INTERNAL ORGANELLES

036158 Cell contains 5 chloroplasts (chromatophores). 036159 Cell contains 6 chloroplasts (chromotophores). 

036187 Stigma are placed in anterior (end that leads while 

036152 Cell has PBB-complex (polar basal body-complex). 

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Calcium, in contrast to magnesium, does not have an important function as an enzyme activator, in accord with its different distribution. It is very important, however, in the control and triggering of biological processes such as muscle contraction and the release of various chemicals, including hormones, defence chemicals and neurotransmitters. This occurs when, in response to some stimulus, the normal selectivity of the cell membrane or the membranes of internal organelles breaks down, and calcium ions are allowed to enter the cell. These bind to specific sites and trigger certain reactions. 

Eukaryotic cells have a membrane-bound nucleus and a number of other membrane-bound subcellular (internal) organelles, each of which has a specific function. 

The first evidence for the arrangement of the lipid molecules on the cell surface came from surface area experiments in the 1920s. Mammalian erythrocytes of uniform size and shape and containing no internal organelles or nucleus were used in these experiments. The only lipid in these samples, therefore, came from the plasma membrane. A comparison of the surface area of a lipid monolayer with the surface area of the erythrocytes from which the lipid was extracted indicated that sufficient lipid was present to make a layer two molecules thick around each cell. It was proposed that the lipid molecules of cell membranes occur as a bilayer. 

A cell cannot divide or enlarge unless it makes sufficient amounts of additional membranes to accommodate the expanded area of its outer surface and internal organelles. Thus the generation of new cell membranes is as fundamentally important to the life of a cell as is protein synthesis or DNA replication. Although the protein components of biomembranes are critical to their biological functions, the basic structural and physical properties of membranes are determined by their lipid components—principally phospholipids, sphingolipids, and sterols such as cholesterol (Table 18-1). Cells must be able to synthesize or import these molecules to form membranes. 

Membrane A sheet structure surrounding individual cells and also their internal organelles. 

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