Acids & Biomolecules: Quick Review

Atoms, Elements, Isotopes, and Molecules

Elements are fundamentally defined by their atomic number, which is the specific quantity of protons they possess. In a neutral atom, the number of protons and electrons is equal. Atoms can bond together to form molecules. An isotope refers to an atom of the same element that has a different number of neutrons than other atoms of that element. A classic example is water, composed of hydrogen and oxygen, represented by the chemical formula H_2O, which illustrates how molecules are formed from bonded atoms.

Bonding: Covalent and Ionic

Covalent bonds involve atoms sharing electrons to achieve a stable, full outer electron shell, often an octet. In this type of bond, neither atom fully donates nor accepts electrons; instead, they both attain an electron configuration that is stable. Conversely, ionic bonds occur when one atom completely donates electrons and another atom accepts them, resulting in the formation of charged particles called ions. These ions are categorized as cations (positively charged, having lost electrons) or anions (negatively charged, having gained electrons), and they form when atoms are no longer electrically neutral.

Water, Hydrogen Bonding, and pH

Water is crucial for life, largely due to its capacity for hydrogen bonding and its role as an excellent solvent. Hydrogen bonds are weaker when compared to the strength of covalent bonds. Water also undergoes self-ionization, a reversible process represented by the equilibrium H_2O \rightleftharpoons H^+ + OH^-. In pure water, the concentration of hydrogen ions [H^+] is equal to the concentration of hydroxide ions [OH^-], resulting in a neutral pH. Acids are substances that increase the concentration of H^+, leading to a lower pH, while bases increase the concentration of OH^-, resulting in a higher pH. The pH scale quantifies acidity or alkalinity, where a high H^+ concentration corresponds to a low pH. For instance, the pH of vertebrate blood is carefully maintained and buffered around pH \approx 7.4.

Acids, Bases, and Buffers

A buffer system plays a vital role in stabilizing pH by reacting with or releasing H^+ or OH^- to counteract the effects of added acids or bases. In vertebrate blood, the carbonic acid-bicarbonate buffer system is critical for maintaining pH homeostasis. Specifically, carbonic acid helps neutralize bases and can form bicarbonate ions, thus working to keep the blood pH around its neutral set point.
The main buffer equilibrium in this system is described by the equation: H2CO3 \rightleftharpoons H^+ + HCO_3^-. This equilibrium allows the system to absorb excess H^+ or OH^-, preventing drastic changes in pH.

Biomolecules: Monomers and Polymers

Biomolecules, the essential components of living organisms, are constructed from smaller repeating units called monomers that are linked together to form larger structures known as polymers. Carbohydrates and lipids serve as primary energy-storage molecules within cells, in addition to fulfilling important structural and signaling roles.

Carbohydrates: Monomers to Polymers

Glucose, a monosaccharide with the chemical formula C6H{12}O_6, is the fundamental monomer of carbohydrates. Carbohydrates are generally characterized by a 1:2:1 ratio of carbon, hydrogen, and oxygen atoms, and their names often end with the suffix -ose. Disaccharides, which are molecules composed of two monosaccharides, can be broken down to glucose and are important for energy transport. Polysaccharides, such as glycogen found in animals and starch in plants, are large polymers of monosaccharides that store energy for longer periods.

Lipids: Structure and Energy Storage

Lipids are a diverse group of biomolecules rich in hydrocarbons, meaning they primarily consist of hydrogen and carbon atoms. Due to their nonpolar nature, they are sparingly soluble in water. Their structure typically involves fatty acids attached to a glycerol backbone, with three fatty acids per glycerol molecule. As energy storage molecules, fats are highly efficient, storing approximately 2–3 times more energy per unit mass than glycogen, and they serve as long-term energy reserves in adipose tissue.

Glucose as Primary Energy Source

Glucose functions as the primary energy source for cells. Cells oxidize glucose through various metabolic pathways to produce adenosine triphosphate (ATP), which is the direct energy currency used to power cellular work. Therefore, glucose is the fundamental starting point for providing both immediate and longer-term energy storage forms within an organism.

Carbohydrate vs Lipid Energy Storage in Cells

Both carbohydrates and lipids are essential for energy storage in cells, but they serve different roles. Disaccharides and polysaccharides facilitate the transport and storage of energy, with glycogen in animals and starches in plants acting as readily accessible reserves for quick energy mobilization. Conversely, fats provide a substantial amount of energy but are less readily mobilized for immediate use. This leads to their differential usage depending on the organism's energy demands and dietary context, with carbohydrates often providing quick energy and lipids providing more prolonged energy storage.

Diffusion and Osmosis: Movement of Molecules and Water

Diffusion describes the passive movement of substances from an area of higher concentration to an area of lower concentration, driven by the desire to reduce differences in concentration. Osmosis is a specific type of diffusion that involves the movement of water across a semipermeable membrane. Water moves towards regions of higher solute concentration to equalize water potential differences. It's important to note that nonpolar, large molecules like fats are less soluble in water, and therefore interact less with water-based solutions, which affects how they influence water movement across membranes.