Carbohydrates

Monosaccharides

Monosaccharides are the simplest form of carbohydrates, consisting of single sugar molecules. They serve as the building blocks for more complex carbohydrates.

• Definition: A monosaccharide is a single sugar molecule, such as glucose or fructose.

• General Formula: The molecular formula for glucose is .

• Structure: Glucose forms a hexagonal (ring) structure in solution.

• Examples: Glucose, fructose, and galactose are common monosaccharides.

Application: Only glucose is typically used as the monomer for polysaccharide synthesis in biological systems.

Disaccharides

Disaccharides are formed when two monosaccharides are joined together by a covalent bond known as a glycosidic linkage.

• Formation: Two monosaccharides undergo a dehydration synthesis (condensation) reaction, releasing a molecule of water () and forming a glycosidic bond.

• Hydrolysis: The glycosidic bond can be broken by hydrolysis, which adds water to split the disaccharide into two monosaccharides.

• Examples: Sucrose (glucose + fructose), lactose (glucose + galactose), maltose (glucose + glucose).

Equation for Dehydration Synthesis:

Polysaccharides

Polysaccharides are long chains of monosaccharide units linked by glycosidic bonds. They serve as energy storage or structural components in organisms.

• Formation: Polysaccharides are formed by repeated dehydration synthesis reactions joining monosaccharide monomers.

• Energy Storage Polysaccharides:

  • Starch: The main energy storage polysaccharide in plants. Composed of alpha-glucose monomers.

  • Glycogen: The main energy storage polysaccharide in animals. Highly branched, stored in liver and muscle cells.

• Structural Polysaccharides:

  • Cellulose: The main component of plant cell walls. Composed of beta-glucose monomers. Most animals cannot digest cellulose due to the beta linkage.

  • Chitin: Found in fungal cell walls and the exoskeletons of arthropods (e.g., lobsters, crabs, insects).

Comparison Table: Energy Storage vs. Structural Polysaccharides

Additional info: The branching in glycogen and starch allows for rapid addition or removal of glucose units, which is important for energy regulation.

Lipids

Lipids are a diverse group of hydrophobic molecules that include fats, phospholipids, and sterols. They play roles in energy storage, membrane structure, and signaling.

Triglycerides

Triglycerides are the main form of stored energy in animals. They are composed of one glycerol molecule and three fatty acids.

• Structure: Glycerol backbone attached to three fatty acid chains via ester bonds.

• Function: Long-term energy storage.

• Formation: Dehydration synthesis joins fatty acids to glycerol, releasing water.

Equation for Triglyceride Formation:

Saturated vs. Unsaturated Fatty Acids

• Saturated Fatty Acids: All single bonds between carbon atoms; straight chains; solid at room temperature (e.g., butter).

• Unsaturated Fatty Acids: At least one double bond between carbon atoms; bent chains; liquid at room temperature (e.g., olive oil).

Comparison Table: Saturated vs. Unsaturated Fatty Acids

Phospholipids and Sterols

• Phospholipids: Major component of cell membranes. Composed of a hydrophilic (polar) phosphate head and two hydrophobic fatty acid tails.

• Sterols: Include cholesterol and steroid hormones. Characterized by a four-ring structure.

Application: The amphipathic nature of phospholipids (having both hydrophilic and hydrophobic parts) is essential for forming biological membranes.

Proteins

Proteins are polymers of amino acids and perform a vast array of functions in living organisms, including catalysis, structure, transport, and signaling.

Amino Acids

• Definition: The monomer of proteins. Each amino acid contains an amino group (), a carboxyl group (), a hydrogen atom, and a variable R group attached to a central carbon.

• General Structure:

• Variety: There are 20 different amino acids, each with a unique R group.

Peptide Bonds and Protein Structure

• Peptide Bond: A covalent bond formed between the carboxyl group of one amino acid and the amino group of another via dehydration synthesis (releasing ).

• Hydrolysis: Peptide bonds can be broken by the addition of water.

• Polypeptide: A chain of amino acids linked by peptide bonds. The average human polypeptide is about 400 amino acids long.

Equation for Peptide Bond Formation:

Levels of Protein Structure

• Primary Structure: The linear sequence of amino acids in a polypeptide chain.

• Secondary Structure: Local folding into alpha-helices and beta-sheets, stabilized by hydrogen bonds.

• Tertiary Structure: The overall 3D shape of a polypeptide, stabilized by interactions between R groups (hydrophobic interactions, disulfide bridges, ionic bonds, hydrogen bonds).

• Quaternary Structure: The association of multiple polypeptide chains to form a functional protein (not all proteins have this level).

Additional info: A change in the amino acid sequence (primary structure) can alter protein folding and function, potentially leading to diseases.