Glycolysis is the process in which glucose is broken down to produce energy. It produces two molecules of pyruvate, ATP, NADH and water. The process takes place in the cytosol of the cell cytoplasm, in the presence or absence of oxygen. Glycolysis is the primary step of cellular respiration. In the absence of oxygen, the cells take small amounts of ATP through the process of fermentation. This metabolic pathway was discovered by three German biochemists- Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas in the early 19th century and are known as the EMP pathway (Embden–Meyerhof–Parnas).

Glycolysis stages:

Step 1: 

A phosphate group is added to glucose in the cell cytoplasm, by the action of enzyme hexokinase.

In this, a phosphate group is transferred from ATP to glucose forming glucose,6-phosphate.

Step 2: Glucose-6-phosphate is isomerized into fructose,6-phosphate by the enzyme phosphoglucomutase.

Step 3: The other ATP molecule transfers a phosphate group to fructose 6-phosphate and converts it into fructose 1,6-bisphosphate by the action of enzyme phosphofructokinase.

Step 4: The enzyme aldolase converts fructose 1,6-bisphosphate into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, which are isomers of each other.

Step 5: Triose-phosphate isomerase converts dihydroxyacetone phosphate into glyceraldehyde 3-phosphate which is the substrate in the successive step of glycolysis.

Step 6: This step undergoes two reactions:

The enzyme glyceraldehyde 3-phosphate dehydrogenase transfers 1 hydrogen molecule from glyceraldehyde phosphate to nicotinamide adenine dinucleotide to form NADH + H⁺.

Glyceraldehyde 3-phosphate dehydrogenase adds a phosphate to the oxidized glyceraldehyde phosphate to form 1,3-bisphosphoglycerate.

Step 7: Phosphate is transferred from 1,3-bisphosphoglycerate to ADP to form ATP with the help of phosphoglycerokinase. Thus two molecules of phosphoglycerate and ATP are obtained at the end of this reaction.

Step8: The phosphate of both the phosphoglycerate molecules is relocated from the third to the second carbon to yield two molecules of 2-phosphoglycerate by the enzyme phosphoglyceromutase.

Step 9: The enzyme enolase removes a water molecule from 2-phosphoglycerate to form phosphoenolpyruvate.

Step 10: A phosphate from phosphoenolpyruvate is transferred to ADP to form pyruvate and ATP by the action of pyruvate kinase. Two molecules of pyruvate and ATP are obtained as the end products.

The Krebs cycle or Citric acid cycle is a series of enzyme catalysed reactions occurring in the mitochondrial matrix, where acetyl-CoA is oxidised to form carbon dioxide and coenzymes are reduced, which generate ATP in the electron transport chain.

Krebs cycle was named after Hans Krebs, who postulated the detailed cycle. He was awarded the Nobel prize in 1953 for his contribution.

It is a series of eight-step processes, where the acetyl group of acetyl-CoA is oxidised to form two molecules of CO₂ and in the process, one ATP is produced. Reduced high energy compounds, NADH and FADH₂ are also produced.

Two molecules of acetyl-CoA are produced from each glucose molecule so two turns of the Krebs cycle are required which yields four CO₂, six NADH, two FADH₂ and two ATPs.

Krebs cycle steps:

It is an eight-step process. The Krebs cycle takes place in the matrix of mitochondria under aerobic conditions.

Step 1: The first step is the condensation of acetyl CoA with 4-carbon compound oxaloacetate to form 6C citrate, coenzyme A is released. The reaction is catalysed by citrate synthase.

Step 2: Citrate is converted to its isomer, isocitrate. The enzyme aconitase catalyses this reaction.

Step 3: Isocitrate undergoes dehydrogenation and decarboxylation to form 5C 𝝰-ketoglutarate. A molecular form of CO₂ is released. Isocitrate dehydrogenase catalyses the reaction. It is an NAD⁺ dependent enzyme. NAD⁺ is converted to NADH.

Step 4: 𝝰-ketoglutarate undergoes oxidative decarboxylation to form succinyl CoA, a 4C compound. The reaction is catalyzed by 𝝰-ketoglutarate dehydrogenase enzyme complex. One molecule of CO₂ is released and NAD⁺ is converted to NADH.

Step 5: Succinyl CoA forms succinate. The enzyme succinyl CoA synthetase catalyses the reaction. This is coupled with substrate-level phosphorylation of GDP to get GTP. GTP transfers its phosphate to ADP forming ATP.

Step 6: Succinate is oxidised by the enzyme succinate dehydrogenase to fumarate. In the process, FAD is converted to FADH₂.

Step 7: Fumarate gets converted to malate by the addition of one H₂O. The enzyme catalysing this reaction is fumarase.

Step 8: Malate is dehydrogenated to form oxaloacetate, which combines with another molecule of acetyl CoA and starts the new cycle. Hydrogens removed, get transferred to NAD⁺ forming NADH. Malate dehydrogenase catalyses the reaction.

Polymers such as proteins, polysaccharides and nucleic acids are formed when monomers link in long chains. The different types of bonds provide structure and stability to biomolecules and make them functional. Proteins are formed from amino acids when the carboxyl group of an amino acid reacts with the amino group of the next amino acid and form a peptide bond. This bond is formed due to dehydration. Polysaccharides are formed when several monosaccharides linked together by a glycosidic bond. This bond is also formed due to dehydration. The glycosidic bond connects carbohydrate molecules or a carbohydrate and a non-carbohydrate molecule. Nucleic acids are formed when a nitrogenous base, a phosphate group and a pentose sugar are linked to form a polynucleotide chain. Here, the three prime carbon atoms of one sugar of a nucleotide are linked to the five prime carbon atoms of the sugar of the succeeding nucleotide by a phosphate moiety. The phosphate and hydroxyl groups of sugar join to form an ester bond. The ester bonds formed by the phosphate group, between the third and fifth carbon atoms of adjacent pentose sugar rings, are called phosphodiester bonds. This bond links three prime carbon atoms of one sugar molecule with the five prime carbon atoms of an adjacent sugar molecule. The phosphodiester bond forms the backbone of the DNA strands.
The B-form of the DNA exists as a double helix, where two strands of polynucleotides are anti-parallel, that is, one strand runs in three prime to five prime and the other strand runs in five prime to three prime directions. The backbone of the DNA is formed by a sugar-phosphate-sugar chain. The four bases found in the DNA are adenine, cytosine, guanine and thymine. Adenine on one strand base pairs with thymine on the other strand by a double bond. Similarly, cytosine on one strand base pairs with guanine on the other strand by a triple bond. One full 360^o turn of the helical strand would involve ten base pairs. The distance along one turn is known as a pitch, which is thirty-four angstroms. The distance between two base pairs is three-point four angstroms.

Polysaccharides are major classes of biomolecules. They are long chains of carbohydrate molecules, composed of several smaller monosaccharides. These complex bio-macromolecules functions as an important source of energy in animal cell and form a structural component of a plant cell. It can be a homopolysaccharide or a heteropolysaccharide depending upon the type of the monosaccharides. Polysaccharides can be a straight chain of monosaccharides known as linear polysaccharides, or it can be branched known as a branched polysaccharide.

Functions of Polysaccharides-

The polysaccharides serve as a structural organization in animals and plants. Other functions of polysaccharides include:

1. They store energy in organisms.
2. Due to the presence of multiple hydrogen bonds, the water cannot invade the molecules making them hydrophobic.
3. They allow for changes in the concentration gradient which influences the uptake of nutrients and water by the cells.
4. Many polysaccharides become covalently bonded with lipids and proteins to form glycolipids and glycoproteins. These glycolipids and glycoproteins are used to send messages or signals between and within the cells.
5. They provide support to the cells. The cell wall of plants is made up of polysaccharide cellulose, which provides support to the cell wall of the plant. In insects and fungi, chitin plays an important role in providing support to the extracellular matrix around the cells.

Gregor Mendel, through his work on pea plants, discovered the fundamental laws of inheritance. He deduced that genes come in pairs and are inherited as distinct units, one from each parent. Mendel tracked the segregation of parental genes and their appearance in the offspring as dominant or recessive traits. He recognized the mathematical patterns of inheritance from one generation to the next. Mendel’s Laws of Heredity are usually stated as-

The Law of Segregation: Each inherited trait is defined by a gene pair. Parental genes are randomly separated into the sex cells so that sex cells contain only one gene of the pair. Offspring, therefore, inherit one genetic allele from each parent when sex cells unite in fertilization. In the case of flower colour, the Mendel Pea Experiment showed that a cross between a purple flower plant and a white flower plant produced only purple flower plants for the F1 generation. It appeared that the white flower characteristic had disappeared. However, the F2 generation threw up a surprising result; the white flower variant appeared in a quarter of this generation.

The Law of Independent Assortment: Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another. Mendel examined two different traits, seed colour and seed shape. Two alleles at the locus controlling seed shape were studied, the dominant round (R) and recessive wrinkled (r) alleles. Mendel crossed one parent that was homozygous for the dominant alleles of these two different genes (round yellow RRYY) with another parent that was homozygous for the recessive alleles of those two genes (wrinkled green rryy). Re-stating the basic question, do the alleles at each locus always stay together (i.e. round with yellow, wrinkled with green) or do they appear in new combinations in the progeny? As expected from the 1st law, the F1 generation shows a uniform round yellow phenotype, since one dominant and one recessive allele was inherited from the parents. When the F2 progeny are obtained by crossing the F1 generation, the parental phenotypes reappear (as expected from the first law), but two nonparental phenotypes also appear that differ from the parents: wrinkled yellow and round green! The results can be explained by the alleles of each different gene assorting into gametes independently. For example, in the gametes from the F1 generation, R can assort with Y or y, and r can assort with Y or y, so that four types of gametes form: RY, Ry, rY, and ry. These can rejoin randomly with other gametes from the F1 generation.

The Law of Dominance: An organism with alternate forms of a gene will express the dominant form. The genetic experiments Mendel did with pea plants took him eight years (1856-1863) and he published his results in 1865. During this time, Mendel grew over 10,000 pea plants, keeping track of progeny number and type. Mendel’s work and his Laws of Inheritance were not appreciated in his time. It wasn’t until 1900, after the rediscovery of his Laws, that his experimental results were understood.

Single gene disorders are caused by DNA changes in one particular gene and often have predictable inheritance patterns. Over 10,000 human disorders are caused by a change, known as a mutation, in a single gene. These are known as single-gene disorders. The mutated version of the gene responsible for the disorder is known as a mutant, or disease, allele. Individually, single-gene disorders are very rare, but as a whole, they affect about one per cent of the population. Since only a single gene is involved, these disorders can be easily tracked through families and the risk of them occurring in later generations can be predicted. Single gene disorders can be divided into different categories: dominant, recessive and X-linked.

Dominant diseases – Dominant diseases are single-gene disorders that occur in the heterozygous state –when an individual has one mutant copy of the relevant gene and one healthy copy. The effects of the mutant version of the gene (allele) override the effects of the healthy version of the gene. So, the mutant allele causes disease symptoms even though a healthy allele is present. Dominant disorders tend to crop up in every generation of an affected family because very one carrying a dominant mutant allele shows the symptoms of the disease.  Dominant disorders spread vertically down family trees, from parent to child. In rare cases when an individual has two copies of the mutant gene  (also known as being homozygous), the disorder symptoms are generally more severe.  An example of a dominant single gene disorder is Huntington’s disease, which is a disease of the nervous system.

Recessive diseases – Recessive diseases are single-gene disorders that only occur in the homozygous state – when an individual carries two mutant versions (alleles) of the relevant gene. The effects of the healthy allele can compensate for the effects of the mutant allele. The mutant allele does not cause disease symptoms when a healthy allele is also present. However, if a parent inherits two mutant alleles, there are no healthy alleles, so the mutant allele can exert its effect. As shown in the diagram below, affected individuals arise when both of their parents carry a single mutated allele and each pass on that mutated copy to the child so the child then has two mutated copies. In this diagram, the mother of the affected grandson has inherited a mutated copy from the grandmother, and the father has inherited a mutated copy from his family. Recessive diseases are more difficult to trace through family trees because carriers of a mutant allele do not show symptoms of the disease. It, therefore, appears that the disease has skipped a generation when it is seen in groups of children within a family. The risk of an individual having a recessive disorder increases when two closely related people have a child together (consanguinity). This is because there is a much greater chance that the same mutant allele will be present in related parents.

X-linked disorders – X-linked disorders are single-gene disorders that result from the presence of a mutated gene on the X chromosome. Because females (XX) have two copies of the X chromosome, but males (XY) only have one copy, X-linked disorders are more common in males. If a male’s single copy on the X chromosome is mutated, he has no healthy copy to restore healthy function. The inheritance patterns of X-linked diseases are simplified by the fact that males always pass their X chromosome to their daughters but never to their sons. Like other single-gene disorders, X-linked disorders can be either recessive or dominant.

X-linked recessive diseases – Examples of X-linked recessive disorders include red-green color blindness,  hemophilia and the Duchenne and Becker forms of muscular dystrophy.  X-linked recessive disorders are much more common in males than females because two copies of the mutant allele are required for the disorder to occur in females, while only one copy is required in males. The inheritance patterns of X-linked recessive disorders are as follows: The overall pattern of the disease is characterized by the transmission of the disease from a carrier mother, who inherited a copy of the mutant gene from her affected father.