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2.1.1Cell Structure

The Cell Theory — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 47 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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2.1.1 The Cell Theory — AQA A Level Biology 7402 notes: State the cell theory, explain the evidence for it and discuss its exceptions and the way scientific theories develop.The cell theory states that:All living organisms are made of one or more cells.The cell is the basic unit …

  • basic unit of structure and function
  • pre-existing cells
  • Robert Hooke (1665)
  • Antonie van Leeuwenhoek
  • Schleiden (1838)
  • Schwann (1839)

Section 1 of 9The cell theory

The cell theory states that:

The cell theory
Cells
All living organisms are made of one or more cells.
Basic unit of structure and function
The cell is the basic unit of structure and function in all living things.
Pre-existing cells
All cells arise from pre-existing cells by cell division.

Modern versions add that cells contain hereditary material (DNA) which is passed from cell to cell during division, and that energy flow (metabolism) occurs inside cells. The theory is a good example of a scientific theory: it explains a wide range of observations, makes predictions and has been supported repeatedly by evidence.

Section 2 of 9How the theory developed

1Robert Hooke (1665)
Robert Hooke (1665) used an early light microscope to examine cork and named the box-like compartments "cells".
2Antonie van Leeuwenhoek
Antonie van Leeuwenhoek used single-lens microscopes to observe living cells such as bacteria, protozoa and sperm.
3Schleiden (1838)
Schleiden (1838) concluded that all plants are made of cells and Schwann (1839) concluded the same for animals; they combined these ideas into the first statement of cell theory.
4Virchow (1855)
Virchow (1855) proposed that new cells come only from existing cells ("omnis cellula e cellula"), replacing the idea of spontaneous generation.
5Pasteur
Pasteur showed that microorganisms do not appear spontaneously in sterilised broth that is kept protected from air-borne microbes.

Improvements in microscope technology, especially better lenses and later the electron microscope, allowed scientists to see the detailed structures inside cells, so ideas developed as the available evidence increased.

Section 3 of 9Evidence for the theory

Observations of tissues from all kinds of organisms show cell structure.

1
Cell division can be seen directly under the microscope, and DNA replication before division explains how genetic information passes to new cells.
2
Cell culture shows that cells grown from a single cell give rise to a population of identical cells (clones).
3
Stem cell studies show how differentiated cells arise from unspecialised cells.
4
Together these observations support the idea that cells are the unit of life and always arise from other cells.

Section 4 of 9Exceptions and challenges

ExampleWhy it challenges the theory
Skeletal muscle fibresLong, multinucleate structures that are larger than typical cells and formed by fusion of cells
Aseptate fungal hyphaeContinuous cytoplasm with many nuclei and no dividing walls
Giant algae such as AcetabulariaSingle cell up to several centimetres long
VirusesNot cellular, but depend on cells to replicate; usually not considered living
Mature phloem sieve tubes and red blood cellsHave no nucleus, yet function

These do not disprove the theory, but they show that the definition of a cell needs to be flexible. Scientists accept a theory as the best explanation of all current evidence and modify it when needed. The first origin of cells on Earth must be an exception to "cells arise from cells", but that happened billions of years ago.

Section 5 of 9Unicellular and multicellular organisms

Unicellular organisms (e.g. Amoeba, Paramecium, bacteria) carry out all life processes within one cell.

Differentiate
In a multicellular organism cells differentiate and specialise, forming tissues, organs and organ systems.
A larger body gives advantages, such as greater size, the ability to have specialised cells that work more efficiently, and longer life.
However, as an organism grows its surface-area-to-volume ratio decreases, so it needs transport and exchange systems.
This is why multicellular organisms have organ systems such as the circulatory and gas-exchange systems.

Section 6 of 9Links

Cell theory links to cell division (2.3), differentiation of cells (2.1.3), stem cells, and microscopy (2.2), which allowed cells to be seen. Questions often ask you to explain why a particular observation does or does not fit the theory; always state the part of the theory that is in question.

Think also about the surface area to volume ratio as a reason why cells are small.

Links
As a cell grows, its volume increases faster than its surface area,
  • so the rate of diffusion of nutrients and oxygen into the cell and of waste out of it becomes too slow for the cell's needs.
This sets an upper limit to the size of most cells and is the reason cells divide once they reach a certain size.
Specialised cells such as those of the small intestine overcome the problem by having folded membranes (microvilli) that increase surface area.

When writing about the development of a theory, use the format: evidence, who found it, what it showed, and how it changed ideas.

Links
Remember that new technology often drives the development of theories: the microscope was crucial,
  • and later the electron microscope showed organelles and prokaryotic structure, further supporting the idea that all organisms are composed of cells, whether prokaryotic or eukaryotic.
A strong evaluation will also mention that a theory is only provisional, since scientists accept it only as long as it explains the evidence.

Section 7 of 9Exam-style questions

  1. State three ideas of the cell theory. [3]3 marks

  2. Skeletal muscle fibres are multinucleate and can be several centimetres long. Explain why they challenge the cell theory. [2]2 marks

  3. Explain how improvements in technology have led to the development of cell theory. [3]3 marks

Section 8 of 9Key terms

TermMeaning
Cell theoryLiving organisms are made of cells, which are the basic unit of life and arise from pre-existing cells.
CellThe basic structural and functional unit of living organisms.
Spontaneous generationDiscredited idea that living things arise from non-living matter.
MultinucleateContaining many nuclei within one cytoplasm.
DifferentiationSpecialisation of cells for a particular function.
TissueA group of similar cells working together.
OrganA group of tissues forming a functional unit.

Section 9 of 9Exam technique

1
Saying the cell theory is just a guess. In science a theory is a well-supported explanation.
2
Saying Hooke discovered living cells. He saw the walls of dead cork cells.
3
Forgetting that viruses are not cells.
4
Saying all cells have a nucleus. Prokaryotes do not, and red blood cells and sieve tube elements lose theirs.
5
Stating that exceptions disprove the theory instead of refining the definition.

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