Cell Types: Prokaryotic vs. Eukaryotic

Comparison of Prokaryotic and Eukaryotic Cells

Cells are the fundamental units of life and can be classified as either prokaryotic or eukaryotic based on their structural features.

• Prokaryotic (Prok.) Cells:

  • Nucleus: None; DNA is located in a region called the nucleoid.

  • Size: Generally smaller.

  • Structure: Simpler; lack membrane-bound organelles.

  • DNA: Circular.

  • Cell Membrane: Primarily made of phospholipids; may have a rigid cell wall (often made of peptidoglycan in bacteria).

  • Examples: Bacteria and Archaea.

• Eukaryotic (Euk.) Cells:

  • Nucleus: Present, which houses DNA (has nucleolus).

  • Size: Generally larger.

  • Structure: More complex with several membrane-bound organelles.

  • DNA: Linear chromosomes within the nucleus.

  • Cell Membrane: Made of phospholipids; some (like plant cells) have an additional cell wall (made of cellulose).

  • Examples: Animals, Plants, Fungi, and Protists.

Origin of Life and Domains

Abiogenesis and the Three Domains of Life

Life is thought to have emerged from non-living matter through a process called abiogenesis, involving the formation of organic molecules and the development of self-replicating systems.

• Importance of Carbon: Carbon atoms form four bonds, allowing a diversity of bonds and stable structures.

• Development of Inheritance: Central Dogma of Molecular Biology:

  • DNA → Transcription → RNA → Translation → Protein

The three domains of life (Bacteria, Archaea, Eukarya) evolved from a common ancestor known as LUCA (Last Universal Common Ancestor).

Tree of Life and Molecular Tools

Evolutionary Relationships and Molecular Sequencing

The tree of life represents evolutionary relationships among organisms. Molecular tools such as DNA sequencing and gene comparisons are used to determine relatedness. Ribosomes are universal and similar across all life forms.

Eukaryotic Cell Organelles

Structure and Function of Major Organelles

• Nucleus: Contains DNA; directs cell activity.

• Mitochondria: Produces ATP; site of cellular respiration.

• Lysosome: Breaks down waste.

• Endoplasmic Reticulum (ER): Synthesizes and transports proteins (Rough ER) and lipids (Smooth ER).

• Golgi Apparatus: Modifies and packages proteins/lipids.

• Chloroplast: Photosynthesis in plant cells.

• Vacuole: Storage (large in plants).

Cell Membrane Structure and Function

Phospholipid Bilayer and Membrane Properties

• Structure: Phospholipid bilayer with embedded proteins.

  • Hydrophilic (polar) heads face outward toward water.

  • Hydrophobic (non-polar) tails face inward, away from water.

• Function: Regulates the entry/exit of substances, provides protection, and communicates with the external environment.

Transport Across Membranes

Types of Membrane Transport

• Passive Transport: No energy needed. Includes:

  • Diffusion: Small molecules (O2, CO2, H2O) move from high to low concentration.

  • Facilitated Diffusion: Uses protein channels for larger or charged molecules.

• Active Transport: Requires energy (usually ATP). Moves substances against their concentration gradient.

Amino Acids and Proteins

Structure and Function

• Amino Acids: Building blocks of proteins, consisting of a central carbon, amino group, carboxyl group, and a side chain (R group). The side chain determines the amino acid's properties and function.

• Proteins: Polymers of amino acids. Functions include structure, enzymes, transport, signaling, and more.

General Structure of an Amino Acid:

H2N–CHR–COOH

Enzymes: Structure and Function

Enzyme Catalysis and Regulation

• Enzymes: Proteins that act as catalysts to speed up chemical reactions by lowering the activation energy required.

Activation Energy Diagram:

• Uncatalyzed reactions have higher activation energy than catalyzed reactions.

Enzyme Inhibition:

• Competitive Inhibition: Inhibitor binds to the active site, blocking substrate binding.

• Allosteric (Non-competitive) Inhibition: Inhibitor binds elsewhere, changing enzyme shape and reducing activity.

• End-product Inhibition: Final product of a pathway inhibits an earlier enzyme, regulating the pathway (feedback inhibition).

Protein and DNA Structure

Subunits and Levels of Protein Structure

• Proteins: Made of amino acids.

  • Primary Structure: Sequence of amino acids.

  • Secondary Structure: Local folding (alpha helices, beta sheets).

  • Tertiary Structure: 3D folding due to side chain interactions.

  • Quaternary Structure: Multiple polypeptide chains.

• DNA: Made of nucleic acids.

Factors Influencing Protein Folding:

• Heat

• pH

• Salt conditions

• Chemicals

Bioenergetics

Redox Reactions and Energy Pathways

Cells acquire energy through redox reactions (oxidation-reduction) in metabolic pathways such as glycolysis, the citric acid cycle, and the electron transport chain.

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP to form ATP (occurs in glycolysis and citric acid cycle).

• Oxidative phosphorylation: ATP production via electron transport chain.

Metabolic Pathways: Glycolysis and Citric Acid Cycle

ATP Synthesis and Electron Transport

• Glycolysis: Produces 2 pyruvate molecules, which enter the citric acid cycle.

• Citric Acid Cycle: Generates electron carriers (NADH, FADH2).

• Both pathways donate electrons to the electron transport chain, leading to ATP synthesis.

Anaerobic vs. Aerobic Metabolism

Main Differences and Outcomes

• Anaerobic: Occurs without oxygen, produces less ATP, leads to fermentation (e.g., lactic acid in animals, ethanol in yeast).

• Aerobic: Requires oxygen, produces more ATP via citric acid cycle and electron transport chain.

• Glycolysis can occur with or without oxygen, but prefers oxygen for higher ATP yield.

Free Energy and Reaction Spontaneity

Gibbs Free Energy (ΔG)

• ΔG (change in Gibbs free energy):

  • Negative ΔG: Reaction is spontaneous (exergonic).

  • Positive ΔG: Reaction is non-spontaneous (endergonic).

Kinetics vs. Thermodynamics

Energy Changes in Reactions

• Kinetics: Study of reaction rates.

• Thermodynamics: Study of energy changes in reactions.

Graphical Interpretation of Energy Changes

Energy vs. Reaction Progress

• Graphs show the energy profile of reactions, including activation energy and the effect of enzymes on lowering this barrier.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Key Equations

• Gibbs Free Energy: Where is the change in free energy, is the change in enthalpy, is temperature (in Kelvin), and is the change in entropy.