Final answer:
Humans consist of trillions of cells due to cell division that begins with a single zygote, leading to a complex body with specialized cells and functions. Multicellularity allows for the organization of cells into compartments, protecting internal environments and supporting growth and tissue repair. Diverse cell shapes and signaling complexity are characteristic of multicellular life.
Step-by-step explanation:
Humans are composed of trillions of cells because of the process of cell division that begins after a single cell, known as a zygote, is formed by the fertilization of an egg by a sperm. This single cell divides and multiplies into more cells through a highly regulated process that leads to the formation of tissues and specialized cells for different functions. The multicellular nature of humans allows for cells to be organized into compartments, protecting them from external environmental threats and supporting diverse functions that single cells could not manage alone. Furthermore, in multicellular organisms like humans, cells need to be in communication through complex signaling pathways to function together seamlessly.
The diversity in cell shapes and sizes reflects the various functions that these cells perform. As the human body develops from this single cell through billions of divisions, a highly organized structure of trillions of cells emerges, leading to the complexity of the human body. This multicellular arrangement is also essential for growth, maintenance, and repair of tissues. Moreover, this separation into trillions of cells also creates distinct internal compartments, providing barriers that protect internal body fluids from countless microorganisms that grow on the body's external and internal surfaces, such as the intestinal tract.