Water, Salts, Lipids, Proteins, and Nucleic Acids: Functions
Key Biological Molecules and Their Functions
Water is the most abundant molecule in living things, generally comprising about 75% of weight. Chemically, it’s a molecule formed by one oxygen atom and two hydrogen atoms. Due to oxygen’s electronegativity, the molecule has a region of negative charge around the oxygen and positive charges around the hydrogens. This bipolar nature causes water molecules to bond with each other and other molecules through electrostatic interactions, known as hydrogen bonding.
- Universal Solvent: Water dissolves polar molecules (hydrophilic) but not nonpolar molecules (hydrophobic).
- High Specific Heat: A large amount of energy is required to raise the temperature of one gram of water by 1°C. This helps to minimize abrupt temperature changes, preventing thermal shock.
- High Heat of Vaporization: It takes 540 calories to vaporize water at its boiling point (100°C).
- High Surface Tension: Water exhibits surface tension, causing it to behave like an elastic film. For example, insects like water striders can walk on water without sinking.
- Substance Transport: Water is a key component of the internal environment in living organisms, transporting nutrients and waste products as part of the blood.
- Lubricant and Shock Absorber: Water prevents friction between hard parts of the skeleton in joints (synovial fluid in the knee), protecting the bones.
- Lower Density as a Solid: Unlike many other substances, water is less dense in its solid form (ice) than in its liquid form.
Mineral Salts
Mineral salts are formed from dissolved ions such as Cl, Na, K, and Ca. They are involved in regulating water movement into and out of cells, a process related to osmosis. Osmosis is the movement of a solvent from an area of low solute concentration to an area of high solute concentration, separated by a semipermeable membrane. In a hypotonic environment (low ion concentration), water enters the cell to equalize concentrations, causing the cell to swell (turgor). Conversely, in a hypertonic environment (high ion concentration), water leaves the cell, causing it to shrink (plasmolysis).
Lipids
Lipids are composed of C, H, and O and are characterized by their insolubility in water and solubility in organic solvents. Their primary functions are energy storage and structural support. Key types of lipids include:
- Fats: Formed by the combination of glycerol and three fatty acids. Due to their low density and high energy value, they serve as energy reserves.
- Phospholipids: Bipolar molecules with a hydrophilic (water-attracting) end and a hydrophobic (water-repelling) end. These molecules are ideal for forming cell membranes.
- Isoprenoids: Include vitamins such as A, D, E, and K.
Proteins
Proteins are macromolecules formed by the polymerization of amino acids. Each amino acid contains an amino group and a carboxyl group attached to the same carbon atom, and they are distinguished by their R group. Amino acids are linked by peptide bonds. A key characteristic of proteins is their specificity; each species has its own unique proteins, which can even vary within the same species. This specificity is why blood transfusions or transplants can be rejected. A protein’s function depends on its three-dimensional structure, which is its final (quaternary) structure when it consists of multiple amino acid chains.
Functions of proteins:
- Transport of CO2, O2, and Fe
- Immune protection
- Involvement in blood coagulation
- Muscle contraction and relaxation
- Catalysis
- Toxins (e.g., cobra venom)
Nucleic Acids
Nucleic acids are formed by C, H, O, N, and P. There are two types: DNA and RNA. They consist of nucleotide units linked together to form long chains. Each nucleotide is composed of a sugar, a phosphate group, and a nitrogenous base. In DNA, the sugar is deoxyribose, while in RNA, it is ribose.
Function: Nucleic acids carry genetic information and direct the cell to produce proteins.