Lynn Margulis
Endosymbiont Hypothesis
• A. This hypothesis was proposed by Lynn
Margulis in the 1960’s.
• B. Define symbiosis and introduce common
types of symbiotic relationships.
• C. It basically hypothesized that Prokaryotes
came to live together in a symbiotic
relationship, the smaller living inside the
larger, to gain a survival advantage over other
prokaryotes and eventually they evolved into
Eukaryotic cells over many generations that
spanned hundreds of thousands of years.
• 1. Smaller organism gained protection.
• 2. Larger organism gained energy production
or faster motility
Endosymbiont Hypothesis
Surface Area to Volume Ratio
• The surface area to volume ratio refers to the
ratio of the cell’s total surface area in relation
to its volume.
• Maximizing surface area to volume ratios is
important so that the transport systems in
cells can run efficiently
Surface- to- Volume Ratio Importance
• A. Cells can only be so small. (There has to be
enough room (volume) to hold things and to
perform work inside a cell.)
• B. Cells can only be so large. (Larger means more
traffic going in both directions across the cell
membrane)
• C. A cell must be large enough to contain DNA,
Ribosomes, and some cytoplasm. They can only be
so big because we have to be able to move enough
“Food” into and “waste” out of a cell efficiently. If it
is too large the cell becomes inefficient at moving
these things so it divides to get back to a smaller
state.
Surface Area to Volume Ratio
• Reasons for keeping cell sizes small
– Decreases distances that substances have to travel
… increasing the rate of diffusion.
Human Systems
• Maximizing surface area is also important for
the efficiency of several systems in the body
Human Systems
• Cardiovascular system (alveoli in the lungs and
capillaries that surround them aid in the
diffusion of O2 and CO2 in and out of the
blood)
Human Systems
• Digestive system
– villi and microvilli are structures in the small
intestine that aid in the absorption of nutrients
into the body)
Plant Systems
• Structures in plants are also designed to
maximize surface area to volume ratios for
their specific functions.
• Examples:
– Leaf thickness
and shape
Plant Systems
• Root structures
Surface Area to Volume
relationship
Surface Area to Volume Ratio
• If we were to watch nutrients being absorbed at a constant rate in 3 different
size cubes, the nutrients could reach the entire cytoplasm of smaller cells
because, for their volume, they had a much larger surface area.
2 cm3
4cm3
1 cm3
Dead: nutrients can’t get to
all parts of the cell.
Surface Area =
but still
Alive! Nutrients make it to
TheBetter,
smaller
the cell,
dead.
all parts of the cell.
the larger the SA / V ratio, and the
(base) (height)
easieradded
it isup
for
to reach
fordiffusion
all sides
all parts of the cell
S.A.= (4cm)(4cm)(6 sides)
S.A.= (2cm)(2cm)(6 sides)
S.A.= (1 cm)(1cm)(6sides)
= 96 cm2 x 1 cube = 96 cm2
= 24 cm2 x 8 cubes = 192 cm2
= 6 cm2 x 64 cubes = 384 cm2
Volume = (base) (width) (height)
V = (4cm)(4cm)(4cm)
= 64cm3 x 1 cube = 64cm3
Surface Area
Volume
S .A .
V

96
64
 1 .5
V = (2cm)(2cm)(2cm)
V = (1cm)(1cm)(1cm)
= 8cm3 x 8 cubes = 64 cm3
S . A.
V

192
64
3
= 1cm3 x 64 cubes = 64 cm3
S . A.
V

384
64
 6
• Shape also affects surface area to volume ratio.
16
7.11
• Skinnier, flatter
cells large SA / V
ratios.
• Cells with
projections, like
microvilli, have
large SA / V ratios.
• In order for an organism to be large, it must be
multi cellular.
• Which of the equal volumes below has the
greatest SA / V Ratio and is therefore most likely
to survive?