From Cells to Organisms
WALT
Explain:
•The relationship between the size of an organism or
structure and the surface area: volume ratio, and the
significance of this for the exchange of substances
and of heat.
•Changes to body shape and the development of
systems in larger organisms as adaptations to
facilitate exchanges as the ratio reduces
WILF
To be able to
Explain how surface to volume ratio differs from
unicellular organisms to large multicellular
organisms and how this affects exchanges of
substances and heat
Diffusion and the Problem of
Size
• All organisms need to exchange substances
such as food, waste, gases and heat with their
surroundings.
• These substances must diffuse between the
organism and the surroundings.
• The rate at which a substance can diffuse is
given by Fick's law:
Rate of
α
Diffusion
Surface area × difference in concentration
Thickness of membrane
The rate of exchange
• The rate of exchange of substances therefore
depends on the organism's surface area that
is in contact with the surroundings.
• The requirements for materials depends on
the mass or volume of the organism, so the
ability to meet the requirements depends on
Surface area
Thickness of membrane
• which is known as the surface area : volume
ratio.
•As organisms get bigger their volume
and surface area both get bigger, but not
by the same amount.
• Which has the bigger surface area to
volume ratio?
As organisms get bigger their volume and surface area
both get bigger, but not by the same amount.
This can be seen by performing some simple
calculations concerning different-sized organisms.
In these calculations each organism is assumed to
be cube-shaped to make the calculations easier.
8cm
2cm
8cm
2cm
2cm
8cm
Surface area = 2x2 x6 = 24
Volume = 2x2x2 = 8cm
Surface area = 8x8x6 = 384
Volume = 8x8x8 = 512cm
SA:V = 24:8  24/8 = 3
SA:V = 384:512  384/512 = 0.75
length
6 x 10-12
vol
SA/vol (m-1)
(m³)
10-18 6,000,000
(10-4
m)
(10-2
m)
(100 m)
6 x 10-8
10-12 60,000
6 x 10-4
10-6
600
6 x 100
100
6
100 m (102 m)
6 x 104
106
0.06
organism
bacterium 1 mm (10-6
m)
amoeba
fly
dog
whale
100
mm
10
mm
1m
SA (m²)
• So as organisms get bigger their surface
area/volume ratio gets smaller.
• A bacterium is all surface with not much inside,
while a whale is all insides with not much surface.
• This means that as organisms become bigger it
becomes more difficult for them to exchange
materials with their surroundings.
• In fact this problem sets a limit on the maximum size
for a single cell of about 100 µm.
• In anything larger than this materials simply cannot
diffuse fast enough to support the reactions needed
for life.
• Organisms also need to exchange heat with their
surroundings
• Large animals have an advantage in having a small
surface area/volume ratio: they lose less heat than
small animals.
• Large mammals keep warm quite easily and don't
need much insulation or heat generation.
• Small mammals and birds lose their heat very
readily, so need a high metabolic rate in order to
keep generating heat, as well as thick insulation.
• So large mammals can feed once every few days
while small mammals must feed continuously.
• Human babies also loose heat more quickly than
adults, which is why they need woolly hats.
Why do penguins huddle?
• So how do organisms larger than 100 µm exists?
• All organisms larger than 100 µm are multicellular,
which means that their bodies are composed of
many small cells, rather than one big cell.
• Each cell in a multicellular organism is no bigger
than about 30µm, and so can exchange materials
quickly and independently. Humans have about 1014
cells.
Questions
Side
length
Total surface
area
Volume
1
1×1×6=6
2
2 × 2 × 6 = 24
2×2×2=8
3
3
3 × 3 × 6 = 54
3 × 3 × 3 = 27
2
4
4 × 4 × 6 = 96
4 × 4 × 4 = 64
1.5
5
5 × 5 × 6 = 150
5 × 5 × 5 =125
1.2
6
6 × 6 × 6 = 216
6 × 6 × 6 = 216
1
Surface
area :
volume
ratio
6
1
surface area:volume ratio
7
6
5
4
3
2
1
0
0
2
4
side length
6
8
c)
d)
The larger the organism, the smaller its surface
area to volume ratio.
Single-celled organisms are small. Therefore the
surface area is very large when compared with
thevolume. This provides a large gasexchange
surface.
2.
a) For the maximum rate of diffusion, surface area and
difference in concentration should be as large as possible.
The thickness of the membrane should be as small as
possible.
b) In Amoeba: Surface area is large as it is a single-celled
organism (look at thetable in question 1). Difference in
concentration is large as oxygen is being used in
respiration. Thin membrane as diffusion is only into a single
cell. In the human lungs:
• Surface area is large as there are many alveoli and
capillaries.
• Difference in concentration is large as oxygen is being
replaced by the breathing process and removed by the
blood.
• Thin membrane as diffusion is only through the flat
squamous epithelial cells of the alveoli and the cells of
the capillary walls.
c) Low oxygen concentration at high altitudes. The folds of
loose skin, through which the toad breathes, provide a
larger surface area for oxygen absorption.
d) In air, water would be lost very rapidly through the thin
membrane, so the Amoeba would dehydrate.