Cell Biology: Golgi Apparatus and Protein Transport

Golgi Apparatus

• Plays a central role in processing and packaging molecules, particularly proteins, within the cell.
• The Golgi apparatus is a stack of flattened, membrane- enclosed sacs called cisternae.
• The Golgi apparatus has two distinct faces:
  • Cis face: Receives materials in transport vesicles that bud off the ER (endoplasmic reticulum).
  • Trans face: Packages and sends off modified molecules in transport or secretory vesicles to other destinations within or outside the cell.

Protein Transport

• Proteins synthesized by ribosomes on the RER (rough endoplasmic reticulum) are transported into the ER's internal compartment.
• These proteins may be used at a different location within the cell or released outside the cell.
• Transport vesicles bud off from the RER, carrying the proteins to the Golgi's cis face for further processing and packaging.

Cell Components

• Cell Wall: Provides support and shape to the cell.
• Cytoplasm: A semi-fluid solution surrounded by the plasma membrane, containing the cell's internal environment.
• Flagellum: A rotating filament that propels the cell (a similar structure, cilia, is shorter and often more numerous).

Eukaryotic Cell Structure

• Eukaryotic cells are much more complex than prokaryotic cells, characterized by internal compartmentalization.
• This compartmentalization is achieved by an extensive endomembrane system and various organelles.
• Endomembrane system: Includes the nuclear envelope, ER, Golgi apparatus, lysosomes, vacuoles, and plasma membrane.
• Organelles: Specialized structures with specific functions, such as mitochondria, chloroplasts, peroxisomes, ribosomes, and the nucleus.

Similarities Between Prokaryotes and Eukaryotes

• Both have cell membranes, which regulate the movement of substances into and out of the cell.

Differences Between Prokaryotes and Eukaryotes

• Prokaryotes (Bacteria and Archaea):
  • Smaller and simpler.
  • Nucleoid: Single circular DNA molecule, not enclosed in a nucleus.
  • Ribosomes: Responsible for protein synthesis.
  • Plasmids: Small, circular DNA molecules that carry additional genetic information.
  • Cell wall: Composed of peptidoglycan for structural support.
  • Capsule: Outer layer that protects the bacteria.
• Eukaryotes (Plants, Animals, Fungi, Protists):
  • Larger and more complex, with internal membrane-bound compartments (organelles).
  • Nucleus: Contains the genetic material (DNA) and directs cellular activities.
  • Ribosomes: Found both free in the cytoplasm and attached to the RER.
  • Cytoskeleton: A network of protein fibers that maintains cell shape, aids movement, and transports organelles.

Other Cell Components

• Centrioles: Organelles within the centrosome, responsible for organizing microtubules during cell division.
• Chromatin: A complex of DNA and proteins, found in the nucleus.
• Lysosomes: Membrane-bound sacs containing digestive enzymes that break down waste products and cellular debris.
• Vacuoles: Large, membrane-bound sacs for storing nutrients, water, and waste.

Animal Cells

• Mitochondria: The cell's powerhouses, responsible for producing ATP (adenosine triphosphate), the cell's primary energy source.
• Peroxisomes: Small organelles that break down harmful substances, such as hydrogen peroxide.

Plant Cells

• Cell wall: Composed of cellulose, providing additional structural support.
• Chloroplasts: Organelles that contain chlorophyll and perform photosynthesis, converting light energy into chemical energy (glucose).
• Vacuoles: Large, central vacuoles that store water and provide structural support.

Plant Tissues

• Meristematic cells: Specialized cells for growth and development, capable of continuous division.
• Tracheids: Elongated, tapered cells with thick, lignified walls and pits for lateral water movement.
• Vessel elements: Shorter, wider cells with perforated end walls for efficient water flow.
• Cuticle: A waxy coating that reduces water loss.
• Dermal tissue: Protective outer layer of the plant, replacing the epidermis in older regions.
• Periderm: Involved in the transport of sugars and other nutrients.
• Stomata: Small openings in the epidermis that allow gas exchange.

Levels of Organization

• Life is organized in a hierarchical manner.
• From simplest to most complex:
  • Atom → Molecule → Macromolecule → Organelle → Cell → Tissue → Organ → Organ System → Organism

Germ Layers

• During embryonic development, three germ layers form:
  • Ectoderm: Develops into skin, nervous system, and sensory organs.
  • Endoderm: Forms the lining of the digestive tract, respiratory system, liver, and pancreas.
  • Mesoderm: Gives rise to muscles, bones, blood, kidneys, and the reproductive system.

Tissue Types

• Epithelial tissue: Covers surfaces, lines organs, and forms glands.
  • Simple (single layer):
    • Squamous: Thin, flat cells for diffusion.
    • Cuboidal: Cube-shaped cells for secretion and absorption.
    • Columnar: Tall, column-shaped cells for secretion, absorption, and protection.
  • Stratified (multiple layers):
    • Squamous: For protection against abrasion.
    • Cuboidal: For secretion and protection.
    • Columnar: For protection and secretion.
  • Pseudostratified columnar: Appears stratified but is a single layer, with all cells connected to the basement membrane.
  • Transitional: Can stretch and change shape, lining organs that need to expand.

Cell Division: Mitosis

• Mitosis: A type of cell division that produces two daughter cells genetically identical to the parent cell.
• Phases of Mitosis:
  • Prophase:
    • The chromosomes condense, becoming visible.
    • The nuclear envelope breaks down.
    • The spindle fibers form.
  • Metaphase:
    • The chromosomes line up at the metaphase plate (equator of the cell).
    • The spindle fibers attach to the centromeres of the chromosomes.
  • Anaphase:
    • The sister chromatids separate and move toward opposite poles of the cell.
  • Telophase:
    • The chromosomes decondense.
    • The nuclear envelope reforms.
    • The spindle fibers disappear.
  • Cytokinesis:
    • The cytoplasm divides, forming two daughter cells.
    • In animal cells, a cleavage furrow forms.
    • In plant cells, a cell plate forms between the two daughter nuclei, eventually becoming a new cell wall.