Cell Structure: Prokaryotic vs Eukaryotic

Prokaryotic Cell Structure

• Prokaryotic cells are simple, single-celled organisms lacking a nucleus and other membrane-bound organelles.
• Typical prokaryotic cells are significantly smaller than eukaryotic cells.
• Their genetic material (DNA) is located in a region called the nucleoid.
• Key structures include:
  • Cell wall: Provides structural support and protection.
  • Plasma membrane: Regulates the passage of substances into and out of the cell.
  • Ribosomes: Sites of protein synthesis.
  • Capsule: A protective outer layer found in some prokaryotes.
  • Flagella: Whip-like structures used for movement.
  • Pili: Hair-like appendages used for attachment or conjugation (transfer of genetic material).

Eukaryotic Cell Structure

• Eukaryotic cells are complex, typically larger than prokaryotic cells, and possess a nucleus and membrane-bound organelles.
• Genetic material (DNA) is enclosed within a membrane-bound nucleus.
• Organelles are specialized structures within the cell performing specific functions.
• Key structures and their functions:
  • Nucleus: Contains the cell's DNA and directs cellular activities. It contains the nucleolus, site of ribosome assembly.
  • Endoplasmic reticulum (ER): A network of membranes involved in protein and lipid synthesis.
  • Rough ER: Studded with ribosomes, responsible for protein synthesis.
  • Smooth ER: Lacks ribosomes, involved in lipid synthesis and detoxification.
  • Golgi apparatus: Processes, packages, and modifies proteins.
  • Mitochondria: The "powerhouse" of the cell, responsible for cellular respiration and ATP production.
  • Lysosomes: Contain digestive enzymes, breaking down waste and cellular debris.
  • Vacuoles: Membrane-bound sacs involved in storage and transport.
  • Cytoskeleton: A network of protein filaments supporting cell shape and movement.

Cell Membrane Structure and Function

• The cell (plasma) membrane is a selectively permeable barrier separating the internal and external environments of the cell.
• Fluid mosaic model describes the membrane structure: a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.
• Phospholipids form a double layer. Hydrophilic (water-loving) heads face the intracellular and extracellular environments; hydrophobic (water-fearing) tails face each other within the bilayer.
• Membrane proteins have various functions:
  • Transport proteins: Facilitate the movement of molecules across the membrane (e.g., channels, carriers).
  • Receptor proteins: Bind to signaling molecules.
  • Enzymes: Catalyze specific reactions.
  • Structural proteins: Maintain membrane structure.
• Carbohydrates are attached to proteins or lipids on the outer surface, playing roles in cell recognition and communication.

Cell Transport Mechanisms

• Passive transport: Movement of molecules across the membrane without energy input.
  • Simple diffusion: Movement from high to low concentration.
  • Facilitated diffusion: Movement with the help of transport proteins.
  • Osmosis: Movement of water across the membrane.
• Active transport: Movement of molecules against the concentration gradient requiring energy.
  • Pumps: Use ATP to move molecules.
• Endocytosis: Bulk transport into the cell.
• Exocytosis: Bulk transport out of the cell.

Cellular Metabolism

• Cellular metabolism encompasses all biochemical reactions occurring within the cell.
• Catabolism: Breakdown of complex molecules to simpler ones, releasing energy.
• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy.
• ATP (adenosine triphosphate): The main energy currency of the cell.
• Cellular respiration: Process that breaks down glucose to produce ATP. Occurs in mitochondria.

Cell Signaling

• Cells communicate with each other through signaling pathways.
• Signaling molecules (ligands) bind to receptors on target cells.
• This triggers a series of intracellular events, leading to a cellular response.
• Different types of signaling mechanisms exist, including direct contact, paracrine, endocrine, and synaptic signaling.