Cellular homeostasis and regulatory processes

During the previous chapters we have started to build up a picture of life at the cell level. The picture is not yet complete but it has progressed sufficiently for the reader to see a problem how is the cell kept in order Within the tiny space of the cell, many different structures are tightly packed. These structures are continually being made, repaired, broken down, recycled and moved from place to place. Thousands of different types of proteins are being synthesised all the time, moved to...

Photosynthesis

Before many millions of years had passed, some cells had learned to use sunlight to split water. Rocks formed over 3,000 million years ago show that the atmosphere contained traces of oxygen by then. Photosynthesis had begun. Photosynthesis opened an evolutionary door. Without an oxygen-rich atmosphere there could have been no animals. Most of the prokaryotes, protists and fungi alive today could never have evolved without photosynthesis they too depend on oxygen. But oxygen is lethal to all...

Amplification and attenuation of signals

The analogy between cellular signalling and electronic circuitry breaks down in one important respect. If A, B, C etc. in the schematic diagrams were electronic circuit components, there would normally be only one of each. But if they are signalling pathway intermediates this is not the case. A single activated receptor will usually activate many molecules of the type M Each M will activate several of type A, each of which will activate several of type B, and so on. Thus, a very tiny stimulus -...

The driving forces for the evolutionary increase in brain size

Evolutionary psychologists have suggested that the large human brain, like the peacock's tail, is the result of sexual selection. Ancestral peahens were more attracted to mates with bigger and more ornate tails. Therefore, size and ornateness of tail were selected for. Therefore, we now have peacocks with ludicrously exuberant tails. By analogy, hominid females were more attracted to mates with bigger brains. Therefore, bigger brains were selected for. Therefore, we now have humans with...

Other solar systems the supply of planets

Thanks to refined astronomical techniques we now have clear evidence that there are planets elsewhere in our galaxy. Some of these planets are free in interstellar space, occurring either singly or in clusters. By 2004, rather more than a hundred had been discovered in orbit around nearby stars. Free planets are very unlikely to house life because their surface temperatures can be only a few degrees above absolute zero, far too cold for the complicated physical and chemical processes that must...

Distinctive features of human brain function

Human brains are bigger relative to body size, and much more complicated in terms of numbers of synaptic connections, than the brains of other mammals. However, being bigger and more multiply connected does not mean that they are better at everything than the brains of other species. For example, dogs process olfactory information far more efficiently and elaborately than humans. Salient examples of human capabilities are hand control, facial recognition and language we discussed hand control...

A survey of intermediary metabolism

The cardboard box models described in chapters 2 and 3 revealed the relative sizes of cell components. Constructing the models showed how tightly packed cells are. Also, it enabled us to talk about the functions of membranes and the relationships among DNA, RNA, ribosomes and proteins. However, the lentils and gherkins and so on that we put into the boxes were inert. The components of real cells, in contrast, are very active and dynamic. In this chapter we shall start to survey this dynamism....

Has the human brain stopped evolving

The proposal outlined in the previous section presumes a dialogue between brain size and culture. But what we now mean by culture is incomparably more elaborate than anything experienced by our remote ancestors. Does the dialogue still continue As far as we know, H. sapiens has been the only extant species of Homo for at least 20,000 years. Cultural change during this period has been radical. Refinements in stone tools, and probably in social organisation among nomadic groups, led some 9-10,000...

Eukaryotic cells and their contents

Electron Micrograph Animal Cell

We promised not to consider evolutionary theory until later in the book, but we have already invoked it several times, mentioning biological drive, transmission and survival of genes, adaptation, and so on. This shows how difficult it is to survey any part of modern biology without referring at least implicitly to biology's central theory. During the next few chapters, these implicit references will continue. However, we shall defer explicit discussion of the theory until chapter 11. Our...