Ch 8Biomolecules — Class 11 Botany, concept-first.

Every living cell maintains what is called a cellular pool - the collection of molecules, both inorganic and organic, that make up its living matter.

A water molecule (H2O) consists of one oxygen atom covalently bonded to two hydrogen atoms. Oxygen is far more electronegative than hydrogen, so it pulls the shared bonding electrons closer to itself;…

Water shows several distinctive properties that arise directly from its polar, hydrogen-bonded structure.

A metabolite is generally a small organic molecule that is an intermediate or product of metabolism. Metabolites are grouped into two categories based on their role.

Organic molecules found in the cell range from small and simple to large and complex. Small molecules assemble together to build large macromolecules, which fall into four main classes: carbohydrates,…

Carbohydrates are organic compounds built from carbon, hydrogen and oxygen, historically viewed as 'hydrates of carbon' because one molecule of water combines with each carbon atom to give the general…

Monosaccharides are relatively small molecules made of a single sugar unit. Glucose, with the chemical formula C6H12O6, is the best-known example; because it contains six carbon atoms it is classed as…

A disaccharide forms when two monosaccharide units join together. Sucrose (common table sugar) is the classic example, formed from one molecule of alpha-glucose and one molecule of fructose.

Polysaccharides, also called glycans, are made of hundreds of monosaccharide units linked into long chains, which may be branched or unbranched, all held together by glycosidic bonds.

Starch is a storage polysaccharide made of two related polymers of glucose: amylose and amylopectin. Starch grains form from successive layers of amylose and amylopectin laid down over time, visible m…

The presence of starch is tested for by adding a solution of iodine dissolved in potassium iodide (an iodine-potassium iodide solution) to the sample.

Glycogen is a storage polysaccharide, sometimes called 'animal starch', and is the only form in which carbohydrate is stored in animals and fungi.

Cellulose is a structural polysaccharide made of thousands of glucose units. Unlike starch or glycogen, cellulose's glucose units are in the beta-glucose form, and successive units are linked by beta-…

Chitin is a homopolysaccharide, but unlike cellulose its repeating monomer carries an amino group, technically making it a mucopolysaccharide.

Aldoses and ketoses (i.e., most monosaccharides) are classed as reducing sugars because their free aldehyde or ketone group can chemically reduce other compounds.

The word lipid comes from the Greek 'lipos', meaning fat. What unites this chemically diverse group is a shared physical property, not a shared structure: lipids do not dissolve in polar solvents such…

Triglycerides - the fats and oils of everyday diet - are composed of a single glycerol molecule bound to three fatty acid chains.

Phospholipids are the major structural lipid of the cell membrane. Structurally they resemble a triglyceride but with one key difference: only two of glycerol's three binding sites carry fatty acid ch…

Steroids are structurally complex lipids built around a characteristic multi-ring carbon skeleton, quite different from the long straight chains of triglycerides and phospholipids.

Waxes are esters formed between a long-chain alcohol and a saturated fatty acid - chemically similar in principle to a triglyceride's ester bonds, but built from just one fatty acid and one long-chain…

Proteins are the most diverse class of macromolecule found in the cell, making up roughly two-thirds of a cell's total dry mass.

Amino acids are classified according to the chemical nature of their R group (side chain) into four broad categories: acidic (side chain carries an extra carboxyl group, giving the amino acid a net ne…

A protein is synthesised on the ribosome as a linear sequence of amino acids joined by peptide bonds. After synthesis, this raw chain undergoes a conformational change, folding into a specific three-d…

Denaturation is the loss of a protein's native three-dimensional (secondary/tertiary/quaternary) structure, without necessarily breaking the primary sequence of amino acids itself.

Three (plus one further interaction) main types of chemical bonding hold a protein's folded structure together.

The biuret test is used to detect the presence of protein in a solution, because it specifically reacts with the peptide bond (-C-N-) that links amino acids together.

Enzymes are globular proteins that catalyse the many thousands of metabolic reactions taking place within cells and whole organisms.

Enzymes share a consistent set of properties. They are all globular proteins in their basic form. They act as true catalysts and are effective even when present in very small quantities, since each en…

In an enzyme-catalysed reaction, the starting molecule is called the substrate, and it is converted into a product.

Enzymes are highly sensitive to their environmental conditions, and the rate at which an enzyme-catalysed reaction proceeds can be affected by several factors: temperature, pH, substrate concentration…

Raising the temperature increases the amount of molecular motion in a system, so the molecules of both substrate and enzyme move around more quickly.

The optimum pH of an enzyme is the pH value at which it catalyses its reaction at the maximum possible rate.

For a fixed, unchanging amount of enzyme, the rate of an enzyme-catalysed reaction increases as the concentration of substrate increases - with more substrate molecules available, enzyme active sites…

Provided substrate is available in excess (i.e., is not the limiting factor), the rate of an enzyme-catalysed reaction is directly proportional to the concentration of enzyme present.

When the initial rate (velocity) of an enzyme reaction is measured across a range of substrate concentrations, using a fixed amount of enzyme, and the results are plotted on a graph, a characteristic…

Certain substances present within cells can react with an enzyme and lower the rate of the reaction it catalyses; these substances are called inhibitors.

Competitive inhibitors are molecules that resemble the shape of an enzyme's true substrate closely enough that they can compete with it to occupy the enzyme's active site.

Non-competitive inhibitors are chemically unlike the enzyme's true substrate, yet they can still bind to the enzyme, typically at a site distinct from the active site.

Allosteric enzymes are regulated by compounds that bind at a site distinct from the active site - called an allosteric site - and cause a reversible change in the shape of the enzyme, including its ac…

End-product inhibition, also called negative feedback inhibition, occurs when the final product of a metabolic pathway begins to build up in the cell and, once concentrated enough, acts as an alloster…

Many enzymes require non-protein helper components, called cofactors, for efficient catalytic activity.

Most enzymes are named according to the substrate they act on, with the name ending in the suffix '-ase'.

Enzymes are classified into six major groups based on their overall mode of action - the type of chemical transformation they catalyse - regardless of what specific substrate they act on.

Enzymes extracted from microorganisms are widely used across various industries because of their efficiency and specificity.

DNA and RNA are the two kinds of nucleic acid known to biology. Both were originally isolated from within the cell nucleus, which is the origin of the name 'nucleic acid'.

A nucleoside is the combination of a nitrogenous base and a pentose sugar alone (for example, adenosine = adenine + ribose, or guanosine = guanine + ribose).

The structure of DNA was proposed by James Watson and Francis Crick, working at the Cavendish Laboratory in Cambridge, who built a scale model of DNA's double-helical structure.

DNA's two strands run in opposite chemical directions - if one strand runs 5'-3', the other runs 3'-5' - a property called being antiparallel; the 5' end of a strand carries a free phosphate group, wh…

Ribonucleic acid (RNA) is a polymeric molecule, built from ribonucleotides, that plays essential roles across a range of biological processes connected to genetic information - including coding, decod…

There are three principal types of RNA, each with a distinct structure and role in gene expression. Messenger RNA (mRNA) is single-stranded and carries a working copy of the instructions needed to ass…

The chapter-end Evaluation set draws together the whole chapter's biomolecule chemistry: the classification and testing of carbohydrates (monosaccharides through polysaccharides), the structure and cl…

Sample papers and previous-year board questions for this subject.