Biology: Proteins, Carbohydrates, and Lipids

Proteins, Carbohydrates, and Lipids

•  Macromolecules are large molecules found in living things. Examples include proteins, carbohydrates, lipids, and nucleic acids.
•  Proteins are polymers of amino acids. They have diverse functions, notably catalyzing reactions (enzymes), providing support, recognizing and responding to non-self substances, controlling physiological processes, carrying substances within the organism, determining gene expression rates, and causing movement.
•  Proteins are made up of one or more polypeptide chains, which are single, unbranched chains of amino acids.
•  The 3-dimensional shape of a protein is determined by the sequence of amino acids.
•  Amino acids have a carboxyl group and an amino group, functioning as both acid and base.
•  Side chains (R-groups) also have functional groups that affect protein structure and function.
•  Amino acids are categorized by their side chains.
•  The alpha carbon is asymmetrical, allowing for different isomers like D- and L-amino acids.
•  Cysteine has an -SH group that can react with another cysteine side chain to form a disulfide bridge (S-S). These bridges can be important in protein folding, but most cysteines are not involved.
•  Oligopeptides (peptides) are short polymers of 20 or fewer amino acids.
•  Polypeptides are longer protein polymers.
•  Amino acids bond via peptide linkages (peptide bonds), during condensation reactions.
•  The primary structure of a protein involves the sequence of amino acids. Side chain properties define secondary and tertiary protein structure. The number of possible proteins from the 20 amino acids is substantial.

Macromolecules and Interactions

•  Condensation reactions create covalent bonds between monomers to form polymers, releasing a water molecule.
•  Hydrolysis reactions break down polymers into monomers, consuming a water molecule.

Carbohydrates

•  Carbohydrates have the formula (C₁H₂O₁)ₙ. They are sources and transporters of stored energy, and components of many other molecules. They also form extracellular structures like cell walls.
•  Monosaccharides are simple sugars.
•  Disaccharides are formed when two simple sugars are linked by covalent bonds.
•  Oligosaccharides have 3 to 20 monosaccharides.
•  Polysaccharides have hundreds or thousands of monosaccharides.
•  All cells use glucose as an energy source in a straight chain or ring form (more stable), which exists as a- or B-glucose, and can interconvert.
•  Pentoses are five-carbon sugars (e.g., ribose, deoxyribose found in RNA and DNA).
•  Hexoses are six-carbon sugars (e.g., glucose, fructose, mannose), some of which are structural isomers.
•  Monosaccharides link via condensation reactions to form glycosidic bonds.
•  Oligosaccharides are attached to proteins/lipids and function as recognition signals (e.g., ABO blood types).
•  Polysaccharides are large polymers of monosaccharides linked by glycosidic bonds.
• Examples are starch (plant glucose storage), glycogen (animal glucose storage), and cellulose (plant structural component).
•  Carbohydrates are modified via functional group addition, yielding sugar phosphates, amino sugars, and chitin.

Lipids

•  Lipids are nonpolar hydrocarbons; insoluble in water.
•  Weak but additive van der Waals forces hold lipids together in aggregates.
•  Lipids include fats and oils (store energy), phospholipids (structural role in cell membranes), carotenoids and chlorophylls (capture light energy, Vitamin A), steroids and modified fatty acids (hormones, vitamins), and waxes.
•  Animal fats provide thermal insulation. Lipid coating around nerves insulates electrically. Oil and waxes on skin/fur/feathers repel water, slowing evaporation.
•  Fats and oils are triglycerides consisting of three fatty acids and glycerol.
•  Fatty acids are nonpolar hydrocarbon chains with a polar carboxyl group.
•  Carboxyls bond with glycerol hydroxyls via ester linkages (condensation reactions).
•  Saturated fatty acids lack double bonds between carbons, packed tightly (solid).
•  Unsaturated fatty acids have one or more double bonds between carbons; kinks prevent packing; liquid at room temperature; cis vs trans isomers affect properties.
•  Polyunsaturated fatty acids have two or more double bonds. Omega-3 fatty acids protect against heart disease.
•  Phospholipids are similar to triglycerides, but one fatty acid is replaced by a phosphate group, creating a hydrophilic head and hydrophobic tails, these being amphipathic molecules.
•  Phospholipids form bilayers in water, creating cell membranes.
•  Lipoproteins in animals transport lipids (e.g., cholesterol) in the blood.
•  Carotenoids are light-absorbing pigments in plants, converting to Vitamin A in humans.
•  Steroids have multiple rings and include cholesterol and hormones like estradiol.
•  Waxes are long-chain alcohols bound to unsaturated fatty acids.

Protein Structure

•  Proteins can bind non-covalently to other molecules via shape and chemistry (surface R group interactions via ionic, hydrophobic, or hydrogen bonds) factors.
•  Denaturation occurs when heat or pH changes disrupt the secondary, tertiary, and quaternary structures of proteins. Some proteins revert to their original structure upon cooling (reversible denaturation). However, sometimes the change is irreversible.
•  Chaperones (e.g., heat shock proteins) help proteins fold correctly and prevent inappropriate binding.