topic 01 - surface area to volume ratio

• where does exchange between an organism and its surroundings occur?

  at the surfaces.
• what makes up the volume of an organism?

  cells.
• what uses the materials absorbed in through the surfaces?

  the cells.
• how does the body ensure that enough required material is absorbed quickly enough?

  the surfaces through which materials enter has to be large in relation to the volume of an organism as a whole.
• why can't diffusion be the sole source of exchange of materials in larger organisms?


  it wouldn't supply enough materials for the organism quickly enough because lots of the body's cells are a distance from the exchange surfaces so diffusion to these inner cells would be hard and slow.
• why can smaller organisms have diffusion as their sole source of exchange?

  smaller organisms have a reasonable ratio of SA:V so diffusion through the surface will be enough to meet its needs.
• what happens to surface area and volume as organisms get larger?

  as they get bigger, the volume increases at a quicker rate than surface area does these organisms have to adapt to aid their exchange with their environment.
• how have some organisms adapted and developed to aid diffusion through surfaces? eg?

  some may have developed a flattened shape so their cells are close to the exchange surfaces. eg: leaves of a plant.
• what have other larger organisms developed? eg?

  specialist exchange systems.

  eg: insects - tracheae, mammals - lungs and fish - gills.
• what do these internal exchange systems comprise of and what do they do?

  they comprise of additional exchange surfaces and they help to increase the overall surface area to volume ratio.
• how do you calculate surface area of a cube?

  area of 1 side x 6 sides.

  cm2
• how do you calculate volume of cube?

  length x width x height

  cm3
• how do you calculate SA to volume ratio?

  SA/V
• what happens as the size of an organism increases?

  the ratio between SA and V quickly reaches equality before flipping so that volume is far greater than surface area so diffusion through surfaces isn't sufficient.
• what do all adaptations that allow larger organisms to exchange materials more efficiently all demonstrate?

  specific characteristics to achieve this.

  • a large surface area relative to organisms' volume.
  • thin exchange surfaces, allows for optimum transfer.
  • selectively permeable, makes them selective of the materials they allow to cross in either direction.
• how are diffusion gradients maintained?

  by moving the environmental medium and internal medium, for mammals this would be environmental = air and internal = blood.
• diffusion is proportional to what?

  diffusion is proportional to SA x difference in conc / length of diffusion path
• what shape are most cells?

  spherical
• what is radius?

  half the diameter
• what is the equation for surface area of a sphere?

  4 pi r^2
• what is equation for ratio?

  ratio = SA/vol