Wednesday, September 14, 2016

I Am Your Density -- Life On Ice

Biology concepts – density of water, latent heat, stratification

Ernest Rutherford showed that atoms were
mostly space by shooting alpha particles at
a sheet of gold foil. Only a few particles struck
something solid, most just passed straight
through – because the atom is mostly the
absence of matter.
It is amazing to know that atoms are mostly empty space. Atoms make up everything around us, including the stuff that hurts when it hits me in the head, but even those things are mostly empty space... or maybe its my head that's empty.

When atoms fit together to form molecules and molecules fit together to form solids and liquids there is also space. How massive the molecules are and how much space is between them determines a substance’s density.

Density (mass per unit volume) has a big impact on biology, and we have been talking about water for a few weeks, so let’s talk about the density of water. Simply put, without water’s unique density properties, life as we know it on Earth would not be possible.

Pure liquid water has a density of 1 g/cm3 (or 1 g/ml). This is 800x times the density of air, so moving around in water is much harder and requires more energy than moving around on a land. Try running in the pool – we just aren’t built for moving in water.

Gram for gram, fish have more muscle than
any other vertebrate animal. Notice how the
muscle fibers are arranged in different
directions to provide forward movement as
the skeleton changes orientation.
But fish have adapted streamlined shapes and big muscles in order to move through water a little easier. The skeleton of a fish is the most complex of all vertebrates. The skull anchors the waving of the vertebral column and the attached muscles. The muscle fibers (myomeres) are arranged so that the muscles can contract in several different directions as the swaying motion passes down the fish body. In all, a fish is about 80% muscle. If you are a marine fish, you’d better be even stronger, since ocean water is slightly more dense (between 1.02 and 1.03 g/cm3, depending on the salinity).

But here is the amazing part - when water freezes, its density goes down. Most substances are denser as solids than as liquids, but water is the exception. As ice crystals form, the water molecules arrange themselves in a very particular order, and this order places slightly more space between them as compared to when they are in liquid form. More space means less mass per unit volume, ie. lower density (0.92 g/ml)….. and this is a key to life on Earth.

Water will form ice crystals in a definite structure,
with more space between the molecules than when
in liquid form. Snow crystals form from water vapor,
not liquid water, and retain a more hexagonal lattice
shape that may stack on one another.
Imagine for a moment that ice was denser than water. Then as the winter came, the winds would blow, the surface water in the pond behind your house would start to cool down, but the deeper water would be a little warmer (remember that water has a high specific heat, it likes to retain its heat. As the surface water arranged itself into a crystal form, ie. turned to ice, it would sink. The warmer water would then be pushed up higher and exposed to the colder temperatures, freeze, and fall to the bottom. Eventually the pond would fill with ice, and be completely frozen.

Few animals or plants could survive in a solid block of ice, so life would cease to exist in the pond. What is more, when spring came, the sun’s energy and warmer temperatures would have to penetrate to bottom of the pond in order to melt all the ice, and this would take longer than a spring summer and fall to occur. Most bodies of water would stay somewhat frozen all year long.

Our food webs (who eats who) depend so much on the growth in water, and half of the Earth’s oxygen’s production oxygen depends so much on phytoplankton, the one celled plants that float on the water’s surface and release oxygen as a by product of photosynthesis. So we couldn't survive for long with completely frozen bodies of water. What is more, frozen lakes and bays would eliminate huge heat sinks that normally keep the surface of the earth warm, so we would plunge into another ice age.

Can you imagine if the massive number of aquatic organisms died as a result of their environment being frozen year round? The animals that feed on them would then die, and the animals that feed on them would die, etc. Eventually the animals on the land that feed on the amphibians and fish would die, and so on.  What’s more, we humans would be looking for more warm clothing while we gasped for enough oxygen to survive! Relax, we are all just fine, and it is because ice floats. Surface water freezes, trapping heat below and keeping the aquatic organisms comfy and cozy until spring.

The North American wood frog can freeze
solid in a long Arctic winter, but once it thaws,
it has work to do. It must find a find a mate and
then fertilize the eggs. The fertilized eggs have
to develop from to tadpoles and then to adults
during the short warm period. Then they can
freeze next winter.
You might have noticed that above I mentioned that MOST organisms can’t survive being frozen, but there is an exception. The wood frog (Rana sylvatica) winters in shallow burrows that are not protected from the cold. To survive, the frog actually freezes solid!

Nucleating proteins in the frog’s blood act as point for ice to form as soon as the frost touches the amphibian’s porous skin. Since the frog is still above 0˚C at this point, the freezing is slower, and the frog can control it. As the liquids freeze, the water is pulled out of the frog’s cells.

It replaces the water with huge amounts of glucose and sugar alcohols, that keep the cells from forming ice crystals (they are sharp and would puncture the cells causing permanent damage and death). Eventually, the frog is 65% frozen and the internal organs are surrounded by a pool of ice until spring, when it takes about 10 hours for the frog to thaw and hop away. Scientists are now using this process to freeze and thaw rat hearts and livers without damage, in hopes to use to the process in human organs for transplant.

But freezing and thawing a whole organism is harder than using a glucose bath to freeze individual organs. Research from early 2013 shows the energy that R. sylvatica must spend to accomplish this feat. In response to cooling near the freezing point, the wood frog increases its metabolism to prepare for freezing. But this increase in metabolism is nothing compared to the increase the frog undergoes when freezing is first detected in its tissues. Carbon dioxide (a sign of metabolism) is increased by 5.8 fold during freezing, as to the period just before freezing. This increase is needed to mobilize glucose into the tissues as the cryoprotectant.

The same thing happens when R. sylvatica thaws, metabolism increases to exactly the same degree as during freezing. But in this instance, the increased cellular activity is necessary for re-establishment of homeostasis and for tissue repair (no anti-freezing strategy is perfect). We have a long way to go to mimic the wood frog's entire preservation strategy, especially since the frog may go through these increases as many as twenty times each winter!

The wood frog takes advantage of freezing in order to survive. Humans can also take advantage of freezing water (other than keeping your drink cold); in fact, your orange juice may depend on it. Freezing of oranges or grapes ruins them for the same reason it kills animals, it causes frostbite. Ice crystals stab through the cell membrane and cell contents spill out. This isn’t conducive to continued function.

To prevent oranges and grapes from freezing, farmers will spray them with water when their frost warning systems sound the alarm. Does that make sense, spraying with water to keep something from freezing? It has to do with a property of freezing called latent heat. This is an amount of energy taken up or given off when a substance changes phase (solid to liquid to gas). The energy goes to changing the arrangement of molecules with no change in temperature.

Oranges can be protected from freezing by
spraying them with water which then freezes!
In a controversial use of genetic modification,
bacteria that do not permit ice crystal formation
can be sprayed on the oranges to compete with
the normal bacteria there. These "ice-minus"
Pseudomonas syringae can reduce frost damage
on oranges, but have not been used commercially.
As water surrounding the orange or grape changes from liquid to solid, the formation of crystals gives off heat (539.4 gram-calories per gram of water frozen).  The latent heat of the freezing mist is enough to keep the fruit above 0˚C. This technique doesn’t work if the temperature falls much below 0˚C or stays at 0˚C for an extended time, but it does work well enough to save millions of dollars per year in freezing damage.

Thermal changes have more to do with differences in water density than salt concentration does, so seasonal changes can alter density in both freshwater and salt water. Even if the changes are not enough to form ice or boil the water, differences in temperature can result in different layers of water within a freshwater body or an ocean.

Both salt water and freshwater are affected by the sunlight that strikes their surfaces. As water warms, it’s density decreases, and the nutrients in the water stay close to the surface. This supplies phytoplankton and algae with lots of food, and blooms can occur.

As winter approaches, the surface water cools and becomes more dense (down to 4˚C). The dense water drops to the bottom and taking nutrients down to the benthic organisms. When all the water reaches 4˚C, the surface can begin to freeze.

In the spring, the process is reversed, and the temperature layers (stratification) can churn again. In salt water, the differences in salinity are added to the differences in density to bring complex stratifications, both in salt content and temperature.

Stratification shows how temperature can set up
layers of water of different density (least dense is
the epilimnion). In the winter, the water is churned,
and then churned again in spring. These churnings
based on changing density move the nutrients around
so everyone gets fed.
Different organisms thrive in different temperature and salinity layers. In order to stay put, some floating organisms (planktonic) and swimming organisms (nektonic) can adjust their buoyancies. Fish can use swim bladders, which are air filled cavities to help them stay buoyant. The size of the bladder is regulated by the CO2 and O2 in the blood that can remain dissolved or leave the blood as a gas.

Bladderwort plants also use air filled cavities to keep part of themselves afloat. Sharks, on the other hand, produce large amounts of oil in their livers to reduce their density; oil is less dense than water, just look at your salad dressing layers.

Plankton can also slightly adjust their densities, but floating is easier for very small things. To them, water is thick, the polar charges have a larger effect on their small bodies. It would be like us trying to swim in molasses. They still have to adapt to seasonal changes in density, but they do it in more subtle (and harder to explain) ways.

Just because there is water around, it doesn’t mean that life will be easy. Next week we will look at a continent-sized exception to idea of water availability.

Sinclair, B., Stinziano, J., Williams, C., MacMillan, H., Marshall, K., & Storey, K. (2012). Real-time measurement of metabolic rate during freezing and thawing of the wood frog, Rana sylvatica: implications for overwinter energy use Journal of Experimental Biology, 216 (2), 292-302 DOI: 10.1242/jeb.076331

For more information, classroom activities and laboratories on the density of water, latent heat, North American wood frog, or stratification, see:

Density of water –

latent heat –

North American wood frog –

stratification –

Wednesday, September 7, 2016

Do You Drink Like A Fish?

Biology concepts – fish osmoregulation, shark osmoregulation, semelparity, iteroparity

The irony of fish drinking is not lost on this café in
the Hotel Portofino at Universal Orlando. What I
really like is the eye patch.
You’d think that fish would never be thirsty; if he needs a drink, he just opens his mouth. But some fish don’t drink a drop! Wouldn’t that be similar to some birds never breathing? Ridiculous.

Fish are good examples of the problems of maintaining proper water and salt concentrations. Some fish live in freshwater, and some in saltwater. These are opposite sides of the same coin when dealing with osmoregulation.

Freshwater fish live in a hypotonic (low salt) environment. The flesh of the fish contains more salt than does the water. Diffusion and osmosis work to equalize salt concentrations in different compartments. Therefore, water will move from the lake or river into the fish’s tissues in order to balance the salt concentrations by osmosis. Salt will not move out of the tissues, since there are molecular mechanisms that work to keep the inside.

Like the kangaroo rat, freshwater fish don’t drink. They do take in water when they eat and move water across their gills, but they don’t take in water just for the water. Even without drinking specifically, freshwater fish take in way more water than they need. Anywhere freshwater contacts a fish cell, water will move inward; this includes the gills, the mouth and gut, and the skin.

In a situation like this, kidney-mediated concentration of urine would be counterproductive; why retain water when water is exactly what you have too much of? Therefore, freshwater fish excrete large amounts of urine. Their kidneys have large glomeruli, which move lots of water into the collecting tubules for excretion.

Saltwater and freshwater fish have different ways of
dealing with salt and water loss and conservation.
Freshwater fish must conserve salt, while saltwater
fish must conserve water. The kidneys play a role,
but so do the chloride cells in the gills.
But if the freshwater fish aren’t drinking, how do they get their salt, which is present in low concentrations in the water? You’d think they would have to be drinking all the time just to collect enough salt.  To get around this, they conserve the salt they ingest through the food they eat. They also take in salts through their gill chloride cells, actively pumping sodium and chloride out of the freshwater and into cells that have a lot of mitochondria (to provide energy to pump the salts). The relatively short collecting tubules of the freshwater fish kidney allow for reuptake of a lot of salt, while excluding almost all the water.

Marine (saltwater) fish have the opposite problem. Their tissues are of much lower salt than they surrounding hypertonic ocean, so osmosis wants to dry them out, sending water out of their bodies. The amount of available drinking water is extremely low - can you imagine dying of dehydration while surrounded by water. Just ask anyone who has survived a shipwreck and prolonged float in the ocean; drinking seawater can be lethal.

However, marine fish must drink all the time in order to keep enough water in their body. Retaining water would be an essential function of marine fish kidneys. They are all fish, but their kidneys work in exactly opposite ways.  Marine kidneys have small or absent glomeruli, so little water is taken out of the blood, but long collecting tubules in order to excrete as much salt as possible.

Drinking a lot of saltwater leaves marine fish with way too much salt; more than their kidneys can get rid of. To aid in salt excretion, they also have chloride cells in their gills. In the opposite fashion of the specialized gill cells of freshwater fish, the chloride cells of saltwater fish actively sequester salts from the blood, and then pump the sodium and chloride out into the seawater.

Sharks have unique ways of maintaining
salt and water. I have no idea of their
mechanisms for pepper regulation.
But sharks are an exception among marine fish. They have a different way to combat high salt concentrations. Remember that osmosis means that water moves from areas of low solute (high water concentration) to areas of high solute (lower water concentration). For many marine fish, this would mean a constant loss of body water to the ocean and quick death by dehydration; much like pouring salt on a slug.

To overcome this movement, sharks produce and retain a huge amount of a chemical called urea; it is one of the soluble wastes that animals normally get rid of. This molecule doesn’t affect the electrical potential that salts create, but increases the solute concentration in the shark’s tissues at levels higher than in the seawater, so water (without the salt) will diffuse into the shark’s body. This is its source of fresh water.

Therefore, sharks are osmoconformers; they maintain an osmotic balance with their environment. If the shark becomes too salty and salt needs to be excreted, it has a salt gland, much like that of birds and reptiles, but the shark’s gland is located in it anus, not near its eyes or nose – that’s a big difference! Taken together, there is no force for movement of water in or out of the shark’s tissues, and the shark remains shark-shaped instead of shriveling or swelling up.

Here is a bullshark caught in the Potomac River.
And you thought that sharks in Washington D.C.
were just in the federal buildings.
An exception to this rule for sharks is the bull shark; it can live in both saltwater and freshwater. Most sharks put into in freshwater would absorb too much water and die of water toxicity. However, the bull shark’s kidneys can adjust to the salinity of the water within a short period of time. Their kidneys will remove less salt and more urea from their blood and tissues and into their urine. They move from being osmoconformers to osmoregulators.

A shark that can live in freshwater; this can present a real problem. There have been many bull shark attacks in rivers and estuaries (video), where people don’t expect to encounter sharks. It is suggested that this behavior and physiology is an adaptation that gives the bull shark a protected nursery for their young, away from predators.

Most fish are stenohaline (Greek, steno = narrow and haline = salt), which means they are restricted to either salt or fresh water and cannot survive in water with a different salt concentration than to that which they are adapted. However, there are exceptions- like the bull shark mentioned just a second ago.

Some salmon species are born in freshwater, then move to saltwater for several years, and then return to freshwater to spawn. Other fish, like some eels, are born in a marine environment, move to freshwater, and then go back out to sea to reproduce. If freshwater and saltwater fish kidneys work opposite of one another, how can there be fish that can do both?

Salmon returning upstream to spawn have many obstacles
to overcome. Their spawning grounds are usually a thousand
feet or more above sea level so they must leap up many
waterfalls. Oh, there are hungry bears too.
Salmon are famous for migrating to and from the sea. Almost all the species are semelparous (in Latin, semel = once and parous = breeding); this means that they return to their freshwater streams to spawn only once, and the trip and the reproduction kills them. The one exception is the Atlantic Salmon (Salmo salar). This species is spawned in, and returns to, the calm streams along the Atlantic coast several times in its life to spawn. This reproductive strategy is call iteroparity (itero = repeated). Iteroparous species lay fewer eggs at a time, the advantage is that survival chance is increased by repeated spawning – one bad year doesn’t destroy a big proportion of the population.

The migratory species of salmon are osmoregulators, as are most freshwater fish; their physiology demands a certain salinity level, and use energy to produce that level in their tissues. However, they can also adapt to various salinity levels. As such, these salmon as well as bull sharks are known as euryhaline (eu = good, haline = salt). Their physiology changes with the salt concentration.

While in freshwater the salmon will not drink, and will produce copious amounts of urine to get rid of the excess water it absorbs through osmosis.  But when it migrates to the ocean, it drinks all the time, and its kidneys work hard to remove the excess salts.

Chloride cells in euryhaline fish can sequester or
excrete salt, based on the hormone signals they receive.
This helps some fish move from aquatic to marine
environments and back again.
But the gills are the key to survival in the both the freshwater and saltwater environments. Energy consuming reactions will transport both Na+ and Cl- against their gradients, so they pump Na+ and Cl- into the fish’s tissues in freshwater and out of the fish’s tissues in saltwater. It is an adaptation of the marine fish’s chloride cells to work in both directions. This switch, as well as the kidney’s change in urine concentration, takes time. Therefore, salmon will spend days or weeks in intermediate zones, or estuaries, before going out to the ocean, and before returning to the rivers.

These are difficult lifestyle choices for salmon, the trips and the spawning kills them. So what is the advantage? The movement to oceans provides the growing salmon with readily available sources of food, so competition is reduced. The return to where they were spawned is just a good bet; if the stream was good enough to spawn them, then it is still probably a good place to lay eggs. Finally, working so hard to get to the spawning ground just a single time allows for selection of strong individuals, allows for huge numbers of eggs to be laid (the chance that some survive goes up), and the death and decomposition of the adults provides nutrients for the hatched fry (baby salmon). But these are human interpretations, I bet there are other advantages and disadvantages. However,  one thing is for sure, the balance sheet for these species comes out in favor of these adaptations – if it did not, nature would adapt further.

The eggs that don’t hatch and the carcasses of the mated
Adults create nutrient rich waters for the fry to develop in
before they head out to sea.
How about one more exception for today? Some individuals in semelparous species of salmon (Chinook, Coho, Pink, Steelhead, etc.) will not die after spawning, and will return again to the ocean. These individuals are often females, and are often smaller than average. These gals reverse their salt and water conservation strategies several times in their lives, making them prize winners for osmoregulatory exceptionality.

Next week, let’s tackle how the properties of hard water affect all life on Earth.

Sakamoto T, Ogawa S, Nishiyama Y, Akada C, Takahashi H, Watanabe T, Minakata H, & Sakamoto H (2015). Osmotic/ionic status of body fluids in the euryhaline cephalopod suggest possible parallel evolution of osmoregulation. Scientific reports, 5 PMID: 26403952

Cozzi RR, Robertson GN, Spieker M, Claus LN, Zaparilla GM, Garrow KL, & Marshall WS (2015). Paracellular pathway remodeling enhances sodium secretion by teleost fish in hypersaline environments. The Journal of experimental biology, 218 (Pt 8), 1259-69 PMID: 25750413

For more information and classroom activities on osmoregulation in fish and sharks, chloride cells, and reproduction strategies, see:

Osmoregulation in fish –

Chloride cells –

Osmoregulation in sharks –

semelparity and iteroparity –

Wednesday, August 31, 2016

Don’t Eat The Yellow Snow

Biology concepts – osmoregulation, tonicity, phytohormones, avian kidney, pinnieds, cetaceans

African elephants are larger than asian elephants, but their
urine production is similar. A 2007 study found that
African elephants can differentiate family members
based on their urine. It is similar to marking territory,
but they use urine to keep track of family members who
may be out of sight.
The asian elephant can urinate as much as 55 liters/day. That's about 3/4 of the volume of the average size bathtub! By comparison, the vaunted racehorse can only manage about 6 liters/day. Maybe we should rethink that old saying.

We know from the posts of the last few weeks that both salts and water are necessary for life, and that they work together to keep their concentrations within safe limits; a process called osmoregulation. You suspect correctly that kidneys and urination is involved, but what about plants – they don’t use the restroom.

For many animals, the kidney is the major organ of osmoregulation. The average adult human voids 1-2 liters of urine each day, but an uncontrolled diabetic with polyuria (poly=much and uria=urine) might expel 5-6 liters. Maybe we should bet on diabetics at the racetrack.

We get rid of water and soluble wastes via our kidneys. The kidneys filter the blood; nearly 800 liters of the red stuff each day. The basic filtering unit of the kidney is the nephron, who we met previously (Sorry, I Don’t Drink), made up of the Bowman’s capsule and sets of tubules.

Solutions of different tonicity have similar effects on plant
and animal cells, but plant cells can handle it better because
they have a rigid cell wall.
If the body is low on water, more water is reabsorbed in the tubules. Likewise, if the body has too much salt, few of the salt ions are reabsorbed in the tubules. In this way, our kidneys are basically concentrating our wastes in a small amount of water for excretion from the body. The amount of water depends on many factors, including the need to keep the cells at the right level of tonicity (concentration of salt relative to outside the cells).

Solutions can be hypertonic, meaning they have more salt than the cytoplasm, and water will flow out of cells by osmosis. Solutions can also be hypotonic, with less salt than in the cells (water will flow in to the cells) or isotonic, with the same osmotic pressure inside the cells as outside.

We all know that we don’t urinate the same amount all the time – drink more, go more. However, you don’t urinate the same amount you drink; your urine is concentrated by your kidneys in order to conserve water. Therefore, there must be some control mechanism. The answer is hormones. A hormone (“to set in motion” in Greek) is a small protein that is released from one cell and then acts as a chemical signal on other cells, either through the bloodstream (endocrine hormones) or through a duct (exocrine hormones) to the bloodstream or directly to other cells.

The angiotensin system. 1. The body senses that water
is low. 2. The kidney releases renin. 3. Renin  and
angiotensin converting enzyme produce angiotensin II
from angiotensin I in the lung. 4. Angiotensin II stimulates
aldosterone in the adrenal glands. 5. Aldosterone causes
more water and salt to be reabsorbed in the Loop of Henle;
this increases the blood volume and solves the problem.
Aldosterone is produced by the adrenal glands and acts on the distal collecting tubules of the kidneys. This endocrine hormone acts to conserve sodium and water and secrete postassium, thereby reducing urine volume but increasing the loss of potassium.  Aldosterone is released in response to angiotensin levels in the plasma, which in turn are controlled by sodium and water levels in the blood.

Arginine vasopressin (AVP, also called antidiuretic hormone) is another endocrine hormone that reduces the amount of water to be lost in the urine. This hormone is produced in the pituitary gland of the brain and also works to conserve water. By reducing the amount of water lost, the blood volume (which is mostly water) is increased, so blood pressure increases. This is why people are given intravenous fluids when they have lost a lot of blood.

The exceptions to this mechanism of kidney function are the mammals that live in hypertonic (saltwater) environments, like whales and dolphins (cetaceans) and seals or walruses (pinnipeds, latin for feather- or fin-footed). It is hard to study the urination in these animals in their native environment; they urinate in the ocean. Are you going to measure their individual contribution to the ocean – I think not.

Water wants to flow out of the cells and into the sea (hypertonic as compared to the cells), trying to balance the salt concentrations in both places. Therefore, the marine mammals must conserve freshwater or they become dehydrated. Both pinnipeds and cetaceans have large kidneys with enough renal tubule length to produce very concentrated urine, thereby conserving water. However, it appears cetaceans don’t really take advantage of this. Instead, they make a lot of metabolic water (Gimme Some Dihydromonoxide) and can keep from dehydrating by using the water they produce through cellular respiration.

Here is an inside view of a seal kidney. It’s huge! The many
lobules provides much tubular area to take up freshwater
and concentrate the urine.
Pinnipeds don’t drink water saltwater to any degree at all, they get their freshwater from their diet and their metabolic water.  Scientists use to think this was also true for cetaceans, but recent studies show that they do drink a small bit of seawater – not enough meet their water needs, but also not more than their kidney’s can handle.

Don’t think that marine (saltwater) mammals have it so bad. If they were to abandon the seas for freshwater sources, they would just trade one problem for another. Freshwater mammals have too much of a good thing, they run the risk of losing too much salt by being in so much salt poor (hypotonic) water all the time. This is why the kidney is so amazing, it can adapt functionally and anatomically to get rid of too much water or too much salt, depending on where you are. That is not to say the kidney is the only anatomic mechanism needed to maintain osmolarity within a tight range. Many organisms need more than kidneys, and have developed completely different mechanisms of osmoregulation.

Bird kidneys may be small, but they represent an evolutionary
intermediate, Some parts have short loops, like most mammals,
and some have long loops, like pinnipeds and cetaceans. However,
most of the kidney has reptile-like nephrons with long loops.
Birds share some water conserving and salt regulating apparatus with mammals. Avian (bird) kidneys have about 75% of their nephrons with reptilian structure, and 25% mammalian nephrons, containing a Loop of Henle. Therefore, avian kidneys are not as good at removing water and regulating salts as mammals are. Mammal urine can be concentrated 20-50x as compared to blood (the Kangaroo rat can produce a 9000x concentration), but birds can only manifest about a 2-3 fold concentration.

Therefore, birds have another mechanism to get rid of salt and maintain an osmotic potential within its limits. The salt gland is found in birds and reptiles. In many birds it is located near the eyes or nostrils (in crocodiles, salt is excreted through their tongues – everything tastes salty to them).  The salt gland removes Na+ and K+ from the blood, allowing birds and reptiles to consume saltwater or animals that live in saltwater.

Some organisms have it easier, like amphibians. With semi-permeable skin, they just leak salt out through their entire skin surface. Other organisms aren’t so lucky, like plants.

Plants must also regulate salt concentration, but they don’t have a familiar excretory system; in fact, they don’t have a specific osmoregulatory system. Water is lost via transpiration (Sorry, I Don’t Drink), and adjustments can be made to alter the amount of evaporation that occurs. Unfortunately, transpiration of water is linked to moving nutrients such as salts up the plant from the roots to the leaves. Therefore, shutting down transpiration will also shut down movement of nutrients. 

Plants in high temperature, low humidity, high wind environments have the highest rates of transpiration and are in danger of losing too much water. Once again, hormones are the answer. Plants do have hormones (phytohormones), so they probably have to deal with teenager issues just like human parents. Abscisic acid is an important hormone which shuts off transpiration. This phytohormone closes the stomata (stoma = mouth in Greek) on the upper sides of leaves, from which water evaporates and gases are exchanged. Abscisic acid also promotes water absorption from roots and root growth.

Some plants are cryptophytes by surviving unfavorable
seasons either underground (geophytes), hide their
seeds in the marshy mud (helophytes) or underwater
(hydrophytes). Hydrophytes in general are plants that
have their roots in water or water-logged soil.
Many xerophytes (plants that live in hot, dry places) have adapted to resolve these issues. They have leaf modifications to reduce water loss; needle-shaped leaves, sunken stomata, and waxy cuticles to cover the leaves. On the other hand, in hydrophytes (plants that live completely or almost completely in water), salts and water can be absorbed in the entire plant, not just the roots.

In terms of cations (Na+, K+), plants have a problem. They use potassium as their primary intracellular cation, but dirt is usually potassium-poor. Therefore, plants have K+ transporters to actively take up this ion. Unfortunately, the transporters don’t discriminate very well between K+ and Na+, so often times too much Na+ is taken up into plants.

Red mangroves have impermeable roots that help keep
out salt, and can also secrete some salt from there leaves,
but their most visible mechanism is the yellow salt leaves.

Excess Na+ can be toxic to cells, so measures must be taken to deal with these ions. Glycophytes are plants that are salt-sensitive, and include many of the plants that we cultivate. Therefore, soil salinity is an important factor in agriculture and gardening. Much research and breeding continues to an effort to produce crops that are better at differentiating uptake of K+ and Na+. Halophytes (halo=salt, phyte=loving), on the other hand, will allow the uptake of the excess ions, and then sequester them in vacuoles to prevent cellular damage.

Some plants live in extremely high salt environments. One example, the red mangrove tree, is a facultative halophyte. Facultative is a fancy way of saying “optionally.” These trees live in estuaries, where the river meets the sea. The water is quite salty there, and the mangroves are rooted in the water, so excess salt could be a problem. To deal with the toxicity of the excess Na+, the mangrove will store the salts in selected leaves, called the “kidney leaves.” When a toxic level is reached, the leaves turn yellow and just drop off. The tree must constantly invest energy in producing new leaves, so there is a cost to this way of life, but it seems to work for them.

If plants that live in or near seawater have adaptive mechanisms to maintain proper salt concentrations, then how about fish? We'll look at the osmoregulatory tricks by these organisms next week.

Ben Hamed-Laouti I, Arbelet-Bonnin D, De Bont L, Biligui B, Gakière B, Abdelly C, Ben Hamed K, & Bouteau F (2016). Comparison of NaCl-induced programmed cell death in the obligate halophyte Cakile maritima and the glycophyte Arabidospis thaliana. Plant science : an international journal of experimental plant biology, 247, 49-59 PMID: 27095399

Peña-Villalobos I, Valdés-Ferranty F, & Sabat P (2013). Osmoregulatory and metabolic costs of salt excretion in the Rufous-collared sparrow Zonotrichia capensis. Comparative biochemistry and physiology. Part A, Molecular & integrative physiology, 164 (2), 314-8 PMID: 23103672

Takei Y (2015). From aquatic to terrestrial life: evolution of the mechanisms for water acquisition. Zoological science, 32 (1), 1-7 PMID: 25660690

For more information, classroom activities or laboratories about osmoregulation, tonicity, abscisic acid, avian kidney, pinnipeds, cetaceans, see:

Osmoregulation –

tonicity and osmotic pressure –

abscisic acid –

avian kidney –

pinnipeds –

cetaceans –

Wednesday, August 24, 2016

Keeping Your “Ion” The Ball – Salts and Life

Biology concepts – salts in biology, osmotic potential, action potential, transpiration

Dietary salt – crucial for survival;
Veruca Salt – not so much.
In Latin, verruca means wart, so Roald
Dahl was probably trying to tell us something
when he wrote her character into Charlie
and the Chocolate Factory.
We have learned that one of the crucial functions of water in living organisms is to help regulate the salt concentration in and between the cells (Gimme Some Dihydromonoxide). But why do living things require salts? We all know that we must have a source of salt (sal in Latin) in our diet or we die; the Romans gave it so much importance that part of a soldiers pay was to be used specifically for buying salt – his salary.  But what are its functions?

Water tends to flow from where salts are in low concentration (high water concentration) to where salts are high concentration (low water concentration). Just like other molecules, water diffuses to where its concentration is lower (It’s All In The Numbers-Sizes in Nature). Osmosis (osmo = push in Greek) is the special name given to the diffusion of water, for every other molecule it is just called diffusion.

Too much salt is destructive to cells and organisms, so water helps control the salt held in the body. On the other hand, too much water is also bad for living things (water toxicity), so salts help to control the water concentration. Together, this ratio of salt and water inside and outside of the cell leads to a controlled imbalance called the osmotic potential of the cell. Every living thing has systems to maintain this osmotic potential within a small range (osmoregulation, we will discuss this in more detail soon).

The osmotic potential is measured in units
of pressure (bars). It is equal to the amount
of water that will move in response to a
difference in solute concentration across
a membrane.
When in water, sodium chloride (NaCl, table salt) dissociates into Na+ and Cl- ions, and it is these ions, along with K+ (potassium ion from KCl) that perform many functions in living organisms. Sodium is 10x more concentrated outside the cell, while potassium is 20x more concentrated inside. The slight difference in the charges of the two ions (and the fact that most Cl- is outside cells) sets up a membrane potential in cells.

An important function of this membrane potential is in the neuron (nerve cell), as rapid reversal of the potential along the cell membrane (through ion specific channels) produces an electrical current that we know as the action potential (neural impulse). It is the rapid change in concentrations of Na+ and K+ cations (positively charged ions) inside and outside of the neurons that sends the messages from our muscles to our brains and back, as well as all the thought processes in our brain.

The action potential of the neuron is not simple.
Sodium is higher outside and potassium is higher inside.
When a signal is received (usually from another neuron),
sodium leaks in and potassium leaks out. The slight
difference in the the charge of each means that the neuron
goes from -70 mV to +40 mV. This depolarization travels
down the neuron’s membrane for the entire cell.
Salt's importance is illustrated when their concentrations get out of whack. Too little salt produces symptoms similar to dehydration, with cramping, nausea and confusion. Too much salt results in hallucinations and insanity. The classic example of too much salt intake is being lost at sea. Not having a supply of freshwater, people may start to drink seawater. The salt concentration is too high; their kidneys can’t get rid of all the excess, and the action potentials in the brain begin to misfire. People will see things that aren’t there, and will make critically bad decisions. Many end up swimming away from relative safety and subsequently drown.

We can get rid of some salt through our skin. Is your dog is happy to see you when licking your face after you arrive home, or does he just want the salt? Athletes will often eat bananas to augment their potassium stores and keep the cramps away after exercising. They should really follow that run with a bowl of lima beans; they have much more potassium.

However, munching on black licorice is alot like running a long distance. Glycyrrhizin is the main glycoside (a sugar bound to a non-carbohydrate) in licorice root and is 20x sweeter than sucrose. Glycyrrhizin prevents potassium reuptake in the kidney, so you end up urinating out most of your potassium stores. You could cramp up due to excessive snacking.

The source of glycyrrhizin’s effect on potassium reuptake has to do with cortisol, a stress hormone. Cortisol is converted to cortisone, but glycyrrhizin inhibits this conversion. The increased cortisol makes it appear like your body has too many salts in the blood, and you adjust. This isn’t just a problem for the people who eat a lot of licorice.

A 2010 study indicates that pregnant women who eat licorice can permanently affect their children’s hormone control in their brains. The hypothalamic-pituitary-adrenocortical axis (HPAA), is a relay that controls the child’s production of cortisol, aldosterone and other hormones. These work to control the osmotic potential of the blood and therefore the blood pressure (as well as other things).

The researchers data shows that maternally ingested licorice inhibits the fetal barrier to maternal cortisol. More cortisol then passes to the fetal blood system, and programs the HPAA to have a higher baseline. From then on, the babies make more cortisol, a stress hormone that puts pressure on the physiology, sodium and potassium levels, and can lead to weight gain. Moms – take care – what you eat does affect your baby.

Na+ and K+ work in muscle function; cramping and paralysis may result from too little or too much salt. Your heart is a muscle, so changes in salt concentration in the cell can cause heart attacks as well. Many a mystery movie has included the injection of potassium chloride to induce a heart attack. Sodium and potassium cations help maintain proper blood pressure, proper acid/base levels, and proper movement of carbon dioxide from the blood to the lungs. There are precious few functions in which these positive ions don’t play a role.

Collagen and elastin help to make your skin and
joints pliable. O.K., maybe not this elastic – this is
the result of Ehlers-Danlos syndrome, which is
often a genetic disease.
When we think of salt, we usually think of table salt (NaCl), but there are more functions for K+ than there are for Na+, and it is present in higher concentrations in the cell. Potassium is important for the formation and crosslinking of collagen and elastin proteins. These connective tissue proteins hold all your tissues together; they keep your skin from tearing when someone pokes you in the arm, and allow your lungs to expand without ripping when you inhale. So K+ is pretty important even when not working with Na+. It is interesting then that potassium is the only major mineral nutrient for which there is not a recommended daily allowance.

Remember that we often take in these salts as NaCl or KCl. Does the Cl- play a role in organism function? – you bet it does. Chloride anion (a negatively charged ion) is used to produce the hydrochloric acid (HCl) that breaks down the food in our stomachs. Chloride also works in the immune system, hypochlorite (the same active molecule as in bleach) in the white blood cells helps to kill infectious agents and activates other immune system molecules. Chloride is required for the uptake of vitamin B12 and iron and helps control your blood pressure; therefore, Cl- isn’t just that other ion that comes in with Na+ or K+ (or Ca2+).

Chloride ion is elemental chlorine that has gained one electron. This doesn’t seem like much of a change, but it is the difference between life and death. Chlorine itself is a yellowish green gas and it can kill you in a matter of seconds. Chlorine really wants that extra electron, and it doesn’t care if it has to rip it from your lung proteins to get it. When you breathe in chlorine, it reacts with the water in your lungs to produce hydrochloric acid that eats away the cells. It will also react with almost any carbon-containing molecule and further destroy the lung tissue. It was suggested during the American Civil War that chlorine gas could be useful, but it wasn’t until World War I that it was used as a weapon.

Chlorine is poisonous, but we use it to disinfect drinking water and pools. When diluted greatly in water, chlorine does not have the strongly deleterious effect on our cells as it does as a gas, but can still react with and kill microorganisms. Chlorination of water began in the Chicago stockyards around 1908, when the decaying meat and gut bacteria were getting into the drinking water and making the residents sick. The bleach used to disinfect surfaces is much the same as the chlorine used to disinfect 75% of the drinking water in the U.S.; it’s just there in lower concentration. Now chlorine is used in pools as well, and you know it is working because your eyes get red and sting.

Did you know that plants had openings in their leaves called
stomata? Turgor pressure caused by the flow ions in and
out of the guard cells makes the stomata open or close. Their
shape changes based on the amount of water in the guard cell.
There are no exceptions to the rules of salt requirements (weird, isn’t it). All living things need to take in Na+, K+, Ca2+, and even Cl-. Plants use potassium and sodium for water balance, especially to bring morphologic changes like the blooming of flowers. These cations, along with chloride, work in the opening and closing of pores in the leaves (stomata) for the uptake of carbon dioxide and the release of oxygen and water during transpiration (Gimme Some Dihydromonoxide), and in the chemical splitting of water during photosynthesis. It seems that other organisms rely on these ions even more than animals.

All bacteria require potassium and sodium for osmotic regulation and cellular activities.
As the concentration of Na+ in a bacteria’s environment goes up, its dependence on Cl- becomes greater. Fungi, protists, and even viruses depend on salts to remain alive, even though viruses are technically not a form of life. Viruses carry nucleic acid, and salts are needed to balance the charges of the DNA or RNA so it can be stuffed into the viral package, a function within the area of molecular biology.

Giardia lamblia and other protozoa use salt ions
to control their osmotic potentials and for other
biochemical functions. Giardia can also change
your potassium levels by causing intense diarrhea
after drinking contaminated stream water.
Molecular biology involves replication of DNA, the transcription of DNA to RNA, and the activities of RNA translation to proteins. K+, Cl-, and Na+ are involved in all these areas. In a feedback mechanism, salt ions control the switches that turn on genes that then control the levels of the ions. If one ion is too high, it will turn on the genes that code for proteins which remove that ion from the cell. Isn’t evolution nifty?

Tightly regulating salt concentration in the cell is important for life, and we have to drink water (kangaroo rats excepted) in order to stay alive. These are the peanut butter and jelly of biology and we will start to see how they work together next time.

Räikkönen, K., Seckl, J., Heinonen, K., Pyhälä, R., Feldt, K., Jones, A., Pesonen, A., Phillips, D., Lahti, J., Järvenpää, A., Eriksson, J., Matthews, K., Strandberg, T., & Kajantie, E. (2010). Maternal prenatal licorice consumption alters hypothalamic–pituitary–adrenocortical axis function in children Psychoneuroendocrinology, 35 (10), 1587-1593 DOI: 10.1016/j.psyneuen.2010.04.010

For more information and classroom activities on salts in biology, osmotic potential, action potentials, or chloride ion in biology, see:

Salts in biology –

Osmotic potential –

Action potential –

Chloride in biology -

stomata –