluid volume. There is a movement of water through semi permeable membrane from a region of dilute solution to a region of concentrated solution. So when the solute concentration becomes high in one cell, water molecules travel from another cell of lower solute concentration to this cell to maintain tonicity.

Osmotic pressure refers to the pressure exerted by the contents of the cell on the cell membrane to stop the entry of water from outside. Constant regulation is necessary to ensure a constant osmotic pressure inside the cell. Homeostasis is achieved when the internal Osmotic pressure (OPi) is equal to the external Osmotic pressure (OPe).

Starfish, spider crab and sea anemones
Sea water salinity does not have significant variation. Marine animals like the star fish, spider crab and sea anemones are confined to the saline sea water from birth. Their internal body fluid is therefore isotonic* with sea water. This helps them achieve homeostasis, and there is then no special need for adaptation.

Fresh water fish
Fresh water fish are mostly gill breathers. The membranes lining their mouth and gills are susceptible to the external osmotic variations. They manage the fluctuations effectively by:

• Increasing their ultra filtration rate at the kidney tubules (formation of urine).
• By reabsorbing lot of salts from the renal fluid back into the blood by making the urine hypotonic.
• Active uptake of salt by the special cells situated in the gill. These take up the salts from the external medium and move them against concentration gradient into the blood stream.

Fresh water fishes therefore regulate tonicity by taking in salts against concentration gradient and producing very dilute urine.

Marine bony fish
Marine bony fish, in contrast, have body fluids that are hypotonic to their surrounding (OPi

Swallowing sea water and reducing the rate of ultrafiltration in their kidneys.

Actively excreting salts by means of chloride secretoy cells in the gills, just exactly the opposite way the fresh water fishes do.

Eliminating nitrogenous waste in the form of a compound called trimethylamine which requires very little water to excrete, whereas the fresh water species excrete ammonia which requires a lot of water to excrete.

A special case is the migratory Salmon.[4] Salmon spend most of their life in the open ocean, where they reach sexual maturity, but lay their eggs on the upper reaches of (fresh water) streams. When the eggs hatch, the young salmon spend several months migrating downstream to the ocean where they remain for some 3-5 years. When mature, the adult salmon return to the mouth of stream where they hatched (they remember the taste/smell of the water in the stream), migrate upstream to its headwaters, spawn, and die.

There are some serious physiological challenges presented by habitats as different as freshwater streams and the open ocean. The salmon has incredible adaptations to keep the concentration and composition of their body fluids within homeostatic limits while migrating from fresh to salt water and back again.

These adaptations include:

• Differing water intake: Drinking copious amounts of water when in the ocean, but very little when in fresh water.
• Differing kidney function: They produce large volumes of dilute urine when in fresh water, but extremely little in the ocean, therefore using relatively little water to get rid of all of the excess ions.
• Special enzyme: In their gill epithelial cells, salmon have a special enzyme that hydrolyzes ATP and uses the released energy to actively transport both Na⁺ and Cl^– (ion) against their concentration gradients. In the ocean, these Na⁺-Cl^– ATPase molecules ‘pump’ Na⁺ and Cl^– out of the salmon’s blood into the salt water flowing over the gills, thereby causing NaCl to be lost to the water and offsetting the continuous influx of NaCl. In fresh water, these same Na⁺-Cl^– ATPase molecules ‘pump’ Na⁺ and Cl^– out of the water flowing over the gills and into the salmon’s blood, thereby offsetting the continuous diffusion-driven loss of NaCl that the salmon is subject to in fresh water habitats with their vanishingly low NaCl concentrations.

Marine cartilaginous fish
Marine cartilaginous fish have a hypertonic body fluid compared to the surrounding sea water. They maintain it this way to prevent water loss by storing the nitrogenous waste urea in their cells so that the osmotic pressure is raised. Though there may be water influx due to this it is quickly expelled through urine. Why is this a great feat? Under normal conditions, storing urea in the body alters the shape of the proteins by breaking the peptide bonds. This will normally destroy the cells’ metabolic processes but these fish’s proteins are immune to these effects.

Shore crab
The shore crab, Carcinus, lives in the brackish waters of the estuary. It is able to eliminate water and salts with the help of special glands at the base of its antennae called antennal glands.

It’s not just the more complex biological systems, but single celled protists too have contractile vacuoles to excrete to maintain water balance.

So if you don’t live underwater, do you still need osmoregulation?
The answer is yes. Much like the marine fish, terrestrial animals also have to cope with water loss in the form of sweat. The various adaptations that enable them to osmoregulate are:

• Water proof integument: Keratinous body scales of reptiles, cornified epithelium of mammals provide both protection and prevent loss of water.
• Regulating urine production: This involves reducing the formation of urine as well as production of non toxic waste urea less toxic and less water for removal. Reptiles and birds save more water by excreting nitrogenous water in the form of uric acid which is insoluble. So these animals excrete this and faeces together in the semi solid form through a common opening called cloaca.
• Reabsorption of water: Insects have special tubules called malpighian tubules which open into the gut and their free ends in the body fluid. So much of water is reabsorbed by the tubules. This remarkable ability to conserve water has enabled the insects to survive in all habitats.
• Behavioural means: Many animals change their lifestyle to suit the environment. For example, the kangaroo rat cuts down evaporation from the lungs by as much as 25 per cent by remaining in its burrow during the heat of the day. The earthworm burrows deeper when the surface of the soil is dry. The African and South African lungfishes can survive the complete drying up of the swamps in which they live by burrowing into the bottom and encasing themselves in a cocoon of hard mud lined with mucus.



• Metabolic water conservation: When one gram of fat is metabolized, twice as much water is produced as against the one gram of carbohydrate which yields 0.56 g of water. The kangaroo rat manages to lose less water than it gains by metabolism. This it achieves by reducing evaporation from the lungs, not sweating and producing highly concentrated urine and very dry faeces. It saves the metabolic water so efficiently that it never needs to drink water.
• Tissues tolerant to water loss: The camel is very efficient in conserving water. The assumption that the camel stores water in its hump is not correct. It stores fat there and the fat is converted into water during metabolism. As discovered by Knut Schmidt-Nielsen#, a camel can go without water for a long period of time by surviving on the metabolic water obtained by burning 30 per cent of its body mass. The camel’s tissues are extraordinarily tolerant to dehydration. In an observation made by Nielsen, a camel drank 40 liters of water in 10 minutes after walking without water for 16 days.

Summary
In this part, we discussed some of the many fascinating adaptations that aid homeostasis, as it relates to tonicity stabilization. We explore so many more in the next one!

Activities to demonstrate osmosis in plant cells

Experiment 1

1. This activity involves observation of raisins in water and after they have swollen up put them in hypertonic solution. The raisins swell up by absorbing water as the concentration of sugar inside the raisin is higher when compared to water, so water from outside enters the raisin.
2. But when the same raisin is again put in a hypertonic solution it shrinks back.
3. This can be a good opportunity to ask the students to think about the application of this process in food preservations.
4. The teacher can draw the attention of the children as to why salt is added in greater amounts directly to the cut vegetable pieces during pickle making.
5. Some fruit slices are preserved in high concentrate of sugar syrup.
6. The bacteria or fungal cells that spoil food cannot withstand the hypertonic surrounding, which will initiate osmosis, and their cells will be plasmolysed.

Experiment 2

1. Take one half of a potato and scoop the middle portion to make a small well deep enough to accommodate some sugar or salt crystals.
2. Keep a quarter teaspoon of sugar in the well of the potato and keep a pin to mark the initial position of the sugar.
3. Place the potato in a shallow container containing water, taking care to see that the water level is not on top of the cut end of the potato.
4. Leave the setup for 2 to 3 minutes.

Observation: Water would have entered the well of the potato showing that osmosis has taken place. When water outside is tasted, it would be seen that no sugar has gone out.

#Knut Schmidt-Nielsen (September 24, 1915 – January 25, 2007) was a prominent figure in the field of comparative physiology and Professor of Physiology Emeritus at Duke University.

*Isotonic – iso implies equal and tonicity refers to the solute concentration. Two fluids are Isotonic when they have the same solute concentration.

References
1) https://www.britannica.com/science/biology#ref498653
2) https://www.britannica.com/science/homeostasis
3) https://www.bioexplorer.net/importance-of-homeostatis-examples.html/
4) http://www.unm.edu/~toolson/salmon_osmoregulation.html

Bibliography
1. M. Roberts, Biology – A Functional Approach, Nelson Publication

The author currently teaches at Vidyaranya High School for Boys and Girls, Hyderabad. She has the experience of teaching different curricula for the past 36 years, teaching biology and chemistry to the children of high school and middle school. She loves to instill curiosity and a passion for learning in children. She can be reached at viji.nandakumar@gmail.com.