From Cells to Organisms WALTWALTExplain:Explain:•The relationship between the size of an organism or The relationship between the size of an organism or structure and the surface area: volume ratio, and the structure and the surface area: volume ratio, and the significance of this for the exchange of substances significance of this for the exchange of substances and of heat. and of heat. •Changes to body shape and the development of Changes to body shape and the development of systems in larger organisms as adaptations to systems in larger organisms as adaptations to facilitate exchanges as the ratio reducesfacilitate exchanges as the ratio reduces

WILFWILFTo be able toTo be able to

Explain how surface to volume ratio differs from Explain how surface to volume ratio differs from unicellular organisms to large multicellular unicellular organisms to large multicellular

organisms and how this affects exchanges of organisms and how this affects exchanges of substances and heatsubstances and heat

Diffusion and the Problem of Size

• All organisms need to exchange substances All organisms need to exchange substances such as food, waste, gases and heat with their such as food, waste, gases and heat with their surroundings. surroundings.

• These substances must These substances must diffusediffuse between the between the organism and the surroundings. organism and the surroundings.

• The rate at which a substance can diffuse is The rate at which a substance can diffuse is given by given by Fick's lawFick's law::

Rate of Diffusion

α Surface area × difference in concentration

Thickness of membrane

The rate of exchangeThe rate of exchange

• The rate of exchange of substances therefore The rate of exchange of substances therefore depends on the organism's surface area that depends on the organism's surface area that is in contact with the surroundings. is in contact with the surroundings.

• The requirements for materials depends on The requirements for materials depends on the mass or volume of the organism, so the the mass or volume of the organism, so the ability to meet the requirements depends on ability to meet the requirements depends on

• which is known as the which is known as the surface area : volume surface area : volume ratioratio..

Surface area

Thickness of membrane

•As organisms get bigger their volume As organisms get bigger their volume and surface area both get bigger, but not and surface area both get bigger, but not by the same amount.by the same amount. • Which has the bigger surface area to Which has the bigger surface area to

volume ratio?volume ratio?

As organisms get bigger their volume and surface area As organisms get bigger their volume and surface area both get bigger, but not by the same amount. both get bigger, but not by the same amount.

This can be seen by performing some simple This can be seen by performing some simple calculations concerning different-sized organisms. calculations concerning different-sized organisms. In these calculations each organism is assumed to In these calculations each organism is assumed to be cube-shaped to make the calculations easier. be cube-shaped to make the calculations easier.

2cm2cm

2cm2cm

2cm2cm

8cm8cm

8cm8cm

8cm8cm

Surface area = 2x2 x6 = 24 Volume = 2x2x2 = 8cm

SA:V = 24:8 24/8 = 3

Surface area = 8x8x6 = 384 Volume = 8x8x8 = 512cm

SA:V = 384:512 384/512 = 0.75

organismorganism lengthlength SA (m²)SA (m²) vol vol (m³)(m³) SA/vol (mSA/vol (m-1-1))

bacteriumbacterium 1 mm1 mm (10(10-6-6 m)m)

6 x 106 x 10-12-12 1010-18-18 6,000,0006,000,000

amoebaamoeba 100 100 mmmm

(10(10-4-4 m)m)

6 x 106 x 10-8-8 1010-12-12 60,00060,000

flyfly 10 10 mmmm

(10(10-2-2 m)m)

6 x 106 x 10-4-4 1010-6-6 600600

dogdog 1 m1 m (10(1000 m) m) 6 x 106 x 1000 101000 66

whalewhale 100 m100 m (10(1022 m) m) 6 x 106 x 1044 101066 0.060.06

• So as organisms get bigger their surface So as organisms get bigger their surface area/volume ratio gets smaller. area/volume ratio gets smaller.

• A bacterium is all surface with not much inside, A bacterium is all surface with not much inside, while a whale is all insides with not much surface. while a whale is all insides with not much surface.

• This means that as organisms become bigger it This means that as organisms become bigger it becomes more difficult for them to exchange becomes more difficult for them to exchange materials with their surroundings. materials with their surroundings.

• In fact this problem sets a limit on the maximum size In fact this problem sets a limit on the maximum size for a single cell of about 100 for a single cell of about 100 µµm.m.

• In anything larger than this materials simply cannot In anything larger than this materials simply cannot diffuse fast enough to support the reactions needed diffuse fast enough to support the reactions needed for life. for life.

• Organisms also need to exchange heat with their Organisms also need to exchange heat with their surroundings surroundings

• Large animals have an advantage in having a small Large animals have an advantage in having a small surface area/volume ratio: they lose less heat than surface area/volume ratio: they lose less heat than small animals. small animals.

• Large mammals keep warm quite easily and don't need Large mammals keep warm quite easily and don't need much insulation or heat generation.much insulation or heat generation.

• Small mammals and birds lose their heat very readily, Small mammals and birds lose their heat very readily, so need a high metabolic rate in order to keep so need a high metabolic rate in order to keep generating heat, as well as thick insulation.generating heat, as well as thick insulation.

• So large mammals can feed once every few days while So large mammals can feed once every few days while small mammals must feed continuously. small mammals must feed continuously.

• Human babies also loose heat more quickly than Human babies also loose heat more quickly than adults, which is why they need woolly hats. adults, which is why they need woolly hats.

Why do penguins huddle?Why do penguins huddle?

• So how do organisms larger than 100 So how do organisms larger than 100 µµm exists? m exists?

• All organisms larger than 100 All organisms larger than 100 µµm are m are multicellularmulticellular, , which means that their bodies are composed of which means that their bodies are composed of many small cells, rather than one big cell. many small cells, rather than one big cell.

• Each cell in a multicellular organism is no bigger Each cell in a multicellular organism is no bigger than about 30than about 30µµm, and so can exchange materials m, and so can exchange materials quickly and independently. Humans have about 10quickly and independently. Humans have about 101414 cells. cells.

Questions

Side Side lengthlength

Total surface Total surface areaarea

VolumeVolume Surface Surface area : area :

volume volume ratioratio

11

22

33

44

55

66

1 × 1 × 6 = 61 × 1 × 6 = 6 11 66

2 × 2 × 6 = 242 × 2 × 6 = 24 2 × 2 × 2 = 82 × 2 × 2 = 8 33

3 × 3 × 6 = 543 × 3 × 6 = 54 3 × 3 × 3 = 273 × 3 × 3 = 27 22

4 × 4 × 6 = 964 × 4 × 6 = 96 4 × 4 × 4 = 644 × 4 × 4 = 64 1.51.5

5 × 5 × 6 = 1505 × 5 × 6 = 150 5 × 5 × 5 =1255 × 5 × 5 =125 1.21.2

6 × 6 × 6 = 2166 × 6 × 6 = 216 6 × 6 × 6 = 2166 × 6 × 6 = 216 11

0

1

2

3

4

5

6

7

0 2 4 6 8

side length

surf

ace

area

:vo

lum

e ra

tio

c)c) The larger the organism, the smaller its surface The larger the organism, the smaller its surface area to volume ratio.area to volume ratio.

d)d) Single-celled organisms are small. Therefore the Single-celled organisms are small. Therefore the surface area is very large when compared with surface area is very large when compared with thevolume. This provides a large gasexchange thevolume. This provides a large gasexchange surface.surface.

2.2.a)a) For the maximum rate of diffusion, surface area and difference in For the maximum rate of diffusion, surface area and difference in

concentration should be as large as possible. The thickness of the concentration should be as large as possible. The thickness of the membrane should be as small as possible.membrane should be as small as possible.

b)b) In In AmoebaAmoeba: Surface area is large as it is a single-celled organism (look at : 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 thetable in question 1). Difference in concentration is large as oxygen is being used in respiration. Thin membrane as diffusion is only into a being used in respiration. Thin membrane as diffusion is only into a single cell. In the human lungs:single cell. In the human lungs:• Surface area is large as there are many alveoli and capillaries.Surface area is large as there are many alveoli and capillaries.• Difference in concentration is large as oxygen is being replaced by Difference in concentration is large as oxygen is being replaced by

the breathing process and removed by the blood.the breathing process and removed by the blood.• Thin membrane as diffusion is only through the flat squamous Thin membrane as diffusion is only through the flat squamous

epithelial cells of the alveoli and the cells of the capillary walls.epithelial cells of the alveoli and the cells of the capillary walls.c)c) Low oxygen concentration at high altitudes. The folds of loose skin, Low oxygen concentration at high altitudes. The folds of loose skin,

through which the toad breathes, provide a larger surface area for through which the toad breathes, provide a larger surface area for oxygen absorption.oxygen absorption.

d)d) In air, water would be lost very rapidly through the thin membrane, so In air, water would be lost very rapidly through the thin membrane, so the the Amoeba Amoeba would dehydrate.would dehydrate.