Showing posts with label adaptation. Show all posts
Showing posts with label adaptation. Show all posts

Thursday, December 21, 2017

The Life Of The Party

Biology concepts – plant adaptations, osmosis, parthenogenesis

Last week we discussed the biological implications of an old Christmas carol. Today’s post is a hodgepodge of holiday biology, but we can still find some exceptions.


From a distance, spruce, fir, and pine Christmas
trees look similar. The differences are mostly in
the needles, both shape and number.
Christmas trees – There are many different types of trees used for Christmas, but they are all evergreens. This is the reason they were used in the first place. The tradition sprung from old pagan ceremonies that reminded us that spring would come and there would be a rebirth of greenery.

Evergreens have a thick wax coating on their needles (these are actually their leaves). This adaptation, as well as the low surface area of each leaf, helps to reduce water loss during the arid winter.

The resin of evergreens is higher in sugar than in other trees species. This keeps the liquids in the tree from freezing solid during the cold months. The higher sugar content oozes from the bark and at the collars of the branches, and is very sticky (picture Chevy Chase in Christmas Vacation).

Evergreen is a characteristic not a botanical grouping. They tend to photosynthesize all winter long, given enough water and sunlight. In deciduous trees there are hormonal (phytohormonal) signals that induce cleavage of the leaves from the stems (abscission) when there is not enough sunlight to justify making chlorophyll. In evergreens, there is some of this signal present, and pines do lose leaves in the winter, just not all of them. When cut and kept indoors, the abscission signal is increased, and together with the reduced water – all the needles end up on your carpet.


The leaves of cedar Christmas trees
look different from other evergreens.
If you choose a red cedar, just remember
that there is actually no evidence that
they keep moths away.
The groups of trees used for Christmas are members of the conifers – cedar, fir, and pine, and spruce. In general, pines have two or three needles coming from the same place on the twig, while fir and spruce usually have just one. To tell fir from a spruce, try to roll a needle in your fingers; if flat and won’t roll, it is probably a fir, but if it is four sided and can be rolled, it is a spruce. Cedars look different from the other three, they have scale-like leaves and ball cones, and their bark is more splintered.

Christmas cactus – This is a small genus of plants, comprised of two groups, the truncata and the buckleyi. In the wild, they grow on other plants (epiphytic) or on rocks (epilithic). They don’t have leaves, common in cacti, their flattened green stems serve as their photosynthetic elements. They occur in naturally in eastern Brazil, along the coast of the Atlantic Ocean. Those for sale in the U.S. are cultivars, bred for hardiness and different colors, different plants will bloom in red, yellow orange, or pink.



Thanksgiving cactus stem is shown on the
top, while the bottom stem is from a
Christmas cactus.
In Brazil, the cacti are called May Flowers, reflecting the month in which they bloom in the Southern Hemisphere. In the northern latitudes, they flower from November through January, depending on the cultivar. This presented a classic opportunity for commercialization.

You might want to look at your Christmas cactus a little more closely; you might actually have a truncata when you think you have a buckleyi. The Christmas cactus has stem segments that are rounded, with more symmetric points. The flowers hang down low and their pollen is pink. These flowers generally bloom later and these buckleyi cultivars therefore termed the Christmas cactus.


The yellow pollen on the left is characteristic of a Thanksgiving
cactus. The pink pollen of the flower on the right is typical of
the Christmas cactus.
In contrast, truncata cacti have much sharper stem segments. If it hurts to prune your cactus, you may have one of these. The flowers stay closer to horizontal, or even rise up on the plant. The pollen grains are yellow, so there are several ways to tell these plants apart. Perhaps the best way is by the blooming time. The truncata will bloom closer to the end of November. For this reason, they are often called Thanksgiving cacti. Still think you have a Christmas cactus?

Fruitcake – I am an unapologetic fruitcake fanatic. To everyone who isn’t - stop making fun and just send them to me.


Fruitcake! It may be my favorite
holiday treat.
The biology of fruitcake is based on bacteria, or more correctly, the lack of bacteria. The candied fruits used in fruitcake are not just dried, they are preserved. For many centuries, fruits were precious commodities, especially in the winter. The vitamin C and other nutrients were needed for good health, but spoilage kept most people from having them during the colder weather.

Meats were preserved with salt, called curing, since the days of the ancients. Fruits, on the other hand, don’t taste so good when salt cured. It turned out sugar that could preserve fruits just as salt cured meats. Either liquid syrup or crystalline sugar would do the job, but sugar was very costly. Honey could do the job, but not as well, and it wasn’t much more available. Therefore, preserved fruits were a luxury for some period of time.

With the advent of sugar beet production in the Americas in the late 1500’s and the resulting availability of sugar in Europe, there was a candied fruit glut in Europe. It became more common to use them in baking. Italian pannatone, and fruitcakes were common uses.

So how do salt and sugar preserve foods? It all has to do with water. Bacteria need water to survive; if you remove the water, you stop (or at least slow) bacterial growth. An osmotic gradient is set up when cells are placed in high salt (hypertonic) or high sugar environment. If the salt or sugar content is higher outside the cell, it means that the water concentration is higher inside the cell.


In osmosis, water flows from where there
is little solute toward where there is
much solute. In hypertonic solution,
this means water leaves the cell.
Water will flow from areas of high concentration to areas of low concentration, just as the salts and sugars will. This is diffusion, but in the case of water it is called osmosis (Plants That Don't Sleep Well). The solvent (water) and solutes (those things dissolved in the solvent) try to balance their concentrations, so water flows out of the cell and salts or sugars flow in. The result is pandemonium, chemical reactions are not possible under these conditions, and the organism either dies or goes into stasis.

Dehydration by salt and sugar work in several ways. One, removing water through osmotic pressure will turn the bacteria, fungi, and parasites already on the food to dried up corpses by pulling out their water. Second, the lack of water in the preserved food stops bacteria and other microbes that might land on them from propagating; no water, no cell division.

Third, the high salt or sugar concentrations, even with some water present, limits the species of organisms that could grow there. Only a few microbes, called halophiles (hal = salt, and phile = lover) can grow in high salt environments. Similarly, honey is only about 30% water, so not many bacteria can grow in this low water/high sugar environment (but some important bacteria can, so don't give raw honey to infants). Finally, the loss of water in the foodstuffs reduces the oxidation reactions that might take place to age the food. Fats are especially susceptible to oxidation, they go rancid in not too long. The curing of meats slows this process, but is less a problem in fruits due to the low fat content.

Those fruitcakes deserve a little more credit, don’t they? And by the way, fruitcakes are not the doorstops everyone thinks they are, they actually float in water.

Virgin birth – I will only touch on this subject, as the blog will soon be delving into a series of stories on mating and reproduction. There are many species of animal that can give birth to viable young without mating. This is called parthenogenesis (partheno = virgin, and genesis = birth).


In 2005, a komodo dragon in a zoo laid some
eggs. No big deal, except she hadn’t been housed
with a male for 2 years! Apparently, they can
reproduce sexually, or by parthenogenesis if
no males around. This has changed how
komodos are housed in zoos.
Parthenogenesis occurs when the unfertilized egg receives the messages necessary to begin to divide and form an embryo. The offspring have only their mother’s DNA with which to work, so they are all clones and all female. The egg does have two copies of the chromosomes, but this can occur in two ways. If the egg is haploid but undergoes chromosome doubling, the resultant offspring is a half-clone of the mother. But if the egg is produced only by mitosis, with no meiotic event to result in a haploid gamete, then the offspring is a full clone.

Many species use parthenogenesis exclusively, or in response to environmental or population conditions. Whiptail lizards, as well as aphids and some plants, are famous for undergoing parthenogenesis. No cases of mammalian parthenogenesis have been documented in the wild, but stem cells have been developed by parthenogenesis in the laboratory. Anyway, if the Christmas story was going to rely on parthenogenesis, then Jesus should have been a baby girl.


Mistletoe is an evergreen that grows
on other plants. It can draw water
from the host even in winter. It also
draws animals to the tree in winter.
Mistletoe – These are evergreen, hemi-parasitic plants that grow in many parts of the world. They have photosynthetic leaves, so they produce their own carbohydrates and energy, but they rely exclusively on their host tree for water and minerals. The mistletoe roots bore into the host bark and vascular tissue to obtain the water and minerals it needs.

The mistletoe can serve to hurt the host plant, especially if it grows too well, but they can also help the host. Junipers that harbor mistletoes produce more berries than those without. This is due to the large number of birds that come to eat the mistletoe berries; the juniper takes advantage. This makes it hard to determine of the symbiosis of mistletoe/host is parasitism or perhaps mutualism.


As the berry passes through the bird,
it releases sticky cellulose fibers that
help the seed stick to an unfortunately
placed branch.
The name, mistletoe, is not something commonly brought up at a holiday party. From the Old English word, “mistiltan,” the name tells it all. Birds eat the fruit and seeds of the plant and some of them pass through the GI tract unaltered. When excreted (mistil means dung), the sticky seeds may germinate on a limb (tan means branch). Interesting, but try not to mention it over a bowl of holiday punch.

The white berries of the mistletoe played a role in the 18th century Christmas kissing tradition. In Scandinavia, the maid under the mistletoe could be kissed, but the gentleman had to pull off a berry each time. While the berries were gone, the kissing privilege was lost. 

Next time we will finish our stories on sleep and activity by talking about introduced species. Then we will start a series of posts on the incredible worlds of water and salts in biology. Our fruitcake discussion above may serve as a great introduction, but it is just the tip of the iceberg.





The concepts discussed here will be discussed in more detail in other posts. Resources will be provided on those occasions.
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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 –
www.csun.edu/~msteele/classes/Ich530/lectures/14_reproduction.pdf
http://web2.uwindsor.ca/courses/biology/weis/55-324/lecture9.htm
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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 –
http://what-when-how.com/marine-mammals/osmoregulation-marine-mammals/
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Wednesday, August 17, 2016

Sorry, I Don’t Drink

Biology concepts – water conservation, kidney function, metabolic water, adaptation, water uptake


“Koala” in aborigine means “no drink.” The
moist eucalyptus leaves are poisonous 
to most animals, but koalas have a special 
bacteria that can break down the toxic
eucalyptus oil.
We all know we need water to survive (see Gimme Some Dihydrogen Monoxide), so why is it that koala bears have decided they don’t need to drink?

Koalas eat eucalyptus leaves, as well as mistletoe and a few other leaves. The leaves contain a good amount of water, and the koalas can survive on just this source of moisture. It also helps that they sleep about 18 hours each day, have a very slow metabolism, and feed about 80% of the time they are awake - it is apparent that they have evolved into teenagers. This doesn’t mean that koalas can’t or don’t drink, they just don’t require drinking to get their daily requirement of water unless a drought dries up the leaves.

However, there exist species that never drink. The kangaroo rat and the spinifex hopping mouse take temperance to the extreme. These rodents can live out their entire life (5-7 years) and never use the water fountain. They have chosen their lifestyles wisely, considering that the hopping mouse lives in the Australian outback and the kangaroo rat lives in Death Valley! We will use the kangaroo rat as our exemplar for this exception.

Unlike the koala that gets its water from its diet, the kangaroo rat eats seeds- not a great source of water. Therefore, it must have other strategies for survival. Foremost, it has developed ways to prevent water loss. Its kidneys super-distill its urine so it is up to 17 times more concentrated than its blood; the best we can do is 3-4 times concentration.


Please meet the nephron. The blood vessels form a
glomerulus, which is surrounded by the Bowman’s capsule.
Notice how the blood vessels surround the Loop of
Henle to take the retained water and salts back into
the blood.
The kidney is made up of thousands of filtering units called nephrons (Greek nephros = kidney). Each nephron has a Bowman’s capsule that filters the blood of waste,and removes some of the water and salt. The filtrate then flows through a series of tubules that adjust the concentration of the salts and water according to what the body needs to retain or dispose of at that particular moment. The portion of the kidney that removes water from the urine back to the blood are called the Loop of Henle, and these loops are much longer in the kangaroo rat’s kidney as compared to those in human kidneys. Therefore, more water is returned to the blood and the urine wastes are more concentrated.


The kangaroo rat doesn’t look thirsty, 
even though it doesn’t look like his 
burrow has seen water for years. 
I would imagine that despite the hot 
weather and the fur coat, kangaroo 
rats don’t sweat; they can’t afford the 
water loss.





The kangaroo rat doesn't stop there. He burrows deep and keeps his burrow small. This helps to trap and moisture that escapes via his exhalations. If you breathe on a mirror, it will show condensation; you invest a lot of water in keeping your lungs moist and functional. The rat can reabsorb some of the moisture present in its burrow via its skin, respiratory tract, and his seeds. 

The dry seeds that the kangaroo rat finds are stored in a pouch in its mouth and taken back to the burrow. Here they are stored for several days in a corner, during which time they also absorb moisture from the burrow’s air. This is just another way the rat recycles some of its own moisture. 

Finally, the kangaroo rat makes the most of the water it produces. Yes, it generates water – but so do you. Think of the production of ATP (aerobic respiration) as the opposite of photosynthesis. In the building of carbohydrates (during photosynthesis). In photosynthesis, water is split and the hydrogen is added to the growing carbohydrate. But in the electron transport chain for oxidative phosphorylation (making ATP) oxygen accepts an electron and then reacts with hydrogen to form water. Water made this way is called metabolic water. In humans, metabolic processes like generation of ATP produce about 2.5 liters of water each day. In the kangaroo rat, this process is more efficient and the water produced is kept in house.


As the electrons from the breakdown of glucose travel down the
electron transport chain in the mitochondrial membrane, they
help to move protons (H+) out. As they leak back in through the
ATPase, they help make ATP. The electron needs some place to go,
and an oxygen atom is a good place to go. This makes 
the oxygen reactive; it picks up hydrogens to form water.
Add all these measures up and the kangaroo rat changes its habitat from Death Valley to Life Valley. Unfortunately,  not many other organisms can join it there.

Just because it doesn't drink or eat watery foods doesn’t necessarily mean that an organism doesn’t take in water. Amphibians absorb environmental (air or surface) water through their skin. Frogs are a group of amphibians that can be used as good examples. Frog skin is smooth, without hair or feathers, and is permeable to water. A ventral patch (sometimes called a seat patch) of skin is located on the underside of the frog between its two hind legs. This skin patch has a higher concentration of blood vessels just beneath the surface, ready to suck available water into the bloodstream.

To get to the blood vessels below the skin, the water passes through a series of aquaporin (aqua = water, pore = opening) protein channels in the skin cells. These proteins also control water entry into bacteria; they are evolutionarily very old and therefore must be important. The frog splays its legs and lays down on a surface that is moist from dew or rain, and the water flows through the ventral patch aquaporins and into the bloodstream. Interestingly, water doesn’t flow the other direction, although some water does evaporate through amphibian skin. That is why frogs must live close to water. Toad skin is much less likely to lose water, so they can live farther from water.

Some plants also garner water in unconventional ways. Non-vascular plants (mosses, lichens, liverworts, hornworts) as well as many epiphytes (bromeliads, orchids, some ferns and mosses, mistletoe) are plants without roots. However, a lack of roots or vessels doesn’t stop these plants, they have evolved marvelous adaptations to procure the water they must have.

Non vascular plants are just that – plants without vascular tissues (xylem and phloem). Plant vascular tissues are tubes inside the stem that transport water (phloem) and sugars (xylem) throughout the plant. Non-vascular plants don’t have roots and vessels to absorb and transport water and minerals, although mosses and ferns may have rhizoids that serve that purpose. In general, non-vascular plants grow close to water so that they can use all their structures to absorb water by capillary action as well as by absorbing water directly from the air.

Epiphytes are even better at pulling water from the air, although they still use pooled rainwater as well. This group of plants may have dense root systems, but some are not anchored in the ground to give support to the plant. Instead, many of them use other plants for support. Orchids are particularly good at storing water in their thick stems and absorbing water through their exposed roots. Velamen (latin for veil or cover) layer root cells of orchids are adapted to prevent water loss while a few cells in this layer and the layer below are hollow and allow water to pass through.

Bromeliad epiphytes are better at absorbing pooled water and humidity through their leaves than in taking water in through their roots. In tropical regions, they have two adaptations to aid this process. One, many bromeliads have near vertical leaves shaped to trap water at their bases (together called a tank) that may hold over a liter of water. Second, they have specialized cells at the base of the leaves to transfer this water (and minerals) to the interior of the plant. The most economically important of this Bromelioideae subfamily is the pineapple, which is a terrestrial bromeliad. It can absorb water through its roots in the ground, but if you are growing one, try to keep the tank from drying out as well.


The top picture is looking down on a bromeliad trichome. 
The middle picture is looking from the side. See how they 
curl up to allow water in. When they fill with water, 
they fold down (lowest picture), to prevent water loss 
from the cells underneath.
Bromeliads living in areas with less rain, such as Spanish moss, have a different adaptation. Their leaves store the water that is absorbed through specialized structures called trichomes on the surface of each leaf. Trichomes have shields made of non-living cells, much like our outer layers of skin. Other cells form a disc and are mostly a void, capable of rapidly taking in water. When these cells swell, their tips curl downward (remember turgor pressure from Plants That Don’t Sleep Well).

Curling forms a small cavity under the disc that draws water in to the protected foot cells under the disc by capillary action. These cells also have aquaporin proteins that draw the water into the interior tissues. When there is less water around, the disc cells flatten out and cover the stalk cells, preventing water loss. The whole structure acts like an anti-umbrella!

So organisms can get water from air, food, or metabolism - but we can go them one better. There is an animal that doesn’t eat or drink during its entire adult life, can you imagine? O.K. – so its life is only five minutes long, but it doesn’t eat or drink during that five minutes.

Adult female sand burrowing mayflies (Dolania Americana) emerge from their water-borne larval form and seek two things, a male for mating, and a place to deposit her eggs. Since all larvae are evolved to mature at once, males are around in large numbers; problem 1 solved. And since they live near water, place to lay eggs are also plentiful; problem 2 solved. Within five minutes, her work is done and she dies – not a glamorous life.


The American sand burrowing mayfly lives a year or more
as a larvae in the water, but when it metamorphoses into
the sexually mature form and leaves the water, 5 minutes
is all she gets. There may be species with shorter sexual
reproductive life span, but it would be hard to spot, and
harder to study.
Different species of mayfly live varying amounts of time – some live as adults for up to 2 days - oldtimers! But even if the mayfly wanted to invest some of their precious time in eating and drinking, they couldn’t do it. Adult mayfly mouthparts are vestigial (having become nonfunctional through evolution) and their digestive systems disappear as they mature. So in this biological case, a lack of form follows a lack of function.

There is another crucial element of life that interacts with water, and ocean going organisms are intimately familiar with it. Salt is just as important for life as is water, but why? We will begin looking into the functions of salts and how they interact with water next time.



Banta MR (2003). Merriam's kangaroo rats (Dipodomys merriami) voluntarily select temperatures that conserve energy rather than water. Physiological and biochemical zoology : PBZ, 76 (4), 522-32 PMID: 13130431


King RF, Cooke C, Carroll S, & O'Hara J (2008). Estimating changes in hydration status from changes in body mass: considerations regarding metabolic water and glycogen storage. Journal of sports sciences, 26 (12), 1361-3 PMID: 18828029


For more information, classroom activities, and laboratories about water uptake, renal function, trichome, or mayflies:

Animals that don’t drink –


Kidneys –

Aquaporins –

Trichomes –

Mayflies -
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