Q.No: Write a note on structure and function of cell wall of plant cells.
Answer:
CELL WALL:
The outer most boundary in the plant cell is cell wall. It is secreted by the proto-plasm of the cell but itself cell wall is not a living structure. Its thickness varies in different cells of the plant.
STRUCTURE AND CHEMICAL COMPOSITION:
It is composed of three main layers:
1. Middle Lamella .
2. Primary Wall
3. Secondary Wall.
1. Middle Lamella: The middle lamella is first to be formed in between the primary walls of the neighboring cells and binds them together. It is mainly composed of magnesium pectate. (So, it is not true cell wall).
2. The Primary Wall: The primary wall sed of cellulose and some ii. SI deposition of pectin and lignin. Cellulose molecules are arranged in a criss cross arrangement. The primary wall is a true wall and develops in newly growing cells.
3. The Secondary Wall: The secondary wall is formed on its inner surface and is comparatively thick and rigid. Chemically it is composed of inorganic salts, silica, waxes, cutin etc. (in addition to cellulose and lignin).
FUNCTIONS OF CELL WALL:
1. Shape & rigidity: Cell wall provides a definite shape to the cell and keeps it rigid.
2. Permeable: It does not act as a barrier to the materials passing through it.
3. Support & strength: Cell wall provides mechanical strength and skeletal support for the individual cell and to the plant as a whole.
4. Movement of water & minerals: Water and minerals can move through an interconnected system of cell wall (i.e. the apoplast).
5. Protection: Cell wall protects the cells from osmotic lysis.
Q.No. Write down the chemical composition and functions of the cytoplasm?
Answer:
PROTOPLASM:
The living contents of the eukaryotic cell are divided into nucleus and the cytoplasm, the two collectively form protoplasm.
CYTOPLASM: Cytoplasm consists of an aqueous ground substance containing a variety of cell organelles and other inclusions such as insoluble wastes and storage products.
Cytoplasm consists of:
a. Cytosol Soluble part or the liquid matrix of the cytoplasm.
b. Inclusions Insoluble wastes and storage products.
c. Organelles- Mitochondria, Ribosomes, etc.
Cytosol: The soluble part of the cytoplasm is called cytosol. It forms the ground substance of the cytoplasm.
Chemical composition: Chemically it is about 90% water. It forms a solution containing all the fundamental molecules of life. In the cytosol, small molecules and ions may form true solutions, and some large molecules form colloidal solutions. Colloidal solution may be a sol (non-viscous) or a gel (viscous). Peripheral parts of the cell are often like a gel.
FUNCTIONS:
1. Store House: The most important function of the cytoplasm is to act as a storehouse of vital chemicals.
2. Site for Glycolysis: It is also a site for certain metabolic processes (e.g. glycolysis).
3. Site for Anabolism: Many substances such as amino acids, fatty acids and nucleic acids are also synthesized here.
4. Organelles: In living cells the cytoplasm contains several cell organelles such as endoplasmic reticulum, mitochondria, Golgi complex, nucleus, plastids, ribosomes, lysosomes and centriole.
5. Cytoskeleton: The cytoskeleton present in cytoplasm maintains the shape and helps the movement of cell organelles.
ENDOPLASMIC RETICULUM (ER):
STRUCTURE: Under an electron microscope a network of channels is seen extending throughout the cytoplasm. These channels are often continuous with plasma membrane and also appear to be in contact with the nuclear membrane. This entire system of channels is the endoplasmic reticulum. These membranes vary widely in appearance from cell to cell. The material present in these channels is separated from the cytoplasmic materials by the spherical or tubular membranes, called cisternae. )
FORMS OF ENDOPLASMIC RETICULUM:
There are two morphological forms of endoplasmic reticulum.
1. Rough ER form (RER): Ribosomes are attached on it.
2. Smooth ER form (SER): This form of ER is without ribosomes.
FUNCTIONS OF ENDOPLASMIC RETICULUM:
1. PROTEIN SYNTHESIS: The rough endoplasmic reticulum (RER) is involved in the synthesis of proteins.
2. TRANSPORT OF PROTEIN: After synthesis the proteins are either stored in the cytoplasm or exported out of the cell through these channels.
3. TRANSPORT OF OTHER MATERIALS: SER plays an important role in the transport of materials from one part of the cell to the other.
4. LIPID METABOLISM: The smooth endoplasmic reticulum (SER) helps in metabolism of a number of different types of molecules particularly lipids.
5. DETOXIFICATION: The smooth endoplasmic reticulum also helps to detoxify the harmful drugs and transport.
6. IMPULSE TRANSMISSION: In some cells SER is responsible for transmission of inmpulses e.g. muscle cells, nerve cells.
7. MECHANICAL SUPPORT: Endoplasmic reticulum also provides mechanical support to the cell so that its shape is maintained.
RIBOSOMES:
STRUCTURE: Cell contains many tiny granular structures known as ribosomes.
DISCOVERY: Palade (1955) was the first person to study them. Their size ranges from 150 to 200°A in diameter.
CHEMICAL COMPOSITION: They are composed of an almost equal amount of RNA and protein hence they are ribonucleic-proteins particles. The RNA present in ribosome is called ribosomal RNA. FORMS Ribosomes exist in two forms: Freely dispersed in cytoplasm Attached with RER as tiny granules. The ribosomes are attached to messenger RNA through small ribosomal subunit .
SUB-PARTICLES OF RIBOSOMES: Each eukaryotic ribosome consists of two sub-units. The larger subunit sediments at 60S (S= Svedberg unit used in ultracentrifugation), while smalle subunit sediments at 40S. Two subunits on attachment with each other form 80S particle. This attachment is controlled by the presence of Mg ions.
PRODUCTION OF RIBOSOMES: New ribosomes are assembled in the nucleolus of the nucleus from where they are transported to the cytoplasm via the pores in nuclear membrane.
The factory of ribosomes is the Nucleolus ,while that of protein synthesis is the ribosomes.
POLYSOMES: A group of ribosomes attached to MRNA are known as polysomes.
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| Polysome |
GOLGI BODIES:
DISCOVERY: Camillo Golgi discovered Golgi apparatus in 1898.
STRUCTURE: This apparatus, which was found virtually in all eukaryotic cells, consists of stacks of flattened; membrane bound sacs, called cisternae. These cisternae together with associated vesicles are called Golgi complex. It is a complex system of interconnected tubules around the central stacks.
Cisternae stacks are continuously formed by fusion of vesicles, which are probably derived by the budding of SER. Their outer convex surface is the forming face, while the inner concave surface is the maturing face. The cisternae break up into vesicles from the latter. The whole stack consists of a number of cisternae thought to be moving from the outer to the inner face.
FUNCTIONS:
1. Cell Secretions: Golgi complex is concerned with cell secretions, which are products formed within the cell on ribosomes and then passed to the outside through endoplasmic reticulum and Golgi apparatus. The secretions are converted into finished product and are packed inside membrane, before export.
2. Modification: The most important function of this apparatus is to modify the proteins and lipids by adding carbohydrates and converting them into glycoproteins or glycolipids.
3. Cell Wall Synthesis: In plants, Golgi apparatus is involved in the synthesis of cell wall. It is called dictyosome in plants.
4. Lysosome Formation: They are involved in the synthesis of primary lysosomes.
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