Cell

cell, cell types, structure &function

1. Shri Shankaracharya Mahavidyalaya , Junwani , Bhilai C ell Dr. Sonia Bajaj (Head of Department)

2. Introduction   Cytology:  ( G.k . kyios = cell ; logas = study) is the branch of biology which comprises the study of cell structure and function. All the living organisms are made up of fundamental unit of life called  “ cell”. The cell is a Latin word for  “a little room”. The scientist Robert Hooke (1665)-saw a little room in the cork (the bark of a tree) resembled the structure of a honeycomb. The use of the word “Cell” to describe these units is used till this day in Biology as” Cell Biology”. Robert Brown in 1831 discovered the nucleus in the cell. The scientist  Leeuwenhoek saw free living cells  in the pond water for the first time. (father of microbiology) The  cell theory  states that all the plants and animals are composed of cells, it was  proposed by Schleiden and Schwann . The  cell theory  was further expanded by Virchow by suggesting that “ all cells arise from the pre-existing cells”.   Cells are the basic structural and functional unit of life. Cell was discovered by Robert Hooke. A number of cells can work together to form tissues and organs. Cell is termed as the structural unit of life as it  provides structure to our body. Cell is considered as the functional  unit of life as all the functions of the body take place at cell level.

3. Scientist name Year Discovered By Robert Hooke 1665 noticed the presence of cells in a cork slice Leeuwenhoek 1674 found the presence of living cells in the pond water Robert Brown 1831 recognized the existence of a nucleus in the cell Purkinje 1839 invented the term ‘ Protoplasm’ which is the liquid present in a cell Schleiden and Schwann 1838, 1839 presented the cell theory that all organisms are actually made up of cells Virchow 1855 suggested that all cells come from cells that already exist in nature

4. The Size and shape of the cell may vary depending upon the type of function they perform in an organism . They may be oval, spherical, rectangular, spindle shaped, or totally irregular like the nerve cell.   Types of cells: Onion cells, Smooth muscle cell, Blood cells, Bone cell, Fat cell, Nerve cell, Ovum, Sperm etc. Each kind of cell performs specific function . A single cell may constitute a whole organism as in  Amoeba, Chlamydomonas , Paramecium and Bacteria ; these are called as  unicellular organisms . Whereas in multi-cellular organisms (Human beings) division of labor is seen . The feature in almost every cell is same- Plasma membrane, nucleus and cytoplasm. Cells can have different Shaped and Sizes

5. Two Types of Organisms on the Basis of Number of Cells (i) Unicellular Organisms:  The organisms that are made up of single cell and may constitute a whole organism, are named as unicellular organisms. For example: Amoeba, Paramecium, bacteria, etc . (ii) Multicellular Organisms:  The organisms which are composed of a collection of cells that assume function in a coordinated manner, with different cells specialized to perform particular tasks in the body, are named as multicellular organisms. For example: Plants, human beings, animals, etc.

6.                            Plant cell                                   Animal cell Cell wall present. Cell wall absent. Nucleus usually lies near periphery due to vacuole. Nucleus present near the centre. Centrosome is usually absent from higher plant cells, except lower motile cells. Usually centrosome is present that helps in formation of spindle fibres. Plastids are present, except fungi. Plastids are absent. Single large central vacuole is present. Many vacuoles occur, which are smaller in size. Number of mitochondria from 200 – 2000. Number of mitochondria is approximately 1600 – 16000 in liver cells. Cytoplasm during cell division usually divides by cell plate method. Cytoplasm divides by furrowing or cleavage method. There is no contractile vacuole. Contractile vacuole may occur to pump excess water. Spindle formed during cell division is an astral. Spindle formed during cell division are amphi astral . Lysosomes present in less number. Lysosomes present in more number. Chromosomes are larger in size. Chromosomes are smaller in size.

7. Types of Cells The cells can be categorized in two types : 1. Prokaryotic Cell  2. Eukaryotic Cell Prokaryotic cell Prokaryotic cells are cells in which true nucleus is absent. They are primitive and incomplete cells. Prokaryotes are always unicellular organisms . These cells are small and multiply at a faster rate. For example, Mycoplasma  and PPLO ( Pleuro Pneumonia Like Organisms ). archaebacteria , bacteria, blue green algae are all prokaryotes .   2. Eukaryotic Cell Eukaryotic cells are the cells in which true nucleus is present. They are advanced and complete cells. Eukaryotes include all living organisms (both unicellular and multicellular organisms) except bacteria and blue green algae .

8. Characteristics of Prokaryotic Cell Prokaryotic cells are the cells that do not have a true nucleus and membrane-bound organelles .” They lack a nuclear membrane. Mitochondria, Golgi bodies, chloroplast, and lysosomes are absent. The genetic material is present on a single chromosome. The histone proteins, the important constituents of eukaryotic chromosomes, are lacking in them. The cell wall is made up of carbohydrates and amino acids. The plasma membrane acts as the mitochondrial membrane carrying respiratory enzymes. They divide asexually by binary fission. The sexual mode of reproduction involves conjugation.

9. Prokaryotic Cell Structure A prokaryotic cell does not have a nuclear membrane. T he   genetic material is present in a region in the cytoplasm known as the nucleoid. They may be spherical, rod-shaped, or spiral . A prokaryotic cell structure is as follows: Capsule – It is an outer protective covering found in the bacterial cells, in addition to the cell wall. It helps in moisture retention, protects the cell when engulfed, and helps in the attachment of cells to nutrients and surfaces. Cell Wall – It is the outermost layer of the cell which gives shape to the cell. Cytoplasm – The cytoplasm is mainly composed of enzymes, salts, cell organelles and is a gel-like component. Cell Membrane – This layer surrounds the cytoplasm and regulates the entry and exit of substances in the cells. Pili – These are hair-like outgrowths that attach to the surface of other bacterial cells. Flagella – These are long structures in the form of a whip, that help in the locomotion of a cell . Ribosomes – These are involved in protein synthesis. Plasmids – Plasmids are non-chromosomal DNA structures. These are not involved in reproduction . Nucleoid Region – It is the region in the cytoplasm where the genetic material is present. A prokaryotic cell lacks certain organelles like mitochondria, endoplasmic reticulum, and Golgi bodies.

10. Reproduction in Prokaryotes A prokaryote reproduces in two ways : 1. Asexually by binary fission 2. Sexually by conjugation Binary Fission The DNA of an organism replicates and the new copies attach to the cell membrane. The cell wall starts increasing in size and starts moving inwards. A cell wall is then formed between each DNA, dividing the cell into two daughter cells . Recombination In this process, genes from one bacteria are transferred to the genome of other bacteria. It takes place in three ways-conjugation, transformation, transduction . Conjugation  is the process in which genes are transferred between two bacteria through a protein tube structure called a pilus . Transformation  is the mode of sexual reproduction in which the DNA from the surroundings is taken by the bacterial cell and incorporated in its DNA . Transduction  is the process in which the genetic material is transferred into the bacterial cell with the help of viruses. Bacteriophages are the virus that initiates the process.

11. Characteristics of Eukaryotic Cells Eukaryotic cells are the cells that contain a membrane bound nucleus and organelles.” The features of eukaryotic cells are as follows : Eukaryotic cells have the nucleus enclosed within the nuclear membrane. The cell has mitochondria. Flagella and cilia are the locomotory organs in a eukaryotic cell. A cell wall is the outermost layer of the eukaryotic cells . The cells divide by a process called mitosis . The eukaryotic cells contain a cytoskeletal structure . The nucleus contains a single, linear DNA, which carries all the genetic information.

12. Eukaryotic Cell  Structure Eukaryotic cell diagram mentioned below depicts different cell organelles present in eukaryotic cells. The nucleus, endoplasmic reticulum, cytoplasm, mitochondria, ribosomes, lysosomes are clearly mentioned in the diagram .

13.                                Prokaryotic cell                                  Eukaryotic cell It is a single membrane system. It is a double membrane system. Cell wall surrounds the plasma membrane. Cell wall surrounds the plasma membrane in some protists, most fungi and all plant cell. Animal cell lack it. Cell wall composed of peptidoglycans. Strengthening material is mureir . It is composed of polysaccharide. Strengthening material is chitin in fungi & cellulose in others plants. Cell membrane bears respiratory enzymes. It lacks respiratory enzymes. Cytoplasm lacks cell organelles e.g., Mitochondria, ER, Golgi body etc. Cytoplasm contains various cell organelles. Ribosomes are 70 S type. Ribosomes are 80 S type. There are no streaming movements of cytoplasm. Cytoplasm show streaming movements. Mitotic spindle is not formed in cell division. Mitotic spindle is formed in cell division. Nuclear material is not enclosed by nuclear envelope and lies directly in cytoplasm. It is called nucleoid. It is enveloped by nuclear envelope. Nucleus is distinct from cytoplasm. DNA is circular and not associated with histone proteins. Nuclear DNA is linear and associated with histone proteins extranuclear DNA is circular and protein free. Replication of DNA occurs continuously throughout cell cycle. Replication of DNA occurs during S- Phase of cell cycle only. Sexual reproduction absent but parasexuality present.  Sexual reproduction is present. Plasmids and pili occur in many prokaryotes Example – E. coli There are no plasmids and pili in eukaryotic cells Example – Spirogyra, Chlorella Cell division mostly amitotic. Cell division is typically mitotic.