Describe the structure of the following with the help of labelled diagrams
Nucleus - Nucleus is the control unit of the cell. It controls the cellular activities of the cell.
Structure - It is spherical in shape. It is composed of the following structures:
Nuclear membrane: It is a double membrane separating the contents of the nucleus from the cytoplasm. The narrow space between the two membranes is called the perinuclear space. Nuclear membrane has tiny holes called nuclear pores. These holes allow specific substances to be transferred into a cell and out from it.
Nucleoplasm/Nuclear matrix: It is a homogenous granular fluid present inside the nucleus. It contains the nucleolus and chromatin. Nucleolus is a spherical structure that is not bound by any membrane. It is rich in protein and RNA molecules and is the site for the formation of ribosomes. Chromatin is an entangled mass of thread-like structures. It contains DNA and some basic proteins called histones.
Centrosome consists of two cylindrical structures called centrioles. Centrioles lie perpendicular to each other. They are surrounded by amorphous pericentriolar materials. Each has a cartwheel-like organisation.
A centriole is made up nine evenly spaced peripheral fibres of tubulin protein. The adjacent triplets are linked together. There is a proteinaceous hub in the central part of a centriole. The hub is connected to the triplets via radial spokes. These centrioles help in organising the spindle fibres and astral rays during cell division. They form the basal body of cilia and flagella.
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.
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.