1.1 – Cell Theory

1.1 – Cell Theory

1.1.1 – Outline the cell theory

The cell theory is that cells are the basic unit structure and function of every living thing. This contains three main ideas:

     1. Cells are the building blocks of structure in living things

     2. They are the smallest unit of life

     3. Cells are formed from other, pre-existing cells by division

Two additional concepts have been added today:

     1. Cells store all the information they require for growth, development and behaviour

     2. Cells are the location for all the chemical reaction needed for life, metabolism.

1.1.2 – Discuss the evidence for the cell theory

Cells are the Building Blocks of Structure in Living Things

All living things observed through microscopes have been found to be made up of cells, proving that living things are made up of cells.

Cells are the Smallest Unit of Life

Viruses: These are non-cellular, crystalline structures. They can only reproduce at the expense of the cell’s metabolic machinery. They cannot obtain life from any other source.

Cell Substructures: The lifespan of organelles has been found to be extremely short when they attempt to function outside the cell. This was shown through biochemical investigations of these organelles.

All Cells are Formed from Pre-existing Cells

Pasteur’s observations: He proved that the apparent ‘spontaneous’ generation of microorganisms was in fact due to the presence of unnoticed cells.

Resistant Spore Phase: This is a stage in the life cycle of many organisms. A related discovery was that spores of many microorganisms are found everywhere, but will only develop in favourable conditions.

Behavior of Chromosomes: Observing chromosome during cell division (mitosis and meiosis) and during reproduction has shown that new cells contain information from old cells.

Function of Genes: Each cell contains the blueprint needed for growth, development and behaviour in DNA. Research has also established the nature and roles of genes in the day-today control of cells and in the process of heredity. Experimental evidence through genetic engineering of the effects on cells of the deliberate transfer of genes between organisms also supports this.

Cells are the site of the necessary chemicals needed for life: The discovery of enzymes and
their machinery, which are used in the chemical processes with a cell. This includes aerobic respiration and fermentation. The discovery of biochemical events with cells also proved this. Examples are the formation of proteins from amino acids

Cell Ultrastructure: the presence of discrete organelles and the biochemical events located in particular organelles shows that cells are designed to function independently. Scientists have shown that, because every cell contains organelles which are the locations of specific
chemical reactions, they are the place where all these necessary processes of life take place.

1.1.3 – State that unicellular organisms carry out all the functions of life
These organisms are capable of carrying out all the necessary processes needed in living things.

  • Metabolism – this includes the respiration and synthesis of ATP
  • Response – to any change in the environment
  • Homeostasis – the maintenance and regulation of internal cell conditions
  • Growth – when the cell increases in size and volume
  • Reproduction – unicellular organisms are asexual, therefore they reproduce through division to form a clone
  • Nutrition – this is either the synthesis of organic molecules or the absorption of organic matter


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


Relative Sizes

1 nm – Molecule

10 nm – Cell membrane

100 nm – Virus

1000 nm – Bacteria (1μm)

10000 nm – Organelles (10μm)

100000 nm – Cells (100μm)


1.1.5 – Calculate the linear magnification of drawings and the actual size of specimens in images of know magnification

1.1.6 – Explain the importance of the surface area to volume ratio as a factor limiting cell size
As the size of a structure increases, the surface area to volume ratio decreases. For example, using a cube:


As organisms get bigger, their volume and surface area both get bigger, but not by the same amount. As a result, larger organisms have a slower rate of exchange (diffusion/radiation) with their outside surroundings.
This is true for organelles, cells, tissues, organs and organisms. All organisms need to exchange substances such as food, waste, gases and heat with their surroundings. The rate of exchange of substances depends on the surface area of the organism which is in contact
with its surroundings.

For this reason, cells are very small so that they are able to exchange substances efficiently.


1.1.7 – State that multicellular organisms show emergent properties

Emergence is the unexpected occurrence of characteristics or properties in a complex system, which could not be expected based on the properties of the constituents. Multicellular organisms have properties that exceed the sum of the properties of its constituents. Therefore, only when these parts are combined that we can determine the properties, as this cannot be done by individually analysing the properties of the parts. The properties emerge as a result of the interaction of the parts.

For example, a mitochondrion makes excess energy in the form of ATP, but for no purpose. It needs other things to give this to.


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

Multicellular organisms are large and need to have specialised parts to their structure so that all the necessary functions of life can be performed.

Differentiation – The cells can become specialised to perform their function. These cells switch on, or express, particular genes that correlate with these specific functions. The expression of these genes will influence the shapes, functions and adaptations with that cell.
For example, a muscle cell will only express muscle genes, but not nerve cell genes.

Specialization in multicellular organisms is more efficient for organisms competing for a specific resource. Movement of nutrients, water, etc, can happen faster and more effectively than passing between cells through diffusion.


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

Stem cells can divide, however they have not yet expressed any of their genes so that they might specialise in a particular function. They will express particular genes under the right conditions and differentiate into a particular type of cell. They can be obtained from a variety of places including blastocyte, or even the placenta. Children possess more stem cells than adults.

Stem cells used for research are usually human embryonic stem cells, which come from embryos only a few days old. These are more flexible, and can grow into any type of mature cell. The techniques are controversial, thus there are international principles regarding this


1.1.10 – Outline one therapeutic use of stem cells

Stem cells could be used to treat and perhaps cure many diseases. One of these possibilities is with cystic fibrosis. Patients would be treated with their own stem cells Cells are removed and genetically engineered, then planted back into the patient, which might lead to the healthy formation of cells in the airway of the lungs. This therapy would also eliminate the problem of tissue rejection in transplant surgery, as the cells are from the patient Stem cells also hold promise for the

Stem cells also hold promise for the testing of new drugs. This would lead to safer testing, but would be more difficult because the conditions must be carefully controlled. Other cells are already used in this way. However, current knowledge does not allow us to be able to precisely control these conditions to generate pure populations of cells for testing.