Which of the following cellular structures is present in prokaryotic cells such as bacteria?

Introduction / Context:
Cells are broadly divided into prokaryotic and eukaryotic types. Prokaryotic cells, such as bacteria, are structurally simpler and lack many membrane bound organelles that are present in eukaryotic cells. Exam questions often ask you to identify which structures are present in prokaryotes and which are unique to eukaryotes. This question focuses on that distinction, specifically asking about ribosomes, chloroplasts, and mitochondria.

Given Data / Assumptions:

• The question refers specifically to prokaryotic cells.
• Options list ribosomes, chloroplasts, mitochondria, and none of the above.
• We assume basic knowledge of cell structure in bacteria versus plants and animals.

Concept / Approach:
Prokaryotic cells lack a true nucleus and membrane bound organelles such as mitochondria, chloroplasts, and endoplasmic reticulum. However, they still need to synthesize proteins, so they contain ribosomes in the cytoplasm. These ribosomes are slightly smaller (70S) than the 80S ribosomes found in eukaryotic cytoplasm but serve the same basic function. Chloroplasts are plastids found in plant and algal cells; mitochondria are double membrane organelles in eukaryotic cells that carry out aerobic respiration. Therefore, among the listed structures, ribosomes are present in prokaryotes, while chloroplasts and mitochondria are not.

Step-by-Step Solution:
Step 1: Recall that prokaryotic cells do not have membrane bound organelles but do carry out basic functions like protein synthesis. Step 2: Recognise that ribosomes are non membrane bound structures and are present in both prokaryotic and eukaryotic cells. Step 3: Understand that chloroplasts and mitochondria are classic examples of membrane bound organelles found only in eukaryotic cells. Step 4: Evaluate the option that states ribosomes are present in prokaryotic cells and involved in protein synthesis. Step 5: Confirm that this is consistent with the known features of bacterial cells.

Verification / Alternative check:
Microbiology texts describe bacterial cell structure as including a cell wall, plasma membrane, cytoplasm, nucleoid region containing DNA, plasmids, and ribosomes. Electron micrographs clearly show numerous ribosomes as small granules in the cytoplasm. These sources also emphasise the absence of mitochondria and chloroplasts in bacteria and archaea. The endosymbiotic theory suggests that mitochondria and chloroplasts originated from prokaryotes, but in modern prokaryotic cells these organelles are not present. This evidence supports the conclusion that ribosomes are the only listed structure present in prokaryotes.

Why Other Options Are Wrong:

• Chloroplast, a membrane bound plastid for photosynthesis: Chloroplasts occur in plant and algal cells, not in free living prokaryotes. Photosynthetic bacteria use internal membrane systems but not true chloroplasts.
• Mitochondria, double membrane organelles for aerobic respiration: Mitochondria are found only in eukaryotic cells; prokaryotes carry out respiration using their plasma membrane.
• None of the above cellular structures are present in prokaryotes: This is incorrect because ribosomes are clearly present and essential in prokaryotic cells.

Common Pitfalls:
Students sometimes wrongly assume that because prokaryotes lack membrane bound organelles, they also lack structures like ribosomes. It is important to remember that ribosomes are not bound by membranes and are universal to all living cells. Another confusion can arise from photosynthetic bacteria, which have thylakoid like membranes, leading some to think they have chloroplasts. In reality, these are different structures and not true chloroplasts.

Final Answer:
The cellular structure present in prokaryotic cells such as bacteria is ribosomes, for protein synthesis in the cytoplasm.