C3 and C4 plants

C3 plants are simply the plants we have been looking at already in photosynthesis. In C3 plants, CO2 is fixed from the atmosphere into the 3 carbon compound GP using the RuBisCO enzyme. Remember the problem here; above 25OC the RuBisCO starts to prefer oxygen over CO2 fixing (the CO2 and O2 are competing), reducing the rate of photosynthesis. C4 plants avoid this problem by having a more efficient way to fix the CO2.

Essentially, the CO2 is used in another biochemical pathway using pyruvate (3C) which is converted eventually to oxaloacetate (4C), and eventually to the compound malate (4C). This allows a ‘store’ of CO2 to be built up that can be released directly into the Calvin cycle. In these higher concentrations the CO2 is more readily absorbed by the RuBisCO in preference to O2.

The problem is the CO2 has to be fixed twice, which requires more energy than a C3 pathway. C3 plants require 18 molecules of ATP to synthesise 1 molecule of glucose; C4 plants require around 30 ATP per molecule.  It turns out to be a balancing act – C4 plants are found in places where there is a higher temperature that would favour RuBisCO taking O2, for example the tropics. C3 plants are found in more temperate regions. C4 plants are also more water efficient, doing better in dry conditions.

Despite being only around 3% of all terrestrial plants, C4 plants are responsible for around 30% of terrestrial carbon fixation.

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Abominable research

You may be wondering why there isn’t much in the course on cryptozoology. This is the study of animals that have not yet been shown to exist, for example unicorns and the Loch Ness monster. This week has seen some attention on the hunt for the Abominable Snowman, or Yeti, and claims from some scientists that they have uncovered evidence for their existence. Unfortunately (and this will explain why we don’t cover cryptozoology in A levels) they have done no such thing.

Science relies on the methodical gathering of evidence, testing and re-testing of hypotheses. The ‘scientists’ involved have gone straight past the boring bit of gathering valid data and gone straight to the headline grabbing phase. Their evidence consists of some hair (unidentified as yet, but it wouldn’t be difficult to do), an alleged nesy made from twigs (Really? Do you have any photos? No? How odd.) and footprints (guess they didn’t get any photos of those either). Science does not get to the truth by making unsubstantiated claims or listening to anecdotes (the Royal Society, probably the oldest scientific society in the world, has for its motto ‘Nulius in verba’, translated as ‘not by words’. This is a good description of the core of science; it is not enough to simply say or assert something, you must back it up with evidence. The evidence must be clearly available to other people to test (it’s no good making special pleading claims that only you can see the evidence, or ‘it did that when I tried it.’

It is possible that the yeti exists, but no valid evidence has been presented to demonstrate this. The scientific position remains that the yeti does not exist. If valid evidence emerges for the existence of these animals, then scientists will change their minds. It is however unlikely to be found. Resorting to boring old things like facts and evidence, creatures of the size proposed (and the same goes for Nessie) would have well understood need for territory, mating groups and reasonably sized gene pools to avoid in-breeding problems. From what we know of comparable sized animals, it is unlikely that something this big would have remained undetected.

New animals are discovered all the time, but it is rare indeed for legendary creatures like bigfoot and Nessie to be found living happily in our world.

Genome sequencing

The human genome (the entire DNA code of a human) was first mapped fully in 2003. When it was started in 1990, it was envisioned that by sequencing an entire genome we would rapidly gain an understanding of the workings of genes and quickly start to develop new medicines and techniques to deal with not just genetic diseases but also various disorders and pathological conditions.

As with many discoveries in science, the immediate benefits of knowing the genome have taken a while to filter through into useful work. One interesting effect is that it is now possible to identify genomes of other organisms much more rapidly, as the technology has advanced. Published this week in the magazine Nature was a study that sequenced the genome of the bacteria responsible for the Black Death, Yersinia pestis. It turns out that the genome is not too dissimilar to modern strains of the bacterium.

Scientists extracted the DNA from skeletons removed from medieval burial grounds. It is of interest particularly to try and identify why certain strains of pathogen seem to cause so many more problems and are so infectious (the Black Death killed an estimated 30-60% of the population of Europe at the time). It is also instructive in showing evolutionary pathways for bacteria, allowing scientists to trace back from today’s descendents and compare DNA sequences. As more and more organisms are sequenced, we are able to gain a better understanding of the similarities between organisms and see further into what comprises ‘life’.