Maintaining adequate surface/volume ratio

Cell Sizes and Shapes

  • Cells vary in size and shape.
  • Smallest cells: Bacteria (0.2-0.3 microns).
    • Require 100x objective lens for observation due to their small size.
  • Longest cells: Nerve cells (e.g., from spine to big toe).
  • Average animal cell size: 10-100 microns.

Limitations on Cell Size

  • Three factors limit cell size:
    • Surface area to volume ratio.
    • Rate of molecule diffusion.
    • Maintaining adequate local concentrations of substances.

Surface Area to Volume Ratio

  • Cells must transport materials in and out.
  • Adequate surface area is needed for the cell membrane to facilitate transport based on cell volume.
  • If the volume becomes too large relative to the surface area, transport becomes insufficient, and the cell cannot survive.
  • Surface area (area) is calculated by length \times width, and volume is calculated by length \times width \times height.
  • As cell size increases:
    • Surface area increases by a power of 2 (squared).
    • Volume increases by a power of 3 (cubed).
  • Volume increases faster than surface area.
  • Cells must maintain a sufficient surface area relative to their volume.

Example: Cells Lining the Small Intestine

  • These cells absorb nutrients (glucose, amino acids) into the bloodstream.
  • Require a large surface area for absorption.

Microvilli

  • Cells lining the small intestine have microvilli, which are ridges that increase surface area.
  • Microvilli increase surface area without significantly increasing volume.
  • This adaptation allows for efficient absorption of nutrients.