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Study of Plant Cell Organelles with Emphasis on Cell Wall, Chloroplast, Mitochondria and Nucleus

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Study of Plant Cell Organelles with Emphasis on Cell Wall, Chloroplast, Mitochondria and Nucleus Unit I: Cell and Organelles: Study of plant cell organelles with emphasis on cell wall, Chloroplast, Mitochondria and Nucleus. Cell Wall Cell wall is outermost, rigid, protective and semi­transparent covering of plant cells and cell of fungi, bacteria and some protists. It was discovered by Robert Hooke in 1665. The cell wall is the rigid, semi­ permeable protective layer in some cell types. This outer covering is positioned next to the cell membrane (plasma membrane) in most plant cells, fungi, bacteria, algae, and some archaea. Animal cells however, do not have a cell wall. The cell wall conducts many important functions in a cell including protection, structure, and support. Cell wall composition varies depending on the organism. In plants, the cell wall is composed mainly of strong fibers of the carbohydrate polymer cellulose. Cellulose is the major component of cotton fiber and wood and is used in paper production. Fungal cell wall – of chitin Bacterial cell wall – of peptidoglycan or mucopeptide or murein Plant cell wall – of cellulose, hemicelluloses and protein. Cell wall has the following different layers: 1. Middle Lamella . It is outermost thin cementing layer between adjacent cells . Formed during cytokinesis . This outer cell wall layer that contains polysaccharides called pectins. Pectins aid in cell adhesion by helping the cell walls of adjacent cells to bind to one another. This is also composed of magnesium and calcium. 2. Primary Wall . More or less elastic layer formed after the formation of middle lamella. Composed of protein, hemicelluloses and loose network of microfibril of cellulose. Primary wall and middle lamella are found in all types of plant cells. The primary cell wall provides the strength and flexibility needed to allow for cell growth. 3. Secondary wall . Much thicker, rigid and inelastic layer formed inner to primary wall. Is formed only when cells attain maximum size . Is common in aclerenchyma, collenchymas, tracheid and vessels. Composed of cellulose, hemicellular, pectin and lignin. S1, S2, and S3 are its sub­layers. It may contain wax, cutin and suberin and silica. This rigid layer strengthens and supports the cell. In addition to cellulose and hemicellulose, some secondary cell walls contain lignin. Lignin strengthens the cell wall and aids in water conductivity in plant vascular tissue cells. 1 Plasmodesmata: Primary and secondary walls are not formed continuously. They form gaps, known as plasmodesmata. It is a cytoplasmic bridge between two cells. Plasmodesmata are known for long time playing a passive role in permitting free movements of small metabolites and growth hor­mones between plant cells. Desmosomes: It is the protoplasmic bridge between two cells of animal cells. FUNCTIONS OF CELL WALL A major role of the cell wall is to form a framework for the cell to prevent over expansion. Cellulose fibers, structural proteins, and other polysaccharides help to maintain the shape and form of the cell. Additional functions of the cell wall include: 1. Support ­ the cell wall provides mechanical strength and support. It also controls the direction of cell growth. 2. Withstand turgor pressure ­ turgor pressure is the force exerted against the cell wall as the contents of the cell push the plasma membrane against the cell wall. This pressure helps a plant to remain rigid and erect, but can also cause a cell to rupture. 3. Regulate growth ­ sends signals for the cell to enter the cell cycle in order to divide and grow. 4. Regulate diffusion ­ the cell wall is porous allowing some substances, including proteins, to pass into the cell while keeping other substances out. 5. Communication ­ cells communicate with one another via plasmodesmata (pores or channels between plant cell walls that allow molecules and communication signals to pass between individual plant cells). 6. Protection ­ provides a barrier to protect against plant viruses and other pathogens. It also helps to prevent water loss. 7. Storage ­ stores carbohydrates for use in plant growth, especially in seeds. 8. Shape ­ It provides definite shape to cell due to its rigidity. It protects the protoplasm against mechanical injury.Cell wall of root hairs absorbs water. It has some enzymatic activity. 9. Transpiration ­ Cutin and suberin of cell wall of stem and leaves help to reduce the rate of transpiration. The permeable nature of cell wall allows the exchange of any substance through it. Primary cell wall Primary cell wall is the first cell wall laid down by the protoplast inner to the middle lamella. The primary wall is thin and elastic and composed of cellulose, hemicellulose, pectic substances, lipids, proteins, some minerals and water. During development of middle lamella and primary wall, certain openings are left at places between the adjacent cells. These are called plasmodesmata (Strasberger 1901). Through these pores cytoplasmic continuity is maintained between the neighbouring cells. It is also made of cellulose, hemicellulose, proteins and polysaccharides. 2 Secondary cell wall Secondary cell wall is formed towards inner side primary wall and is made of several layers of cellulose, hemicellulose. Deposition of lignin and suberin takes place after the primary wall is fully formed. The wall is thick and nonelastic and provides additional strength. Tertiary cell wall In addition to primary and secondary cell wall, tertiary cell wall is deposited in a few cells. First reported by Butchen (1955) in tracheids of some gymnosperms, it is considered to be a dried residue of the protoplasm. It looks like swollen nodules on the inner side of the secondary wall. Besides cellulose and hemicellulose, xylan is also present in the tertiary cell wall. Cell wall depositions The cell wall is very thin and delicate in the beginning. As the cell grows, it is stretched and new substances are deposited on the primary cell wall. The various substances found on the cell wall are: 1. Lignin: It is a complex chemical substance deposited in the secondary wall. With the lignin deposition, the protoplasm is lost and the cells become woody and hard. Lignin deposition is generally not uniform and may result in annular, spiral, scalariform, reticulate or pitted patterns. These thickenings provide mechanical support to the cell. Most of the vegetable fibres are lignified. 2. Cutin: It is a waxy substance, forms a thin or thick layer called cuticle on the stem or leaf surfaces. Cutin is impervious to water and checks evaporation. Deposition of cutin is found to be more in xeric plants. 3. Suberin: This fatty substance is also waxy in nature. This is deposited on cork cells and is impermeable to water and gases. 4. Mucilage: It is a slimy substance made of complex carbohydrates. It absorbs water and stores it. It becomes hard when dry and viscous when moist. Mucilage is present in leaves of Aloe, flowers of Hibiscus, seeds of Linum, etc. 5. Mineral crystals: Silica, calcium oxalate and calcium carbonate, etc. are deposited in the cell wall in the form of crystals. The main function of cell wall is to provide mechanical strength. It is also capable of imbibing water and thus, helps the movement of water and solutes inside the cell. 3 Chloroplast Chloroplasts are organelles present in plant cells and some eukaryotic organisms. Chloroplasts are the most important plastids found in plant cells. It is the structure in a green plant cell in which photosynthesis occurs. Chloroplast is one of the three types of plastids. The chloroplasts take part in the process of photosynthesis and it is of great biological importance. Animal cells do not have chloroplasts. All green plant take part in the process of photosynthesis which converts energy into sugars and the byproduct of the process is oxygen that all animals breathe. This process happens in chloroplasts. The distribution of chloroplasts is homogeneous in the cytoplasm of the cells and in certain cells chloroplasts become concentrated around the nucleus or just beneath the plasma membrane. A typical plant cell might contain about 50 chloroplasts per cell. The chloroplasts are cellular organelles of green plants and some eukaryotic organisms. These organelles conduct photosynthesis. They absorb sunlight and convert it into sugar molecules and also produce free energy stored in the form of ATP and NADPH through photosynthesis. Chloroplasts are unique organelles and are said to have originated as endosymbiotic bacteria. Chloroplast Structure Chloroplasts found in higher plants are generally biconvex or planoconvex shaped. In different plants chloroplasts have different shapes, they vary from spheroid, filamentous saucer­shaped, discoid or ovoid shaped. They are vesicular and have a colorless center. Some chloroplasts are in shape of club, they have a thin middle zone and the ends are filled with chlorophyll. In algae a single huge chloroplast is seen that appears as a network, a spiral band or a stellate plate. The size of the chloroplast also varies from species to species and it is constant for a given cell type. In higher plants, the average size of chloroplast is 4­6 µ in diameter and 1­3 µ in thickness. Structure of Chloroplast The chloroplast are double membrane bound organelles and are the site of photosynthesis The chloroplasts have a system of three membranes: the outer membrane, the inner membrane and the thylakoid system. The outer and the inner membrane of the chloroplast enclose a semi­gel­like fluid known as the stroma. This stroma makes up much of the volume of the chloroplast, the thylakoids system floats in the stroma. Outer membrane ­ It is a semi­porous membrane and is permeable to small molecules and ions, which diffuses easily. The outer membrane is not permeable to larger proteins. Intermembrane Space ­ It is usually a thin intermembrane space about 10­20 nanometers and it is present between the outer and the inner membrane of the chloroplast.
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