Chloroplast

Plastids, Chloroplast Plastids (Gr. plastikos : molded) Term was coined by Haeckel (1886). Major cell organelles found in plant cells and some protists (e.g. Euglena). Carry out photosynthesis, synthesize and store starch and oil contribute products for other biosynthetic pathways. Vary in size, shape, content, and functions. Shapes: Varies widely in shapes. Cup-shaped in Chlamydomonas. Spiral ribbon like Spirogyra Girdle shaped Ulothrix Branched star like Zygnema Reticulate Oedogonium Higher plants spherical, oval or discoidal. Possess a great capacity to differentiate, dedifferentiate, and redifferentiate. On the basis of functions and pigments plastids are of two types: A) Leucoplast B) Chromoplast. All types of plastids are formed from common precursors known as protoplastids. Etioplasts: Plastids for which the development from protoplastids to chloroplast has been arrested by the absence of light. Found only in white or pale yellow etiolated tissues kept in dark. Lack chlorophyll. When exposed to light, begin to develop chloroplast. Gerontoplast: Formed from chloroplast during senescence of foliar tissues. Involves controlled dismantling of photosynthetic apparatus. Degraded thylakoids and stroma proteins provide a significant source of amino acids and nitrogen NUMBER: Varies in number. One in Chlamydomonas and Ulothrix. 1-16 in Spirogyra 20-40 in higher plants Internodal cells of Chara may have several hundred. Size: Also varies Large sized in polyploidy cells and shade plants than diploid and sun plants. Average size 4-6µm. Biogenesis: Chloroplast DNA: Ris and Plaut were first to demonstrate. Cp DNA is double stranded circular in nature. Without histones, 5-methyl cytosine is absent. Replication takes place in a semi conservation way. Size ranges between 120,000 and 150,000 base pairs. Contains some genes for herbicide resistant. Never originate de novo. Multiply by fission. During development first structure to appear is proplastid. Then develop into chloroplast. #chloroplast#