2.1 Cell Theory
2.1 Cell Theory
2.1.1 Outline Cell theory
Define Outline: to give a brief summary, or account of
Cell theory is the phrase used to describe four concepts:
o Living organisms are composed of cells
 Cells are the building blocks of all living organisms
o Cells are derived from pre-existing cells, through the process of cell division
o 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
o 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
Concept
Living things are composed of cells
Cells are the smallest unit of life
Cells are derived from pro-existing
cells
Cells are the site of chemical
reactions of life
Evidence
Observations using electron microscopy and light microscopy appear
consistently to be made up of cells
Exceptions to the rule:
Muscle cells: do not conform the rule that all cells have one nuclei
o Skeletal muscle cells have more than one nuclei per cell
o Made up of fibres, which can be over 300mm long
o 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
o Made up of hyphae, which are thread like structures
o Do not contain dividing walls
o Multi-nucleated
o Cell walls made up of chitin
o 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
o Can carry out all functions of life
Cells are the smallest functioning unit of life, where the cell carries out
all the chemical equations of life
Organelles require the co-operation and efficient work of other organelles
in the cell to survive
Observations on the behavior of cells at cell division (reproduction,
cytology, mitosis, and meiosis)
Spontaneous generation of life is impossible
o 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
The discovery of enzymes, and the part they play in cellular processes
e.g. aerobic respiration
The discovery of cell ultrastructure, as the presence of organelles
designed to carry out a specific function
2.1 Cell Theory
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:
o Nutrition: either the absorption of organic matter, or the production (synthesis) of organic substances
o Reproduction: Largely asexual, thus through cell division to form a clone
o Homeostasis: Regulation and maintenance of internal conditions
o Metabolism: includes respiration: the synthesis of ATP
o Growth: Increase in cell size and volume
o 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
Unit
1 meter (m)
1 millimeter (mm)
1 micrometer (μm)
1 nanometer (nm)
Units of length (International system, SI Units)
Metres
1
-3
10 (0.001)
10-6 (0.000001)
10-9 (0.000000001)
Equivalent
1000 millimeters
1000 micrometers
1000 nanometers
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
40 μm
20 μm
1 μm
Average diameter of a plant cell
Average diameter of an animal cell
Average diameter of a
mitochondrion, chloroplasts or
bacteria
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
o Image size: Measured length from the picture, or photograph
o 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:
o Measure the length of the scale bar
o Work out the magnification
o 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
o
As the size of a structure increases, its surface area to volume ratio decreases
2.1 Cell Theory

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.
Heat
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Same concept as the exchange of materials
Cells that generate heat may not be able to release it into the surrounding environment fast enough
Conclusion
o As an organism gets larger, its surface area to volume ratio decreases
o This is a limiting factor
o 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
o A large cells requires more oxygen than a smaller one
o 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
o While the cell may benefit from being large, it cannot get larger, as the cell is limited by its gas
exchange rate
o 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.
o 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
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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
Examples
2.1 Cell Theory
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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
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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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Patient requires heavy does of radiation and or chemotherapy. This will destroy health blood tissue as
well as the diseased tissue
Blood is filtered for the presence of peripheral stem cells. Cells in the general circulation that can still
differentiate into different types of blood cell otherwise known as stem cells.
Bone marrow can be removed before treatment
Chemotherapy supplies toxic drugs to kill the cancerous cells.
Radiation can be used to kill the cancerous cells. In time however the cancerous cells adapt to this
treatment so that radiation and chemotherapy are often used together
Post radiation/ chemotherapy means that the patients health blood tissues is also destroyed by the
treatment
Health stem cells or marrow cells can be transplanted back to produce blood cells again