Describe the structure of the following with the help of labelled diagram.

Nucleus
 

(i) Cytology is the branch of biology that deals with the formation, structure, and function of cells.

(ii) The cell was discovered by Robert Hooke in $1665$.

(iii) The cell nucleus was discovered by Robert Brown in $1831$.

(iv) The cell theory was proposed by Theodor Schwann and Matthias Jakob Schleiden in $1838$.

(v) Cell is the structural and functional unit of life.

(vi) Cells vary in their shape, size and activities/functions.

2. Types of cells:

(i) Based on the presence or absence of a membrane-bound nucleus and other organelles, cells and hence organisms can be named as eukaryotic or prokaryotic.

(ii) A typical eukaryotic cell consists of a cell membrane, nucleus and cytoplasm.

(iii) The most widely accepted current model, the fluid mosaic model of cell membranes, was proposed by Singer and Nicolson in $1972$.

3. Plasma Membrane:

(i) The plasma membrane is selectively permeable and facilitates transport of several molecules.

(ii) There are four main types of cell-cell junctions: occluding junctions (zonula occludens or tight junctions), adhering junctions (zonula adherens), desmosomes (macula adherens) and gap junctions.

(iii) The plasma membrane is covered by a rigid cell wall in cells of bacteria, fungi, algae and plants.

(iv) Cytoplasmic connections from cell to cell are known as plasmodesmata.

4. Endoplasmic Reticulum, Golgi bodies and Lysosomes:

(i) Endoplasmic reticulum contains tubules or cisternae.

(ii) ERs are of two types: rough and smooth.

(iii) ER helps in the transport of substances, synthesis of proteins, lipoproteins and glycogen.

(iv) The secretions of cells are packed in Golgi bodies and transported from the cell.

(v) Lysosomes, commonly known as ‘suicide bags’, are single membrane structures containing enzymes for intracellular digestion of all types of macromolecules.

5. Mitochondria:

(i) Mitochondrion is bound by a double membrane; the outer membrane is smooth, and the inner one folds into several cristae.

(ii) Mitochondrion is known as the ‘powerhouse of cell’; the energy is released in it through cellular respiration.

(iii) Mitochondrion helps in oxidative phosphorylation and generation of adenosine triphosphate.

6. Ribosomes:

(i) Ribosomes occur freely in the cytoplasm or are associated with ER.

(ii) Ribosomes are the ‘protein factories’ of the cell.

7. Plastids:

(i) Plastids are found in plant cells only. In plant cells, chloroplasts are responsible for trapping light energy essential for photosynthesis.

(ii) The grana, in the plastid, is the site of light reactions and the stroma of dark reactions.

(iii) The green coloured plastids are chloroplasts, which contain chlorophyll, whereas the other coloured plastids are chromoplasts, which may contain pigments like carotene and xanthophyll.

8. Cytoskeleton and Centrosome:

(i) Cytoskeleton is an intracellular framework composed of microtubules, microfilaments and intermediate filaments.

(ii) Cytoskeleton helps to define and control the shape of the cell, is involved in cell movements and is important in cell division.

(iii) Centrosome and centriole form the basal body of cilia and flagella that facilitate locomotion.

(vi) In animal cells, centrioles also form spindle apparatus during cell division.

(v) Cilia and flagella are hair-like extensions from the cell surface that produce movement.

9. Nucleus:

(i) Nucleus is the most important organelle of the cell, and it plays a vital role in inheritance and gene expression.

(ii) The nucleus consists of four parts: nuclear membrane, nucleoplasm, chromatin and nucleolus.

(iii) The nucleus is enclosed by a nuclear envelope, a double membrane structure with nuclear pores.

(iv) The inner membrane encloses the nucleoplasm and the chromatin material.

(v) The nucleus contains a viscous fluid the nucleoplasm (nuclear sap or karyolymph) which keeps the nucleus turgid.

10. Chromosomes

(i) Prokaryotic cells contain their chromosome as a circular DNA.

(ii) Eukaryotic chromosome consists of chromonema, chromomere, centromere, secondary constriction, telomere, satellite, etc.

(iii) The chromosome (chromatin) is formed of a series of repeating units called nucleosomes.

(iv) The complete complement of genetic information in a haploid chromosome set is called the genome.