You asked part 2

Jack raised the point about theories in the lesson this week. In science, a theory has a different meaning to how we use the word in everyday language. Theory can be used to mean a conjecture, idea, speculation or simply a guess about why something happens. This use is fine as long as you realise that words can have multiple meanings, and the context the word is used in is important.

In science, a theory denotes something else. It used to describe statements or principles that not only describe what is happening, they also provide an explanation. It has predictive power, in other words a theory can be used to predict an event or observation that has not yet happened. Theories are well tested and have evidence to support the statements. If you wanted to show that a theory is incorrect, you could show that the predictions it makes do not happen under experimental conditions.

To give an example, let’s take the good old theory of evolution. Broadly speaking, it would predict that organisms closely related would share more genetic similarities than more distantly related organisms. If you found a monkey more closely related to a fish than a gorilla, you could start questioning the theory. Older fossils should be found in older rocks – if you found a human remains mixed in with a T rex’s bones then you may have a problem. So far these things have not occurred, but if they did and were shown to be valid (e.g. not faked) then the theory would be changed. Theories are changed in reponse to new evidence, that’s how science works.

In the example Jack brought up (ATP synthase mechanism) the theory is incomplete because it doesn’t provide an explanation for all observations. Although there is no hierachy of theories, some have been tested more than others so we often consider them to be more robust. Many accepted theories are used simply because they work well enough for the time being, in other words they hold up to testing so far.

Scientific theories are often described as falsifiable. This means that you could potentially cause a theory to be changed or abandoned by evidence that shows its predictions do not hold up in experiment or observation. Knocking down established theories is a good way to get a Nobel prize. But there’s a reason why the big theories are rarely abandoned…

You asked part 1

Since Charlotte asked about the proton pumps, I thought I’d go into a bit more detail.

The pumps are referred to as complexes, given Roman numerals I, II, III and IV. Complex I is really an enzyme, called NADH dehydrogenase (why is it called a complex? There are 45 separate polypetide chains). 2 electrons are passed onto a substance called ubiquinone (or just Q), which is reduced to ubiquinol (QH2). This is lipid soluble, and moves easily through the membrane. 4 protons are pumped through the membrane.

Complex II (aka succinate dehydrogenase)  gives additional electrons to Q, which is then passed to Complex III (aka cytochrome bc complex). Here the electrons are passed on to another molecule, cytochrome C. 6 protons in total are translocated (moved across the membrane) at this point. Finally cytochrome C (whgich is a water soluble, integral protein membrane) electrons on to Complex IV, where the electrons are removed and used in reducing oxygen to water. Cytochrome C is inhibited by cyanide – a poison which will stop aerobic respiration.

Here’s a nice summary from wikipedia:

In eukaryotes, NADH is the most important electron donor. The associated electron transport chain is

NADHComplex IQComplex IIIcytochrome cComplex IVO2 where Complexes I, III and IV are proton pumps, while Q and cytochrome c are mobile electron carriers. The electron acceptor is molecular oxygen.

Remember, you will be tested on what is in the specification, but reading further in your subject will always help.

If you want to get ahead, read a book.

If you are interested in science (and since you are doing at least one A level in science, I presume you are), then you really need to start looking beyond what you have to do as a part of the course and read around your subject. This doesn’t have to mean looking at textbooks; popular science books are very good at informing and entertaining. There is also a rich tradition of fiction that has its roots in science (you think it’s an accident that physicists all like Star Trek?). While TV and fiction cannot replace understanding and knowledge when it comes to exams, get reading over the summer. Ask for a book for Christmas (yes, it is legal, you’re allowed!). Visit the LRC, the you’ll find many of these books there for free. Apart from the possibility of being asked about your wider understanding of a subject in a university interview, it’s worth it to broaden your mind.

Some suggestions:
A Short History of Nearly Everything, Bill Bryson

An entertaining guide to most of the big ideas of science; biology, chemistry and physics. Gives the background to discoveries, including some interesting biographical information of scientists.

The Selfish Gene, Richard Dawkins

Getting old now, this book is still a classic in how it introduced genetic ideas to a wider audience. Dawkins’ other books are worth a look, such as Climbing Mount Improbable and  The Greatest Show on Earth.

Genome, Matt Ridley

Looks at the human genome through the idea of looking at one gene per chromosome. A good outline of this exciting area of biology. Some other good stuff by Matt Ridley as well.

Power, sex, suicide:mitochondria and the meaning of life, Nick Lane

You know you want to read a book with a title like that.

Not clear?