You are in: Cavendish Outreach » Physics At Work » 2006
Biological and Soft Systems (Biological Physics)
http://www.phy.cam.ac.uk/research/bss
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DNA and proteins are types of molecules that are cells of every
living thing (you, me, sheep, potatoes, bananas, bacteria... everything).
They are examples of bio-molecules. ['bio' means 'life']
These bio-molecules
do all the jobs in your body that keep you alive and make you who you are.
They digest your food, work your muscles, make you think and allow you to
grow, to name just a few... In biological physics we investigate the structure
and properties of these molecules to see how they do their amazing jobs.
All molecules are very small, so even relatively big ones like DNA and
proteins are much too small to be seen by most microscopes. So, we need
ways to see
which molecules we have in our test tubes (and in our bodies) and ways to
follow them around as they do their jobs.
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Strand of DNA looped around a protein
molecule.
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There are A LOT of different bio-molecules. In fact, there may be more than 10,000
different types of protein in each cell in your body. We need ways to tell them
apart and a good way to do that is by their colour. DNA and proteins do have colours
although to our eyes they look plain white. This is because their colours are beyond
the range of colours that we can see, somewhere beyond the blue end of the rainbow
in the ultra-violet region of the electromagnetic spectrum.
Using a sensitive
instrument, called an absorbance spectrophotometer, we can tell the colour of the
molecules by looking at the wavelengths of light they absorb in the ultra-violet
region. Different types of molecules absorb different wavelengths and this is how
we can tell which ones we have.
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The electromagnetic spectrum
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Once the light has been absorbed, certain types of molecules can give the light
back out again. However, the molecule has removed some of the energy in the process,
which makes the colour of the light given out different from that which went in.
This change acts like a finger-print of the molecule, allowing us to identify it -
even if it's mixed in with other similar molecules. We call this property
fluorescence and we measure it using an instrument called a fluorescence
spectrophotometer.
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Only some bio-molecules are fluorescent,
(we call this intrinsic fluorescence). However, if the molecule we're
interested in is not
fluorescent we can make it so by attaching a small fluorescent molecule
to it. We call this labelling and say that the molecule now has
extrinsic
fluorescence. We can label many different bio-molecules with different
fluorescent molecules and watch them all at the same time; this is very
useful as it allows us to see how bio-molecules interact with each other
to do their jobs in our bodies.
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Each dot is a fluorescently labelled DNA molecule
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So by using absorbance and fluorescence we can identify which bio-molecules are
present, where they are in the cell, and how they interact with other bio-molecules.
Polymers are long chain molecules made up
from many small repeat units, which are called monomers.
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Synthetic polymers are made from products of the petrochemical industry.
They include polystyrene, polythene, PVC, nylon and many other plastics.
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Biological polymers come from nature, and may be made up of protein,
polysaccharides or DNA. Materials such as wool, silk, wood, starch,
skin and muscle are all mainly made out of biological polymers.
Polymers can adopt a huge variety of structures and can have many different
physical properties. They therefore have a wide range of applications. The job
of a polymer physicist is to understand why they behave in the way they do, and
what may be done to alter and control their properties.
In a glassy polymer, all the molecules are fixed firmly in position. These
materials are therefore hard and rigid. Polycarbonate is a polymer that is used
in its glassy state to make compact disks. PVC can be moulded into different
shapes and set to make window frames.
When glassy polymers are heated, the polymer molecules can move more and more,
until they can flow past each other. The polymer becomes softer and softer and
eventually becomes a liquid, like a wax candle when it is heated.
It is possible to join together the ends of the polymer molecules chemically to
make a network. This process is called cross-linking. When this happens, the molecules
cannot flow past each other. When cross-linked polymers are heated until they
stop being glassy, they do not become liquid. Instead they become rubbery polymers.
Squash balls, elastic bands, rubber tubing and car tyres are all made of rubbery
polymers. You will see in the talk that when you cool down rubber tubing, it goes
back to being glassy and brittle and it can be smashed with a hammer.
Gels are rubbery materials. They act as solids, but they also contain a lot of
solvent. Many foods are made from gel-forming polymers. When you dissolve the
protein gelatin in water and then cool it, the mixture goes from being a thick,
viscous solution to being a wobbly rubbery solid. This is because when the mixture
is cooled, triple helix knots between different gelatin chains act as cross-links.
The images below show some of the kinds of materials that we are interested in.
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images taken using a conventional camera:
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images taken using an electron microscope:
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Toothpaste
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Toothpaste
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Socks
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Socks
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Ice cream
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Ice cream
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