Back to cells …

IB topics 2.1.4 -2.1.6

Relative sizes

Some organelles can be seen with the light microscope, others cannot We need an electron microscope

Although even the largest cell is too small to see with the unaided eye, it is important to have an understanding of the relative sizes of cells and organelles. Why?

Relative Sizes

Eukaryotic Cell 10-100 μm μm = 10-6 m

Prokaryotic Cell 1-5 μm

Nucleus 10-20 μm

Bacteria 1-4 μm

Cell Membrane 7.5 nm thick

Large Virus (HIV) 100 nm nm = 10-9 m

DNA Double Helix 2 nm diam.

Mitochondrion 0.5 – 5 μm

Remember the metric system?

μm = 10-6 m Micrometer; one millionth of a meter

nm = 10-9 m Nanometer; one thousand millionth of a meter

The largest known bacterium was found in fish in tropical waters surrounding Lizard Island, Australia (over 1 mm in length)

Some eukaryotic cells are larger than indicated in the table The yolk of an egg is one cell Each sap vesicle of an orange is one cell

Metric System Practice

3 m = _____ mm 30 mm = ____ μm 30 μm = _____ nm

Linear Magnification

Diagrams and photographs can be shown larger or smaller than reality

To indicate the real size, the magnification can be indicated next to the diagram Scale bar

Calculating Linear Magnification

See hand out; we’ll finish with this.

Surface Area : Volume

The size of a cell is limited by its needs to exchange materials with its environment

If a cell becomes too large, its diffusion distance becomes too great Why is this a problem?

Volume increases more quickly than surface area This is a problem So what do cells do?