Cells and Organelles

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-

Organelle Functions
Fig. 6.8. Prokaryotic and eukaryotic cells on the same scale (modified after de Duve 1991)

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.

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  • nicholas
    How eukaryotic cell function?
    8 years ago
  • aman aziz
    How are eukaryotic and prokaryotic flagella different?
    8 years ago
  • Marguerite
    What are the cell organelles functions?
    8 years ago

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