Cell Organelles: Mitochondria, Plastids and Endoplasmic Reticulum - Biology Grade XI

Science Notes

Cell Organelles: Mitochondria, Plastids and Endoplasmic Reticulum - Biology Grade XI

Cell Organelles: Mitochondria, Plastids and Endoplasmic Reticulum

Cell Organelles:

They are living sub cellular structures which are embedded within cytoplasm.

a) Mitochondria (Mitochondrion):

Its shape is variable and the size range from 2-6 micrometer. It is double membrane structure. Its outer membrane is smooth and regular. Its inner membrane gives finger like projections towards inner side called cristae. The surface of the cristae contains number of stalked and globular proteins called oxysomes. The cavity bounded by membrane is filled with a liquid mass called matrix. The matrix contains DNA RNA, inorganic and organic molecules. The mitochondria have its own DNA so it expresses its nature and characters own self. It is distributed in the cytoplasm.

Function of mitochondria:

It stores various types of enzymes used in aerobic respiration. It releases energy during oxidation of food material in aerobic respiration and stores energy in the form of ATPs so it is considered as the powerhouse of the cell. It can synthesize some proteins and it is carrier of genetic material.

b) Plastids

Plastids are photosynthetic pigments containing bodies found in the cytoplasm of plant cell. There are three types of plastids

  • Chloroplasts: Plastids, which contain green colored photosynthetic pigments, are called chloroplast. It is double membraned. A kind of fluid is enclosed by membrane is called stroma. In the stroma number of sac like membranes found called thyllakoid or lamellae. The group of thyllakoid is called granum. The thyllakoid that connects one granum to another granum is called fret. The stroma also caontains DNA RNA and other organic and inorganic substances.
  • Function of chloroplast: The main function of chloroplast is photosynthesis. The green colored pigments, the chlorophylls traps solar energy and the energy is then converted into chemical energy C6H12O6 with the combination of CO2 and H2O. It is also carrier of genetic materials, as it possesses its own DNA.
  • Chromoplast: The plastid, which contains colored photosynthetic pigment other than green, is called chromoplast. Yellow coloured pigments are called Xanthophylls. Orange colored pigments is called Carotenes Red colored pigments are called Erythrocyanine and blue colored pigments are called Phycocyanine. The chromoplasts are also doubling membrane. In the stroma grana are absent and the thyllakoids are found degenerating. They are present in colored parts of the plant body. Their main function is to make colorful the body parts like colorful parts of the flower attracts insects.
  • Leucoplast: The plastids which contain colorless pigments are called leucoplast. It is doubling membrane. The grana are absent. Developing thyllakoids are present in the stroma. In presence of light the leucoplast changes into chloroplast. The main function of the leucoplast is to store food materials. It is present in the stem and root. The leucoplast which store starch is called amyloplast, which stores oil, is called elaioplast and which stores protein called aleuroplast.

c) Endoplasmic Reticulum:

It is the network of tube-like structures and sac like structures. It arises from nuclear membrane and extends up to cell membrane. The endoplasmic reticulum is formed by three structures.
  • Cisternae: They are long elongated flattened unbranched sac like structures lie parallel to each other.
  • Vesicles: They are minute spherical or oval sac like structures
  • Tubules: They are branched tube like structures they form net work with cisternae and vesicles.

Function of Endoplasmic Reticulum:

It provides mechanical support to the cell. Therefore, it is also known as cytoskeleton. It synthesizes proteins and hormones. It helps to transport ions, molecules and impulse or metabolites. It forms nuclear membrane. It also forms new cell membrane and components.

There are two types of Endoplasmic Reticulum:

Rough ER Smooth ER

It bears ribosomes on its surface

It is found near nuclear membrane It is formed by nuclear membrane In it mainly cisternae are found

It bears no ribosomes

It is found near cell membrane It is formed by rough ER

In it mainly tubules are found


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