Fig. 1.6. Dna structure

A strong chemical bond forms between the phosphate group of one nucleotide and the deoxyribose sugar of another. These bonds are not easily broken and join neighboring nucleotide units into a permanent strand. Two of these strands become joined together by weaker hydrogen bonds formed between these bases. However this union is temporary in those hydrogen bonds and can be easily broken when it becomes necessary. Each base can join with only one of the other types of the base: A always bonds with T, and G always bonds with C. A-T and G-C is called base pairs. Each member of a pair is complementary to its partner. The pairs of nucleotides forming the DNA ladder can appear in any order. The sequence of the nucleotides is the code that controls the production of all the proteins of an organism.

The resultant double stranded molecule is DNA and its two strands are arranged like a twisted ladder or coil. This twisted coil is called a double helix. The sugar and phosphate groups make up the sides of the ladder. The bases make up the rungs.

Chemically, RNA is very similar to DNA, but there are two differences in its chemical groups. One difference is in the sugar component; instead of deoxyribose, RNA contains ribose, which has an additional oxygen atom. The other difference is in nitrogen base - instead of thymine, RNA contains the closely related uracil (Fig. 1.7). The third, and very important, difference between the two is that most RNA does not possess a regular helical structure and is usually single-stranded (Fig. 1.8). Unlike DNA which is found primarily in the nucleus, most of the RNA is found in the cytoplasm and it is here that protein manufacture takes place.

Three types of RNA are involved: messenger RNA, transfer RNA and ribosomal RNA.

Cell making large amounts of protein have numerous ribosomes, and ribosomes are rich in RNA.

phosphate ribose sugar

Base

Fig. 1.7. Structure of RNA nucleotide

Many molecules of adenosine triphosphate (ATP) are present in every living cell. ATP is composed of adenosine, ribose and three inorganic phosphate groups. ATP is able to immediately provide the energy required for muscle contraction whereas glucose, despite being an energy-rich compound, is unable to do so. ATP is also regenerated and used during photosynthesis.

Carbohydrates are molecules that supply much of the cell’s energy. The smallest carbohydrate molecules are sugars. Many sugars are joined together to form large starch molecules.

Cellulose is the principal structural polysaccharide of plants. It is formed within a plant cell and deposited outside of the cell membrane. Like starch and glycogen, cellulose is composed of units of glucose, but because of slight differences in molecular configuration, cellulose cannot be broken down by the enzymes that hydrolyze the storage polysaccharides.

Fig. 1.8. Structure of RNA

One of the earliest conclusions from biology as a modern experimental science was that even the tiniest microbe comes from others of the same type—that is, that life begets life. The conditions on primeval Earth that may have enabled life to arise from inanimate self-replicating chemicals no longer exist. Today all life comes from pre-existing life.

Summary and test questions

Current scientific evidence indicates that life as we know it cannot exist without water, and that life on Earth originated in the water of the planet’s primordial oceans. Earth began forming about 4.6 billion years ago, and the first signs of life are 3.8–4 billion years old.

Matter is composed of atoms. Each atom consists of a positively charged nucleus of protons and neutrons surrounded by electrons bearing negative charges. There are many elements in nature, but only a few of them make up the bulk of living systems. Isotopes of an element differ in their numbers of neutrons. Some isotopes are radioactive, emitting radiation as they decay. Electrons are distributed in shells consisting of orbitals. Each orbital contains a maximum of two electrons. In losing, gaining, or sharing electrons to become more stable, an atom can combine with other atoms to form molecules.

Covalent bonds are strong bonds formed when two atomic nuclei share one or more pairs of electrons. Covalent bonds have spatial orientations that give molecules three-dimensional shapes. Nonpolar covalent bonds are formed when the electronegativities of two atoms are approximately equal. When atoms with strong electronegativity (such as oxygen) bond to atoms with weaker electronegativity (such as hydrogen), a polar covalent bond is formed, in which one end is + and the other is –. Hydrogen bonds are weak electrical attractions that form between a + hydrogen atom in one molecule and a – nitrogen or oxygen atom in another molecule or in another part of a large molecule. Hydrogen bonds are abundant in water. Ions are electrically charged bodies that form when an atom gains or loses one or more electrons. Ionic bonds are electrical attractions between oppositely charged ions. Ionic bonds are strong in solids, but weaker when the ions are separated from one another in solution. Nonpolar molecules interact very little with polar molecules, including water. Nonpolar molecules are attracted to one another by very weak bonds called van der Waals forces.

Test questions

1. The most abundant molecule in the cell is

a. carbohydrate.

b. lipid.

c. nucleic acid.

d. protein.

e. water.

2. All lipids are

a. triglycerides.

b. polar.

c. hydrophilic.

d. polymers of fatty acids.

e. more soluble in nonpolar solvents than in water.

3. All carbohydrates

a. are polymers.

b. are simple sugars.

c. consist of one or more simple sugars.

d. are found in biological membranes.

e. are more soluble in nonpolar solvents than in water.

4. Which of the following is not a carbohydrate?

a. Glucose

b. Starch

c. Cellulose

d. Hemoglobin

e. Deoxyribose

5. All proteins

a. are enzymes.

b. consist of one or more polypeptides.

c. are amino acids.

d. have quaternary structures.

e. are more soluble in nonpolar solvents than in water.

6. Which of the following statements about the primary structure

of a protein is not true?

a. It may be branched.

b. It is determined by the structure of the corresponding DNA.

c. It is unique to that protein.

d. It determines the tertiary structure of the protein.

e. It is the sequence of amino acids in the protein.

7. The amino acid leucine

a. is found in all proteins.

b. cannot form peptide linkages.

c. is likely to appear in the part of a membrane protein that

lies within the phospholipid bilayer.

d. is likely to appear in the part of a membrane protein that

lies outside the phospholipid bilayer.

e. is identical to the amino acid lysine.

8. The quaternary structure of a protein

a. consists of four subunits—hence the name quaternary.

b. is unrelated to the function of the protein.

c. may be either alpha or beta.

d. depends on covalent bonding among the subunits.

e. depends on the primary structures of the subunits.

9. All nucleic acids

a. are polymers of nucleotides.

b. are polymers of amino acids.

c. are double-stranded.

d. are double-helical.

e. contain deoxyribose.

10. Which of the following statements about condensation

reactions is not true?

a. Protein synthesis results from them.

b. Polysaccharide synthesis results from them.

c. Nucleic acid synthesis results from them.

d. They consume water as a reactant.

e. Different condensation reactions produce different kinds of

macromolecules.

For Discussion

1. Phospholipids make up a major part of most biological membranes; cellulose is the major constituent of the cell walls of plants. How do the chemical structures and physical propertiesof phospholipids and cellulose relate to their functions in cells?

2. Suppose that, in a given protein, one lysine is replaced by aspartic acid. Does this change occur in the primary structure or in the secondary structure? How might it result in a change in tertiary structure? In quaternary structure?

3. If there are 20 different amino acids commonly found in proteins, how many different dipeptides are there? How many different tripeptides? How many different trinucleotides? How many different single-stranded RNAs composed of 200 nucleotides?

4. Contrast the following three structures, emphasizing the surfaces they present to their environment: hemoglobin; a DNA molecule; a protein that spans a biological membrane.

5. Why might RNA have preceded proteins in the evolution of biological macromolecules?