Chloroplast: Definition, Structure, and Function

Chloroplasts are organelles specializing in the conversion of radiant energy to chemical energy. The chloroplast is involved in photosynthesis and consequently, cells that contain chloroplasts are autotrophic, which means that they are able to make their own food from inorganic molecules by using the light energy of sunlight. The chloroplast converts the radiant energy of the sun into chemical energy by producing organic matter from carbon dioxide and water.

Distribution

The chloroplasts remain distributed homogeneously in the cytoplasm of plant cells. But in certain cells, the chloroplasts become concentrated around the nucleus or just beneath the plasma membrane. The chloroplasts have a definite orientation in the cell cytoplasm. Since chloroplasts are motile organelles, they show passive and active movements.

Structure of Chloroplast

What is the Shape of Chloroplast?

Higher plant chloroplasts are generally biconvex or plano-convex. However, in different plant cells, chloroplasts may have various shapes, viz., filamentous, saucer-shaped, spheroid, ovoid, discoid, or club-shaped. They are vesicular and have a colorless center.

What is The Size of Chloroplast?

The size of the chloroplasts varies from species to species. The chloroplasts generally measure 2–3μm in thickness and 5–10μm in diameter (e.g., Chlamydomonas). The chloroplasts of polyploid plant cells are comparatively larger than the chloroplasts of the diploid plant cells. Generally, chloroplasts of plants grown in the shade are larger and contain more chlorophyll than those of plants grown in sunlight.

How Many Chloroplast in the Cell?

The number of the chloroplasts varies from cell to cell and from species to species and is related to the physiological state of the cell, but it usually remains constant for a particular plant cell. The algae usually have a single huge chloroplast. The cells of the higher plants have 20 to 40 chloroplasts.

According to a calculation, the leaf of Ricinus communis contains about 400,000 chloroplasts per square millimeter of surface area. When the number of chloroplasts is inadequate, it is increased by division; when excessive, it is reduced by degeneration.

Ultrastructure

A chloroplast comprises the following three main components:

1. Envelope

The entire chloroplast is bounded by an envelope which is made of a double unit membrane. Across this double membrane, envelope occurs the exchange of molecules between chloroplast and cytosol (cytoplasmic matrix).

Isolated membranes of the envelope of chloroplast lack chlorophyll pigment and cytochromes but have a yellow color due to the presence of small amounts of carotenoids. They contain only 1 to 2% of the total protein of the chloroplast.

2. Stroma

The matrix or stroma fills most of the volume of the chloroplasts and is a kind of gel-fluid phase that surrounds the thylakoids (grana). It contains about 50% of the proteins of the chloroplast, most of which are soluble type.

The stroma also contains ribosomes and DNA molecules (i.e., 80 DNA molecules per chloroplast per cell of Chlamydomonas; 20 to 40 DNA molecules per chloroplast per cell of the leaf of maize), both of which are involved in the synthesis of some of the structural proteins of the chloroplast.

The stroma is the place where CO2 fixation occurs and where the synthesis of sugars, starch, fatty acids, and some proteins takes place.

3. Thylakoids

The thylakoids (thylakoid = sac-like) consists of flattened and closed vesicles arranged as a membranous network. The outer surface of the thylakoid is in contact with the stroma, and its inner surface encloses an intrathylakoid space (the third compartment).

Thylakoids may be stacked like a neat pile of coins, forming grana or they may be unstacked, intergranal, or stromal thylakoids, forming a system of anastomosing tubules that are joined to the grana thylakoids. There may be 40 to 80 grana in the matrix of a chloroplast.

The number of thylakoids per granum may vary from 1 to 50 or more. For example, there may be single thylakoid (e.g., red alga), paired thylakoids (e.g., Chrysophyta), triple thylakoids and multiple thylakoids (e.g., green algae and higher plants).

Molecular Organization of Thylakoids

Molecular organization of the membrane of thylakoids is based on the fluid-mosaic model of the membrane which represents the following main characteristics: fluidity, asymmetry, and economy (i.e., lack of movement in the third dimension).

Lipids represent about 50% of the thylakoid membrane; these include those directly involved in photosynthesis (called functional lipids) such as chlorophylls, carotenoids, and plastoquinones. Structural lipids of thylakoids include glycolipids, sulpholipids, and a few phospholipids. Most of these structural lipids are highly unsaturated which confer to the membrane of thylakoids a high degree of fluidity.

The protein components of thylakoid membrane are represented by 30 to 50 polypeptides which are disposed in the following five major supramolecular complexes, which can be isolated with mild detergent:

1. Photosystem (PS I).

This complex contains a reactive center composed of P700 (Type of pigment which is bleached at the wavelength of 700 nm), several polypeptides, a lower chlorophyll a/b ratio, and β-carotene. It acts as a light trap and is present in unstacked thylakoid membranes. In its light-induced reduction of NADP+ takes place.

2. Photosystem II (PS II).

This complex comprises two intrinsic proteins that bind to the reaction center of chlorophyll P680 (The pigment that bleaches when absorbing light at 680 nm). It contains a high ratio of chlorophyll a/b and β- carotene. Frequently, the PS IIs are associated with the light-harvesting complex and are involved in the light-induced release of O2 from H2O (i.e., photolysis of water). PS II works as a light trap in photosynthesis and is mainly present in the stacked thylakoid membranes of grana.

3. Cytochrome b/f.

This complex contains one cytochrome F, two cytochromes of b 563, one FeS center, and a polypeptide. It is uniformly distributed in the grana and acts as the electron carrier.

These three complexes are related to electron transport and are linked by mobile electron carriers (i.e., plastoquinone, plastocyanin, and ferredoxin). Electron transport through PS II and PS I finally results in the reduction of the coenzyme NADP+. Simultaneously, the transfer of protons from the outside to the inside of the thylakoid membrane occurs.

4. ATP synthetase.

As in mitochondria, this complex consists of a CF0 hydrophobic portion, a proteolipid that makes a proton channel, and a CF1 (or coupling factor one) that synthesizes ATP from ADP and Pi, using the proton gradient provided by the electron transport. ATP synthetase complexes are located in stacked membranes (grana).

5. Light harvesting complex (LHC).

The main function of the LH complex is to capture solar energy. It contains two main polypeptides and both chlorophyll a and b. LH complex is mainly associated with PS II, but may also be associated with PS I. LHC is localized in stacked membranes and lacks photochemical activity.

Mutation and chloroplast structure.

The organization of chloroplasts and other plastids is often modified due to mutation. D. Von Wettstein (1956) reported that the plastids of normal barley plants have a well-organized system of grana and stroma.

But the plastids of an albino mutant of barley, fail to develop beyond a particular stage and there occurs no differentiation of grana and stroma. Further, the plastids of a yellow-green mutant of barley develop somewhat further than plastids of an albino plant.

Comparison of Chloroplasts and Mitochondria

Chloroplasts carry out their energy inter-conversions by chemiosmotic mechanisms in much the same way that mitochondria do and they are organized on the same principles. Thus, each chloroplast contains three distinct membranes which define three separate internal compartments—the intermembrane space, the stroma, and the thylakoid space.

The thylakoid membrane contains all the energy generating systems of chloroplasts. Like the mitochondria, chloroplasts have a highly permeable outer membrane; a much less permeable inner membrane, in which special carrier or transport proteins are embedded; and a narrow intermembrane space in between.

The inner membrane surrounds a large space called the stroma, which is analogous to the mitochondrial matrix and contains various enzymes, ribosomes, RNAs, and DNA. However, there is an important difference between the two: the inner membrane of the chloroplast is not folded into cristae and does not contain an electron-transport chain.

Instead, the photosynthetic light-absorbing system, the electron-transport chain, and an ATP synthetase are all contained in a third distinct membrane that forms a set of flattened disc-like sacs, the thylakoids.

In a general way, one might view the chloroplast as a greatly enlarged mitochondria in which the cristae are converted into a series of interconnected submitochondrial particles in the matrix space. The knobbed ends of the chloroplast ATP synthetases (F0 – F1 coupling factors), where ATP is made, protrude from the thylakoid membrane into the stroma, just as they protrude into the matrix from the membrane of each mitochondrial crista.

Function of Chloroplast:

It is well evident now that the process of photosynthesis consists of the following two steps:

1. Light reaction.

It is also called Hill reaction, photosynthetic electron transfer reaction, or photochemical reactions. In light reaction solar energy is trapped in the form of chemical energy of ATP and as reducing power in NADPH. During it, oxygen is evolved by photolysis or splitting of water molecules. A light reaction occurs in thylakoid membranes.

2. Dark reaction.

It is also called Calvin reaction, photosynthetic carbon reduction cycle (PCR cycle), carbon-fixation reaction or thermochemical reaction. In dark reaction, the reducing capacity of NADPH and the energy of ATP are utilized in the conversion of carbon dioxide to carbohydrate. Such a process of “carbon fixation” or “CO2-fixation” occurs in the stroma of a chloroplast.

Chloroplast as Semiautonomous Organelle

Like the mitochondria, the chloroplasts have their own DNA, RNAs, and protein synthetic machinery and are semiautonomous in nature.

1. DNA of chloroplast.

Recently the chloroplasts of the algae and higher plants are found to contain DNA molecules. First of all, Ris and Plant (1962) have reported a DNA molecule in the chloroplast of the Chlamydomonas. Later on, a DNA molecule has been reported from the chloroplasts of other algae and higher plants.

In general, chloroplasts have a double-helical DNA circle with an average length of 45 μm (about 135,000 base pairs). The replication of chloroplast DNA has been followed with 3H-thymidine.

Maps of the location of genes (genetic maps) have been made in several chloroplast DNAs with the help of restriction enzymes. The gene for the large subunit of carboxy dismutase enzyme has been fully sequenced and is found to contain 1425 nucleotides.

2. Ribosomes of chloroplasts.

The chloroplasts contain the ribosomes which are smaller than the cytoplasmic ribosomes. The ribosomes of the chloroplast are of 70S type and resemble with then bacterial ribosomes. The ribosomes of the chloroplasts consist of two ribosomal RNAs, 23S rRNA and 16S rRNA.

Lyttleton (1962) has also separated polyribosomes or polysomes from the chloroplast. The chloroplasts also contain aminoacyl-tRNAs, aminoacyl-tRNA synthetases, methionyl-tRNA.

3. Protein synthesis.

According to most recent studies (see Hall, et al., 1974). the DNA of chloroplast codes for chloroplast mRNA, rRNA, tRNA, and ribosomal proteins. It also codes for certain structural proteins of thylakoid membranes.

The synthesis of other chloroplast components as chlorophyll, carotenoids, lipids, and photosynthetic and starch synthesizing enzymes, is controlled by nuclear genes. The 70S ribosomes of Euglena chloroplast are found to require Mg++ for their stability and also have a requirement for N-formy1 methionyl-tRNA in chain initiation protein synthesis like the bacteria.

The protein synthetic mechanism of chloroplasts is inhibited by chloramphenicol like that of mitochondria and bacteria the mode of synthesis of proteins of chloroplasts indicates towards their semiautonomous or symbiotic nature.

For example, of the 30 known thylakoid polypeptides that function in photosynthesis, so far 9 have been demonstrated to be synthesized on chloroplastic ribosomes and 9 are coded by nuclear genes and synthesized on cytoplasmic ribosomes.

Synthesis of carboxydismutase (C Dase) presents a good case of cooperative action of two genetic systems (i.e., chloroplastic and nuclear genetic systems). C Dase comprises 16 subunits: 8 subunits of high molecular weight (55,000 daltons) and 8 subunits of much smaller molecular weight (14,000 daltons).

The large subunit is coded by genes present in chloroplastic DNA, while the small subunit is produced by nuclear genes. The small subunit (called P20) is synthesized as a precursor weighing 20,000 daltons on free ribosomes; it then enters post-translationally into the stroma to be cleaved to attain its final size.

It is postulated that the chloroplastic envelope has receptor sites that recognize the proteins that are to be incorporated into the organelle. The extra sequence (acting as the signal) that is present in P20 is composed of acidic amino acids, in contrast to the hydrophobic ones in the signal sequence of secretory proteins.

After entering the chloroplast the signal sequences are removed by a protease enzyme, which is present in the envelope of a chloroplast, and the small subunit of C Dase is released into the stroma. Thus, chloroplast proteins may be synthesized by three avenues:

  1. by an exclusive chloroplastic mechanism,
  2. by a mechanism involving nuclear genes and chloroplastic ribosomes, and
  3. by nuclear genes and cytoplasmic ribosomes.

Protein transport into chloroplasts resembles transport into mitochondria in many respects:

both occur post-translationally, both require energy, and both utilize hydrophilic amino-terminal signal peptides that are removed after use. However, there is at least one important difference that while mitochondria exploit the electrochemical gradient across their inner membrane to help drive the transport, chloroplasts (which have an electrochemical gradient across their thylakoid but not their inner membrane) appear to employ only ATP hydrolysis to import across their double-membrane outer envelope.

Translocation of proteins into the thylakoid space of chloroplasts requires two signal peptides and two translocation events. The precursor polypeptide contains an amino-terminal chloroplast signal peptide followed immediately by a thylakoid signal peptide.

The chloroplast signal peptide initiates translocation into the stroma through a membrane contact site by a mechanism similar to that used for translocation into the mitochondrial matrix. The signal peptide is then cleaved off, unmasking the thylakoid signal peptide, which initiates translocation across the thylakoid membrane.

Biogenesis of Chloroplast

The chloroplasts never originated de novo. Since the classic work of Schimper and Meyer (1883), it has been accepted that chloroplasts multiply by fission, a process that implies the growth of the daughter organelles.

This is easily observed in the alga Nitella, which contains a single huge chloroplast. In Nitella, a division cycle of 18 hours has been cinematographically recorded for the chloroplast. During the development of the chloroplast, the first structure to appear is the so-called proplastid, which has a double membrane. Development of proplastid into chloroplast takes place in the following steps:

  1. In the presence of light, the inner membrane grows and gives off vesicles into the matrix that are transformed into discs. These intrachloroplastic membranes are the thylakoids which, in certain regions, pile closely to form the grana. In the mature chloroplast, the thylakoids are no longer connected to the inner membrane, but the grana remain united by intergranal thylakoids.
  2. In the absence of light, a reverse sequence of changes takes place. This is the process of etiolation, in which the leaves lose their green pigment and the chloroplast membranes become disorganized. The chloroplasts are transformed into etioplasts, in which there is a paracrystalline arrangement of tubules forming the so-called prolamellar body.

Attached to these bodies are young thylakoid membranes that lack photosynthetic activity. The regular crystal lattice of two prolamellar bodies surrounded by young thylakoid membranes is observed by Osumi et al., (1984). If etiolated plants are re-exposed to light, thylakoids are reformed and the prolamellar material is used for assembly.

The symbiotic origin of the chloroplast.

In certain characteristics, the chloroplasts are comparable with that of a semiautonomous or symbiotic organism living within the plant cells. They divide, grow, and differentiate; they contain circular DNA, ribosomal RNA, messenger RNA and are able to conduct protein synthesis.

By visualizing these similarities between chloroplast and microorganism, it has been suggested that chloroplast might have resulted from a symbiotic relationship between an autotrophic micro-organism, one which is able to transform radiant energy from sunlight and heterotrophic host cell.

The symbiotic origin of the chloroplast appears very justified but Kirk (1966) has shown that certain important enzymes that are necessary for the development of the chlorophyll and for the photosynthetic mechanism are synthesized according to the codes of the nuclear DNA. There still exists certain doubt about the symbiotic origin of the chloroplast.

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