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Carbohydrates
Polysaccharides: Complex carbohydrates are long chains (polymers) of monosaccharides (simple sugars) linked by glycosidic bonds.
Diversity: Polysaccharides can be linear (e.g., cellulose) or branched (e.g., starch, glycogen), leading to different properties.
Function:
Energy Storage: Starch in plants and glycogen in animals serve as short-term energy reserves.
Structural Support: Cellulose provides rigidity to plant cell walls, while chitin forms the exoskeleton of insects and the cell walls of fungi.
Monomer Variation: While glucose is the primary building block, modifications like those in chitin create diverse structures.
Proteins
Primary Structure
The linear sequence of amino acids, which is determined by the genetic code (DNA). The primary structure ultimately influences all other levels of protein structure.
Polypeptides: Proteins are made up of one or more polypeptides, which are linear chains of amino acids connected by peptide bonds.
Peptide Bonds: These bonds form between the carboxyl group (-COOH) of one amino acid and the amino group (-NH₂) of another.
R-Group Importance: The unique properties of each amino acid R-group determine how the polypeptide folds and interacts, ultimately dictating the protein's function.
Secondary Structure
Coiling and Folding: The polypeptide chain begins to fold and coil due to hydrogen bonds between the amino and carboxyl groups in the backbone of the polypeptide (not the side chains).
Alpha Helix: A spiral-shaped structure where hydrogen bonds form every fourth amino acid.
Beta Pleated Sheet: A flat, sheet-like structure where hydrogen bonds form between parallel or antiparallel stretches of the polypeptide chain.
Tertiary Structure
R-Group Interactions: The overall 3D shape of a polypeptide is determined by interactions between R-groups (side chains) of the amino acids. These interactions include:
Hydrophobic interactions: Nonpolar side chains cluster together in the protein's interior, away from water.
Disulfide bridges: Covalent bonds between sulfur atoms in the side chains of cysteine amino acids.
Hydrogen bonds and ionic bonds between other side chains.
Denaturation: When these interactions are disrupted by heat, pH changes, or other factors, the protein loses its shape and function, a process called denaturation.
Quaternary Structure
Multiple Polypeptides: Some proteins consist of multiple polypeptide subunits that come together to form a functional complex.
Example: Hemoglobin, the protein responsible for carrying oxygen in blood, consists of four polypeptide subunits.
Function: All four levels of protein structure (primary, secondary, tertiary, and quaternary) contribute to the protein's final shape and function.
Takeaway: The intricate folding and interactions of polypeptides are essential for proteins to carry out their diverse roles in cells and organisms.
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