TheCell7e Ch02 Lecture

2 Molecules and Membranes

The Molecules of Cells

• Essential molecules for cellular functions include water, ions, organic compounds like carbohydrates, lipids, proteins, and nucleic acids.

Enzymes as Biological Catalysts

• Enzymes serve as catalysts in biochemical reactions, increasing reaction rates without being consumed.

Cell Membranes

• Composed primarily of phospholipid bilayers with proteins; they control substance movement in and out of cells.

Introduction

• Cells display a high degree of complexity and diversity.

• They adhere to the same physical and chemical laws as nonliving systems.

• Modern cell biology examines cellular processes through the lens of chemical and physical reactions.

The Molecules of Cells

Water

• Most abundant molecule in cells, exhibiting polar characteristics due to the positive hydrogen and negative oxygen charges.

• Capable of forming hydrogen bonds with itself or with other polar molecules, influencing solubility and interaction with ions.

Characteristics of Water

• Hydrophilic: Ions and polar molecules dissolve easily in water.

• Hydrophobic: Nonpolar molecules fare poorly in water.

• Importance in biological systems; water’s properties support life processes.

Inorganic Ions

• Comprise less than 1% of cellular mass, vital for cell function.

• Key ions include:

  • Sodium (Na+)

  • Potassium (K+)

  • Magnesium (Mg2+)

  • Calcium (Ca2+)

  • Phosphate (HPO4^2−)

  • Chloride (Cl−)

  • Bicarbonate (HCO3−)

Organic Molecules

• Unique constituents of cells, categorized into four classes:

  • Carbohydrates

  • Lipids

  • Proteins

  • Nucleic Acids

Carbohydrates

• Subdivided into simple sugars (monosaccharides) and complex polymers (polysaccharides).

• Basic formula: (CH2O)n; glucose serves as a primary energy source.

Formation of Polysaccharides

• Monosaccharides are linked via glycosidic bonds formed through dehydration reactions.

• Types of polysaccharides include:

  • Glycogen (animal glucose storage)

  • Starch (plant glucose storage)

  • Cellulose (structural component of plant cell walls)

• Chitin in animal exoskeletons parallels cellulose in structure and function.

Lipids

• Key roles include energy storage, structural cell membrane components, and involvement in cell signaling.

• Types of fatty acids:

  • Saturated: No double bonds.

  • Unsaturated: One or more double bonds; affects fluidity.

• Triacylglycerols: Formed from three fatty acids and glycerol, serving as energy storage.

Phospholipids

• Fundamental for cell membrane structure, featuring hydrophilic heads and hydrophobic tails, creating bilayers.

• Glycolipids and Cholesterol also play significant roles in membrane fluidity and stability.

Nucleic Acids

• DNA and RNA are essential for genetic information storage and transmission.

• DNA comprises two strands; RNA exists as single-stranded with varied functions in protein synthesis and gene regulation.

Proteins

• Most diverse macromolecules directing nearly all cell functions, made from 20 different amino acids.

• Enzymatic activities, transport, structural support, and immune defense are key protein roles.

• Proteins fold into specific 3D structures, crucial for their function, influenced by amino acid sequences.

Protein Structure Levels

1. Primary Structure: Sequence of amino acids.

2. Secondary Structure: Localized folding (α-helix/beta-sheet).

3. Tertiary Structure: Overall 3D shape formed by amino acid side chain interactions.

4. Quaternary Structure: Assembly of multiple polypeptide chains.

Enzymes as Biological Catalysts

• Enzymes enhance reaction rates, lower activation energy, and bind substrates to form enzyme-substrate complexes.

• Specificity of enzymes determined by active site shape, requiring precise substrate interactions.

• Feedback inhibition and phosphorylation are key mechanisms for enzyme regulation, ensuring cells react to different physiological conditions.

Cell Membranes

• Phospholipid bilayers with proteins facilitate a variety of biological functions and maintain selective permeability.

• Membrane fluidity is influenced by lipid composition and temperature, with cholesterol playing a critical role.

• Integral and peripheral proteins have distinct functions, from transport to signaling, maintaining cellular integrity and responsiveness.

Summary Insights

• Understanding cellular components and mechanisms is crucial for grasping biological processes.

• Interactions between different biological molecules form the basis of cell function.