Carbohydrates

Classification and Structure of Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, and serve as a primary energy source for living organisms. They are classified based on their structure and the number of sugar units present.

• Monosaccharides: Simple sugars with a single polyhydroxy aldehyde or ketone unit (e.g., glucose).

• Disaccharides: Composed of two monosaccharide units joined by a glycosidic bond (e.g., sucrose).

• Polysaccharides: Large polymers of monosaccharides (e.g., starch, glycogen, cellulose).

Monosaccharides are further classified by:

• Aldose: Contains an aldehyde group.

• Ketose: Contains a ketone group.

• Number of carbons: Triose (3C), tetrose (4C), pentose (5C), hexose (6C).

Chirality and Isomerism

• Chiral Molecule: A molecule that is non-superimposable on its mirror image.

• Chiral Carbon: A carbon atom bonded to four different groups.

• Enantiomers: Mirror-image isomers of chiral molecules.

• D- and L- Isomers: Determined by the position of the -OH group on the chiral carbon farthest from the carbonyl group in Fischer projections.

Example: D-glucose vs. L-glucose differ in the configuration at all chiral centers.

Cyclic Forms and Anomerism

• Monosaccharides can cyclize to form ring structures (Haworth projections) in solution.

• These cyclic forms are in equilibrium with their open-chain (Fischer projection) forms.

• Anomers: Isomers differing at the new chiral center formed during ring closure (α and β forms).

Example: α-D-glucose vs. β-D-glucose differ in the orientation of the -OH group on the anomeric carbon.

Polysaccharides and Glycosidic Bonds

• Glycosidic Bond: Covalent bond joining monosaccharide units (e.g., α(1→4) linkage).

• Starch: Plant polysaccharide; amylose (unbranched, α(1→4)), amylopectin (branched, α(1→4) and α(1→6)).

• Glycogen: Animal storage polysaccharide; highly branched (similar to amylopectin).

• Cellulose: Linear polymer with β(1→4) glycosidic bonds; not digestible by humans.

Table: Major Glucose Polysaccharides

Additional info: Branched polysaccharides provide more ends for enzymatic breakdown, allowing rapid glucose release.

Lipids

Classification and Properties of Lipids

Lipids are a diverse group of hydrophobic biomolecules, insoluble in water, and serve as energy storage, structural components, and signaling molecules.

• Fatty Acids: Long hydrocarbon chains with a carboxylic acid group.

• Waxes: Esters of fatty acids and long-chain alcohols.

• Triacylglycerols (Triglycerides): Esters of glycerol and three fatty acids; main storage form of fat.

• Steroids: Four fused carbon rings; cholesterol is the most abundant steroid.

Fatty Acids: Types and Properties

• Saturated: No double bonds; higher melting points; solid at room temperature (animal fats).

• Monounsaturated: One double bond.

• Polyunsaturated: Multiple double bonds; lower melting points; liquid at room temperature (plant oils).

• Omega-3 and Omega-6: Classified by the position of the first double bond from the methyl end.

Example: Oleic acid (monounsaturated), linoleic acid (polyunsaturated, omega-6).

Reactions of Triacylglycerols

• Hydrogenation: Addition of H2 to unsaturated fatty acids, converting double bonds to single bonds (used to make margarine).

• Cis vs. Trans: Hydrogenation can produce trans fats, which have higher melting points and are less healthy.

• Hydrolysis: Splitting of ester bonds by water, producing glycerol and fatty acids.

• Saponification: Base-catalyzed hydrolysis of triacylglycerols, producing soap and glycerol.

Equation for Saponification:

Phospholipids and Cell Membranes

• Glycerophospholipids: Main component of cell membranes; contain a polar head and two nonpolar fatty acid tails.

• Form lipid bilayers with hydrophobic tails inward and hydrophilic heads outward.

• Lipoproteins: Transport cholesterol and other lipids in the bloodstream.

Amino Acids, Proteins, and Enzymes

Structure and Classification of Amino Acids

Amino acids are the building blocks of proteins, each containing a central (α) carbon, an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group).

• Classification: Based on side chain properties: nonpolar, polar, acidic, or basic.

• Essential Amino Acids: Nine amino acids that must be obtained from the diet.

• At physiological pH (7.4), amino acids exist as zwitterions: and .

Peptides and Protein Structure

• Peptide Bond: Amide bond linking the carboxyl group of one amino acid to the amino group of another.

• Dipeptide: Two amino acids joined by a peptide bond.

• Polypeptide: Chain of many amino acids.

• Protein: One or more polypeptide chains folded into a functional structure.

• Written from N-terminus (left, ) to C-terminus (right, ).

Levels of Protein Structure

• Primary: Sequence of amino acids.

• Secondary: Local folding (α-helix, β-pleated sheet) stabilized by hydrogen bonds.

• Tertiary: Overall 3D shape stabilized by hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges.

• Quaternary: Association of multiple polypeptide chains.

Table: Forces Stabilizing Protein Structure

Protein Denaturation

• Loss of native structure due to changes in pH, temperature, or chemical exposure.

• Disrupts secondary, tertiary, and quaternary structure, leading to loss of function.

Enzymes and Catalysis

• Enzymes: Biological catalysts that speed up reactions by lowering activation energy.

• Active Site: Region where substrate binds and reaction occurs.

• Enzyme-Substrate Complex: Temporary association during catalysis.

• Enzyme Inhibitors: Molecules that decrease enzyme activity by blocking or altering the active site.

• Enzyme activity is sensitive to pH and temperature (optimal at 37°C and pH 7.4).

Nucleic Acids and Protein Synthesis

Structure of Nucleic Acids

Nucleic acids (DNA and RNA) are polymers of nucleotides, which store and transmit genetic information.

• Nucleotide: Composed of a pentose sugar, phosphate group, and nitrogenous base.

• DNA: Deoxyribonucleic acid; double helix; bases A, T, C, G.

• RNA: Ribonucleic acid; single-stranded; bases A, U, C, G.

Base Pairing and Structure

• DNA strands held together by hydrogen bonds between complementary bases: A=T (2 H-bonds), C≡G (3 H-bonds).

• RNA uses uracil (U) instead of thymine (T).

DNA Replication and Transcription

• Replication: DNA makes an exact copy of itself during cell division.

• Transcription: DNA sequence is copied into mRNA in the nucleus.

• Translation: mRNA is decoded by ribosomes to synthesize proteins in the cytoplasm.

Types of RNA:

• mRNA (messenger RNA): Carries genetic code from DNA to ribosome.

• tRNA (transfer RNA): Brings amino acids to ribosome during translation.

• rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.

Genetic Code and Mutations

• Genetic Code: Sequence of three bases (codon) on mRNA specifies an amino acid.

• Mutations are changes in DNA sequence that can lead to nonfunctional proteins and genetic diseases.

Viruses and Disease

• Viruses infect cells by injecting their genetic material, hijacking the host's machinery for replication.

• Some viruses can cause cancer by disrupting normal cell regulation.

Identification of Biomolecules

Recognizing Biomolecules by Structure

• Monosaccharide: Single ring or chain with multiple -OH groups.

• Disaccharide: Two monosaccharide units joined by a glycosidic bond.

• Polysaccharide: Long chains of monosaccharide units (e.g., starch, glycogen, cellulose).

• Fatty Acid: Long hydrocarbon chain with terminal carboxylic acid; can be saturated or unsaturated.

• Triacylglycerol: Glycerol backbone with three fatty acid esters.

• Wax: Ester of a fatty acid and a long-chain alcohol.

• Steroid: Four fused carbon rings.

• Amino Acid: Central carbon with amino, carboxyl, hydrogen, and R group.

• Polypeptide/Protein: Chain of amino acids; may show α-helix or β-sheet structure.

• DNA Nucleotide: Deoxyribose, phosphate, and nitrogenous base (A, T, C, G).

• RNA Nucleotide: Ribose, phosphate, and nitrogenous base (A, U, C, G).

• DNA Double Helix: Two antiparallel strands coiled into a helix.