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Understanding Surface Area to Volume Ratio in Organisms and Its Biological Implications

This article explores why certain organisms, like bacteria and elephants, have distinct physiological requirements based on their size and surface area to volume ratio (SA:Vol). We analyze how SA:Vol affects the movement of substances and heat loss, explaining why small organisms may rely less on circulatory systems compared to larger ones. Additionally, the practical experiment comparing the diffusion rates of substances in gelatin cubes demonstrates these concepts. The relationship between metabolism, respiration rates, and SA:Vol is also examined, with implications for adaptation in various species.

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Understanding Surface Area to Volume Ratio in Organisms and Its Biological Implications

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  1. Why… …do bacteria not have a circulatory system?

  2. Why… …are polar bears really big?

  3. Why… …does a Robin have a higher rate of respiration than a Golden Eagle?

  4. Why… …do elephants have lungs?

  5. Small mammals or birds Why… …is the oxygen dissociation curve of a mouse shifted to the right? Human % sat Hb pO2

  6. Surface area to Volume Ratio • calculate the SA:Vol for a simple cube and identify how this changes with the size of the cube. Grade D. • describe how SA:Vol affects movement of substances and heat loss. Grade C • relate the anatomy of different organisms to their SA:Vol. Grade B

  7. Surface Area to Volume Ratio SA:Vol for short. -How does the SA: Vol change with the size of an organism? 3 2 1 54:27 54/27 = 2 24:8 24/8 = 3 6:1 6/1 = 6

  8. “the bigger the organism, the smaller the SA:Vol” 3 2 1 54:27 54/27 = 2 24:8 24/8 = 3 6:1 6/1 = 6

  9. The affect of SA:Vol on the transport of substances. • Cut out 2 cubes of gelatine (lengths of equal sizes). • Calculate their SA:Vol. • Half fill a petri dish with hydrochloric acid (goggles on!) • Place the cubes in the acid. • Time how long it takes for each cube to turn completely red.

  10. Practical analysis. • Which cube took up the acid the quickest? • Relate this to the SA:Vol of the cubes. How does the rate of diffusion change with the SA:Vol. • Use the above rule to answer the following questions… • Explain why bacteria do not need a circulatory system. • Explain why Elephants have lungs.

  11. The affect of SA:Vol on the transport of substances. • Substances will diffuse more quickly into organisms with abigger SA:Vol. • Bacteria are small-they have a large SA:Vol. Substances will diffuse very quickly into the centre of them through the plasma membrane. There is no need for a mass transport system such as the blood. • Elephants are very large-they have a small SA:Vol. Substances would take a long time to diffuse into the centre of the an elephant. They therefore need a mass transport system (blood and lungs)

  12. The affect of SA:Vol on Heat loss. Which organism is going to loose heat more quickly and why? (note: these both live in china!) or Mouse. It has a large surface area-more area to lose heat from and a smaller body-heat will travel to the surface more quickly.

  13. The affect of SA:Vol on heat loss • Organims with a bigger SA:Vol (smaller organisms) will lose heat more quickly. • Smaller organisms therefore tend to have a higher metabolic rate (faster rate of respiration) as this generates heat. • Their haemoglobin is adapted for this-the oxygen oxygen dissociation curve is shifted to the right- haemoglobin has a decreased affinity for O2so it is unloaded more easily at the muscles.

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