SA:Vol Ratio

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Key Knowledge:
  • Surface area to volume ratio as an important factor in the limitations of cell size and the need for internal compartments (organelles) with specific cellular functions


Cells need to produce chemical energy (via metabolism) to survive and this requires the exchange of materials with the environment

  • The rate of metabolism of a cell is a function of its mass / volume  (larger cells need more energy to sustain essential functions)
  • The rate of material exchange is a function of its surface area  (large membrane surface equates to more material movement)


Surface Area : Volume Ratio

The relationship between metabolic energy requirements and capacity for material exchange is represented by the SA:Vol ratio

  • Cells need a high SA:Vol ratio, as it means there is sufficient capacity to exchange the materials needed for vital metabolic processes
  • As a cell grows, the volume (units3) will increase faster than the surface area (units2), leading to a lower SA:Vol ratio
  • Hence growing cells tend to divide and remain small in order to maintain a sufficiently high SA:Vol ratio necessary for survival


Cells and tissues that are specialised for gas or material exchanges will increase their surface area to optimise material transfer

  • Intestinal tissue of the digestive tract may form a ruffled structure (villi) to increase the surface area of the inner lining
  • Alveoli within the lungs have membranous extensions called microvilli, which function to increase the total membrane surface


Organelles will also increase their surface area to volume ratio in order to improve the efficacy of their specialised functions

  • The inner mitochondrial membrane is highly folded into cristae in order to maximise ATP production via aerobic respiration
  • Chloroplasts contain membrane discs (thylakoids) arranged into stacks (grana) to increase the light dependent reactions


SA-Vol ratio


Cell Size

Cells and their components are measured according to the metric system (all measurements relative to the standard of 1 metre)

  • Most cells and organelles are typically measured in micrometres (μm), which represent one millionth (10–6) of a metre
  • Smaller components such as cell membranes, viruses and DNA molecules may be measured in nanometres (10–9)


cell size


Microscopes

Microscopes are scientific instruments that are used to visualise objects that are too small to see with the naked eye

  • There are two main types of microscope: optical (light) microscopes and electron microscopes


Light Microscopes

  • Use lenses to bend light and magnify images by a factor of roughly 100-fold
  • Can be used to view living specimens in natural colour
  • Chemical dyes and fluorescent labelling may be applied to resolve specific structures


Electron Microscopes

  • Use electromagnets to focus electrons resulting in significantly greater magnifications and resolutions
  • Can be used to view dead specimens in monochrome (although false colour rendering may be applied)
    • Transmission electron microscopes (TEM) pass electrons through specimen to generate a cross-section
    • Scanning electron microscopes (SEM) scatter electrons over a surface to differentiate depth and map in 3D


chloroplast micrograph


Magnification

To calculate the linear magnification of a drawing or image, the following equation should be used:

  • Magnification = Image size (with ruler) ÷ Actual size (according to scale bar)


To calculate the actual size of a magnified specimen, the equation is simply rearranged:

  • Actual Size = Image size (with ruler) ÷ Magnification