Lesson 1 of 7 / Cells and stem cells
What makes the cell the unit of life?
Why does a tissue grow by making cells?
I can apply the three statements of cell theory to growth and reproduction.
I can apply the three statements of cell theory.Syllabus
Syllabus 8876, Core 1(a). Apply the three statements of cell theory.
About 4 min
Make a guess. You are not marked.
A cut on your skin heals with new tissue. Where do the new skin cells come from?
Show the answer
From existing cells dividing.
Cell theory says new cells arise from pre-existing cells. Nearby cells divide to fill the wound.
The key idea
Living 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.
Watch out for this
A student says: "Every structure that uses energy is a cell." What is wrong with this?
Show the answer
Energy use alone does not define a cell. Organelles carry out particular processes within the organised living cell.
Check your understanding
Which observation directly supports the origin-of-cells statement?
Show the answer
One bacterial cell divides into two cells.
The new cells arise from an existing cell.
Quick recall
Why does a tissue grow by making cells?