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How Large Molecules Support Cell Function

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we will talk about the different types of large molecules and the ways that they are responsible for cell function. We will meet four broad categories of large biomolecules: nucleic acids, proteins, carbohydrates, and lipids. We will also learn about the building blocks that make up these biomolecules.

Monomers and Polymers in the Body

Have you ever been told that the human body is a well-oiled machine? You might imagine your cells as a kind of factory floor, with all of your organelles performing their functions in perfect harmony. But there's a huge difference between your cells and a factory. In a factory, someone's in charge. There's a person, with a mind and perhaps a to-do list, telling everyone else what they should be doing. In your cells, there's no one in charge! How on earth do cells check off everything on their to-do list, if they can't even think for themselves?

Complex molecules in our cells have very specific functions. These help make sure cells are doing exactly what they are supposed to do. Often, these molecules are polymers, or long chains consisting of many similar units called monomers. There are four main categories of complex macromolecules, or large molecules, in the body.

Nucleic Acids

Let's start at the beginning with nucleic acids. Monomers called nucleotides join together to make polymers called nucleic acids. Each nucleotide consists of a 5-carbon sugar, a phosphate group (phosphorus with oxygens attached), and a nitrogenous base. These nitrogenous bases form the 'alphabet' your DNA and RNA use to build you!

Nucleic acids come in two flavors: DNA and RNA. DNA, or deoxyribonucleic acid, is relatively stable. It can hold onto genetic information for a long time. DNA can then be used to build RNA, ribonucleic acid. RNA is slightly less stable than DNA. RNA is useful to the cell because it does break down more quickly. RNA can then be used short-term as a template to build our next category of macromolecules, the proteins.

DNA, a nucleic acid
DNA, a nucleic acid

Proteins

Proteins are biologically active polymers made of monomers called amino acids. An amino acid looks like a carbon atom with a hydrogen on one side, a functional group called an amino group on one side, a functional group called a carboxyl group on one side, and, finally, what is called an R group on the last side. The R group is unique to the amino acid. There are 23 different amino acids that combine to make all the proteins we know today (twenty common ones, and three rare ones). Proteins can be very short chains of amino acids, or they can be chains of over a thousand amino acids.

Proteins fold into complex shapes, which is how they have such diverse functions.
Proteins fold into complex shapes, which is how they have such diverse functions.

Since proteins are the ultimate product of your DNA and RNA, you might guess that proteins have lots of different functions. You'd be right! Some proteins are enzymes, which help chemical reactions in your body go faster. Some proteins provide structure. Some proteins, called membrane proteins, transmit signals that allow cells to communicate with each other. Some proteins, called motility proteins, are responsible for movement within the cell - or responsible for movement of the cells themselves, such as the contraction of muscle cells.

Carbohydrates

Carbohydrates are simple sugar molecules or polymer chains made up of sugar molecules. Unlike DNA, RNA, and proteins, your body can't make simple sugars - unless you've learned to photosynthesize. However, you can modify existing sugars and make polymers.

Simple sugars are used for energy in the cell. Long chains of sugars are called polysaccharides, and are often used for energy storage. Starch is a polysaccharide used by plants to store energy; glycogen is the polysaccharide that animals use to store energy. Some polysaccharides, such as cellulose in plants or chitin in fungi, are used for structural support.

Shorter chains of sugars are called oligosaccharides. Oligosaccharides are often attached to other molecules such as proteins. These can help proteins identify each other or help the proteins to stick to surfaces. They can also be used to aid in immune system function.

Lipids

There are lots of different kinds of lipids. What all lipids have in common is that they are hydrophobic, meaning that they don't mix well with water. Some common lipids are steroids, waxes, fats, and phospholipids.

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