Plastid - Cell Biology PDF

# CH 11. PLASTIDS ## I. GENERAL DESCRIPTION OF PLASTIDS In higher plants and algae, there is a family of organelles named plastids comprising several types (e.g. chloroplast, amyloplast, chromoplast...). Plastids are all characterized by being bounded by two membranes that enclose a stroma. Most o...

# CH 11. PLASTIDS ## I. GENERAL DESCRIPTION OF PLASTIDS In higher plants and algae, there is a family of organelles named plastids comprising several types (e.g. chloroplast, amyloplast, chromoplast...). Plastids are all characterized by being bounded by two membranes that enclose a stroma. Most of these organelles are easily seen by light microscopy; however, their ultrastructure is revealed by electron microscopy only. In the stroma, there are several copies of circular DNA, ribosomes, tRNA and all enzymes and actors of DNA replication and gene expression (transcription and translation). In addition, there are membrane-bounded sacks which contain photosynthetic chain and light absorbing pigments. Plastids, similarly to mitochondria, proliferate by division of previously existing organelles after duplication of their content. **Etioplast**, **chloroplast**, **chromoplast**, **proteoplast** and **amyloplast** are all plastids that develop from a common proplastid but have different ultrastructures and functions. Proplastids are small (1 µm in diameter), colorless organelles, containing irregular membranous structures that correspond to primitive thylakoids. Proplastids which are found in young and dividing plant cells (meristem) are delimited by two membranes, the inner one is at the origin of the thylakoids membrane. Differentiation of proplastids into either one of the diverse plastid types depends on many factors including light intensity. For instance, **etioplasts** are prevalent in leaves of plants that grow in the dark and contain yellow pigment named **protochlorophyll** which is a chlorophyll precursor. Their shape is ellipsoid and do not contain genuine thylakoids, instead, they have a **prolamellar body** and certain flattened vesicles, considered as primary thylakoids. Etioplasts develop into chloroplasts upon exposure to suitable light intensity. **Leucoplastes** are simply enlarged proplastids found in internal tissues as well as in epidermis cells. **They play a main role in terms of storage**. An example of leucoplasts is the **amyloplast** which are abundant in storing tissues such as roots and play an important role in the response of plants to gravity (determination of growth direction). Amyloplast ultrastructure is almost destroyed by the storage of large starch granules (Figure 11.2). What remains is the outer membrane that delimits the stroma which contains several starch granules composed of concentric layers of starch. Even the outer membrane may be ruptured when the starch granules get larger in certain plant tissues such as potato tuber. Other less frequent leukoplasts have been described such as **elaioplasts** (containing oil), and **proteoplasts** (containing proteins). **Chromoplasts** are elongate irregular structures containing **carotenoids** and are responsible for the yellow, red and orange colors of certain plant organs such as roots, petals and fruits. For instance, the carrot roots orange color is due to orange chemicals, carotenoids (e.g. ẞ-carotene), that are present in chromoplasts. Tomato color is also due to chromoplasts that are rich in a specific carotenoid named **lycopene** which belongs to terpenes (Figure 2.20). The pigments are in form of lipid droplets in the chromoplast stroma (Figure 11.2). ## II. CHLOROPLAST STRUCTURE Chloroplasts are found in green tissues of higher plants, they are especially abundant in leaf mesophyll cells. They are also found in bryophytes (e.g. mosses) and algae. Chloroplasts are rich in chlorophyll pigments which are responsible for the plant green color, but they do contain other types of pigments. Chloroplasts are variable in morphology (size and shape) and number depending on species, on cell type and on physiological conditions such as light intensity. In higher plants, a chloroplast is usually lens-shaped (Figure 11.3), approximately 2 to 4µm wide and 5 to 10µm long, and typically numbering 20 to 40 per cell. Frequently, the simpler plants such as algae contain only one or few chloroplasts that are irregular in shape (e.g. helical and star-like, lamellar). By contrast, vascular plants (flowering plants) have many chloroplasts per cell. Chloroplast may have uneven distribution in the cytoplasm. Moreover, they usually move due to the flow of cytoplasm known as Brownian movement (cyclose). ### 1. Chloroplast membranes Similarly to mitochondria, chloroplasts and plastids are bounded by two membranes. Thickness of each membrane is about 5 nm and the intermembrane space is about 7 to 10 nm. Chloroplast membranes are rich in **galactolipids** (a type of glycolipid). A minor percentage of the chloroplast proteins are found in its outer and inner membranes. Most of them occur in the stroma and the thylakoids. The outer chloroplast membrane (OCM), like the OMM, contains porins which makes it permeable to molecules up to 10 kDa in weight. Of course, many other proteins are present in the OCM. In contrast, the inner chloroplast membrane (ICM) is far less permeable and has different properties since it has different composition. Therefore, it is the ICM which regulates the transport of material between the cytoplasm and the stroma of the organelle. In fact, the ICM contains diverse transporter types responsible for transport of diverse organic and mineral molecules inwardly and outwardly. ### 2. Stroma The stroma contains a complex system of membranes whose structure, organization and density differ depending on species, cell type and physiological condition (Figure 11.3). It consists of lamellae that are suspended in a granular fluid matrix named stroma. The stroma contains a variety of particles such as starch granules resulting from polymerization of glucose produced by photosynthesis. There are also lipid droplets combined with many protein types (named plastoglobules or plastidial globules, actively involved in thylakoid function from biogenesis to senescence). There are in the stroma several copies of circular DNA and all the enzymatic equipment for replication and gene expression such as tRNA and ribosomes that may be free and attached to thylakoid membrane. In addition, there is the **Rubisco** (ribulose bisphosphate carboxylase) complex which is the most abundant enzyme on earth. As suggested by its name, Rubisco catalyzes CO2 fixation. It catalyzes a key step in photosynthesis. Similarly to mitochondria, not all chloroplast proteins are encoded by the chloroplast genes. Actually, up to 100 proteins are encoded by chloroplast genes, and nuclear genes encode all of the other chloroplast proteins which are delivered post-translationally to the six chloroplast subcompartments. In fact, the chloroplast has six subcompartments which are: the OCM, the ICM, the intermembrane space, the stroma, the thylakoid membrane and the thylakoid lumen. ### 3. Import of proteins# Import of chloroplast proteins resembles import of mitochondrial proteins in many respects. There are translocation complexes in both OCM and ICM named TOC and TIC complex (translocase), respectively (Figure 11.5). In addition, chaperones are involved in the import process as well as in the folding process of the polypeptide once it arrives to the stroma. With respect to the polypeptide itself, it must contain an N-terminal sequence named transit peptide (address code) which targets the protein to chloroplast, and more precisely to the right subcompartment. ### 4. Thylakoids The membrane system inside chloroplasts of higher plants consists of disk-shaped sacks called thylakoids (Figure 11.4). It is differently organized in lower plants and protists (Figure 11.3). The thylakoids are delimited by a lipid bilayers containing 40% lipids (mainly glycolipids including monogalactosyldiglyceride, few phospholipids), 50% proteins (ATP-synthetases, electron and proton transporters, proteins associated with pigments, ...). Thylakoids are arranged in orderly piles called grana (plural of granum which is one stack). Each pile has a diameter of about 300 to 600 nm and resembles to a stack of coins. A typical chloroplast has between 40 to 60 grana, each one consists of 2 to 100 thylakoids. Certain thylakoids, called stroma thylakoids, extend laterally and interconnect the grana to one another. Thylakoid membrane is a lipid bilayer that contains little amount of phospholipids and high amount of a specific glycolipids which is the monogalactosyl diacylglycerol. The two fatty acids of this molecule are unsaturated so that the membrane is fluid enough to allow interaction between photosynthetic chain elements. Among the proteins of this membrane there is the ATP synthase complex (cF1 at the stromal face) coupled to cF0 channel which is intrinsic. This organization of ATP synthase is similar to that in the IMM. The internal space of a thylakoid is named lumen. The photosynthetic chain35 is a metabolic chain present in the thylakoid membranes (Figure 11.7) which also contain light-absorbing pigments named carotenoids (orange) and xanthophylls (yellow)36. A large number of distinct chlorophylls 35 Diverse elements of the photosynthetic chain are targets for development of electron transport inhibitors that are destined to be used as herbicides. 36 In certain algae, carotenoids and xanthophylls are more abundant than chlorophylls. As a result, chloroplast color is brown or red. have been identified in a variety of different plants. However, the structure of all chlorophyll is basically the same (Figure 11.6). It consists of a magnesium-containing green porphyrin ring (distinct from the iron-containing red porphyrin of hemoglobin and myoglobin) linked to a phytol tail which is a hydrocarbon tail of about 18 carbon atoms. Phytol group is anchored in the lipid bilayer. All chlorophylls have this basic structure but differ by the side groups on the porphyrin ring. Chlorophyll pigments absorb light because of their alternating double bonds that result in an electronic cloud (electron delocalization) at the edge. However, because of their electronic properties, chlorophylls do not absorb all wavelengths (Figure 11.8). Actually, they absorb blue and red light (400 to 500 nm and 650 to 700 nm), and each chlorophyll is characterized by a specific absorption peak because of its specific chemical groups. All chlorophylls reflect green light (500 to 550 nm) which makes plants green. Moreover, another class of pigments named **carotenoids** is found in the thylakoid membranes of chloroplasts and serve as secondary light collectors. Carotenoids belong to terpenes (see Ch 2. VI. 2. ) and absorb light in the blue and green (400 to 550 nm, Figure 11.8). In addition to absorption of light energy, carotenoids absorb the excess of energy in order to prevent formation of oxygen singlet that destroys biological molecules 37. Clearly, diversity of light-absorbing pigments helps increasing photosynthesis efficiency over the entire visible spectrum. It must be kept in mind that pigments are not free in the thylakoid membrane; rather, they are complexed with many proteins such as photosynthetic systems (PSI and PSII). ## III. CHLOROPLAST FUNCTIONS# ### a. Autotrophs and heterotrophs The organisms that have chloroplasts are **photoautotrophs** since they use light energy and mineral compounds (H2O, CO2, N2,...) to produce their own organic compounds. By contrast, **chemoautotrophs** do not have chloroplast, do not use light energy, they use 37 Algal cells that lack carotenoids (by mutation) are highly sensitive to aerobic conditions energy of mineral compounds (e.g. H2S, nitrites, ammonia) to produce their organic compounds. All chemoautotrophs are prokaryotes and have little contribution to biomass formation on earth. In order to live, heterotrophs require, in addition to the mineral molecules, organic compounds produced by autotrophs which are the beginning of the nutrition chain. It is worth mentioning here that prokaryotic photoautotrophs do not have chloroplasts, instead, they have lamellar structures that contain photosynthesis enzymatic system (Figure 3.5). ### b. Overview of photosynthesis Chloroplasts carry out diverse functions and store many different compounds such as carbohydrates and lipids. However, their main function is absorption of light energy, fixation of CO2, and production of O2 and carbohydrates. Production of O2 by chloroplasts was discovered by Engelman (1881). He discovered that aerobic bacteria gather around illuminated alga (spyrogyra) near its chloroplast in order to use the produced O2. The overall reaction called photosynthesis is the reverse of cell respiration in mitochondria (Figure 11.9) and is as follows: 6 CO2 + 6 H2O → C6H12O6 + 6 02 light energy Similarly to cell respiration, the reactions of photosynthesis are oxidation-reduction reactions induced by light energy which displaces electrons and protons from H2O38 to CO2. The latter is therefore reduced into carbohydrates (Figure 11.9). Thus, light energy is converted into chemical energy and stored as carbohydrates. The O2 emanating from photosynthesis becomes the ultimate oxidizing agent for cellular metabolic reactions, especially in mitochondria. It is worth mentioning here that glucose is utilized by plant cells in order to produce lipids, proteins and nucleic acids needed by the cell. Photosynthesis is divided into two phases: the light-dependent reactions and the light independent reactions (dark reactions), Figure 11.10. The first phase occurs in the thylakoid while the second one takes place in the stroma. Light-dependent reactions are carried out in the thylakoid by the photosynthetic chain elements (PSII, plastoquinones, cytochromes, PSI) and are responsible for photolysis. Photolysis is the splitting 38 Another form of photosynthesis displaces e- and H+ from H2S as follow: CO2 + 2 H2S + light → CH2O + H2O+ 2S of H2O from the thylakoid lumen (due to photon energy) into O2, protons and electrons (NADP). Therefore, a part of light energy is converted into reduced coenzymes (NADPH). Simultaneously, like the respiratory chain, there is generation of proton gradient in the thylakoid lumen. Thus, a part of the captured light energy is converted into proton gradient that serves to produce ATP by ATP synthase (cF0-cF1 complexes) exposed on the stroma face in the thylakoid membrane. Production of ATP by use of proton gradient is named photophosphorylation since the gradient derives from light energy. The produced ATP and NADPH are utilized during light-independent reactions in order to reduce CO2 into glucose (Figure 11.10) through reactions of the Calvin cycle. Note that ATP generated by the light-dependent reactions is utilized in the chloroplast. The other cell compartments have their ATP supply from cell respiration and oxidative phosphorylation that permanently occur in the mitochondria. ### c. Other undesired functions Photorespiration is an undesired function performed by the chloroplast. It leads to O2 consumption and CO2 release by chloroplast, which is considered as waste of energy. Photorespiration is the result of non specificity of Rubisco with respect to O2 and CO2 and other gases. Actually, the enzyme has no preference for CO2 over O2 because gases have no binding sites in Rubisco. Instead, Rubisco has RuBP binding site where RuBP is bound and presented to gases to react with. Clearly, the CO2/O2 ratio will determine which gas molecule will react with RuBP. Normally, a CO2 reacts with RuBP, which will ultimately lead to glucose synthesis. If an O2 reacts with RuBP, one CO2 will be produced. Such a situation occurs when the stomata are closed because of the hot and dry climate. In this case, the CO2/O2 ratio decreases significantly thereby making more probable the fixation of O2 on RuBP instead of CO2. Photoinhibition is another undesired abnormal function. It is produced by a very high light intensity that results in over excitation of PSII system of the photosynthesis chain. Overexcitation will lead to generation of strong oxidizing agents (highly toxic oxygen species) that attack the photosystems and other proteins causing their damage. As a result, photosynthesis is blocked and the plant dies. Photoinhibition symptoms appear 39It is a ketose of five carbons (pentose). clearly on the leaves of a plant that used to grow in the shadow and which is suddenly exposed to direct intense sunlight.