The Levels of Organization is a term used in basic biology-3, anatomy, and physiology. It is used with five basic concepts, the smallest the cell. Many specialized cells make up a tissue, many tissues an organ, many organs an organ system, and finally, many organ systems an organism. In bizarre cases microorganism(microorganism) and superorganism, only one cell or many organisms may occur within a single species.

• peroxisome
• ventricle-2

Other less common or varying structures are listed in theMain article: organelle

A tissue is a collection of specialized cells that perform a specific function. The four types of tissues are muscle, nervous-tissue-1, epithelium-1, and connective-tissue. When all these tissue join together they form an organ

An organ is a group of tissues that perform a specific function or groups of functions. Animal organs include the brain, the lungs, the liver, and the stomach and some plant organs are leafs, roots, and the stem. Your senses are also organs- your ears, tongue, eyes, nose, and skin are sense. Organs inside an animals body are often called internal organs or more scientifically viscera (pl. viscus). Plant organs are split into both vegetative (root, stem, etc.) and reproductive organs (flower, seed, etc.). A list of human organs is provided below in alphabetical order:

• Arteries
• Bronchi
• Ears
• Heart
• Lungs
• Muscles
• Skin
• Tongue

An organ system is a collection of organs that perform a specific function- the circulatory system or digestive system for example. Several organ systems are present in humans and other creatures have different kinds of systems. Plants have circulatory-systemand sharkhave a kind of "electromagnetismsystem". An organ system cannot live by itself and is dependent on other organ systems to form an organism. Here is a list of human organ systems and the organs they are made up of:

• circulatory-system: the heart, blood, and blood vessels
• digestion: the salivary glands, esophagus, stomach, liver, gallbladder, pancreas, intestines, rectum, and anus
• endocrine-system: the hypothalamus, pituitary gland, pineal body, thyroid, parathyroids, and adrenals
• integumentary-system: the skin, hair, and nails
• lymphatic-system: the leukocytes, tonsils, adenoids, thymus, and spleen
• muscular-system: the muscles
• nervous-system: the nerves, brain, spinal cord, and peripheral nerves
• reproductive-system: the ovaries, fallopian tubes, uterus, vagina, mammary glands, testes, vas deferens, seminal vesicles, prostate, and penis
• respiratory-system: the pharynx, larynx, trachea, bronchi, lungs, and diaphragm
• skeleton-1: the bones, cartilage, ligaments, and tendons
• urinary-system: the kidneys, ureters, bladder, and urethra

Organ systems may be closely intertwined and called things like the human-musculoskeletal-systemor neuroendocrinology.

An organism is made up of cells and are either unicellular or multicellular. The fifth level of organization is referring to multicellular organisms. Organism is also the largest level of organization known- it is highly likely that superorganism will be accepted as the sixth level of organization. An organism is a type of living thing that is made of cells and is close enough- genetically- to be considered a species. Millions of organisms are currently known, from parameciumto ourselves. All generally known or excepted organisms fit into six monarchyor regnum: Animalia (stuffed-animal), Plantae (plant), Protista (protist), Fungi (fungus), archaeon(or Archaebacteria), and bacteria. virusand virus-classificationare not generally considered organisms. Scientists are currently looking into the creation of artificial-lifeand cybornetics and may end up looking towards the levels of organization for guidance and research.

Depending on how you measure them - and what you mean by 'alive' - the smallest thing is either a bacterium, a virus or a bit of mobile DNA. And the biggest is either a tree, a fungu, or the entire planet!

Boo! That's cheating. I mean, bacteria and viruses aren't really alive, are they? They're just kind of... well... there.

That depends on how you look at it. Biologists will tell you that bacteria - at the very least - are definitely alive. And some might even go as far as saying viruses are alive too.

But I was thinking of a tiny mouse or a flea or something. You know - something that actually moves around. Eats things. Lives.

But bacteria do do all those things. Even though they don't have legs, many can contract tiny muscle-like rods and fibres inside their bodies to squidge around and look for food. Some even have little protein propellers called flagella, which drive them through liquids like a motorboat through water. And although they don't have mouths, they can certainly eat. Most absorb nutrients (which could be anything from sugars to metals), digest them using proteins called enzymes, and use them for energy or to build things inside their bodies. And, if you think about it, that's just what other living things do when they eat. Including us. We just eat and digest things in a more complicated way than bacteria do. Besides that - living things aren't identified by their ability to move around and eat things.

They're not? How do you know if something is alive or not, then?

Well, it's more to do with whether or not the thing organizes itself, and more importantly, reproducesand maintains itself so that it (and it's descendants) can go on 'living'. Bacteria definitely do this, since they absorb nutrients, cope their own DNA and reproduce by splitting in two, producing bacterial 'daughters' that will go on to do the same. Viruses copy their DNA, assemble themselves and reproduce too. And if you want to go even further, there are tiny strings of mobile DNA called transposons (or 'jumping genes') which insert themselves into a cell's DNA, copy themselves and 'jump' out again to insert themselves elsewhere. In a way, they're also orgnanizing, reproducing and maintaining themselves.

So are they alive, then?

Well, sort of. But since viruses and transposons can't do all this alone (they have to hijack the copying machinery inside the living cells they invade), many biologists don't consider them to be 'alive' at all. A few say they are, and perhaps most say they contains elements of living and non-living things. So they're kind of on the edge or boundary of life as well know it. So the safe bet for the smallest things alive is bacteria, which are - in any case - waaaay smaller than mice or fleas.

Like how small?

The average mouse is about 12cm long, including the tail, and the average flea is about 3mm across. Bacteria come in a range of shapes and sizes, but a typical one (like E. coli, a bacterium that lives in your guts) is about 2 micrometres (two millionths of a metre, or two thousandths of a millimetre) across. To give you some idea of how small that is - you could lay about a thousand of them end to end across the head of a pin. Viruses and transposons are even smaller - 100-200 nanometres (or billionths of a metre) long. But we're disqualifying them here and crowning bacteria as the world's tiniest life forms. At least for now.

All right - so that's the small stuff sorted. What about the biggest things alive? Don't dinosaurs get a look in?

Well, some of them were pretty big, but you did say the biggest things alive...

Fine - whales, then. Aren't they bigger than trees and funguses?

Fungi.

Whatever.

That depends on the whale, the tree and the fungus. At over 34m long, the blue whale is the largest animal alive today. But, if we're talking about the largest living things, then some trees are way bigger than that. Giant sequoia trees grow up to 90m tall, up to 9m wide, and weigh more than sixteen blue whales. And that's just if you're looking at individualtrees.

What do you mean by that?

Some trees, like quaking aspen trees, can clone themselves. They grow in clumps or stands that share the same roots. Since all the bodies or trunks in the clump are identical, you could say that they're all part of one big tree-body. If so, then one quaking aspen clump covers over 170,000 square metres and weighs nearly 6,000 tonnes - about the same weight as a giant sequoia tree, but much larger in volume.

And what's with the fungus?

Just like the aspen, some fungi clone themselves and grow to cover enormous areas of land. And since it's not clear where one fungal body ends and another begins, you could say that these are all one living thing too. One colony of honey mushroom, which grows in North America, has been found to weigh about 540 tonnes - or about one tenth as much as a giant sequoia. But the fungus covers almost 9 square kilometres of land, making it the biggest living thing in terms of how much space it takes up.

Is that the biggest, then?

Maybe. But one or two scientists have suggested that you could even think of the entire Earth - including it's soil, atmosphere, oceans and all the things living in them - as one huge living thing or superorganism. If that's the case, then the largest living thing is about 12,800km wide and has a mass of about 5.5 billion trillion tonnes.

So the biggest living thing in the world is... the world?

You could see it that way, yes.

It must be pretty lonely, then.

Maybe not - there could be other living worlds out there in Space waiting for it. Perhaps we can help introduce them one day...