A cell must have the following structural and functional attributes to be called a living cell:
i. Plasma Membrane: A cell should have a plasma membrane. Without the plasma membrane, the contents of the cell cannot get a container and cells won't be able to maintain its independent existence.
ii. Functional Attributes of Plasma Membrane: The plasma membrane should be selectively permeable so as to facilitate the exchange of materials between the cell and its environment.
iii. Protoplasm: A cell must have a protoplasm to be called a living cell. It is the protoplasm which contains all the materials necessary for life to continue. If protoplasm dries up, the cell becomes dead; as in the case of sclerenchymatous cells.
iv. Endoplasmic Reticulum: Endoplasmic reticulum is necessary for the exchange of materials between nucleoplasm and cytoplasm. It is also necessary for the synthesis of various materials; like protein and lipid. Thus, a cell must have an ER to be considered a living cell.
Mitochondria are not mandatory for all living cells. For example; Red Blood Cells do not have mitochondria yet they are living cells. The prokaryotes do not have mitochondria but they are living cells.
The nucleus is not mandatory for all living cells. For example; mitochondria lack a nucleus. Similarly, prokaryotic cells lack a nucleus. Another important aspect of cells is that a cell is always formed from a pre-existing cell; as proposed in the Cell Theory.