Primary cell wall
Secondary cell wall
1. It is found in young dividing cells.
2. It lies internal to the middle lamella.
3. It is present in all plant cells.
4. It is elastic and capable of expansion in a growing cell.
5. It is thin.
6. It is composed of mainly cellulose and lacks additional material.
1. It is found in a mature and non dividing cell.
2. It lies internal to the primary wall.
3. It is present in certain cells only.
4. It is rigid and incapable of expansion.
5. It is thick.
6. It has additional materials like hemi-cellulose lignin, suberin etc.
Passage of polar molecules : Due to bilayer of nonpolar lipid molecules, the polar molecules cannot pass through biomembranes. They require carrier molecules to facilitate their transportation across membrane. The transport of certain molecules takes place against their concentraion gradient from lower to higher concentration, this process requires energy and is an active transport process. The energy is provided by ATP.
Characteristics of Prokaryotic cell are as follows:
1. The nucleus is not bounded by a membrane.
2. Their genetic material is naked and present in the form of nucleoid in the cytoplasm.
3. They are generally smaller and divide more rapidly as compared to the eukaryotic cell.
4. They are usually surrounded by cell wall except for mycoplasma.
5. Besides genomic DNA many prokaryotic cell have small circular DNA called plasmids outside the genomic DNA.
6. Eukaryotic organelles or membrane bound organelles are not found in the prokaryotic cells. The ribosomes found in these are of 70S type.
7. Mesosomes which are the invaginations of plasma membrane may be present.
8. They have special inclusion bodies.
Functions of nuclear pores :
1. The materials are exchanged between cytoplasm and nucleus through nuclear pores.
2. The proteins are passed in and out of the nucleus through nuclear pores.
3. The enzymes for synthesis of DNA and RNA are passed into nucleus through nuclear pores.
4. Ribosomal subunits and RNA are passed out from nucleus via nuclear pores.
5. m RNA and t RNA are passed out into cytoplasm via nuclear pores.
Depending upon the location of centromere in a chromosome, the chromosome is classified into the following four types :
(a) Telocentric: Centromere is present at the terminals.
(b) Acrocentric: Centromere is present near the terminals resulting in two short arms and two long arms.
(c) Sub- metacentric : Centromere divides the chromosomes into two unequal arms. It lies either below centre or above centre.
(d) Metacentric : Centromere divides the chromosomes into two equal arms with the centromere in the middle.