17.1.4: Water Absorption - Biology

17.1.4: Water Absorption - Biology

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Learning Objectives

  • Explain the function of root hairs.
  • Define root pressure and explain its mechanism.
  • Contrast the three pathways of water movement through the roots and identify each cell type or tissue involved.

Most plants secure the water and minerals they need from their roots. Water moves from the soil to the roots, stems, and ultimately the leaves, where transpiration occurs. The roots absorb enough water to compensate for water lost to transpiration. Rapid absorption is aided by root hairs, which extend from epidermal cells, increasing surface area (Figure (PageIndex{1})). As discussed earlier in this chapter, roots draw water from the soil because they have lower water potential than the soil. Much of this difference in water potential is an indirect result of transpiration, but roots can also water potential by decreasing solute potential.

Root Pressure

When a tomato plant is carefully severed close to the base of the stem, sap oozes from the stump (Figure (PageIndex{2})). The fluid comes out because roots are constantly absorbing water, drawing it into the vascular cylinder, and pushing it up the xylem. This is called root pressure, and it is created by the osmotic pressure of solutes trapped in the vascular cylinder by the Casparian strip.

Although root pressure plays a small role in the transport of water in the xylem in some plants and in some seasons, it does not account for most water transport. As evidence, few plants develop root pressures greater than ~0.2 kPa, and some develop no root pressure at all. Additionally, the volume of fluid transported by root pressure is not enough to account for the measured movement of water in the xylem of most trees and vines. Furthermore, the highest root pressures occur in the spring, but water moves through the xylem most rapidly in the summer (when transpiration is high).

As discussed in the Cohesion-Tension Theory section, transpiration, rather than root pressure, is typically the driving force for upward water movement in a plant. However, when transpiration rates are very low, such as in cool and humid weather, root pressure pushes water up the xylem faster than water is lost through the stomata. As a result water droplets are forced out of openings on the leaf margin, a phenomenon called guttation (Figure (PageIndex{3})). Droplets resulting from guttation are not to be confused with dew droplets, which result from the condensation of water vapor when the air becomes colder and has less capacity to hold water. In other words, guttation results from water that was inside of the plant, but dew droplets form from water vapor that was in the surrounding air.

Pathways of Water Movement

Water can move through the roots by three separate pathways: apoplast, symplast, and transmembrane (transcellular). In the apoplast pathway (apoplastic route), water moves through the spaces between the cells and in the cells walls themselves. In the symplast pathway (symplastic route), water passes from cytoplasm to cytoplasm through plasmodesmata (Figure (PageIndex{4})). In the transmembrane pathway, water crosses plasma membranes, entering and exiting each cell. Water moving through the transmembrane pathway thus moves through both the symplast (interconnected cytoplasms) and apoplast (cell walls and spaces in between cells). Water may also cross the tonoplast, entering the central vacuole as part of the transmembrane pathway.

Water from the soil is absorbed by the root hairs of the epidermis and then moves through the cortex through one of the three pathways. However, the inner boundary of the cortex, the endodermis, is impervious to water due to the Casparian strip. Regardless of how the water moved up to this point (apoplast, symplast, transmembrane), it must enter the cytoplasms of the endodermal cells. From here it can pass via plasmodesmata into the cells of the vascular cylinder (stele). Once inside, water is again free to move through the apolast, the symplast, or both (transmembrane).

Once in the xylem, water with the minerals that have been deposited in it (as well as occasional organic molecules supplied by the root tissue) move up in the vessel elements and tracheids. At any level, the water can leave the xylem and pass laterally to supply the needs of other tissues. At the leaves, the xylem passes into the petiole and then into the veins of the leaf. Water leaves the finest veins and enters the cells of the spongy and palisade layers. Here some of the water may be used in metabolism, but most is lost in transpiration.

Figure (PageIndex{4}) illustrates minerals moving through the apoplast or symplast, but minerals typically move through the symplast. Minerals enter the root by active transport into the symplast of epidermal cells and move toward and into the vascular cylinder through the plasmodesmata connecting the cells. They enter the conducting cells of the xylem from the pericycle and other parenchyma cells via active transport through specialized transmembrane channels.

17.1.4: Water Absorption - Biology

Diarrhea is an increase in the volume of stool or frequency of defecation. It is one of the most common clinical signs of gastrointestinal disease, but also can reflect primary disorders outside of the digestive system. Certainly, disorders affecting either the small or large bowel can lead to diarrhea.

For many people, diarrhea represents an occasional inconvenience or annoyance, yet at least 2 million people in the world, mostly children, die from the consequences of diarrhea each year.

There are numerous causes of diarrhea, but in almost all cases, this disorder is a manifestation of one of the four basic mechanisms described below. It is also common for more than one of the four mechanisms to be involved in the pathogenesis of a given case.

Osmotic Diarrhea

Absorption of water in the intestines is dependent on adequate absorption of solutes. If excessive amounts of solutes are retained in the intestinal lumen, water will not be absorbed and diarrhea will result. Osmotic diarrhea typically results from one of two situations:

  • Ingestion of a poorly absorbed substrate: The offending molecule is usually a carbohydrate or divalent ion. Common examples include mannitol or sorbitol, epson salt (MgSO 4 ) and some antacids (MgOH 2 ).
  • Malabsorption: Inability to absorb certain carbohydrates is the most common deficit in this category of diarrhea, but it can result virtually any type of malabsorption. A common example of malabsorption, afflicting many adults humans and pets is lactose intolerance resulting from a deficiency in the brush border enzyme lactase. In such cases, a moderate quantity of lactose is consumed (usually as milk), but the intestinal epithelium is deficient in lactase, and lactose cannot be effectively hydrolyzed into glucose and galactose for absorption. The osmotically-active lactose is retained in the intestinal lumen, where it "holds" water. To add insult to injury, the unabsorbed lactose passes into the large intestine where it is fermented by colonic bacteria, resulting in production of excessive gas.

A distinguishing feature of osmotic diarrhea is that it stops after the patient is fasted or stops consuming the poorly absorbed solute.

Secretory Diarrhea

Large volumes of water are normally secreted into the small intestinal lumen, but a large majority of this water is efficienty absorbed before reaching the large intestine. Diarrhea occurs when secretion of water into the intestinal lumen exceeds absorption.

Many millions of people have died of the secretory diarrhea associated with cholera. The responsible organism, Vibrio cholerae , produces cholera toxin, which strongly activates adenylyl cyclase, causing a prolonged increase in intracellular concentration of cyclic AMP within crypt enterocytes. This change results in prolonged opening of the chloride channels that are instrumental in secretion of water from the crypts, allowing uncontrolled secretion of water. Additionally, cholera toxin affects the enteric nervous system, resulting in an independent stimulus of secretion.

Exposure to toxins from several other types of bacteria (e.g. E. coli heat-labile toxin) induce the same series of steps and massive secretory diarrhea that is often lethal unless the person or animal is aggressively treated to maintain hydration.

In addition to bacterial toxins, a large number of other agents can induce secretory diarrhea by turning on the intestinal secretory machinery, including:

  • some laxatives
  • hormones secreted by certain types of tumors (e.g. vasoactive intestinal peptide)
  • a broad range of drugs (e.g. some types of asthma medications, antidepressants, cardiac drugs)
  • certain metals, organic toxins, and plant products (e.g. arsenic, insecticides, mushroom toxins, caffeine)

In most cases, secretory diarrheas will not resolve during a 2-3 day fast.

Inflammatory and Infectious Diarrhea

The epithelium of the digestive tube is protected from insult by a number of mechanisms constituting the gastrointestinal barrier, but like many barriers, it can be breached. Disruption of the epithelium of the intestine due to microbial or viral pathogens is a very common cause of diarrhea in all species. Destruction of the epithelium results not only in exudation of serum and blood into the lumen but often is associated with widespread destruction of absorptive epithelium. In such cases, absorption of water occurs very inefficiently and diarrhea results. Examples of pathogens frequently associated with infectious diarrhea include:

  • Bacteria: Salmonella, E. coli, Campylobacter
  • Viruses: rotaviruses, coronaviruses, parvoviruses (canine and feline), norovirus
  • Protozoa: coccidia species, Cryptosporium , Giardia

The immune response to inflammatory conditions in the bowel contributes substantively to development of diarrhea. Activation of white blood cells leads them to secrete inflammatory mediators and cytokines which can stimulate secretion, in effect imposing a secretory component on top of an inflammatory diarrhea. Reactive oxygen species from leukocytes can damage or kill intestinal epithelial cells, which are replaced with immature cells that typically are deficient in the brush border enyzmes and transporters necessary for absorption of nutrients and water. In this way, components of an osmotic (malabsorption) diarrhea are added to the problem.

Diarrhea Associated with Deranged Motility

In order for nutrients and water to be efficiently absorbed, the intestinal contents must be adequately exposed to the mucosal epithelium and retained long enough to allow absorption. Disorders in motility than accelerate transit time could decrease absorption, resulting in diarrhea even if the absorptive process per se was proceeding properly.

Alterations in intestinal motility (usually increased propulsion) are observed in many types of diarrhea. What is not usally clear, and very difficult to demonstrate, is whether primary alterations in motility are actually the cause of diarrhea or simply an effect.

Tannic acid modification of metal nanoparticles: possibility for new antiviral applications

Malgorzata Krzyzowska , . Jaroslaw Grobelny , in Nanostructures for Oral Medicine , 2017

2.1 Tannins

Tannins are water-soluble phenol derivatives naturally synthesized and accumulated by higher plants as secondary metabolic products. From a chemical point of view, tannins are polyphenols with molecular weights between 500 and 3000 Da. In complexes with saccharides, alkaloids, and proteins its molecular weight can increase even up to 20,000 Da ( Haslam Edwin, 1998 ) that exhibit characteristic reactions for phenols. The chemical structure of tannic acid depends on the plant species producing the compound. Currently, more than 8000 different tannins have been isolated and chemically characterized. However, there are certainly many more tannins with chemical structures that have not been precisely determined yet.

All tannins have some common features, which enable classification of these types of compounds in two main groups, three types of hydrolysable tannins: gallotannines, ellagitannines, and complex tannins (sugars derivatives—mainly glucose, gallic acid, and ellagic derivatives) and condensed tannins (nonhydrolysable) called procyanidins containing condensed carbon chain typical for flavonoids ( Khanbabaee and Van Ree, 2001 ). Condensed tannins are much more resistant to microbial degradation than hydrolysable tannins and exhibit stronger antibacterial, antiviral, and antifungal activity.

The chemical structure of tannins determines its biological activity. Moreover, tannins can act as potential: metal ion chelating agent, biological antioxidant, or, depending on its concentration, as a complexing or precipitating agent (in low concentrations as a complexing, and in high as a precipitating, agent). Because of the variety of possible tannin structures and diverse biological effects, it is difficult to predict the effects of tannins on any living organism. The combination of tannins and metal NPs is also very weakly presented in literature. However, the combination of tannins with metallic NPs allows it to prepare new types of nanomaterials with improved antimicrobial activity of NPs and the antiinflammatory properties of tannic acid.

Neural Control of Thermoregulation

The nervous system is important to thermoregulation. The processes of homeostasis and temperature control are centered in the hypothalamus of the advanced animal brain.

Practice Question

Figure 1. The body is able to regulate temperature in response to signals from the nervous system.

When bacteria are destroyed by leuckocytes, pyrogens are released into the blood. Pyrogens reset the body’s thermostat to a higher temperature, resulting in fever. How might pyrogens cause the body temperature to rise?

The hypothalamus maintains the set point for body temperature through reflexes that cause vasodilation and sweating when the body is too warm, or vasoconstriction and shivering when the body is too cold. It responds to chemicals from the body. When a bacterium is destroyed by phagocytic leukocytes, chemicals called endogenous pyrogens are released into the blood. These pyrogens circulate to the hypothalamus and reset the thermostat. This allows the body’s temperature to increase in what is commonly called a fever. An increase in body temperature causes iron to be conserved, which reduces a nutrient needed by bacteria. An increase in body heat also increases the activity of the animal’s enzymes and protective cells while inhibiting the enzymes and activity of the invading microorganisms. Finally, heat itself may also kill the pathogen. A fever that was once thought to be a complication of an infection is now understood to be a normal defense mechanism.

Different Cells Have Various Mechanisms for Controlling Cell Volume

Animal cells will swell when they are placed in a hypotonic solution (i.e., one in which the concentration of solutes is lower than it is in the cytosol). Some cells, such as erythrocytes, will actually burst as water enters them by osmotic flow. Rupture of the plasma membrane by a flow of water into the cytosol is termed osmotic lysis. Immersion of all animal cells in a hypertonic solution (i.e., one in which the concentration of solutes is higher than it is in the cytosol) causes them to shrink as water leaves them by osmotic flow. Consequently, it is essential that animal cells be maintained in an isotonic medium, which has a solute concentration close to that of the cell cytosol (see Figure 5-22).

Even in an isotonic environment, all animal cells face a problem in maintaining their cell volume. Cells contain a large number of charged macromolecules and small metabolites that attract ions of opposite charge (e.g., K + , Ca 2+ , PO4 3− ). Also recall that there is a slow leakage of extracellular ions, particularly Na + and Cl − , into cells down their concentration gradient. As a result of these factors, in the absence of some countervailing mechanism, the cytosolic solute concentration would increase, causing an osmotic influx of water and eventually cell lysis. To prevent this, animal cells actively export inorganic ions as rapidly as they leak in. The export of Na + by the ATP-powered Na + /K + pump plays the major role in this mechanism for preventing cell swelling. If cultured cells are treated with an inhibitor that prevents production of ATP, they swell and eventually burst, demonstrating the importance of active transport in maintaining cell volume.

Unlike animal cells, plant, algal, fungal, and bacterial cells are surrounded by a rigid cell wall. Because of the cell wall, the osmotic influx of water that occurs when such cells are placed in a hypotonic solution (even pure water) leads to an increase in intracellular pressure but not in cell volume. In plant cells, the concentration of solutes (e.g., sugars and salts) usually is higher in the vacuole than in the cytosol, which in turn has a higher solute concentration than the extracellular space. The osmotic pressure, called turgor pressure, generated from the entry of water into the cytosol and then into the vacuole pushes the cytosol and the plasma membrane against the resistant cell wall. Cell elongation during growth occurs by a hormone-induced localized loosening of a region of the cell wall, followed by influx of water into the vacuole, increasing its size (see Figure 22-33).

Although most protozoans (like animal cells) do not have a rigid cell wall, many contain a contractile vacuole that permits them to avoid osmotic lysis. A contractile vacuole takes up water from the cytosol and, unlike a plant vacuole, periodically discharges its contents through fusion with the plasma membrane (Figure 15-31). Thus, even though water continuously enters the protozoan cell by osmotic flow, the contractile vacuole prevents too much water from accumulating in the cell and swelling it to the bursting point.

Figure 15-31

The contractile vacuole in Paramecium caudatum, a typical ciliated protozoan, as revealed by Nomarski microscopy of a live organism. The vacuole is filled by radiating canals that collect fluid from the cytosol. When the vacuole is full, it fuses for (more. )

Transpiration - What and Why?

What is transpiration? In actively growing plants, water is continuously evaporating from the surface of leaf cells exposed to air. This water is replaced by additional absorption of water from the soil. Liquid water extends through the plant from the soil water to the leaf surface where it is converted from a liquid into a gas through the process of evaporation. The cohesive properties of water (hydrogen bonding between adjacent water molecules) allow the column of water to be ‘pulled’ up through the plant as water molecules are evaporating at the surfaces of leaf cells. This process has been termed the Cohesion Theory of Sap Ascent in plants.

Picture of water molecules exiting stomata - side view

Why do plants transpire?

Evaporative cooling: As water evaporates or converts from a liquid to a gas at the leaf cell and atmosphere interface, energy is released. This exothermic process uses energy to break the strong hydrogen bonds between liquid water molecules the energy used to do so is taken from the leaf and given to the water molecules that have converted to highly energetic gas molecules. These gas molecules and their associated energy are released into the atmosphere, cooling the plant.

Accessing nutrients from the soil: The water that enters the root contains dissolved nutrients vital to plant growth. It is thought that transpiration enhances nutrient uptake into plants.

Carbon dioxide entry: When a plant is transpiring, its stomata are open, allowing gas exchange between the atmosphere and the leaf. Open stomata allow water vapor to leave the leaf but also allow carbon dioxide (CO2) to enter. Carbon dioxide is needed for photosynthesis to operate. Unfortunately, much more water leaves the leaf than CO2 enters for three reasons:

  1. H2O molecules are smaller than CO2 molecules and so they move to their destination faster.
  2. CO2 is only about 0.036% of the atmosphere (and rising!) so the gradient for its entry into the plant is much smaller than the gradient for H2O moving from a hydrated leaf into a dry atmosphere.
  3. CO2 has a much longer distance to travel to reach its destination in the chloroplast from the atmosphere compared to H2O which only has to move from the leaf cell surface to the atmosphere.

This disproportionate exchange of CO2 and H2O leads to a paradox. The larger the stomatal opening, the easier it is for carbon dioxide to enter the leaf to drive photosynthesis however, this large opening will also allow the leaf to lose large quantities of water and face the risk of dehydration or water-deficit stress. Plants that are able to keep their stomata slightly open, will lose fewer water molecules for every CO2 molecule that enters and thus will have greater water use efficiency (water lost/CO2 gained). Plants with greater water use efficiencies are better able to withstand periods when water in the soil is low.

Water uptake: Although only less than 5% of the water taken up by roots remains in the plant, that water is vital for plant structure and function. The water is important for driving biochemical processes, but also it creates turgor so that the plant can stand without bones.

The Water in You: Water and the Human Body

Water is indeed essential for all life on, in, and above the Earth. This is important to you because you are made up mostly of water. Find out what water does for the human body.

The Water in You: Water and the Human Body

​​​​​​​Water serves a number of essential functions to keep us all going

Think of what you need to survive, really just survive. Food? Water? Air? Facebook? Naturally, I'm going to concentrate on water here. Water is of major importance to all living things in some organisms, up to 90% of their body weight comes from water. Up to 60% of the human adult body is water.

According to H.H. Mitchell, Journal of Biological Chemistry 158, the brain and heart are composed of 73% water, and the lungs are about 83% water. The skin contains 64% water, muscles and kidneys are 79%, and even the bones are watery: 31%.

Each day humans must consume a certain amount of water to survive. Of course, this varies according to age and gender, and also by where someone lives. Generally, an adult male needs about 3 liters (3.2 quarts) per day while an adult female needs about 2.2 liters (2.3 quarts) per day. All of the water a person needs does not have to come from drinking liquids, as some of this water is contained in the food we eat.

Water serves a number of essential functions to keep us all going

  • A vital nutrient to the life of every cell, acts first as a building material.
  • It regulates our internal body temperature by sweating and respiration
  • The carbohydrates and proteins that our bodies use as food are metabolized and transported by water in the bloodstream
  • It assists in flushing waste mainly through urination
  • acts as a shock absorber for brain, spinal cord, and fetus
  • forms saliva
  • lubricates joints

According to Dr. Jeffrey Utz, Neuroscience, pediatrics, Allegheny University, different people have different percentages of their bodies made up of water. Babies have the most, being born at about 78%. By one year of age, that amount drops to about 65%. In adult men, about 60% of their bodies are water. However, fat tissue does not have as much water as lean tissue. In adult women, fat makes up more of the body than men, so they have about 55% of their bodies made of water. Thus:

  • Babies and kids have more water (as a percentage) than adults.
  • Women have less water than men (as a percentage).
  • People with more fatty tissue have less water than people with less fatty tissue (as a percentage).

There just wouldn't be any you, me, or Fido the dog without the existence of an ample liquid water supply on Earth. The unique qualities and properties of water are what make it so important and basic to life. The cells in our bodies are full of water. The excellent ability of water to dissolve so many substances allows our cells to use valuable nutrients, minerals, and chemicals in biological processes.

Water's "stickiness" (from surface tension) plays a part in our body's ability to transport these materials all through ourselves. The carbohydrates and proteins that our bodies use as food are metabolized and transported by water in the bloodstream. No less important is the ability of water to transport waste material out of our bodies.

Photosystem I

The light absorption processes associated with photosynthesis take place in large protein complexes known as photosystems. The one known as Photosystem I contains a chlorophyll dimer with an absorption peak at 700 nm known as P700.

Photosystem I makes use of an antenna complex to collect light energy for the second stage of non-cyclic electron transport. It collects energetic electrons from the first stage process which is powered through Photosystem II and uses the light energy to further boost the energy of the electrons toward accomplishing the final goal of providing energy in the form of reduced coenzymes to the Calvin cycle.

The above sketch depicts the setting of Photosystem I in the electron transport process which provides energy resources for the Calvin cycle.

Photosystem I is the light energy complex for the cyclic electron transport process used in some photosynthetic prokaryotes.

The protein complex that constitutes Photosystem I contains eleven polypeptides, six of which are coded in the nucleus and five are coded in the chloroplast. The core of Photosystem I contains about 40 molecules of chlorophyll a, several molecules of beta carotene, lipids, four manganese, one iron, several calcium, several chlorine, two molecules of plastoquinone, and two molecules of pheophytin, a colorless form of chlorophyll a .(Moore, et al.)

The Modeled Ocean

Rather than huddling over bottles of water on a ship in the middle of winter, Corinne Le Quéré huddles in front of a computer screen in her office at the University of East Anglia. “I have an easy life compared to the people who do observations,” she notes. “I’m a modeler. I stay in my office all day.” With the click of a mouse, Le Quéré gathers real-world observations into a computer program that simulates ocean processes. The model helps scientists put together measurements like Feely’s—snapshots of the ocean at specific places and times—to see how individual processes come together to create the conditions they observed. In 2007, Le Quéré announced that her model helped her uncover the first evidence that human activity is changing the ocean carbon sink.

Le Quéré and a number of colleagues were studying a natural weather cycle, the Antarctic Circumpolar Wave, which circles Antarctica in the Southern Ocean. “We were trying to see if we could detect which direction the carbon dioxide flux changed when the circumpolar wave passed through,” says Le Quéré. In other words, did the phenomenon cause the ocean to take up more or less carbon dioxide? Unlike Feely and other observational oceanographers who had been trying to measure the human impact on the ocean carbon cycle for decades, Le Quéré simply wanted to understand how natural processes could change the way the Southern Ocean takes carbon from the atmosphere.

The problem, Le Quéré found, was there was not enough ocean data to simulate changes in the ocean as the Antarctic Circumpolar Wave changed. “There’s not very much data in the Southern Ocean because people don’t want to go there in the winter. There’s too much wind. But you need winter data to get changes in the carbon sink,” says Le Quéré. “I can complain that there are no data in the winter, but there’s no way I would go there myself,” she adds with a laugh. Instead, Le Quéré inferred ocean carbon dioxide levels based on atmospheric measurements.

Scattered across the southern hemisphere, a number of isolated weather stations track concentrations of carbon dioxide in the atmosphere. Many are automated, says Le Quéré, but in some cases, hardy samplers trek outside at the same time every day to pump a sample of atmosphere into a flask, most of which are shipped to the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory in Boulder, Colorado. For Le Quéré, the more remote stations—in places like the South Pole, Palmer Station on the tip of the Antarctic Peninsula, Amsterdam Island in the south Indian Ocean, and Ascension Island in the South Atlantic—provided key measurements.

Corinne Le Quéré studies the carbon cycle through computer simulations that incorporate direct observations. (Photograph ©2008 Sheila Davies, University of East Anglia.)

In these remote places, the biggest thing changing atmospheric carbon dioxide levels is the ocean. The plants whose seasonal cycles dominate atmospheric carbon dioxide concentrations in other parts of the world, simply don’t exist in such places. “If there is one place in the world where you can [measure changes in the ocean carbon sink with atmospheric measurements], it is over the Southern Ocean,” says Le Quéré. “It is the place where you have the least contaminated air, so to speak.”

When Le Quéré plugged atmospheric measurements from the Southern Ocean between 1981 and 2004 into her model, she was startled by the result—something far more interesting than the Antarctic Circumpolar Wave. “The Southern Ocean carbon sink has not changed at all in 25 years. That’s unexpected because carbon dioxide is increasing so fast in the atmosphere that you would expect the sink to increase as well,” says Le Quéré. But it hadn’t. Instead, the Southern Ocean held steady, while atmospheric carbon dioxide concentrations climbed. Why?

Continuous measurements of the atmosphere are obtained at cold and remote observatories on the shores of the Southern Ocean, such as Palmer Station on the Antarctic Peninsula. These atmospheric data complement the direct measurements of ocean water made during research cruises. (Photograph courtesy Jeffrey Kietzmann, National Science Foundation.)

Le Quéré expected to see a steady increase in the amount of carbon dioxide absorbed by the Southern Ocean between 1981 and 2004 (blue line). Instead, weather station measurements (red line) suggested year-to-year variability, but no long-term increase over time. (Graph by Corrine Le Quéré, University of East Anglia.)

Future research

This article highlights our current views about the mechanisms of salt and water absorption in healthy and inflamed human colon. Colonic electrolyte transport processes and their regulation remain an active research area. Studies are usually performed in laboratory animals, but species dependent as well as segment dependent differences in basal transport processes mean that much of the data cannot be extrapolated readily to the human colon. In addition to the above-mentioned in vivo and in vitro techniques, the application of molecular biological techniques to colonoscopic biopsy material means that we are poised to make considerable progress in understanding human colonic Na + (as well as other electrolyte) transport processes at the intact epithelial, cellular, and molecular levels. Some areas ripe for study have already been mentioned. At present we know little about the role of dysfunctional Na + transport in the pathogenesis of diarrhoea in ulcerative colitis. We therefore need to map the distribution of apical Na + channel subunits, Na + –H + exchange isoforms and basolateral Na + ,K + -ATPase (at the levels of both transport proteins and their corresponding mRNAs) along the surface cell–crypt cell axis in different regions of the human colon, and determine the effects of mucosal inflammation. Although right hemicolectomy removes the main site of SCFA production and the most efficient region for Na + absorption, the implications for colonic Na + salvage remain unclear. Determining the effects of mineralocorticoid and glucocorticoid hormones on the distribution of colonic Na + transport proteins will improve our understanding about the changes in Na + transport that occur during Na + deprivation, following segmental resection, and during corticosteroid treatment of patients with inflammatory bowel disease.

Watch the video: Rectum, Functions of the Large Intestine u0026 Water Absorption (December 2022).