Robert Hooke was the first scientist to discover cell. This discovery was made possible by the use of microscope which he invented. With this microscope, Robert examined a cork and noticed pores which filled the entire volume of the thin slices of cork; he called the pores cells. This discovery was made in January 1653.
This varies from baby to baby, but it is approximated to be the ages of 4 to 7 months.
Plants maintain homeostasis by keeping their stomata open just enough to allow photosynthesis to take place but not so much that they lose an excessive amount of water.
When plant cells use the food they make, the cells respire, taking in oxygen and giving off carbon dioxide.
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If a large animal such as a human being had a thin light exoskeleton, there would be several problems. Since the exoskeleton would not be able to hold its shape, it would be difficult to keep the vital organs protected and the organism would be subject to damaging levels of stress just by moving around. They are not strong enough to hold organs or hold its shape.