Biological Molecules in Cell Biology

Objectives

• Explain what makes water polar and the properties that arise from its polarity, making it essential for cellular life.

• Define salt, acid, base, anion, and cation.

• Understand the pH scale and its importance for maintaining stable cellular environments.

• Describe the structure and function of carbohydrates, lipids, proteins, and nucleic acids.

Water and Its Unique Properties

Polarity of Water (H2O)

Water is indispensable for life due to its molecular structure and chemical properties. Its polarity underlies many of its biological functions.

• Polar molecule: A molecule with an unequal distribution of electrical charge, resulting in regions of partial positive and negative charges.

• In water, oxygen is more electronegative than hydrogen, attracting electrons more strongly. This makes the oxygen atom slightly negative and the hydrogen atoms slightly positive.

• Hydrogen bonds form between water molecules due to this polarity.

Key Properties of Water Due to Polarity

• Universal Solvent: Water dissolves many substances, facilitating chemical reactions in cells.

• Medium for chemical reactions: Most cellular reactions occur in aqueous environments.

Example: Sodium chloride (NaCl) dissolves in water, dissociating into sodium (Na+) and chloride (Cl-) ions.

Cohesion and Adhesion

Water molecules interact with each other and other substances through cohesion and adhesion.

• Cohesion: Attraction between molecules of the same kind (e.g., water molecules sticking together).

• Adhesion: Attraction between molecules of different kinds (e.g., water molecules interacting with cell membranes).

• These properties are critical for the structure of cell membranes and the function of DNA and proteins in cells.

Example: Hydrophilic heads of phospholipids interact with water, while hydrophobic tails avoid water, forming the basis of biological membranes.

High Specific Heat Capacity

Water can absorb and retain heat due to hydrogen bonding, helping cells resist temperature fluctuations.

• Specific heat capacity: The amount of heat required to raise the temperature of water by one degree Celsius.

• This property stabilizes cellular and environmental temperatures.

Ions, Acids, Bases, and pH

Definitions

• Salt: A compound formed from the reaction of an acid and a base, typically dissociating into ions in water.

• Acid: A substance that releases hydrogen ions (H+) in solution.

• Base: A substance that releases hydroxide ions (OH-) or accepts H+ ions.

• Anion: A negatively charged ion (e.g., Cl-).

• Cation: A positively charged ion (e.g., Na+).

The pH Scale

The pH scale measures the concentration of hydrogen ions in a solution, indicating its acidity or basicity.

• pH: Defined as

• Scale ranges from 0 (most acidic) to 14 (most basic); 7 is neutral.

• Each pH unit represents a tenfold change in [H+].

• Cells must maintain pH between 6.5 and 8.5 for optimal function.

• Buffers: Substances that help maintain stable pH in cells.

Organic Molecules in Cells

Carbon and Organic Molecules

Organic molecules are carbon-based compounds essential for life. Carbon's ability to form four covalent bonds allows for diverse structures such as chains and rings.

• Macromolecules: Large molecules formed by linking smaller units (monomers) via dehydration synthesis; broken down by hydrolysis.

• Main types: Carbohydrates, Proteins, Lipids, Nucleic acids.

Carbohydrates

Carbohydrates are the primary energy source for cells and serve structural roles.

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

• Disaccharides: Two monosaccharides joined together (e.g., sucrose).

• Polysaccharides: Long chains of monosaccharides (e.g., cellulose in plant cell walls, chitin in fungi).

Lipids

Lipids are hydrophobic molecules important for energy storage, membrane structure, and signaling.

• Triglycerides: Composed of glycerol and three fatty acids; stored in fat cells.

• Phospholipids: Contain two fatty acids and a phosphate group; major component of cell membranes.

• Steroids: Lipids with carbon ring structures (e.g., cholesterol), help maintain membrane fluidity.

Proteins

Proteins perform diverse functions, including catalysis, transport, movement, and defense.

• Composed of amino acids (20 types) linked by peptide bonds.

• Levels of organization:

  • Primary structure: Sequence of amino acids.

  • Secondary structure: Folding into alpha helices and beta sheets via hydrogen bonding.

  • Tertiary structure: Overall 3D shape due to interactions among side chains.

  • Quaternary structure: Association of multiple polypeptide chains.

• Proteins are functional at tertiary and quaternary levels.

Nucleic Acids

Nucleic acids store and transmit genetic information and direct protein synthesis.

• Made of nucleotides, each containing a pentose sugar, phosphate group, and nitrogenous base.

• DNA: Double-stranded, contains deoxyribose sugar, bases A, T, C, G; stores genetic material.

• RNA: Single-stranded, contains ribose sugar, bases A, U, C, G; involved in protein synthesis.

Additional info: Some explanations and table entries were expanded for clarity and completeness based on standard cell biology curriculum.