The Core Biomolecules: Carbohydrates, Proteins, and More
Carbohydrates
Definition
Optically active polyhydroxy aldehydes or ketones, or compounds that yield these upon hydrolysis.
General Formula and Name Origin
General Formula: Cₓ(H₂O)ᵧ
Name Origin: The name comes from “carbo” (meaning carbon) and “hydrate” (meaning water).
Classification of Carbohydrates
| Type | Definition | Examples |
|---|---|---|
| Monosaccharides | Cannot be hydrolyzed further into simpler units. | Glucose, Fructose, Ribose |
| Oligosaccharides | Yield 2–10 monosaccharide units upon hydrolysis. | Disaccharides: Sucrose, Maltose, Lactose |
| Polysaccharides | Yield many monosaccharide units upon hydrolysis. | Starch, Cellulose, Glycogen |
Reducing and Non-Reducing Sugars
Reducing sugars are carbohydrates that can reduce Fehling’s or Tollens’ reagent. All monosaccharides are reducing sugars.
Non-reducing sugars do not have a free carbonyl group and cannot reduce these reagents (e.g., sucrose).
Classification by Carbon Number
| Carbon No. | Aldose | Ketose |
|---|---|---|
| 3 | Aldotriose | Ketotriose |
| 4 | Aldotetrose | Ketotetrose |
| 5 | Aldopentose | Ketopentose |
| 6 | Aldohexose | Ketohexose |
Fructose
- Formula: C₆H₁₂O₆
- Type: A ketohexose.
- Ring Structure: Forms a five-membered ring known as a furanose structure.
- Sources: Commonly found in fruits, honey, and vegetables.
- Anomers: Exists as α- and β-anomers, represented by Haworth structures.
Glucose (Dextrose)
- Formula: C₆H₁₂O₆
- Sources: Found in fruits, honey, and grapes.
Preparation Methods
- From Sucrose: By boiling sucrose with a dilute acid.
- From Starch: Through the hydrolysis of starch under pressure.
Structural Evidence
- An aldehydic group is confirmed by oxidation, which forms gluconic acid.
- The presence of five -OH groups is confirmed by the formation of pentaacetate.
- A straight six-carbon chain is indicated by its reaction with HI, which forms n-hexane.
- It forms a cyclic hemiacetal, leading to the existence of α and β anomers.
Cyclic Structure
Glucose typically exists in a six-membered ring structure known as a pyranose ring. The α-D-glucose and β-D-glucose forms can interconvert in a process called mutarotation.
Note: Despite having an aldehyde group, glucose does not give a positive Schiff’s test because the group is involved in its cyclic hemiacetal form.
Disaccharides
| Name | Units | Bond | Type |
|---|---|---|---|
| Sucrose | Glucose + Fructose | α1→β2 | Non-reducing |
| Maltose | Glucose + Glucose | α1→4 | Reducing |
| Lactose | Galactose + Glucose | β1→4 | Reducing |
Invert Sugar
The hydrolysis of sucrose results in a mixture that changes its optical rotation from dextrorotatory (+) to levorotatory (–). This mixture is known as invert sugar.
Polysaccharides
| Type | Composition | Feature | Example |
|---|---|---|---|
| Starch | α-glucose | The main food reserve in plants. | Composed of Amylose (linear) and Amylopectin (branched). |
| Cellulose | β-glucose | A structural component of plant cell walls; non-digestible by humans. | Found in the cell walls of plants. |
| Glycogen | α-glucose | The main form of glucose storage in animals. | Stored in the liver and muscles; highly branched structure. |
Exceptions to the General Formula
- Acetic acid fits the general formula Cₓ(H₂O)ᵧ but is not a carbohydrate.
- Rhamnose does not fit the general formula but is classified as a carbohydrate.
Proteins
Definition
Polymers of α-amino acids linked together by peptide bonds.
Sources
Common dietary sources include milk, eggs, pulses, meat, and fish.
Amino Acids
- Structure: Contain two functional groups, a carboxyl group (–COOH) and an amino group (–NH₂), attached to the same alpha (α) carbon.
- Nature: They are amphoteric and can exist in a zwitterion form.
- Chirality: All naturally occurring amino acids are chiral, except for glycine.
Types of Amino Acids
| Type | Example |
|---|---|
| Neutral | Glycine, Alanine |
| Acidic | Aspartic acid, Glutamic acid |
| Basic | Lysine, Arginine |
Essential Amino Acids
There are ten essential amino acids that must be obtained from the diet:
- Valine
- Leucine
- Isoleucine
- Methionine
- Phenylalanine
- Tryptophan
- Threonine
- Lysine
- Arginine
- Histidine
Protein Structure Levels
| Level | Description | Example |
|---|---|---|
| Primary | The linear sequence of amino acids in a polypeptide chain. | Insulin |
| Secondary | Local folding of the polypeptide chain into structures like the α-helix or β-pleated sheet. | Silk (β-sheet) |
| Tertiary | The overall three-dimensional folding of a single polypeptide chain, often into a globular shape. | Myoglobin |
| Quaternary | The arrangement of several polypeptide chains (subunits) to form a functional protein complex. | Hemoglobin |
Denaturation
The loss of the secondary and tertiary structures of a protein, which results in the loss of its biological activity. An example is the boiling of an egg.
Enzymes
Definition
Enzymes are biocatalysts that accelerate the rate of biochemical reactions under mild physiological conditions.
Examples
- Maltase: Catalyzes the hydrolysis of maltose.
- Sucrase: Catalyzes the hydrolysis of sucrose.
- Oxidoreductase: Catalyzes oxidation-reduction reactions.
Mechanism of Action
An enzyme functions by lowering the activation energy of a reaction, thereby speeding it up.
Vitamins
Definition
Organic compounds required in small quantities for proper metabolic functioning.
Types of Vitamins
| Type | Vitamins | Solubility | Storage |
|---|---|---|---|
| Fat-soluble | A, D, E, K | Soluble in fats and oils. | Stored in the liver and adipose tissue. |
| Water-soluble | B-complex, C | Soluble in water. | Generally not stored in the body (except for B₁₂). |
Vitamin Sources and Deficiency Diseases
| Vitamin | Sources | Deficiency Disease |
|---|---|---|
| A | Milk, butter, carrots | Night blindness |
| B₁ (Thiamine) | Yeast, milk | Beriberi |
| B₂ (Riboflavin) | Green vegetables, cereals | Cheilosis (cracking of lips) |
| B₆ | Gram, cereals | Convulsions |
| B₁₂ | Meat, eggs | Pernicious anemia |
| C | Citrus fruits | Scurvy |
| D | Sunlight, fish | Rickets |
| E | Vegetable oils | Increased fragility of Red Blood Cells (RBCs) |
| K | Leafy green vegetables | Delayed blood clotting |
Nucleic Acids
Definition
Polymers of nucleotides that are responsible for controlling heredity and protein synthesis.
Components of Nucleic Acids
A nucleotide consists of three components: a nitrogenous base, a pentose sugar, and a phosphate group.
Comparison of DNA and RNA
| Type | Sugar | Bases | Structure |
|---|---|---|---|
| DNA | Deoxyribose | Adenine (A), Guanine (G), Cytosine (C), Thymine (T) | Double helix |
| RNA | Ribose | Adenine (A), Guanine (G), Cytosine (C), Uracil (U) | Single strand |
Base Pairing Rules
In DNA, specific base pairing occurs via hydrogen bonds:
- Adenine (A) pairs with Thymine (T) via two hydrogen bonds.
- Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.
Types of RNA
There are three main types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA).
Functions of Nucleic Acids
- DNA (Deoxyribonucleic Acid): Stores genetic information.
- RNA (Ribonucleic Acid): Plays a crucial role in protein synthesis.
Hormones
Definition
Hormones are chemical messengers secreted by endocrine glands directly into the bloodstream to regulate various physiological processes.
Types of Hormones
| Type | Examples | Function |
|---|---|---|
| Peptide | Insulin, Endorphins | Regulate blood glucose levels. |
| Steroid | Estrogen, Testosterone, Cortisol | Control growth, development, and metabolism. |
| Amino acid derivatives | Thyroxine, Epinephrine | Regulate metabolism and stress responses. |
Common Hormonal Disorders
- Hypothyroidism: Caused by low levels of thyroxine, leading to obesity and lethargy.
- Hyperthyroidism: Caused by high levels of thyroxine.
- Goiter: An enlargement of the thyroid gland, often due to iodine deficiency.
- Addison’s disease: Caused by adrenal gland deficiency.