Why do complex organisms require cells, tissues, organs, and body systems to work together to ensure all their cells have everything required to make the things our body needs, such as energy and structures?

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Complex organisms, such as human beings, animals, and plants, are much bigger in size and complex in structure and functions compared to a simple unicellular organism such as a bacterium. In the case of unicellular organisms, the transport of material to and from the environment can take place through the cell membrane. The cell, in case of unicellular organisms, is in direct contact with the environment and can exchange material easily as a result. In comparison, the complex organisms have a very large number of cells distributed throughout their body. Since they are away from the environment, it is very difficult for them to exchange anything with the environment. As a result, complex organisms need elaborate transport mechanisms consisting of organs (which are made up of tissues), tissues (which are made up of cells), and cells. This way, they ensure the cells receive nutrients and can get rid of wastes, in order to survive and function properly.

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A complex organism has a number of different cells and tissues all working together to keep the organism alive. If these parts were separated, they would quickly perish on their own. For example, in a plant like a tree, energy is generated through photosynthesis, when the plant combines carbon dioxide, water, and light to yield sugar and oxygen. Water is supplied by the root system of the tree, transported up the trunk through the xylem, and directed into cells on the leaf. Similarly, in the human body, cells require oxygen and sugar to function. Oxygen is captured by the lungs, directed into red blood cells, and transported through the body by the cardiovascular system. Eventually, it gets to a cell where it is used.

Sources if you are curious for more information:

Cardiovascular System

Xylem

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