Lesson 1 of 7 / Cells and stem cells
What makes the cell the unit of life?
Why does a tissue grow by making cells?
In this lesson: Apply the three statements of cell theory to growth and reproduction.
About 4 min
The key ideaLiving organisms consist of cells; a cell is the smallest unit of life; new cells arise from pre-existing cells.
A structure can perform one useful reaction without being a living cell. Cell theory concerns the organised unit that sustains living processes.
Explore the idea
A new cell has a cellular source
The membrane bounds an organised living unit. An isolated internal component is not a whole cell.
Original schematic. Structures, cell numbers and distances are illustrative, not to scale.
Explanation
An organism may be one cell or many cells. In a multicellular organism, specialised cells cooperate, but each cell remains a bounded unit with the machinery for its activities. A tissue is an organised group of cells, not a replacement for them.
Cells are the smallest units of life. An isolated enzyme can catalyse a reaction, and a mitochondrion can carry out parts of respiration, but neither is an independently complete cell. The activities of a living cell depend on coordinated molecular systems.
New cells come from existing cells by cell division. Growth can involve increases in cell size and cell number; repair replaces lost cells through division of suitable surviving cells. Cell theory does not say that every mature specialised cell can divide indefinitely.
Step by step
- 1
Identify the biological level
Distinguish an organelle, a whole cell and a tissue.
- 2
Trace new cells
Look for division of existing cells, rather than spontaneous appearance from non-living material.
Worked example
A skin wound closes
Explain how cell theory applies when new skin cells replace cells lost from a small wound.
One way to explain it
The new cells arise by division of existing cells in the tissue. They become part of a multicellular organism made of cells; they do not form spontaneously from the wound fluid.
Why this answer works
- Name the source of the new cells.
- Connect division to replacement of tissue.
Is this true? "Every structure that uses energy is a cell."
Energy use alone does not define a cell. Organelles carry out particular processes within the organised living cell.