As we have seen in preceding chapters, the plastid plays a major role in the plant cell in carrying out photosynthesis and enabling the plant to increase in biomass as a result of carbon dioxide assimilation. In addition, the plastid also plays a crucial role in carrying out a variety of metabolic processes, which give rise to a myriad of different molecules, which are used both inside the plastid or are exported into the cytosol. As a result, the original endosymbiont that was taken up by an early eukaryotic cell has become so important to the functioning of the modern-day plant cell that it is generally considered that cells lacking plastids are non-functional and not viable. Thus the plastid is a prerequisite for plant cell function.
Furthermore, it is important to realise that a significant proportion of the plastids resident in the cells of a higher plant are not photosynthetic chloroplasts but other non-photosynthetic types such as leucoplasts, amyloplasts, root plastids or chromoplasts and that these plastids carry out many critical parts of cellular metabolism in these non-photosynthetic tissues which are essential to cell function. These plastids are generally termed non-green plastids.
Research on a diverse range of plastid metabolism has generated a huge amount of information about plastid biochemical pathways, the details of many of which are beyond the scope of this book. In this chapter, therefore, we will consider an overview of the main biochemical pathways that occur in plastids, especially those associated with the chloroplast.