Why does the Northern Hemisphere experience summer during the Southern Hemisphere's winter?

Because of the tilt of the earth on its axis. When the North Pole is tilted towards the Sun, the northern hemisphere experiences summer solstice. At this time, since the south Pole is tilted away from the Sun, the southern hemisphere experiences winter solstice.

That is why, When it is summer in the northern hemisphere, it is winter in the southern hemisphere. It's all because the Earth is tilted on its axis.

unicellular organisms, like bacteria, are able to perform all life functions within one single cell. multicellular organisms need many different types of cells to carry out the same life processes. each of these special types of cells has a different structure that it perform a specific function.