PLASTIDS

INTRODUCTION

The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a major organelle found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell. They often contain pigments used in photosynthesis, and the types of pigments present can change or determine the cell's colour. They possess a double-stranded DNA molecule, which is circular, like that of prokaryotes.

PLASTIDS IN PLANTS

Those plastids which contain pigments can carry out photosynthesis. Plastids can also store products like starch and can synthesise fatty acids and terpenes, which can be used for producing energy and as raw material for the synthesis of other molecules. For example, the components of the plant cuticle and its epicuticular wax, are synthesized by the epidermal cells from palmitic acid, which is synthesized in the chloroplasts of the mesophyll tissue. All plastids are derived from proplastids which are present in the meristematic regions of the plant. Proplastids and young chloroplasts commonly divide by binary fission, but more mature chloroplasts also have this capacity.

In plants, plastids may differentiate into several forms, depending upon which function they play in the cell. Undifferentiated plastids (proplastids) may develop into any of the following variants:

Chloroplasts green plastids: for photosynthesis; see also etioplasts, the predecessors of chloroplasts

Chromoplasts coloured plastids: for pigment synthesis and storage

Gerontoplasts: control the dismantling of the photosynthetic apparatus during senescence

Leucoplasts colourless plastids: for monoterpene synthesis; leucoplasts sometimes differentiate into more specialized plastids:Amyloplasts: for starch storage and detecting gravityElaioplasts: for storing fatProteinoplasts: for storing and modifying proteinTannosomes: for synthesizing and producing tannins and polyphenols

Depending on their morphology and function, plastids have the ability to differentiate, or redifferentiate, between these and other forms.

Each plastid creates multiple copies of a circular 75–250 kilobase plastome. The number of genome copies per plastid is variable, ranging from more than 1000 in rapidly dividing cells, which, in general, contain few plastids, to 100 or fewer in mature cells, where plastid divisions have given rise to a large number of plastids. The plastome contains about 100genes encoding ribosomal and transfer ribonucleic acids (rRNAs and tRNAs) as well as proteins involved in photosynthesis and plastid gene transcription and translation. However, these proteins only represent a small fraction of the total protein set-up necessary to build and maintain the structure and function of a particular type of plastid. Plantnuclear genes encode the vast majority of plastid proteins, and the expression of plastid genes and nuclear genes is tightly co-regulated to coordinate proper development of plastids in relation to cell differentiation.

Plastid DNA exists as large protein-DNA complexes associated with the inner envelope membrane and called 'plastid nucleoids'. Each nucleoid particle may contain more than 10 copies of the plastid DNA. The proplastid contains a single nucleoid located in the centre of the plastid. The developing plastid has many nucleoids, localized at the periphery of the plastid, bound to the inner envelope membrane. During the development of proplastids to chloroplasts, and when plastids convert from one type to another, nucleoids change in morphology, size and location within the organelle. The remodelling of nucleoids is believed to occur by modifications to the composition and abundance of nucleoid proteins.

Many plastids, particularly those responsible for photosynthesis, possess numerous internal membrane layers.

In plant cells, long thin protuberances called stromules sometimes form and extend from the main plastid body into the cytosol and interconnect several plastids. Proteins, and presumably smaller molecules, can move within stromules. Most cultured cells that are relatively large compared to other plant cells have very long and abundant stromules that extend to the cell periphery.

CHLOROPLAST Chloroplasts /ˈklɔrəplæsts/ are organelles, specialized subunits, in plant and algal cells. Their main role is to conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, and stores it in the energy storage molecules ATP and NADPH while freeing oxygen from water. They then use the ATP and NADPH to make organic molecules from carbon dioxide in a process known as the Calvin cycle. Chloroplasts carry out a number of other functions, including fatty acid synthesis, much amino acid synthesis, and the immune response in plants.

A chloroplast is one of three types of plastid, characterized by its high concentration of chlorophyll. (The other two types, the leucoplast and the chromoplast, contain little chlorophyll and do not carry out photosynthesis.) Chloroplasts are highly dynamic—they circulate and are moved around within plant cells, and occasionally pinch in two to reproduce. Their behaviour is strongly influenced by environmental factors like light colour and intensity. Chloroplasts, like mitochondria, contain their own DNA, which is thought to be inherited from their ancestor—a photosyntheticcyanobacterium that was engulfed by an early eukaryotic cell. Chloroplasts cannot be made by the plant cell, and must be inherited by each daughter cell during cell division.

With one exception (a member of the genus Paulinella), all chloroplasts can probably be traced back to a single endosymbiotic event (the cyanobacteria being engulfed by the eukaryote). Despite this, chloroplasts can be found in an extremely wide set of organisms, some not even directly related to each other—a consequence of many secondary and even tertiary endosymbiotic events.

The word chloroplast is derived from the Greek words chloros (χλωρός), which means green, andplastes , which means "the one who forms".[1]

Functions (1) Absorption of light energy and conversion of it into biological energy.

(2) Production of NAPDH2 and evolution of oxygen through the process of photosys of water.

(3) Production of ATP by photophosphorylation. NADPH2 and ATP are the assimilatory powers of photosynthesis. Transfer of CO2 obtained from the air to 5 carbon sugar in the stream during dark reaction.

(4) Breaking of 6-carbon atom compound into two molecules of phosphoglyceric acid by the utilization of assimilatory powers.

(5) Conversion of PGA into different sugars and store as stratch. The chloroplast is very important as it is the cooking place for all the green plants. All heterotrophs also depend on plasts for this food.

CHROMOPLAST

Chromoplasts are plastids, heterogeneous organelles responsible for pigment synthesis and storage in specific photosynthetic eukaryotes. It is thought that like all other plastids including chloroplasts and leucoplasts they are descended from symbiotic prokaryotes.

Functions 1.Chromoplast imparts colour and found in some cells of more complex plants.

The three types of plastids are chloroplasts, chromoplasts and leucoplasts.

2.Chromoplasts are coloured plastids other than green and found in coloured parts of plants such as petals of the flower, pericarp of the fruits etc. Due to the presence of carotenoid pigments they are orange, yellow or red in color.

3.The primary functions of chromoplasts are synthesis and accumulation of carotenoid pigments.

4.Though similar to chloroplasts in size, chromoplasts differ significantly in shape, often appear angular.

5.Found in fruits, flowers, roots, and stressed and aging leaves; available in large quantities in ripe tomatoes, carrot tap roots, daffodil flower petals, sweet potato tubers or red peppers.

LEUCOPLAST Leucoplasts  are a category of plastid and as such are organelles found in plant cells. They are non-pigmented, in contrast to other plastids such as the chloroplast.

Lacking pigments, leucoplasts are not green. They are colourless, so they are predictably located in roots and non-photosynthetic tissues of plants. They may become specialized for bulk storage of starch, lipid or protein and are then known as amyloplasts,elaioplasts, or proteinoplasts [also called a leuroplast] respectively. However, in many cell types, leucoplasts do not have a major storage function and are present to provide a wide range of essential biosynthetic functions, including the synthesis of fatty acids, many amino acids, and tetrapyrrole compounds such as haem. In general, leucoplasts are much smaller than chloroplasts and have a variable morphology, often described as amoeboid. Extensive networks of stromules interconnecting leucoplasts have been observed in epidermal cells of roots, hypocotyls, and petals, and in callus and suspension culture cells of tobacco. In some cell types at certain stages of development, leucoplasts are clustered around the nucleus with stromules extending to the cell periphery, as observed for proplastids in the root meristem.

Etioplasts, which are pre-granal, immature chloroplasts but can also be chloroplasts that have been deprived of light, lack active pigment and can be considered leucoplasts. After several minutes exposure to light, etioplasts begin to transform into functioning chloroplasts and cease being leucoplasts.

function 1. Leucoplasts are another type of plastid - the colourless plastids and common to cells of higher plants.

2 .These colourless plastids are involved in the storage of various materials (carbohydrates, fats, oils and proteins), especially starch.

3. Plastids storing carbohydrates are called amyloplasts, plastids storing fats and oils are called elaioplasts and plastids storing protein are called proteinoplasts.

4. If leucoplasts are exposed to light they can develop into chloroplasts and vice versa.

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