Small Molecules - Online Tutor, Practice Problems & Exam Prep

Water is a universal solvent, crucial in cell biology, comprising 70% of cell weight. It exhibits polarity, allowing hydrophilic molecules to dissolve while hydrophobic ones do not. Water's cohesive properties contribute to its high specific heat, stabilizing temperature. Carbon, capable of forming four covalent bonds, creates diverse structures, including stereoisomers from asymmetric carbon atoms. Functional groups attached to carbon impart unique characteristics, making water and carbon essential small molecules in biological systems.

Water and Carbon

Hi. In this video, we're going to be talking about small molecules. So the first molecule that we are going to talk about is water, which of course is extremely important in biology. I'm going to list a few properties of water that are important in cell biology specifically. First, water is a universal solvent, which means that it can dissolve many types of molecules. This is extremely important, as you can tell by looking at how much of the cell weight is actually composed of water. 70 percent of the cell weight is made up of water.

Now with water, we talk about some classifications, and there's these terms that continually pop up in bio and intro bio, and again, in cell biology, and that's hydrophilic and hydrophobic. I know you probably have been beat over the head with this, but it's really important that you understand these terms. In water, hydrophilic molecules dissolve, and in hydrophobic molecules do not dissolve.

Now water is also polar, which means that there is an uneven distribution of charge within the water molecule. Here's how this works: you have a water molecule, and oxygen draws electrons towards it, giving it a negative charge, whereas the hydrogen that water is attached to gains a slight positive charge. Water molecules are cohesive, meaning that because of their polar nature, they can stick together, and that cohesion actually gives water unique properties and makes it really a temperature stabilizing molecule.

One of the ways we measure this is through specific heat, which you don't need to know necessarily how to calculate that in cell biology. But what you do need to know is just that it exists and that it's a value that represents the amount of heat absorbed per gram to raise the temperature by 1 degree Celsius. This value is really high, so it takes a lot of heat to raise the temperature of water by 1 degree.

An example of one water molecule interacting through its cohesive properties with other water molecules shows how you can measure the polarity because the oxygens have the slight negative charge, whereas the hydrogens have the slight positive charge. This is what allows cohesion between water molecules because the slight negative charge of oxygen can interact with the slight positive charge of the nearby hydrogen. Water is extremely important in cell biology.

Now, let's talk about another important small molecule, and that is carbon. Carbon is extremely important and can bind up to 4 molecules at once. Now in cell biology, this typically includes oxygen, hydrogen, nitrogen, sulfur, and a few other types. Carbon bonds form through covalent bonds, and these are extremely stable. One way to measure the stability of a bond is through bond energy, which measures the amount of energy necessary to break it. More stable bonds have a higher bond energy.

In chemistry, you'll see bond energy demonstrated in joules per mole, but in cell biology, you typically see it as calories per mole. You won't be doing any kind of calculations of this in cell biology, but just in case your professor or textbook mentions it, I just wanted to throw that out there. It's a measure of bond strength and covalent bonds are strong and stable.

Now, because carbon molecules can bind to 4 other molecules, they can have multiple configurations. These configurations can be called stereoisomers, which are formed when there is a carbon molecule bound to different things and, therefore, they have the same chemical nature but can be mirror structures. You can see this best in this image here: this carbon molecule is bound to 4 different things, but the position of these four things actually creates stereoisomers because obviously this one here is a mirror image of this one here. Two stereoisomer conservations are possible, this one here and the second one here, for what we term an asymmetric carbon atom.

Now in chemistry, you'll often see a 'chiral center' a lot more, but in cell biology and in your textbook, you'll be reading a lot about the asymmetric carbon atom, which is just this carbon atom. By asymmetric, it means that on each side, it's bound to different things. Stere oisomers are really important and found in a lot of biological molecules, and we're going to talk about them a lot moving forward. Additionally, there are groups called functional groups, and they are found on carbon molecules, and they are unique in that they give particular characteristics to the molecule that contains them.

So these are really two important small molecules in cell biology: water and carbon. So now let's move on.

Which of the following is not a property of water?

• A. It is a universal solvent
• B. It is polar
• C. It dissolves hydrophobic molecules
• D. It has a high specific heat

Which type of molecule dissolves in water?

• A. Hydrophobic
• B. Hydrophilic
• C. Amphipathic

A carbon atom can bind how many independent molecules at once?

• A. 1
• B. 2
• C. 3
• D. 4

Here’s what students ask on this topic:

What makes water a universal solvent in cell biology?

Water is considered a universal solvent in cell biology due to its polarity and ability to form hydrogen bonds. The polar nature of water molecules, with a slight negative charge on the oxygen atom and a slight positive charge on the hydrogen atoms, allows them to interact with various solutes. Hydrophilic (water-loving) molecules, such as salts and sugars, dissolve easily in water because they can form hydrogen bonds with water molecules. This property is crucial for cellular processes, as it enables the transport and reaction of various biomolecules within the cell. Additionally, water's high specific heat helps stabilize cellular temperatures, further supporting its role as a universal solvent.

How does the polarity of water contribute to its cohesive properties?

The polarity of water molecules contributes to their cohesive properties through hydrogen bonding. Each water molecule has a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms. This polarity allows water molecules to attract each other, with the negative oxygen of one molecule forming a hydrogen bond with the positive hydrogen of another. These hydrogen bonds create a cohesive force, making water molecules stick together. This cohesion is responsible for water's high surface tension and its ability to form droplets. It also plays a role in various biological processes, such as the transport of water in plants through capillary action.

What are stereoisomers and why are they important in cell biology?

Stereoisomers are molecules that have the same chemical formula and sequence of bonded atoms but differ in the three-dimensional arrangement of their atoms. In cell biology, stereoisomers are important because they can have different biological activities and properties. For example, enzymes and receptors in cells are often specific to one stereoisomer over another. This specificity can affect how drugs interact with their targets, how metabolic pathways function, and how signals are transmitted within cells. The concept of stereoisomers is crucial for understanding the structure-function relationship of biomolecules and their roles in cellular processes.

What role do functional groups play in the properties of carbon-based molecules?

Functional groups are specific groups of atoms within molecules that have characteristic properties and reactivity. In carbon-based molecules, functional groups determine the chemical behavior and properties of the molecule. For example, hydroxyl groups (-OH) make molecules more hydrophilic, while carboxyl groups (-COOH) can act as acids. Functional groups are essential in cell biology because they influence how molecules interact with each other, participate in chemical reactions, and contribute to the structure and function of biomolecules like proteins, nucleic acids, and lipids. Understanding functional groups helps in predicting the behavior of complex biological molecules.

Why is carbon considered a versatile element in biological systems?

Carbon is considered a versatile element in biological systems because it can form four covalent bonds with other atoms, allowing for a wide variety of stable and complex structures. This versatility enables carbon to form long chains, branched molecules, and rings, which are the backbone of organic molecules. Carbon's ability to bond with elements like hydrogen, oxygen, nitrogen, and sulfur further increases the diversity of possible compounds. This structural diversity is crucial for the formation of biomolecules such as carbohydrates, proteins, lipids, and nucleic acids, which are essential for life. Carbon's versatility underpins the complexity and functionality of biological systems.