Main Organelles of Cytoplasm | Plant Physiology

The following points highlight the nine main organelles of cytoplasm. The Organelles are: 1. Endoplasmic Reticulum 2. Mitochondria 3. Plastids 4. Golgi Apparatus 5. Ribosome 6. Peroxisomes and Glyoxysomes 7. Lysosomes 8. Nucleus 9. Hydrogenosome.

Organelle # 1. Endoplasmic Reticulum:

The cytoplasm of a eukaryotic cell consists of an elaborate network of double membrane system called the ergastoplasm, or endoplasmic reticulum (ER) (Plate 2).

The endoplasmic reticulum is continuous with the outer nuclear membrane and extends to the cell surface and through the primary cells even to the neighbouring cells (Plate 2). The inclusion of the nuclear membrane with the ER provides extensive surface contact between nuclear material and the cell cytoplasm.

When some strands of the ER extend from one cell to the next through plasmodesmata, the nuclei of the two cells may be said to be in a direct contact with each other. Sieve tube element furnishes an excellent example where it is in connection with the companion and phloem parenchyma cells.

The endoplasmic reticulum may be visualized as a sheet-like membrane which divides the cytoplasm into several small compartments. Within these compartments certain enzymes and metabolites may be accumulated or excluded. This compartmentalization is of vital importance to the cell function.

Cells active in protein systhesis are packed with ER while other cells have only a few of them in the form of discontinuous vesicle. The membranes of ER are of two types: granular or smooth. To the former are bound many ribosomes on which protein synthesis occurs while smooth is concerned with the synthesis of cell wall material in differentiating cells.

ER forms a conducting system and is also concerned with the storage, secretion and the transport of proteins synthesized by the cell to the exterior for the extra-cellular use. ER also contains enzymes which play a role in lipid metabolism, their storage in the plastids and their export from the cell. ER acts as an intra-and intercellular channel system for the transport of metabolites within the cell and from cell to cell.

When cells are ultra-centrifuged, the ER with its associated ribosomes breaks up into smaller pieces called microsomes. Mitochondria and Golgi apparatus, appear to have originated from ER.

Functions:

i. Provides supplementary mechanical support to colloidal structure of the cytoplasm

ii. Provides added surface area for the operation of metabolic activities

iii. Concerned with the synthesis of lipids, glycogenolysis, some drugs detoxification, regulation of Ca²⁺availability

iv. Acts as transitory storage compartments of metabolites

v. Involved in formation of peroxisomes

vi. Concerned with synthesis and intracellular transport of many proteins.

Organelle # 2. Mitochondria:

Mitochondria are regarded as the ‘power house’ of the cell. Their main function in a cell is to provide usable energy, in the form of energy-rich compound, adenosine triphosphate (ATP), which is synthesized in them during the respiratory oxidation of the proteins, fats and carbohydrates.

The ATP from mitochondria is dispersed throughout the cell and energy stored in it is readily released and utilized to drive energy-consuming reactions of the cell. Mitochondria abound in a cell, sometimes exceeding 1,000 or more and are in constant motion. They range in size from 0.2 to 7.0 µm in diameter. In shape they are spherical or rod-like (Plate 2).

When seen through the electron microscope mitochondrion is surrounded by a thin double membrane (lipo-protein) which encloses inner matrix (Fig. 3-6). Numerous folds forming partition-like bridges develop from the inner membrane which project deep into the matrix. The projecting folds of the inner membrane are collectively called the cristae.

Spread over the inner surface of the cristae and between the cristae is small particles, each consisting of a stem and a rounded head. These particles are called the inner membrane spheres. The membrane of cristae contains the enzymes of the electron transport system and oxidative phosphorylation.

These enzymes together constitute respiratory assemblies. The inner membrane spheres contain a coupling factor which is necessary for the coupling of phosphorylation to oxidation. The semi-fluid matrix contains the enzymes of the Krebs cycle, β- oxidation of fatty acids, amino acid transferases, etc.

Oxidative phosphorylation (synthesis of ATP) and reactions of Krebs cycle (Fig. 3-7) are dependent upon the double membrane structure of mitochondrion (Fig. 3-6).

Both the processes stop if this structure is lost. The fragments of mitochondria can carry on oxidative phosphorylation partially.

A part of the oxidative enzyme system still remains intact in the membranes of these fragments. So i; is believed that the functional unit of the mitochondria i.e. the enzyme is the same as the structural unit, the membrane. Mitochondria swell and contract as they carry out these processes.

Functions of Mitochondria:

Cell respiration:

Mitochondria are centres of cell respiration, which consists of 4 phases e.g. glycolysis; pyruvic acid oxidation; citric acid cycle and oxidative phosphorylation (Fig. 3-8).

ATP transport:

Cellular respiration results in the accumulation of ATP in the mitochondria. It is squeezed out of this organelle due to internal hydrostatic pressure.

Lipid synthesis:

Mitochondria abound in enzymes which control lecithin and phosphatidyl- ethanolamine synthesis from fatty acids, etc.

Based on several similarities between mitochondria and bacteria, it is believed that they might have evolved from bacteria through symbiotic association of the two. Mitochondria show autonomous replication of chromosome; origin from promitochondrion; origin through budding or division and nuclear origin.

Mitochondria contain DNA (mit-DNA), RNA and RNA-polymerase, mitoribosomes and some enzymes, etc. The DNA resembles bacterial DNA and is usually associated with extra- chromosomal mitochondrial inheritance. In a way they possess their autonomous translational and transcriptional machinery.

Organelle # 3. Plastids:

Different kinds of plastids occur in many plant cells. They are complex, microscopic organelles which are divisible on the basis of coloration and function.

Chromoplasts are variously pigmented and may or may not contain chlorophyll. In comparison leucoplasts are colourless and their chief function is to store starch grains, oil and even protein. The chromoplasts are devoid of chlorophyll and have several types of fat-soluble carotenoid pigments. The latter may be red, yellow or orange. They also contain oils. The chomoplasts are present in corolla, carrot root, etc.

Plastids which contain green pigment chlorophyll are known as chloroplasts. These plastids are photosynthetically active. In addition to chlorophylls, chloroplasts may contain phycocyanin, fucoxanthin, and carotenoids, phycoerythrin, etc. It also contains 20-50% lipids, nucleic acids, etc. The protein function is associated with Fe, Cu, Mn atoms. It is believed that these metals are associated with photosynthetic enzymes.

Chloroplast:

Chloroplasts are typically disc-shaped bodies. The contents of the chloroplast are enclosed in an “envelop” consisting of two membranes with an enclosed space. Inside the envelope there is a colourless matrix or stroma. A cross-section of the chloroplast shows several membranes stacked on top of each other.

These membranes are paired forming stacks of discs. The photosynthetic pigments are confined to the lamellar system of the chloroplast (Fig. 3-9).In the lower plants, the pigments are evenly distributed over the entire surface of the lamellae, while in the higher plants; they are restricted to certain areas of the lamellae.

If these restricted areas are layered one on top of another, the complete stack is known as a granum. Lamellae are thin in the region of stroma and are called stroma lamellae and thick in the region of a granum and are called granum lamellae. In addition to the lamellar system, granules, lipid droplets, starch grains and vesicles may be found in the matrix (Plates 3, 4).

Chloroplasts, like mitochondria, also contain RNA-polymerases. Each chloroplast has 20-60 copies of a circular double-strandard DNA molecule. The amount of DNA in a circle is sufficient to code for 125 proteins or so. It is pertinent to add that this DNA does not code for all the chloroplast proteins.

Consequently chloroplast formation is dependent upon nuclear and chloroplast genomes. Recently, some protein synthesis has been demonstrated in the isolated chloroplast and this provides a proof of its autonomy.

Leucoplast:

Leucoplasts are a class of plastids which do not contain any visible pigment and are chiefly concerned with the synthesis and storage of various types of metabolites.

Leucoplasts are further classified on the basis of stored and synthesized metabolites. For instance, amyloplasts are involved in the synthesis and storage of starch while elaiplasts are concerned with the synthesis and storage of oil.

The third category includes aleuroplasts which are primarily involved in the synthesis and storage of protein granules.

Of the three types stated above, amyloplasts are very important and resemble proplastids.

They possess a few lamellae and on exposure to light turn green after developing thylakoid structures.

Chloroplastids are also capable of synthesizing starch and store it in the stroma region. This is, however, transitory starch. The starch granules in the leucoplasts can be stored for longer durations e.g. in potato tubers.

The starch development is initiated when a particle is formed in the stroma. This particle has several tubuli and thylakoids and is surrounded by additional rings of starch.

It keeps on growing till the amyloplast is filled with starch. Consequently tubuli are pressed against the wall and ultimately the starch granules exceed the amyloplast size. At this stage the wall of the amyloplast breaks and starch granules are clearly visible.

Chromoplast:

These plastids possess coloured pigments other than chlorophyll. They may originate from chloroplast or leucoplast.

An examination of young and mature petals clearly shows such a transition. Carrot roots abound in chromoplastids. During the transformation of chloroplast into chromoplast, yellow coloured droplets called globulins appear. Chromoplasts appear to be the outcome of disintegration of chloroplast or leucoplast.

There are several views regarding the origin of plastids. They may have originated from nuclei or from the pre-existing ones through budding.

There is also a view that plastids are autonomous structures which could change from one form to another. The origin of plastids from the proplastids has also been proposed.

Organelle # 4. Golgi Apparatus (Golgi Complex):

Structure:

Under the electron microscope, the Golgi apparatus consists of two distinct structures, a stack of flattened membrane-bound cisternae known as dictyosomes and several small spherical vesicles which appear clustered around the edges of the cisternae.

Whaley (1959) assumes that the vesicles “pinch off’ from the edges of the cisternae membrane (Fig. 3-10, 10A).

Functions:

i. Take part in the synthesis and distribution of membranes.

ii. Biosynthesis of some polysaccharides and sulphated muco-polysaccharides.

iii. Synthesis of glycol-proteins.

iv. Accept vesicles from ER and process them in specialized organelles e.g. Lysosomes, secretory vesicles.

v. Enzymic processing of some proteins.

Organelle # 5. Ribosome (Microsome):

These are found in every kind of cell and are associated with the ER and floating free in the cytoplasm.

According to Whaley (1960), microsomal fractions of the cytoplasm contain 40-50% of the cell RNA, 15% of the cell protein and about 50% of the cell phospholipid.

The ribosome can be described as a “phospho-lipid-ribonucleoprotein complex”. Ribosomes have an important role in protein synthesis.

They occur in the cytoplasm in groups of two, three, four or five. The groups are called polysomes, ergasomes or ribosomal clusters.

Prokaryotic cells have ribosome of 70 S units whereas eukaryotic cells have 80 S ribosomes.

The organization of RNA and proteins in the ribosomes is not clear. The 70 S ribosome of bacteria consists of two subunits with sedimentation coefficient of 50 S and 30 S, respectively. Ribosomal RNA initially appears in the nucleus. In chloroplast the ribosomes are of 70 S types compared with 80 S cytoplasmic ribosomes (Fig. 3-11).

Figure 3-11 A shows ribosomal cycle showing formation of polyribosome. The two ribosome types are distinguishable in their Mg⁺⁺ requirements stability and response to antibiotics like chloramphenicol. The mechanism of protein synthesis in chloroplast resembles prokaryotic cells.

Functions:

i. Carry out protein synthesis.

ii. Provide enzymes and other factors to catalyse protein synthesis.

iii. Facilitate interaction between mRNA and tRNA, help alighning their Codon-Anticodon. Large subunit is seat of peptidyl transferase, or peptide synthetase which catalyses the formation of peptide bond.

Spherosomes:

These are recognized as tiny vesicles pinched off from the ends of the ER, which expand and develop into bodies called spherosomes.

They are surrounded by a membrane that was originally part of the ER. They are abundant in the cells rich in fats. Their function in lipid synthesis and storage is suggested.

Organelle # 6. Peroxisomes and Glyoxysomes:

These are microbodies which have single membrane and about 1 µm in diameter. They occur in leaf mesophyll cells where they contain catalase together with the enzymes of glycolate pathway.

It is suggested that this pathway brings about the formation of glycine and serine from the non-phosphorylated intermediates of the photosynthetic carbon reduction cycle i.e. glycolate to glycine and serine.

So far these microbodies have failed to show the presence of nucleic acids, ribosomes, etc.

Figure 3-12 shows role of different peroxisome membrane proteins. Fig. 3-12A shows gluconeogenesis and glyoxylate cycle.

Their origin is still speculative though some people regard their production from ER. Catalase breaks down hydrogen peroxide formed during oxidation of glycolate.

Glyoxysomes contain certain enzymes of glyoxalic pathway of fat metabolism. The process occurs during germination of fat-rich seeds.

Functions:

i. In plants involved in Glycolate metabolism during photo-respiration along with Chloroplast and Mitochondria.

ii. Carry out β-oxidation of fatty acids.

iii. Concerned with conversion of fat into carbohydrate, as in seed germination.

iv. Involved in biosynthesis of phospholipids.

Organelle # 7. Lysosomes:

Good amount of biochemical and cytochemical evidence is available to suggest their role in several physiological processes. (Table 3-3).

Matile (1968) has described as many as nine hydrolases in the fraction from maize root tip. Studies of Matile (1974) and Poux (1975) have clearly demonstrated that plant cells contain a wide range of particles, varying in size, enzyme activity and in internal contents (Table 3-3).

It is also proposed that the plant vacuole represents one type of plant lysosome. The origin of lysosomes is not vivid but it is believed that they possibly originate from the endoplasmic reticulum.

Types:

There are four types of lysosomes (Fig. 3-13 A).

Primary lysosomes:

These are small bodies which contain enzymes synthesized by the ribosomes and accumulated in the ER. Subsequently these are passed on to Golgi apparatus and then to lysosomes.

Digestive vacuoles:

These are also called heterophagosomes and are formed by phagocytosis or pinocytosis of foreign bodies by the all (Fig. 3-13 A).

Residual bodies:

These are formed following complete digestion of the material.

Cytolysosomes:

These are called autophagic vacuoles. In such lysosomes part of cell or its components is enclosed inside.

i. Concerned with digestion.

ii. Involved in cellular secretion.

Meristematic cells lack vacuoles or they are small to be easily observed. As the cell develops, small vacuoles appear and fuse to give rise to bigger vacuole (Fig. 3-14).

Vacuoles are rich in solutes, inorganic ions, amino acids, sugars, water soluble pigments and several types of crystals.

The old concept that vacuoles are an inert entity is not all true. They abound in several types of enzymes especially hydrolase. Vacuoles are also rich in potassium, soluble sugars, amides, etc. They play an important role in cell metabolism. Matile (1978) has written an interesting review on biochemistry and function of vacuoles in Ann. Rev. Plant Physiology.

Vacuoles may act as storage compartments and also as lysosomes. In the former state they have variable constituents and their tonoplast has specific properties to bring about membrane transport.

They are also involved in turgor and detoxification. On the other hand, vacuoles abound in several different types of hydrolases and are concerned with intracellular digestion.

The role of compartmentation and their autophagic properties have also been demonstrated in several systems.

Based on these findings, the vacuole should be viewed as an important subcellular organelle which participates in active metabolic process of the cell.

Organelle # 8. Nucleus:

Nucleus is the largest inclusion in most cells discovered in plant cell by Robert Brown in 1831.

It contains most part of genetic material of a cell. It controls or directs the synthesis of the enzymes that catalyze most of the metabolic reactions of the cell.

In a young cell, the nucleus appears as a spherical body centrally located in the cytoplasm; in the mature cell it is located to one side of the cell as a result of cytoplasm being pressed against the cell wall by vacuole.

The nucleus is surrounded by a thin double-membrane of lipoprotein composition. The nuclear membrane separates the cytoplasm from the granular and semi-fluid substance (nucleoplasm) of the nucleus.

The membrane contains minute, pore-like performations and the outer nuclear membrane is continuous with the ER. On account of these two features, there exists a direct communication between cytoplasm and nucleoplasm (Plate 2).

Three structures (chromosomes, ribosomes, nucleoli) are embedded in the nucleoplasm. The nucleoplasm consists of a structural and a structureless phase. The structural phase has a network of threads called the chromatin network. This phase of the nucleoplasm appears as a network or as distinct chromosome depending upon the mitotic stage of the nucleus.

On the other hand, the structureless phase of the nucleoplasm appears as a granular substance similar to, but a little denser than the cytoplasm. This phase is also called the nuclear sap. Chemical analysis of the nucleoplasm shows that it contains large amount of lipids, particularly phospholipids and proteins.

Several hydrolytic enzymes, viz. ribonuclease, dipeptidase and phosphatase are present in the nucleus and may be the specific components of the nucleoplasm. Chromatin is the hereditary material. Chemically it consists of RNA, DNA, histone-a low molecular weight protein and a more complex protein.

The interphase nucleus contains one or more nucleoli, which are without any membrane. Sometimes several nucleoli may be present in the nucleus. The nucleolus is formed during the telophase of mitosis as a result of the activity of certain areas on specific chromosomes, called the nucleolar chromosomes.

It disappears when the nucleus is about to divide. Nucleolus is mainly composed of RNA and a high concentration of protein. Although nucleolus is capable of some RNA synthesis, most of the nucleolar RNA is of chromatin origin.

Nucleolus, however, is the chief source of nuclear protein which is utilized in the manufacturing of ribosomes. It is believed that nucleoli may function in passing genetic materials and informations from the nucleus to the cytoplasm especially during cell division when the nucleus disappears.

Organelle # 9. Hydrogenosome:

These are widely distributed and were first identified in 1997 from parasitic flagellated protists. They possess their own genome but lack mitochondria. Hydrogenosomes possess double membrane and are supposedly derived from mitochondria. Unlike mitochondria they do not contain TCA cycle, cytochromes or oxidative phosphorylation.

These organelles produce large amount of hydrogen and possess the enzymes as are characteristic of anaerobes.