From the discussion of the building materials, we now turn to the assembled product, the cell. All living organisms are made of cells. By enclosing itself in a cell membrane, a basic unit of life defines its identity by separating the outside world from its own self. From the absence or presence of a nucleus, one classifies the cells as either prokaryotic cells or eukaryotic cells, respectively. Figure 6.8 shows some examples of cells, magnified to the same scale. Prokaryotic cells have diameters between 0.1 and 10 |im. Simple prokaryotes such as mycoplasmas possess only a scanty internal structure, while more advanced prokaryotes have a cell wall and a sizeable number of organelles.
Eukaryotic cells possess a nucleus, and with diameters between 10 and 100 | m are much larger than prokaryotic cells. They show a very complicated internal structure, with a large number of different organelles (see Fig. 6.8). Organelles are organ-like subunits in the cells in which specialized processes take place. Typical organelles of eukaryotic cells, as well as their form and function, are listed in Table 6.3. The most conspicuous organelle is the nucleus (Fig. 6.8), where the chromosomes reside and where the transcription to RNA is carried out. It is enclosed by the nuclear envelope, which allows communication with the other cell parts through small openings, the nuclear pores.
Arrays of protein filaments, microtubules, and actin fibers, called the cytoskeleton, give the cell its shape and support, and allow internal movements. Whip-like flagella provide the cell with the means to move through the external fluid medium. Lysosomes contain enzymes which carry out intracellular digestion. Mitochondria are the power plants of the cell. By burning food using oxygen, they produce ATP. Peroxisomes are involved in the lipid metabolism; in the endoplasmic reticulum lipid synthesis and protein formation are carried out; while in the Golgi apparatus the manu-
Table 6.3. Organelles of eukaryotic cells, their form and function
Endoplasmic reticulum Golgi apparatus
Enclosed by envelope Tubes, fibers Tubes, fibers Enclosed by membrane Enclosed by membranes
Enclosed by membrane Enclosed by membrane Enclosed by membrane
Plast ids (only plants) Enclosed by membranes
Archive, DNA, RNA synthesis Internal transport, support Cell movement Digestion
Burns food using oxygen, produces ATP Fat metabolism Lipid and protein synthesis Storage and modification of lipids and proteins Photosynthesis factured proteins and lipids are modified, sorted, and packaged for delivery to other organelles. All these organelles are enclosed by membranes. This and their similar size hints that a long time ago their ancestors had been independent prokaryotic cells, that managed to survive inside the eukary-otic cell by becoming endosymbionts. Endosymbionts are organisms which live in a symbiotic relationship (a relationship with mutual benefits) with their host.
Was this article helpful?
Are You Sick And Tired Of All The Fat-Burning Tricks And Trends That Just Don’t Deliver? Well, Get Set To Discover The Easy, Safe, Fast, And Permanent Way To Mega-Charge Your Metabolism And Lose Excess Fat Once And For All! This Weight Blasting Method Is Easy AND Natural… And Will Give You The Hot Body And Killer Energy Levels You’ve Been Dreaming Of.