The Parts

In a hierarchical order the parts of plants are molecules, membranes, organelles, cells, tissues, and organs. Macro-molecules such as polynucleic acids, proteins, polysac-charides (carbohydrates), and lipids may have both structural and functional roles. Biological membranes composed of lipids and proteins (lipoprotein membranes) border the living cells at their surface (the plasma membrane) and separate and conceal various compartments inside the cells, for example the central cell sap vacuole (the tonoplast) typical of plant cells. Important organelles within cells are the mitochondria and the chloroplasts. The plant cells are surrounded by cell walls composed of polysaccharides, most importantly cellulose. Individual cells can already be independent autotrophic organisms, such as prokaryotic photosynthetically active bacteria and cyanobacteria, which as endosymbionts also have become the evolutionary precursors of chloroplasts, and eukaryo-tic unicellular algae. In the pluricellular algae, bryophytes, and vascular plants, many cells build up tissues, different tissues form organs and various organs, such as roots, stems, leaves, and flowers make up the whole vascular plant. Eukaryotic plant cells have a nucleus with chromosomes where the central genome is located, but they have two additional genomes in the two organelles, the mitochondria and the chloroplasts, which as original endosymbionts in the phylogenetic history of the eukaryotic cells have retained their own deoxyribo-nucleic acid (DNA) carrying genetic information.

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