Constituents of the Living Body (With Diagram)

The following points highlight the six important constituents of the living body. The constituents are: 1. Matter 2. Elements 3. Atoms, Molecules and Energy 4. Chemical Reactions 5. Enzymes 6. Mixtures and Compounds.

Constituent # 1. Matter:

It is defined as something which occupies space and has some weight. Matter may be formed of one kind of substance or of differ­ent kinds of substances. Matter may exist in any one of the three states—solid, liquid and gaseous.

A matter may change from one state to the other. When this happens due to tem­perature and pressure, it is called a physical change, which does not alter the basic char­acteristics of the matter. A second type of change may alter the fundamental character­istics of the matter which is called chemical change.

Constituent # 2. Elements:

When matter is made up of same kind of sub­stances it is called an element. An element cannot be further broken down by chemical means. More than hundred elements are known to occur and 25% of them are present in the living body.

Constituent # 3. Atoms, Molecules and Energy:

Each element is made up of a particular kind of atoms. Atom is defined as the smallest part of an element which participates into the chemical combinations with other elements. Atom consists of positively charged nucleus in the centre and one or more negatively charged electrons around it.

The nucleus, which constitutes the mass of the atom, is composed of positively charged proton and electrically neutral neutron. The electrons are in constant motion along a definite orbit around the nucleus. There is a limit to the number of electrons which may be present in a particular shell. This is expressed by the for­mula—maximum number = 2 x n². Here, ‘n’ represents the number of shells in the atom.

It means that if the first shell contains 2 electrons, second shell may hold 8 electrons, third shell may retain 18, fourth shell 32. But according to another rule the outermost shell cannot possess more than eight electrons.

This means that in an atom having four shells, the outer one though able to hold 32 electrons will not have more than 8. Thus, atoms which have the full number of electrons are called stable atoms, e.g., helium, neon, argon.

On the con­trary, the atoms which do not have the com­plete set of electrons in the outer shell com­bine with other atoms by gaining or losing electrons and thus retain stability (Fig. 3.2).

The atoms which lose electrons are called electro-positive and the atoms which gain electrons are electro-negative. This tendency of atoms to gain stability is the basis of all chemical reactions in which atoms of differ­ent elements unite with each other (Fig. 3.3).

Such union of two or more atoms results in the formation of molecule. Molecules are defined as the smallest part of a substance which can remain independently and exhibit the properties of the original substance.

The attachment of atoms needs energy which comes from external source. Thus within a molecule, the atoms retain some form of en­ergy as the binding force. When in a chemical reaction this bond breaks, energy is released.

The binding capacity varies in different atoms and is fixed for each kind of atom. Thus hydrogen has one bond and oxygen, nitrogen, carbon possess 2, 3, 4 bonds respectively. The nature of a molecule depends upon the number, type and the spatial arrangement of the atoms.

Some very large molecules may contain thousands of atoms. Some atom may be present in two different molecules but in different arrangements. Such molecules which have same number and kind of atoms but with different arrangements are called iso­mers (as seen in the case of monosaccharides).

Constituent # 4. Chemical Reactions:

All compounds are formed either by the un­ion of atoms or of molecules. When atoms of one kind unite to form a compound, they re­move the outer electrons from other atoms. This gain in one and loss in other affect the electronic charge in them and they are held together by opposite charges.

These charged atoms are called—ions (Fig. 3.4) and they are held together by ionic bonds. The compounds produced in this way are called ionic com­pounds. When molecules of the ionic com­pounds are dissolved in water and are disso­ciated from one another they still retain the properties of the substance.

When two or more molecules are mixed, they interact in such a way that a different kind of substance is formed. This interaction of molecules is called chemical reaction. Four types of chemical reactions are known.

A. Exchange reaction:

Atoms interchange their position in the contact of two molecules.

H—CI + Na—O—H = Na—CI + H—O—H

B. Synthesis reaction:

Several molecules unite together to form a single molecule.

CO₂ + H₂O = H₂CO₃

C. Decomposition reaction:

This is just opposite to synthesis reaction. Here one mol­ecule breaks up into smaller molecules.

H₂CO₃ = CO₂ + H₂O

D. Reversible reaction:

In this type of re­action, both the synthesis and decomposition continue simultaneously.

For example, when Hydronium ion (H₃O) and bicarbonate (HCO₃) are kept together in a test tube, they react in the following way:

H₃O + HCO₃ = H₂O + H₂CO₃

hydronium + bicarbonate = water + carbonic acid

As H₂CO₃ decomposes into CO₂ + H₂O, the carbon dioxide escapes in the air.

But in a closed chamber, this CO₂ reacts with water to form H₂CO₃ (carbonic acid), which in turn again combines with water to form Hydronium ion and bicarbonate ion. Thus, reactions are taking place in two direc­tions and it ultimately reaches an equilibrium. Such reactions are indicated by double ar­rows.

Thus we can write the formula in the following way:

In some of these reactions where new mol­ecules are formed, energy is stored; in others during breakdown of the molecules, energy is released. The reaction where energy is stored is called endergonic or energy regain­ing, whereas when energy is released it is called exergonic or energy-yielding reaction.

All these chemical reactions depend upon cer­tain factors—temperature, pressure and con­centration of reacting molecules. Within a test tube all chemical reactions are activated by heat energy.

There are certain substances called catalysts, which reduce the amount of activation energy needed. Living body is provided with a vast array of catalysts, called enzymes. These enzymes help all chemical reactions to occur quickly and at safe tempera­ture (Fig. 3.5).

Constituent # 5. Enzymes:

Enzymes are proteins and they act as cata­lytic agents. They are found only in the living body. They speed up different chemical reac­tions but remain unchanged at the end of re­actions. The word enzyme was first coined by Kuhne in 1880. Uptil now more than 700 en­zymes are known from different living sys­tems. All enzymes are not seen in all living bodies.

On the contrary, there exists a defi­nite enzymatic pattern in relation to the ana­tomical plan of each species in response to its particular adaptation. Enzymes may be named on the basis of their substrates (sub­stance on which enzyme acts).

For example, the enzymes which split proteins are called proteolytic enzymes; which break down carbo­hydrates are called amylolytic and which take part in the breakdown of fat are called lipoly­tic enzymes.

Enzymes are again named accord­ing to the nature of the compound on which they are effective. For example, enzymes which act on proteins are called proteases; which act on nucleic acids are called nuclease, etc.

Enzymes are specific in their action, i.e., one enzyme is capable of taking part in one chemical reaction only. Enzymes act with a great speed and a single enzyme molecule may be used repeatedly. A minute amount of enzyme can break down a very large quan­tity of substrate.

Constituent # 6. Mixtures and Compounds:

A mixture is formed by two or more sub­stances where each substance retains its property. Compounds are also formed by the un­ion of two or more substances but the prop­erties of the compound become different from those of its constituents. Within a mixture the properties of the constituent may vary but in the compound it is always the same.

Follow­ing mixtures are important from the point of view of biology:

A. Solution:

It is the homogeneous mix­ture at molecular level of two or more sub­stances having different molecular orientations. Generally, it is formed by the mixture of a solid or gas in a liquid. The liq­uid is called the solvent and the substance which is dissolved is known as the solute. Water is the major solvent in living body.

B. Suspension:

It is a mixture which con­sists of very minute solid particles scattered in a liquid dispersion medium. A mixture of sand and water is a perfect example of sus­pension. The constituents of a suspension separate by the action of gravity.

C. Emulsion:

It is a mixture of a liquid in a liquid. Here the two liquids remain separate in minute droplets. Oil in mixture with water forms emulsion. Two such substances are al­ways intimately united. But this may be aug­mented with the addition of an emulsifier. Within the body the bile salts act as emulsify­ing agent.

D. Colloids:

The colloid is the mixture of particles which remain larger than in solutions but smaller than in suspension. The particles in a colloid never precipitate down at the bot­tom as in case of suspension, e.g., milk (col­loid of fat and protein in water), fog (colloid of water in air). The particles may be seen under microscope but cannot be filtered through ordinary filter paper.

The protoplasm is partly colloid and partly suspension. Many substances remain dis­solved in the watery medium of protoplasm which also contains numerous insoluble ma­terials in colloidal state. The protoplasm is thus a colloid of both solid and liquid in a liquid medium.

The colloidal consistency of proto­plasm exhibits following features:

1. Brownian Movement:

Gravitational force always tends the colloidal particles in the protoplasm to settle down, but finer par­ticles in the suspension, which are always under constant thermal agitation kick them. This counteracts gravitational force and pro­duces a rotating motion called Brownian movement.

2. Similar Electric Charges:

The dispersed colloidal particles possess similar electrical charges which repel each other. This together with Brownian movement causes the colloi­dal particles to remain in dispersed state.

3. Sol-Gel Conversion:

The cellular col­loids have the ability to change from a solid to liquid phase. When water is withdrawn, the particles stay together and form a stiff gel phase. Immediately with the coming of wa­ter in between the particles they disperse and the fluid sol phase appears.

4. Formation of Membrane:

The colloids within living body always tend to form a membrane, which separates inner part from the outer environment. The membrane thus makes the interphase by tight packing of molecules. When broken due to injury it is soon repaired before the inner contents flow out.

The membrane regulates the flow of dif­ferent materials. It has been noticed that though many small molecules are not permit­ted yet several large molecules are allowed to enter. Such selective filtration involves in one hand physical processes like osmosis and dif­fusion and on the other hand complicated chemical reactions.