Cell theory- all organisms are made of one or more cells and the products of those cells. An organism may be a single cell, such as a bacterium, or many cells organized to function together as in an animal or plant. In many celled organisms, there may be intercellular material made by the cells. All cells carry on life activities. The life activities of a many celled organism are the combined result of the activities of its individual cells. New cells only arrive from other living cells by the process of cell division.
             This is where the DNA is kept and RNA is transcribed. RNA is transported out of the nucleus through the nuclear pores. Proteins needed inside the nucleus are transported in through the nuclear pores. The nucleolus is usually visible as a dark spot in the nucleus (note the dark nucleolus in this electron microscope photo of a nucleus), and is the site of ribosome formation.
             Ribosomes are the sites of protein synthesis, where RNA is translated into protein. Protein synthesis is extremely important to cells, and so large numbers of ribosome's are found throughout cells (often numbering in the hundreds or thousands). Ribosome's exist floating freely in the cytoplasm, and also bound to the endoplasmic reticulum (ER). ER bound to ribosomes is called rough ER because the ribosomes appear as black dots on the ER in electron microscope photos, giving the ER a rough texture. These organelles are quite small, made up of 50 proteins and several long Ranks intricately bound together. Ribosome's have no membrane. Ribosomes disassemble into two subunits when not actively synthesizing protein.
             Mitochondria (singular: mitochondrion) are the sites of aerobic respiration, and generally are the major energy production center in eukaryotes. Mitochondria have two membranes, an inner and an outer, clearly visible in this electron microscope photo of a mitochondrion. Note the reticulations, or many enfolding, of the inner...