HL 1 Biology

2.1a Cell Theory

Cells were first discovered after the invention of microscopes, yet the term “cell” was first used only after Robert Hooke’s examination of cork and plant tissues. Later as the cells were further observed, the cell theory, was developed.

Cell Theory:

  • Cells are the smallest unit of life, thus nothing smaller  can survive by itself.
  • All living things consists of one or more cells. (Exceptions: Extracellular matrices; e.g. bone, blood, cartilage and multinuclear cells; e.g. muscles, fungus)
  • Cells are all made from pre-existing cells. (It is not possible to produce new cells with non-living chemical substances)

Theory is a broad and greater amount of ideas that are united, which are used to explain observations. Hypothesis is a more specific prediction that can be tested by experiments/observations. Evidence of cell theory can be provided from observations and experiments.

  • All living things are composed of cells: 
    • Evidence: microscopes enabled us to examine how all living things consists of cells.
  • Cells are the smallest unit of life
    • Evidence: nothing smaller than a cell are able to survive. Subunits (nuclei, ER, golgi, etc.) cannot survive independently.
  • Cells are all from pre-exisiting cells
    • Evidence: experiments show how cells cannot be developed from non-living materials when all cells are killed and entry of cells are denied.

Amoeba and Paramecium are examples of unicellular organisms, which are organisms that are composed with only a single cell. Thus a single cell within these organisms must complete the function of life, things all organisms must be able to do in order to survive.

Fuction of Life: 

  • Growth: increase in size
  • Homeostasis: keep conditions inside the organism stable
  • Metabolism: chemical reactions in cells (e.g. cell respiration which releases energy)
  • Nutrition: consume food, in order to provide energy or any materials that lead to cell’s growth
  • Reproduction: produce offsprings (sexually or asexually)
  • Sensitivity: accept and respond to changes in the environment

Multicellular organisms, on the other hand, are organisms made up of multiple cells. Each cell only has to complete one task, and thus the cells in multicellular organisms can be develop differently: differentiation.

Most of the cells are not visible without a microscope for us. When examining the cells with the microscope, it is important to have properly placed the cells onto the slide by carefully positioning the cover slip. The magnification of microscopes can be calculated with the following formula:  Magnification of Eye Piece Lens x Objective Lens. By looking at the structure of the cells, we can differentiate it between a plant and animal cell. Although all cells contain cytoplasm, nucleus, and a plasma membrane (which surround these materials), the plant cells also consist of chloroplast and all is enclosed by a cell wall. The chloroplast of the plant cells enable these cells to perform photosynthesis, a way to produce food by using the energy from light.

Globally, scientists use the SI units to measure the sizes of structures. Each unit is 1000 times smaller than the previous unit. From largest to smallest: metres (m), millimetres (mm), micrometres (µm), and nanometres (nm). 

Examples of typical sizes:

  • 10-100µm: cells of eukaryotes
  • 1-10µm: organelles
  • 1µm: bacteria (size vary)
  • 100nm: viruses (size vary)
  • 10nm: thickness of cell membranes
  • 2nm: DNA molecule in human choromosome
  • 1nm: molecules
  • 0.1nm: atoms

Magnification is how much larger the image is than the species actual size. The ‘size of the image’ is how large the specimen looks like in a photograph/drawing. The ‘actual size of specimen’ is how large it really is.

  • Magnification= size of image/ acutal size of specimen

Scale bars most often appear on drawings/micrographs to show actual size of structures. It is simply a line which represents the actual size of that line.


1 a) As the scale bar is 2omm long, it is the size of the cell in this micrograph. By dividing this number by the scale bar length of 0.2mm, which is also the actual size of the Thiomargarita cell, we will come to a conclusion of 100X magnification. (20mm/0.2mm=100x)

b) The width of the string of cells were measured as 26mm as the size in the micrograph. Since we have found the magnification of 100x in the previous question, we could use that information to find the actual size: 100x = 26mm/ (unknown actual size). Thus,  x= 26/100 and the width of the string of cells is 0.26mm.

2 a) This electron micrograph is about 63mm and we know that the actual length of the mitochondrion is 8µm, which can be converted to 0.008mm. By inserting these two numbers into the formula, we can find the magnification: 63mm/0.008mm= 7875X.

b) As the information of the actual size is given as 5µm (0.005mm) for the scale bar and we found the magnification of 7875X from the previous question, we can use the formula to find the length of the scale bar. Thus, 7875= x  (unknown size)/ 0.005mm which will become 7875 x 0.005 = 39.375mm.

c) We know that the magnification is 7875X from question 2a. The width of the mitochondrion was 22mm when measured on the micrograph. Using the formula we know that the equation will be like the following: 7875 = 22/ x (the actual size)  and thus x= 22/7875 which is about 0.0028mm or 2.8µm.

3 a) As the magnification 2000 X and the actual size 20µm is given, we can find the length of the scale bar by multiplying these two numbers together. Therefore, the scale would be 40000µm or 40mm.

b) We know that the magnification is 2000 X and I have measured 35mm for the size of the cell of the image. By inserting these two numbers into the formula, we will be able to find the actual length of this cheek cell: 2000= 35/ x (unknown actual size). With further calculations, we come to a solution of 0.0175mm or 17.5µm.

4 a) The width of the hen’s egg size is about 8mm, which is approximately three times smaller than that of an ostrich egg (measured as 25mm).

b) On average, a hen’s egg would be about 57mm in length and yet in the image, it is only 8mm. Dividing the two numbers, we can find the estimate magnification of 0.14X.


Extracellular components are components that form part of its structure outside the plasma membrane (which is the barrier between the inside/outside of cells). These components are the main objects in which it keeps the cells in a tissue stick together. Examples of extracellular matrices includes bone, cartilage, and the cell wall of plant cells. As for the cell wall, the cellulose microfibrils passes through the plasma membrane in order to make the wall more thick. Pressure built by the water entering the plant cell by osmosis actually help the plant to stand up with the pressure being built up inside.

Emergent properties are made by the interaction between component parts/ “the whole is greater than the sum of its parts”. Examples include: surface tension is an emergent property of water molecules, shown in multicellular organisms, and life.

*Textbooks: AA-CC: 7, 9-13, 19-21; DBQ p13 • AA-SG: 3, 5

One thought on “2.1a Cell Theory

  1. Grade 6 A consistent and thorough understanding of the required knowledge and skills, and the ability to apply them in a wide variety of situations.

    1 a Magnfication = size of image / actual size of the specimen
    Size of the image (scale bar) = 20 mm
    Actual size = 0.2 mm
    Magnification 20 / 0.2 = 100 ×

    b Width of thiomargarita in the image (image size) = 26 mm
    magnification = 100 ×
    actual size = 26/100 = 0.26 mm

    2 a Magnification = length mitochondrion in the image
    (63 mm) / actual size of the specimen (8 µm / 0.008 mm)
    = 63 / 0.008 = × 7875

    b Scale bar 5 µm × 7875 = 39 375 µm (approx. 40 mm)

    c Width on the image 23 mm / magnification 7875
    = 0.0029 mm (2.9 µm)

    3 a 20 µm × 2000 (magnification) = 40, 000 µm
    (or 40 mm scale bar)

    b Actual size of specimen 34 mm/2000 = 0.017 mm

    4 a Hens egg is 16 mm in diagram. Ostrich egg is
    46 mm long in diagram. Real hen egg is about
    50 mm wide.
    Ostrich egg (50 × 46)/16 5 144 mm approx.

    b Magnification = size image / actual size of the
    Hens egg : Magnification 16 mm /50 mm = × 0.32

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