Surface area to volume ratio

• Why is surface area to volume ratio important in biology?

  Because diffusion is a key method of movement of molecules in biology and surface area to volume ratio is important for maximising the rate of diffusion.
• What happens to the surface area to volume ratio of organisms as they go from single celled to multicellular?

  The more cells an organism has, the smaller the surface area to volume ratio is.
• State Fick's Law.

  $Rate\ of\ diffusion\ \alpha\ \frac{SA\ x\ concentration\ gradient}{diffusion\ dis\tan ce}$
• Define concentration gradient.

  Concentration gradient describes the difference in concentration between two areas.
• According to Fick's Law, what is the relationship between rate of diffusion and SA x concentration gradient?

  Rate of diffusion is directly proportional to SA x concentration gradient.
• According to Fick's Law, what is the relationship between rate of diffusion and diffusion distance?

  Rate of diffusion is inversely proportional to diffusion distance.
• What happens to the rate of diffusion if surface area or concentration gradient increases?

  Rate of diffusion increases
• What happens to the rate of diffusion if diffusion distance increases?

  Rate of diffusion decreases
• How do organisms increase their surface area (adaptions)?

  • They have lots of small structures (increases number of membranes)
  • They have additional or larger structures E.g. folds, microvilli, big ears
  • They have folded membranes/structures E.g. villi
• How do organisms increase concentration gradient?

  • Constant blood flow -> takes molecules away from the site of exchange so structures are often surrounded by many capillaries
  • Ventilation (breathing)
  • Chemical reactions -> reactants constantly being used up & products constantly being produced
• How do organisms decrease diffusion distance?

  • Thin walls (walls are one cell thick)
  • Leaves and single celled organisms are flat/thin
  • Structures are pushed next to each other
• Why is SA:Vol ratio so important to organisms?

  • Heat from organisms is transferred to the environment through their surface area
  • Water will also evaporate from surfaces exposed to air
  • The SA:Vol ratio of most animals will be a compromise between maximising the ratio for efficient diffusion & reducing it to conserve water and heat
• What is the SA:vol ratio?

  • The surface area of an organism divided by its volume
  • the larger the organism, the smaller the ratio