Question

Which side in a cell does luminal and extra luminal compartments are situated respectively? (a) Cytoplasm, inside ER (b) Inside ER, cytoplasm (c) cytoplasm, plasma membrane (d) Nucleus, cytoplasm

Short answer

(b) Inside ER, cytoplasm

Step-by-step solution

Understanding Cell Structure

The Luminal space refers to the interior of any tubular structure within the cell, while extraluminal is the space outside of it. The Endoplasmic Reticulum (ER) is a membrane-bound tubular structure in the cell, typically involved in protein synthesis and lipid metabolism.

Identifying the correct option

Upon reviewing the answer options, we can see that (b) Inside ER, cytoplasm correctly reflects the positions of the luminal space (inside ER) and the extraluminal space (cytoplasm) in a cell.

Key concepts

Cell Structure

The cell is the basic unit of life, comprising numerous specialized structures that carry out distinct functions. Understanding cell structure is essential to grasp how cells operate and interact within themselves and their environments.

At the heart of cell structure is the cell membrane, a protective boundary that controls the entry and exit of substances. Within this membrane lies the cytoplasm, a jelly-like substance hosting various organelles. One of the key organelles is the nucleus, which contains genetic material and acts as the command center for the cell. Each organelle has a specific role, whether it's energy production in mitochondria, waste removal via lysosomes, or photosynthesis in chloroplasts in plant cells.

What's relevant to our problem is the intracellular compartmentalization which is fundamental for cell function. Organelles like the endoplasmic reticulum (ER) and Golgi apparatus are crucial in compartmentalizing chemical reactions and pathways. Luminal refers to 'within the lumen' – the interior of these organelles, whereas extraluminal refers to spaces such as the cytoplasm, outside such structures.

Endoplasmic Reticulum

The Endoplasmic Reticulum (ER) is an expansive network of membranes found throughout the cell but is particularly concentrated around the nucleus. It serves a multifaceted role in both the synthesis of proteins and lipids and the detoxification of harmful substances. The ER can be classified into two types - Rough ER (RER) and Smooth ER (SER).

The RER is studded with ribosomes on its cytoplasmic side, giving it a 'rough' appearance under a microscope. These ribosomes are the sites of protein synthesis. The synthesized proteins are threaded into the lumen of the RER where they are folded and modified. This space within the RER is an example of a luminal compartment.

Conversely, the SER lacks ribosomes and is tasked with the synthesis of lipids and steroids, as well as the detoxification processes. The space outside the ER, where the cytoplasm resides, constitutes the extraluminal compartment as it lies external to the tubular lumens of the ER.

Protein Synthesis

Protein synthesis is a critical biological process that takes place within every cell, producing the proteins necessary for various cellular functions. This complex mechanism involves two main stages: transcription and translation.

During transcription, the genetic code from DNA is transcribed into messenger RNA (mRNA) inside the nucleus. The mRNA then travels to the cytoplasm and attaches to a ribosome. In eukaryotic cells, ribosomes may be found freely in the cytoplasm or bound to the RER, which is where translation occurs.

Translation is the process by which ribosomes read the mRNA sequence and translate it into a polypeptide chain using amino acids. These chains fold into functional proteins, often with the help of the ER. In the case of secretory or membrane-bound proteins, the folding and processing occur within the luminal space of the ER before being transported to the Golgi apparatus for further modifications and sorting.