Water accounts for about 70 % of a cell’s weight. Given how complicated life is, it seems quite surprising that the remaining 30 % are mainly made up of only four small molecular building blocks. These building blocks are nucleotides, amino acids, sugars, and fatty acids. Life’s diversity is created by assembling these building blocks into larger molecules.

Nucleotides are linked to one another to produce DNA and RNA, amino acids are linked to create proteins, sugars are the units that yield complex carbohydrates or polysaccharides, and fatty acids are the basic ingredient to make fats and membranes.

Nucleotides
Nucleotides are the building blocks of DNA and RNA, the molecules that are responsible for storage and retrieval of genetic information. DNA encodes this information and RNA, in the form of messenger RNA (mRNA) and transfer RNA (tRNA), makes sure the information is properly translated into proteins.

Nucleotides are also involved in energy metabolism. Adenosine triphosphate (ATP) transfers energy between different places in the cell. ATP is generated where sugars and fats are broken down. It then travels to the places in the cell where energy is needed. Cleavage of ATP releases a large amount of energy, which is used to drive energy consuming chemical reactions in the cell.
 

Amino Acids
Amino acids are small molecules that can be linked head to tail into proteins. Proteins fulfill a countless number of different functions in the cell. They can be enzymes, biological catalysts, which facilitate all the chemical reactions that support life. Proteins decorate the cell surface, where they sense environmental signals. They give a cell internal structure by assembling into the cytoskeleton and they pump small molecules across membranes. Proteins also regulate when and where genes are switched on.

Sugars
Sugars play a major role in energy production. Breakdown of glucose into carbon dioxide (CO2) is one of the most important pathways that fuel energy-consuming processes in the cell.

Therefore, polysaccharides are used for energy storage. Starch is the energy store produced by plants; glycogen, the energy store of animals.

Furthermore, polysaccharides are used for mechanical support. Cellulose, the structural component of the plant cell wall, as well as chitin, the main constituent of the exoskeleton of insects, are polysaccharides.

Polysaccharides also play a role in cell-cell recognition. Each cell is coated with a characteristic set of sugars that can be recognized by other cells. For example, the A and B antigens that determine a person’s blood type are cell surface polysaccharides.

Fatty Acids
Fatty acids are the main components of animal fats and plant oils. The main function of fats and oils is energy storage. Just like sugars, fats are broken down into carbon dioxide (CO2) when the cell needs energy. However, breakdown of fat yields a lot more energy than breakdown of sugar. Fat contains about 6-times as much energy – weight per weight- as glucose.

Fatty acids are also the major constituent of membranes. Membranes surround all living cells and are used to divide the interior of eukaryotic cells into specialized compartments.

To learn more about the chemical structure of these molecules,
click on the icons and articles below:

Nucleotides / DNA, RNA
(click on the image to enlarge)

Learn more: Nucleotide structure

Amino Acids / Proteins
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Learn more: Amino acid structure

Sugars / Polysaccharides
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Learn more: Sugar Structure

Fatty acids / Fats and membranes
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Learn more: Fatty Acid Structure