Maximizing Surface Area-to-Volume Ratios (College Board AP Biology)

• As organisms increase in size, their surface area to volume ratio decreases
  • Small cells have a high surface area-to-volume ratio
  • Larger cells have a lower surface area-to-volume ratio 
• The surface area-to-volume ratio affects the exchange of materials between cells or organisms and the environment
  • This might include materials such as oxygen, carbon dioxide, water, or heat exchange with the environment
• Some single-celled organisms can rely on simple diffusion alone to exchange materials with their environment
• As organisms evolved from single-celled to multicellular, more sophisticated exchange systems (eg, lungs) evolved to maintain diffusion rates that were high enough to sustain life

Examples of Sophisticated Exchange Structures

• Exchange structures may evolve to be specially adapted to maximize exchange.
• Some examples of specialized structures might include
  • membrane folds
  • exchange organs

Membrane Folds

• A cell on the inside surface of the small intestine (the intestinal epithelium) is in contact with the flow of food passing down the lumen of the intestine
• The cell's principal role is to absorb important food molecules for distribution around the rest of the organism's body
• Only one surface of the cell (see diagram below) is in contact with the food flow; this is the upper surface as shown
  • The remaining surfaces of the cell are in direct contact with other cells in the tissue, so cannot absorb food molecules directly from the lumen
• So the surface area of the food-contacting surface is maximized by folded structures called microvilli
• These increase the internal surface area of the small intestine by a factor of around 100
  • Microvilli are highly significant in ensuring full absorption of food molecules

Membrane Folds Diagram

Folds in the upper surface of the cell's membrane increase the surface area over which materials can be exchanged by the cell

Exchange Organs

• Lungs in air breathing animals give an increase in the effective surface area that gases can diffuse across
• Gills in fish and other aquatic organisms perform the same role for exchange of dissolved gases
• Spongy mesophyll tissue in leaves allows a greater exchange of gases by increasing the effective surface area available
• Even primitive multicellular organisms like sea sponges are full of pores to allow ocean water to pass through their bodies, allowing a greater surface area for exchange