The Spark of Life: Electricity in the Human Body (24 page)

Bacillus thurigiensis
is widely used as a biological control agent to limit caterpillar populations in commercial greenhouses, to destroy mosquito larvae, and to kill the blackflies that carry river blindness. More recently, the gene that codes for the bacterium’s toxin has been engineered into plants, which then manufacture the toxin themselves. Pesticide-producing strains of maize, potato and cotton are commonly grown in the USA and enable the use of synthetic insecticides to be dramatically reduced. This has had clear environmental benefits. Nevertheless, the practice has been quite controversial, in part because of anxiety about genetically modified crops. Another concern is that continual exposure of insects to the pesticide creates a strong evolutionary selection pressure that favours toxin-resistant insects. Any insect developing a mutant receptor that does not bind the toxin has a clear reproductive advantage, and insects resistant to the pesticide have already been reported. As is the case for antibiotics, countering resistance is a constant battle.

Cell Suicide

Long ago, before you were born, you had webbed hands and feet like those of a duck. As you developed inside your mother’s womb, the cells that made up the web of soft tissue between your digits were killed off in a process known as programmed cell death (or apoptosis) so that you ended up with separate fingers and toes. If this process of body sculpting fails, as occasionally happens, you end up born with webbed fingers.

Everyone who has kept tadpoles has seen such cell suicide in action for the gradual disappearance of the tadpole’s tail as it develops into a baby frog occurs by apoptosis and reabsorption of the dying cells. Similarly, apoptosis is drawn to the attention of a woman every month, for the sloughing off of the lining of the womb that occurs at the start of her period is also the result of programmed cell death. Perhaps most important of all, cell suicide plays a key role in the development of the nervous system and in how your brain is wired up. Early in development, many nerve cells are born and send forth their axons towards their destination in an exploratory manner. If they find their correct targets, a tentative connection is established, impulses speed excitedly down the lines, chemical kisses are exchanged, and the link is cemented. Nerve cells whose axons fail to find their correct targets produce more feeble impulse activity and simply wither away through lack of use. Many die during brain development and without such cell suicide the brain could not function correctly. Apoptosis is also a way to ensure that damaged cells that might threaten an organism’s survival are eliminated. Your immune system can kill cells infected with viruses this way, and cells whose DNA is damaged are encouraged to commit suicide to prevent cancers forming.

At the cellular level, then, death is far from being a negative event. It is an essential part of the life of every multicellular organism and every day several billion cells in our bodies die by apoptosis. Without it, multicellular life is not possible. If we are no closer to understanding the meaning of life, at the cellular level, at least, we might claim to understand the meaning of death.

A Time to Live, a Time to Die

When a cell commits suicide it shrinks, its membrane lifting away from the underlying cytoplasm in ugly bubble-like blebs. The DNA is broken down so that no more proteins can be produced, and the mitochondria, the cell’s powerhouses, are disabled. Specific lipids appear on the surface of the cell membrane that signal to scavenger cells to come and gobble up the broken fragments of the dying cell for recycling.

There are several ways in which a cell can self-destruct but, as you have probably guessed, one of them is mediated by an ion channel. It also involves the mitochondria, tiny intracellular organelles, about the size of a bacterium, that are found in almost every cell of your body. The ancestors of mitochondria were once free-living entities, rather similar to the blue-green algae (the cyanobacteria) that form the familiar green scum on lakes in hot summers, but around two billion years ago these ancestral mitochondria gave up the solitary life and became incorporated within early cells. Thus like the
Star Trek
aliens known as the Trill, we live our lives in partnership with another organism – but this is no science fiction and our symbionts are microscopic. Almost all plant and animal cells contain mitochondria and they are essential for life: without them, multicellular organisms could not function, as mitochondria act as molecular furnaces where fuels such as sugar and fats are burned with oxygen to produce chemical energy. Cells that require a lot of energy, like muscle cells, have large numbers of mitochondria.

But mitochondria also have their dark side. They are surrounded by two membranes, whose integrity is important for the ability of the mitochondrion to produce energy. When a cell decides to commit suicide a large pore forms in the outer mitochondrial membrane known as the mitochondrial apoptosis-induced channel. The hole is so big that relatively large chemicals can leak out of the mitochondria into the cytoplasm, where they create mayhem, triggering a cascade of events that leads inexorably to cell death. Importantly, however, the decision to commit suicide is not decided by the mitochondria itself. It is a process that is initiated and tightly controlled by the cell, which simply co-opts the mitochondrial machinery to serve its own ends.

Blighted Harvest

Mitochondria are also targeted by the Southern Corn Leaf Blight toxin, which wreaks such havoc on cytoplasmic male sterile (CMS) strains of maize. CMS maize plants are sterile because they possess a unique ion channel that sits within their inner mitochondrial membrane. Like a silent timebomb, this channel is normally closed and does not affect organelle function. However, binding of the Southern Corn Leaf Blight toxin activates the timebomb, opening the channel and destroying the ability of the mitochondria to make energy. Without energy, the cell dies. As the fungus spreads, the toxin destroys the plant, cell by cell. Only those plants that possess the ion channel gene, that is the CMS varieties, are susceptible. It is an inescapable association, for toxin sensitivity and male sterility result from the same process. Even in the absence of the toxin, the ion channel is activated in the mitochondria of the cells that supply the developing pollen grains with nutrients, and when these cells wither and die, so too does the pollen.

Despite the wide devastation caused by Southern Corn Leaf Blight in 1970 in the USA, the country was lucky. More than 85 per cent of maize plants at that time carried the gene. Only the dry September in the northern and western states, which limited the spread of the fungus, prevented an almost total destruction of the crop. As Paul Raeburn points out in his thought-provoking book
The Last Harvest
, the size of the Southern Corn Leaf Blight epidemic and its enormous economic impact resulted from the fact that the Corn Belt in the USA was largely planted with a single variety of maize. The genetic uniformity of modern crops and the practice of planting only one or two varieties over a wide area means that if one plant is susceptible to a new disease, all plants will be. Consequently, the whole crop is at risk. More traditional methods of agriculture, which use many different local varieties, preserve considerable genetic variability so that although some plants may succumb to infection, many others will be resistant. A good reason, then, for preserving as many wild crop species and indigenous cultivars as we can, for without the genes that these plants contain, plant breeders may be unable to adapt crops to the new dangers we will assuredly encounter in the future.

Green Electricity

Almost all life on the planet depends on the ability of plants to capture the energy of the sun and store it as sugar molecules. This process, known as photosynthesis, is the ultimate source of all the food we eat, all the molecules from which our bodies are built, and most of the oxygen in the atmosphere. Photosynthesis involves the conversion of carbon dioxide and water into sugar and oxygen in a reaction powered by sunlight, and it takes place in specialized organelles, known as chloroplasts, that lie within plant cells.

To prevent excessive water loss, the leaves of most plants are covered with a thick waxy cuticle. However, this also restricts the diffusion of oxygen and carbon dioxide into and out of the leaf, so that gas exchange can only take place via dedicated pores on the underside of the leaf, known as stomata, that act like microscopic windows. The problem that plants face is that the stomata not only allow carbon dioxide to enter and oxygen to leave, they also provide a very effective pathway for water vapour to escape. This can put a considerable strain on the plant, for water lost this way must be replaced by absorbing more from the ground. Some desert plants have solved the problem by opening their stomata only at night, greatly restricting water loss during the heat of the day. But this poses another difficulty because photosynthesis normally requires carbon dioxide and sunlight to be present at the same time. It’s a classic catch-22 situation. Consequently, most plants balance photosynthesis and water stress by opening and closing their stomata throughout the day, as the ambient light and humidity conditions dictate.

Stomata are composed of two ‘guard’ cells that both form the aperture of the pore and regulate its opening and closing by adjusting the amount of water they contain. When the guard cells are swollen and turgid the pore between them is forced open, whereas when they lose water and become flaccid the pore collapses shut. The water movements that influence guard cell volume, and thereby stomatal opening, are controlled by a combination of pumps and channels. An increase in light intensity causes positively charged hydrogen ions to be pumped out of the cell, creating a negative potential across the cell membrane. In turn, this change in membrane potential opens potassium channels, allowing potassium ions to enter the guard cells. Water follows the potassium ions, so that the guard cells swell by as much as 40 per cent, forcing open the stomatal pore. As long as the potassium channels remain open, the pore remains ajar. However, when light levels fall or the plant experiences water stress the potassium channels close. Consequently, water leaves the cell, the guard cells shrink and the stomatal pore closes.

In a sense then, by controlling the turgidity of the guard cells, plant potassium channels regulate photosynthesis. Arguably, they are some of the most important ion channels on Earth. I find it strangely pleasing that these potassium channels belong to the same superfamily as the ones I am most passionate about. They must stem from a common ancestor that evolved long ago, before the animal and plant kingdoms divided.

Life in the Slow Lane

Remarkably, a few plants not only have ion channels, they also have the ability to generate action potentials. However, the electrical impulses of plants differ from those of nerves in that they are of longer duration, travel more slowly and are carried by different ions. That of the alga
Nitella
, for example, is initiated not by an influx of positively charged sodium ions, but instead by the loss of negatively charged chloride ions from the cell. There is a good reason why this is the case. Unlike animal cells, the cells of most terrestrial plants are not bathed in a salty extracellular fluid. Ions are present at very low levels in plant cell walls and thus an influx of sodium ions would not be a viable means of producing an action potential. Instead, plants must rely on chloride efflux.

Carnivorous plants have exploited action potentials to capture their prey. One of the most fascinating is the Venus flytrap, a favourite of Charles Darwin. This plant, he wrote, ‘from the rapidity and force of its movements, is one of the most wonderful in the world’. To cope with the nitrogen-poor soils of the bogs in which it lives, the Venus flytrap supplements its diet by capturing small insects. It attracts them with an enticing ‘trap’ formed from a modified leaf that consists of two brilliant crimson lobes, like the two halves of a cockleshell, fringed by long pinkish-green hairs. At rest, the trap sits invitingly open. No sooner has an unwary fly landed on its sweet, sticky surface, however, than the two halves snap shut, imprisoning the insect inside. The long hairs at the edge of the lobes interlock tightly together like the teeth of a rat-trap, preventing large insects from escaping. Small insects can squeeze out, presumably because it would not be energetically favourable to process a tiny morsel, but larger insects are slowly digested to provide the nitrogen the plant needs to make its own proteins. About seven days later the trap reopens, releasing the indigestible remains.

As you will know if you have ever tried to swat a fly, insects move fast. Thus, to catch one, the Venus flytrap must move even faster and it has evolved a specialized electrical signalling system that enables it to do so. Each lobe of the trap bears several triangular hairs projecting up from its surface that are exquisitely sensitive to touch. If more than two of these are distorted at roughly the same time – for example, by the movement of an insect – the lobes clap shut faster than the blink of an eye.
²
The hairs possess mechanosensitive ion channels and touching them elicits an action potential that spreads throughout the lobe cells to the centre of the trap. At rest, the lobes of the trap are bowed upwards, but when the electrical signal arrives at the midline of the trap they flip from a convex to a concave shape, forming a pocket that entraps the prey. Precisely how this happens is still debated, but ion channels that trigger ion and water movements that lead to differential swelling and shrinking of the lobe cells, and thus to dramatic changes in pressure across the leaf, have been invoked.