I had seen this question in an exam:

A living cell has a protoplasm which is water based and demarcated by a lipid bilayer membrane. If a cell is pierced to 1/5th of its diameter with a very sharp needle, after taking the needle out (a) no effect will be observed. (b) protoplasm will leak out from the hole made by the needle for a few minutes until the cell heals the wound. (c) protoplasm will keep on leaking out till the cell is dead (d) the cell will burst like a balloon.

I understand that the answer is option (a) as the cell membrane will reseal itself after getting pierced. But my doubt is that if in the question 1/5th was replaced by 1/2 or by any number greater than 1/2, will the answer be different. Or in other words, I want to know whether the number 1/5 significant in this question.

  • 2
    $\begingroup$ This is a homework question. Unless you show that you made an effort to solve the question yourself it is likely to be closed. Please look here on how to properly ask homework questions. $\endgroup$
    – Thawn
    Feb 5, 2016 at 11:43
  • $\begingroup$ Depends on the diameter and the shape of the needle, and the hydrostatic pressure inside the cell. $\endgroup$
    Feb 5, 2016 at 18:49
  • 1
    $\begingroup$ I don't believe the edits satisfy the need for an attempt at an answer, there's no effort or justification, therefore I don't think it should be reopened $\endgroup$
    – rg255
    Feb 5, 2016 at 20:15
  • $\begingroup$ @rg255 Why not? The OP is asking what (to me) appears to be a valid question about the fraction of lost cell membrane that results in cell death. $\endgroup$
    – March Ho
    May 5, 2016 at 12:40
  • $\begingroup$ Because there is no attempt to work out an answer and is homework $\endgroup$
    – rg255
    May 5, 2016 at 12:43

1 Answer 1


Generally speaking, there is no effect in microinjections.

This is relevant in the real world example of micro-injections. A video of a pronucleus injection can be seen here, or here is a more thoroughly explained procedure involving cloning. As you can see in the video the membrane, almost no effect will be observed - molecular dynamic simulations show that the membrane can spontaneously reform from fragments in the tens-of-nanoseconds timescale.

1/5 refers to the question "is the needle small enough?"

The question does include the term "1/5", which is a bit unusual. I guess they want you to recall a specific piece of information about the correct ratio size of needle should be. Here is a video of a microinjection at approximately that ratio. However the exit hole is unintentionally filled with the inserted material and cannot close immediately. The cell disappears out of frame before we can confirm the hole is closed.

Cell diameters are very variable

Cell sizes vary greatly.

Table describing different cell sizes. Smallest cell volume is sperm cell at 30 micrometers per meter cubed. The largest is an oocyte at 4,000,000 micrometers per meter cubed.

From using these values, assuming a cell is a sphere an oocyte has a diameter of approximately 196µm, whereas a neutrophil has a diameter of 8µm. It's a bizarre question given this range because usually micro-needles are between 0.5 and 5 µm diameter in length, except in the case of oocytes where larger needles are used (according to wikipedia). I don't think this is relevant, and it is just arbitrarily picking a needle size that wouldn't completely maul the cell!


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