Construct an explanation, supported by evidence, for the cell theory and the idea that the cell is the basic structural and functional unit of all organisms (Louisiana Student Standards for Science, High School Biology, HS-LS1).

What this topic is asking

Louisiana's LS1 standards treat the cell as the starting point for all of biology: the basic structural and functional unit of every living thing. For LEAP 2025 Biology you should be able to state the cell theory, describe the evidence that supports it, explain the role of the microscope, and connect cells to the larger levels of organization in an organism. Because the standards are three-dimensional, the test often gives you an observation and asks which part of the theory it supports, which is a constructing-an-explanation practice.

The three parts of the cell theory

These three statements together explain a huge range of observations. The first says cells are universal: bacteria, plants, fungi, and animals are all cellular. The second says the cell is where the activities of life actually happen, so to understand an organism you study its cells. The third, added later, says cells do not appear from nonliving material; a new cell is always produced when an existing cell divides. On LEAP, expect to be given a single observation and asked which of these three claims it best supports.

The evidence and the role of the microscope

The cell theory is a good example of how a scientific theory is built: not by one person, but by many scientists gathering repeatable evidence over time. The key technology was the microscope. Early microscopists described cells in cork and saw "tiny animals" (microorganisms) in pond water; later scientists, using better lenses, concluded that plant and animal tissues are both made of cells, and finally that cells come only from other cells.

Cells as the unit of structure and function

Calling the cell the basic unit of life means it is the smallest thing that can carry out all the processes of life on its own: taking in nutrients, releasing energy, responding to the environment, and reproducing. A single-celled organism does all of this in one cell. In a multicellular organism, cells specialize and work together, which leads to the levels of organization.

This connects directly to the structure-and-function crosscutting concept that runs through all of LS1: the shape and contents of a cell suit the job it does, just as the shape of an organ suits its role in the body.

Levels of organization

In a multicellular organism, cells of the same type group into a tissue; different tissues form an organ; organs that work together form an organ system; and the organ systems together make the organism. This hierarchy (cells to tissues to organs to organ systems to organism) is the LS1 idea of organisms as systems of interacting parts, and it is examined in more depth in the body-systems topic.

Try this

Q1. State the three parts of the cell theory. [3]

• Cue. All living things are made of one or more cells; the cell is the basic unit of structure and function; all cells come from pre-existing cells.

Q2. Explain why the development of the microscope was essential to the cell theory. [2]

• Cue. Cells are too small to see with the unaided eye, so the microscope's magnification was needed to observe them and gather the evidence for the theory.