Cell Biology: Endoplasmic Reticulum & Golgi Apparatus

Endoplasmic Reticulum (ER)

• A series of interconnected channels and sacs called cisternae.
• Two forms: Rough ER (rER) and Smooth ER (sER)
• Rough ER (rER): Covered with ribosomes and synthesizes new membrane for the cell.
• Smooth ER (sER): No ribosomes. Involved in:
  • Synthesis of membrane components like fatty acids and phospholipids
  • Synthesis of steroid hormones
  • Metabolism of carbohydrates
  • Detoxification of poisons, particularly in liver cells

Golgi Apparatus

• Composed of flattened, disk-like vesicles stacked together, located near the rER.
• Plays a crucial role in modifying and packaging proteins made on ribosomes attached to the rER (bound ribosomes).
• Transport vesicles fuse with one side of the Golgi and leave from the opposite side.
• Sorts and modifies lipids and proteins as they move from the cis to the trans side.
• Functions: Modify, sort, package, and manufacture/synthesize.
• Often referred to as the "mail center" of the cell.

Vacuoles

• Space or cavity in the cytoplasm enclosed by a single membrane.
• Larger than vesicles, can be derived from the ER and Golgi complex.
• Specific functions vary depending on the cell type.
• In plants:
  • Temporary storage organelles for proteins, nutrients, water, sugars, etc.
  • Store wastes and poisons to prevent toxicity to the cytoplasm (used for defense against predation).
  • Internal strength through turgor: Central vacuole takes up water, enabling plant cells to increase in size and provide rigidity to leaves and stems.
• In animals: Involved in exocytosis and endocytosis (secretory, excretory & storage functions).

Vesicles

• Small membrane-enclosed transport units that transfer molecules between different cellular compartments.
• Can fuse with other cell membranes.
• Often referred to as "shuttle vehicles".

Lysosomes

• Membrane-enclosed spheres bound by a single membrane.
• Contain about 40 different enzymes that work best in an acidic pH (4.5).
• Function in the cell by:
  • Digesting other cells through phagocytosis.
  • Recycling cellular materials that have exceeded their lifespan and recycling cellular organic material through autophagy.

Endosomes

• Membrane-bound compartment inside eukaryotic cells.
• Provide an environment for material taken into the cell by receptor-mediated endocytosis to be sorted before reaching the lysosome.

Secretory Vesicles

• Transport molecules to the cell membrane for secretion.

Mitochondria

• Sites of cellular respiration, a process that generates ATP (the energy molecule of the cell).
• Found in all eukaryotic cells.
• Approximately 1–10 µm long.
• Structure:
  • Large organelles containing circular DNA.
  • Surrounded by a double membrane:
    • Inner membrane: Highly folded, folds are called cristae.
    • Outer membrane: Encloses the organelle.
• These membranes divide the mitochondrion into two compartments:
  • Central matrix: Contains DNA in the form of a circular molecule.
  • Intermembrane space: Space between the inner and outer membranes.
• Mitochondrial DNA: Encodes many of the components for mitochondrial function, while nuclear DNA encodes for the remaining components.

Peroxisomes

• Found in both plant and animal cells.
• Break down fats and amino acids into smaller molecules that can be used for energy production by mitochondria.
• Cytoplasmic vesicles where metabolic reactions occur that transfer hydrogen from various substrates to oxygen, producing hydrogen peroxide (H2O2) as a by-product.
• H2O2 is highly reactive and toxic and is rapidly broken down by the enzyme catalase.

Ribosomes

• Translate the DNA genetic code into proteins.
• Composed of RNA and protein.
• Two subunits:
  • Eukaryotic ribosomes: 60S and 40S subunits that come together during protein synthesis to form an 80S ribosome.
  • Prokaryotic ribosomes: 50S and 30S subunit that come together during protein synthesis to form a 70S ribosome.
• Occur in the cell either free in the cytoplasm or bound to the ER.
• Bound ribosomes produce proteins destined for:
  • Plasma membrane
  • Nuclear membrane
  • Lysosomes
  • Golgi
  • Vacuoles
  • Secretory vesicles.
• Free ribosomes synthesize proteins destined to be enzymes used in:
  • Cytoplasm (cytoplasmic enzymes)
  • Mitochondria
  • Chloroplasts.

Cytoplasm

• Region in eukaryotic cells between the cytoplasmic membrane and nuclear membrane.
• Contains all the cell’s internal substructures (organelles) except the nucleus.
• Liquid part: Cytosol.
• Site of almost all the chemical activity occurring in a eukaryotic cell.

Nucleoplasm

• Similar to the cytoplasm of a cell.
• Found inside the nucleus.

Cytosol

• Aqueous component of the cytoplasm of a cell.
• Within which various organelles and particles are suspended.

Cytoskeleton

• Consists of:
  • Microtubules
  • Intermediate fibers
  • Microfilaments
• Functions:
  • Maintaining cell shape
  • Anchoring organelles
  • Facilitating cell movement.