E2science

If you pick any two points, the difference in gravitational potential between them tells you the amount of work that needs to be done against gravity to get from one point to the other, or the amount of gravitational energy released in moving between the two points in the other direction. It is usually measured in Joules of energy per kilogram of mass. For example, the top of a waterfall has a higher gravitational potential than the bottom, so every kilogram of water that falls gains that much kinetic energy on the way down, minus whatever is absorbed by drag - at Niagara Falls, that would be 500J per kilogram, energy that is ripe for harvesting as hydroelectric power.

Electrical potential is a close analogue in electromagnetism - the potential difference between two points on a circuit is what we are all familiar with as voltage. Again, this is a measure of the amount of energy that is gained or lost by a given amount of stuff moving from one point to another - a coulomb of electrons travelling from one pole of a 9V battery to the other will gain 9J, for example.

At sea level on Earth, a kilogram of matter that drops a metre will gain 9.8J of kinetic energy, because the local gravitational field strength is 9.8J/kg m, which is the same as saying that it is 9.8N/kg, or 9.8m/s². The fact that the three units are equivalent might not be immediately obvious, but it is fairly easy to prove, and it brings out three different ways of looking at gravitational field strength, each with their uses. It can be thought of as a force that is proportional to mass; a constant rate of acceleration; or the rate of change of gravitational potential. Going upwards can hence be seen either as pushing against the force of gravity, or working your way towards the top of a potential 'gravity well'.

So far we have only talked about the difference in gravitational potential from one point to another. Sometimes it is useful to be able to talk about it in absolute terms, and for that, the convention is to compare the current location with a point so far from anything that it is completely unaffected by gravity. The gravitational potential in that case tells us how much energy it would take to escape from the local gravity well completely. To take an example, if it would take 1,000J of energy to fling a kilogram of matter out to the edge of space from here, then the gravitational potential right here is -1,000J/kg. That minus sign might seem like an oddity, but there is a good reason why gravitational potential energy is conventially considered to be negative; otherwise it would be almost impossible to define it absolutely. We need to define it with reference to something, and a point at infinity is really the only way that makes sense.

One curious consequence of defining potential (and potential energy) as negative is that it makes it possible for the total amount of energy in the universe to be exactly zero. This makes the creation of the universe - or indeed, a multiverse - surprisingly compatible with the conservation of energy.

The gravitational potential (or more accurately its relativistic equivalent) is also what determines the degree of gravitational time dilation according to Einstein's theory of General Relativity. The curvature of spacetime causes time to slow down around any massive body, and the amount of slowing depends on gravitational potential.

Single-celled animal of changing shape, whose very name means change. 

The cytoplasm is differentiated into an inner, granular, more fluid endoplasm and an outer, clear, firmer ectoplasm, but even this structure is dynamic, for the two types are interconvertible.  In a moving amoeba, the ectoplasm in a given area of the periphery will suddenly become fluid, permitting the endoplasm to flow forward into a projecting lobe, called a pseudopod ("false foot").  Near the tip of the pseudopod, the endoplasm flows to the sides and becomes firmer, becomes ectoplasm, so that the pseudopod extends as a tube of firm ectoplasm with endoplasm inside.  The animal can progress at about 2 centimeters per hour at this pace, hardly racing along, but moving nonetheless.  (They don't usually go in straight lines, though, having little sense of what we would call purpose.)

There are stranger things yet.  If you put an amoeba in a centrifuge at high speed, you can take the thing apart completely, with the various organelles (component parts) at different levels in the test tube, with the nucleus floating in there too.  No surprises here: but if you leave this mixture alone, the animal will re-assemble itself, apparently without lasting harm.  (Think of the horror movies that could be made if people could do this!) 

Still, there are limits.  The nucleus is important.  If you cut an amoeba into two pieces, one with the nucleus and one without, the one with the nucleus goes on as before, but the piece without, while it can move and feed for a time, soon becomes inactive, being unable to digest, grow or divide.

Amoebas eat by surrounding a food particle and taking it inside in a "food vacuole."  Digestive enzymes (which float around inside the amoeba in small packages, ready for use) are released into the food vacuole and break the food down into usable form.  Organic molecules can also be absorbed from the surrounding medium.

As everyone knows, amoebas reproduce by fission.  No sexual reproduction has been observed in amoebas.  Every individual, then, is, in a way, its own species, and in the same way, is immortal, never born, never dying.

There are quite a number of variations on this basic theme.  There are giant amoebas such as Chaos carolinense which have about 1,000 nuclei.  When such a giant divides, one daughter cell doesn't take 500, the other the other 500, as you might expect, but each nucleus divides, so each new cell gets a full set.  Some amoebas live collectively for a time ("slime molds"); some have shells; they are found nearly everywhere. 

We think of them as primitive, but really they're not.  They've been around a lot longer than we have, and carry out every organic function we do, with a charming economy of scale, without specialized equipment, making do, enduring indefinitely.

Conventional scientific classification falls down with these things, partly because they do not breed, partly because we don't understand them very well.  Every authority has a slightly different way of classifying them.  All agree so far that they are in the Kingdom Protista.

Leaf mold is pretty much what it sounds like: leaves that have decomposed through fungal growth.¹ It is the next step in leaf decomposition after leaf litter, and is midway to humus.

Leaves² take a long time to decompose; while many gardeners will find ways to compost leaves more quickly, in the woods leaves take at least two to three years to break down to soil. Leaf mold will be identifiable as the remains of leaves, but will show significant degradation. Technically this is the F layer of the O Horizon (or, if you prefer, the O1 horizon); the plant matter is largely decomposed and some of the original structures are becoming difficult to recognize. This process will involve any number of decomposers; earthworms, bacteria, snails and slugs, small mammals, and others. However, the major player in this process will be filamentous fungi³, as the fungi are the most effective producers of the enzyme that breaks down the tough lignin in the leaves.

Leaf mold is usually dark brown to black, is crumbly but may be somewhat cohesive (due to the aforementioned filamentous fungi), and has an earthy aroma that is usually described as pleasant. In most forests the layer of leaf mold is rather thin and full of roots, so it is generally not harvested from its natural habitat. However, gardeners may use a separate compost pile just for leaves so that they can 'grow' leaf mold. This generally involves shredding leaves and leaving them to rot for about a year, watering occasionally to make sure that the pile remains damp. You can get just as good leaf mold without shredding (and, in most environments, without watering), but it may take 2-3 years to get the decomposition you want. When leaf mold is added to the soil it increases the water retention and encourages earthworms and beneficial bacteria, but it does not have much nutritive value. When talking about gardening, usually there is no strong distinction made between leaf mold and leaf litter; if someone says they are using leaf mold as mulch, they are probably actually referring to leaf litter.

Warning: These footnotes are unusually pedantic. Do not read if prone to boredom.

1. Well, I lie. Mold, in this case, refers to loose earth, and the name doesn't actually have anything to do with fungi. (The two senses of mold may have shared a root ~900 years ago. Or maybe not.) But as it happens, fungus is a major component of healthy leaf mold.

2. Leaf mold is most often used when referring to leaf litter in deciduous forests or compost from deciduous trees; however, the same terminology is used for coniferous forests/trees. Grasses and forbs have much less lignin, and are generally not considered to form leaf mold.

3. In most cases the most effective decomposers of leaf litter are assorted members of the Basidiomycota, although various members of the Ascomycota also help. This sounds very specific, but together these phyla comprise the entire subkingdom Dikarya, AKA the higher fungi. In other words, Dikarya includes all fungi big enough to see, including all mushrooms, along with a good sampling that you will need a microscope to locate.

Methane is constantly produced in the earth's biosphere, by the decomposition of organic material, sometimes assisted by the fermentation processes of various bacteria and archaea, sometimes living freely, sometimes living in symbiosis with other creatures, ranging from termites to cattle. Since organic materials are carbon chains, and methane is just the simplest form of carbon chain, it makes sense that it would be a byproduct.

But once it reaches our atmosphere, it only exists for a limited time. The quickest source I could get lists the half-life of atmospheric methane as six years. This has probably varied across the earth's history, depending on temperature and atmospheric oxygen levels. But, overall, since methane is a reducing chemical, and we have an atmosphere that is an oxidizing atmosphere, at some point, they will have to meet and neutralize and produce water and carbon dioxide.

This is basic chemistry and earth science. Where methane gets interesting is in the rapidly growing field of astrobiology. Because methane's existence involves being pushed up a thermodynamic hill, there is a limited amount of reasons it could exist inside a planet's atmosphere.

• It is constantly being created by some process. One of which would be life, but it can also be created by, for example, ultraviolet light interacting with water and carbon dioxide. However, Life is the most likely process to create methane.
• The planet is cold enough that even if methane and oxygen are both present, the reaction between them is occurring at a slow enough rate that they can both exist indefinitely. This is the case, for example, on the gas giants of our own solar system.
• The methane exists with no oxidizer to counterbalance it. It forms the atmosphere, and is thus stable.

When the Curiosity Rover went to Mars, it carried equipment to detect atmospheric methane. If methane does exist in quantity on Mars, it might be a byproduct of deeply buried microbial life, slowly releasing it into the atmosphere. Of course, since on Mars there is no oxidizing atmosphere, the only way it would be destroyed is through photodisintegration. Currently, the experiments are not turning up elevated levels of methane, but if they do, the argument about whether it is biogenic will certainly be fascinating.

Much further away, the hunt for exoplanets is now reaching the point where scientists are beginning to be able to take a spectrum of an exoplanet's atmosphere, which is an incredible feat. And if methane shows up in the spectrum of some exoplanets, it would be a sign that the planet could have life. A small, rocky planet in the habitable zone of a star with both oxygen and methane in its spectrum would either have some form of life, or else very unusual non-biological chemical processes. NASA currently has a mission, called FINESSE, to analyze atmospheric spectrums for signs of life.

And so it is that the first sign of life in the universe we might ever get is the spectrum of methane, one of the simplest and most common of molecules, barely detectable in the atmosphere of a planet orbiting a star scores of lightyears away.

Now, we're talking about parasites here: not the most attractive organisms, admittedly. But everyone has to make a living.

The phylum Platyhelminthes is a big one, and includes such things as free-living flatworms as well as tapeworms, liver and blood flukes, and other disgusting beings. Disgusting to us, largely because we're more likely to be the victims than the perpetrators of parasitic behavior.

The flatworms in question here, Diplozoon, live parasitically on the gills of fish. Each has both male and female sex organs; but the plan is not self-fertilization. When immature individuals meet and fall in love (say, who am I to say that they don't?) they thereupon become firmly and permanently physically attached to each other, their tissues fuse, and their reproductive systems grow together so that each fertilizes the other. They live the rest of their lives together in a state of perpetual copulation, through sickness and health, and so forth. They may be the only completely monogamous species.

Love is so important to these tiny beings (they're only 2 or 3 centimeters long) that the hapless young worm who does not find a mate fails to mature, and soon dies.

(PHYLUM, Platyhelminthes; CLASS, Monogenea; SUBCLASS, Polyopisthocotylea; GENUS, Diplozoon)

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Buchsbaum, Animals Without Backbones, University of Chicago Press, Third Edition 1987

New York Times