Cell Theory

notes on cell theory

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Robert Hooke was the first to use the term cell in 1665, after examining cork and other plant tissues under the microscope. In centuries that followed, the cell theory was developed.

  • Cells are the basic fucntional unit of life and nothing smaller can survive independently
  • All living things consist of cells, although the smallest organisms may consist of one cell only.
  • All cells come from other pre-existing cells, by division, and therefore new cells cannot be constructed from non- living chemical substances.
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The function of microscopes- to magnify objects invisible to the naked eye.


Resolution means magnification. The higher the resolution the greater the magnification. Resolution therefore refers to the ability to seperate two objects that are close together. The higher the resolution the closer the two objects can be and still be seen as seperate entities.

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The light microscope

The first light microscopes were buolt in 1595 by the englishman Hooke and his dutch counterpart Swammerman. They managed to achieve magnifications of X 20. Later Leeuwenhook working in Holland in the 17th century achieved magnifications of X 200

The Electron Microscope 

The Transmission Electron Microscope (TEM) has resolution of about 2nm, and looks at replicas of dead cells or thin smears, preceded by fixation and heavy metal ion staining. Electrons are scattered as they pass through a thin section of the specimen, and then detected and projected onto an image on a fluorescent screen. 

The Scanning Electron Microscope (SEM) has a limit of 2nm, it allows you to look at replicas of dead cells, after fixation and heavy metal ion staining. Electrons are reflected off the surface of the specimen to produce a 3D image.

Hard specimens such as pollen grains require no further treatment.

Soft objects need to be coated in gold before observation.

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Light microscope

Living things can be observed
Colour observation
Resolution is not very high

Electron Microcrope

Extremely high resolustion
Specimens are always dead
Cannot see things in colour

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10x 4 =40
Value of one gratical unit:
25 um

10 x 10= 100
Value of one gratical unit:
10 um

10x 40= 400
Value of one gratical unit:
2.5 um

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Approx 10 um in diameter
Nucleus houses the genetic information for the cell

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6um long and 3 um wide
Production of ATP/ energy

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5 um long and 3 um wide
Function- to photosynthesise

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Various sizes, contents and functions


Involved in cell division
2+9 structure
2 per cell
perpendicular to each other long
1um long

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80 svedbergs in eukaryotic cells
70 svedbergs in prokaryotic cells
Involved in protein synthesis

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Smooth endoplasmic reticulum (SER)
Rough endoplasmic reticulum (RER)
and Golgi apparatus

various size
Cells with secretory function
have more than others
Sites of molecular conjugation

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ribosomes attatched? YES
Continuous with nuclear membrane? YES

ribosomes attatched? NO
Cotinuous with nuclear menbrane? NO

Golgi app
ribosomes attatched? YES
Continuous with nuclear membrane? NO

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Calculating the magnification

actual size of the object / apparent size of the object = magnification

Metre (m) 1m= 1000mm
Millimetre (mm) 1mm= 1000 um
Micrometre (um) 1um= 1000 nm
Nanometre (nm)

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Limits to cell size

Cells aquire substance by duffusion across their surface. As the cell increase in size, the surface area increases so more substances can duffuse at a disproportionate rate, resulting in reduction of the surface area to volume ratio. (SA:V) As a result there is a limit on the ability of the cell to acquire sufficient substance by diffusion in order to supply the volume.

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Multicellular oganisms are composed of many cells. This organisation can take two forms:

1. The cells are joined in such a way that each on has acces to the environment. ie cells linked in a long ribbon. eg. The algae Volvox aureus

2. Where there are networks through the organism that act as exchange surfaces between the organisms and the environment.

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Tissues, Organs, systems and organisms

Cells are the basic functional unit

TISSUES are collections of cells all of which have the same function within an organism. Eg. muscle tissue, bone tissue

ORGANS are made up of different tissues whose functions are amalgamated to perform the overall function of the organ. Eg. Leaf

Organs are designed into SYSTEMS. The organs work together to fulfil a function within the organism. Eg. Respiratory system.

All systems put together are ORGANISMS

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Eukaryotic Cell

Main areas of a eukaryotic cell

The cytoplasm:
Cytoplasm (aqueous solution)

Cell membrane (or plasma membrane)
Cell wall(support- will let anything in) 

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Prokaryotic and Eukaryotic cells

Magnification: size of image/ size of specimen

Prokaryotic cells

  • Genetic material: naked loop of DNA in nucleoid
  • Mitochondria: not present
  • Ribosomes: small- 70s
  • Internal membranes: few/ none

Eukaryotic cells

  • Genetic material: Chromosomes in nucleus
  • Mitochondria: always present
  • Ribosomes: small- 80s
  • Internal membranes: golgi apparatus/ ER/ lysosomes etc.
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