Macromolecules

Up to this point we have considered only small molecules. Many of the molecules important to biological processes are HUGE. These are known as macromolecules. Most macromolecules are polymers, which are long chains of subunits called monomers. These subunits are often very similar to each other, and for all the diversity of polymers (and living things in general) there are only about 40 - 50 common monomers.

Making and breaking polymers

Joining two monomers is achieved by a process known as dehydration synthesis. One monomer gives up a hydroxyl (OH) group and one gives up a (H). These combine to make a water molecule. Hence the name dehydration synthesis.

Polymers are broken apart by a process known as hydrolysis. Bonds between monomers are broken by the addition of water. (3.3, pg 36)

There are four major categories of organic compounds found in living cells.

Carbohydrates

Carbohydrates are the sugars and their polymers. Simple sugars are called monosaccharides. These can be joined to form polysaccharides (3.5, pg 38). Glucose is an important monosaccharide. Sucrose, a disaccharide (consisting of two monosaccharides), is table sugar. (Note the ending "ose" common to most sugars.)

Polysaccharides may be made from thousands of simple sugars linked together. These large molecules may be used for storage of energy or for structure. First a couple of storage examples:

Starch is a storage polysaccharide of plants. Its is a giant string of glucoses. The plant can utilize the energy in starch by first hydrolyzing it, making the glucose available. Most animals can also hydrolyze starch. That's why we eat it.

Animals store glycogen as a supply of glucose. It is stored in the liver and muscles. (3.7, pg 39)

And some examples of structural carbohydrates:

Cellulose is a polysaccharide produced by plants. Its is a component of the cell walls. Cellulose is also a string of glucose molecules. Because the glucoses are joined together differently cellulose has a different shape, and therefor different properties, than starch or glycogen. The enzymes (we'll learn more about these soon) that are used to hydrolyze starch don't work on cellulose. Most organisms cannot digest cellulose and it passes right through them (roughage). Goats and termites don't really digest cellulose, they have bacteria that do it for them.

Chitin is an important polysaccharide used to make the exoskeletons of arthropods.

Lipids

Lipids are all similar in that they are (at least in part) hydrophobic. There are three important families of lipids: fats, phospholipids and steroids.

Fats

Fats are large molecules made of two types of molecules, glycerol and some type of fatty acid. The fatty acid has a long chain of carbon and hydrogen, usually referred to as the hydrocarbon tail, with a carboxyl group head. (The carboxyl group is why its called an acid). Glycerol has three carbons (3.8b, pg 40) so it can get three fatty acids. These can be the same three or different. This arrangement of three is why fats are called triglycerides.

Fats may be saturated or unsaturated. This has to do with the amount of hydrogen in the tail. Unsaturated fatty acids have some hydrogen missing, with double bonds replacing them. The double bond give the fatty acid a kink (3.8c, pg 40). Saturated fats are solid at room temperature and come from animals, unsaturated fats come from plants and are liquid at room temperature.

Fats are used as a high density energy storage in animals and in plants (seeds). It may also be used in animals for insulation.

Phospholipids

Phospholipids are like fats but they have two fatty acids and a phosphate group joined to glycerol. The fatty acid tails are hydrophobic but the phosphate part is hydrophilic. This is an important feature of these molecules.

More about phospholipids when we cover membrane structure.

Steroids

Steroids are also lipids but they have a carbon skeleton of four connected rings (no glycerol here) (3.9, pg 41). The different properties of different steroids are due to the attached functional groups. Cholesterol is a steroid that can be modified to form many hormones.

Proteins

Proteins are extremely important. They are large, complex molecules that are used for structural support, storage, to transport substances, and as enzymes. They are a sophisticated, diverse group of molecules, and yet they are all polymers of just 20 amino acids.

Amino acids have a carbon attached to a hydrogen, an amino group, a carboxyl group and something else (R). Its the something else that give the amino acid its characteristics (3.12a&b, pg 42).

Amino acids are joined together by peptide bonds (dehydration synthesis) (3.13, pg 43). Polypeptide chains are strings of amino acids, joined by peptide bonds.

Proteins are formed by twisting up one or more poly peptide chains. It is the shape, or conformation, of the protein that gives it its properties. There are four levels of protein structure.

Primary structure is the unique series of amino acids. The secondary structure results from hydrogen bonds along the chain which cause repeated coiled or folded patterns. The tertiary structure is superimposed on the secondary structure. It is the irregular contortions formed by bonding between the R groups. Some R groups of amino acids have sulfhydryl groups which bond together to for disulfide bridges. Quaternary structure results when the protein is made up of more than one polypeptide subunits (for example hemoglobin, which has four polypeptide subunits). Quaternary structure is the relationship of these subunits. (Figure on pg 45 for summary) When a protein's structure has been altered we say it has been denatured. Denaturing occurs when the hydrogen bonds that are holding parts of the molecule to other parts come apart. Usually as a result of exposure to extremes of pH or heat. Some denaturing is reversible some is irreversible. Cooking eggs denatures the proteins in the egg whites. They cannot be uncooked. A high fever can denature proteins (enzymes) in the human body which can be fatal.

Nucleic Acids

Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are polymers of nucleotides (3.20a, pg 47). Later we will learn in more detail the roles these nucleic acids play in protein synthesis.

Nucleotides are made of three parts: a phosphate, a pentose sugar, and a nitrogenous base. The pentose sugar of DNA is deoxyribose. The pentose sugar of RNA is ribose.

CELLS

All organisms are made of cells. Subcellular structures are called organelles. Cytology is the study of cell structure. The cell's "anatomy" is referred to as its ultrastructure.

There are two types of cells: prokaryotic cells and eukaryotic cells. Four of the five kingdoms, protists, plants, fungi, and animals are made up of eukaryotic cells. The other kingdom, Monera (bacteria & cyanobacteria), consists of prokaryotic cells. Prokaryotic cells have no true nucleus. They have genetic material (DNA) but it is in a nucleoid region. The eukaryotes have their DNA in a nucleus which is enclosed by a membranous nuclear envelope. The nucleus of the eukaryotes is surrounded in the cell by the cytoplasm. The organelles are located in the cytoplasm. Many of the organelles that are found in eukaryotes are not found in prokaryotes.

Cells are usually very small. The size of the smallest of cells is constrained by the minimum amount of genetic material need to keep the cell going. At the large end, cell size is constrained by the passage of materials through the plasma membrane. All cells are enclosed in a plasma membrane and it is through this membrane that all the nutrients and wastes must pass. As a three dimensional object grows in size its surface area does not keep up with is volume. Thus cells reach a limit to their maximum size. The partitioning of various cellular functions into other membrane enclosed structures allows for larger cells. This is why eukaryotic cells are usually larger than prokaryotic cells. Another factor that limits the size of cells is that the cell must be controlled by the nucleus. You should look at section 4.2 on varieties of cell sizes.