IMAT 2015 Q37 [Structure Sizes]

Place the following structures in a human sperm cell in descending order of size (left to right):

  1. mitochondria
  2. nucleus
  3. ribosome

A. 1, 2, 3
B. 1, 3, 2
C. 3, 1, 2
D. 2, 3, 1
E. 2, 1, 3

The answer to this question is choice E.

To rationalize this question, let’s look at each structure (in descending order in terms of size), and look at what function they serve:

  • Nucleus: It would be important to be reminded of where the nucleus of sperm cells are. It is located in the “head” of the sperm cell, which has an oval shape. Undeniably, the head is the most important part of the sperm cell because it carries the nucleus, which subsequently carries the genetic material! If you recall, sperm cells are haploid cells with 23 chromosomes, which will combine with the 23 chromosomes of the ovum which makes sense why this is the biggest structure out of the choices, given its importance in the essence of fertilization.

  • Mitochondria: Motility is one of the most important characteristic of sperm cell and is a significant determinant of its viability. But how exactly do sperm cells move? Sperm cells have a tail, and within this tail is the flagellum. Within this flagellum, there are microtubules that slide relative to other microtubules during motion, which is only possible via the energy from ATP, which is provided by the mitochondria! The abundance of tightly packed mitochondria can be found in the midpiece of the sperm cell, which is the structure in between the head and the tail. In terms of length, both the head and the midpiece are the same, but in terms of SIZE, the head is larger than the midpiece.

  • Ribosomes: You are going to have to think about the main goal of sperm cells, which is the delivery of DNA to the ovum for fertilization to ensue. Therefore, the sperm cell’s structure is optimized in such a way to achieve this objective, which includes reducing as much unnecessary organelles that serve no function in terms of the delivery of genetic material. The only organelle that can be found in abundance is the mitochondria which provides the energy needed for sperm cell to be motile. However, there is not much demand for protein, which makes sense why ribosomes will be the least in terms of number AND size.

Thus, the correct order of size (in descending order) is Nucleus → Mitochondria → Ribosomes, which is choice E.