the lowest temperature at which a substance burns is called its

the smallest cells in human body

ovary produce female gametes called

The lowest temperature at which a substance burns is called its ignition temperature.

The smallest cells in human body are sperms in males.

Ovary produce female gametes called ova or eggs.

Explanation:

Ignition Temperature

If you have ever tried a small piece of paper, you must have seen that it initially doesn't catch fire. It catches fire at a certain temperature and start burning. This temperature is known as Ignition temperature.

The ignition temperature of various substances varies. Kerosene oil catches fire at lower temperature as compared to a wooden stick. This is because of their different ignition temperatures. That's why substances like kerosene oil, petrol etc are kept more secure as they can easily catch fire  with their lower ignition temperature.  

Sperms: The smallest cells in human body

The sperms generated in a human male body are the smallest among all cells of human body. On the other side, a female ovum or egg is known as the largest cell in the human body.

They are smaller than the red blood cells (RBCs). The female ovum is around 20 times larger than a normal sperm.

Ova: Female Gametes

The female gamete produces in a human body are termed as ova or eggs that exist inside the ovaries of a uterus. The ovum (a single egg) moves through the Fallopian tube to the uterus and is responsible for bearing the genetic material.

Initially, the number of ova in a female body is around 1 million at the time of birth. It decreases with time. Unlike sperms, female can not reproduce ova.