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3.1.1Unit 3: Practical Skills in Biology I

Cell Theory — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 4 parts Lesson 3 of 27 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Part 1 of 4The Cell as the Unit of Life

3.1.1 Cell Theory

Cell theory is a unifying concept that developed as microscopes improved.

Early observations
  • Robert Hooke was among the first to see cells using a microscope, and he used the word cell to describe them.
Schleiden and Schwann
  • In 1837 these scientists proposed that all living organisms are built from cells, which became known as cell theory.
Idea 1
  • All living organisms are made up of one or more cells.
Idea 2
  • The cell is the basic functional unit of living organisms, since the processes of life happen within cells.
Idea 3
  • New cells are produced only from pre-existing cells, usually by cell division.
A unifying concept
  • Cell theory is accepted by all biologists and it links the study of every type of organism.

Part 2 of 4Features Common to All Cells

Features Common to All Cells
Cell surface membrane
  • A partially permeable boundary that controls the movement of substances into and out of every cell.
Cytoplasm
  • The jelly-like fluid inside the membrane where many metabolic reactions of the cell take place.
DNA
  • Genetic material that carries the instructions for making proteins and is copied before the cell divides.
Ribosomes
  • Structures where proteins are made by translation; they are found in all cells, although their size differs.
FeatureProkaryotic cellEukaryotic cell
Internal membranesNoneMany membrane-bound organelles
RibosomesSmaller (70S)Larger (80S)
DNAFree in cytoplasmIn a nucleus

Part 3 of 4Ultrastructure and Evidence

1
Improved lenses allowed scientists to see small cells and to compare cells from many different tissues and species.
2
Observations of plants and animals showed repeating cellular units, which led to the proposal that all living things are made of cells.
3
Later work showed that new cells arise from existing ones by division and not by appearing spontaneously.
4
Electron microscopes revealed the ultrastructure of cells, meaning the detailed internal structures such as organelles.
5
Comparing ultrastructure explains differences between cell types, such as the lack of membrane-bound organelles in prokaryotes.

Part 4 of 4Exam Checklist & Tips

3.1.1 – Key Points for the Exam

  • Cell theory The idea that all organisms are made of cells, which are the basic units of life and arise from existing cells.
  • Ultrastructure The detailed internal structure of a cell seen with an electron microscope.
  • Organelle A structure within a cell that performs a specific function.
  • Prokaryote An organism whose cells have no nucleus or membrane-bound organelles.

Common Mistakes

  • Saying that cell theory states that cells can arise spontaneously; new cells come from existing cells.
  • Writing that Hooke proposed cell theory, when it was Schleiden and Schwann in 1837.
  • Stating that prokaryotes have no DNA, when their DNA is not enclosed in a nucleus.
  • Stating that all cells have a nucleus, when prokaryotic cells do not.

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Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.

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