### 2.1.4: Relative sizes

```2.1.4: Relative sizes
CONVERTING UNITS
mm
x 1,000
÷ 1,000
µm
x 1,000
÷ 1,000
nm
Compare the relative sizes of molecules, cell membrane thickness, viruses, bacteria,
organelles and cells, using the appropriate SI unit.
A molecule
1nm
http://mendosa.com/glucose_molecule.jpg
Glucose (1nm)
A molecule
10nm
http://www.biolibogy.com/images/structure_of_plasma_membrane.JPG
A virus – the T
bacteriophage
100nm
10nm
http://oceanworld.tamu.edu/resources/oceanography-book/Images/BacteriophageCartoon.jpg
Bacteriophage (virus)
E.coli (bacteria cell)
100nm
0.1µm
1000nm
1µm
http://bio1903.nicerweb.com/doc/class/bio1903/Locked/media/ch18/18_01T4PhageEColi_LP.jpg
Bacteria (1µm)
Cell ( up to
100µm)
http://www.cic-caracas.org/departments/science/images/08eukaryote.jpg
Organelle (10µm)
1mm = 1,000 µm
1 µm = ___1 mm
1,000
1 µm = 1,000 nm
1 nm = ___1 µm
1,000
1 nm
10 nm
100 nm
1 µm
(up to) 10 µm
(up to) 100 µm
molecules
membrane thickness
virus
Molly
Met
Virgil
bacteria
organelles
cells
But
Organised
Cells
IB Question: State the typical size of
(i) a bacterium
[1]
(ii) An average eukaryotic cell [1]
(i) (approximately) 1μm [1]
Accept any value between .5μm and 1.0μm.
(ii) (approximately) 10μmto 100μm [1]
Accept any value within this range.
2.1.5: Scale bar
15 µm
2.1.6: Surface area : volume
2.1.6: Surface area : volumeLets look
at Surface areas and volumes………
• Fill in the table below:
a
Size of
side (a)
a
a
Surface
area of
the box
(a x a x 6)
Volume of Surface
the box
area to
(a x a x a) volume
ratio
1
6
1
2
24
8
3
54
27
4
96
64
5
150
125
6
216
216
7
294
343
6:1
2.1.6: Surface area:volume
IB Question: Explain the importance of the surface area to volume ratio as a factor limiting cell
size. [7]
as size increases both surface area and volume increase, but volume increases
more / ratio of surface area to volume decreases as size of cell increases;
rate of metabolism is a function of its mass to volume ratio;
surface area limits/affects the rate at which substances can enter (or leave) the
cell;
volume determines the rate at which material is produced/used;
oxygen/nutrients/substances will take too long to diffuse into/out of the centre of
the cell if it is too big;
excretory products would take too long to be eliminated;
heat will take too long to be eliminated;
example of cell adaptation to increase the ratio of surface area:volume e.g. root
hair cell; [7 max]
```