Читать книгу Wonders of Life - Andrew Cohen - Страница 12

Isaac Newton was the first to demonstrate that white light is made up of a multitude of colours when he famously revealed the rainbow hiding in sunlight in 1671 using a simple glass prism. This multicoloured rain illuminates everything on Earth, but why is life so good at selecting only certain colours to reflect into our eyes?

As a particle physicist, I feel I am permitted to think of everything in terms of the interactions between particles. This is a sensible thing to do, since every experiment conducted in the history of science has shown that the elementary building blocks of nature are particles. To be sure, these particles do not behave like little grains of sand or billiard balls; they are quantum particles, and this allows them to exhibit wave-like behaviour. But they are particles nonetheless, and this applies to light as well as electrons, quarks and Higgs bosons.

I will therefore choose to picture the light from the Sun as a rain of particles – an endless stream of photons that rain down on the surface of the Earth after a 150-million km journey from the surface of the Sun. At a subatomic level, when a photon hits something – a leaf, for example – it hits an electron around an atom or molecule and, if the structure of the molecule is just right, the photon will transfer all of its energy to that electron. If the structure of the molecule isn’t right, the photon will not be absorbed. In this way, only photons of certain energies interact and are absorbed, and those energies are determined by the structure of the molecules themselves.

As we have already seen, a photon’s wavelength is directly related to its colour. So, another way of saying that pigment molecules interact only with photons of particular energies is to say that they absorb only particular colours of light, reflecting the rest away. This is how pigment molecules work – they interact only with photons of particular energies, and therefore absorb only particular colours of light.

There is a dazzling array of pigment molecules in nature, from carotenes that colour a carrot orange, to polyene enolates, a class of red pigments unique to parrots. In some cases the animals and plants produce the pigments themselves, but in many cases they are absorbed into the organism through its diet. If flamingos didn’t ingest beta-carotene from blue-green algae in their diet, their trademark pink colour would quickly turn white.

The selective nature of pigment molecules’ interactions with photons is the reason for life’s rich and varied colour palette. Think about a green leaf. We see it as green because green photons do not interact with the molecules in the leaf. Red and blue photons do – they are both absorbed by a pigment called chlorophyll. If the rain of photons falls on a surface that reflects the majority of them back (such as the feathers of a swan or the sclera of an eye) we perceive the surface as white. If the light falls on a surface that absorbs photons of all energies (such as a raven’s feathers) the surface appears black. The Mexican tiger flower (Tigridia pavonia) absorbs all but the lower-energy red photons in sunlight, and so this flower is red. The feathers of the Mexican blue jay (Aphelocoma wollweberi) absorb low-energy photons but reflect the higher-energy blue photons back into your eye.

A light micrograph of melanocytes (pigment cells), which produce the pigment melanin. It is melanin that absorbs the harmful ultraviolet rays found in sunlight.

One class of pigments is unique to parrots, and the brightly coloured rainbow lorikeet (Trichoglossus haematodus) dramatically illustrates life’s varied palette.

Pigment molecules perform a large variety of functions in living things. Some, as in the case of melanin, evolved to absorb light for protection. It is not known whether the first pigments were used for protection, although many biologists think this was the case. Protection is a simple function, needing no additional complexity such as a nervous system to respond to light’s stimulus. There are also pigments that simply make organisms colourful, in order to attract a mate, warn off a predator, entice insects to nectar or invite animals to consume vivid-coloured fruit. But some pigments do not simply dissipate the energy from the Sun as harmless heat or reflect it for display. Chlorophyll, the pigment that lends the natural world its verdant hue, is such a molecule, and its ability to absorb photons and, when integrated into a complex set of molecular machines, use their energy to do something useful, changed the world.

Flamingos are pink because they ingest beta-carotene from blue-green algae in their diet.

FROM THE SMALLEST BEGINNINGS…

Van Leeuwenhoek’s development of the single-lens microscope led to the possibility of viewing microbial life. This image shows merismopedia – a genus of cyanobacteria whose cells are arranged in perpendicular rows one cell thick to form rectangular colonies.

Antonie van Leeuwenhoek was a draper who sold cloth in the Dutch city of Delft in the 1660s. He had no scientific training, was notoriously bad-tempered and could speak only Dutch. And yet this grumpy man literally changed the way we look at the world: he discovered microbes, the most ancient life forms on the planet.

Although the microscope had been invented at the beginning of the seventeenth century, little of much importance had been discovered with it. Then, in Holland, people began to use single-lens microscopes, in which the lens was a tiny ball of glass about 1 millimetre across. Like other Dutch microscopists, such as Jan Swammerdam or the great philosopher Benedict de Spinoza, Van Leeuwenhoek both ground his lenses and pulled them from thin rods of heated glass. But he had an extra trick to produce large quantities of high-quality tiny glass spheres and was so protective of his technique that he kept it secret. What exactly it was is still unknown: his refusal to share his technique may have held back the development of science. This is why, ultimately, the ability to keep secrets is not a common or desirable trait in a scientist – something that legions of conspiracy theorists would do well to remember.

Despite its simplicity, the single-lens microscope gave unparalleled access into the world of the small and revealed amazing wonders – they could magnify up to 500X, revealing incredible detail in the anatomy of insects and plants. In 1672, Van Leeuwenhoek had been introduced to the Royal Society in London, which invited the Dutchman to turn his powerful device to all sorts of substances, including pepper.

The single-lens microscope gave unparalleled access into a previously unseen world. This image is a micrograph of the bacterium Clostridium tetani, a rod-shaped, anaerobic bacterium of the genus species Clostridium that causes tetanus.

Bacteria are organisms known as prokaryotes, which do not have a cell nucleus. The twisted, thread-like spirochaete bacterium shown in this image (here magnified 4,000 times actual size) causes syphilis in humans.

Bacteria have existed for almost the entire history of life on Earth. This computer-generated image depicts a small group of the bacteria Treponema pallidum, which causes diseases such as syphilis, bejel, pinta and yaws.

Bacteria are found in even the most inhospitable places on Earth. This image shows psychrophilic (cold-loving) bacteria, discovered in Ace Lake, Antarctica, in 1992.

The single-lens microscope was the forerunner of the scanning electron microscope (SEM), which provides a far greater level of magnification. This image shows Clostridium botulinum, the cause of botulism in humans.

In the spring of 1676, Van Leeuwenhoek tried to discover why pepper was hot; he assumed that there must be some tiny spikes on the surface of the pepper that would explain the tongue-tingling sensation of heat generated by peppercorns. His initial attempts to confirm the ‘spiky pepper hypothesis’ failed when he used dried pepper, so he put a handful of peppercorns in water and let them soften for three weeks. On 24 April, he drew some of the ‘pepper-water’ into a glass capillary tube with an extremely fine bore, fixed the tube in front of the metal plate that held his tiny lens in place, and held the apparatus up to the light. To his amazement he saw that the water was full of an incredible number of ‘animalcules’, or tiny ‘animals’. As he wrote to the Royal Society, they ‘were incredibly small, nay so small, in my sight, that I judged that even if 100 of these very wee animals lay stretched out one against another, they could not reach to the length of a grain of coarse sand.’ These were in fact protists and bacteria. The scale of life had just become almost infinitely smaller than had been imagined.

When the Royal Society got news of Van Leeuwenhoek’s astonishing discovery, they instructed their resident microscopist, Robert Hooke, to replicate the ‘pepper-water’ experiment. He failed, because Van Leeuwenhoek had neglected – perhaps deliberately – to make clear that he had used a capillary tube. Eventually, Hooke realised what was missing from his set-up and was able to confirm the observation. For many years it was thought that the use of pepper infusion was necessary to observe the animalcules – some classes of bacteria are still called infusoria. Of course, the bacteria and protists were in the water all along.

Although he didn’t realise it at the time, Van Leeuwenhoek was the first to observe bacteria, the most numerous and ancient life forms on the planet. He went on to explore and detail many uncharted aspects of the biological world – a year later he made the momentous discovery of spermatozoa – but it is rightly for his discovery of the bacteria that he is remembered.

It is estimated that there are around 10³¹ bacteria alive on Earth – a hundred million times the number of stars in the observable Universe.

Bacteria have been around for almost the entire history of life on Earth. The oldest known bacterial fossils are almost 3.5 billion years old. Typically just a few millionths of a metre in length, these single-cell life forms come in a multitude of forms, from spheres and rods to spirals or even cuboidal shapes. A single drop of water contains, on average, a million bacteria; a gram of soil may be home to 40 million; in your body there are ten times as many bacteria as there are human cells. It is estimated that there are currently around 10³¹ bacteria alive on Earth – a hundred million times the number of stars in the observable Universe. By mass, they are comfortably the dominant organisms on our planet.

Bacteria are organisms known as prokaryotes, which means that they do not have a cell nucleus. They share this trait with another group of single-celled organisms known as archaea. The lack of a nucleus, and indeed virtually any complex structures inside their cells, distinguishes them from all other forms of life, which are known as eukaryotes. All animals, plants, fungi and algae – in fact, anything that we would regard as ‘complex’ – are eukaryotes. The overwhelming majority of biologists today believe that eukaryotes emerged from prokaryotes around 2 billion years ago, and that this fundamental and revolutionary change happened only once. We’ll return to this quite remarkable claim later in this chapter. For now, we’ll remain in the domain of the prokaryotes, and explore a more ancient yet no less epochal leap in life’s capabilities, achieved purely by the seemingly lowly bacteria, that turned the planet green and paved the way for the eukaryotes to flourish.