Biomolecules: Carbohydrates and Monosaccharides

• Biomolecules are molecules produced by living organisms
• The four major classes of biomolecules are carbohydrates, lipids, proteins, and nucleic acids
• Biomolecules are essential for life and perform a wide range of functions, including energy storage, structural support, catalysis, and information storage

Carbohydrates

• Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen, typically with a hydrogen-oxygen ratio of 2:1.
• They are a major source of energy for living organisms.
• They play important structural roles in cells and tissues.
• Carbohydrates are classified into monosaccharides, disaccharides, oligosaccharides, and polysaccharides based on the number of sugar units they contain

Monosaccharides

• These are the simplest carbohydrates, also known as simple sugars
• They are single sugar units that cannot be hydrolyzed into smaller carbohydrates
• Examples include glucose, fructose, and galactose
• Glucose is the primary source of energy for cells and is transported in the bloodstream
• Fructose is found in fruits and honey and is sweeter than glucose
• Galactose is a component of lactose, the sugar found in milk
• Monosaccharides are classified by the number of carbon atoms they contain: trioses (3 carbons), tetroses (4 carbons), pentoses (5 carbons), hexoses (6 carbons), and heptoses (7 carbons)
• Glucose, fructose, and galactose are hexoses, with the formula C6H12O6
• Monosaccharides can exist in linear and ring forms, with the ring form being more stable in aqueous solutions
• The ring formation occurs through a reaction between a carbonyl group (aldehyde or ketone) and a hydroxyl group in the same molecule

Disaccharides

• These consist of two monosaccharides joined together by a glycosidic bond
• A glycosidic bond is a covalent bond formed between two monosaccharides through a dehydration reaction, where a water molecule is removed
• Examples include sucrose, lactose, and maltose
• Sucrose (table sugar) is composed of glucose and fructose
• Lactose (milk sugar) is composed of glucose and galactose
• Maltose is composed of two glucose molecules
• Disaccharides must be broken down into monosaccharides before they can be absorbed into the bloodstream and used for energy
• The enzyme sucrase breaks down sucrose into glucose and fructose
• Lactase breaks down lactose into glucose and galactose
• Maltase breaks down maltose into two glucose molecules

Polysaccharides

• These are complex carbohydrates composed of many monosaccharide units linked together by glycosidic bonds
• They serve as energy storage molecules (e.g., starch and glycogen) and structural components (e.g., cellulose and chitin)
• Starch is the main storage polysaccharide in plants, consisting of glucose monomers in α(1→4) glycosidic linkages, with α(1→6) branches
• Amylose is a linear form of starch, while amylopectin is a branched form
• Glycogen is the main storage polysaccharide in animals, similar to starch but more highly branched
• It is stored in the liver and muscles and broken down into glucose when energy is needed
• Cellulose is a structural polysaccharide found in the cell walls of plants, consisting of glucose monomers in β(1→4) glycosidic linkages
• The β linkages make cellulose indigestible to many animals, including humans, as they lack the enzyme cellulase needed to break it down
• Chitin is a structural polysaccharide found in the exoskeletons of arthropods and the cell walls of fungi
• It is similar to cellulose, but the glucose monomers are modified with a nitrogen-containing group

Proteins

• Proteins are complex biomolecules composed of amino acids
• They perform a wide variety of functions in living organisms, including catalyzing biochemical reactions (enzymes), transporting molecules, providing structural support, and regulating gene expression
• Proteins are polymers of amino acids, linked together by peptide bonds

Amino Acids

• These are the building blocks of proteins
• Each amino acid has a central carbon atom (α-carbon) bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom (-H), and a unique side chain (R-group)
• There are 20 common amino acids found in proteins, each with a different R-group that determines its chemical properties
• Amino acids are classified as nonpolar, polar, acidic, or basic based on the properties of their R-groups
• Nonpolar amino acids have hydrophobic R-groups that tend to cluster together in the interior of proteins, away from water
• Polar amino acids have hydrophilic R-groups that can form hydrogen bonds with water and other polar molecules
• Acidic amino acids have negatively charged R-groups at neutral pH
• Basic amino acids have positively charged R-groups at neutral pH

Peptide Bonds

• These are covalent bonds that link amino acids together to form polypeptide chains
• They are formed by a dehydration reaction between the carboxyl group of one amino acid and the amino group of another
• The sequence of amino acids in a polypeptide chain determines the protein's primary structure

Protein Structure

• Primary Structure: The linear sequence of amino acids in a polypeptide chain
• Determined by the genetic code and is unique for each protein
• Secondary Structure: Localized folding patterns within a polypeptide chain, stabilized by hydrogen bonds between the amino and carboxyl groups of the peptide backbone
• Common secondary structures include alpha helices and beta sheets
• Alpha helix: A coiled structure stabilized by hydrogen bonds between amino acids four residues apart
• Beta sheet: A pleated structure formed by hydrogen bonds between parallel or antiparallel strands of the polypeptide chain
• Tertiary Structure: The overall three-dimensional shape of a single polypeptide chain, determined by interactions between the R-groups of amino acids
• Interactions that contribute to tertiary structure include hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges
• Quaternary Structure: The arrangement of multiple polypeptide chains (subunits) in a multi-subunit protein
• Not all proteins have quaternary structure, as some consist of a single polypeptide chain
• Examples of proteins with quaternary structure include hemoglobin (four subunits) and antibodies

Protein Folding

• Protein folding is the process by which a polypeptide chain acquires its functional three-dimensional structure
• It is driven by the interactions between amino acid R-groups and the surrounding environment
• Chaperone proteins assist in protein folding by preventing aggregation and ensuring proper folding
• Misfolded proteins can lead to non-functional proteins and can contribute to diseases such as Alzheimer's and Parkinson's
• Denaturation is the loss of a protein's native structure, which can be caused by changes in temperature, pH, or exposure to chemicals
• Denatured proteins are usually non-functional

Nucleic Acids

• Nucleic acids are biomolecules that store and transmit genetic information
• The two main types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA)
• DNA contains the genetic instructions for the development and function of living organisms
• RNA plays a role in protein synthesis and gene regulation
• Nucleic acids are polymers of nucleotides, linked together by phosphodiester bonds

Nucleotides

• These are the building blocks of nucleic acids
• Each nucleotide consists of a nitrogenous base, a pentose sugar (deoxyribose in DNA, ribose in RNA), and one or more phosphate groups
• The nitrogenous bases are adenine (A), guanine (G), cytosine (C), thymine (T) in DNA, and uracil (U) in RNA
• Adenine and guanine are purines, which have a double-ring structure
• Cytosine, thymine, and uracil are pyrimidines, which have a single-ring structure
• In DNA, adenine pairs with thymine (A-T) via two hydrogen bonds, and guanine pairs with cytosine (G-C) via three hydrogen bonds
• In RNA, adenine pairs with uracil (A-U)
• The sequence of nucleotides in a nucleic acid determines the genetic information it carries

DNA Structure

• DNA is a double-stranded helix, with two strands running antiparallel to each other (5' to 3' and 3' to 5')
• The sugar-phosphate backbone forms the outside of the helix, and the nitrogenous bases are located on the inside
• The two strands are held together by hydrogen bonds between complementary base pairs (A-T and G-C)
• The double helix is twisted, forming major and minor grooves
• DNA is organized into chromosomes in the nucleus of eukaryotic cells
• The sequence of nucleotides in DNA determines the genetic information that is passed from one generation to the next

RNA Structure

• RNA is typically single-stranded, although it can fold into complex three-dimensional structures
• RNA contains ribose as the sugar and uracil (U) instead of thymine (T)
• There are several types of RNA, each with a specific function in the cell
• Messenger RNA (mRNA) carries genetic information from DNA to the ribosomes, where proteins are synthesized
• Transfer RNA (tRNA) carries amino acids to the ribosomes during protein synthesis
• Ribosomal RNA (rRNA) is a component of ribosomes, which are the sites of protein synthesis
• RNA plays a role in gene regulation, DNA replication, and other cellular processes

Nucleic Acid Function

• DNA stores the genetic information that is needed for the development and function of living organisms
• The genetic information in DNA is copied and passed on to new cells during cell division
• RNA plays a crucial role in protein synthesis, translating the genetic information in DNA into proteins
• RNA also regulates gene expression, controlling which genes are turned on or off in a cell
• Nucleic acids are essential for life, enabling the storage, transmission, and utilization of genetic information