SURFACE AREA TO VOLUME
RATIO
 Why can't cells continue to grow larger and larger
to become giant cells, like a blob?
 Why are most cells, whether from an elephant or
an earthworm, microscopic in size?
 What happens when a cell grows larger and what
causes it to divide into two smaller cells rather
than growing infinitely larger?
 This investigation provides you with a 'hands-on'
activity that simulates the changing relationship of
Surface Areas -to- Volume for a growing cell.
Background information
 Cells are roughly cubic or spherical in shape and
are limited in size by the ratio between surface
area and volume (SA/V); cells divide when they
reach a certain surface area.
 The volume of a cell is related to the amount of
surface area-to-volume ratio it has.
 The chemical activity within the cell and the
surface area of the cell limits the exchange of
nutrients, gases and waste products with its
environment.
 As a cell grows larger, its rate of producing wastes
and requiring nutrients increases faster than the
surface area through which molecules must exit
and enter.
 Volume increases by the cube, while surface area
increases by the square.
 As a cell grows larger, its surface area becomes too
small to maintain life functions.
 Cells remain small and therefore maintain a large
surface-area-to volume ratio
 As the cell increases in size, the SA/V ratio
decreases (the V ↑ faster than the SA, i.e. cubed
function vs. squared function).
 There is less surface (CM) to meet the needs of the
volume (chemical activity within the cytoplasm &
organelles).
 If chemical reactions within the cytoplasm cannot
be supplied with nutrients or remove wastes, the
cell will die OR it can divide into two cells and thus
obtain a favorable, i.e. large SA/V ratio.
 The important point is that the surface area to
volume ratio gets smaller as the cell gets larger.
 In summary, increasing cell size results in a
decrease in the SA/V ratio for the cell. This
reduction in the SA/V ratio combined with the
increased distance to the cell center makes
diffusion of materials into & out of the cell less
efficient.
 Some cells have unique shapes & structures to
overcome SA/V ratio. E.g. intestinal epithelial cells
on villi contain microvilli (a “brush border”) to ↑
SA for diffusion of nutrients; RBC’s are biconcave
shape to ↑ SA/V ratio for more efficient gas
diffusion.
 What other shape might cells have to ↑ SA/V ratio?
Practical Activity
 You will be given copies of the Cubic Cell Models
on heavy, colored paper. The four cell models are
then cut out, folded, and taped together. The
models represent one cube-shaped cell at
increasing stages of growth. The smallest stage
represented is 1cm long on a side; the largest stage
is 4cm on a side.