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The cell theory
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
Discuss the evidence for the cell theory
Microscopes:

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
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
Compare the relative sizes of molecules, cell membrane thickness, viruses, bacteria,
organelles and cells, using appropriate SI units
Relative sizes:

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
Unit Conversion Table:
Diagram of the Relative Sizes and Scale of Biological Materials
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
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
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
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
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
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 immunocompromised as a result of chemotherapy.