Exercise | Q 4. (A) | Page 57
Label the diagram and write down the detail of concept in your word.
Mitochondrion is known as the power house of the cell. It plays significant role in aerobic respiration. Mitochondria are absent in prokaryotic cells and red blood corpuscles (RBCs).
The structure of mitochondrion:
1. Shape of the mitochondria may be oval or spherical or like a spiral strip.
2. It is a double membrane-bound organelle.
3. Outer membrane is permeable to various metabolites due to the presence of a protein-Porin or Parson’s particles.
4. The inner membrane is selectively permeable to a few substances only.
5. Both membranes are separated by intermembrane space.
6. The inner membrane shows several fingers like or plate-like folds called as cristae which bears numerous particles exosomes and cytochromes/electron carriers.
7. Inner membrane encloses a cavity called inner chamber, containing a fluid-matrix.
8. Matrix contains few coils of circular DNA, RNA, 70S types of ribosomes, lipids, and various enzymes of Krebs’ cycle and other pathways.
Chloroplast plays a significant role in the synthesis of starch in plants.
Structure of chloroplast:
1. In plants, the chloroplast is found mainly in mesophyll of leaf.
2. A chloroplast is lens-shaped but it can also be oval, spherical, discoid, or ribbon-like.
3. A cell may contain a single large chloroplast as in Chlamydomonas or there can be 20 to 40 chloroplasts per cell as seen in mesophyll cells.
4. Chloroplasts contain a green pigment called chlorophyll along with other enzymes that help in the production of sugar by photosynthesis.
5. Inner membrane of double membraned chloroplast is comparatively less permeable.
6. Inside the cavity of inner membrane, there is another set of membranous sacs called thylakoids.
7. Thylakoids are arranged in the form of stacks called grana (singular: granum).
8. The grana are connected to each other by means of membranous tubules called stroma lamellae.
9. Space outside thylakoids is filled with stroma.
10. The stroma and space inside thylakoids contain various enzymes essential for photosynthesis.
11. Stroma of chloroplast contains DNA and ribosomes (70S).
Golgi complex or Golgi apparatus or Golgi body act as an assembly, manufacturing cum packaging, and transport unit of the cell.
Structure of the Golgi complex:
1. Golgi complex consists of stacks of membranous sacs called cisternae.
2. Diameter of cisternae varies from 0.5 to 1µm.
3. A Golgi complex may have few to several cisternae depending on its function.
4. The thickness and molecular composition of membranes at one end of the stack of a Golgi sac differ from those at the other end.
5. The Golgi sacs show specific orientation in the cell.
6. Each cisterna has a forming or ‘cis’ face (cis: on the same side) and maturing or ‘trans’ face (trans: the opposite side).
7. Transport vesicles that pinch off from transitional ER merge with cis face of Golgi cisterna and add its contents into the lumen.
Location of Golgi complex:
Golgi bodies are usually located near the endoplasmic reticulum.
Functions of Golgi complex:
1. Golgi body carries out two types of functions, modification of secretions of ER and production of its own secretions.
2. Cisternae contain specific enzymes for specific functions.
3. Refining (modification) of the product takes place in a sequential manner.
4. For example, certain sugar component is added or removed from glycolipids and glycoproteins that are brought from ER, thus forming a variety of products.
5. Golgi bodies also manufacture their own products. Golgi bodies in many plant cells produce noncellulose polysaccharides like pectin.
6. Manufactured or modified, all products of Golgi complex leave cisternae from trans face as transport vesicles.
structure of Endoplasmic Reticulum:
1. Endoplasmic reticulum is a network present within the cytosol.
2. It is present in all eukaryotic cells except ova and mature red blood corpuscles.
3. Under the electron microscope, it appears like network of membranous tubules and sacs called cisternae.
4. This network of ER divides the cytoplasm in two parts viz. one within the lumen of ER called laminal cytoplasm and non-laminal cytoplasm that lies outside ER.
5. Membrane of ER is continuous with nuclear envelope at one end and extends till cell membrane. It thus acts as intracellular supporting framework and helps in maintaining position of various cell organelles in the cytoplasm.
6. Depending upon the presence or absence of ribosomes, endoplasmic reticulum is called rough endoplasmic reticulum (RER) or smooth endoplasmic reticulum (SER) respectively.
Smooth endoplasmic reticulum (SER):
1. Depending on cell type, it helps in synthesis of lipids for e.g. Steroid secreting cells of cortical region of adrenal gland, testes and ovaries.
2. Smooth endoplasmic reticulum plays a role in detoxification in the liver and storage of calcium ions (muscle cells).
Rough Endoplasmic Reticulum (RER):
1. Rough ER is primarily involved in protein synthesis. For e.g. Pancreatic cells synthesize the protein insulin in the ER.
2. These proteins are secreted by ribosomes attached to rough ER and are called secretory proteins. These proteins get wrapped in membrane that buds off from transitional region of ER. Such membrane bound proteins depart from ER as transport vesicles.
3. Rough ER is also involved in formation of membrane for the cell. The ER membrane grows in place by addition of membrane proteins and phospholipids to its own membrane. Portions of this expanded membrane are transferred to other components of endomembrane system.