water is the most abundant molecule in any cell. The unique chemical properties of water enable it to act as a carrier for dissolved molecules inside and outside the cell, as a raw material in essential cell reactions, and as a lubricant between organs, tissues, and individual cells. Edit
- Carbon: Can bond with up to four other elements and forms the main structure of many organic molecules
- Hydrocarbon: Organic molecules containing only carbon and hydrogen which come in many sizes and shapes, including open-ended chains and closed rings
Air is about 20% oxygen and 78% nitrogen which is why many organic molecules contain these elements.
Living cells make and use a variety of organic molecules such as glucose (a sugar). The cells of plants and some other organisms manufacture glucose through the process of photosynthesis. Both plants and animals use glucose as a food from which they obtain energy.
The organic molecules contained in carbohydrates, lipids, poteins, and nucleic acids are large molecules, or macromolecules, composed of smaller subunits.
Sugars and starches always contain carbon, hydrogen, and oxygen and almost always in the same proportion: two atoms of hydrogen and one of oxygen for every carbon atom or CH2O. Since the formula for water is H2O scientists concluded that sugars and starches consist of carbons with water attached to them or carbohydrates (hydro means water). Carbohydrates provide short- or long-term energy storage for living organisms.
A carbohydrate molecule with three to seven carbon atoms and the corresponding number of hydrogen and oxygen atoms is called a monosaccharide, or simple sugar (sakkharon means sugar). Some examples are glucose, fructose, and galactose.
A disaccharide, or double sugar, is made up of two simple sugars. Examples are maltose, two linked glucose units, and sucrose, a linked glucose and fructose.
A polysaccharide is a complex carbohydrate consisting of many simple sugars linked together. Starch stores energy in plants while glycogen does the same thing in animals. Glycogen has a branching structure of glucose subunits which allows them to store more energy in a cell than starch does.
Plants produce a larger carbohydrate called cellulose which they use to build their cell walls. Cellulose is considered a structural molecule because it protects individual cells and provides support to the whole plant. As a polysaccharide made up of glucose units, cellulose also stores a great deal of energy. However, only a few bacterial species produce the digestive chemicals needed to break cellulose down into glucose units and release energy. So to obtain nourishment from cellulose-rich plant materials animals such as cattle, rabbits, and termites must host these bacteria in their guts. The human gut does not host these bacteria, so the food energy in cellulose is not directly available to them.
Lipids are a group of macromolecules that do not dissolve in water. Living organisms use lipids for long-term nutrient and energy storage, insulation, cushioning of internal organs, and as hormones to send messages around the body. They are also the main component forming each cell membrane.
Fats include substances like butter and oils. Whether in solid or liquid form, one gram of fat contains 2.25 times as much energy as one gram of carbohydrate.
All fat molecules have a backbone of the alcohol glycerol and three fatty acid molecules which are hydrocarbon chains with an acidic CO2H group on one end. This acidic group attaches to one of three reaction sites on the glycerol molecule. This structure is called a triglyceride and is non-polar which is why it is insoluble in water.
Glycerol always has the same composition but fatty acids can be of varying lengths and can be saturated or unsaturated. A saturated fatty acid has four atoms bonded to each of its carbon atoms. An unsaturated fatty acid has double bonds where hydrogen atoms could be attached.
If a fat is mostly unsaturated the fat will likely be liquid at room temperature and if they are mostly saturated the fat will likely be solid at room temperature.
Most cellular structures are made of various types of protein which also serve many other functions in cells. Hair and fingernails are made of the same type of protein, keratin, each with distinctive properties. The bones and muscles in hands as well as the ligaments and tendons connecting them contain different kinds of proteins.
In additon to structural functions proteins also:
- Function as enzymes to facilitate chemical reactions
- Help transport substances across cell membranes or to different parts of an organism
- Act as chemical messengers (some hormones are proteins rather than lipids)
Like other macromolecules, proteins are assembled from small units, in this case amino acid molecules. An amino acid is made of an amine group (NH2) and a carboxyl group (CO2H) with a side chain or remainder group (R group) that determines the type of amino acid.
A chemical linkage called a peptide bond join amino acids together between the amine and carboxyl groups to form a polypeptide chain. Because of the chemical interactions between amino acids and their environment the polypeptide folds into a pleated sheat or coils into a helix and forms a protein. The structure of the protein determines its properties and function.
If a protein is exposed to extreme temperatures, pH conditions, or harsh chemicals, it will unfold or change shape becoming denatured losing its ability to perform its normal function.
Whether a protein is soluble in water depends on how it folds. When R groups that can interact with water are on the outside of the protein it will be soluble in water and conversely if the parts that cannot interact with water end up on the outside it will not be soluble in water.
Humans need 20 amino acids, known as common amino acids, to make proteins. However they can only synthesize 12 of these and must obtain the other 8 pre-formed in food. These 8 amino acids are called the essential amino acids.
Nucleic acids direct the growth and development of every living thing by means of a chemical code. Cells contain two types of nucleic acid: ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Nucleic acids contain hereditary instructions for building proteins.
Nucleic acids consist of long chains of linked subunits called nucleotides. DNA consists of four nucleotides: adenine, cytosine, guanine, and thymine while RNA uses uracil in place of thymine. A nucleotide is composed of a pentose sugar and phosphate backbone attached to a nitrogen-containing base.
RNA is a single-stranded chain of nucleotides while DNA is a double-stranded coiled ladder-like structure (double helix).