1.1 Cell theory

1.1 Introduction to cells

Lesson one: Cell theory

Essential questions: What is the basic unit of life?

Objectives: Understand what is meant by cell theory. Know exceptions to cell theory.


  • Cell theory: the theory that the cell is the basic unit of life for all living things
  • micrometre: one thousandth of a milimetre
  • nanometre: one thousandth of a micrometre 
  • Tissue: a group of highly similar cells with the same function
  • Protista: a kingdom of ‘amoeba-like’ single celled organisms eg. paramecium or Scenedesmus

Cell theory states that living organisms are made from cells. This establishes cells as the building blocks of living organisms. The most simple organisms are only made from one cell, such as Protists (in the kingdom Protoctista).

All cells have:

  • a cell membrane,
  • contain genetic material which stores all of the information needed for the cell’s activities,
  • have enzymes to catalyse their metabolic reactions,
  • and generate their own energy.

Life is an emergent property (it emerges from the interactions of their cellular components, as something extra to them).

Can you think of other examples of emergent properties?

The implicit assumption of cell theory is that nothing smaller than a cell can survive eg. a cell membrane cannot live on it’s own.

Single-celled organisms carry out all the functions of life:

The seven characteristics of life are below:

  1. Movement: to transport oneself from place to place.
  2. Reproduction:  to reproduce more organisms of the same species
  3. Sensitivity:  to respond to stimuli
  4. Growth:  to increase in dry mass or number of cells
  5. Respiration: to release energy from sugars
  6. Excretion: to remove waste substances from blood or cells
  7. Nutrition: to assimilate nutrients from the environment in order to survive

it is also assumed that cells come from pre-existing cells. Cells cannot spontaneously appear from nothing (it was once thought that this was possible, it was called the theory of spontaneous generation).

This gives us a more complex view of cell theory. A summary:

  • Living things are made of cells
  • The cell is the basic unit of life (nothing smaller than a cell is alive).
  • Cells carry out all lifes functions (ie. MRSGREN)
  • Cells can only come from pre-existing cells

TASK: PLASTICINE ALIEN: You have five minutes as a group to create a model of an alien made from plasticine. You need to explain the basic natural history of the alien, give it a name, and then explain how it carries out the seven characteristics of life.

CASE STUDY: How do two protists carry out all the processes of life

Task: Investigating two single celled organisms, Paramecium and Scenedesmus.


image credit: bioninja

image credit: bioninja

An introduction: Meet paramecium


Exceptions to cell theory:

Some organisms cannot be neatly divided into cells. They are exceptions to cell theory

  1. Giant algae (A). Several species of Giant algae exist eg. Caleurpa.  which seem to have only one nucleus, and so seem to be made of one giant cell. They can be as long as 100mm.
  2. Striated muscle. Striated muscle is a type of muscle tissue found in skeletal muscle in mammals. It is not divided into cells, but compartments called sarcomeres instead. (B)
  3. Aspetate fungi. Some fungi are made out of thread like structures called hypha. Usually these are divided into cell like components by divisions called ‘septa’. Sometimes they aren’t, and the hypha appears to be multinucleate (many nuclei), without divisions into cells.(C)


giant algae





Understanding relative sizes: Follow this link to compare the relative sizes of cells / organelles in the general framework of things.

Lab Microscopy (see lab handout):

You must know how to prepare a wet slide of plant and animal tissue, focus on the relevant cells, estimate cell size and make a clearly labelled tissue drawing ( a tissue drawing is where no individual cells are drawn). Please see the lab skills webpage of MrHorrocks.com for more details.

Examples of specimens prepared by IB Junior students

cheek cellIMG_8789


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