Cell Size

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

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

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

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

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.

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

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

Surface Area to Volume

Cell types

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

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

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

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

Common structures in plant and animal cells:

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

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

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

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