Q.Briefly describe the cell theory.

The cell theory states that all living organisms are made of cells and the products of cells, and that every cell arises from a pre-existing cell.

• Matthias Schleiden, studying plants, and Theodore Schwann, studying animal cells, worked out that both plant and animal bodies are made up of cells and cell products.
• Their combined formulation, however, left a gap: it did not explain how new cells actually come into existence.
• Rudolf Virchow filled this gap by showing that cells arise through division of pre-existing cells, captured in the idea that every cell comes from an earlier cell. …

The cell theory did not emerge from a single discovery but was built up gradually through the work of several scientists in the mid-nineteenth century. Matthias Schleiden, a botanist, studied many plants and concluded that they were all built from different kinds of cells forming the plant's tissues. Around the same time, Theodore Schwann, examining a wide range of animal cells, noticed that cells are bounded by a thin outer plasma membrane, and recognising the cell wall as a feature special to plants, he proposed that both animal and plant bodies are composed of cells and the products of cells. Working together, Schleiden and Schwann formulated the cell theory in this combined form. …

Method: Tracing an Idea Through the Scientists Who Built It

When a question asks you to "describe" a theory that has a known history (cell theory, evolution, etc.), organising the answer chronologically by contributor is usually clearer and easier to reconstruct under exam pressure than trying to state the finished theory as one abstract block. Ask yourself: who contributed which piece, and in what order did the gaps get filled?

Here, start with Schleiden's observation from plants and Schwann's observation from animals — note that they were looking at different kingdoms and arrived at a compatible conclusion, which is itself worth mentioning since it's part of what made their combined claim persuasive. Their joint statement (bodies are made of cells and cell products) is the first half of the theory, but flag explicitly what it left unanswered: it says nothing about where a new cell actually comes from. …