Transport of Amino Acids Across Cell Membranes in Plants

Transport of Amino Acids Across Cell Membranes in Plants 

Only limited information is available regarding the transport of amino acids across cell membranes in plants 243, 263). Active carrier-mediated transport of amino acids into plant cells has been demonstrated in a number of different systems. In some cases competition experiments have indicated a single transport system for all amino acids, but in other cases there is evidence to indicate the existence of different transport systems for different groups of amino acids. 

The active transport of amino acids and their accumulation in vacuoles has been demonstrated with carrot slices 16, 17, 33). Active transport of amino acids has been shown for leaf strips of barley (2, 273, 308) and for leaf slices of barley 200). In the latter investigation competition experiments indicated a single carrier system for all amino acids. However, experiments on the uptake of amino acids into barley roots indicated different transport systems for lysine -i- arginine, proline, and methionine, respectively 310). 

A single carrier-mediated system has been found in leaves of Egeria densa 260) and a single uptake system for all a-amino acids has been 

Studies on the uptake of amino acids and oligopeptides into embryos of Hordeum vulgare indicate independent transport systems (97) whilst uptake studies with chloroplasts from Pisum sativum have indicated the presence of two carrier systems, one transporting glycine and aliphatic amino acids (L-alanine, L-leucine, L-isoleucine, and l-valine) and the other transporting L-serine, L-threonine and L-methionine. The carrier systems may not perform active transport but merely facilitated diffussion (244). Other studies have also indicated the possibility of transport of amino acids into chloroplasts (285). 

---

Finally, y-glutamyl peptides may be involved in the transport of amino acids across cell membranes in plants (cf. Section VII. 7.). However, this fact offers no explanation for their occurrence in high concentrations, since the operation of the y-glutamyl cycle in animals does not lead to the accumulation of y-glutamyl peptides. 

Since in mammalian species metals first need to be assimilated from dietary sources in the intestinal tract and subsequently transported to the cells of the different organs of the body through the bloodstream, we will restrict ourselves in this section to the transport of metal ions across the enterocytes of the upper part of the small intestine (essentially the duodenum), where essentially all of the uptake of dietary constituents, whether they be metal ions, carbohydrates, fats, amino acids, vitamins, etc., takes place. We will then briefly review the mechanisms by which metal ions are transported across the plasma membrane of mammalian cells and enter the cytoplasm, as we did for bacteria, fungi and plants. The specific molecules involved in extracellular metal ion transport in the circulation will be dealt with in Chapter 8. 

---