Carbohydrates, Lipids & Proteins

Carbohydrates — Definition & Classification

Carbohydrates occur abundantly in living organisms and are found in almost all parts of the cell. Examples include cellulose in wood and cotton, starch in cereals and tubers, and sugars such as cane sugar and milk sugar. The word carbohydrate literally means "hydrated carbons" — they are composed of carbon, hydrogen, and oxygen, with the ratio of hydrogen to oxygen being the same as in water.
General formula of carbohydrates, where x ranges from 3 to thousands and y is a whole number (same as or different from x)
=Carbon atoms(—)
=Water-like hydrogen-to-oxygen ratio(—)
=Number of carbon atoms (3 to thousands)(—)
=Whole number (may equal x or differ)(—)
Chemical Definition: Polyhydroxy aldehydes or ketones, or complex substances that on hydrolysis yield polyhydroxy aldehyde or ketone subunits
Sources: Green plants produce carbohydrates as the primary product of photosynthesis; all other plant compounds are derived from them through chemical changes
Conjugated Molecules: Carbohydrates combine with proteins to form glycoproteins and with lipids to form glycolipids, both having structural roles in extracellular matrices and biological membranes

Three Classes of Saccharides

1
Monosaccharides — simple sugars, cannot be hydrolysed further
2
Oligosaccharides — yield 2–10 monosaccharides on hydrolysis
3
Polysaccharides — polymers of many monosaccharide units
Monosaccharides are the simplest sugars — sweet in taste, easily soluble in water, and cannot be hydrolysed into simpler sugars. Chemically they are either polyhydroxy aldehydes (aldose sugars) or polyhydroxy ketones (ketose sugars). All carbon atoms except one carry a hydroxyl group; the remaining carbon is part of either an aldehyde group or a keto group.
Carbon Number Classification: Trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), and heptoses (7C)
Aldose vs Ketose: A sugar with an aldehyde group (—CHO) is an aldose; with a keto group (C=O) it is a ketose
Ring Structures: In solution, pentoses form five-cornered rings (furanose form, e.g., ribofuranose) and hexoses form six-cornered rings (pyranose form, e.g., glucopyranose)

Key Monosaccharides and Their Roles

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Glyceraldehyde (triose) — intermediate in respiration and photosynthesis; aldehyde form
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Dihydroxyacetone (triose) — ketone form of a triose; intermediate in metabolism
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Ribose (pentose, ) — component of RNA and nucleotides; forms ribofuranose ring
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Glucose (hexose, ) — most important hexose; aldose sugar; found free in fruits (grapes, figs, dates) and at 0.08% in blood; forms glucopyranose ring
Oligosaccharides are less sweet and less soluble than monosaccharides. On hydrolysis they yield from two to ten monosaccharide units. The covalent bond joining two monosaccharides is called a glycosidic bond, formed with the release of a water molecule. Those yielding two monosaccharides are called disaccharides, three yield trisaccharides, and so on.
Disaccharides: The most physiologically important oligosaccharides; include maltose, sucrose, and lactose
Sucrose: Molecular formula ; on hydrolysis yields glucose and fructose; formed by a 1,2-glycosidic linkage between glucose and fructose
Maltose: A disaccharide composed of two glucose units
Lactose: Milk sugar; a disaccharide composed of glucose and galactose

Important Disaccharides

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Maltose — glucose + glucose
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Sucrose (cane sugar) — glucose + fructose ()
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Lactose (milk sugar) — glucose + galactose
Polysaccharides are the most complex and most abundant carbohydrates in nature. They are usually branched, tasteless, have high molecular weights, and are only sparingly soluble in water. They are formed by many monosaccharide units linked by glycosidic bonds and yield glucose on complete hydrolysis.
Starch: Found in fruits, grains, seeds, and tubers; main source of carbohydrates for animals; gives blue colour with iodine
Amylose: Unbranched chains of glucose; soluble in hot water
Amylopectin: Branched chains of glucose; insoluble in hot or cold water
Glycogen: Also called animal starch; stored abundantly in liver and muscles; insoluble in water; gives red colour with iodine; yields glucose on hydrolysis
Cellulose: Most abundant carbohydrate in nature; cotton is pure cellulose; main constituent of plant cell walls; insoluble in water; yields glucose on hydrolysis; gives no colour with iodine; not digested by humans (no cellulase enzyme) but digested by herbivores through microorganisms secreting cellulase

Iodine Test for Polysaccharides

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Starch → blue colour
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Glycogen → red colour
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Cellulose → no colour change

Biological Roles of Carbohydrates

Carbohydrates play both structural and functional roles in living organisms. They are the primary source of energy in cells and serve as key structural components of plant cell walls and bacterial cell walls.
Photosynthesis equation — green plants synthesize glucose using carbon dioxide, water, and light energy
=Carbon dioxide from air(—)
=Water from soil(—)
=Glucose produced(—)
=Oxygen released as byproduct(—)
synthesis of 10 g glucose
→
717.6 kcal of solar energy is consumed and stored as chemical energy in glucose
Energy Source: Simple carbohydrates (monosaccharides and disaccharides) are the main source of energy in cells; the chemical energy stored in glucose by photosynthesis becomes available when oxidized in the body
Structural Role: Cellulose is the main constituent of plant cell walls; polysaccharides form the structural framework of plants and microorganisms
Conjugated Molecules: Glycoproteins and glycolipids (carbohydrates combined with proteins and lipids) are components of biological membranes and play structural roles in the extracellular matrix of animals and bacterial cell walls
Storage: Starch stores energy in plants; glycogen stores energy in animals (chiefly in liver and muscles)

Summary of Carbohydrate Functions

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Primary energy source for cells (via oxidation of glucose)
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Structural component of plant cell walls (cellulose)
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Energy storage in plants (starch) and animals (glycogen)
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Component of biological membranes (glycoproteins, glycolipids)
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Structural role in extracellular matrix and bacterial cell walls

Lipids — Definition & Classification

Lipids are a heterogeneous group of compounds related to fatty acids. They are insoluble in water but soluble in organic solvents such as ether, alcohol, chloroform, and benzene. Lipids include fats, oils, waxes, cholesterol, and related compounds. Because of their higher proportion of C—H bonds and very low proportion of oxygen, lipids store double the amount of energy compared to the same amount of any carbohydrate.
Hydrophobic Nature: Lipids are hydrophobic compounds and are key components of cellular membranes
Energy Storage: Lipids store more energy than carbohydrates per unit mass due to higher C—H bond content
Protective Functions: Some lipids provide insulation against heat and cold, and act as waterproof material (waxes in insect exoskeletons, cutin on plant cuticles)

Lipid Classification

1
Acylglycerols (triglycerides/neutral lipids)
2
Waxes
3
Phospholipids
4
Sphingolipids
5
Glycolipids
6
Terpenoid lipids (carotenoids, steroids)
Acylglycerols are esters of fatty acids and the alcohol glycerol. The most widespread acylglycerol is the triglyceride (triacylglycerol or neutral lipid), composed of one glycerol molecule and three fatty acid molecules. An ester bond is formed when the hydroxyl group (—OH) of the alcohol reacts with the carboxyl group (—COOH) of a fatty acid, releasing a water molecule.
Three fatty acids react with glycerol via ester bonds, releasing three water molecules to form a triacylglycerol
=Three-carbon alcohol with three —OH groups(—)
=Carboxylic acids with long hydrocarbon chains(—)
=Water molecules released (one per ester bond)(—)
Glycerol Backbone: A three-carbon alcohol; each carbon carries a hydroxyl group that forms an ester bond with a fatty acid
Ester Bond Formation: The —OH from glycerol and the —H from the fatty acid carboxyl group combine to form water, leaving a C—O—C ester linkage
Neutral Lipids: Triglycerides are called neutral lipids because they have no charged groups
Fatty acids are one of the most important components of triglycerides. They contain even numbers of carbon atoms (from 2 to 30) in a straight chain, with hydrogen atoms attached and a terminal carboxylic acid group (—COOH).
Saturated Fatty Acids: Contain no double bonds between carbon atoms; animal fats are typically saturated; solid at room temperature
Unsaturated Fatty Acids: Contain up to six double bonds in the chain; plant fats are typically unsaturated; liquid at room temperature (oils)
Chain Length Effects: Solubility in organic solvents and melting point increase with increasing chain length — palmitic acid (, m.p. 63.1°C) vs butyric acid (, m.p. −8°C)
Plant Fatty Acids: May be branched or ringed, unlike animal fatty acids which are straight chains

Example Fatty Acids

1
Acetic acid () —
2
Butyric acid () — ; m.p. −8°C
3
Palmitic acid () — saturated; m.p. 63.1°C
4
Oleic acid () — unsaturated (double bond between and )
Waxes are widespread as protective coatings on fruits and leaves, and some insects secrete wax. Chemically, waxes are mixtures of long-chain alkanes (with odd numbers of carbon atoms, to ) along with alcohols, ketones, and esters of long-chain fatty acids.
Plant Protection: Waxes protect plants from water loss and abrasive damage on leaves and fruits
Animal Protection: Waxes provide a water barrier for insects, birds, and animals (e.g., wool wax in sheep)
Phospholipids are derivatives of phosphatidic acid, composed of glycerol, two fatty acids (on C₁ and C₂), and phosphoric acid (on C₃) of glycerol. Nitrogenous bases such as choline, ethanolamine, and serine are important components attached to the phosphoric acid. Phospholipids are widespread in bacteria, animal, and plant cells and are frequently associated with membranes. Phosphatidylcholine (lecithin) is one of the most common phospholipids.
Amphipathic Nature: Phospholipids have a polar hydrophilic head (phosphate-containing region with nitrogenous base) and a non-polar hydrophobic tail (fatty acid chains)
Membrane Component: This amphipathic structure makes phospholipids the fundamental building blocks of all biological membranes

Phospholipid Components

1
Glycerol backbone (3-carbon alcohol)
2
Two fatty acids (on C₁ and C₂) — non-polar (hydrophobic) tails
3
Phosphoric acid (on C₃)
4
Nitrogenous base (choline, ethanolamine, or serine) — polar (hydrophilic) head
Terpenoids are a large and important group of compounds made up of simple repeating isoprenoid units. By condensation in different ways, these units give rise to compounds such as rubber, carotenoids, steroids, and terpenes.
Building Block: The isoprenoid unit is the fundamental repeating subunit of all terpenoids
Diverse Products: The same basic unit produces an enormous variety of compounds — from rubber (polymer) to carotenoids (pigments) to steroids (hormones)

Biological Roles of Lipids

Lipids serve multiple critical functions in living organisms beyond their role as structural membrane components.
Energy Storage: Lipids are the major source of stored energy; they store approximately double the energy per unit mass compared to carbohydrates due to a higher proportion of C—H bonds and lower oxygen content
Membrane Structure: Phospholipids form the bilayer of all biological membranes; glycolipids are also membrane components
Insulation: Lipids provide thermal insulation against atmospheric heat and cold in animals
Waterproofing: Waxes in insect exoskeletons and cutin on plant epidermis (leaves, fruits, seeds) act as waterproof barriers
Mechanical Protection: Waxes protect against abrasive damage to plant surfaces
Specific Gravity: Fats and oils are lighter than water with a specific gravity of about 0.8

Summary of Lipid Functions

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Long-term energy storage (more energy-dense than carbohydrates)
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Structural component of biological membranes (phospholipid bilayer)
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Thermal insulation in animals
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Waterproofing (waxes on insects, cutin on plants)
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Mechanical protection against abrasion

Amino Acids & Peptide Bonds

Proteins are the most abundant organic compounds in cells, comprising over 50% of total dry weight. They are polymers of amino acids — compounds containing carbon, nitrogen, oxygen, and hydrogen. About 170 types of amino acids occur in cells and tissues, of which about 25 are constituents of proteins, and most proteins are made from just 20 types.
Amino Group: All amino acids have an amino group () attached to the alpha carbon
Carboxyl Group: All amino acids have a carboxyl group () attached to the same alpha carbon
Alpha Carbon: The central carbon atom to which both the amino and carboxyl groups are bonded
R Group (Side Chain): The variable group that differentiates one amino acid from another; may be a hydrogen atom (glycine), (alanine), or any other group
Amino acids are linked together by peptide bonds to form polypeptides, which fold into functional proteins. The peptide bond forms when the amino group of one amino acid reacts with the carboxyl group of another, releasing a molecule of water. The linkage involves the hydroxyl (—OH) of the carboxyl group and the hydrogen (—H) of the amino group, producing a C—N bond.
Peptide Bond Formation: A condensation (dehydration) reaction — water is removed as the bond forms
Dipeptide: Two amino acids linked by one peptide bond (e.g., glycylalanine from glycine + alanine)
Reactive Ends: A dipeptide has a free amino group at one end and a free carboxyl group at the other, allowing further peptide bonds to form
Polymer Growth: Adding more amino acids produces tripeptides, tetrapeptides, and eventually long polypeptide chains

Structure of Proteins

The primary structure of a protein comprises the specific number and sequence of amino acids in the polypeptide chain. This sequence is determined by the order of nucleotides in DNA and is highly specific — if any amino acid is not in its normal position, the protein may fail to function.
Sanger's Discovery: F. Sanger was the first to determine the amino acid sequence of a protein; insulin has 51 amino acids in two chains (21 in one chain, 30 in the other) held together by disulphide bridges
Haemoglobin Structure: Composed of four polypeptide chains — two alpha chains (141 amino acids each) and two beta chains (146 amino acids each)
Sickle Cell Example: In sickle cell haemoglobin, just one amino acid substitution out of 574 total causes the protein to fail in carrying sufficient oxygen — demonstrating the critical importance of correct sequence
Protein Diversity: Over 10,000 proteins in the human body, each with a unique and specific arrangement of 20 amino acid types

Key Primary Structure Facts

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Defined by the number and sequence of amino acids
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Sequence determined by DNA nucleotide order
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Incorrect sequence can cause loss of function (e.g., sickle cell haemoglobin)
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Disulphide bridges (—S—S—) can hold separate chains together
The secondary structure involves regular coiling or folding of the polypeptide chain, maintained by hydrogen bonds between amino acids in successive turns of the spiral.
Alpha-Helix: A spiral formation of the polypeptide chain; 3.6 amino acids per turn; hydrogen bonds form between the carbonyl oxygen of one amino acid and the amide hydrogen of another four residues away, maintaining the helical shape
Beta-Pleated Sheet: Formed by folding back of the polypeptide chain; adjacent chains are held together by hydrogen bonds, creating a sheet-like structure
The tertiary structure results from further bending and folding of the polypeptide chain upon itself, forming a compact globular shape. It is maintained by three types of bonds: ionic bonds, hydrogen bonds, and disulphide bonds (—S—S—).
Stabilising Bonds: Ionic bonds, hydrogen bonds, and disulphide bridges (—S—S—) all contribute to maintaining the tertiary conformation
Hydrophobic Effect: In aqueous environments, the most stable conformation buries hydrophobic amino acids inside the molecule while hydrophilic amino acids remain on the surface
The quaternary structure arises when multiple polypeptide tertiary chains aggregate and are held together by hydrophobic interactions, hydrogen bonds, and ionic bonds.
Multi-Subunit Proteins: Only proteins with more than one polypeptide chain exhibit quaternary structure
Haemoglobin Example: The oxygen-carrying protein of red blood cells has quaternary structure — four polypeptide chains (two alpha, two beta) arranged in a specific three-dimensional configuration
Stabilising Forces: Hydrophobic interactions, hydrogen bonds, and ionic bonds hold the subunits together

Classification of Proteins

Due to the complexity of their structure and diversity of function, proteins are classified according to their overall shape into two main categories: fibrous proteins and globular proteins.
Fibrous Proteins: Consist of one or more polypeptide chains in the form of fibrils; secondary structure is the most important structural level; insoluble in aqueous media; non-crystalline; elastic in nature; perform structural roles. Examples: silk fibre, myosin (muscle cells), fibrin (blood clot), keratin (nails and hair)
Globular Proteins: Spherical or ellipsoidal shape due to multiple folding of polypeptide chains; tertiary structure is the most important structural level; soluble in aqueous media (salt solutions, acids, bases, aqueous alcohol); can be crystallised; disorganise with changes in physical and physiological environment. Examples: enzymes, antibodies, hormones, haemoglobin

Fibrous vs Globular Proteins Comparison

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Fibrous — fibril-shaped; Globular — spherical/ellipsoidal
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Fibrous — secondary structure dominant; Globular — tertiary structure dominant
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Fibrous — insoluble in water; Globular — soluble in water
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Fibrous — structural roles; Globular — functional roles (enzymes, hormones, transport)
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Fibrous — elastic; Globular — crystallisable

Biological Roles of Proteins

Proteins are involved in virtually every process in living organisms. Their diverse functions arise from the enormous variety of shapes and chemical properties that different amino acid sequences can produce.
Structural Role: Proteins build many cellular structures; fibrous proteins like keratin (hair, nails) and silk fibre provide mechanical support
Enzymatic Function: All enzymes are proteins; they control the whole metabolism of the cell by catalysing biochemical reactions
Hormonal Regulation: Some proteins act as hormones that regulate metabolic processes (e.g., insulin, growth hormone)
Transport and Carriers: Haemoglobin carries oxygen; other proteins transport lipids, metal ions, and specific substances
Defense: Antibodies are proteins that defend the body against pathogens
Blood Clotting: Clotting proteins (e.g., fibrin) prevent blood loss after injury
Movement: Proteins cause movement of organs and organisms and movement of chromosomes during anaphase of cell division (e.g., myosin in muscle cells)

Summary of Protein Functions with Examples

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Structural — keratin (hair, nails), silk fibre
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Enzymatic — all enzymes are proteins
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Hormonal — insulin, growth hormone
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Transport — haemoglobin (oxygen), carrier proteins (lipids, metal ions)
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Defense — antibodies (immunoglobulins)
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Blood clotting — fibrin
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Movement — myosin (muscles), spindle proteins (chromosome movement)