(a) Cell organelles with no membrane In eukaryotic cell, ribosomes occur freely in the cytoplasmic matrix and are attached to the outer cytoplasmic surface of the rough endoplasmic reticulum and nuclear envelope. The ribosomes are also found in the matrix of mitochondria and stroma of plastids called as mitoribosomes and plastidoribosomes, respectively.

(b) Cell organelles with single membrane A lysosome is a tiny sac bounded by a single unit membrane of lipoprotein. The lysosomes occurs in all animal cells and protozoans. Prokaryotic cell lack lysosomes.

Number of lysosomes varies with the cell type. Lysosome contain a dense, finely granular fluid consisting of glycoproteinaceous hydrolytic (digestive) enzymes called acid hydrolases. Cell organelles like sphaerosomes and microbodies also possess single membranes.

(c) Cell organelles with double membrane Mitochondria are double membrane bound structures with the outer membrane and inner membrane dividing its lumen distinctly into two aqueous compartments.

The inner compartment is called the matrix whereas the outer membrane forms the continuous limiting boundary of the organelle. Chloroplast and nucleus are also double membrane bound organelles.

In bacteria, additional to the genomic DNA, small circular DNA molecules are present in cytoplasm. These small molecules are called plasmids. They confer unique phenotypic character to bacteria, i.e., resistance to antibiotics. It is also used to monitor bacterial transformation with foreign DNA. In eukaryotes, the extra DNA molecules are present in chloroplast (stroma) and mitochondria (matrix). Because of the presence of this DNA molecule, they are treated as self autonomous organelles. In case of highly active organism, the DNA extrachromosomal is found to be more as compared to the less active ones.

Mitochondria is a sausage shaped or cylindrical structure having a diameter of 0.2 to 1.0 $\propto \mathrm{m}$ and length 1.0-4.1 $\propto \mathrm{m}$. Each mitochondrion is a double membrane bound structure with outer membrane and inner membrane dividing its lumen distinctly into two aqueous compartments. The inner compartment is called matrix and outer one forming infoldings is called cristae towards the matrix. These cristae are associated with an increase in surface area.

The mitochondria are the actual sites of aerobic respiration. They produce cellular energy in the form of ATP, hence are called as power house of the cell.

This ATP generated is used in performing vital functions by the organism. The matrix of mitochondria also possess a single circular DNA molecule of itself and a few RNA molecules, ribosomes (70S) and the compartments required for the protein synthesis.

Plastids are species specific and are found in all plant cells and in euglenoids. They bear some specific pigments thus, imparting specific colours to the part of the plant which possess them. Based on the type of pigments plastids are classified into three main types, i.e., leucoplasts, chromoplasts and chloroplasts.

Leucoplasts They are colourless plastids which store food material based on there storage products, they are of three types

(a) Amyloplasts Stores starch, e.g., tuber of potato, grain of rice, grain of wheat.

(b) Elaioplasts These store fats, e.g., rose

(c) Aleuroplasts They are protein storing plastids, e.g., castor endosperm

Chromoplast These are non photosynthetic coloured plastids which synthesise and store carotenoid pigments. They appear orange, red or yellow. These mostly occur in ripe fruits (tomato and chilles) carrot roots, etc.

Chloroplasts These are green colour plastids which help in synthesising food material by photosynthesis. They contain chorophyll and carotenoid pigments which trap light energy. Each chloroplast is oval or spherical, double membrane bound cell organelle.

The space present inside inner membrane is called stroma. A number of organised flattened membranous sacs called thylakoids are present in the stroma. Thylakoids are arranged in stacks are called grana.

The thylakoids of different grana are connected by membranous tubules called the stroma lamellae. The stroma of the lamellae contain the enzymes that are required for the synthesis of carbohydrates and proteins.

(a) Centromere It is required for proper chromosome segregation. The centromere consists of two sister chromatids. It is also necessary at the point chromosome attaches to the spindle apparatus during mitosis and meiosis.

(b) Cell wall It gives a definite shape to the cell and protects the cell from mechanical injury and infections. It also helps in cell to cell interaction and act as a barrier for undesirable macromolecules.

(c) Smooth ER It helps in synthesis of lipids, metabolism of carbohydrates, regulation of calcium concentration, drug detoxification and attachment of receptor on cell membrane proteins.

The smooth ER also contain enzymes-glucose 6 phosphatase, which converts glucose 6 phosphate to glycogen, essential in glucose metabolism.

(d) Golgi apparatus It is the important site for the formation of glycoprotein and glycolipids. It is also involved in the synthesis of cell wall materials and also play an important role in formation of cell plate during cell division.

(e) Centrioles They form the base body of cilia and flagella and spindle fibres that gives rise to spindle apparatus during cell division in animal cells. They help in formation of microtubules and sperm tail. They also help in cell division by forming asters, which acts as spindle pole.