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Fundamental Building Blocks


Biomolecules are organic compounds produced by living things. The four biomolecule families are carbohydrates, lipids, proteins, and nucleic acids, which form the basic structural units for DNA and RNA.
Biomolecules are organic compounds produced by living things, including microorganisms (microbes). By definition, organic molecules contain one or more carbon-hydrogen bonds. There are four families of biomolecules, all of which perform essential functions in cells: carbohydrates, lipids, proteins, and nucleic acids. These compounds can be described in part by how they interact with water. Compounds are often described as hydrophobic, hydrophilic, or amphipathic. Hydrophilic is the characteristic of having a strong affinity toward water. Hydrophobic is the characteristic of having a weak or no affinity to water, which occurs when the bonds that connect their atoms are nonpolar. Molecules become hydrophilic when the bonds that connect their atoms are polar, creating a partial charge. An amphipathic molecule has both hydrophobic and hydrophilic portions.


Carbohydrates are compounds consisting of only carbon, hydrogen, and oxygen that have structural and energy storage or transport roles.
A carbohydrate is an organic compound that contains carbon, hydrogen, and oxygen and provides energy to cells. Most carbohydrates exist as polymers, long, chainlike molecules or rings of five or six carbons, which can be bonded together. Three major groups of carbohydrates exist: monosaccharides, disaccharides, and polysaccharides. Monosaccharides consist of a single sugar molecule, such as glucose. Disaccharides, such as lactose, consist of two sugar molecules bonded together. Polysaccharides, such as cellulose, are composed of many sugar molecules. Common types of carbohydrates include sugars, starches, and cellulose. Sugars include glucose, the critical component of cellular respiration. Sugars provide both immediate energy and long-term energy storage for microbes. Carbohydrates also contribute to the physical framework of many biological structures. For example, the carbohydrate polymer peptidoglycan composes the cell walls of most bacteria. Cellulose is a primary component in rigid cells walls that support the vertical growth of many plants. Humans cannot digest cellulose, but it provides fiber needed for the health of the intestinal tract.


Carbohydrates, such as 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.

Lipids and Phospholipids

Lipids are nonpolar and hydrophobic chains of carbon with bound hydrogen atoms that are used to construct membranes and store chemical energy. Phospholipids, composed of two fatty acid chains and a phosphate group attached to a glycerol molecule, are used in the construction of the plasma membrane.
A lipid is a long-chain hydrocarbon that is soluble in nonpolar solvents, that is, solvents without charge. Lipids are a class of hydrophobic compound that are involved in building cell membranes and storing energy. Lipids include fats, waxes, and sterols. Fats, which can be used to store energy, are a combination of fatty acids and the alcohol glycerol. Waxes are composed of fatty acid and a long-chain alcohol. They provide a water-repellent surface for the fur or feathers or some animals as well as the leaves and fruits of some plants. Sterols, such as cholesterol, consist of a hydroxyl group (OH-\rm OH) bonded to a fused four-ring structure.


Lipids include fats, waxes, and sterols. Lauric acid is a typical lipid known as a fatty acid, consisting of a long chain of carbon atoms, linked end to end. Attached to these are bonds where hydrogen can attach. Cholesterol is an example of a sterol, which includes a hydroxol group (OH-\rm OH) and four fused rings.
In all cells, whether prokaryotic (Archaea and Bacteria) or eukaryotic (all other organisms), the main structure of the plasma membrane is formed of phospholipids. A phospholipid is a molecule composed of a phosphate group, fatty acids, and glycerol. The thin plasma membrane surrounds cells and acts as a selective barrier that controls passage of ions and molecules into and out of cells. Phospholipids consist of glycerol molecule connected to two fatty acids and to a phosphate group. This combination makes phospholipids amphipathic. They are arranged in a bilayer in the membrane, with hydrophobic fatty acid tails pointing inward and hydrophilic phosphate heads pointing outward.

Phospholipid Bilayer

The cell membrane consists of a phospholipid bilayer that contains both hydrophobic (water-fearing) fatty acid tails and hydrophilic (water-loving) heads. The hydrophilic heads are found toward the more aqueous sides of the cell (inside and outside).


Proteins are complex folded chains of amino acids that provide structure and perform most of the functions in cells.
A protein is a large molecule composed of amino acids. Most enzymes are proteins that act as biological catalysts and speed up reactions. Transport proteins move materials in and out of cells. The amino acid chains of proteins fold into complex three-dimensional structures. For example, some proteins form cable-like shapes that provide scaffolding in the cell and others form pipe-like shapes that form channels in membranes allowing chemical transport. Other proteins form subcellular structures in membranes, cell walls, organelles, the cytoplasmic scaffolding skeleton, flagella, and pili. Proteins make up approximately 55 percent of the dry weight of prokaryotic cells.

Levels of Protein Structure

A protein can be defined by four levels of structure: the sequence of amino acids, the inclusion of beta sheets or alpha helices, the attraction between beta sheets and alpha helices, and the number of amino acid chains.
The shape and function of a protein is determined by the specific sequence of amino acids that make up the protein. Different types of amino acids possess different chemical properties that help shape the final protein, such as having hydrophilic or hydrophobic sections, or differences in polarity. For example, cysteine is an important amino acid in structural proteins, as it is capable of forming very strong bonds with other cysteine molecules, called disulfide bridges. Different amino acids are more frequent in some groups of organisms than others, depending on their function. Twenty amino acids are used by all organisms and two additional amino acids are used by subsets of organisms. For example, the amino acid pyrrolysine is found only in some archaea species and one bacterium. Pyrrolysine is used by these species to produce an enzyme that correctly positions the methyl group of a compound called methylamine during a reaction that produces methane.

Structure of an Amino Acid

Proteins are composed of sequences of organic molecules called amino acids. Amino acids contain a carboxyl group and an amino group.

Nucleic Acids

Nucleic acids provide the instructional material that guides the development, growth, and reproduction of cells.
A nucleotide is an organic compound consisting of a sugar, a phosphate, and a nitrogenous base. It is the basic structural unit of genetic sequences. A nucleic acid is a molecule made of nucleotides. Nucleic acids exist in cells in two forms: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA employs the nucleotides thymine (T), cytosine (C), adenine (A), and guanine (G). RNA employs the nucleotides uracil (U), cytosine (C), adenine (A), and guanine (G).

Molecular Structure of DNA and RNA

DNA and RNA are nucleic acids that encode and communicate the genetic information that sustains the structure and function of cells in living organisms. Though similar in their makeup, they do have some notable differences. DNA contains deoxyribose sugar, a 5-carbon sugar with a hydrogen attached to its fourth carbon atom. RNA contains ribose, a 5-carbon sugar with an OH-\rm OH group attached to its fourth carbon atom. Additionally, while DNA utilizes the nucleotide thymine (T), RNA utilizes uracil (U).
Together, DNA and RNA encode and communicate the genetic information necessary to sustain living things. Nucleic acids are the second most common component of prokaryotic cells, composing approximately 24 percent of cellular dry weight. In bacterial chromosomes, DNA is frequently circular, whereas DNA in chromosomes of eukaryotes is generally linear. Bacterial cells also contain plasmids, small circular pieces of DNA that can replicate on their own and transfer between cells. Unlike eukaryotic cells, where DNA is packed into nuclei, prokaryotic cells—which lack nuclei—store DNA only in the cytoplasm. The viral genome is made of DNA or RNA that can be double or single stranded.

Nucleotides Form Nucleic Acids

DNA consists of two long chains of nucleotides bonded together in a double helix pattern. DNA in bacteria is found in cytoplasm and is typically circular.