Biology · Cell: The Unit of Life · NEET
The smallest living cell is Mycoplasma (also called PPLO). NCERT gives its size as about 0.3 µm (0.3 micrometre). It is so small that it can pass through a filter with pores smaller than 1 micron. For NEET, always remember the exact number: 0.3 µm.
NCERT is very careful with the wording. It says the ostrich egg is the "largest isolated single cell." So the correct phrase is largest isolated single cell, not just 'largest cell'. If a NEET option says 'largest cell', check the full wording — the safe, exact answer is 'largest isolated single cell = ostrich egg'.
No — do not mix these two. The smallest cell overall is Mycoplasma (0.3 µm), which is a free-living organism, not a human cell. The commonly quoted smallest human cell is the sperm cell (by volume). The human red blood cell (RBC) is about 7.0 µm in diameter. NEET usually asks the overall smallest = Mycoplasma.
NCERT says nerve cells (neurons) are some of the longest cells. 'Longest' is about length, not size or volume. So Mycoplasma = smallest, ostrich egg = largest single cell, and nerve cell = longest. Keep these three facts separate.
The shape of a cell may vary with the function it performs. NCERT lists these shapes: disc-like, polygonal, columnar, cuboid, thread-like, or even irregular. Example: RBCs are disc-like to carry oxygen easily; nerve cells are thread-like to carry signals over long distances.
Because these are exact, quotable NCERT facts that are easy to test: 0.3 µm (Mycoplasma), 3 to 5 µm (bacteria), 7.0 µm (human RBC), ostrich egg (largest single cell). NEET loves fact-recall questions, and cell size gives clean one-line answers, so learn the numbers by heart.
Which among the following are the smallest living cells, known without a definite cell wall, pathogenic to plants as well as animals and can survive without oxygen?
Statement I: Mycoplasma can pass through less than 1 micron filter size. Statement II: Mycoplasma are bacteria with a cell wall. Choose the most appropriate answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
About 0.3 µm (0.3 micrometre) in length, as stated in NCERT. It is the smallest living cell.
The largest isolated single cell is the egg of an ostrich. Use the full phrase 'isolated single cell' for NEET accuracy.
Human red blood cells (RBCs) are about 7.0 µm in diameter, and they are disc-like in shape.
Nerve cells (neurons) are some of the longest cells in the body, as they carry signals over long distances.
NCERT lists disc-like, polygonal, columnar, cuboid, thread-like, or even irregular. The shape depends on the function of the cell.
