Biological Molecules

Cheatsheet Content

### Water - **Structure:** Polar molecule due to uneven sharing of electrons between O and H (O is slightly negative, H slightly positive). - **Hydrogen Bonding:** Weak bonds between water molecules, responsible for many properties. - **Properties & Importance:** - **High Specific Heat Capacity:** Buffers temperature changes (stable internal environment, aquatic habitats). - **High Latent Heat of Vaporisation:** Evaporation cools organisms (sweating). - **Cohesion:** Water molecules stick together (transport in plants - transpiration stream). - **Adhesion:** Water sticks to other polar surfaces. - **Solvent:** Dissolves polar molecules and ions (transport medium for nutrients, wastes). - **Metabolite:** Involved in hydrolysis and condensation reactions. - **High Surface Tension:** Supports small organisms. - **Density Anomaly:** Ice is less dense than water (aquatic life survives under ice). ### Carbohydrates - **General Formula:** $(CH_2O)_n$ - **Monosaccharides:** Simple sugars (e.g., glucose, fructose, galactose). - **Glucose:** Alpha ($\alpha$) and Beta ($\beta$) isomers. Ring structure. - **Role:** Primary energy source for respiration. - **Reducing Sugars:** All monosaccharides and some disaccharides (e.g., maltose, lactose) have a free aldehyde or ketone group that can reduce other compounds. Detected by Benedict's test. - **Disaccharides:** Two monosaccharides joined by a glycosidic bond (condensation reaction). - **Maltose:** Glucose + Glucose - **Sucrose:** Glucose + Fructose - **Lactose:** Glucose + Galactose - **Polysaccharides:** Many monosaccharides joined. - **Starch:** Energy storage in plants. - **Amylose:** Unbranched, helical (compact). - **Amylopectin:** Branched (rapid hydrolysis). - **Glycogen:** Energy storage in animals. Highly branched (rapid hydrolysis). - **Cellulose:** Structural component in plant cell walls. - Long, unbranched chains of $\beta$-glucose. - Alternate glucose molecules are inverted, allowing H-bonds between parallel chains. - Forms microfibrils, providing high tensile strength. - **Chitin:** Structural component in fungal cell walls and insect exoskeletons. ### Lipids - **Insoluble** in water (non-polar). Soluble in organic solvents. - **Triglycerides:** - **Structure:** One glycerol molecule + three fatty acid molecules. - **Bond:** Ester bond (condensation reaction). - **Fatty Acids:** - **Saturated:** No C=C double bonds (straight chains, solid at room temp). - **Unsaturated:** One or more C=C double bonds (kinks, liquid at room temp). - **Role:** Long-term energy storage (high energy/mass ratio), insulation, protection. - **Phospholipids:** - **Structure:** One glycerol + two fatty acids + one phosphate group. - **Amphipathic:** Hydrophilic phosphate head, hydrophobic fatty acid tails. - **Role:** Forms cell membranes (phospholipid bilayer). - **Cholesterol:** Steroid lipid. - **Role:** Maintains fluidity of animal cell membranes, precursor for steroid hormones. ### Proteins - **Monomers:** Amino acids. - **Structure of Amino Acids:** Central carbon, amino group (-NH2), carboxyl group (-COOH), hydrogen atom, R-group (side chain). - **Peptide Bond:** Forms between amino group of one AA and carboxyl group of another (condensation reaction). - **Levels of Protein Structure:** - **Primary:** Sequence of amino acids in polypeptide chain. Determined by gene sequence. - **Secondary:** Folding into $\alpha$-helix or $\beta$-pleated sheet due to hydrogen bonds between backbone atoms. - **Tertiary:** 3D folding of polypeptide chain due to interactions between R-groups: - Hydrogen bonds - Ionic bonds - Disulfide bridges (covalent between cysteine R-groups) - Hydrophobic interactions - **Quaternary:** Arrangement of multiple polypeptide chains (subunits). E.g., Haemoglobin. - **Denaturation:** Loss of 3D structure (secondary, tertiary, quaternary) due to extreme pH or temperature, leading to loss of function. - **Functions:** Enzymes, structural (collagen, keratin), transport (haemoglobin), hormones, antibodies. ### Nucleic Acids - **Monomers:** Nucleotides. - **Nucleotide Structure:** Pentose sugar (deoxyribose in DNA, ribose in RNA), phosphate group, nitrogenous base. - **Nitrogenous Bases:** - **Purines:** Adenine (A), Guanine (G) (double-ring). - **Pyrimidines:** Cytosine (C), Thymine (T) in DNA, Uracil (U) in RNA (single-ring). - **DNA (Deoxyribonucleic Acid):** - **Structure:** Double helix. Two polynucleotide strands held by hydrogen bonds between complementary bases. - **Base Pairing:** A-T (2 H-bonds), G-C (3 H-bonds). - **Sugar-Phosphate Backbone:** Covalent phosphodiester bonds. - **Antiparallel Strands:** One runs 5' to 3', other 3' to 5'. - **Role:** Stores genetic information. - **RNA (Ribonucleic Acid):** - **Structure:** Single polynucleotide strand. - **Bases:** A, U, G, C. - **Role:** Involved in protein synthesis (mRNA, tRNA, rRNA). ### ATP (Adenosine Triphosphate) - **Structure:** Adenine, ribose sugar, three phosphate groups. - **Role:** Universal energy currency of the cell. - **Hydrolysis:** ATP + H$_2$O $\rightarrow$ ADP + P$_i$ + Energy. - Energy released is used for metabolic processes (active transport, muscle contraction, synthesis). - **Synthesis:** ADP + P$_i$ + Energy $\rightarrow$ ATP + H$_2$O (phosphorylation). - Occurs during respiration and photosynthesis.