Life on our planet is an interesting thing and in order to truly understand the intricacies of how life works you need to understand the basics of the components that make life possible and one of those very important molecular components is water. All the way back to the beginning of life on our planet the first cell that was created was created in and with water. And since that point, water has continued to be the main medium in which the processes of life take place. Let's start with the basics and talk about why water is such a key component of life, from its structure to its properties.
Water molecules are made up of three atoms, one oxygen and two hydrogen bound together by chemical bonds. One of the most important properties of water that we need to know for biology is that water molecules are polar. which is a consequence of their asymmetrical structure and polar covalent bonds.
This means that there is an unequal distribution of charges throughout the molecule, causing one side of the water molecule to be partially negative and the other side to be partially positive. The electrons shared between the bonds of the hydrogen and oxygen are pulled closer to the oxygen because it is more electronegative. And because the general shape of a water molecule is asymmetrical, the pull of these electrons causes charges to form.
This does not happen with evenly distributed bonds like we see in methane, where the poles of the electrons are evenly distributed and it does not create polar ends. An easy way to tell if a molecule is polar or not is to ask yourself, can I draw a straight line through this molecule and have all positive charges on one side and all negative charges on the other? If you can do that, the molecule, or at least the part of the molecule, should be polar.
Illustrating that here, we can see that a line can be drawn through the water molecule to separate the charges. But there is nowhere I can accomplish that with the methane molecule, which means it is nonpolar. Okay, back to water.
The polarity is important because within the world of physics and chemistry, opposite charges are attracted to each other. So when multiple water molecules get together, the negatively charged side of the molecule with the oxygen is attracted to the positively charged hydrogen end of another water molecule. This forms a weak but very relevant bond called a hydrogen bond. The hydrogen is important here, as our hydrogen atom at this point is a positively charged proton, because its electron is being pulled closer to the oxygen.
If this attraction takes place between this proton and another atom that is negatively charged, we can classify it as a hydrogen bond. Now all of this information about water being polar as a molecule is important for life, because it gives water a few important properties, which we will cover on the next few slides. Cohesion, which describes the ability of water molecules to attract and stick to other water molecules, is the first important property.
As we can see in this image, this is mainly due to the polarity of each water molecule and the formation of hydrogen bonds. These negative and positive charges keep the water molecules together, making them more difficult to pull apart compared to molecules of other liquids. Cohesion between water molecules plays an important role in biological systems, like when water is moved within plants.
Plants have specialized tissue within their root, stem, and leaf systems called xylem that is used to transport water and nutrients. Water is absorbed by the roots of the plant and moves up through the stem to the leaves to transport nutrients and be used for photosynthesis. As plants stand erect to grab as much sunlight as they can, the upward movement of water is fighting against the force of gravity, which is pulling it down. The process of transpiration, paired with the property of cohesion, allows for water to make its way up despite the pull of gravity. Just like the blood in our bodies, the water within plants needs to constantly be moving.
Transpiration is a process that describes how plants give off water vapor through the stomata of their leaves. With water constantly evaporating and leaving the plant, this leaves room for more water molecules to move up and replace what was lost. This tension created by transpiration pulls the next water molecule up, and because all of the water molecules are held together by cohesion, it ends up pulling all of them up together, all the way down to the root.
There are other forces at play here which we will discuss later, but this is how cohesion plays an important role. In addition to plant xylem function, the property of cohesion, is also utilized in different ways for different organisms. The water strider is an insect that leverages the property of cohesion to move across bodies of still water without sinking, meaning they can literally walk on water.
This is possible because the cohesion of the water molecules, built up by the hydrogen bonds, require more energy to break than water. but is exerted by the insect's specialized hydrophobic legs. So the water molecules continue to stick to each other instead of breaking formation to wrap around and submerge the insect. We call this characteristic surface tension, and without this cohesive property, this surface habitat would not be possible.
Another similar important property of water is adhesion, which describes how water molecules can be attracted to and stick to other surfaces. Just like cohesion, Adhesion can be explained by the structure of water molecules and their polarity and hydrogen bonds. Surfaces that are polar or possess charges can attract water molecules, causing them to stick. To talk through a few examples, we can see this happening in soil and also in plant xylem. Let's start with soil.
Soil is a mixture of many substances that includes organic materials, minerals, water, and air. Of all of the materials found within soil, some of them possess polar properties. For this reason, water is able to create hydrogen bonds and stick to the soil, allowing it to move through porous, air-filled spaces, replacing it with water and making the soil wet. This type of movement is called capillary action and can even force the water to move up towards the surface of the soil if it's dry compared to deeper layers that are wet. This is good for plants to be able to continuously absorb available water through their roots.
Our next example of adhesion is within plant xylem. We already talked about how cohesion helps the water molecules stick to each other, to be pulled up, but the property of adhesion is also at play here. The wall of the xylem tube is made out of polar molecules, and because water is also polar, the xylem wall and the water molecules are attracted to each other based on their charges.
This pull and attraction from the water molecules to the wall helps create tension that supports the upward movement against gravity, very similar to what happens in the soil. Water has the ability to dissolve particles within it in a liquid state. We call this the solvent property.
This works, just like all the other properties, due to the fact that water is polar and creates hydrogen bonds. Substances that are attracted to the polar charges of the water molecules are called hydrophilic. Take a salt crystal for example.
When you shake some salt into a cup of water and swirl it around, you will notice that it disappears. This isn't a magic trick, and the atoms that were in the salt cube are not actually gone. they were just separated by the water and spread out evenly into tiny pieces that are too small to see with the naked eye. This works because the salt itself is made out of sodium and chlorine. And when these ionic bonds are broken apart, they separate into two charged ions.
Sodium has a positive charge and chlorine has a negative charge. And per our charge pairing rule, the positively charged sodium ions will be attracted to the partial negative charge of the water molecule near the oxygen end. and the negatively charged chlorine atoms, properly called chloride, will be attracted to the partial positive charges of the hydrogen end of the water molecule. The water molecules end up wrapping around and enclosing each ion, which is the process of dissolving.
This makes the water the solvent, and the sodium and chloride the solute, the substance that is dissolved in the water. This separation of the salt crystal will continue to happen until each atom is separated. assuming that there are enough water molecules to do the job. But this does not only happen with salt. It happens with many other ions within many different biological systems.
It is for this reason that we call water a universal solvent. Two examples of this property are metabolism and transport. Metabolism describes the collection of reactions that happen within the body, and these reactions are happening at the cellular level.
And because water is the main component and solvent, it supports the movement of charged particles to support all of these reactions, many of which are carried out by enzymes. Water is also used as a transport medium within organisms, like the xylem within a plant and the blood within animals. The xylem tube moves water and other mineral ions that can easily dissolve within the water. Blood plasma within our body can easily transport polar substances that can dissolve in water.
like the sodium chloride we discussed earlier, along with other substances like sugar and amino acids. It's important to note that this solubility property of water does not apply to every molecule that enters the system. Some molecules do not dissolve in water because they are nonpolar, and are more attracted to other nonpolar molecules than to polar ones, like water. We classify these molecules as hydrophobic. These hydrophobic molecules also play important roles as well.
A great example of this are the phospholipids that make up the cell membrane. With aqueous solutions existing both inside and outside the cell, these lipids contain structures that are hydrophobic and are more attracted to each other than the surrounding water. This makes the creation of the barrier possible because they do not dissolve in water, keeping the structure of the membrane intact and separating the space that is inside the cell from the exterior. This would not be possible if the entire phospholipid molecule was polar. In addition to cohesion, adhesion, and solvent properties, water also has other physical properties that are important for organisms.
Let's talk about each property individually and go over a few examples that you will likely see on the IB exam. First, we have specific heat. which describes the amount of energy or heat needed to raise the temperature of a substance by 1°C. Water has a specific heat that is higher than many other substances, meaning that it takes more energy to heat up water compared to something else like air or rock, which take less energy to heat up. The specific heat values for these substances are 4.186 Joules per gram degree Celsius and 1.005 Joules per gram degree Celsius.
So, it takes about four times more energy to heat up water to the same degree that you can heat up air molecules. Water has a relatively high specific heat because of the hydrogen bonds that hold water molecules together. It takes energy to break these bonds and to get water molecules to move. This is good for organisms because water can withstand large swings in temperature, keeping things relatively constant.
This is especially important to support the chemical reactions that happen in the body to ensure stable internal temperature is maintained. Next is the thermal conductive property, which outlines the rate at which heat is able to pass through a material. Water has a relatively high thermal conductivity capacity, meaning it can easily transfer heat through it.
We can contrast this to air which has lower thermal conduction and cannot transfer heat as easily. As we already established, much of our internal structures are made up of water, like our blood, which means it has the ability to carry and transfer heat. An example of this can be seen with the fennec fox of North Africa.
It has large ears that are loaded with blood vessels, helping it dissipate heat from blood to the exterior environment, allowing it to cool down on a hot day, along with obviously having a keen sense of hearing. The water-based solution of the blood helps radiate the heat from the vessels to the atmosphere. Next up we have buoyancy, which describes the ability of a material or substance to float on water based on an upward force exerted back on the object from the water molecules.
The basics of this revolve around density. If an object is more dense than water, it will break that upward force placed upon it and sink, where if it is less dense than water, the upward force exerted will be great enough to keep it afloat. The density of water is 997 kilograms per meter cubed, and because living organisms are made up of water and use water as a solute for their systems, their overall density ends up being pretty close to water. The average density of human tissue is around 1078 kilograms per meter cubed, which of course can vary.
We can contrast that to the density of air, which varies but is upwards of hundreds of times less dense than water. Taking a look at an example, we can see that green algae can float due to the oxygen they produce during photosynthesis. The oxygen gets trapped between the algae's filaments, making it lighter and causing it to float up to the top of the water.
Algae also have gas vacuoles that increase their buoyancy. The gas vacuole volume cell volume ratio determines whether the cells rise, sink, or remain at the same depth, which is used to their advantage to obtain the proper amount of sunlight throughout the day. The last physical property is viscosity. The viscosity of a substance describes its ability to flow based on internal friction between molecules.
When molecules start to move, if there is a great amount of friction between them, they will tend to stick together and not easily flow, having a high viscosity. You can think about the difference between pouring water and tree sap out of a bottle. The water will flow pretty easily where the sap will take longer because it sticks together, meaning the sap has a higher viscosity than the water.
Water does have some internal forces that hold it together, like hydrogen bonds, but tends to increase in viscosity when other solutes are added. Blood plasma has many different solutes and is more resistant to flow than pure water. Adaptations around viscosity are especially important for organisms that live in ocean water, which is within it, making it, again, more viscous than pure water.
We can again compare this to the viscosity of air, which is much less than that of pure water. For the IB exam there are two specific organisms that you need to know and be able to compare in terms of how these physical properties of water impact them. They are Gavia arctica, common name black-throated loon, and Pusa Hispida, common name Ringed Seal.
The seal is a mammal and the loon is a bird, and both of these organisms spend varying amounts of time interacting with the land and water in the same ecosystem. They both find food in the water, but the seal spends more time in the water diving to greater depths while hunting. Due to this reason, the physical properties of the water have a greater impact on the seal. The water is more viscous to move through compared to the air, so the seal spends more energy moving throughout the water.
and fighting against buoyancy forces than the loon does flying through the air. In addition, the water has a higher heat conductivity than the air, so the seal will have more heat pulled from its body faster than the loon in the air. Adaptations for the seal, like the thick coat of fur, help it retain its body heat so it can regulate its temperature under the cold water. Be sure to know these comparisons for the test, and also think about how they can apply to other organisms that you learn about throughout the curriculum.