Edexcel A 3.1; Edexcel B 2.1 (i, ii), relevant background for AQA and OCR A & B.

Cell theory

It may be difficult to believe there was once a time that scientists were unaware that cells are the most fundamental unit of life. It was not, in fact, until 1838 that Schleiden and Schwann (German botanist and physiologist, respectively) first proposed cell theory, which consisted of three main arguments:

  • Cells are the most basic and fundamental units of life
  • Through biogenesis (more on this later), cells come from pre-existing cells
  • Every living organism is composed of at least a single cell

Whilst today many augmentations and additions of these three basic principles exist (often to address the impact of natural selection on cells), these principles still stand today.

There are approximately 60 trillion cells in the human body – each containing 10,000 different types of molecules. In such complex, multicellular organisms these cells are organised into tissues (a group of similar cells which together carry out a specific function), which are organised into organs (a collection of tissues which carry out a specific function(s)), which are organised into the organism.

Cell size

So why are cells so tiny? Cells are often 1-100 micrometres in diameter (0.001-0.1 millimetres), condemning the biologist to eternally squinting down microscopes to observe them (this is particularly not fun for the long-sighted biologist) – however, there is good reason. Cell size is largely confined by the surface area-to-volume ratio. What is meant by this is that the increase in the volume of the cell is not proportionate to the increase in surface area. For example, a cell with a diameter of just 1 μm (‘μ’ is the letter ‘mi’ in the Greek alphabet and in scientific connotation is used to represent ‘micro’. It is also used in statistics to represent the population mean) will have a 6:1 surface area-to-volume ratio. But, when cells get larger this ratio decreases – at 3 μm the ratio is just 2:1.

So why is this an issue? Well the activity of the cell is proportional to its volume – meaning it needs more resources the larger it gets. However, these resources can only be obtained from the surface of the cell, as molecules both enter and exit the cell this way (more on transport across the cell membrane later), and will have a larger distance to move across in cells with greater volume. The result of this is the cell not being as efficient when the surface area-to-volume ratio is smaller, hence their generally small size.

Next up, we’ll look at the history, diversity and uses of the instrument that first allowed the observation of cells – the microscope!

References

Sadava, Hillis, Heller & Berenbaum (2012). Life: The Science of Biology. 10th ed. Sunderland, MA: Sinauer. 78-79.