The Cell: The Unit of Life
Structure and Functions of Cell
- In 1838, Schleiden and Schwann put forward the cell theory stating that plant or animal body is ultimately made up of minute cells and concluded that the cell is the structural unit o[life.
- The living organisms are of two types, either unicellular or multicellular.
- In multicellular organisms, the life activities are performed by co-ordination of several organs, these organs are made up of tissues, while the tissues in turn are aggregates of similar cells. Thus, a cell is a structural unit of life. While in unicellular organisms all activities are performed by the same cell.
Structure of Cell
The cell has a definite shape of its type with a few exceptions like amoeba, leucocytes, etc. Various shapes like oval, spherical, polyhedral, columnar, cylindrical, stellate and several others are found in plant as well as in animal cells. Normally the size may vary from 0.5 to 20 p and very exceptionally up to 200 — 300 mm as in the case of plant fibers and latex cells.
There are two types of cells. These are:
(i) Prokaryotic, (ii) Eukaryotic.
Prokaryotic cells are characterized by the absence of true nucleus, obviously the nucleolus, nuclear membrane are missing, DNA is without protein sheath and nuclear matter is in direct contact with the cytoplasm. The ribosomes are scattered in the matrix while other organelles and endoplasmic reticulum are missing in prokaryotic cells. Respiratory enzymes and photosynthetic pigments are present. Meiosis and mitosis are not observed in this type of cells.
Primitive type of organisms like blue-green algae, bacteria, represents this type of cells. Eukaryotic cells possess well-marked true nucleus, with nuclear membrane, while DNA is covered with protein sheath. There is a distinct nucleolus in the nucleus, while plastids and mitochondria are represented in the cytoplasm. The other important and major component of the cytoplasm is endoplasmic reticulum associated with ribosomes. In eukaryotic cells, cell wall is made up of cellulose. Meiosis and mitosis are observed in these types of cells. All plant and animal cells belong to eukaryotic type of cells.
Difference between plant and animal cell
Before knowing the structure and functions of various cell organelles, one must know the structural difference between a plant cell and an animal cell.
Functions of Cell
- 1. Cell wall:
- (i) It offers rigid framework and protection to protoplast.
- (ii) Thick and lignified cells of the plant provide mechanical support to the organ.
- (iii) Checks the rate of transpiration due to cuticular sheath.
- (iv) The developing wall pressure prevents the distention of protoplast. Due to various contents like cut in, lignin, wax etc. the cell wall results in permeability which is ultimately responsible for life of cell.
- 2. Plasma membrane:
- (i) Being selectively permeable controls transport of materials across it.
- (ii) Permits diffusion of water- and fat-soluble components. Fat insoluble components pass through the membrane by forming reversible compounds with membrane proteins.
- 3. Endoplasmic reticulum:
- (i) Due to ribosomes, it is involved in protein synthesis, also in glycogens and fat metabolism.
- (ii) Gives mechanical support to cytoplasm.
- (iii) Participate in exchange of materials by active and passive transport.
- 1. Ribosomes: Degradation and synthesis of proteins takes place in ribosomes.
- 2. Golgi complex:
- (i) Condensation of lipids, carbohydrates hormones takes place in golpe bodies.
- (ii) Participates in formation of Lysosomes.
- 3. Mitochondria:
- (i) Mainly responsible for transformation of chemical energy into biological energy in the form of ATP compounds.
- (ii) All enzymes involved in Krebs cycle are present in mitochondria. It is also responsible for transmission of hereditary characters (extra-nuclear).
- 4. Plastids:
- Plays vital role in plant metabolism. Chloroplasts capture solar energy and convert it in chemical energy (photosynthesis).
- 5. Nucleus:
- The nucleus controls all activities of the cell. Biogenesis of ribosomal proteins take place in nucleolus only, nucleolus takes part in cell division.
- 6. Nucleolus: Within the nucleus a single or more spherical bodies called nucleoli are present. The nucleolus is the aggregation of portions of chromosomes which are responsible for the secretion of the ribosomes, the subunits. Nucleolus is composed of two important nucleic acids the deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) RNA is in granular form while DNA is in chromosome form.
- 7. Centrosome:
- Centrosomes are associated with the nuclear membrane durinp prophase of the cell cycle. In mitosis, the nuclear membrane breaks down and the centrosome nucleated microtubules (parts of the cytoskeleton) can interact with the chromosomes to build the mitotic spindle.
- 8. Chromosomes:
- These play very important role in heredity, mutation and variation. Chromosomes have capacity of self-reproduction.
- Golgi complex is present in all eukaryotic cells except mammalian red blood corpuscles. In plant cells as well as in invertebrate tissues, it is not present as a single body but there are many such organelles scattered throughout the cytoplasm.
- Lysosomes are present in animal cells and not in the plant cells. Plastids are the organelles which are only present in plant cells. They are absent in bacteria and blue green algae.
Cell Division
- Types of Cell Division (Mitosis and Meiosis) Cell division includes mitosis and cytokinesis.
Mitosis
- In mitosis, cell nucleus is replicated and divided into two identical nuclei containing genetically identical material.
- 1. Prophase:
- It is first step which follows G2 phase of interphase.
- Characteristics of Prophase:
- 1. Chromosomal material condenses and seen to be composed of two chromatids and centromere.
- 2. Nuclear envelope breaks up. Spindle fibers begin to form and extends from centrioles. These are made up of microtubules and attached to centromere of sister chromatids.
- 3. Centrioles slowly migrate to opposite side of cell. At the end of prophase, cells do not show under microscope, Golgi complex, endoplasmic reticulum, nucleolus and nuclear envelope.
- 1. Metaphase:
- Characteristics of Metaphase:
- 1. In this phase, chromosomes move and lined up along equatorial plate or metaphase. (Equatorial plate is imaginary line in the center of cell.)
- 2. In this phase, chromosomes move with the help of spindle fires and centrioles. The spindle fibers are attached to kinetochores (disc shaped structure at centromere and serve as site of attachment of spindle fibers) of chromosomes.
- 3. Anaphase:
- Each chromosome, arranged at metaphase, is splits and pulled apart by spindle fibers, causing sister chromatids to separate and create two daughter chromosomes. The daughter chromosome further migrates towards opposite poles. This ensures that daughter cells will have identical sets of chromosomes and is also identical to parent cell.
- 4. Telophase:
- New nuclei begin to form around new set of chromosomes. Spindle fiber disappears at the end of this phase and chromosomes cluster at opposite poles and back into their loose form. Nuclear envelopes assemble around chromosome and nucleolus, golpe complex and ER reform.
Meiosis
- Meiosis is formation of gametes (production of haploid cells) from specialized diploid cells. This is performed by reproductive cells only. Meiosis involves two cycles of nuclear and cell division (Meiosis I and Meiosis II) and only single cycle of DNA replication.
- Meiosis T:
- It is a four-step process:
- 1. Prophase T:
- It is longer and complex process as compared to prophase of mitosis. It is further divided into more steps where; chromosomes thicken and are visible under microscope. Then chromosomes start pairing (synapsis) and are known as homologous chromosomes. These homologous chromosomes move towards equatorial plate. Thus, four sisters.
- 2. Metaphase T:
- During this stage, homologous pairs are lined up along equatorial plate.
- 3. Anaphase:
- The homologous chromosomes separate, while sister chromatids remain associated at their centromeres' (each chromosome has two sister chromatids).
- 4. Telophase T:
- The nuclear membrane and nucleolus reappear and then this follows cytokinesis resulting in two new cells each with haploid number of chromosomes and are in the form of sister chromatids.
- 2. Metaphase T:
- During this stage, homologous pairs are lined up along equatorial plate.
- 3. Anaphase !:
- The homologous chromosomes separate, while sister chromatids remain associated at their centromere (each chromosome has two sister chromatids).
- 4. Telophase T:
- The nuclear membrane and nucleolus reappears and then this follows cytokinesis resulting in two new cells each with haploid number of chromosomes and are in the form of sister chromatids.
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