Guide to organismal biology

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Diversity of Life

guide to organismal biology

Table of Contents

Intro How to be an Organism 3
1 Evolution 11
2 Prokaryotes, Protista 16
3 Primitive Invertebrates 27
4 Molluscs, Annelids 41
5 Nematodes and Arthropods 52
6 Echinoderms, Chordates 63
7 Fungi 75
8 Mosses, Ferns & Fern Allies 84
9 Gymnosperms & Angiosperms 99
10 Plant Structure 113

How To Be An Organism

Multicellularity introduces a new element into the natural order of things. The snake has entered the bacterial Garden of Eden. We can no longer live forever. But in exchange for leaving immortality behind, a vast array of evolutionary pathways opens up, different ways for multicellular organisms to live, in an ever-increasing number of different environments. Each of these new environments poses a different set of challenges, and only those organisms that can adapt to these changing conditions will survive to carry on the species. And all of the amazing diversity we see in nature, all of the millions of different ways to be a living thing, represent the many ways in which organisms have solved those basic environmental challenges.

We are accustomed from birth to look at plants and animals as very different sorts of beings, that somehow animals are a different order of creation from plants. But if we look under the surface, if we think about what plants and animals really are, in the most basic and fundamental sense, we might find that we are more alike than we think. All multicellular organisms, whatever their environments, share a common set of evolutionary problems. And the differences we see between them are a result of the different evolutionary strategies they have used to solve those problems.

All organisms face the same basic challenges:

1) Find and digest food

2) Find a mate and reproduce

3) Avoid being eaten while you are doing number 1 and number 2

4) Maintain a balance between the fluids in the body and the salts dissolved in them (osmotically stable environment)

5) Circulate nutrients from one part of the body to the other

6) Remove waste products generated by metabolism (especially nitrogen compounds)

Evolutionary Challenges

Plants and animals have adopted very different strategies to solve these problems. And different groups of animals have come up with some solutions that are truly radical. The possible solutions, however, are not infinite. Any engineer can tell you that the number of solutions to an engineering problem is finite. The basic laws of physics and chemistry are not repealed when we put up a building. If you push something hard enough, it will fall over!

For example, there are three very fundamental modes of existence that an organism can adopt:

1) Sessile or Motile

2) Aquatic or Terrestrial
3) Small or Large

Sessile or Motile

Sessile (attached) organisms are usually radially symmetric. Radial symmetry means the animal can be folded along any plane into mirror image halves. Like a wagon wheel. Bilateral symmetry means that there is only one plane that can divide the organism into mirrored halves, like a wagon. The Phylum Cnidaria is a large group of organisms that are sessile for all or part of their existence. Sea anenomes, for example, or coral polyps live out their lives attached to the same spot. Radial symmetry in these organisms is probably a fundament adaptation to a sessile existence. Your awareness of your environment is omnidirectional. You can sense and get food from any direction.

The down side is that when danger threatens, you've got nowhere to go. Cnidarians solved this problem by evolving a variety of stinging cells, loaded with nasty little microscopic harpoons, which they can use to stun prey and attack predators. You also have to find a way to disperse your young when you reproduce. They can't simply walk away. So, many sessile animals have motile larvae.

Sessile animals, like sea anenomes, don't have to invest in complicated structures like legs or wings in order to move about and look for food. But being sessile limits them to one type of food source, the kind that just happens to float by. Sessile animals are usually filter feeders or suspension feeders. Some higher organisms, especially the echinoderms (sea lilies, starfish) have gone back to a sessile mode of existence, and in the process have lost their bilateral symmetry, returning to a more primitive radial symmetry.

Motile organisms are usually bilaterally symmetric, a group which includes most higher animals. This is a much more efficient shape for moving through the environment. Animals in motion can actively seek out food and mates, and run away from predators. Animals in motion generally have a specific direction. And if you are moving forward, it makes good sense to concentrate your awareness of your environment in that direction.

So bilaterally symmetric animals become cephalized. They develop a head end, where the sensory organs are located, as well as the brain to which those senses are wired. That is why vertebrates have a central nervous system, and sea anemones do not. Forward motion allows different parts of the body to become specialized for different purposes, with senses and awareness at the anterior end, and functions like excretion and reproduction at the posterior end. Such organisms also have a dorsal or top surface (remember the dorsal fin of the shark), and a ventral surface, or, to use its scientific name, the tummy.

Aquatic or Terrestrial

Consider the second mode of existence, being an aquatic organism or a terrestrial one. One of the few things we know for certain about the earliest history of life is that it began in water. Most probably in the sea, as the salt content of every cell in our body suggests. The great leap from water to land required a radically different set of adaptations, problems both plants and animals had to solve:

1) Desiccation
2) Gravity
3) Excretion

Desiccation becomes a big problem for aquatic organisms as soon as they leave the water. In the ocean, your body is constantly bathed in an isotonic salt solution, one whose concentration of salts is uniform and stable. On land, you instantly lose water to evaporation, and every cell exposed to the air begins to dry out. A protective outer layer of epidermal cells, or a thick cuticle helps prevent this. Animals have skin. Respiratory surfaces must be kept moist in order for gases to be dissolved in water and enter the cells. That's why our lungs are on the inside.

Desiccation also poses a problem for reproducing on land. When it came time to reproduce in the water, you could just dump all your gametes overboard, and let the currents do the rest. The larvae would develop in a nurturing saline bath, the ultimate womb of the ocean. But terrestrial organisms must find a way to keep their gametes from drying out. Aquatic organisms can rely on external fertilization. Terrestrial organisms have to develop some sort of internal fertilization to guard against desiccation.

Some primitive plants get around this problem by relying on a thin film of water, like dew, to give their gametes a moist place to swim through. Such plants, like mosses and ferns, are limited to moist environments. As are some animals, like amphibians, which must return to the water for at least part of their life cycle. In a very basic sense, many terrestrial organisms have never actually left the water. They live in the thin films of water that cling to moist places, like the tiny pore spaces between grains of moist soil.
Organisms also had to evolve new ways to protect their embryos from drying up on land. Higher animals evolved the amniotic egg, sealed in a shell and bathed in nutritious liquid. Amphibians must return to the water to lay their eggs, but reptiles can lay their eggs anywhere. Higher plants evolved the seed, a tiny time capsule filled with food and sealed against the elements. The reptilian egg and the seeds of gymnosperms allowed organisms to break the last link with their aquatic heritage.

Gravity is another basic fact of life that is not a very big deal in the ocean, but of paramount importance on land. Aquatic organisms rely on the natural buoyancy of water to support their weight. In general, they don’t need to invest much energy in support structures. Unless, of course, they need to move very rapidly, like vertebrates. On land, gravity requires a support system. Plants developed the root-shoot system, roots holding you in place while the stiff tissues of the stem lift your body up into the air. Animals on land developed sturdy skeletal systems, whether internal, like our own (endoskeleton), or external, like that of an insect (exoskeleton).

Getting rid of wastes is not a big problem in the ocean - dump it overboard. Waste material is generally excreted in a solution of water, and is usually high in nitrogen compounds. Aquatic organisms usually excrete nitrogenous wastes in the form of ammonia. Ammonia requires large amounts of water to dissolve in, but if you're floating in the ocean - no problem! On land, however, organisms have to conserve water, in any way they can. So terrestrial animals excrete liquid wastes as urea. Even urea, however, requires a fair amount of water to dissolve. The evolution of the sealed amniotic egg in reptiles required an even more compact way to store nitrogenous wastes inside the egg shell. So birds and other animals came to rely on uric acid to get rid of nitrogenous wastes, which uses very little water (the white part of bird droppings). Excretory systems themselves pose certain critical problems. The water that carries off the waste stream also takes with it essential salts that the organism must replace. So animals have developed excretory organs like nephridia, simple tubes through which the wastes pass and in which precious salts can be recovered.

Small or Large

Related to all of these basic environmental challenges is the problem of size. If you remain small, you can rely on simple diffusion to absorb nutrients and excrete wastes. This is true for both plants and animals. Increasing size brings increasing control over your environment, and allows for greater complexity. But larger and thicker organisms can no longer rely on diffusion. Cells that are too far away from the surface will starve to death, or drown in their own poisons before they can be carried off. And to make matters worse, the surface area across which gases, nutrients, and wastes must be exchanged rapidly decreases as you get larger.

As organisms become larger, their volume increases much more rapidly than their surface area. Cells become farther removed from the outside at an exponential rate. Consider a spherical creature. The formula for the surface area of a sphere? (4 pi r2). The formula for the volume of a sphere? (4/3 pi r3) The animals volume increases as a cube of its radius, but its surface area only increases as the square of the radius. So as it gets bigger, more and more volume is covered by less and less surface area.

Organisms have solved this problem in several ways:

1) Folding the respiratory, digestive, and other surfaces to increase the amount of surface area that can be packed into a limited space (lungs, brains, intestines)

2) Being very thin or very flat

3) Developing vascular systems - Plants and animals have solved this problem in a basically similar way. They rely on a network of tubes that runs throughout the body of the organism, a vascular system. These closed tubes can circulate water and nutrients, and carry off wastes.

4) Developing coelomic systems, fluid-filled cavities that can be used to circulate materials and hold the internal organs, along with a variety of other useful functions. (fr. Greek koiloma = cavity)

Kingdom Animalia

Kingdom Animalia includes 36 phyla and over 1 million species. If all the various worms and insects were finally found and described, the number of named animal species might grow as high as 10 million species! Most of these animals, about 95-99% of all known species, are invertebrates, animals without backbones (a term coined by Lamarck). And most of these are different types of aquatic worms! Animals are diploid, eukaryotic and multicellular. All animals are heterotrophic. And all animals respond to external stimuli. All animals are motile, moving about at some point in their life cycle. Only animals can fly. J.B.S. Haldane once called animals “wanderers in search of spare parts.” All animals reproduce sexually by forming haploid gametes of unequal size, the egg and the sperm. The gametes fuse into a zygote, which develops into a hollow ball of cells called a blastula. In most animals, this hollow ball folds inward to form a gastrula, a hollow sac with an opening at one end, the blastopore, and an interior space or blastocoel.

All animals share a common ancestor. The clade Opisthokonta includes the Kingdom Animalia as wello as the two groups most closely related to animals, the Kingdom Fungi, and the Phylum Choanoflagellata. The choanoflagellates are free-living protozoans, usually tucked away in the Kingdom Protista. They bear a striking resemblance to the feeding cells of the sponges. Colonial forms of this protozoan are now considered the most likely ancestor of all multicellular animals.

We can divide the entire animal kingdom into two subkingdoms. The Subkingdom Parazoa contains the sponges, and one or two obscure groups of rudimentary animals. Parazoa literally means “animals set aside”. These animals that are so strange, so unlike all other animal life, that we tuck them away in their own little group.

All other animals belong to the Subkingdom Eumetazoa, the “true” metazoans (meta - zoan = animals that came “after”, as opposed to “proto” -zoans, = first animals). Eumetazoans have a definite symmetry, which sponge animals lack, and share common patterns of embryonic development. There are two branches of Eumetazoans: one includes animals like sea anemones that have radial symmetry, and the other branch including all other animals, all of whom have (like ourselves) bilateral symmetry.

The bilaterally symmetric animals can be further divided into three grades. Grade is not a formal taxonomic term. Grades represent a level of organization. The group of all animals that fly, for example, could be called a grade. These three grades represent three basic types of body plan found in all animals. These body plans differ mainly in the presence or absence of an internal body cavity, or coelom. So what is a coelom, and how does it form?

The Coelomic Body Plan

In the embryos of all bilaterally symmetric animals, there are three tissue layers distinctly visible in the developing organism. These are the endoderm (= the skin within), which gives rise to the gut and most digestive organs; the mesoderm, (= the skin in the middle), which gives rise to the skeleton and body muscles, and the ectoderm (= the skin outside), which gives rise to the epidermis or outer covering of the animal as well as the nervous system. Many primitive animals, like flatworms, have no coelom at all, just a rudimentary internal pouch, like sticking your finger deep into a ball of clay. We call such animals acoelomate, because they lack a coelom. Another group of animals, including nematode worms and rotifers, have a large body cavity that looks like a coelom, and functions like a coelom, but is actually formed in a different fashion. This pseudocoelom is a remnant of the hollow space inside the blastula, the blastocoel. We call these animals pseudocoelomates. All higher animals have a true coelom, a body cavity formed from within the mesoderm tissue layer. This cavity is lined by mesodermal membranes, and surrounds most internal organs. Such animals are called coelomate.

If we look at the overall pattern of animal evolution, we see that all of the coelomate animals are split into two distinct groups, one called protostomes, the other called deuterostomes. This is a very ancient split within the animal kingdom, going back at least 570 million years to the early Cambrian. These groups are separated by what happens to the blastopore, the small hole that opens in the gastrula, connecting the embryonic gut to the outside. In protostomes, this opening gives rise to the animals mouth, hence proto=first, stoma=mouth. This group includes the annelid worms, the mollusks, and the arthropods. In deuterostomes, the first opening becomes the anus, and the mouth opens up later on in development at the opposite end of the embryo, hence deutero=other, stoma=mouth. This group includes the echinoderms and the chordates.

Another fundamental difference between protostomes and deuterostomes has to do with the fate of the early cells in the developing embryo. Protostomes show a pattern of spiral cleavage, in which new cells are staggered in a spiral fashion, overlapping one another like bricks in a brick wall. These cells are determinate, their fate is determined early on in development. Removing them results in an incomplete organism. Deuterostomes show a pattern of radial cleavage, in which cells appear directly over other cells, like a stack of coins. These cells are indeterminate. If you separate them at an early stage, each one can develop into a complete functioning organism. This is how identical twins are created, by cell separation at a very early stage of deuterostome development. The coelom in protostomes develops as a split in the mesoderm (schizocoels). The coelom in deuterostomes develops from outpocketing of the gut (enterocoels)

Advantages of a Coelom

The fluid-filled coelom represents a big evolutionary advance.

1) The coelomate body plan is a “tube within a tube”. Because this tube is filled with fluid, it allows fluid circulation, even in primitive animals that lack circulatory systems.

2) Fluids (like water) are relatively incompressible. The fluid-filled coelom can therefore act as a type of rigid skeleton, or hydrostatic skeleton. The muscles now have something solid to push against.

3) The coelom allows for an open digestive tract, with a mouth at one end, an anus at the other, and this tract can be increased by coiling within the coelom so that it is many times longer than the animal itself.

4) Animals like flatworms, on the other hand, with one opening into a hollow cavity, are limited in how fast they can eat, digest, and excrete.

5) A coelom allows digestion independent of movement. Gut action need no longer depend on the muscular contractions generated by the animals movements.

6) There is more space for the internal organs to develop, especially the gonads, and large numbers of eggs and sperm can be stored in the coelom as well.

7) And finally, the combination of a coelom and bilateral symmetry opens up an entirely new evolutionary pathway, in which parts of the body can be adapted to perform special functions. This new pathway, which has ultimately given insects dominion over all other life, is called segmentation, and we’ll discuss it further when we talk about annelids and arthropods



One of the ways we can demonstrate the reality of evolution is to simply consider biodiversity, the large numbers of species, many of which may have similar forms, but are reproductively isolated from one another - like lions, tigers, leopards, cheetahs, house cats, lynx, mountain lions, bobcats, and all the other members of the Family Felidae. It is difficult to imagine any process other than evolution that could have produced such an amazing number of ways to be a cat.

Organisms who live on different continents, but in similar environments, are often very similar to one another. Animals like the American bison and the African wildebeest, both large mammalian grazers who browse in open grasslands, hint at the broader patterns of evolution.

Further evidence of that pattern comes from a detailed study of biogeography, the geographic distribution of plants and animals. The plants and animals that we see in a particular place often traveled there from somewhere else, where conditions were somewhat different, and then evolved to adapt to their new environment.

An important line of evidence for evolution is the fossil record. The fossil record shows us the evolutionary history of life on earth. We find many extinct forms which are obviously related to more modern forms, evidence of descent with modification.

Another line of evidence for evolution comes from the study of embryology. In vertebrates, for instance, the early stages of development are extremely similar to one another, even though the adult stages are very dissimilar (like mammals and reptiles and birds). This implies a common ancestry for all mammalian species. Among the invertebrates, there are several similar examples. Certain annelid worms have a larval stage called a trochophore larva, which is essentially identical to the trochophore larval stage of the mollusks. This suggests that these two groups share a common ancestry. All the various types of crustaceans share a common larval stage, called a nauplius larva, which is one of the characteristics uniting that diverse group into a single class.

Comparative anatomy also provides evidence of evolution. We find the same bones in many different types of animals, but these bones are often modified to do different things. The hopping legs of the frog contains the same bones as our own legs, but the frog's legs are highly modified to fulfill a different function (hopping). The wing of a bird and the forelimb of a bat contain exactly the same bones as the arm of a human, but the size, shape, and even internal structure of these bones are all adapted to play a different role in each animal.

We call structures like the wings of a bird and the forelimbs of a bat homologous structures. Homologous structures are structures that are derived from a common ancestor. Even if they are superficially different, they are developmentally related. Homology does not mean that these structures must share the same function. You can alter the same pieces to make different biological structures. The flippers of a whale are supremely designed to cut through the water, but they are homologous with our own human arms. You can trace out the same bones in each, in the same relative positions, and they develop in roughly the same fashion. This is strong evidence that we are closely related to whales.

But very often in nature we find structures that are superficially similar, even though the organisms are completely unrelated to one another. These structures may even serve the same function, like flying. We call these analogous structures. The wing of a bird and the wing of an insect are good examples of analogous structures. In every physical and biological way, these wings are radically different from one another. One is a flat plane of exoskeletal material, the other is a chordate forelimb shaped into an airfoil, with hollow internal bones and an outer covering of feathers. But they can both be used to fly. If you can fly, you have a huge advantage over animals that can't fly. You can escape from ground predators, grab your food out of mid air, and nest in relative safety in the treetops. So wings are a good idea, whether they evolve on an insect or a bird.

We often find unrelated animals converging on the same form or structure, because that form is very adaptive in their common environment. This special case of evolution is called convergent evolution. Another example of convergent evolution is the streamlined shape of sharks and dolphins. One is a fish, the other is a mammal, and they are related to one another in only the most distant sense. But if your life depends on swift movement through the water, then a streamlined shape is pretty much essential.

Convergent evolution produces analogous structures. Divergent evolution produces homologous structures. The same bones can be used in many ways, leading to several divergent evolutionary paths - frogs, bats, birds, men and so on. But this causes a real problem for evolutionary biologists. Just because two organisms have a similar structure, like a wing, does not necessarily mean that they are related to one another. We have to be very careful not to let these analogies confuse us when we puzzle out which animals are related to one another.

One final line of evidence for evolution, completely lacking in Darwin's day, is molecular evolution. Molecules themselves change over time, because the genes that code for them are changing or mutating. Mutations are an alteration in the genetic instructions that shape the molecules of living systems. Changes at the molecular level occur slowly. If these building blocks are altered too radically, they might lose their ability to work at all. So the pace of molecular evolution is often very slow. The greater the similarities between the biochemistry of two organisms, the more likely it is that they are related. For example, consider the reaction between antibodies and the invading antigen. If you take blood from one animal, and mix it with blood from another, you will get an antibody reaction, because some elements in the blood of the other species will be different enough for the blood of the first species to sense that it is "not-me", and attack it chemically. The more distantly these animals are related, the greater the reaction will be. So we can use the measure of the intensity of the reaction as a clue to the degree of their relatedness.

Another molecular test of common descent depends on the simple fact that proteins are made up of sequences of simpler molecules, the amino acids. Proteins are the molecules that compose the structural elements of living systems, and control the rate and direction of biochemical reactions in living tissue.

By comparing the sequence of amino acids that compose various proteins in organisms, we can get a better idea of how closely they are related. The more similar the same protein is between two species, the more likely those species are closely related. Cytochrome c, for example, is an enzyme essential in cellular metabolism. The closer that two organisms are related, the fewer the differences between their version of this basic molecule. Modern phylogenetic analysis is often based on the S16 subunit of ribosomal RNA.

There are several strong lines of circumstantial evidence that the branching pattern of relationships between organisms are an expression of a fundamental pattern. As Darwin discovered, that branching pattern is a simple consequence of their shared descent from a common ancestor. There is unity in diversity.

Why and how do we classify organisms?

Why do we bother classifying organisms? It seems like a tremendous amount of time and effort spent to fill museum cabinets with neatly labeled specimens. But unless we are willing to take the time to sort out all the ways in which organisms are alike or differ from one another, we can never hope to truly understand them.

Biologists have always been fascinated by the diversity of living things. In the early days of biology, systematic biologists felt a moral imperative to catalog all the creatures they encountered. By identifying and comparing all the organisms on earth, they hoped to illuminate the divine plan they believed lay behind the natural world. The big names in early biology were expert systematic biologists, people like Linnaeus, Lamarck, Buffon, and even Charles Darwin, who became the world’s leading authority on the classification of barnacles.
As other fields of biological research opened up, taxonomy (the description, naming, and classification of organisms) became less glamorous, and was sadly neglected at most universities. The recent discovery of molecular tools for the systematic comparison of organisms has revitalized the field, and added a wealth of new information about how organisms are related to one another.
Figuring out how organisms can be grouped together will ultimately allow us to map their phylogeny, their evolutionary history or lineage. This knowledge also allows us to better communicate with one another about organisms of all types. By clearly identifying and naming organisms, we no longer need to rely on their common names, which can run to a dozen or more different names for the same creature in different parts of the world. Taxonomy turns out to be an extremely valuable tool for anyone involved in the study or exploitation of organisms (living or extinct), including biology, the environmental sciences, business, medicine and even the legal profession.
What traits do organisms share in common, and what traits set them apart? The modern system of classification, cladism, tries to identify characteristics that organisms share in common, traits that are derived from a common ancestor. These shared derived characteristics are called synapomorphies. Cladists seek to identify monophyletic groups (one lineage), groups of organisms that include the common ancestor and all of its descendant species. Cladists try to avoid paraphyletic groups (similar lineage), that include the common ancestor, but exclude some of its descendants. Paraphyletic groupings usually occur because one or more of the descendant species do not resemble their closest relatives. Cladists also try to avoid polyphyletic groups (many lineages), which include organisms that may resemble one another, but do not share a common ancestor. Convergent evolution often results in unrelated species that superficially resemble one another in form or function.

Things to Remember

Know the several lines of evidence supporting the theory of evolution.

Consider This

Conscious awareness is the ultimate product of evolution. Born from stardust, we are truly the universe becoming aware of itself.

2 - Kingdoms Bacteria, Archaea, and Protista

Introduction to bacteria

Bacteria are the oldest group of organisms on Earth. They have a very simple physical structure. Although they are generally similar to higher organisms in their basic organization, they differ from higher organisms in their metabolic chemistry. Different types of bacteria also differ radically from one another in their metabolic pathways. Bacteria may represent a range of early evolutionary experiments in cellular chemistry.

Bacteria are extremely small, about 1/1000 of a millimeter, and are the most abundant organisms on the planet. All bacteria are haploid. Bacteria reproduce by simply dividing into replicas of themselves, a process called binary fission, much simpler than mitosis (and probably ancestral to it). Some can also undergo an exchange of genetic material known as conjugation. Bacteria are solitary organisms in the sense that they do not form true social groupings or colonies. They often stick together after fission, to form long chains or clumps. They were first classified as Kingdom Monera, from the Greek moneres (meaning single or solitary). This clumping together is not true colonial organization, because the cells do not communicate or interact in any complex way. Some forms are motile, they swim by means of a rudimentary flagella. There are three basic types of bacteria that we can easily recognize: Bacillus (-i) = rod shaped; Coccus (-i) = spherical; Spirillum (-i) = spiral shaped

All bacteria are prokaryotes (pro=first). All higher organisms are eukaryotes (eu = true). Prokaryotes are unicellular, lack a cell nucleus (no nuclear membrane around their single circular chromosome), and lack cellular organelles that are bound by membranes (ex. no chloroplasts, no mitochondria). Eukaryotes can be unicellular, but are usually multicellular, have a cell nucleus bounded by a nuclear membrane, and have cellular organelles bound by membranes (chloroplasts and mitochondria).

Both chloroplasts, which contain the photosynthetic machinery, and mitochondria, which produce energy for the cells, function as little autonomous and self-replicating units inside eukaryotic cells. The theory of endosymbiosis (endo = within, sym = same or shared, biosis = life) suggests that these organelles were actually free-living bacteria in the distant past, which were captured and ingested by larger bacterial cells. Instead of being digested, they somehow took up residence, providing cells with new energetic pathways, and providing the organelles with nourishment and a relatively safe shelter. So in a fundamental sense, every cell in the body of a higher eukaryotic organism like ourselves is itself a colonial organism, the heritage of an ancient confederation between different types of bacteria.

Bacteria have a rigid cell wall made of polysaccharides and amino acids, which protects them against mechanical and osmotic damage. Some bacteria have a second cell wall, consisting of polysaccharides and lipids. This second cell wall makes these species of bacteria especially resistant to antibiotics, so this group of bacteria contain some dangerous disease causing organisms.

Bacteria get their energy in a variety of ways. Some bacteria are autotrophs, or “self feeders”. They produce their own energy from sunlight (photosynthetic), or from inorganic compounds (like Hydrogen Sulfide, H2S). Other bacteria are heterotrophs, (= fed by others), they use energy produced by other organisms. Autotrophic bacteria can be photosynthetic (use H2O) or chemosynthetic (use H2S instead of water as an electron source). Photosynthetic bacteria, especially the cyanobacteria, played a major role in creating our oxygen atmosphere.

Bacteria are also of critical ecological importance, because they are at the base of many food chains. Both autotrophic and heterotrophic forms include species capable of nitrogen fixation. These nitrogen fixers can change atmospheric Nitrogen, N2, into a form that can be used by plants (NH3, Ammonia). Rhizobium is a common genus that forms nodules on the roots of legumes, like the common clover, alfalfa, and soybeans. Nitrogen fixation is essential for agricultural crops. So bacteria do some very good things for the planetary ecosystem. Many of our common food products would not exist were it not for bacteria, foods such as yogurt, pickles and most types of cheeses.

Bacteria are also among the most dangerous organisms on planet Earth. Cholera, diphtheria, syphilis, botulism, strep throat, tetanus, scarlet fever, meningitis, toxic shock syndrome, dysentery, and bubonic plague, the Black Death-are only a few of the more memorable diseases caused by bacteria. And ironically, we also owe many of our most effective antibiotics to bacteria: streptomycin, aureomycin, and neomycin, to name a few.

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