Lesson 1 of 6 / Cells and microscopy
Cells are the units of life
Why does a cell count as living when an isolated ribosome does not?
I can apply cell theory and distinguish cells from their components.
I can explain the three statements of cell theory.Syllabus
Syllabus 9477, Core 1(a). Explain the three statements of cell theory.
About 5 min
Make a guess. You are not marked.
An isolated ribosome in a test tube, given amino acids, tRNA, mRNA and ATP, joins amino acids into a polypeptide. Is the ribosome alive?
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It cannot maintain or reproduce itself.
Carrying out one reaction is not life. A cell maintains a boundary, runs metabolism and arises from another cell.
The key idea
A cell is the smallest unit able to carry out the coordinated processes of life; new cells arise from existing cells.
Explore the idea
Trace a cell back to an existing cell
A cell is the smallest unit of life. An isolated nucleus or ribosome is a component of a cell, not a complete independently functioning cell.
Unicellular organisms consist of one cell. Multicellular organisms organise many cells into a functioning whole.
Original schematic. Features, cell sizes and distances are not to scale. Use several visible clues when interpreting an unfamiliar section.
Explanation
Cell theory links three claims: organisms consist of cells, the cell is the smallest unit of life, and cells arise from pre-existing cells. A unicellular organism performs its life processes in one cell. A multicellular organism distributes tasks among specialised cells, which depend on one another.
An organelle performs only part of a cell's work. A ribosome can assemble a polypeptide when supplied with the necessary molecules, but cannot independently regulate a boundary, maintain metabolism and reproduce as a complete living system. Small size alone does not define life.
Cells grow and divide by organised processes that transmit genetic information. The statement about pre-existing cells describes the continuity of present-day cellular life; it is not an explanation of the first origin of life.
A specimen with DNA is not automatically a cell. Establish whether it has a cellular organisation, such as cytoplasm and a membrane enclosing biochemical machinery. Viruses make this distinction challenging because they have genetic information but depend on host cells for reproduction.
Step by step
- 1
Identify the unit
Distinguish a whole cell from one component or a multicellular tissue.
- 2
Look for coordinated functions
Link a boundary, metabolism and hereditary continuity.
- 3
State the limit
Cell theory does not claim every DNA-containing particle is a cell.
Worked example
Work through the evidence
A tissue fragment contains 200 cells. A purified sample contains 200 ribosomes. Which contains units of life?
One way to explain it
The tissue contains 200 cellular units. The ribosome sample contains components of cells, not 200 living organisms.
Why this answer works
- Count the relevant biological unit.
- An isolated structure may have activity without independent cellular organisation.
Watch out for this
A student says: "Every structure that carries out a reaction is alive." What is wrong with this?
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Chemical activity is necessary for life but does not make an isolated molecule or organelle a living cell.
Check your understanding
Which observation directly supports the origin of cells from existing cells?
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One cell divides to produce two daughter cells.
Division connects new cells to a pre-existing cell.
Quick recall
Why does a cell count as living when an isolated ribosome does not?