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4.2.74 · Cell Membranes and Transport

Surface Area to Volume Ratios — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 14 of 20 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

4.2.7 Surface Area to Volume Ratios — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Calculate surface area to volume ratios and explain how they limit cell size and shape adaptations in organisms.

  • Effect of size
  • Doubling the side
  • Presenting a ratio
  • Single-celled organisms
  • Multicellular organisms
  • Flat or thin bodies

Part 1 of 6Why the ratio matters

SA:V
  • SA:V the surface area to volume ratio compares the outer surface of a cell or organism with the volume it encloses.
Supply
  • Supply oxygen and nutrients enter across the surface but are used by the whole volume of living contents.
Loss
  • Loss heat and waste are produced throughout the volume and lost through the surface.
Effect of size
  • Effect of size as an object gets bigger its volume increases faster than its surface area, so SA:V decreases.
  • Small organisms can exchange substances by diffusion across the body surface, but large organisms cannot.

Part 2 of 6Calculating the ratio

ShapeSurface areaVolumeSA:V
Cube (side l)6l2l36 ÷ l
Cuboid2(lw + lh + wh)l × w × harea ÷ volume
Sphere4πr2(4/3)πr33 ÷ r
Cylinder2πr2 + 2πrhπr2harea ÷ volume
Doubling the side
  • Doubling the side it multiplies the surface area by four and the volume by eight, so the ratio halves.
Presenting a ratio
  • Presenting a ratio give units (cm2, cm3) and simplify so the volume term is 1.
Sphere
  • Sphere a sphere has the highest SA:V of any shape of a given size, so it is best suited to diffusion.

Part 3 of 6Consequences for organisms

1Single-celled organisms
  • Amoeba and bacteria have a large SA:V, so oxygen and nutrients diffuse in fast enough to supply the whole cell and waste diffuses out.
2Multicellular organisms
  • Multicellular organisms they have a small SA:V, so diffusion across the body surface alone would be too slow.
  • They have specialised exchange surfaces such as lungs, gills, villi and root hairs.
  • They have a transport system (circulatory system, xylem and phloem) and often ventilation to maintain gradients.
3Flat or thin bodies
  • Flat or thin bodies a flatworm is thin, so its ratio is large enough for diffusion across the surface without a circulatory system.
4Metabolic rate
  • Metabolic rate a higher metabolic rate needs more oxygen per unit volume, so it needs a larger ratio or extra exchange adaptations.

Part 4 of 6Heat exchange and size

Small mammals
  • Small mammals they have a large SA:V and lose heat quickly, so they have high metabolic rates, thick fur or hibernation.
Cold climates
  • Cold climates large mammals such as the polar bear have a small SA:V, which reduces heat loss.
Hot climates
  • Hot climates animals such as the elephant have large ears, which increase heat loss.
Benefit of large size
  • Benefit of large size large organisms hold more water and heat, store more food and have complex organs.

Part 5 of 6Increasing the surface area

StrategyExamples
FoldingCristae in mitochondria, villi in the ileum
FlatteningLeaf blade, flatworms
ElongationRoot hairs, microvilli
Branching or many small unitsAlveoli, gill filaments and lamellae
  • Effect
    • each raises the area without a proportional rise in volume, and many also reduce the diffusion distance, so exchange is faster.

Part 6 of 6More from the specification

Surface Area to Volume Ratio and Cellular Exchange
  • Smaller cells have a higher surface area to volume ratio, facilitating rapid diffusion of respiratory gases and nutrients across the partially permeable plasma membrane without requiring specialized internal transport systems.
Osmosis and Water Potential Principles
  • Osmosis is the net movement of water molecules from a region of higher water potential to an area of lower water potential (more negative / higher solute concentration) across a selectively permeable membrane.
    • (less negative / dilute solution)
Osmosis in Plant Cells (Potato Cylinder Assay)
  • When potato plant cylinders are placed in a series of sucrose solution concentrations, cells in hypotonic solutions gain water by osmosis becoming turgid,
  • while cells placed in hypertonic solutions lose water,
  • resulting in flaccidity and plasmolysis.
Surface Area to Volume Ratio and Cellular Exchange
  • Smaller cells have a higher surface area to volume ratio, facilitating rapid diffusion of respiratory gases and nutrients across the partially permeable plasma membrane without requiring specialized internal transport systems.
Osmosis and Water Potential Principles
  • Osmosis is the net movement of water molecules from a region of higher water potential to an area of lower water potential (more negative / higher solute concentration) across a selectively permeable membrane.
    • (less negative / dilute solution)
Osmosis in Plant Cells (Potato Cylinder Assay)
  • When potato plant cylinders are placed in a series of sucrose solution concentrations, cells in hypotonic solutions gain water by osmosis becoming turgid,
  • while cells placed in hypertonic solutions lose water,
  • resulting in flaccidity and plasmolysis.
Channel protein
  • Hydrophilic pore for specific ions or water.
Dilute solution
  • Solution with a high water concentration and low solute concentration.
Animal cell
  • A cell with a nucleus, cytoplasm, cell membrane and mitochondria but no cell wall, chloroplasts or large vacuole.
Sucrose
  • The sugar mainly transported in phloem.
Indicator
  • A chemical that changes colour depending on pH.
Endocytosis
  • Bulk transport into a cell using a membrane vesicle.
Bilayer
  • A double layer of phospholipids with tails facing inwards.
Selectively permeable
  • Allowing only some substances through.
Active transport
  • Movement of particles against a concentration gradient using energy from respiration.
Facilitated diffusion
  • Passive movement through transport proteins.
Phospholipid bilayer
  • Two layers of phospholipids with hydrophobic tails inwards
Osmosis
  • The net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
Permeable
  • Allowing substances to pass through.
Protein
  • A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.
Movement
  • An action by an organism or part of it causing a change of position or place.
Carrier
  • A female with one recessive sex-linked allele who is unaffected.

Key terms

TermMeaning
Surface area to volume ratioThe surface area of an object divided by its volume.
Exchange surfaceA surface specialised for passage of substances, with a large area and thin barrier.
Diffusion distanceThe distance over which diffusion has to occur.
Metabolic rateThe rate at which an organism uses energy.
VillusFinger-like fold of the small intestine that increases surface area for absorption.
AlveolusAir sac in the lung where gas exchange occurs.

Avoid these mistakes

1
Saying a larger organism has less surface area — it has a larger surface area but a smaller surface area to volume ratio.
2
Using surface area alone when the question is about the ratio.
3
Forgetting units for surface area (cm²) and volume (cm³).
4
Stating that all large animals have small ears — heat-exchange adaptations depend on climate.

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