General Biology Study Notes: pH, Buffers, and Biomolecules

pH and Buffers

Introduction to pH and Buffers

The regulation of pH is essential for biological systems, as many biochemical reactions are sensitive to changes in hydrogen ion concentration. Buffers play a critical role in maintaining stable pH levels in living organisms.

• pH is a measure of the hydrogen ion concentration in a solution, defined as $pH = -\log[H^+]$.

• Buffers are molecules that resist changes in pH by absorbing or releasing hydrogen ions.

• Buffers typically consist of a weak acid and its conjugate base.

Dynamic Equilibrium and Buffer Systems

Biological buffers operate through dynamic equilibrium between their acid and base forms. The carbonic acid-bicarbonate system is a key buffer in human blood.

• Dynamic equilibrium refers to the balance between the dissociated and undissociated forms of a molecule.

• Example: aspirin (acetylsalicylic acid) exists in equilibrium between its undissociated (acid) and dissociated (ion + proton) forms.

• The carbonic acid-bicarbonate buffer system is represented by the equation:

$CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow HCO_3^- + H^+$

• Rapid breathing (hyperventilation) removes protons from solution, affecting the equilibrium and blood pH.

• Infusion of bicarbonate can help restore equilibrium and correct blood acidity.

Law of Mass Action and Buffer Response

The Law of Mass Action states that the direction of a chemical reaction is influenced by the concentrations of reactants and products. In buffer systems, adding or removing components shifts the equilibrium.

• Adding bicarbonate ($HCO_3^-$) drives the reaction to the left, reducing acidity.

• Removing $CO_2$ (as in hyperventilation) also shifts the equilibrium, affecting pH.

• Buffers are most effective when only partially dissociated.

Buffer Function in Blood

Carbonic acid ($H_2CO_3$) and bicarbonate ($HCO_3^-$) maintain blood pH within a narrow range, crucial for physiological function.

• When excess acid is present, bicarbonate absorbs protons, reducing acidity.

• When the solution is too basic, carbonic acid donates protons.

• Buffering capacity is highest when the acid and base forms are present in similar concentrations.

Biomolecules: Structure and Function

Introduction to Biomolecules

Biomolecules are essential for life, serving as building blocks and functional molecules in cells. The four major classes are carbohydrates, proteins, nucleic acids, and lipids.

• Polymers are large molecules made of repeating subunits called monomers.

• Carbohydrates, proteins, and nucleic acids are polymers; lipids are not true polymers.

Polymerization and Hydrolysis

Polymers are formed and broken down by specific chemical reactions involving water.

• Dehydration reaction: Monomers are joined by removing a water molecule, forming a covalent bond.

• Hydrolysis reaction: Polymers are broken down by adding water, cleaving the covalent bond.

Example equation for dehydration:

$\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + H_2O$

Example equation for hydrolysis:

$\text{Polymer} + H_2O \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio. They serve as energy sources and structural components.

• Monosaccharides: Simple sugars (e.g., glucose, fructose, galactose) with 3-7 carbon atoms.

• Disaccharides: Two monosaccharides joined by a glycosidic bond (e.g., sucrose, maltose, lactose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).

Monosaccharides

• Glucose is the primary energy source for cells; its ring form is common in biological systems.

• Deoxyribose and ribose are sugars found in DNA and RNA, respectively.

Disaccharides

• Formed by dehydration reactions between two monosaccharides.

• Sucrose (table sugar) = glucose + fructose.

• Maltose (found in beer) = glucose + glucose.

• Lactose (milk sugar) = glucose + galactose.

Polysaccharides

• Starch: Storage form of glucose in plants.

• Glycogen: Storage form of glucose in animals, found in liver and muscle cells.

• Cellulose: Structural component in plant cell walls.

• Chitin: Structural polysaccharide in insect exoskeletons.

Hydrocarbons

Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen. They are important in biology for energy storage and as components of cell membranes.

• Hydrophobic: Hydrocarbons do not mix with water due to nonpolar bonds.

• Hydrophilic: Molecules that mix well with water, typically containing polar bonds (e.g., O-H).

• Adipose tissue stores energy in the form of hydrocarbon-rich fat molecules.

Summary Table: Types of Carbohydrates

Type	Structure	Example	Function
Monosaccharide	Single sugar unit	Glucose, Fructose	Energy source
Disaccharide	Two sugar units	Sucrose, Lactose	Transport, energy
Polysaccharide	Many sugar units	Starch, Glycogen, Cellulose	Storage, structure

Additional info:

• Buffers are crucial in maintaining homeostasis in biological systems.

• Hydrolysis reactions are essential for digestion and cellular metabolism.

• Polysaccharides can have branched or unbranched structures, affecting their function.