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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.
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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.
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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.
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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.
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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. Kierdemonstrates 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?).
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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
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