by Molly Sargen
figures by Daniel Utter
Water makes up 60-75% of human toàn thân weight. A loss of just 4% of total toàn thân water leads vĩ đại dehydration, and a loss of 15% can be fatal. Likewise, a person could survive a month without food but wouldn’t survive 3 days without water. This crucial dependence on water broadly governs all life forms. Clearly water is vital for survival, but what makes it sánh necessary?
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The Molecular Make-up of Water
Many of water’s roles in supporting life are due vĩ đại its molecular structure and a few special properties. Water is a simple molecule composed of two small, positively charged hydrogen atoms and one large negatively charged oxygen atom. When the hydrogens bind vĩ đại the oxygen, it creates an asymmetrical molecule with positive charge on one side and negative charge on the other side (Figure 1). This charge differential is called polarity and dictates how water interacts with other molecules.
Water is the “Universal Solvent”
As a polar molecule, water interacts best with other polar molecules, such as itself. This is because of the phenomenon wherein opposite charges attract one another: because each individual water molecule has both a negative portion and a positive portion, each side is attracted vĩ đại molecules of the opposite charge. This attraction allows water vĩ đại size relatively strong connections, called bonds, with other polar molecules around it, including other water molecules. In this case, the positive hydrogen of one water molecule will bond with the negative oxygen of the adjacent molecule, whose own hydrogens are attracted vĩ đại the next oxygen, and sánh on (Figure 1). Importantly, this bonding makes water molecules stick together in a property called cohesion. The cohesion of water molecules helps plants take up water at their roots. Cohesion also contributes vĩ đại water’s high boiling point, which helps animals regulate toàn thân temperature.
Furthermore, since most biological molecules have some electrical asymmetry, they too are polar and water molecules can size bonds with and surround both their positive and negative regions. In the act of surrounding the polar molecules of another substance, water wriggles its way into all the nooks and crannies between molecules, effectively breaking it apart are dissolving it. This is what happens when you put sugar crystals into water: both water and sugar are polar, allowing individual water molecules vĩ đại surround individual sugar molecules, breaking apart the sugar and dissolving it. Similar vĩ đại polarity, some molecules are made of ions, or oppositely charged particles. Water breaks apart these ionic molecules as well by interacting with both the positively and negatively charged particles. This is what happens when you put salt in water, because salt is composed of sodium and chloride ions.
Water’s extensive capability vĩ đại dissolve a variety of molecules has earned it the designation of “universal solvent,” and it is this ability that makes water such an invaluable life-sustaining force. On a biological level, water’s role as a solvent helps cells transport and use substances lượt thích oxygen or nutrients. Water-based solutions lượt thích blood help carry molecules vĩ đại the necessary locations. Thus, water’s role as a solvent facilitates the transport of molecules lượt thích oxygen for respiration and has a major impact on the ability of drugs vĩ đại reach their targets in the toàn thân.
Water Supports Cellular Structure
Water also has an important structural role in biology. Visually, water fills cells vĩ đại help maintain shape and structure (Figure 2). The water inside many cells (including those that trang điểm the human body) creates pressure that opposes external forces, similar vĩ đại putting air in a balloon. However, even some plants, which can maintain their cell structure without water, still require water vĩ đại survive. Water allows everything inside cells vĩ đại have the right shape at the molecular level. As shape is critical for biochemical processes, this is also one of water’s most important roles.
Water also contributes vĩ đại the formation of membranes surrounding cells. Every cell on Earth is surrounded by a membrane, most of which are formed by two layers of molecules called phospholipids (Figure 3). The phospholipids, lượt thích water, have two distinct components: a polar “head” and a nonpolar “tail.” Due vĩ đại this, the polar heads interact with water, while the nonpolar tails try vĩ đại avoid water and interact with each other instead. Seeking these favorable interactions, phospholipids spontaneously size bilayers with the heads facing outward towards the surrounding water and the tails facing inward, excluding water. The bilayer surrounds cells and selectively allows substances lượt thích salts and nutrients vĩ đại enter and exit the cell. The interactions involved in forming the membrane are strong enough that the membranes size spontaneously and aren’t easily disrupted. Without water, cell membranes would lack structure, and without proper membrane structure, cells would be unable vĩ đại keep important molecules inside the cell and harmful molecules outside the cell.
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In addition vĩ đại influencing the overall shape of cells, water also impacts some fundamental components of every cell: DNA and proteins. Proteins are produced as a long chain of building blocks called amino acids and need vĩ đại fold into a specific shape vĩ đại function correctly. Water drives the folding of amino acid chains as different types of amino acids seek and avoid interacting with water. Proteins provide structure, receive signals, and catalyze chemical reactions in the cell. In this way, proteins are the workhorses of cells. Ultimately proteins drive contraction of muscles, communication, digestion of nutrients, and many other vital functions. Without the proper shape, proteins would be unable vĩ đại perform these functions and a cell (let alone an entire human) could not survive. Similarly, DNA needs vĩ đại be in a specific shape for its instructions vĩ đại be properly decoded. Proteins that read or copy DNA can only bind DNA that has a particular shape. Water molecules surround DNA in an ordered fashion vĩ đại tư vấn its characteristic double-helix conformation. Without this shape, cells would be unable vĩ đại follow the careful instructions encoded by DNA or vĩ đại pass the instructions onto future cells, making human growth, reproduction, and, ultimately, survival infeasible .
Chemical Reactions of Water
Water is directly involved in many chemical reactions vĩ đại build and break down important components of the cell. Photosynthesis, the process in plants that creates sugars for all life forms, requires water. Water also participates in building larger molecules in cells. Molecules lượt thích DNA and proteins are made of repetitive units of smaller molecules. Putting these small molecules together occurs through a reaction that produces water. Conversely, water is required for the reverse reaction that breaks down these molecules, allowing cells vĩ đại obtain nutrients or repurpose pieces of big molecules.
Additionally, water buffers cells from the dangerous effects of acids and bases. Highly acidic or basic substances, lượt thích bleach or hydrochloric acid, are corrosive vĩ đại even the most durable materials. This is because acids and bases release excess hydrogens or take up excess hydrogens, respectively, from the surrounding materials. Losing or gaining positively-charged hydrogens disrupts the structure of molecules. As we’ve learned, proteins require a specific structure vĩ đại function properly, sánh it’s important vĩ đại protect them from acids and bases. Water does this by acting as both an acid and a base (Figure 4). Although the chemical bonds within a water molecule are very stable, it’s possible for a water molecule vĩ đại give up a hydrogen and become OH–, thus acting as a base, or accept another hydrogen and become H3O+, thus acting as an acid. This adaptability allows water vĩ đại combat drastic changes of pH due vĩ đại acidic or basic substances in the toàn thân in a process called buffering. Ultimately, this protects proteins and other molecules in the cell.
In conclusion, water is vital for all life. Its versatility and adaptability help perform important chemical reactions. Its simple molecular structure helps maintain important shapes for cells’ inner components and outer membrane. No other molecule matches water when it comes vĩ đại unique properties that tư vấn life. Excitingly, researchers continue vĩ đại establish new properties of water such as additional effects of its asymmetrical structure. Scientists have yet vĩ đại determine the physiological impacts of these properties. It’s amazing how a simple molecule is universally important for organisms with diverse needs.
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Molly Sargen is a first-year PhD Student in the Biological and Biomedical Sciences Program at Harvard Medical School.
Dan Utter is a fifth-year PhD student in Organismic and Evolutionary Biology at Harvard University.
For More Information:
- To learn more about the importance of drug solubility see this article.
- Check out these articles for more information about proteins and how water impacts their folding.
- Learn more about phospholipids here.
- Learn more about water affects DNA structure here.
- Learn more about acids and bases here.
- Check out the unique properties of water at this page or recently discovered properties of water at this article.
This article is part of our special edition on water. To read more, kiểm tra out our special edition homepage!