IB Biology: Cell Theory

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2.1 Cell Theory

2.1.1 Outline Cell theory

The cell theory is a scientific theory about the properties of cells and revolves around the following statements

  • Living organisms are composed of cells
    • Cells are the building blocks of all living organisms
    • Cells are derived from pre-existing cells, through the process of cell division
    • Cells are the smallest functioning unit of life
      • Nothing smaller than the unit of a cell can survive
      • Chemical reactions (that allow the cell to live, metabolism) take place within the cell
      • Cells contain hereditary material
        • Hereditary material contains information which is used as instructions for growth, functioning, development and behavior


2.1.2 Discuss the evidence for the cell theory




Living things are composed of cells Observations using electron microscopy and light microscopy appear consistently to be made up of cellsExceptions to the rule:Muscle cells: do not conform the rule that all cells have one nuclei

  • Skeletal muscle cells have more than one nuclei per cell
  • Made up of fibres, which can be over 300mm long
  • Surrounded by plasma membranes, yet contain more than one nucleus i.e. multi-nucleated

Fungal cells: do not conform to the rule that a cell is a singular unit

  • Made up of hyphae, which are thread like structures
  • Do not contain dividing walls
  • Multi-nucleated
  • Cell walls made up of chitin
  • Cytoplasm is continuous, as it continues along the hyphae, with no ending cell wall or membrane

Protocista: do not conform to the generalization that all cells carry out one, specific function

  • Can carry out all functions of life
Cells are the smallest unit of life Cells are the smallest functioning unit of life, where the cell carries out all the chemical equations of lifeOrganelles require the co-operation and efficient work of other organelles in the cell to survive
Cells are derived from pro-existing cells Observations on the behavior of cells at cell division (reproduction, cytology, mitosis, and meiosis)Spontaneous generation of life is impossible

  • E.g. Pasteur’s observations: ‘spontaneous’ generation of life in the form of micro-organisms in ponds or puddle water were later attributed to the existence of previously un-noticed cells
Cells are the site of chemical reactions of life The discovery of enzymes, and the part they play in cellular processes e.g. aerobic respirationThe discovery of cell ultrastructure, as the presence of organelles designed to carry out a specific function


2.1.3 State that unicellular organisms carry out all the functions of life

Define state: to express something clearly or definitely

Unicellular organisms can carry out all the chemical processes of life, and demonstrate all the characteristics of a living organism, such as:

  • Nutrition: either the absorption of organic matter, or the production (synthesis) of organic substances
  • Reproduction: Largely asexual, thus through cell division to form a clone
  • Homeostasis: Regulation and maintenance of internal conditions
  • Metabolism: includes respiration: the synthesis of ATP
  • Growth: Increase in cell size and volume
  • Response: to a change in the environment (sensitivity)


2.1.4 Compare the relative sizes of molecules, cell membrane thickness, viruses, bacteria, organelles and cells using the appropriate SI unit

Define Compare: noting the similarities, and differences between two or more items

Units of length (International system, SI Units)




1 meter (m)


1000 millimeters

1 millimeter (mm)

10-3 (0.001)

1000 micrometers

1 micrometer (μm)

10-6 (0.000001)

1000 nanometers

1 nanometer (nm)

10-9 (0.000000001)

1000 picometers


With cells and their organelles only occupying a very small amount of space, they are usually measured in micrometers and nanometers


Relative size

Average diameter of a plant cell

40 μm

Average diameter of an animal cell

20 μm

Average diameter of a mitochondrion, chloroplasts or bacteria

1 μm

Average virus size

100 nm

Average thickness of a membrane

7-10 nm

Generally, plant cells are larger than animal cells


2.1.5 Calculate the linear magnification of drawings and the actual size of specimens in images of known magnification

Magnification = Image size / Actual size

  • Image size: Measured length from the picture, or photograph
  • Actual size: The genuine size of the cell

Scale bar: shows the distance represented by the length of the bar, so the magnification of the photograph is shown indirectly. You have to:

  • Measure the length of the scale bar
  • Work out the magnification
  • Calculate how many times greater the length of the scale bar is, and then the distance it represents


2.1.6 Explain the importance of surface area to volume ratio as a factor limiting cell size

  • As the size of a structure increases, its surface area to volume ratio decreases
    • If a cell becomes too large, it may develop problems because the surface area to volume ratio becomes too small, resulting in an inefficient, unsustainable cell

The rate at which materials enter (or leave) a cell depends on its surface area, however the rate at which materials are produced or used depends on its volume. So, if a cell becomes too large, the resulting small surface area to volume area doesn’t allow the cell to function properly. It may not be capable of taking in enough essential materials, or excreting waste substances quickly enough.


  • Same concept as the exchange of materials
  • Cells that generate heat may not be able to release it into the surrounding environment fast enough



  • As an organism gets larger, its surface area to volume ratio decreases
  • This is a limiting factor
  • If the surface area to volume ratio decreases, the rate of exchange is decreased
    • E.g. Gas exchange of oxygen for respiration

Cells are small because

  • A large cells requires more oxygen than a smaller one
  • Big cells thus require a higher rate of oxygen diffusion across the cell membrane, however, this is unattainable as the cell has a low surface area compared to its volume
  • While the cell may benefit from being large, it cannot get larger, as the cell is limited by its gas exchange rate
  • This scenario can also be applied to heat, nutrients and waste
    • Excretion of waste molecules e.g. carbon dioxide, urea
    • Obtaining nutrients e.g. glucose


2.1.7 State that multicellular organisms show emergent properties

Emergent properties: arise from the interaction of smaller component parts to form the living organism. Cell organelles individual functions collate to allow the cell to live – all the small parts collect together for one function.

  • The interaction of smaller components
    • E.g. Water molecules and surface tension
    • Cell organelles and cells


2.1.8 Explain that cells in multicellular organisms differentiate to carry out specialized functions by expressing some of their genes, but not others

  • Cells in a multicellular organism develop in different ways and can therefore carry out different functions
  • Different genes are required to develop in different ways
  • Each cell has all these genes, yet only the ones required for its ‘pathway for development’ are used
    • Once a pathway of development has begun, it is fixed
    • The cell is ‘committed’
    • Rather than all cells carrying out all functions, cells are specialized – all organs and tissues are integrated to demonstrate emergent properties
    • Differentiation: when cells within a multicellular organism carry out a specialized function
      • Specialized cells can switch on, or express certain genes that allow them to become specialized
      • Specific gene functions correlate to, or dictate shapes, functions and adaptations
      • For a large multi-cellular organism, differentiation and specialization is efficient



  • The process by which a cell becomes specialized to carry out a particular function is known as differentiation
  • Red Blood Cells, or erythrocytes
    • It’s shape as a bi-concave disk allows it to fold in on itself, to increase its haemoglobin, or oxygen capacity. The absence of a nucleus also allows the cell to fit in more haemoglobin (thus increasing the amount of oxygen transported around the body through the blood stream, and overall efficiency) as well as maintaining a high surface area to volume ratio
    • When cells differentiate and become specialized, they become more efficient at carrying out a particular function
    • Differentiation changes both the shape and physiology of the cell
    • Every cell in the human body (with the exception of gametes) contains the same genetic information, yet differentiation occurs because different genes are expressed
    • While differentiation allows cells to become more efficient, they lose the ability to carry out other functions
      • If the need arises for a cell to carry out a new, particular function, it can no longer adapt to carry this out


2.1.9 State that stem cells retain the capacity to divide and have the ability to differentiate along different pathways

  • Stem cells have the capacity to self-renew by cell division and differentiate
  • Found mostly in skin cells, bone marrow and the liver
  • Give human tissue a partly limited ability to regenerate and repair


2.1.10 Outline one therapeutic use of stem cells 











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One Response to IB Biology: Cell Theory

  1. Varshini says:

    This is really good! I’m in IGCSE right now (going to take my exams in June) and I think this is really useful to prepare me for IB! Thank you!!!

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