2) It sorts proteins according to their destination.
3) It provides a control passage for the export of mRNA molecules from nucleus to rough endoplasmic reticulum.
4) Several enzyme molecules, e.g. glucose-6-Phosphatase, AT- Pase etc. are embedded in the membranes of ER.
Ribosomes are particles of about 200 A° diameter; they are composed of RNA and protein. Generally ribosomes are attached to the outer surfaces of ER membranes. This converts smooth ER elements into rough ER.
Ribosomes are essential for protein synthesis, as mRNA can support protein synthesis only when they are attached to ribosomes.
III) Golgi body:
It consists of 2-7 flat cisternae stacked close to each other. Golgi bodies originate from ER elements.
A major function of Golgi bodies is protein sorting.
Lysosomes are vesicles of 400-800 ?m formed by budding of golgi bodies and they contain hydrolytic enzymes.
The main enzyme present in lysosomes is acid phosphatase, other enzymes are acid DNAase, acid RNAase and ? galactosidase etc.
The function of lysosomes is to digest (lyse) the food particles and microorganisms ingested by a cell and also to cause autolysis of cells, if required.
The term was first used by Benda in 1897 (mitos = thread + chondrian = granule). These are cylindrical bodies with an average diameter of 0.2 to 1 ? and ordinarily 3-10 ? in length.
The average composition of mitochondria is protein: 70%, lipids : 25-30%, RNA : 1% and DNA less than 1%.
The main function of mitochondria is the oxidation of carbohydrates, amino acids and fatty acids and the production of ATP.
VI) Plastids (Chloroplast):
Chloroplast contain the single most important pigment on earth i.e. Chlorophyll. They impart the characteristic green color to plants and carry out photosynthesis, the ultimate source of all organic compounds.
Chloroplast is typically biconvex lens shaped of about 5 ? diameter and 3 ? thicknesses. The average cell may have 20-40 chloroplasts.
It consists of protein: 50-59%, lipid : 21-34%, chlorophyll : 5-8 %, carotenoids : 0.7- 1.1%, RNA: 1-7.5% and DNA : 0.2 to 1%.
The main function of chloroplast is photosynthesis.
It is a denser, rounded or spherical protoplasmic body enclosed in the protoplasm. Robert Brown (1883) was discovered it for the first time.
On the basis of presence or absence of well defined nucleus, living organisms have been classified into two groups viz. prokaryotes; individuals which do not have well organized nucleus such as viruses, bacteria, blue green algae and eukaryotes; which have very well organized nucleus e.g. plants, animals etc. Its shape and size differs greatly according to size of cell. It is composed of following organelles:
1) Nuclear Membrane:
Nucleus is bounded by a membrane on the outside called nuclear membrane, which is double walled and having numerous minute pores.
It is relatively permeable and may allow large molecules of protein. It breaks down at the end of prophase and is reformed at the end of nuclear division.
It is viscous, non staining, granular, colorless fluid inside the nuclear membrane. It is also known as nuclear sap or karyolymph.
The chromatin network and nucleolus are suspended in it. Nucleoplasm occupies maximum portion of nucleus and composed of protein, RNA and enzymes.
3) Chromatin network:
The threads like bodies forming a reticulum are suspended in nucleoplasm, which are network of chromosomes.
They are long, fine and thread like structures. The numbers of chromosomes are constant in a particular species.
A spherical round body usually single but may be double. It plays important role in protein synthesis. It is associated with a particular nuclear organizing chromosome.
Functions of nucleus:
1. It is responsible for life.
2. It is controlling centre of all the vital activities of the cell.
3. It takes direct part in growth and cell division.
4. It takes initiative in cell division.
5. It contains chromosomes and genes i.e. hereditary material.
B) Non living
It contains the pigments other than chlorophyll, e.g. Phucoxanthin, phycocyanin etc.
Plant cells have one or more vacuoles of variable size. The material contained in vacuole is called as cell sap.
3) Cell sap:
Cell sap is relatively less dense than the surrounding cytoplasm. It contains sugars, salts, proteins and phenols as well as some specific pigments e.g. anthocyanin.
4) Cell wall:
Plant cell is surrounded by a non-living and rigid coat called cell wall. The main function of the cell wall is to provide plant cell a definite shape and mechanical support and strength to tissues and organs.
Types of organism:
There are two types of organism; prokaryotes and eukaryotes. There differences are given below.
a. Prokaryotes have small cells (
b. They are always unicellular
c. No nucleus or any membrane- bound organelles, such as mitochondria
d. DNA is circular, without proteins
e. Ribosomes are small (70S)
f. No cytoskeleton
g. Motility by rigid rotating flagellum made of flagellin
h. Cell division is by binary fission
i. Reproduction is always asexual
j. Huge variety of metabolic pathways
a. Eukaryotes have larger cells (> 10 mm)
b. They are often multicellular
c. Always have nucleus and other membrane-bound organelles
d. DNA is linear and associated with proteins to form chromatin
e. Ribosomes are large (80S)
f. Always has a cytoskeleton
g. Motility by flexible waving undulipodium, made of tubulin
h. Cell division is by mitosis or meiosis
i. Reproduction is asexual or sexual
j. Common metabolic pathways