Question

Each of the following is part of the endomembrane system except a. the nuclear envelope. b. the endoplasmic reticulum. c. the Golgi apparatus. d. lysosomes. e. mitochondria.

Short answer

\(e.\) Mitochondria is not part of the endomembrane system.

Step-by-step solution

Identify the constituents of the endomembrane system

The endomembrane system includes the nuclear envelope, endoplasmic reticulum, golgi apparatus, lysosomes, vesicles and the cell membrane.

Evaluate given options

\(a.\) The nuclear envelope: It is part of the endomembrane system. \(b.\) The endoplasmic reticulum: This is also part of the endomembrane system. \(c.\) The Golgi apparatus: This too is included in the endomembrane system.\(d.\) Lysosomes: They are part of the endomembrane system as well. \(e.\) Mitochondria: Crucial for energy production in the cell, but it is not part of the endomembrane system, as it is not involved in protein and lipid synthesis or modification.

Select the correct option

From the evaluation in Step 2, it's clear that all options except 'Mitochondria' are part of the endomembrane system. So, 'Mitochondria' is the correct answer to the exercise.

Key concepts

Nuclear Envelope

The nuclear envelope is a critical component of a cell's architecture. It surrounds the nucleus, which houses the cell's genetic material. This double membrane structure provides both protection and regulation.

By containing nuclear pores, it controls the flow of substances in and out of the nucleus.

• It allows mRNA and ribosomal subunits to exit the nucleus.
• It permits the import of proteins and regulatory factors.

The nuclear envelope plays a significant role in maintaining the shape of the nucleus and ensuring genetic material is securely enclosed. In sum, it is part of the endomembrane system, emphasizing its role in cellular organization.

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a vast network of membranes spread throughout the cell, crucial for protein and lipid synthesis. It comes in two forms:

• Rough ER: Studded with ribosomes, it plays a pivotal role in protein synthesis, modifying them to be transported to their needed locations.
• Smooth ER: Lacks ribosomes and is vital for lipid synthesis, detoxification processes, and calcium ion storage.

Its interconnected nature allows it to transport these synthesized molecules to various parts of the cell, further integrating itself into the endomembrane system, alongside the nuclear envelope.

Golgi Apparatus

The Golgi apparatus is often referred to as the cell’s "post office." This is because it modifies, sorts, and packages proteins and lipids received from the endoplasmic reticulum.

It consists of flattened membrane sacs called cisternae. Proteins and lipids are further processed in the Golgi apparatus to become functional products.

• Glycosylation: Addition of carbohydrates to proteins.
• Phosphorylation: Addition of phosphate groups to molecules.

After this processing, they are packed into vesicles and dispatched to their final destination, either inside or outside the cell. This vital role in managing cellular products places the Golgi apparatus firmly within the endomembrane network.

Lysosomes

Lysosomes are cellular organelles filled with enzymes that can digest biomolecules. They act as the waste disposal system of the cell.

They break down unwanted or damaged cellular components through a process of digestion as they are equipped with a wide range of enzymes.

• Autophagy: The process of cleaning out damaged parts of the cell.
• Phagocytosis: Engulfment and digestion of pathogens or debris.

When cellular components are no longer needed or are defective, lysosomes help in recycling these components. Their role in breaking down and recycling cellular waste ensures they are an integral part of the endomembrane system.

Mitochondria

Mitochondria are not part of the endomembrane system. They are powerhouse organelles of the cell, generating adenosine triphosphate (ATP) through cellular respiration. Unlike organelles in the endomembrane system, their primary function is energy production rather than involvement in protein and lipid synthesis.

Key features of mitochondria include:

• Two membranes: An outer membrane and a highly folded inner membrane.
• Possession of their own DNA, reflecting a prokaryotic origin.

While they play a critical role in providing energy for cellular functions, mitochondria operate independently of the endomembrane system, distinctively responsible for energy metabolism processes.