Assertion: The cell is the basic structural and functional unit of all living organisms.

Reason: Cells are responsible for carrying out all the necessary functions required for an organism to survive and thrive.

Explanation:
- Cells are the building blocks of life, as they make up all living organisms and perform essential functions such as metabolism, growth, and reproduction.
- Each cell is enclosed by a membrane that regulates the passage of substances in and out of the cell, maintaining its internal environment.
- Within the cell, organelles such as the nucleus, mitochondria, and endoplasmic reticulum carry out specific functions necessary for the cell's survival.
- Cells can be categorized into prokaryotic cells, which lack a nucleus, and eukaryotic cells, which have a defined nucleus containing genetic material.
- The study of cells, known as cell biology, helps us understand how living organisms function and provides insights into diseases and potential treatments.
- Cell division is a crucial process for growth, development, and repair of tissues, allowing for the formation of new cells to replace old or damaged ones.
- Cells communicate with one another through chemical signals and interactions, coordinating their activities to maintain homeostasis within an organism.
In conclusion, the assertion that cells are the basic structural and functional unit of all living organisms is supported by the reason that cells carry out essential functions necessary for life. Understanding the complexities of cells is fundamental to comprehending the biology of living organisms.