Cell The Unit Of Life

Statement I: Cell wall is freely permeable.
The cell wall in plants is freely permeable to water and solutes. This allows the movement of substances through the apoplast pathway, ensuring easy transport across the cell wall. Hence, Statement I is correct. Statement II: Plasma membrane is selectively permeable.
The plasma membrane regulates the entry and exit of substances into the cell. It is selectively permeable, allowing only specific molecules (e.g., ions, nutrients) to pass through, while restricting others. Hence, Statement II is correct. Conclusion:
Since both statements are correct, the correct answer is:

In eukaryotic cells, the rough endoplasmic reticulum (RER) is primarily responsible for synthesizing proteins destined for secretion, membrane integration, or lysosomal targeting. The RER is studded with ribosomes, which translate mRNA into polypeptide chains. These proteins enter the RER lumen, where they are folded and modified (e.g., glycosylated) before being transported to the Golgi apparatus for further processing and sorting.
The roles of the other organelles are:
- Mitochondrion: Generates ATP through cellular respiration but does not synthesize secretory proteins.
- Golgi apparatus: Modifies, packages, and sorts proteins received from the RER for secretion or other destinations, but it is not the primary site of protein synthesis.
- Lysosome: Contains hydrolytic enzymes for intracellular digestion, not involved in protein synthesis.
Thus, the organelle primarily responsible for synthesizing proteins destined for secretion is the rough endoplasmic reticulum.

The correct answer is mitochondria.

Mitochondria contain 70S ribosomes, which are similar to those found in prokaryotic cells, such as bacteria. This is an important feature because it supports the theory of endosymbiosis, which suggests that mitochondria are the evolutionary descendants of ancient bacteria that were engulfed by a primitive eukaryotic cell.

Details about 70S ribosomes:

• 70S ribosomes: These ribosomes are smaller than the 80S ribosomes found in the cytoplasm of eukaryotic cells. They consist of a 50S large subunit and a 30S small subunit. The "S" (Svedberg unit) refers to the sedimentation rate, which is a measure of how quickly particles move in a centrifuge.
• Prokaryotic ribosomes: The 70S ribosomes are characteristic of prokaryotic organisms like bacteria. This feature is shared by mitochondria, which, like bacteria, are believed to have evolved from a prokaryotic ancestor.
• Endosymbiotic theory: According to this theory, mitochondria were once free-living bacteria that were engulfed by a host cell in a symbiotic relationship. Over time, the engulfed bacteria became mitochondria, but they retained some of their original characteristics, including the 70S ribosomes.

Why do mitochondria have 70S ribosomes?

Mitochondria replicate independently from the host cell and contain their own DNA, which codes for some of their proteins. To produce these proteins, mitochondria use their 70S ribosomes to translate the mitochondrial DNA. This process is similar to how bacterial ribosomes function, further supporting the idea that mitochondria evolved from prokaryotic ancestors.

In contrast, other organelles in eukaryotic cells, such as the endoplasmic reticulum and the Golgi apparatus, contain 80S ribosomes, which are larger and structurally different from the 70S ribosomes found in mitochondria.