The elements involved in life processes can, and do, form millions ofdifferent compounds. Thankfully, these millions of compounds fall intofour major groups: carbohydrates, proteins, lipids, and nucleic acids.Though all of these groups are organized around carbon, each group hasits own special structure and function.
Carbohydrates
Carbohydrates are compounds that havecarbon, hydrogen, and oxygen atoms in a ratio of about 1:2:1. If you’restuck on an
SAT II
Biology question about whether a compound is acarbohydrate, just count up the atoms and see if they fit this ratio.Carbohydrates are often sugars, which provide energy for cellularprocesses.
Like all of the biologically importantclasses of compounds, carbohydrates can be monomers, dimers, orpolymers. The names of most carbohydrates end in “-ose”:
glucose, fructose, sucrose, and maltose are some common examples.
Monosaccharides
Carbohydrate monomers are known as monosaccharides. This group includes glucose, C6H12O6,which is a key substance in biochemistry. Sugars that an animal eatsare converted into glucose, which is then converted into energy to fuelthe animal’s activities by respiration (see Cell Processes).
Glucose has a cousin called fructose with the same chemical formula. But these two compounds have different structures:
Glucose and fructose differ in one importantway: glucose has a double-bonded oxygen on the top carbon, whilefructose has its double-bonded carbon on the second carbon. Thisdifference is most apparent when the two monosaccharides are in theirring forms. Glucose generally forms a hexagonal ring (six sided), whilefructose forms a pentagonal ring (five sided). Whereas fructose is thesugar most often found in fruits, glucose is most often used as themajor source of energy for cellular activities.
Disaccharides
Disaccharidesare carbohydratedimers. These dimers are formed from two monomers by dehydrationsynthesis. Any two monosaccharides can form a disaccharide. Forexample, maltose is formed by the dehydration synthesis of two glucosemolecules. Sucrose, common table sugar, comes from the linkage of onemolecule of glucose and one of fructose.
Polysaccharides
Polysaccharides can consist of as few asthree and as many as several thousand monosaccharides. Depending ontheir structure and the monosaccharides they contain, polysaccharidescan function as a means of storing excess energy or provide structuralsupport.
When cells ingest more carbohydrates thanthey need for fuel, they link the sugars together to formpolysaccharides. The structure of these polysaccharides is different inplants and animals: in plants, polysaccharides take the form of
starch, whereas in animals, they are linked in a structure called
glycogen.
Polysaccharides can also have structuralroles in plants and animals. Cellulose, which forms the cell walls ofplant cells, is a structural polysaccharide. In animals, thepolysaccharide chitin forms the hard outer armor of insects, crabs,spiders, and other arthropods. Many fungi also use chitin as astructural carbohydrate.
Proteins
More than half of the organic compounds incells are proteins, which play an important function in almost everycellular process. Proteins, for example, provide structural support tothe cell in the cytoskeleton and make up many of the hormones that sendmessages around the body.
Enzymes, which regulate chemical reactions in the cell, are also proteins.
Amino Acids
Proteins are made up of monomers calledamino acids. The names of many, but not all, amino acids end in -ine:methionine, lysine, serine, etc. Each amino acid consists of a centralcarbon atom attached to a set of three designated groups: an atom ofhydrogen (–H), an amino group (–NH2), and a carboxyl group (–COOH). The final group, designated (–R) in the diagram below, varies between different amino acids.
It is possible to make an infinite number of amino acids by attaching different compounds to the R position of the central carbon. However, only 20 types of R groups exist in nature, so there are only 20 naturally occurring amino acids.
Polypeptides
All proteins are made of chains of some orall of these 20 amino acids. The bond formed between two amino acids bydehydration synthesis is known as a
peptide bond.
A particular protein has a specific sequence of amino acids, which is known as its
primary structure.Every protein also winds, coils, and folds in three-dimensional spacein specific and predetermined ways, taking on a unique secondary(initial winding and coiling) and tertiary structure (overall folding).In harsh conditions, such as high temperature or extreme pH, proteinscan lose their normal tertiary shape and cease to function properly.When a protein unfolds in harsh conditions, it has been “denatured.”
Lipids
Lipids are carbon compounds that do notdissolve in water. They are distinguished from other macromolecules bycharacteristic
hydrocarbon chains—long strings of carbonmolecules with hydrogens attached. Such chains do not dissolve well inwater because they are nonpolar.
Triglycerides
Triglycerides consist of three long hydrocarbon chains known as
fatty acids attached to each other by a molecule called glycerol.
Because they include three fatty acids, fatsand oils are also known as triglycerides. As you might expect by thispoint, glycerol and each fatty acid chain are joined to each other bydehydration synthesis.
Some fats are saturated, while others areunsaturated. These terms refer to the presence or absence of doublebonds in the fatty acids of fats. Saturated fats have no double bonds,whereas unsaturated fats contain one or more such bonds. In general,plant fats are unsaturated and animal fats are saturated. Saturatedfats are generally solid at room temperature, while unsaturated fatsare typically liquid.
Phospholipids
Phospholipids, which are importantcomponents of cell membranes, consist of a glycerol molecule attachedto two fatty acid chains and one phosphate group (PO4–2):
Like all fats, the hydrocarbon tails ofphospholipids do not dissolve in water. However, phosphate groups dodissolve in water because they are polar. The different solubilities ofthe two ends of phospholipid molecules allow them to form the bilayersthat make up the cell membrane.
Steroids
Steroids are the primary structure inhormones, substances that play important signaling roles in the body.Structurally, steroids are made up of four fused carbon rings attachedto a hydrocarbon chain.
The linked rings indicate that each carbonatom is attached to other carbon atoms that form multiple loops.Cholesterol, the steroid in the image above, is the central steroidfrom which other steroids, such as the sex hormones, are synthesized.Cholesterol is only found in animal cells.
Nucleic Acids
Cells use a class of compounds callednucleic acids to store and use hereditary information. Individualnucleic acid monomers, known as
nucleotides, consist of three main units: a
nitrogenous base (a compound made with nitrogen), a phosphate group, and a sugar:
There are two main types of nucleotides, differentiated by their sugars:
deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA).DNA nucleotides have one less oxygen than RNA nucleotides. The “deoxy”in deoxyribonucleic acid refers to the missing oxygen molecule. Interms of function, DNA molecules store genetic information for thecell, while RNA molecules carry genetic messages from the DNA in thenucleus to the cytoplasm for use in protein synthesis and otherprocesses.
Within both DNA and RNA, there are furthersubdivisions of nucleotides by nitrogenous bases. For DNA, there arefour kinds of nitrogenous bases:
- adenine (A)
- guanine (G)
- cytosine (C)
- thymine (T)
The nitrogenous base of a nucleotideprovides it with its chemical identity, so the nucleotides are calledby the name of their nitrogenous base. RNA also has four nitrogenousbases. Three—adenine, guanine, and cytosine—are identical to thosefound in DNA. The fourth, uracil, replaces thymine.
DNA and RNA
In 1953, James Watson and Francis Crickpublished the discovery of the three-dimensional structure of DNA.Watson and Crick hypothesized that DNA nucleotides are organized into apolymer that looks like a ladder twisted into a coil. They called thisstructure the
double helix.
Two separate DNA polymers make up each sideof the ladder. The sugar and phosphate molecules of the DNA form thevertical supports, while the nitrogenous bases stick out to form therungs. The rungs attach to each other by hydrogen bonding.
The nitrogen bases attach to each otheraccording to two simple rules: adenine (A) pairs with thymine (T), andguanine (G) pairs with cytosine (C). The exclusivity of the attachmentsbetween nitrogen bases is known as
base pairing.
The rules of base pairing are frequently tested on the SAT II Biology. A test question might ask, “What is the
complementaryDNA strand to ‘CAT’?” Following the rules of DNA base pairing, you candeduce that the answer is “CAT.” (“DOG” is the wrong answer, smartguy.)
RNA Structure
Unlike the double-stranded DNA, RNA issingle stranded. It looks like a ladder cut down the middle. As youwill see when we discuss protein synthesis in the chapter on CellProcesses, this structure of RNA is very important to its functions asa messenger from the DNA in the nucleus to the cytoplasm.
| DNA | RNA |
| Bases | Adenine, guanine, cytosine, thymine | Adenine, guanine, cytosine, uracil |
| Structure | Double helix | Single helix |
| Function | Stores genetic material and passes it from generation to generation | Carries messages from the nucleus to the cytoplasm |
Summary of the Molecules of Life
| Proteins | Lipids | Nucleic Acids | Carbohydrates |
| Function | Structure, signaling, catalysis | Energy storage, signaling, membrane constituents | Store genetic material | Energy source, energy storage, structural |
| Monomer | Amino acid |
| Nucleotide | Monosaccharide |
| Polymer | Polypeptide, protein |
| RNA, DNA | Polysaccharide |
| Example | Insulin, transcriptase (an enzyme) | Corn oil | A chromosome | Glucose |
