Biomolecules– Class 11 NEET Free Complete High Quality Notes

1. Introduction to Biomolecules

Definition of Biomolecules

Biomolecules are naturally occurring chemical compounds that form the basis of life. They are present in all living organisms and are essential for carrying out various biological functions. These molecules are mainly composed of carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Biomolecules are responsible for building the structure of cells, performing metabolic processes, storing genetic information, and transmitting signals. Simply put, biomolecules are the molecules that make life possible.

There are four major types of biomolecules: carbohydrates, proteins, lipids, and nucleic acids. Each of these biomolecules plays a vital role in the maintenance and development of life. Carbohydrates provide energy, proteins perform structural and functional roles, lipids serve as energy storage and components of cell membranes, and nucleic acids store and transfer genetic information.

Biomolecules can be large or small. Some are simple and consist of only a few atoms, while others are large macromolecules made up of many smaller subunits. For example, a simple sugar like glucose is a small biomolecule, while DNA is a complex and large biomolecule composed of many nucleotides.

In our biosphere, there is a huge diversity of living organisms, and a natural question arises: Are all these living beings made of the same elements and compounds? From chemistry, we know about elemental analysis, which can tell us what chemical elements are present in a substance. If we do such an analysis on plant tissues, animal tissues, or microbial material, we get a list of elements such as carbon, hydrogen, oxygen, and others, along with their amount per unit mass of the living tissue. Interestingly, if we perform the same analysis on a piece of earth’s crust, which is an example of non-living matter, we also get a list of elements that looks quite similar. So, are there any differences between living and non-living matter? At first glance, it seems there are no major differences, because all the elements present in the earth’s crust are also found in living tissues. However, when we look more carefully, we notice an important difference: the relative amount of carbon and hydrogen is much higher in living organisms compared to the earth’s crust. This shows that while the types of elements may be the same in both living and non-living matter, their proportions are different, which plays a key role in the structure and function of living beings.

2. Analysis of Chemical Composition – Methods and Techniques

To understand what organic compounds are present in living organisms, scientists perform chemical analysis. For example, a small piece of living tissue—like a vegetable or liver—can be ground in trichloroacetic acid (Cl₃CCOOH) using a mortar and pestle, forming a thick slurry. When this slurry is strained, it separates into two parts: the filtrate (also called the acid-soluble pool) and the retentate (or acid-insoluble fraction). Scientists have found thousands of organic compounds in the acid-soluble pool. In advanced studies, one learns how to extract, separate, and purify a particular organic compound from a tissue, which then allows scientists to determine its molecular formula and probable structure. All these carbon-containing compounds from living tissues are called biomolecules.

Besides organic compounds, living tissues also contain inorganic elements and compounds. To identify these, a slightly different experiment is done. A tissue is weighed (wet weight) and then dried to remove water, giving the dry weight. If the tissue is completely burnt, all the carbon compounds are converted into CO₂ and water vapor and removed. The remaining material, called ash, contains inorganic elements like calcium, magnesium, and compounds like sulfates and phosphates. Therefore, elemental analysis gives the composition of elements (like hydrogen, oxygen, carbon, chlorine) in living tissue, while compound analysis shows the types of organic and inorganic constituents. From a chemistry perspective, scientists identify functional groups such as aldehydes, ketones, and aromatic compounds, but from a biological perspective, these are classified into amino acids, nucleotide bases, fatty acids, and others.

Amino acids are organic compounds containing an amino group (-NH₂) and an acidic carboxyl group (-COOH) attached to the same carbon (α-carbon), which is why they are called α-amino acids. Each α-carbon is attached to four substituents: hydrogen, carboxyl group, amino group, and a variable group (R group). There are many amino acids depending on the R group, but in proteins, only 20 amino acids are common. For example, glycine has hydrogen as R, alanine has a methyl group, and serine has a hydroxy-methyl group. The chemical and physical properties of amino acids mainly depend on their amino, carboxyl, and R groups. Based on the number and type of functional groups, amino acids can be acidic (like glutamic acid), basic (like lysine), or neutral (like valine). Some are aromatic, such as tyrosine, phenylalanine, and tryptophan. A unique property of amino acids is that their –NH₂ and –COOH groups can ionize, meaning that their structure changes with pH.

Lipids are a group of organic compounds that are generally insoluble in water. One type of lipid is fatty acids, which have a carboxyl group (-COOH) attached to a variable R group. The R group can be a methyl group (-CH₃), an ethyl group (-C₂H₅), or a longer chain of –CH₂– units, ranging from 1 carbon to 19 carbons. For example, palmitic acid has 16 carbons including the carboxyl carbon, and arachidonic acid has 20 carbons. Fatty acids can be saturated (without double bonds) or unsaturated (with one or more C=C double bonds). Another simple lipid is glycerol, which is trihydroxy propane. Many lipids are combinations of glycerol and fatty acids, where fatty acids are esterified with glycerol to form monoglycerides, diglycerides, and triglycerides. These are commonly called fats and oils, depending on their melting points. Oils have a lower melting point, like gingelly oil, and remain liquid in winters. Some lipids contain phosphorus and a phosphorylated organic compound, called phospholipids, which are important components of the cell membrane, with lecithin as a well-known example. Certain tissues, especially neural tissues, contain lipids with more complex structures.

Besides lipids, living organisms also contain carbon compounds with heterocyclic rings, such as nitrogen bases like adenine, guanine, cytosine, uracil, and thymine. When a nitrogen base is attached to a sugar, it is called a nucleoside. If a phosphate group is also attached to the sugar, it becomes a nucleotide. Examples of nucleosides are adenosine, guanosine, thymidine, uridine, and cytidine, and examples of nucleotides are adenylic acid, guanylic acid, thymidylic acid, uridylic acid, and cytidylic acid. Nucleic acids like DNA and RNA are made entirely of nucleotides. Their main function is to serve as genetic material, storing and transmitting hereditary information in living organisms.

3. Primary and Secondary Metabolites – Types and Functions

One of the most exciting areas of chemistry is the study of biomolecules from living organisms. Scientists can isolate thousands of compounds, both small and large, determine their structure, and sometimes even synthesize them in the laboratory. If we make a list of biomolecules, it would include thousands of organic compounds such as amino acids, sugars, and many others. These compounds are often referred to as metabolites because they are involved in the metabolic processes of organisms. In animal tissues, we mainly find compounds that serve essential functions and are part of normal physiological processes; these are called primary metabolites. However, when scientists analyze plants, fungi, and microbes, they find thousands of other compounds beyond primary metabolites. Examples include alkaloids, flavonoids, rubber, essential oils, antibiotics, coloured pigments, scents, gums, and spices. These are known as secondary metabolites.

The difference between primary and secondary metabolites is that primary metabolites have clear functions and roles in growth, development, and reproduction, whereas the functions of many secondary metabolites in the host organism are not fully understood. Despite this, many secondary metabolites are extremely useful for humans, such as rubber, medicines, spices, fragrances, and pigments. Additionally, some secondary metabolites play important ecological roles, helping organisms survive and interact with their environment. As you progress in biology, you will learn more about their specific functions and applications.

4. Biomacromolecules – Structure and Functions

All the compounds found in the acid-soluble pool of living tissues share one common feature: they have molecular weights ranging from about 18 to 800 daltons (Da). On the other hand, the acid-insoluble fraction contains only four main types of organic compounds: proteins, nucleic acids, polysaccharides, and lipids. Except for lipids, these compounds have very high molecular weights, often in the range of tens of thousands of daltons. Because of this difference, biomolecules in living organisms are classified into two types. Those with molecular weights less than 1000 Da are called micromolecules or simply biomolecules, while compounds found in the acid-insoluble fraction are called macromolecules or biomacromolecules. Most molecules in the insoluble fraction—except lipids—are polymeric substances.

You might wonder why lipids, which have molecular weights below 800 Da, are included in the acid-insoluble or macromolecular fraction. The reason is structural: lipids are not only present as small molecules but also organized into cell membranes and other membranes. When a tissue is ground, these membranes break into vesicles, which are not water-soluble. Therefore, these membrane fragments are separated along with the acid-insoluble pool, making lipids appear in the macromolecular fraction, even though they are not truly macromolecules.

The acid-soluble pool mainly represents the cytoplasmic composition, while macromolecules come from the cytoplasm and organelles. Together, these two fractions give a complete picture of the chemical composition of living tissues. In summary, if we look at the chemical composition of living tissue in terms of abundance and classify the compounds, we find that water is the most abundant chemical in all living organisms, followed by various organic and inorganic biomolecules, each playing specific roles in maintaining life.

5. Proteins – Structure and Functions

Proteins are made up of long chains of amino acids called polypeptides, which are connected by peptide bonds. Each protein is a polymer of amino acids, but because there are 20 different types of amino acids—such as alanine, cysteine, proline, tryptophan, and lysine—a protein is a heteropolymer, not a homopolymer. A homopolymer contains only one type of monomer repeated multiple times, while a heteropolymer has a variety of monomers arranged in a specific sequence. Knowing the amino acid composition of proteins is important because, in nutrition, we learn that some amino acids are essential for health and must be obtained through our diet, while others are non-essential because our body can produce them. Therefore, dietary proteins are the main source of essential amino acids.

Proteins perform many vital functions in living organisms. Some transport nutrients across cell membranes, some fight infections by acting as part of the immune system, some serve as hormones, and others act as enzymes to catalyze chemical reactions. In the animal world, collagen is the most abundant protein, providing strength to tissues. In the entire biosphere, Ribulose bisphosphate Carboxylase-Oxygenase (RuBisCO) is the most abundant protein, playing a crucial role in photosynthesis and the global carbon cycle. Proteins, therefore, are not only structural molecules but also functional molecules, essential for life processes in every organism.

6. Polysaccharides – Structure and Functions

Another important class of macromolecules found in the acid-insoluble fraction of living tissues is polysaccharides, which are long chains of sugars. You can think of them as threads made of monosaccharides. For example, cellulose is a polymer made entirely of glucose units, making it a homopolymer. Starch is another plant polysaccharide that acts as an energy store, while animals have glycogen, a similar energy-storing polysaccharide. Another example is inulin, which is a polymer of fructose. Polysaccharide chains, such as glycogen, have a reducing end on one side and a non-reducing end on the other, and many have branched structures. Starch forms helical secondary structures that can trap iodine (I₂), giving a blue colour known as the starch-iodine test, while cellulose lacks this helical structure and cannot trap iodine. Plant cell walls are mainly made of cellulose, and materials like paper and cotton fibres are cellulosic.

Nature also contains more complex polysaccharides, which are made not only of simple sugars but also of amino sugars or chemically modified sugars such as glucosamine and N-acetyl galactosamine. For example, the exoskeletons of arthropods contain a complex polysaccharide called chitin. These complex polysaccharides are mostly homopolymers, meaning they consist of repeating units of the same type of sugar. Polysaccharides, therefore, serve both structural roles, like in plant cell walls and exoskeletons, and storage roles, like starch in plants and glycogen in animals, making them essential biomolecules in living organisms.

7. Nucleic Acids – Structure and Functions

Another important type of macromolecule found in the acid-insoluble fraction of living tissues is nucleic acids, which are essentially polynucleotides. Along with polysaccharides and polypeptides, nucleic acids make up the true macromolecular fraction of any cell or tissue. The building block of a nucleic acid is a nucleotide, which is made of three chemically distinct components: a heterocyclic compound, a monosaccharide sugar, and a phosphate group.

The heterocyclic compounds in nucleotides are the nitrogenous bases, which include adenine, guanine, cytosine, thymine, and uracil. Among these, adenine and guanine are purines, while cytosine, thymine, and uracil are pyrimidines. The basic ring structures are called purine and pyrimidine, respectively. The sugar in nucleotides is either ribose, a pentose monosaccharide, or 2’-deoxyribose. A nucleic acid containing deoxyribose is called deoxyribonucleic acid (DNA), while one containing ribose is called ribonucleic acid (RNA). These nucleic acids are crucial for storing and transferring genetic information, forming the blueprint for life in all living organisms.

8. Protein Structure – Levels and Organization

Proteins, as we have seen, are heteropolymers made up of chains of amino acids. The term structure can mean different things depending on the field. In inorganic chemistry, it usually refers to the molecular formula (like NaCl or MgCl₂). In organic chemistry, a 2-dimensional representation of the molecule is often shown (like benzene or naphthalene), while physicists consider the 3-dimensional structure. In biology, protein structure is described at four levels.

The primary structure of a protein is the sequence of amino acids, which provides the positional information: which amino acid is first, second, and so on. The first amino acid is called the N-terminal, and the last amino acid is the C-terminal. Proteins are not straight rods; the chain folds in specific ways. Some regions form right-handed helices, known as the secondary structure, while other regions fold into different shapes. The long protein chain can also fold upon itself like a hollow woolen ball, forming the tertiary structure, which provides the 3-dimensional shape necessary for the biological activity of proteins.

Some proteins consist of more than one polypeptide chain or subunits. The way these folded subunits are arranged relative to each other is called the quaternary structure. For example, human hemoglobin (Hb) consists of four subunits: two α-type and two β-type, which together form the functional hemoglobin protein. Understanding these levels of protein structure is crucial because a protein’s function is directly linked to its shape and folding, allowing it to perform roles like oxygen transport, enzyme activity, and signaling in living organisms.

9. Enzymes – Structure and Functions

Proteins, as we have seen, are heteropolymers made up of chains of amino acids. The term structure can mean different things depending on the field. In inorganic chemistry, it usually refers to the molecular formula (like NaCl or MgCl₂). In organic chemistry, a 2-dimensional representation of the molecule is often shown (like benzene or naphthalene), while physicists consider the 3-dimensional structure. In biology, protein structure is described at four levels.

The primary structure of a protein is the sequence of amino acids, which provides the positional information: which amino acid is first, second, and so on. The first amino acid is called the N-terminal, and the last amino acid is the C-terminal. Proteins are not straight rods; the chain folds in specific ways. Some regions form right-handed helices, known as the secondary structure, while other regions fold into different shapes. The long protein chain can also fold upon itself like a hollow woolen ball, forming the tertiary structure, which provides the 3-dimensional shape necessary for the biological activity of proteins.

Some proteins consist of more than one polypeptide chain or subunits. The way these folded subunits are arranged relative to each other is called the quaternary structure. For example, human hemoglobin (Hb) consists of four subunits: two α-type and two β-type, which together form the functional hemoglobin protein. Understanding these levels of protein structure is crucial because a protein’s function is directly linked to its shape and folding, allowing it to perform roles like oxygen transport, enzyme activity, and signaling in living organisms.

Chemical Reactions – Types and Mechanisms

To understand enzymes, we first need to understand what a chemical reaction is. Chemical compounds can undergo two main types of changes. A physical change involves only a change in shape or state, without breaking chemical bonds. For example, ice melting into water or water evaporating into vapour are physical changes. On the other hand, a chemical reaction occurs when bonds are broken and new bonds are formed, creating new substances. For instance, Ba(OH)₂ + H₂SO₄ → BaSO₄ + 2H₂O is an inorganic chemical reaction, while the hydrolysis of starch into glucose is an organic chemical reaction.

The rate of a reaction refers to the amount of product formed per unit time. It can be expressed as rate = δP/δt and is influenced by factors such as temperature. Generally, the rate doubles or halves for every 10°C change. When a reaction is catalyzed, it proceeds much faster than the uncatalyzed reaction. For example, the reaction CO₂ + H₂O → H₂CO₃ is extremely slow on its own, producing only about 200 molecules of H₂CO₃ per hour. However, in the presence of the enzyme carbonic anhydrase, this reaction speeds up dramatically, producing about 600,000 molecules per second, which is roughly 10 million times faster. This shows the incredible power of enzymes.

There are thousands of enzymes, each catalyzing a specific chemical or metabolic reaction. A multistep reaction, where each step is catalyzed by one or more enzymes, is called a metabolic pathway.

For example, in glucose metabolism, glucose (C₆H₁₂O₆) is converted into pyruvic acid (2C₃H₄O₃ + 2H₂O) through ten enzyme-catalyzed steps.

This pathway is studied in cellular respiration. Depending on the conditions, the end products of a pathway can vary: in skeletal muscles under anaerobic conditions, lactic acid is formed; under aerobic conditions, pyruvic acid is produced; and in yeast fermentation, ethanol (alcohol) is generated. This illustrates how enzymes and metabolic pathways can create different products under different conditions, highlighting their flexibility and importance in living organisms.

Enzyme Activity – Mechanism of Accelerating Chemical Reactions

To understand enzymes better, we need to look at how they interact with their substrates. As discussed earlier, an enzyme has a 3-dimensional structure with a specific active site, which is a pocket or cleft where the substrate (S) binds. The substrate is the chemical that undergoes conversion to form a product (P), which can be symbolically represented as S → P. For the reaction to occur, the substrate must diffuse to the enzyme and form a temporary enzyme-substrate complex (ES complex). This ES complex is transient, meaning it exists only briefly.

During this time, the substrate adopts a special structure called the transition state, which is an unstable intermediate between the stable substrate and the final product. Bonds in the substrate are broken and formed during this phase, and after the transformation, the product is released from the enzyme. The reaction pathway may also involve other unstable intermediate states, each with different energy levels. If we represent this process in a graph, the y-axis shows the potential energy, and the x-axis shows the progression from substrate to product. The difference in energy between the substrate (S) and the transition state is called the activation energy, which is the energy barrier that must be overcome for the reaction to proceed.

Here comes the key role of enzymes: they lower the activation energy, making it easier for the substrate to reach the transition state and convert into the product. In this way, enzymes speed up reactions without changing the overall energy difference between the substrate and the product. If the product (P) has lower energy than the substrate, the reaction is exothermic, meaning it releases energy and can occur spontaneously. If the reaction requires energy input, it is endothermic, but enzymes still facilitate the reaction by reducing the energy barrier, ensuring the process happens efficiently under biological conditions.

Enzyme Action – Mechanism and Characteristics

Every enzyme (E) has a specific binding site for its substrate (S), allowing the formation of a highly reactive enzyme-substrate complex (ES). This ES complex is short-lived and eventually transforms into the enzyme-product complex (EP) before finally releasing the product(s) (P) and regenerating the unchanged enzyme. The overall process can be represented as: E + S → ES → EP → E + P.

The catalytic cycle of an enzyme involves several steps. First, the substrate binds to the active site of the enzyme, fitting precisely into the pocket. This binding induces a conformational change in the enzyme, making the active site fit more snugly around the substrate. Once the substrate is properly bound, the enzyme’s active site facilitates the breaking of chemical bonds in the substrate, forming the enzyme-product (EP) complex. Finally, the product(s) are released, and the free enzyme is ready to bind with another substrate molecule, repeating the catalytic cycle. This cycle ensures that enzymes can catalyze multiple reactions efficiently without being consumed in the process, highlighting their remarkable role in speeding up biochemical reactions.

Enzyme Activity – Factors Influencing Rate of Reaction

The activity of an enzyme can be influenced by changes in its environment, which may alter the tertiary structure of the protein. Key factors include temperature, pH, substrate concentration, and the presence of specific chemicals that regulate enzyme activity. Most enzymes work effectively only within a narrow range of temperature and pH, and each enzyme has a specific optimum temperature and optimum pH at which it shows maximum activity. If the temperature is too low, the enzyme becomes temporarily inactive, while high temperatures can denature the protein, destroying its activity.

Substrate concentration also affects enzyme activity. As substrate concentration increases, the rate of the enzymatic reaction rises initially. However, it eventually reaches a maximum velocity (Vmax), beyond which increasing substrate concentration does not increase the rate. This happens because all enzyme molecules are saturated, leaving no free enzymes to bind additional substrate molecules.

The activity of enzymes is also affected by inhibitors, which are chemicals that reduce or stop enzyme activity. When an inhibitor resembles the substrate closely and competes for the substrate-binding site, it is called a competitive inhibitor. This prevents the substrate from binding, thus reducing enzyme activity. An example is malonate, which inhibits succinic dehydrogenase because it structurally resembles succinate, the enzyme’s natural substrate. Competitive inhibitors are often used in controlling bacterial pathogens. In this way, enzyme activity is finely regulated by physical conditions and chemical interactions, ensuring proper biochemical control within living organisms.

Enzymes – Classification and Naming

Thousands of enzymes have been discovered, isolated, and studied, and most of them have been classified based on the type of reactions they catalyze. Enzymes are generally divided into six major classes, each having 4–13 subclasses, and are identified by a four-digit numbering system. The first class, oxidoreductases (also called dehydrogenases), catalyzes oxidation-reduction reactions between two substrates (S and S’), where one substrate is oxidized and the other is reduced. The second class, transferases, catalyzes the transfer of a chemical group (G), other than hydrogen, from one substrate (S) to another (S’), e.g., transferring a phosphate or methyl group. The third class, hydrolases, is involved in hydrolysis reactions, breaking ester, ether, peptide, glycosidic, C-C, C-halide, or P-N bonds with the addition of water. The fourth class, lyases, catalyzes the removal of chemical groups from substrates by mechanisms other than hydrolysis, often forming double bonds in the product. The fifth class, isomerases, catalyzes the conversion of isomers, including optical, geometric, or positional isomers. Finally, the sixth class, ligases, catalyzes the joining of two molecules, such as the formation of C-O, C-S, C-N, or P-O bonds, linking compounds together. These classifications help in understanding enzyme functions and their roles in metabolism within living organisms.

Enzyme Co-factors – Types and Functions

Enzymes are mostly made up of one or more polypeptide chains, but in many cases, they need non-protein components called cofactors to become catalytically active. When the protein part of an enzyme is without its cofactor, it is called an apoenzyme. Cofactors can be of three main types: prosthetic groups, coenzymes, and metal ions. Prosthetic groups are organic molecules that are tightly bound to the apoenzyme and often form part of the active site. For example, in peroxidase and catalase, which break down hydrogen peroxide into water and oxygen, haem acts as the prosthetic group. Coenzymes are also organic molecules, but their association with the enzyme is temporary, usually only during the catalytic reaction. Coenzymes often act in multiple enzyme-catalyzed reactions, and many of them are derived from vitamins. For instance, NAD and NADP contain niacin, a vitamin, and act as coenzymes in several reactions. Some enzymes require metal ions as cofactors, which form coordination bonds with the enzyme’s active site and sometimes with the substrate, e.g., zinc is a cofactor for carboxypeptidase, a proteolytic enzyme. If a cofactor is removed, the enzyme loses its catalytic activity, showing that these cofactors are essential for proper enzyme function.

10. Overview Of This Chapter🥰

Conclusion

Qualitative tests for biomolecules provide a basic yet powerful way to identify important compounds like carbohydrates, proteins, lipids, and nucleic acids. These tests depend on specific color reactions that help distinguish between different types of biomolecules. Although they don’t give exact amounts, they are extremely useful in biology labs, medicine, food science, and research. Understanding these simple tests helps lay the foundation for more advanced biochemical and molecular studies.

Although there is a huge diversity of living organisms, their chemical composition and metabolic reactions are surprisingly similar. When we analyze the elemental composition of living tissues and non-living matter, it looks qualitatively similar, but a closer look shows that carbon, hydrogen, and oxygen are relatively more abundant in living organisms than in non-living matter. The most abundant chemical in living systems is water. Living organisms contain thousands of small biomolecules with molecular weights less than 1000 Da, including amino acids, monosaccharides, disaccharides, fatty acids, glycerol, nucleotides, nucleosides, and nitrogen bases. Among these, there are 20 types of amino acids and 5 types of nucleotides. Fats and oils are glycerides, where fatty acids are esterified to glycerol, and phospholipids also contain a phosphorylated nitrogenous compound.

Only three types of true macromolecules are commonly found in living systems: proteins, nucleic acids, and polysaccharides. Lipids, because of their association with membranes, are also part of the macromolecular fraction. These biomacromolecules are polymers made of specific building blocks. Proteins are heteropolymers of amino acids, while nucleic acids (RNA and DNA) are polymers of nucleotides. Biomacromolecules have a hierarchy of structures—primary, secondary, tertiary, and quaternary. Nucleic acids serve as the genetic material, carrying hereditary information from parent to offspring. Polysaccharides form the cell wall in plants and fungi and the exoskeleton in arthropods, and they also act as energy storage molecules (like starch and glycogen).

Proteins perform a variety of cellular functions: many act as enzymes, some as antibodies, receptors, hormones, or structural proteins. Collagen is the most abundant protein in animals, and Ribulose bisphosphate Carboxylase-Oxygenase (RuBisCO) is the most abundant protein in the entire biosphere. Enzymes are proteins that catalyze biochemical reactions in cells, while some nucleic acids with catalytic activity are called ribozymes. Protein enzymes show substrate specificity and work best at an optimum temperature and pH. They are denatured at high temperatures. Enzymes function by lowering the activation energy of reactions and greatly increasing the reaction rate. Nucleic acids carry genetic information, ensuring it is passed from one generation to the next.

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