2.1 Cell Theory


2.1.1  Outline the cell theory

The cell theory states that:

1.  All living things are composed of cells (or cell products)

2.  The cell is the smallest unit of life

3.  Cells only arise from pre-existing cells

2.1.2  Discuss the evidence for the cell theory


  • Microscopes have increased man's ability to visualise tiny objects
  • All living things when viewed under a microscope have been found to be made of cells and cell products (e.g. hair)
  • Note:  Certain types of cells do not conform to the standard notion of what constitutes a cell
    • Muscle cells contain multiple nuclei
    • Fungal hyphae consist of multiple cells that share a continuous cytoplasm

Light vs Electron Microscopes

Experimental Evidence:

  • Cells removed from tissues can survive independently for short periods of time
  • Nothing smaller than a cell has been found to be able to live independently
  • Experiments by Francesco Redi and Louis Pasteur have demonstrated that cells cannot grow in sealed and sterile conditions

History of the Cell Theory

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

Unicellular organisms (such as amoeba, paramecium, euglena and bacterium) are the smallest organisms capable of independent life.

All living things share 7 basic characteristics:

  • Movement:   Living things show movement, either externally or internally
  • Reproduction:   Living things produce offspring, either sexually or asexually
  • Sensitivity:   Living things can respond to and interact with the environment
  • Growth:   Living things can grow or change size / shape
  • Respiration:   Living things use substances from the environment to make energy
  • Excretion:   Living things exhibit the removal of wastes
  • Nutrition:   Living things exchange materials and gases with the environment

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

Relative sizes:                                                                 Unit Conversion Table:

  • A molecule = 1 nm
  • Cell membrane thickness = 7.5 nm
  • Virus = 100 nm  (range: 20 - 200 nm) 
  • Bacteria = 1 - 5 um
  • Organelles = <10 um
  • Eukaryotic cells = <100 um

Diagram of the Relative Sizes and Scale of Biological Materials

Cell Size and Scale (Learn Genetics)

2.1.5  Calculate the linear magnification of drawings

To calculate the linear magnification of a drawing the following equation should be used:

  • Magnification = Size of image (with ruler) ÷ Actual size of object (according to scale bar)

To calculate the actual size of a magnified specimen the equation is simply re-arranged:

  • Actual size = Size of image (with ruler) ÷ Magnification


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

  • The rate of metabolism of a cell is a function of its mass / volume
  • The rate of material exchange in and out of a cell is a function of its surface area
  • As the cell grows, volume increases faster than surface area (leading to a decreased SA:Vol ratio)
  • If the metabolic rate is greater than the rate of exchange of vital materials and wastes, the cell will eventually die 
  • Hence the cell must consequently divide in order to restore a viable SA:Vol ratio and survive
  • Cells and tissues specialised for gas or material exchange (e.g. alveoli) will increase their surface area to optimise the transfer of materials

Microvilli increase surface area allowing for a more efficient exchange of materials / heat

2.1.7  State that multicellular organisms show emergent properties

Emergent properties arise from the interaction of component parts: the whole is greater than the sum of its parts

Multicellular organisms are capable of completing functions that individual cells could not undertake - this is due to the interaction between cells producing new functions

In multicellular organisms:

  • Cells may group together to form tissues
  • Organs are then formed from the functional grouping of multiple tissues
  • Organs that interact may form organ systems capable of carrying out specific body functions
  • Organ systems carry out the life functions required by an organism

Levels of Anatomical Organisation

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

  • All cells of an individual organisms share an identical genome - each cell contains the entire set of  genetic instructions for that organism
  • The activation of different instructions (genes) within a given cell by chemical signals will cause it to differentiate from other cells like it
  • Differentiation is the process during development whereby newly formed cells become more      specialised and distinct from one another as they mature
  • Active genes are usually packaged in an expanded and accessible form (euchromatin), while inactive genes are mainly packaged in a condensed form (heterochromatin)
  • Differentiated cells will have different regions of DNA packaged as heterochromatin and euchromatin depending on their function

Differential Gene Expression Leading to Specialisation of Cell Structure and Function

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

Stem cells are unspecialised cells that have two key qualities:

1.  Self renewal:  They can continuously divide and replicate

2.  Potency:  They have the capacity to differentiate into specialised cell types

Stem Cells

2.1.10  Outline one therapeutic use of stem cells

Stem cells can be derived from embryos or the placenta / umbilical cord of the mother; also minimal amounts can be harvested from some adult tissue

Stem cells can be used to replace damaged or diseased cells with healthy, functioning ones

This process requires:

  • The use of biochemical solutions to trigger differentiation into desired cell type
  • Surgical implantation of cells into patient's own tissue
  • Suppression of host immune system to prevent rejection of cells
  • Careful monitoring of new cells to ensure they do not become cancerous

Examples of therapeutic uses of stem cells:

1.  Retinal cells:  Replace dead cells in retina to cure diseases like glaucoma and macular degeneration

2.  Skin cells:  Graft new skin cells to replace damaged cells in severe burn victims

3.  Nerve cells:  Repair damage caused by spinal injuries to enable paralysed victims to regain movement

4.  Blood cells:  Bone marrow transplants for cancer patients who are immuno-compromised as a result of chemotherapy