Four main types of organic molecules predominate in living organisms:
Most complex carbohydrates are made of repeating units called sugars. Most simple sugars (monosaccharides) conform to the basic formula (CH2O)n and possess an aldehyde or ketone functional group. Monosaccharides (the simplest carbohydrates) can:
- provide ready energy,
- be converted to other types of organic molecules,
- be used as monomers for polymers (macromolecules).
Examples: glucose, fructose, galactose
Disaccharides consist of two monosaccharide monomers covalently linked by a glycosidic bond. They function in sugar transport.
- maltose (2 glucoses),
- lactose (glucose+galactose),
- sucrose (glucose+fructose)
Polysaccharides serve as storage or structural molecules. Many common polysaccharides are made of the monosaccharide glucose.
- structural polysaccharides: cellulose, chitin
- storage polysaccharides: glycogen (animal starch); amylose (plant starch)
Glucose like other sugars can exist in a straight chain form or it can loop back on itself and become a ring. If the hydroxyl group on the first carbon atom points down when the ring is formed alpha glucose results and if the hydroxyl group points up the result is beta glucose.
When glucose molecules are enzymatically joined to form a disaccharide a water molecule is removed. This process is called dehydration synthesis. The covalent bond that holds sugar molecules together is called a glycosidic linkage.
When glucose molecules are combined in a straight chain by alpha 1-4 linkages the resulting polysaccharide is the edible, somewhat soluble starch called amylose. If the beta form of glucose is used instead, the resulting polysaccharide is an insoluble, indigestible, and tough fiber called cellulose.
The polymer strands of cellulose can hydrogen bonds in groups of 60 or 70 to produce microfibrils, which in turn can join with other microfibrils to make strong cord-like fibers. Plant cell walls are made of crisscross layers of these fibers.
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Lipids are hydrocarbons insoluble in polar solvents.
Lipids constitute a heterogeneous group of hydrophobic molecules that include the neutral fats or triglycerides, the steroids, and the phospholipids.
Lipids serve as energy-storage molecules, as major components of cell membranes, and as hormones. Learn more about lipids with Professor Thomas M. Terry.
Fats or triglycerides are formed by three fatty acids each bonded by an ester linkage to glycerol.
Fats and oils contain a higher proportion of energy-rich carbon-hydrogen bonds than carbohydrates or proteins. Many seeds are rich in oils. Why?
Saturated fatty acids have the maximum number of hydrogen atoms because of single bonding between all the carbons. Unsaturated and polyunsaturated fatty acids (present in oils) have one or more double bonds between the carbons.
Fats are ideal for energy storage requiring only half the mass of glycogen. Fats are also important in cushioning delicate organs like kidneys against shock and for insulation.
Waxes are a form of structural lipid. They form protective coatings on skin, fur, feathers, on the leaves of land plants, and on the exoskeletons of many insects.
Phospholipids substitute the third fatty acid of a triglyceride with a negatively charged phosphate group, which may be joined to another small molecule. Phospholipids may have a hydrophilic and a hydrophobic end making them ideally suited for construction of cell membranes.
Steroids, such as cholesterol and the sex hormones, are classified as lipids. These lipids are characterized by a carbon skeleton consisting of four interconnected rings. Steroids often have a hydroxyl functional group.
Proteins consist of one or more chains of amino acids linked by peptide bonds. These chains are known as polypeptides. Proteins are the most complex and versatile macromolecules.
Each amino acid contains a central carbon singly bonded to four different groups: a hydrogen atom, an amino group, a carboxyl group, and some other chemical group which confers on it unique properties. Because of these four groups amino acids may form either "D" or "L" optical isomers.
Only the "L" optical isomer of an amino acid is used by living matter.
For more concerning amino acids click on this line.
Proteins exhibit three or four levels of structural organization. Primary structure is the first level and is determined by a unique linear sequence of amino acids.
Secondary structure of proteins describes how the primary structure is folded into particular, localized configurations, the alpha helix and the beta pleated sheet, which result from hydrogen bonding.
Tertiary structure describes the additional, less regular contortions of the molecule caused by the side groups in hydrophobic interactions, hydrogen bonds, and disulfide linkages. In many proteins, the tertiary structure produces an intricately folded, globular shape.
Quaternary structure describes how two or more polypeptide chains interact to form a functional structure. Click here for more on proteins
Just how proteins get their shape is a topic for Kimball's Biology Pages
Visit MIT to learn how proteins are sequenced.
Proteins are generally classified as either fibrous or globular. Proteins may also be characterized by their function. Examples of types of proteins include:
- Structural proteins - collagen, silk, microtubules
- Regulatory proteins (hormones) - insulin, growth hormones
- Contractile proteins - Actin, myosin, dynein
- Transport proteins - hemoglobin, myoglobin
- Storage proteins - egg white, seed protein
- Protective proteins - antibodies
- Membrane proteins - membrane-transport, channels
- Enzymes - most proteins ending in -ase
The function of a protein is an emergent property of its conformation, which is sensitive to conditions such as pH, salt concentration, and temperature. If these conditions exceed certain limits the protein's shape may be altered or denatured rendering it biologically inactive. Here's where to find out more about What Proteins are For.
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Nucleic acids are polymers of nucleotides, complex monomers consisting of a pentose (five carbon sugar) covalently bonded to a phosphate group and to one of five different kinds of nitrogenous bases.
DNA and RNA are the only two nucleic acids found in living matter. These large polymers are formed when the pentose of one nucleotide joins to the phosphate of another forming a sugar-phosphate backbone from which the nitrogenous bases project.
The five nitrogenous bases are members of two families, the purines (A and G) and the pyrimidines (C, T, and U).
Turn to this page to learn more about DNA.
Modified July 8, 2005