Study of Cell - Cytology

Study of Cell - Cytology

Cell and Cytology

The study of cells 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, Mitochondria, Mytosis

Cell

• The 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 the human body is the Neuron.

• The biggest cell is the egg of the Ostrich.

• Schilden and Schwan established cell theory in the year 1838-39.

• Germ plasm theory was proposed by August Weismann.

Main Features of the Cell Theory

1. All organisms are composed of cells.

2. The body of every organism is made of cells.

3. Each cell arises from a pre-existing cell.

4. Every organism starts its life from a single cell.

Types of Cells

Cells are of two kinds:

1. Prokaryotic Cell

• These are primitive cells having three basic structures of a typical cell but lack a nuclear membrane.

• Nuclear material is present in a region of cytoplasm called the nucleoid.

Other membrane-bound organelles are absent, such as mitochondria, lysosomes, Golgi bodies, etc.

Examples – Bacteria and Cynobacteria are Prokaryotes.

• Number of Mitochondria in bacterial cell is zero.

• The smallest known prokaryotic organism is Mycoplasma.

• In photosynthetic prokaryotic bacteria, chlorophyll is associated with membranous vesicles.

2. Eukaryotic Cell

These are complete cells 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 a Typical Cell

A cell has the following structures:

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 provides 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 molecules inside and outside of the cell. The membrane is selectively permeable to some molecules 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 the 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.

• 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 structures found in cytoplasm.

• It is surrounded by a double-layered membrane.

• Inner membrane has many folds called cristae.

• The fluid present inside mitochondria is called matrix, which contains many enzymes and co-enzymes.

• 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 the ‘Power House’ of the cell.

• ATP is known as the energy currency of cell.

5. Golgi Bodies

• Discovered by scientist Camilo Golgi.

• Golgi bodies are made up of a group of tubes, vesicles, and vacuoles.

• In plant it is more in number and here it is known as dictyosomes.

Function:

• It works as storage, processing, and packaging of material.

• It is also involved in the synthesis of cell wall, plasma membrane, and lysosome.

6. Endoplasmic Reticulum

• Membranous network of tubules-like structures found in cytoplasm is called endoplasmic reticulum.

• It is attached with the nucleus on one side and on the other side it is joined with plasma membrane.

Function:

• Endoplasmic reticulum helps in the distribution of material.

• It forms a supporting framework of cell.

• Rough endoplasmic reticulum is the site of protein synthesis due to the presence of ribosome on it.

• Smooth endoplasmic reticulum is the major site of lipid synthesis.

Endoplasmic reticulum is responsible for 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 structures bounded by a 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 mammals.

9. Centrosome

• Discovered by Boveri.

• It is only found in animal cell, taking part in cell division.

• It is not bounded by membrane and consists of two centriole.

Function:

• Centriole found in Centrosome initiate cell division forming spindle fiber between the poles.

10. Plastid

Only found in plant cell. It is of three types:

(a) Chloroplast

(b) Chromoplast

(c) Leucoplast

(a) Chloroplasts

• These are green pigments found in green plants involved in photosynthesis.

• So, it is known as the ‘Kitchen of the cell’.

Chloroplast is bounded by two unit membranes having grana and stroma.

• Grana are made up of membrane-bounded sac-like structures known as thylakoids found in stacks.

• It contains chlorophyll molecule.

• Stroma is the matrix present inside the chloroplast which contains 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 kinds:

• Lycopene: In tomato it provides red colour.

• Carotene: Provides 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 organelle of cell.

• In plant cell it is larger in size but in animal it is smaller in size.

• The membrane which surrounds the vacuole is called tonoplast.

Function:

• It helps in osmoregulation. It stores toxic metabolic waste.

• In plant cell, vacuoles provide turgidity and rigidity.

• In amoeba, vacuole plays a role in osmoregulation and nutrition.

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 a double-layered nuclear membrane.

• Within nucleoplasm, chromatin material is present.

• Nucleolus is rich in protein and RNA.

• Chromatin material forms a network made up of DNA and histone protein.

• During cell division, chromatin breaks into pieces and forms chromosome.

Function:

• It controls all the activities of cells. So it is also known as the ‘control room’ of the cell.

• Chromatin transmits hereditary characters from parents to 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 using energy from respiration through cell membrane is called active transport.

Difference between Plant and Animal Cells

Additional Notes

• Cell becomes turgid because of endosmosis.

• The process of imbibition involves both diffusion and capillary action.

• A cell increases in volume when placed in hypotonic solution.

• When a fresh egg is placed in salt water, it floats.

Chromosome

Chromosomes are thread-like structures found in the nucleus as chromatin material.

• They become visible during cell division.

• Each chromosome is made up of two chromatids joined together at a point centromere.

• Bead-like structures found on chromosome are called genes.

• Genes are made up of DNA which carry genetic information.

• In some viruses RNA is the genetic material (called retrovirus).

• Chromosomes were named by Waldeyer in 1888.

Number of Chromosomes in Different Organisms

Nucleic Acid

Nucleic acid is a complex organic compound found in cells.

• It contains special genetic instructions in coded form.

• Nucleic acids are of two kinds:

A. Deoxyribo Nucleic Acid (DNA)

• Frederic Meischer first isolated DNA from the nucleus of pus cells.

• DNA is a macromolecule made of many nucleotides.

A nucleotide has three components:

1. Nitrogen base

2. Deoxyribose or ribose sugar

3. Phosphate group

Nitrogen bases:

• Purines: Adenine and Guanine

• Pyrimidines: Thymine and Cytosine

Watson and Crick gave the structural model of DNA — a double helix structure.

Function:

1. It contains genetic information responsible for heredity.

2. DNA synthesizes RNA.

B. Ribonucleic Acid (RNA)

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

Types of RNA:

1. Messenger RNA (mRNA): Carries message from DNA to cytoplasm.

2. Ribosomal RNA (rRNA): Present in ribosomes, site of protein synthesis.

3. Transfer RNA (tRNA): Carries amino acids to ribosome.

Function: Synthesis of protein.

Cell Cycle and Cell Division

The cell cycle is the sequence of events where a cell duplicates its genetic material, synthesizes other constituents, and divides into two daughter cells.

A. Mitosis

• Occurs in somatic cells for growth, repair, and development.

• Unicellular organisms reproduce asexually by mitosis.

Significance:

1. Produces two daughter cells with the same chromosome number as the parent.

2. Uncontrolled mitosis may cause tumor or cancerous growth.

B. Meiosis

• Occurs in reproductive cells during the formation of gametes (ova & sperm).

• Also known as reduction division.

• Produces four daughter cells with half the chromosome number of the parent.

Terms Related to Cytology

• Karyokinesis: Division of nucleus during cell division.

• Cytokinesis: Division of cytoplasm.

• Diploid: Two complete sets of chromosomes (in somatic cells).

• Haploid: Single set of chromosomes (in gametes).

• Crossing over: Exchange of genetic material during meiosis.

• Homologous chromosome: Pair of chromosomes with same size and gene loci.

• Allele: Alternative form of a gene.

• Phenotype: Observable characteristics.

• Genotype: Genetic constitution of organism.

• Tonoplast: Membrane surrounding the vacuole.

• Unit membrane: Basic trilaminar structure of the cell membrane.

Final Thoughts

The study of cells, called cytology, helps us understand the basic unit of life. Cells vary in structure and function, with plant and animal cells showing key differences like cell wall and plastids in plants, and centrosomes in animals.

Each cell contains vital organelles such as nucleus, mitochondria, ribosomes, and lysosomes, performing essential roles like energy production, protein synthesis, and intracellular digestion. DNA and RNA carry genetic information, while processes like mitosis and meiosis ensure growth, repair, and reproduction.

Learning about the structure, types, and functions of cells allows students to grasp how life works at a microscopic level and how organisms develop, survive, and pass traits to the next generation.