CONTENTS

Introduction
Human Skin Cancer
Sunscreen and Fabric
The Mouse Model of Cancer
Studies Using Skin Tissue
Drugs and Sunlight
Plant and Algae Growth
Conclusion
Glossary
Bibliography

Laser Spectrophotometer

Paul Walker and Phil Lukins are constructing a laser spectrophotometer to  help  Tony Larkum examine the electron transfer processes that take place in  photosynthesis and thereby pinpoint at what stage and where in photosystem  2  the damage is being done by UV light. But to do that it is necessary to  work at  the incredibly short time spans of pico seconds. That is, at a million  millionth of a second.    

When light is absorbed, say a photon comes into the system, it is absorbed well  within a pico second. Photosynthesis involves an electron being shuttled  across  a membrane involving several components including the one that is thought  to  get damaged. Larkum needs to look at this shuttling process to find out  whether or not the particular component he has pin-pointed is, in fact, the  one  that gets especially damaged. This proof is only possible if things can be  followed on the very short time scale provided by the spectrophotometer.    

As physicists, Walker and Lukins bring a different point of view to work that is  generally done by biologists. They became involved in the project after  initial discussions with Tony Larkum. They view photosystem 2 as the plant’s solar cell. It converts light energy into chemical energy. They  plan to  follow the electron movement and energy transfer through photosystem 2.  These  electrons are far too small to see but when they are in different parts of  the  molecular chain, if a lig

ht of variable wavelengths is shined on them, it  is  possible to detect which particular state they are in. But they only remain  in  each state for a very short period of time.    

Their purpose is to be able to characterise the transient passage of electrons  through the system. If a particular portion of a plant’s photosynthetic  system  is damaged by UV then by studying the alteration to that transient  behaviour it  might be possible to detect where the damage has occurred in the chain. All  the  evidence so far indicates that it is quite early in the chain.    

If the damage is as close to the early stages of the process as it seems  then there may be little or no oxidation of water at the water splitting site. Functionally it looks like an electrical series circuit. If something is knocked out  somewhere in the line it effectively stops the current. Biological systems have  quite a lot of complexity and redundancy and they sometimes have the ability to form  new  pathways so it is possible that a diversion of the pathway could develop.  But  the ability to do that would depend partly on where in the chain of events  that  damage occurs. If it occurs very early then there may be no alternative  pathway. This has implications for UV damage in terms of evolutionary time  because if it is very early on in the process then it would be more  difficult  for the plant to genetically re-engineer itself to avoid that damage. The  later  it is in the process the easier it would be to take a detour.    

Walker and Lukins expect to spend 2 or 3 years before they get meaningful results. There have been some papers published on UV damage but these seem  to concentrate on a much slower time scale. One of the impediments, of  course, is the technical side. Very specialised instruments have to be available in  order to see things in this extremely short time scale and this equipment  is not commonly available.    

The laser system they have installed, for instance, will be only the third  of its kind in Australia. But the actual configuration of their new  equipment will be quite different from the others. They see one of the key  aspects of their work as experimental flexibility so they can chase after a  number of different  processes.    

They didn’t have to learn a great deal of biology for this work. This is because the samples they plan to work with will largely be purified samples of  biological materials rather than natural materials such as leaves. Leaves, unfortunately would be too complex because of the interference from  bulk chlorophylls. Chlorophylls act as the receivers of light which they channel to  the photosynthetic system. The problem with doing experiments on the whole  organism is that there is so much interference from other parts and substances  that it tends to swamp the reactions in the specific area under study. To  study  it, photosystem 2 has to be separated out.    

It is always a problem that when things are separated out they might be changed  in the process. But Walker and Lukins claim that photosystem 2 retains its  basic structure and function when it is separated from the living organism.  ”It  is really more of a biochemical problem than a botanical problem,” they  say.    

The task is made more difficult by the fact that the function and structure  of  photosystem 2 are not precisely known yet. So apart from researching how  damage  occurs there are also prospects for clarifying how photosystem 2 actually  works. But despite the gaps in the knowledge there is still a fairly good  guide  to photosystem 2 as a result of the work that has been done over the last  5-10  years by other scientists and also by analogy with bacterial photosynthetic  reactions. Since 1985 chemists have been looking at the function of  bacterial  photosynthetic reactions and there are great similarities between  certain photosynthetic bacteria and plants. So all these previous scientific  studies  have given Walker and Lukins a good idea of how photosystem 2 works.    

“Finding the sites of the damage requires a bit of detective work. It is  really  a matter of inferring what the electrons are doing by looking at the  absorption  or fluorescence from the molecular fragments, not directly at the electrons  themselves. Another way to describe the technique is say it is a matter of  inferring what the dynamics of the electron transfer is by observing what  it  does to a beam as it passes through.”