1. Cell Size

2. Why are Cells Small?
cells must exchange nutrients & other molecules with their environment
food must come in and wastes go out
the cell membrane is the only way in and out of the cytoplasm
the cell membrane has a limited surface area
the cytoplasm has a volume to “feed” or have wastes removed

3. Surface area of a cell
If it were a perfect square, how would you calculate surface area?
Each side is 1 x 1=1unit squared, 6 sides therefore, surface area = 6 units squared
Distance equals on unit

4. Volume of a cell - cytoplasm
Distance equals one unit
Volume equals: Length x Width x Height
Thus the volume equals 1 x 1 x 1
Volume equals 1 unit cubed

6. Comparison of Size, Surface Area & Volume
Cell size
Surface area (units2)
Volume
(units3)
Ratio of SA/V
1 unit 6 1
6/1
2 units 24 8
24/8
3/1
3 units 54 27
54/27
2/1

7. Conclusion
As cell size increases ratio of surface area to volume decreases.
Therefore, the rate of diffusion slows down….
…..thus a cell cannot get rid of wastes fast enough or get nutrients fast enough
. . . so it will either be poisoned by its wastes or starve.

8. Surface Area to Volume
Cell surface area is important in taking in nutrients
Surface area increases as the square of cell diameter
But… entire cell volume needs to be fed
And, cell volume increases as the cube of cell diameter so it increases at a faster rate

9. Consider 2 Cells...
100 µm
diameter
10 µm
diameter

10. Surface Area to Volume

13. Cell types

14. Prokaryotes vs. eukaryotes
All living cells can be divided into two major types:
Prokaryotes – simple cells that lack a nucleus, always single-celled
Eukaryotes – complex cells with a distinct nucleus, can be single-celled or multicellular

15. Prokaryotic cells
lack a nuclear membrane – the DNA floats freely in the cytoplasm
lack any membrane-bound organelles (like mitochondria)

16. Plant vs. animal cells
Within eukaryotes, there are two major kinds of cells:
Plant cells – stiff cell wall gives the cell shape, large vacuoles, special organelles called chloroplasts
Animal cells – lack cell walls and chloroplasts, vacuoles are small, centrioles are visible

17. What they have in common:
both have a cell membrane
both have a nucleus with DNA inside
both have many of the common organelles (ER, Golgi complex, etc.)
both have mitochondria for making energy

18. Common structures in plant and animal cells:

19. Organelles found in plant cells
Cell wall – made of cellulose, gives cell strength and protection
Larger vacuoles – store water and keep cell rigid
Chloroplasts – make sugar from CO2 and sunlight - photosynthesis

20. Organelles found in animal cells
Centrioles – attachment for the cytoskeleton, organise spindle fibres during cell division

21. Do not have chloroplasts
Plant Cells
Animal cells
Have a cell wall
Do not have a cell wall
Have chloroplasts
Do not have chloroplasts
Have large vacuoles
Have small vacuoles
No visible centrioles
Centrioles visible

23. Some exceptions to this!
Protist cells
single-celled eukaryotes (protists) can have organelles typical of both plant AND animal cells
Fungus cells
multicellular eukaryotes with a cell wall made of chitin (the same substance in the exoskeleton of insects)
cell walls can remain open after cell division, and the cells are free to move back and forth