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 –
http://www.apsnet.org/edcenter/intropp/topics/Pages/OverviewOfPlantDiseases.aspx

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 -

Wednesday, August 10, 2016

Gimme Some Dihydrogen Monoxide


Birds need water just like the rest of us,
but beaks make it harder. They may suck
it up like a straw or scoop it up like a bucket,
or by leaning back and letting the rain fall in.
At some point or another we've all said, “I’m about to die of thirst.” Of course we can only survive for a few short days without water, but do you know why?

Cells are full of salt water (saline), but are also crowded with proteins, carbohydrates and lipids (saline + organic molecules = cytoplasm). This suggests the importance of H2O, but it doesn’t say anything about the reasons behind its importance.

Water is the solvent (the liquid part of a solution), while the proteins and carbohydrates are the solutes (the solids dissolved in the solvent). Lipids (a type of fat) are insoluble in water; therefore, they are good for building cell membranes. They help keep what is in in, and what is out out. With a lipid membrane, our cytoplasm doesn't leak out on to the floor.


Cytoplasm isn’t water plus some organelles. As shown in
this electron micrograph, it is more like a gel, packed
with organelles, proteins, minerals, sugars, and nucleic
acids. There is water, but just enough to separate the other
constituents. Photomicrograph credit: Dr. Jeremy Burgess/Science
Photo Library.
The intracellular solutes are surrounded by water. It’s like the green jello with pineapple that your Aunt brought every Christmas, except that it's packed to the gills with pineapple. Cytoplasm is more crowded than the public pool on a 104˚F day when the ice cream vendors have gone on strike. In some cases, there may only be a few molecules of water separating different cellular components, but this water layer is crucial.

Water is the solvent in which most cellular reactions take place. Water is made up of an acid (H+) and a base (hydroxyl, OH-). Together, they are two hydrogen atoms and one oxygen, H2O! Having the H+ around keeps the bases in check, while the OH- keeps the acids in check. This helps keep the cytoplasmic pH within a small range (buffers it), about 7.35-7.45. Buffering the cytoplasm ensures that that reactions proceed in the proper direction and at the proper rate.

Water transports materials within the cell, from cell to cell, and through the blood and lymph. The partial negative and positive charges, the high surface tension, and the cohesive properties of water make it good at its jobs.


Water being sucked up in a capillary tube
uses cohesion (water sticking to water) and
adhesion (water sticking to the glass tube).
Water likes to bond to itself (cohesion) via hydrogen bonds formed between the positive H+’s of one water molecule and the negative OH-‘s of two others. Cohesion is what makes water form drops as it rains, and what gives water its strong surface tension. Surface tension is why some insects can land on water and take off again. Water striders (family Gerridae), walk on water and you can actually see the depression in the surface, like when you stand on your bed. They are helped out in this endeavor by hydrophobic (water-fearing) tiny hairs on their legs and feet.

Water also likes to hydrogen bond other surfaces; this is called adhesion. If you pour water into a small diameter glass, you can see it cling to the side (meniscus, Greek for crescent), and even seem to rise up the side of the glass (see the image above). If the glass tube is narrow enough, like in a capillary tube, the water will climb up the tube against gravity. The force that drives this is adhesion.


Water striders spread their weight over a large area to
reduce their pressure on the water. They are also helped
by the hydrophobic proteins on their legs. But mostly, the cohesive
force of the water raises the surface tension so the strider
remains on the surface.
The adhesive force is driven by the bipolar (a negative end and a positive end) nature of water, just as with cohesion. The positive H+ is attracted to any negative molecules, and the negative OH- is attracted to anything positive. Together, they are attracted to most everything, not just other water molecules.

Hydrogen bonding and the adhesion and cohesion they produce are important for plants. How does water absorbed by a redwood’s roots get to its leaves way up high? The mechanism has several features, the most important of which is suction. When water in the leaves evaporates, it creates negative pressure that actually pulls the water up from the roots through the plants vessels.

The negative pressure alone isn’t strong enough to keep the water moving against gravity, but when you add in the cohesion of water molecules to one another, and adhesion of the water molecules to the sides of the vessels, it all works out. The sum total of these actions is called transpiration, and is responsible for moving water against gravity in plants.

Water also participates in many cellular reactions, most famously photosynthesis. During the Calvin cycle of photosynthesis (dark reactions) glucose is produced, water is split into hydrogen atoms that are incorporated into the growing carbohydrate and gaseous oxygen (O2) that is released. It is this transformation of water to gas that drives transpiration.  In cellular respiration, when carbohydrates are used to produce chemical energy (ATP), the exact opposite occurs – water is formed from oxygen and hydrogen.

Other cellular reactions, such as the hydrolysis (hydro = water and lyse = split) of fats or proteins are occurring inside cells all the time. In these types of reactions, a water molecule is split into H and OH while the target molecule is also split in two; one part gains a hydrogen and the other gains a hydroxyl group. This is crucial for the normal degradation of cellular proteins by protease enzymes, amongst other things.

If that wasn’t enough, water acts as temperature buffer, helping organisms hold a more constant temperature. Water does not warm up fast and it does not cool down fast; it tends to keep an even temperature. It has a high specific heat (1 calorie/gram C˚), meaning that you must add a lot of energy in order to change its temperature. Water’s high specific heat evens out temperature fluctuations in the body and allows reactions to proceed in a controlled fashion.

Finally, many organisms use water pressure to hold their form, an example of the turgor pressure we learned about several weeks ago (Plants That Don’t Get A Good Night’s Sleep). For instance, you return home from a trip to find your plants have turned brown and are drooping in their pots. Your goldfish are belly up, and the expensive six-pack in your fridge is now a two pack – the neighbor you asked to look after them did a bang up job. If you’re lucky, the plants stand back up a few hours after a good soaking, especially if you fertilize them with your goldfish carcasses. Your plants need the water for everything we have discussed, but also because the water pressure in the cells keeps them the plant stem and leaves standing rigid.


The tube feet of starfish and other eichinoderms have a
suction cup on the end of the podia. The internal portion
is the ampulla, the tube that holds water to regulate the
tube movement.
In a similar fashion, starfish store and move water through a series of hollow tubes to form a hydrostatic skeleton. In the general sense, this type of skeleton is any fluid filled cavity surrounded by muscle, in which the actions of the muscles work against the fluid pressure in the cavity. Worms, and many other invertebrates have this type of support system.

But starfish take the concept a bit further. Not only is water used to maintain the form and structure of the animal; it makes up the water vascular system for locomotion (tube feet), food transport, and respiration. By moving water in and out of specific tubes in the different arms, the muscles contract and extend the tube feet, pushing them against a surface. The movement of water in and out of the tube feet is also the primary way to move oxygen into the tissues of the starfish, and the water pressure can be used to evert their stomach (it will protrude out their mouth and turn inside out) to surround and engulf food. Ugh!


Many types of animals use hydrostatic skeletons, where the pressure of water substitutes for a rigid skeleton. Muscular movements are generated against the in agonist/antagonist form against the pressure of the water, using muscular fibers positioned in several planes. A recent review by William M. Kier demonstrates how the hydrostatic skeletons and muscular arrangements of several different animals work to generate stiffness as well as movement.

For instance, in the tube feet of the starfish, Ludia clathrata, muscular fibers are oriented in longitudinal and circular directions, allowing for extrusion and contraction. But he also discusses the connective tissue fibers that are just as important for the limiting of movement and generation of tension.

We always knew water was crucial for life, and now we know why. Its importance is reinforced when you consider how much water there is in different organisms. Humans are about 60% water by mass, but it varies from person to person. Younger children are normally have a slightly higher percentage of water, maybe 70%, while morbidly obese people have much less water, remember that fat is stored in the absence of water (Is it Hot in Here or is it Just My Philodendron?).


The golden barrel cactus has ribs that can expand and
contract, depending on the hydration state of the plant.
It is also called a mother-in-law’s cushion….that’s just mean.
Plants require even more water. Cactuses can be more than 90% water after a good rainfall. The places where cacti grow have variable water availability, so when water is present, they must take advantage. The endangered golden barrel cactus has ribs that can expand to take in more water. In addition, the golden barrel cactus is round to reduce surface area and has a thick waxy surface, both of which reduce water loss.

Despite these dehydration prevention measures, cacti still lose water over time, and it might not be replaced for a long time. Therefore, cacti have evolved mechanisms to withstand the loss of almost 60% of their water without any negative ramifications. In this area, they are the exception. Typical flowers and trees can only withstand a 20% water loss without damage; however, this is still much better than humans can do.

No matter what your personal water percentage might be, you can only afford to lose about 5% of your water without suffering symptoms. At mild levels of dehydration (5%), you may feel groggy or get a headache. Higher levels of water loss will bring tingling in the muscles, nausea, and confusion. If the loss reaches 10-15%, there can be muscle spasms, delirium, and the kidneys may be permanently damaged (if water loss is held for a sufficient period). Held above 15%, dehydration is usually fatal. However, athletes can lose up to 30% of their body water in the short term, but it must be replenished immediately so that performance or normal function will not be compromised.

When we say normal function, we mean those functions of water we have mentioned, but also several others we haven’t. Water, along with surfactant proteins, works to keep our lungs absorbing oxygen. Water lubricates our joints and tissues to avoid friction damage. People with xerostomia (Greek, xero = dry and stoma = mouth) or xerophthalmia (dry eyes) use artificial saliva or tears to prevent damage to mucous membranes. Finally, water acts as a cushion, absorbing pressure and force to protect our organs from traumatic damage, like a punch to the gut.

Damage can come in many forms when water is low, so all living organisms require water intake to function and remain safe, right?……Or is just most organisms? Next time.


Kier, W. (2012). The diversity of hydrostatic skeletons Journal of Experimental Biology, 215 (8), 1247-1257 DOI: 10.1242/jeb.056549


For more information, classroom activities, or laboratories about water in biology, the properties of water, transpiration, or the Calvin cycle, see:

Water in biology –

properties of water –

transpiration –

calvin cycle –
http://www.educationalrap.com/song/photosynthesis.html