 Cell Size & Cell Membrane
Overview: Surface Area to
Volume Lab
AP Biology
The Work of Life
 What jobs do cells have to do for an organism to live?

“breathe”
 gas exchange: O2 in vs. CO2 out

eat
 take in & digest food

make energy
 ATP

ATP
build molecules
 proteins, carbohydrates, fats, nucleic acids


remove wastes
control internal conditions
 homeostasis


respond to external environment
build more cells
AP Biology  growth, repair, reproduction & development
Cell size comparison
Animal cell
Bacterial cell
most bacteria
 1-10 microns
eukaryotic cells
 10-100 microns
AP Biology
 micron = micrometer = 1/1,000,000 meter
 diameter of human hair = ~20 microns
 How does cell size affect the cells
ability to function optimally?

AP Biology
Lets work on the following handout to
explore Cell Surface Area to Volume
Ratio
AP Biology
FIGURE
TOTAL
NUMBER
OF CUBES
SURFACE
AREA OF
FIGURE (=6
x length x
width)
VOLUME
OF FIGURE
(=length x
width x
height)
SURFACE
AREA TO
VOLUME
RATIO
(= SA / V )
TOTAL
SURFACE
OF
INDIVIDUAL
CUBES* ( =
6* x #cubes)
A
1
6
24
54
96
1
8
27
64
6
3
2
1.5
1
48
162
384
B
C
D
AP Biology
8
27
64
**Because surface area of one cube is 6
Surface Area to Volume Activity Questions
1. What happened to the surface area as the size
increased?
It increased
2. What happened to the volume as the size
increased?
It also increased, but
not as quickly.
3. What happened to the surface area to volume ratio
as the size increased?
It decreased.
4. Now imagine you’re looking at a very large
cell. Next to it is a very small cell. Which one has a
larger surface area to volume ratio? The small one
AP Biology
The Cell Membrane
AP Biology
2007-2008
Phospholipids
 Phosphate head

“attracted to water”
hydrophilic
 Fatty acid tails

Phosphate
hydrophobic
 Arranged as a bilayer
Fatty acid
“repelled by water”
Aaaah,
one of those
structure–function
examples
AP Biology
Arranged as a Phospholipid bilayer
 Serves as a cellular barrier / border
sugar
H 2O
salt
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
impermeable to polar molecules
polar
hydrophilic
heads
waste
AP Biology
lipids
Cell membrane defines cell
 Cell membrane separates living cell from
aqueous environment

thin barrier = 8nm thick
 Controls traffic in & out of the cell

allows some substances to cross more
easily than others (selectively permeable)
 hydrophobic (nonpolar) vs. hydrophilic (polar)
AP Biology
Cell
Compartmentalization
Video- Bozeman
Science
AP Biology
Movement across the
Cell Membrane
AP Biology
2007-2008
Diffusion
 2nd Law of Thermodynamics
governs biological systems

universe tends towards disorder (entropy)
 Diffusion

AP Biology
movement from HIGH  LOW concentration
List 2
examples of
diffusion in
everyday life.
AP Biology
The Special Case of Water
Movement of water across
the cell membrane
AP Biology
2007-2008
Osmosis is just diffusion of water
 Water is very important to life,
so we talk about water separately
 Diffusion of water from
HIGH concentration of water to
LOW concentration of water

AP Biology
across a
semi-permeable
membrane