Microscopic Life
Cells
Just as atoms are the most basic building block of all matter, cells, which are built from molecules of atoms, are the most basic unit of a living thing, and they come in a vast variety. The average human body is made up of trillions upon trillions of over 200 different types of cells. Groups of cells that work together in an organism are called tissues. Groups of tissues make up organs (heart, liver) and the organs make up systems that serve a purpose for the organism (digesting food, circulating oxygen). Only biotic creatures have cells, incredible tiny cretures do so much amazing jobs, like busy little factories cranking out work according to set rules.
Because they are so tiny, there are far more single-celled organisms (bacteria, protists, etc.) than multicellular organisms, like gerbils or banana trees. In fact, microbes are the simplest and most successful organisms on the Earth. For tiny creatures, gravity has little influence, oxygen and nutrients simply pass in and out, and no complicated body systems are needed. Although there are more than 500,000 different kinds of single-celled orgnaisms, because they are so small we usually do not detect them with our unaided senses and are unaware of them.
All cells are either prokaryotic or eukaryotic. In the Kingdoms of Life, only archaeabacteia and bacteria have prokaryotic cells. This is because many of them are among the most ancient life forms. Prokaryotes (Greek: πρό- , pro- before, καρυό, karuon, nut or kernel) existed before nuclei evolved. Their structure is very simple with fewer and less organized internal structures than eukaryotic cells.
Figure 2: Structure of eukaryotic and prokaryotic. Note that eukaryotic cells often contain many more structures than those shown in this illustration.
Eukaryotic comes from the Greek ευ- “well, good, proper” and καρυό, karuon, (still meaning kernel), meaning that they have a well-formed, good or proper nugget inside them (nucleus). Eukaryotic cells have well defined parts, including nuclei (nucleus) and other internal structures called organelles.
While cells and atoms both have a nucleus, they are not the same structure! The meaning of nucleus from Latin is nucula, "little nut.” Many things are referred to as nuclei, including the solid part of the head of a comet. A nucleus is a central, very important part of an object, the basis for it functioning. In atoms, the nucleus is protons and neutrons; in cells, it contains the organism’s genetic material which determines the functioning of the cell.
Organelles
Within many cells are organelles (Latin: organelle or “little tool”) that do many jobs. One can see that organ, organelle, and organism are related words. Plant, animal, fungi and microbes are built from cells which have some organelles in common and some differences.
A cell can be envisioned as a water balloon with filled with fruit cocktail jello instead of water. The balloon parallels the cell membrane, the jello is like the cytoplasm, and the fruit are the different organelles.
The surface of the cell membrane gives the cell shape and is covered with pores, channels and molecules which regulate the flow of food molecules into the cell and waste molecules out. Cytoplasm is a jelly-like substance which surrounds and protects the organelles. Movement can occur within cytoplasm. Cell membranes and cytoplasm are common to both prokaryotic and eukaryotic cells.
Within all eukaryotic cells there is a nucleus which contains the genetic material that directs the functioning of the organism. It contains chromosomes which are made up of the DNA and contains genes which determine the characteristics of an organism. We have around 24,000 genes. Ribosomes direct the production of proteins according to the information in the DNA. Lysomes make enzymes to dispose of cellular garbage while the endoplasmic reticulum moves molecules through the cell, builds proteins and disarms toxins. Mitocondria convert energy. Mitochondria within most eukaryotic cells comes from the mother’s egg, as sperm contains no cytoplasm, hence no mitochondria. Mitochondrial DNA is being used to map patterns of migration of human ancestors.
Scientists believe that ancient life evolved with organelles initially being independent organisms which over long spans of time developed symbiotic relationships and eventually fused to form mutually beneficial cell systems.
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