(8a) The scale of things and orders of magnitude

Comparing the size of various things in biology.

Scale of data: nm = nanometre (10^-9 m), µm = micrometre/micron (10^-6 m), mm = millimetre  (10^-3 m)

 When comparing the size of small objects like cells, scientists refer to differences in sizes as an order of magnitude.

This means that an object is described as being greater of smaller by a factor of 10.

For example, the leaf cell size is one order greater (10 x) in size than a red blood cell.

The common ladybird is two orders greater (100 x or 10 x 10) in size than a plant leaf cell.

The thickness of the cell membrane is three orders smaller {1/1000 or 1/(10 x 10 x10)} in size than the diameter of a red blood cell.

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(8b) A table showing the chemical composition of unicellular microorganisms e.g. an E. coli or an amoeba (albeit a significantly different cell structure)

Although not required for any exam, I hope you biology students will find this chemical analysis of the humble E. coli bacterium cell interesting.

There can be over 6000 different molecules and ions in just one unicellular microorganism.

Awesome !!!! as long as you ignore the very undesirable effects of E. coli bacteria or an amoeba in your body!

Compound or ions in an E. coli bacterium	approximate % by mass	Approximate number of molecular species or ions
Water	70	1
Proteins	15	3000
DNA nucleic acids	1	1
RNA nucleic acids	5	3000
Carbohydrates	3	50
Lipids	2	40
Building blocks and intermediates	2	500
Inorganic ions	1	12

All of these microscopic organisms have very complex structures that are built of, and function, using thousands of different molecules and ions. See also cell structure Part 9

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Key points - Summary of ideas

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

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Scale of Things and Orders of Magnitude in Biology

Biology involves the study of organisms at different scales, from molecules to entire ecosystems.

Understanding the relative sizes of biological structures is crucial for visualising processes such as diffusion, osmosis, and enzyme activity.

Organisms and structures in biology can range from nanometres (nm) to metres (m), covering several orders of magnitude.

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Understanding Orders of Magnitude

An order of magnitude refers to a tenfold difference in size.

Scientists use orders of magnitude to compare the relative sizes of biological structures.

• Nanometres (nm) (10⁻⁹ m) – e.g., DNA molecules, viruses

• Micrometres (µm) (10⁻⁶ m) – e.g., bacteria, animal and plant cells

• Millimetres (mm) (10⁻³ m) – e.g., small multicellular organisms like mites

• Centimetres (cm) (10⁻² m) – e.g., insects, larger tissue structures

• Metres (m) (10⁰ m) – e.g., whole organisms like humans, trees

Comparing Biological Structures by Size

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Biological Molecules and Their Scale

Molecules: The Smallest Biological Units

• Water molecule (H₂O): ~0.3 nm – crucial for life and biochemical reactions.

• Glucose molecule: ~0.9 nm – used in respiration to provide energy.

• DNA (Deoxyribonucleic acid): Width ~2 nm, but can be metres long – carries genetic information.

• Proteins (e.g., haemoglobin): Varies, typically 5-10 nm – essential for cellular functions.

Cells and Their Scale

• Prokaryotic cells (bacteria) are typically between 1-5 µm in size.

• Eukaryotic cells (animal and plant cells) are generally 10-100 µm, with plant cells typically larger than animal cells.

• Organelles inside cells vary in size:

  • Mitochondrion: ~1 µm – site of aerobic respiration.

  • Ribosome: ~20-30 nm – synthesises proteins.

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Scale of Organisms

Living organisms vary greatly in size:

• Microorganisms (e.g., bacteria and yeast) are typically between 1-10 µm.

• Multicellular organisms like insects range from mm to cm.

• Larger animals like humans reach metres in size.

• Plants can be several metres tall, such as oak trees (>30 m).

Why Scale Matters in Biology

Understanding orders of magnitude helps with:

• Comparing structures when studying microscopy.

• Predicting how substances move (e.g., diffusion occurs quickly in small structures).

• Understanding biological interactions (e.g., pathogens vs. immune cells).

• Appreciating evolution and the vast range of life.

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Exam tips

• Be familiar with conversions (1 mm = 1000 µm, 1 µm = 1000 nm).

• Use orders of magnitude calculations when comparing structures:
  Relative size difference = Larger size / Smaller size.
  Example: A typical plant cell (100 µm) is 10 times larger than an animal cell (10 µm).

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Summary of learning objectives and key words or phrases

Using the idea of orders of magnitude scale, the comparative size  of various organisms, hair, cells, viruses, onion cell, yeast cell, thickness of cell membrane, DNA molecule, carbon atom.

The chemical composition of the unicellular bacteria E coli bacterium is tabulated and discussed.

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