字幕列表 影片播放 列印英文字幕 Water is essential for Life. Here are the numbers: We can go weeks without food, but we’ll die in a few days if we don’t have water. A living cell is about 75-95% water, depending on the organism. We humans are about 60% water - and there are some organisms that are as much as 90% water! Water covers about 75% of the Earth’s surface. Good thing for us, then, that water is so well-suited to support life. What is it about water that makes it so special? How does water support life? Water has many special properties that make it the “solvent of life.” Chief among these properties is the extensive hydrogen bonding between water molecules that make water an extremely cohesive liquid. In other words, the water molecules stick together. Let’s take a closer look at what hydrogen bonding in water looks like, diagrammatically. First, remember for individual water molecules, they are held together by polar covalent bonds. Oxygen atoms are more electronegative than hydrogen atoms, so in a water molecule, electrons spend more time around the oxygen atom than around the hydrogen atoms. As a result, the oxygen atom has a partial negative charge, here shown with a lowercase greek delta minus sign. Because the electrons are spending less time around the hydrogens, those atoms have partial positive charges, shown as delta plus. Hydrogen bonding is a chemical behavior that emerges when you have more than one water molecule. The partial positive charge on the hydrogen of one water molecule is attracted to the partial negative charge on an oxygen atom in a second water molecule. This is shown as a dotted line, to let you know this is a weak bond, weaker than a covalent bond which is shown as a straight UNBROKEN line. Imagine hydrogen bonds kind of blinking on and off, like christmas lights. At any one time, a water molecule might be hydrogen bonded to one, two, three, or even four other water molecules. This interconnectedness of water molecules results in a very significant emergent property - water is cohesive. How do we observe water being cohesive? Think about how water beads up on a surface. Because water is so interconnected by hydrogen bonds, the sides of a drop of water pull together, forming a rounded shape. Compare this with a less cohesive liquid, like ethanol or isopropyl alcohol. You can see how those less cohesive fluids flow more rapidly and they don’t bead up. That’s all very interesting, but what does that have to do with how water supports life? Because of water’s cohesive nature, it forms a kind of a skin on its surface. We say water has high surface tension. It can actually support weight on its surface, because the molecules are so interconnected. This creates a new place for life. Have you ever seen a water stick insect, or a Jesus Lizard - they can run across water without sinking? If water were not so cohesive, those ecological niches would not exist. The Jesus lizard wouldn’t be able to escape from a land-based predator. A closely related property is water’s ability to adhere to surfaces, again, by forming hydrogen bonds. Adhesion allows water to crawl along surfaces, for instance, how water grabs onto the walls of xylem tubes in plants. So here’s an example of another emergent property - that is, you don’t see it from one water molecule, but you start to see it when they act in concert - the cohesive nature of water and its adhesion to the sides of tubes allows water to be pulled up through plants in a process called transpiration. The combination of these two properties allows water to reach every part of a plant. That means plants can grow a lot taller - Anything taller than a few inches is due to these phenomena. Think of giant redwoods that can grow hundreds of feet tall. Life forms like these would not exist if not for these unusual properties of water. Another emergent property due to water’s extensive hydrogen bonding is its high Specific Heat. That is, it’s hard to raise the temperature of water. Increased temperature means increased kinetic energy. Basically the molecules are vibrating faster at a higher temperature. You can’t make water molecules vibrate faster unless you first break the hydrogen bonds that are holding them together. So it takes more heat energy to raise the temperature of water than you might think. It takes 1 calorie of heat energy to raise the temperature of 1 gram of water 1 degree Celsius. That may not seem like a lot, when you say it that way, but it’s significantly higher than many other liquids. For instance, ethanol. It takes only 0.59 calories to raise 1 gram of ethanol 1 degree Celsius. So that might sound like an interesting difference in behavior of water in the lab, but what about that makes it well suited for life? This means that anything watery has a pretty stable temperature. This is true whether we’re talking about a beaker of water in the lab, or an ocean, or a swimming pool, or an organism that contains a lot of water - which we do. Water on Earth, and in living things, helps moderate temperature changes. It’s a lot harder to quickly heat up or cool down something that has a high water content. So animals can go out into the desert and not immediately turn 120 degrees Fahrenheit (or 50 degrees Celsius) We have a variety of mechanisms to help keep us cool in these circumstances, but one inherent advantage is that we are made mostly of water, which resists temperature changes. Like a temperature buffer. A related idea is that water has a high heat of vaporization. That is, it takes a lot of energy for water to evaporate - going from a liquid to a gas (water vapor). Again, this is because of water’s extensive hydrogen bonding. Water has to reach a certain temperature before it turns to gas - 100 degrees Celsius. The water molecules have to first be broken free of their hydrogen bonds holding them together, before they can start vibrating faster and have a higher temperature. Then eventually they will reach 100 degrees Celsius and evaporate out of the liquid phase to the gaseous phase. This emergent property of water also helps support life. Many living things take advantage of evaporative cooling to maintain a constant temperature. Dogs pant, we sweat - basically we allow a thin film of water to coat our skin, and when that water reaches a high enough temperature, it evaporates. When the water molecules evaporate, they take a lot of heat energy with them. The hottest molecules leave. So the average temperature of the molecules left behind is lower. That’s evaporative cooling. This is another survival strategy, another way organisms maintain the status quo. We call this homeostasis - in this case, temperature homeostasis. We can only survive if our bodies don’t get too hot or too cold. All of our biochemical reactions that we need to do to stay alive have ideal temperatures at which to run. The fact that our biochemistry is water-based - that water is the solvent of life, and all of our biological molecules are surrounded by water - that makes it easier for us to keep these biochemical reactions happening at the ideal rates. Of course, not all biological molecules are dissolved in water. Some are ionic, or polar and hydrophilic, and so THEY dissolve in water, but some are hydrophobic. So this is one way you can classify biological molecules. Are they hydrophilic or hydrophobic? The hydrophilic molecules can dissolve in water. They find themselves surrounded by a “Shell of Solvation.” Here you can see how a positively charged species is surrounded by water in one orientation, while a negatively charged species is surrounded by water oriented in the opposite way. You can see the partial positives on hydrogen will be attracted to negative regions, and repelled by positive regions. Similarly, the partial negatives on oxygen will be attracted to positive regions, and repelled by negative regions. Hydrophobic molecules repel water. These include lipids - things like fats, oils, waxes. Hydrophobic molecules are especially good at making barriers, so we can have distinct compartments in cells. We’ll talk more about how cell membranes achieve this in another video. There’s one last odd property of water that supports life that we’ll talk about today. That’s the fact that solid water - ice - is less dense than liquid water. That’s not what you normally expect from materials. Generally gases take up the most space. Then when gas particles slow down, and temperature decreases, and the material becomes liquid, the molecules are closer together. So liquids are more dense than gases. You would expect the next step, going from a liquid to a solid, the molecules are moving even slower and so they would pack even closer together. The solid form of a material is generally more dense than the liquid form. But this is NOT the case for water. The liquid form is more dense than the gaseous form. Liquid water takes up less space than water vapor. But this strange thing happens when the molecules cool down and slow down even more, and water becomes ice. Once again, we can blame water’s peculiar behavior on hydrogen bonding. When water molecules cool down enough to start forming solids, the hydrogen bonds lock the molecules into a very open lattice formation. You can see there’s a lot of space between the water molecules in solid ice - more space than there was in the liquid form that had fewer hydrogen bonds. This is why solid ice floats to the top, on top of liquid water. Now again, like the case of the high surface tension of water - one way this unusual property of water supports life is that it provides additional habitats for living organisms. You’ve seen polar bears on ice floes. Or penguins. ...these are places these animals can meet up and do their business. Eat, sleep, mate, all those important things. Ice also acts as an insulator and protects life in small bodies of water like ponds and lakes. In the winter, the top layer of a lake freezes, but once there is a layer of ice on top, the water underneath stays liquid. The liquid water is protected from the freezing cold air and wind.