Study of Cell - Cytology

Study of Cell - Cytology

• Study of cell is called cytology.
• Cell-Cytology, Main features & function, Types of cell, Plant cell, Animal cell, Cell structure, Protoplasm, Cell membrane, Chromosome, DNA, RNA, Protoplasm, Lysosome, Mitochondrea, Mytosis

Cell:

• Cell is the basic structure and functional unit of life.
• The word ‘cell’ was first coined by British scientist Robert Hook in the year 1665.
• Anton Van Leeuwenhook first described a living cell.
• The longest cell in human body is Neuron.
• The biggest cell is egg of Ostrich.
• Schilden and Schwan established cell theory in the year 1838-39.
• Germ plasm theory was proposed by August Weisman.

Main feature of the cell theory:

1. All organisms are composed of cell.

2. Body of every organism is made of cell.

3. Each cell arises from pre-existing cell.

4. Every organism starts its life from single cell.

Cells are of two kinds:

1. Prokaryotic cell:

These are primitive cell havig three basic structures of typical cell but lack nuclear membrane. Nuclear material is present in a region of cytoplasm called nucleoid. Other membrane bound organelles are absent such as mitochondria, lysosome, Golgi bodies etc. Ex – Bacteria and Cynobacteria are Prokaryotes.

• Number of Mitochondria in bacterial cell is zero.
• The smallest know prokaryotic organism is Mycoplasma.
• In photosynthetic prokaryotic bacteria chlorophyll is associated with membranous vesicles.

2. Eukaryotic Cell:

These are complete cell which contain membrane bound organelles and nucleus. Unicellular and multicellular plant and animal have Eukaryotic cell.

• The biggest single celled organism is Acetabularia.
• Nucleus contains chromatin made up of DNA and histone protein.
• Nucleolus is present inside nucleus.

Difference between Prokaryotes & Eukaryotes

Structure of typical cell:

A cell has following structure.

1. Cell wall:

In plant cell there is a rigid cell wall which is non living and freely permeable. It is made up of cellulose or chitin. It provide shape and rigidity to the cell.

2. Cell membrane:

It is thin, elastic, living, double layer, permeable membrane.

• Cell membrane is mainly composed of lipids and protein.
• The major lipids are phospholipids arranged in a bilayer.
• Function: It regulates movement of molecule inside and outside of the cell.
• The membrane is selectively permeable to some molecule present on either side of it.

3. Protoplasm:

The whole fluid present inside plasma membrane is protoplasm. The name protoplasm is given by purkenje in 1839. Protoplasm is made up of various chemical substances like water, ions, salt and organic molecule. It is the living part of cell.

Protoplasm is divided into two parts:

A. Cytoplasm: The fluid found outside the nuclear membrane.

B. Nucleoplasm: The fluid found inside the nuclear membrane.

• 99% of protoplasm is made up of oxygen (76%), carbon (10.5%), hydrogen (10%) and nitrogen (2.5%).
• 80% of protoplasm is water. o knows as plasma membrane which form the outer covering of animal cell. In plant cell it is found within cell wall. It
• The ratio of inorganic and organic compound found in protoplasm is 81 : 19.

4. Mitochondria:

Discovered by Altman in the year 1886. These are cylindrical, rod shaped or spherical structure found in cytoplasm. It is surrounded by double layered membrane. Inner membrane has many fold called cristae. The fluid presents inside mitochondria is called matrix, which contains many enzyme and co-enzyme.

• Mitochondria is considered a prokaryotic cell inside eukaryotic.
• Function: Mitochondria is the respiratory site of cellular respiration. Mitochondria synthesize energy rich compound ATP. It is also known as ‘Power House’ of the cell. ATP is known as energy currency of cell.

5. Golgi bodies:

Discovered by scientist Camilo Golgi. Golgi bodies are made up of group of tubes, vesicles and vacuoles. In plant it is more in number and here it is known as dictyosomes.
Function: It work as storage processing and packaging of material. It also involved in the synthesis of cell wall, plasma membrane and lysosome.

6. Endoplasmic reticulum:

Membranous network of tubules like structure found in cytoplasm is called endoplasmic reticulum. It is attached with the nucleus on one side and on other side it is joined with plasma membrane.
Function: Endoplasmic reticulum helps in the distribution of material. It form supporting framework of cell.

• Rough endoplasmic reticulum is the site of protein synthesis due to presence of ribosome on it.
• Smooth endoplasmic reticulum is the major site of lipid synthesis.
• Endoplasmic reticulum is responsible for the mechanical support, protein synthesis and enzyme transport.

7. Ribosome:

Discovered by Palade. Small granules like structure found attached to the endoplasmic reticulum or in free state. It is made up of ribonucleic acid (RNA).

• Ribosome is not surrounded by membrane.
• Function: Take part in protein synthesis.

8. Lysosome:

Discovered by De Duve. These are sac like structure bounded by single membrane and contain hydrolytic enzyme.
Function: It helps in intracellular digestion. The enzyme found in lysosome may digest the entire cell. So it is also known as ‘suicide bags of the cell.’

• Lysosome is not found in Red blood corpuscles of mammal.

9. Centrosome:

Discovered by Boveri. It is only found in animal cell taking part in cell division. It is not bounded by membrane consist of two centriole.
Function: Centriole found in Centrosome initiate the cell division forming spindle fiber between the pole.

10. Plastid:

Only found in plant cell.. It is of three type :
(a) Chloroplast (b) Chromoplast (c) Leucoplast.

(a) Chloroplasts:

These are green pigment found in green plant involve in photosynthesis. So, it is known as ‘Kitchen of the cell’. Chloroplast is bounded by two unit membrane having grana and storma. Grana are made up of membrane bounded sac like structure known as thylakoid found in stack. It contain chlorophyll molecule. Storma is the matrix present inside the chloroplast which contain photosynthetic enzymes and starch grain. A Granum is composed of 20 to 50 thylokoids,  Granum is the site of light reaction during photosynthesis while stroma is the site of dark reaction.

(b) Chromoplast

Chromoplast Provides various colours to the plant like flower, fruit etc.

Chromoplasts are of different kind-

Lycopene: In tomato it provide red colour.

Carotene: Provide yellow or orange colour in plant Example - Carrot.

Betanin: Found in sugar beet.

(c) Leucoplast

Leucoplast is colourless. It stores the food in the form of starch, fat & protein, granules.

• Leucoplast is found in root and underground stem.

11. Vacuole:

It is fluid filled single membrane bounded, dead organelles of cell. In plant cell it is larger in size but in animal it is smaller in size.

• The membrane which surrounds the vacoule called tonoplast.
  Function: It helps in osmoregulation. It stores toxic metabolic waste.
• In plant cell there is a large central vacuole that or may occupy 90% of total cell volume.
• In plant cell, vacuoles provide turgidity and rigidity.
• In amoeba vacuole play role in osmoregulation and nutrition.
• The membrane which surrounds the vacuoles is called tonoplast.

12. Nucleus:

Nucleus is a spherical, centrally located major structure found in the cell. In plant cell it is shifted towards periphery. It is bounded by double layered nuclear membrane having pre. Within Nucleoplasm chromatin material are present. Nucleolus is rich in protein and RNA. Chromatin material is thin thread like structure forming network. This is made up of genetic substance DNA (deoxyribo nucleic acid) and histone protein. During cell division chromatin breaks into pieces and form chromosome.
Function: It controls all the activity of cells. So it is also known as ‘control room’ of cell. Chromatin transmits hereditary characters from parents to their offspring.

• Other than nucleus DNA is also found in mitochondria and chloroplast.
• Mitochondria is able to produce their own protein.
• The movement of substance against diffusion gradient with the use of energy from respiration through cell membrane is called active transport.

Difference between Plant and Animal cells

• Cell becomes turgid because of endosmosis.
• The process of imbibitions involves both diffusion and capillary action.
• A cell increases in volume when it is placed in hypotonic solution.
• When fresh egg place in salt water, it floats.

Chromosome

• Chromosomes are thread like structure found in the nucleus as chromatin material. It becomes visible during cell division. Each chromosome is made up of two chromatids joined together at a point centromere. Bead like structure found on chromosome is called gene. Genes are made up of DNA (deoxyribo nucleic acid) which are the carrier of genetic information from generation to generation. In some viruses RNA is the genetic material called retrovirus. In prokaryotes there is only one chromosome, like bacteria and virus.
• Chromosomes are made up of DNA and protein.
• Chromosome was named by Waldeyer in 1888 capable of self replication, which transmits coded information from one generation to other.
• Eukaryotic cell possess many chromosome. A particular kind of species have definite number of chromosome in their cell, which are in pair known as diploid. The set of unpaired chromosome is called haploid. Gametes have haploid set of chromosome.

Number of Chromosomes in Different Organisms:

Nucleic Acid:

• Nucleic acid is a complex organic compound found in cell. It contain special genetic instruction in coded form. Nucleic acids are of two kinds-

A. Deoxyribo Nucleic Acid (DNA):

Frederic Meischer was the first who isolated DNA from the nucleus of pus cells. DNA is a macro molecule in which large number of nucleotides are present. Chemically a nucleotide has three components.
1. Nitrogen base 2. Deoxyribose or ribose sugar 3. Phosphate group.

• Nitrogen base are of two types – Purines & Pyrimidines. Purines contain two types of nitrogen base – Adinine and Guanine. Pyrimidine nitrogen base are Thymine and Cytosine.
• Thus there are four kinds of nucleotides present in DNA.

Watson and Crick gave the structural model of DNA –

1. DNA molecule is consist of two polynucleotide strand, forming a double helix. Each strand has a backbone of sugar and phosphate. Nitrogen base is attached to the sugar.

• Sugar + Nitrogen base = Neucleoside.
• Sugar + Nitrogen base (Neucloside) + Phosphate (Nucleoside) = Nucleotide

2. Nitrogenous base of the two strands of a double helix form a pair with the help of hydrogen bonds. Adenine pairs with thymine where as guanine pairs with cytosine. Adenine and thymine are complementary to each other whereas cytosine is complementary to guanine. Hydrogen bonding between nitrogenous base holds the two strands together. This structure can be compared with the steps of spiral staircase.

• Number of hydrogen bond found between guanine and cytosine are three.
• Adenine pairs with thymine or vice versa by double hydrogen bond.
• Hydrophilic nature of DNA is due to presence of phosphate bond.

Function:
1. It contain genetic information is coded form, responsible for controlling hereditary characters.
2. DNA synthesis RNA

Note: DNA is mainly found in nucleus. In small amount it is also found in mitochondria and chloroplast.

• Phosphorous is an essential constituent of nucleic acid.
• Sub unit of DNA is nucleotide.

Gene: Gene is hereditary unit which is made by a special segment of DNA found on the chromosome.

• Any change in the base sequence within gene is called mutation.

B. Ribonucleic Acid (RNA):

RNA is single stranded nucleic acid made up of phosphate, ribose sugar and nitrogen base uracil, adenine, guanine and cytosine. It is found in nucleus as well as cytoplasm.

• RNA molecule which act as enzymes are known as riboenzyme.

RNA is of three kind:

1. Messenger RNA (mRNA): It brings message from DNA found in the nucleus to cytoplasm in the coded form.

2. Ribosomal RNA (rRNA): Present in ribosome which is the site of protein synthesis.

3. Transfer RNA (tRNA): It is the carrier of amino acid and transfer it to the ribosome.

Function: Synthesis of protein.

• The process of copying genetic information from one strand of DNA into mRNA is termed as transcription.

Difference between RNA and DNA

Cell cycle:

It is the sequence of events in which cell duplicates its genetic material, syntheses the other constituents of cell and ultimately divide into two daughter cell.

Cell Division:

The process in which cell increase in their number is called cell division. It is needed for growth, development and repair of body. There are mainly two kind of cell division –

A. Mitosis:

Mitosis cell division generally occur in somatic cell which make part in growth, repair and development. In unicellular organism asexual reproduction takes place by this type of cell division.

Significance of Mitosis:

1. After Mitosis cell division one cell divided into two daughter cell in which number of chromosome is equal to parent cell.

2. Uncontrolled Mitosis may cause tumor or cancerous growth.

B. Meiosis:

1. Meiosis cell division occur in reproductive cell. This type of division takes place during the formation of haploid gamete, i.e. ova & sperm.

2. It is also known as reduction division during which each daughter cell have haploid number of chromosome.

3. Four daughter cells are produced from one meiotic cell division.

Terms related to cytology:

• Karyokinesis: Division of nucleus during cell division is called Karyokinesis.
• Cytokinesis: Division of cytoplasm is called cytokinesis.
• Diploid: Two complete set of chromosome is called diploid, found in somatic cell.
• Haploid: Single set of chromosome in cell is called haploid, found in gametes.
• Crossing over: Exchange of genetic material between two non sister chromatids takes place during meiosis cell division is called crossing over.
• Homologous chromosome: A pair of chromosome having semesize and shape bearing corresponding gene.
• Allete: Alternative form of characters governed by gene.
• Phenotype: The character of organism which can be seen directly.
• Genotype: Genetic constitution of organism is called genotype.
• Tonoplast: The membrane surrounding the vacuole.
• Unit membrane: The basic trilaminar structure of cell membrane.