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PinkMonkey Online Study Guide-Biology

2.1 Structural Organization and the Chemical Basis of Life

(A) Introduction

The universe and living bodies are composed of matter which occupies space and possesses mass. Matter can exist in four forms -solid, liquid, gaseous and plasma. Matter is made up of basic substances called elements. There are over a hundred elements recognized by scientists today. An element is made up of identicle atoms. Calcium, Carbon, hydrogen, iron, sodium, etc. are some elements.

i) Atoms

Each element is made up of one particular kind of atom. Atoms are the smallest part of an element which do not share the properties of the element. An atom consists of 3 basic particles:

a) Protons, which are positively charged and are present in the nucleus.

b) Electrons, which are negatively charged and which rotate around the nucleus.

c) Neutrons, which are present in the nucleus and which donít have any electricle charge. A neutron has approximately the same mass as a proton.


In the nucleus of the atom the proton and neutron are firmly attached to each other. The chemistry of the atom is dependent upon the number of protons and electrons in the atom. Protons and electrons are present in the same number in the atom, which leads to a neutral electrical charge in the atom. The nucleus of an atom is surrounded by shells or orbits of electrons. In the first orbit there are a maximum of 2 electrons. From the second shell to the last shell, every shell has a maximum of 8 electrons. When the final orbit of an atom is complete with electrons it becomes a stable atom. When there is a single electron or more missing in the last orbit of the atom, the atom becomes active allowing for a chemical reaction with another atom. During the reaction there is sharing or exchange of electrons. Atoms are most stable when they have a full outer shell. If they can, they will either share one or more electrons with another atom so that both atoms " think " they have full outer shells, or sometimes an atom that " needs " one or two electrons to have a full outer shell will actually take electrons another atom that has only one or two electrons in its outer shell; therefore, both atoms end up with full outer shells.

ii) Molecules

In biology we study molecules as part of molecular biology, molecular interaction, etc. Atoms combine chemically in a specific order to form molecules. For instance, two atoms of hydrogen combine with one atom of oxygen to form a single molecule of water. A molecule is the smallest particle of a substance existing freely yet retaining the characteristics of that substance. A collection of molecules forms a compound. Properties of the compound depend upon molecules and atoms present in the molecule. Consider this example: the molecular weight of the compound, water, is 18. This weight has been calculated taking into account the weight of its molecular components H and O.

H2O ®H + H + O ® 1 +1+ 16 ® 18

Some molecules are formed from atoms of the same element e.g. oxygen molecule (O2) is formed from 2 atoms of oxygen; ozone (O3) is formed from 3 atoms of oxygen. But, oxygen or ozone cannot be called a compound because in a compound we require atoms of different elements.

When the compound is formed it contains different elements. These elements stay together by means of links between them. In scientific terminology this link is called a bond. There are various kinds of bonds. In a compound, when one atom of an element gives away an electron, this exchange creates a bond between these two elements. This is an ionic bond. This bond consists of an electromagnetic force which is formed due to exchange of charge [electron]; the atoms conducting the exchange are called ions. This eletromagnetic force attracts two opposite charges: a positive charge in the atom which gives away electrons and a negative charge in the atom which takes up electrons. This attraction force forms the ionic bond.

A second important type of bond is called the covalent bond. Such a bond is formed when two atoms share one or more electrons with one another. For example (fig. 21), in water (H2O) 2 atoms of hydrogen share 2 electrons with oxygen. In ammonia 3 hydrogen atoms share one electron each with a single nitrogen atom. This leads to the formation of ammonia (NH3). Between atoms when one pair of electrons is shared, a single covalent bond is formed. When two pairs of electrons are shared a double covalent bond is formed.

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iii) Acids and Bases or Alkalis

If a compound reacts with water and releases hydrogen ions (H+) ions then this compound is called an acid, and is said to be acidic in nature. For example, when hydrogen sulphide is mixed with water it releases hydrogen ions and the solution becomes one of sulphuric acid. Other chemical compounds when dissolved in water attract hydrogen atoms. These substances are called bases or alkalis. For example, when sodium hydroxide (NaOH) is mixed with water, sodium hydroxide attracts hydrogen ions from H2O, and (OH-) ions remain. So these substances that remove (H+) from water act as bases or alkalis.

Table of Contents

2.0 - Introduction
2.1 Structural Organization and Chemical Basis Of Life
2.2 Organic Compounds
2.3 Nucleotides and Nucleic Acid
2.4 Lipids

Chapter 3





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