Molecules of Life

Carbon Compounds

Life on Earth is built on carbon compounds.
The term carbon-based life refers to the organisms that live on Earth. This is because carbon atoms (individual units of carbon) are present in the molecules that living things use to survive, grow, and reproduce. Carbon atoms form the basis of complex molecules because they form four stable bonds and thus can form long, branching chains of infinite variety. This means that carbon can readily form bonds with many other types of atoms and these bonds are difficult to break. Carbon compounds are so important that entire branches of chemistry are devoted their study. Molecules that contain carbon are known as organic compounds.


Carbohydrates provide energy for cells.
A carbohydrate is an organic molecule containing carbon, hydrogen, and oxygen. Common examples include sugars, starches, and cellulose. Carbohydrates are the primary energy source for living things. Carbohydrates are synthesized by different organisms, such as algae and plants, during photosynthesis. Carbohydrates also provide the physical framework of biological structures, such as cellulose giving plants rigid cell walls or chitin forming the hard bodies of insects. Carbon makes up the backbone of carbohydrates. Hydrogen and oxygen bind to this backbone, both as individual atoms or as hydroxyl (–OH) groups. The specific arrangement of atoms determines how the molecule stores energy and releases the energy during cellular respiration. Most carbohydrates exist as long, chain-like molecules or as rings of five or six carbons, which can be bonded together.
Carbohydrates, like glucose and fructose, are organic molecules that provide energy for cellular function. A carbohydrate has carbon, oxygen, and hydrogen atoms. It usually forms long chains or rings of carbon atoms and has hydrogen and hydroxyl groups attached to it. There is usually a double bond found in each molecule connecting to an oxygen atom.


Lipids enclose cells and provide organisms with fuel and protection.
A lipid is a long-chain hydrocarbon that is soluble in nonpolar solvents, i.e., those without charge. Typically, lipids have a large nonpolar region (a region without a charge) and a small polar region (a region with a charge). Fats, waxes, and sterols (such as cholesterol) are lipids.
Lipids have many functions in living things. Fats, waxes, and cholesterol are all lipids. For example, lauric acid, a component of coconut oil, provides stored energy to cells. Cholesterol helps the body produce hormones that regulate growth, digestion, and mental faculties. A typical lipid is a long chain of carbon atoms, linked end to end. Attached to these are bonds where hydrogen can attach. If all of the spaces are filled with hydrogen, the lipid is considered saturated. If there are still empty spaces, the lipid is unsaturated.
Lipids, in conjunction with proteins, form the cell membrane, the structure that encloses the cell. Phospholipids, which make up the cell membrane, have two long, nonpolar "tails" and a polar "head." In the aqueous (water-based) environment both within and surrounding cells, two layers of phospholipids line up with their tails toward each other and their heads facing out. This double layer of lipids that separates the cell interior from the external environment and regulates the passage of substances into and out of the cell is known as the phospholipid bilayer. The lipid molecules slide freely past each other within each side of the bilayer, giving the cell flexibility. The lipid molecules can also move to allow substances to pass through the layer, giving the cell permeability.
The phospholipid bilayer is an important component of cell membranes. The arrangement of the molecules in this bilayer give the cell membrane flexibility and allow substances to pass in and out of the cell. The hydrophilic heads of the phospholipid are "water loving" and therefore can tolerate getting wet. The hydrophobic tails of the phospholipid are "water-fearing" and cannot get wet.
Additionally, lipids in the form of fats store energy for organisms. They also store other nutrients, such as fat-soluble vitamins like vitamin A and vitamin K. Lipids also form the waxy coating found on plants such as cactus and aloe. Lipids make up hormones, such as estrogen and testosterone, which regulate body functions such as sexual reproductive cycles and growth and development.


Proteins assist chemical reactions, transport materials into and out of cells, and provide structure.
A protein is a large molecule that speeds up reactions and transports material in and out of cells. Proteins are composed of amino acids. An amino acid is an organic molecule that contains a carboxyl group (carbon atom bonded to two oxygen atoms in which one oxygen atom is in turn bonded to a hydrogen atom, denoted –COOH) and an amino group (nitrogen atom bonded to two hydrogen atoms, denoted –NH2). Each amino acid also contains a side chain (denoted "R") that gives it a specific shape and function and consists of one or more atoms. The specific sequence of amino acids in a protein determines the protein's shape and therefore its function. A protein's function can be changed by replacing a single amino acid with a different one.

Structure of an Amino Acid

Proteins are composed of organic molecules called amino acids. Amino acids contain a carboxyl group and an amino group.
Humans use 20 amino acids to make the proteins that build, maintain, and repair cells. Nine amino acids have been identified as essential for humans, meaning that our bodies cannot make them and must obtain them from food. An additional seven amino acids are conditionally essential, meaning that they cannot be made in some situations but can be made in others. Under normal conditions the body makes them, but this process stops in times of illness or stress. The remaining four amino acids are made in the human body and do not have to be obtained from food.
Essential Conditionally Essential Non-Essential
Histidine Arginine Alanine
Isoleucine Asparagine Aspartate
Leucine Glutamine Cysteine
Methionine Glycine Glutamate
Phenylalanine Proline
Threonine Serine
Tryptophan Tyrosine

There are 20 amino acids. Nine are considered essential because the human body cannot synthesize them. These nine must be obtained from food. An additional seven amino acids are "conditionally essential," meaning that they cannot be made in some situations but can be made in others. Under normal conditions the body makes them, but this process stops in times of illness or stress. The remaining four amino acids are made in the human body and do not have to be obtained from food.


Proteins play numerous roles in the body. For example, DNA polymerase is the protein that copies genetic material to ensure every cell has a complete set of DNA. Lactase is the enzyme that breaks down milk sugars. People deficient in lactase have the condition lactose intolerance. Protease breaks down proteins from food so that the individual amino acids can be used to build other proteins the body needs. Proteins form the scaffolding around which DNA wraps, making chromosomes. It is important to note that the suffix –ase indicates that a molecule is an enzyme, a specific kind of protein that speeds up chemical reactions. The name of the enzyme often gives a clue as to its function.

Nucleic Acids

Nucleic acids provide instructions for making organisms and building and maintaining cells.
A nucleic acid is a large molecule made of nucleotides. A nucleotide is an organic molecule consisting of a five-carbon sugar, a nitrogen base, and a phosphate group. The nucleic acid people are most familiar with is deoxyribonucleic acid (DNA) an organic molecule containing coded instructions for the life processes of an organism, which consists of nucleotides bonded together forming a double helix. Most living things encode the instructions necessary to carry out the processes vital to life using DNA. In this system, a series of letters make up the code, which is read by proteins. Another type of nucleic acid, ribonucleic acid (RNA), is also present in living things. Ribonucleic acid (RNA) is an organic molecule that carries genetic messages out of the nucleus consisting of a single strand of nucleic acids. RNA functions in many different ways in cells. RNA carries the message DNA sends to the cells to create proteins; this message is known as mRNA. RNA also helps the cell build proteins; this RNA is known as tRNA. RNA is also part of the machinery (ribosomes) where proteins are built; this is known as rRNA.
Nucleic acids, such as DNA and RNA, are carbon compounds that provide the instructions for the processes of life. They are organic molecule consisting of a five-carbon sugar, a nitrogen base, and a phosphate group.
Nucleotides are the building blocks of nucleic acids. A single nucleotide of DNA contains a five-carbon sugar, a nitrogen base, and a phosphate group. The nitrogen base of DNA can be one of four types: adenine (A), thymine (T), guanine (G), or cytosine (C). Each base binds to the sugar molecule, which in turn binds to the phosphate group. The phosphate groups then bind to the sugar in the next nucleotide, creating a backbone that makes a long polymer consisting of many molecules. A polymer is a large molecule made of repeating smaller units of similar structure that are bonded together. Typically, DNA does not exist as a single polymer chain. Rather, the nucleotides selectively form hydrogen bonds (weak bonds between hydrogen atoms): adenine binds to thymine, and guanine binds to cytosine. Thus, DNA forms two long chains with bonded nucleotides making a double helix, the helical structure formed by two strands of DNA as they wind around each other.
The double-helix structure of DNA results from the bonding of nucleotides. It consists of two long chains of nucleotides bonded together in a double helix pattern. The four nitrogen bases found in DNA bond together as follows: adenine bonds with thymine and cytosine bonds with guanine.
RNA has a structure similar to DNA—a nitrogen base attached to a five-carbon sugar that binds with a phosphate group. The sugar in RNA is ribose, which is similar to the sugar in DNA (deoxyribose) except for the addition of an oxygen atom. Further, thymine (T) is not present in RNA but is replaced with uracil (U). RNA forms shorter polymer chains than DNA and is single-stranded rather than double-stranded.
RNA, the molecule that carries messages and makes proteins, is similar to DNA. While DNA contains a deoxyribose sugar, RNA contains ribose. Deoxyribose is a 5-carbon sugar with a hydrogen attached to its 4th carbon atom. Ribose is a 5-carbon sugar with an -OH group attached to its 4th carbon atom. In RNA, the base thymine is replaced by uracil.