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 –

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 -