Cellular Organelles: ER, Golgi, Lysosomes, Vacuoles, Mitochondria, and Chloroplasts

Endoplasmic Reticulum (ER)

Structure of ER

• The Endoplasmic Reticulum (ER) is a membranous system composed of interconnected tubules and flattened sacs, known as cisternae.

• The ER membrane is continuous with the nuclear envelope, as depicted in Figure 6.11.

• It encloses a continuous internal compartment called the ER lumen, also referred to as the cisternal space.

• There are two distinct regions of the ER:

  • Smooth ER: Lacks ribosomes on its outer surface.

  • Rough ER: Is studded on its outer surface with ribosomes.

Functions of Smooth ER

• Lipid Synthesis: The smooth ER is crucial for the synthesis of lipids, including oils, phospholipids, and steroids.

• Carbohydrate Metabolism: It participates in the metabolism of carbohydrates.

• Detoxification of Drugs and Poisons: Enzymes of the smooth ER, particularly in liver cells, detoxify specific drugs and poisons.

  • This process typically involves adding hydroxyl groups to drug molecules, making them more soluble and easier to excrete from the body.

  • Clinical Implication: The proliferation of smooth ER in response to one drug can lead to an increased need for higher dosages of other drugs as well, due to the broader action of its detoxification enzymes. For example, barbiturate abuse can decrease the effectiveness of certain antibiotics and other important medications.

• Calcium Ion Storage: The smooth ER stores calcium ions ($ext{Ca}^{2+}$).

  • In Muscle Cells: The smooth ER membrane actively pumps $ext{Ca}^{2+}$ from the cytosol into the ER lumen. When a muscle cell receives a nerve impulse, $ext{Ca}^{2+}$ rapidly rushes back from the ER lumen across the ER membrane into the cytosol, triggering the contraction of the muscle cell.

  • In Other Cell Types: The release of $ext{Ca}^{2+}$ from the smooth ER can trigger various cellular responses, such as the secretion of vesicles carrying newly synthesized proteins.

Functions of Rough ER

• Protein Secretion: Many cells, especially those specialized for secretion, produce proteins by ribosomes attached to the rough ER.

  • Mechanism: As a polypeptide chain elongates from a bound ribosome, it is threaded into the ER lumen through a pore formed by a protein complex embedded in the ER membrane. The newly synthesized polypeptide then folds into its characteristic functional tertiary structure as it enters the ER lumen.

  • Glycoprotein Formation: Most secretory proteins are glycoproteins, meaning they have carbohydrates covalently bonded to them. These carbohydrates are attached to the proteins within the ER lumen by specific enzymes that are built into the ER membrane.

  • Segregation and Transport: The ER membrane ensures that secretory proteins are kept separate from proteins synthesized by free ribosomes and destined for the cytosol. Upon formation, secretory proteins depart from the ER encased within the membranes of vesicles that bud off from a specialized region called transitional ER (Figure 6.11).

  • Transport Vesicles: These vesicles, in transit from one part of the cell to another, are referred to as transport vesicles.

• Membrane Factory for the Cell: The rough ER serves as a membrane factory, continually expanding by adding its own membrane proteins and phospholipids.

  • Integration of Membrane Proteins: Polypeptides destined to become membrane proteins are inserted directly into the ER membrane as they grow from ribosomes, where they are anchored by their hydrophobic regions.

  • Phospholipid Synthesis: Like the smooth ER, the rough ER also synthesizes membrane phospholipids. Enzymes embedded in the ER membrane assemble phospholipids using precursors available in the cytosol.

  • Membrane Transfer: The ER membrane, as it expands, transfers portions of itself in the form of transport vesicles to other components of the endomembrane system.

The Golgi Apparatus: Shipping and Receiving Center

Role and Structure of the Golgi Apparatus

• The Golgi apparatus is often described as a cellular warehouse responsible for receiving, sorting, shipping, and even some manufacturing of cellular products.

• It extensively modifies and stores products received from the ER, before dispatching them to various other cellular destinations.

• Cells with high secretory activity typically possess a more extensive Golgi apparatus.

• Structure: The Golgi apparatus comprises a group of associated, flattened membranous sacs known as cisternae, resembling a stack of pita bread (Figure 6.12). Individual cells can contain many, sometimes hundreds, of these stacks.

• Each cisterna's membrane separates its internal space from the surrounding cytosol.

• Vesicles concentrated around the Golgi apparatus are involved in the transfer of material between different parts of the Golgi and other cellular structures.

Directionality of the Golgi Stack

• A Golgi stack exhibits clear structural and functional directionality.

• The membranes of cisternae on opposite sides of the stack differ in thickness and molecular composition.

• Cis face: This is the