Carbohydrates

Sugar. Does this look like biological energy?

Essentially, carbohydrates are made of sugar, from a single sugar molecule to thousands of sugar molecules attached together. Why? One reason is to store energy. But that does not mean you should eat it by the spoonful.

Carbohydrates

Carbohydrates are the most common type of organic compound. A carbohydrate is an organic compound such as sugar or starch and is used to store energy. Like most organic compounds, carbohydrates are built of small, repeating units that form bonds with each other to make a larger molecule. In the case of carbohydrates, the small repeating units are called monosaccharides. Carbohydrates contain only carbon, hydrogen, and oxygen.

Monosaccharides

A monosaccharide, the monomer of carbohydrates, is a simple sugar such as fructose or glucose. Fructose is found in fruits, whereas glucose generally results from the digestion of other carbohydrates. Glucose (C₆H₁₂O₆) is used for energy by the cells of most organisms and is a product of photosynthesis.

Another monosaccharide, fructose, has the same chemical formula as glucose, but the atoms are arranged differently. Molecules with the same chemical formula but with atoms in a different arrangement are called isomers. Compare the glucose and fructose molecules in the Figure below. Can you identify their differences? The only differences are the positions of some of the atoms. These differences affect the properties of the two monosaccharides.

Sucrose Molecule. This sucrose molecule is a disaccharide. It is made up of two monosaccharides: glucose on the left and fructose on the right.

Disaccharides

If two monosaccharides bond together, they form a carbohydrate called a disaccharide. An example of a disaccharide is sucrose (table sugar), which consists of the monosaccharides glucose and fructose (Figure below). Monosaccharides and disaccharides are also called simple sugars. They provide the major source of energy to living cells.

Polysaccharides

A polysaccharide is a complex carbohydrate that forms when simple sugars bond together in a chain. Polysaccharides may contain just a few simple sugars or thousands of them. Complex carbohydrates have two main functions: storing energy and forming structures of living things. Some examples of complex carbohydrates and their functions are shown in the Table below. Which type of complex carbohydrate does your own body use to store energy?

Proteins

Proteins as food. To you, these may not look appetizing (or they might), but they do provide a nice supply of amino acids, the monomers of proteins. Proteins have many important roles, from transporting, signaling, receiving, and catalyzing to storing, defending, and allowing for movement. Where do you get the amino acids needed so your cells can make their own proteins? If you cannot make it, you must eat it.

Proteins

A protein is an organic compound made up of small molecules called amino acids, the monomers of proteins. There are 20 different amino acids commonly found in the proteins of living organisms. Small proteins may contain just a few hundred amino acids, whereas large proteins may contain thousands of amino acids. The largest known proteins are titins, found in muscle, which are composed from over 27,000 amino acids.

The Figure below shows the general structure of all amino acids. Only the side chain, R, varies from one amino acid to another. For example, in the amino acid glycine, the side chain is simply hydrogen (H). In glutamic acid, in contrast, the side chain is CH₂CH₂COOH. Variable side chains give amino acids different chemical properties. The order of amino acids, together with the properties of the amino acids, determines the shape of the protein, and the shape of the protein determines the function of the protein.

General Structure of Amino Acids. KEY: White = hydrogen (H), Blue = nitrogen (N), Dark Grey = carbon (C), Red = oxygen (O), R = variable side chain

Functions of Proteins

Proteins play many important roles in living things. Some proteins help cells keep their shape (structural proteins), some, such as connective and motor proteins, make up muscle tissues, and some transport items in and out of cells (transport proteins). Some proteins act as signals, and other proteins receive those signals. Enzymes are proteins that speed up chemical reactions in cells. Other proteins are antibodies, which bind to foreign substances such as bacteria and target them for destruction. Still other proteins carry messages or transport materials. For example, human red blood cells contain a protein called hemoglobin, which binds with oxygen. Hemoglobin allows the blood to carry oxygen from the lungs to cells throughout the body.

Lipids

A lipid is an organic compound such as fat or oil. Organisms use lipids to store energy, but lipids have other important roles as well. Lipids consist of repeating units called fatty acids. Fatty acids are organic compounds that have the general formula CH₃(CH₂)_nCOOH, where n usually ranges from 2 to 28 and is always an even number. There are two types of fatty acids: saturated fatty acids and unsaturated fatty acids.

Saturated Fatty Acids

In saturated fatty acids, carbon atoms are bonded to as many hydrogen atoms as possible. This causes the molecules to form straight chains, as shown in the Figure below. The straight chains can be packed together very tightly, allowing them to store energy in a compact form. This explains why saturated fatty acids are solids at room temperature. Animals use saturated fatty acids to store energy.

Unsaturated Fatty Acids

In unsaturated fatty acids, some carbon atoms are not bonded to as many hydrogen atoms as possible. Instead, they are bonded to other groups of atoms. Wherever carbon binds with these other groups of atoms, it causes chains to bend (see the Figure in the above section). The bent chains cannot be packed together very tightly, so unsaturated fatty acids are liquids at room temperature. Plants use unsaturated fatty acids to store energy.

Types of Lipids

Lipids may consist of fatty acids alone, or they may contain other molecules as well. For example, some lipids contain alcohol or phosphate groups. They include

1. Triglycerides: the main form of stored energy in animals.

2. Phospholipids: the major components of cell membranes.

3. Steroids: serve as chemical messengers and have other roles.

[Figure 7]

Nucleic Acids

A nucleic acid is an organic compound, such as DNA or RNA, that is built of monomers called nucleotides. Many nucleotides bind together to form a chain called a polynucleotide. The nucleic acid DNA (deoxyribonucleic acid) consists of two polynucleotide chains. The nucleic acid RNA (ribonucleic acid) consists of just one polynucleotide chain. Adenosine Triphosphate (ATP) is another important nucleic acid. ATP is described as the "energy currency" of the cell or the "molecular unit of currency." One molecule of ATP contains three phosphate groups.

Structure of Nucleic Acids

Nucleic acids are constructed out of many structures referred to as nucleotides. The nucleotide is the monomer of nucleic acids. Each nucleotide consists of three smaller molecules as shown in the Figure below:

1. sugar (variable depending on the type of nucleic acid)

2. phosphate group

3. nitrogen base

If you look at the Figure below, you will see that the sugar of one nucleotide binds to the phosphate group of the next nucleotide. These two molecules alternate to form the backbone of the nucleotide chain. This backbone is known as the sugar-phosphate backbone.

Structure of a nucleotide

The nitrogen bases in a nucleic acid stick out from the backbone. There are four different types of bases: cytosine (C), adenine (A), guanine (G), and either thymine (T) in DNA, or uracil (U) in RNA. In DNA, bonds form between bases on the two nucleotide chains and hold the chains together. Each type of base binds with just one other type of base: cytosine always binds with guanine, and adenine always binds with thymine. This pattern of how nitrogen bases pair up is often known as the complementary base pairing rules.

Roles of Nucleic Acids

DNA is also known as the hereditary material or genetic information. It is found in genes, and its sequence of bases makes up a code. Between 'starts' and 'stops', the code carries instructions for the correct sequence of amino acids in a protein. DNA and RNA have different functions relating to the genetic code and proteins. Like a computer code, DNA contains the genetic instructions for the correct sequence of amino acids in proteins. RNA uses the information in DNA to assemble the correct amino acids and help make the protein. The information in DNA is passed from parent cells to daughter cells whenever cells divide. The information in DNA is also passed from parents to offspring when organisms reproduce. This is how inherited characteristics are passed from one generation to the next.