Cell Organelles

Lipids contribute to the structures of a variety of membranes in different parts of cells and in different organisms. The composition of these lipids varies considerably depending on the type and function of the membrane.

Eukaryotic cells characteristically contain a range of different cell organelles. All of these are separated from the cytosol (and usually from each other) by membranes. Some organelles such as mitochondria or chloroplasts are surrounded by two distinct membranes whereas others, such as microbodies, have a single membrane only. The relative numbers and detailed structure of most of these organelles vary from cell to cell. Moreover, in some cases, for example chloroplasts, the morphology will change depending on the developmental stage of the cell and will even be affected by the environment and metabolic state of the organelle.

Even in prokaryotes, many species contain a complex internal membrane structure. This is especially true in photosynthetic organisms such as purple or green bacteria and cyanobacteria (Fig. 6.1). Moreover, in some organisms membrane (or

Fig. 6.1 Diagrammatic representation of internal membrane systems of bacteria and cyanobacteria.

organelle) formation can be induced (Table 6.1) -thus providing a convenient experimental system from which to isolate membranes and in which to study membrane biogenesis.

Since organelles and their membranes carry out such a wide range of different functions, it is scarcely surprising that their lipid compositions vary.

Moreover, the percentage of lipid and protein in different membranes can range from 80% lipid in myelin to only about 25% in mitochondria (Table 6.2). Almost all membrane lipids are amphipathic molecules (Section 6.5) such as phospholipids or glycolipids; neutral lipids such as triacylglycerols have little part in membrane structure. In organ-

Table 6.1 Experimental systems for studying membrane induction

Organism

Stimulus

Morphological change

Photosynthetic bacteria

Euglena Yeast

Etiolated plants Animals

Animals

Starvation, light

Starvation, light

Glucose in medium reduced

Light

Fed barbiturates Fed clofibrate

Photosynthetic internal membranes formed

Chloroplasts produced Mitochondria produced

Etioplasts converted to chloroplasts

Liver and endoplasmic reticulum proliferates

Liver peroxisomes proliferate

Table 6.2 Lipid composition of different membranes

Membrane (% total lipid)

Table 6.2 Lipid composition of different membranes

Membrane (% total lipid)

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