All living cells are composed of which substance?

The correct answer is that all living cells are composed of protoplasm. Protoplasm refers to the material that makes up the living part of a cell, which includes both the cytoplasm and the nucleus. It encompasses all the cellular components that are essential for life, including organelles, proteins, water, and other substances necessary for metabolic processes.

Cytoplasm is a component of protoplasm, specifically the gel-like substance that fills the cell and surrounds the nucleus, where many cellular processes take place. However, referring only to cytoplasm does not encompass the entirety of what makes up living cells, since it excludes the nucleus.

Dendrites are extensions of neurons and are not a fundamental component of all living cells. They are specialized structures involved in the transmission of electrical signals in nerve cells.

The cell membrane is also a critical component of cells, as it acts as a protective barrier and regulates the movement of substances in and out of the cell. However, like cytoplasm, the cell membrane is only one part of the overall composition of protoplasm.

Thus, protoplasm accurately reflects the complete substance of living cells, incorporating all essential elements necessary for cellular function.