Cell Types and Bacterial Structure

Cell Types

• Prokaryotic cells lack a membrane-bound nucleus, are smaller and simpler than eukaryotic cells.
• Prokaryotic cells are categorized into two domains: Bacteria and Archaea, the latter inhabiting extreme environments.
• Eukaryotic cells, part of Domain Eukarya, include protists, fungi, plants, and animals.

Structure of Bacteria

• Bacteria are typically 1–1.5 μm wide and 2–6 μm long, with three common shapes:
  • Spherical (coccus)
  • Rod-shaped (bacillus)
  • Spiral (spirillum and spirochete).
• The cell envelope includes:
  • Plasma membrane: lipid bilayer with embedded proteins, forming internal pouches called mesosomes.
  • Cell wall: provides structure and is fortified with peptidoglycan.
  • Glycocalyx: a polysaccharide layer, often structured as a capsule, resistant to removal.

Cytoplasm and Genetic Material

• Cytoplasm is a semifluid solution bounded by the plasma membrane, containing water, organic/inorganic molecules, and enzymes.
• The nucleoid region houses a single, circular DNA molecule.
• Plasmids are small, accessory DNA rings independent of chromosomal DNA.

Appendages in Bacteria

• Flagella: structures aiding in motility.
• Fimbriae: small, bristle-like structures that help adhere to surfaces.
• Sex pili: tubular structures that facilitate DNA transfer between cells.

Eukaryotic Cell Characteristics

• Eukaryotic cells possess a membrane-bound nucleus that contains DNA and specialized organelles.
• Generally larger than prokaryotic cells, some have a cell wall (e.g., plant cells).
• Organelles perform specific functions and compartmentalize cellular processes.

Organelles in Eukaryotic Cells

• Eukaryotic organelles can be divided into:
  • Endomembrane system: organelles that communicate through membrane channels and vesicles.
  • Energy-related organelles: mitochondria and chloroplasts, known for their independence and self-sufficiency.

Cell Fractionation

• Cell fractionation involves breaking apart cellular components for analysis.
• Differential centrifugation separates cell parts based on size and density, akin to a washing machine's spin cycle; faster spins isolate smaller components.

Animal Cell Anatomy

• Plasma membrane: regulates the flow of molecules into and out of the cell.
• Nucleus: serves as the cell's command center, encompassed by a double membrane called the nuclear envelope.
• Cytoskeleton: maintains cell shape and aids in cellular movement and organization.

Energy-Related Organelles: Chloroplasts

• Chloroplasts are double-membrane organelles involved in photosynthesis.
• Inner membrane contains disc-like thylakoids, stacked into structures called grana.
• Stroma is the semi-fluid matrix where thylakoids are suspended, containing chlorophyll.
• Photosynthesis equation: Solar energy + CO2 + H2O → Carbohydrates + O2.
• Only plants, algae, and specific bacteria can perform photosynthesis.

Energy-Related Organelles: Mitochondria

• Mitochondria are smaller than chloroplasts and contain ribosomes and their own DNA.
• Surrounded by a double membrane; the inner membrane is convoluted, forming cristae.
• The matrix, a semifluid interior, houses respiratory enzymes for carbohydrate breakdown.
• Mitochondria are crucial for cellular respiration, producing most ATP utilized by the cell.

Endomembrane System

• Comprises a network of intracellular membranes, compartmentalizing cellular functions.
• Includes: Nuclear envelope, endoplasmic reticulum (ER) membranes, Golgi apparatus, and vesicles.
• Proteins synthesized in rough ER and lipids from smooth ER are transported to Golgi apparatus in vesicles.
• Golgi apparatus modifies, sorts, and packages proteins and lipids for cellular destinations.

Golgi Apparatus

• Consists of 3-20 flattened saccules resembling a stack of pancakes.
• Modifies incoming proteins and lipids from ER, organizing them for transport.
• Packages products in vesicles for delivery within or outside the cell.

Endoplasmic Reticulum (ER)

• Rough ER is studded with ribosomes; involved in protein synthesis and modification (e.g., glycoproteins).
• Smooth ER lacks ribosomes; involved in lipid synthesis, detoxification, and storage processes.
• Both types contribute to the formation of transport vesicles for cellular distribution.

Lysosomes

• Membrane-bound vesicles produced by the Golgi apparatus, absent in plant cells.
• Contain powerful digestive enzymes, crucial for the breakdown of large molecules and cellular recycling.
• Involved in apoptosis, or programmed cell death, for proper development.
• Genetic defects in lysosomal enzymes can lead to storage diseases, such as Tay-Sachs.

Peroxisomes

• Membrane-bounded vesicles containing enzymes synthesized by free ribosomes in the cytoplasm.
• Active in lipid metabolism and catalyze reactions that yield hydrogen peroxide (H2O2), a toxic byproduct.
• H2O2 is broken down into water and oxygen by the enzyme catalase.

Vacuoles

• Larger than vesicles, serving as storage sacs for excess materials.
• Plant cells have a central vacuole that can comprise up to 90% of cell volume.
• Functions: Storage of nutrients, water, pigments, and waste; contributes to turgor pressure.
• Some vacuoles perform functions akin to lysosomes in non-plant eukaryotic cells.