You have always learned that a living system can grow, sustain, and reproduce itself. The most remarkable thing about a living system is that it itself is composed of non-living atoms and molecules. The scientific study of what goes on chemically within a living system falls in the domain of biochemistry.
Living systems are composed of various complex biomolecules such as carbohydrates, proteins, nucleic acids, lipids, enzymes and many more.
Proteins and carbohydrates are extremely crucial constituents of the food that we eat every day. These biomolecules regularly interact with each other and constitute the molecular logic of life processes. Moreover, some simple molecules such as vitamins and inorganic salts also play important roles in the biological functions of organisms.
In this section, we will learn more about the fascinating chemistry of life, what are the living organisms made of and what are the key types of biomolecules.
What are living organisms chemically made of?
We’ve already discussed the diversity of living organisms on earth. Miraculously, all of them are found to be made up of the same elements and compounds.
Whether we chemically analyze a plant tissue, animal tissue, or microbial paste, we discover that they’re composed of elements such as carbon, hydrogen, oxygen and many more.
If we perform this same chemical analysis on a specimen of non-living matter, such as a piece of earth’s crust, then we get another similar list of elements.
On a strictly technical basis, we can’t really make out any significant differences between the two lists above. All the elements present in a sample of the earth’s crust are also found in a sample of living tissue. However, upon more meticulous examination, we discover that the relative abundance of carbon and hydrogen with respect to other elements is higher in any living tissue than in the earth’s crust.
|Element||% weight of the earth’s crust||% weight of the human body|
|Nitrogen (N)||Very little||3.3|
Analyzing the chemical composition of living cells
To discover the different kinds of organic compounds found in living organisms, we need to perform a chemical analysis of living tissues. After obtaining a sample of any living tissue, such as a slice of vegetable or a piece of liver, we grind it in trichloroacetic acid (CCl3COOH) using a mortar and a pestle to obtain a thick slurry.
After straining this slurry through a cheesecloth or cotton, we obtain two different fractions. One is called the filtrate or, more technically, the acid-soluble pool. The second fraction is known as the retentate or the acid-insoluble fraction. Scientists have discovered thousands of organic compounds in the acid-soluble pool.
When we apply analytical techniques to a freshly isolated organic compound, we get an idea about the molecular formula and the probable structure of the said compound. All the carbon compounds that we obtain from living tissues are collectively known as biomolecules.
However, you should know that living organisms have also got inorganic elements and compounds within them.
To prove that, we must perform a slightly different experiment that is destructive in nature. First, we weigh a small amount of a living tissue (let’s say a leaf or a piece of the liver; this is known as the wet weight) and dry it until all the water has evaporated. What’s left behind is the dry weight of the given sample.
We now proceed to fully burn the tissue sample so that all the carbon compounds are oxidized to their gaseous form (CO2 and water vapor) and are thus removed from the sample.
The remaining material, known as the ash, contains inorganic elements such as calcium, magnesium, sodium, potassium, and so on. We also find inorganic compounds like sulfates and phosphates in the acid-soluble fraction.
Thus, performing an elemental analysis gives us the elemental composition of living tissues in the form of hydrogen, oxygen, chlorine, carbon, and so on. On the other hand, carrying out an analysis for compounds gives us an idea of the kind of organic and inorganic constituents that are present in living tissues.
From a chemist’s point of view, you can identify functional groups such as aldehydes, ketones, aromatic compounds, etc. But as biologists, we classify these compounds like amino acids, nucleotide bases, fatty acids, and so on.
Biomacromolecules and micromolecules
All the compounds we find in the acid-soluble pool are found to have molecular weights ranging from 18 to about 800 Daltons. On the other hand, the acid-insoluble fraction has only four types of organic compounds – proteins, nucleic acids, polysaccharides and lipids. These classes of compounds, with the exception of lipids, happen to have molecular weights in the range of 10,000 Daltons and higher.
Because of this finding, scientists have classified biomolecules into two different types. The first type of biomolecules has molecular weights of less than one thousand Daltons; they are usually known as micromolecules or simply biomolecules. The second type of compound, found in the acid-insoluble fraction, is known as macromolecule or biomacromolecule.
All the molecules found in the insoluble fraction are polymeric in nature, with the notable exception of lipids. Naturally, a question subsequently arises in our minds – why do we find lipids in the acid-insoluble fraction or macromolecular fraction if their molecular weights don’t exceed 800 Daltons?
Compared to other compounds, lipids indeed have relatively low molecular weights. They’re present not only in pure form but are also found arranged into the structure of cellular components such as the plasma membrane and other membranes.
When you grind a tissue sample, you are disrupting the structure of the cell. As a result, the plasma membrane and other membranes get disintegrated and form water-insoluble vesicles.
These insoluble pieces of membrane get separated with the acid insoluble pool fragments in the form of vesicles and are subsequently found in the macromolecular fraction. Thus, we can say that lipids are not strictly macromolecules.
The acid soluble pool roughly represents the cytoplasmic composition, whereas the macromolecules from the cytoplasm and organelles form the acid insoluble fraction. Together, they represent the entire chemical composition of living organisms or tissues.
To sum up, if we happen to represent the chemical composition of a living tissue on the basis of the abundance of compounds and arrange them class-wise, we discover that water is the most abundant chemical compound in living organisms.
Primary and secondary metabolites
Most of the biomolecules we’ve discussed so far, such as amino acids, proteins and carbohydrates, are known as primary metabolites. We find them abundantly in animal cells.
However, when we analyze plant, fungal and microbial cells, we notice thousands of compounds other than primary metabolites such as alkaloids, flavonoids, rubber, essential oils, antibiotics, colored pigments, scents, gums, and spices. These compounds are known as secondary metabolites.
Primary metabolites have clearly identifiable functions and play well-known roles in normal physiological processes, while the role or functions of most of the secondary metabolites in host organisms are still not understood. However, many of these molecules such as rubber, drugs, and pigments are extensively used in human welfare.
Some secondary metabolites have ecological significance as well. For example, certain cyanobacterial secondary metabolites have toxic effects on living organisms. Many of these cyanotoxins have ecological roles as insecticides and herbicides.
|Type of secondary metabolite||Examples|
|Essential oils||Lemongrass oil|
|Polymeric substances||Rubber, gums, cellulose|
Primarily produced by plants, carbohydrates form a most extensive group of naturally occurring organic compounds. Some common examples of carbohydrates are cane sugar, glucose, and starch. They are primarily compounds of carbon, hydrogen and oxygen, and are also known as saccharides because their basic components are sugars.
Proteins are physically and functionally complex macromolecules that are heteropolymers of amino acids. Their very name, derived from the Greek word proteios, literally means “holding the first place” and signifies the importance of these molecules.
All lipids are composed of carbon, hydrogen, and a little oxygen. They are insoluble in water but readily soluble in organic solvents such as benzene, acetone, and ether. Like we discussed before, lipids aren’t polymeric substances but are assembled from smaller molecules by dehydration.
Lipids could be simple fatty acids or glycerol. Several lipids are composed of both fatty acids and glycerol, while some of them also contain phosphorus and a phosphorylated organic compound in their structure. Certain lipids possess more complex structures as well.
Nucleic acids are polynucleotides that, together with polysaccharides and polypeptides, form the true macromolecular fraction of any living tissue or cell. Along with proteins, they are the main constituents of chromosomes and are responsible for the transmission of characters from one generation to the next (a phenomenon known as heredity).
Life is possible by virtue of the coordination of various chemical reactions that constantly take place in living organisms, such as the digestion of food, absorption of necessary molecules, and production of energy. This process involves a sequence of reactions that take place within the body under very mild conditions. All of this occurs with the help of certain biocatalysts known as enzymes.