WBCS Preliminary (Biology): Cell-Unit of Life

1. All organisms are made of smaller parts called organs.
2. Organs are made of still smaller parts. The smallest living part of an organism is a ‘cell’.
3. The word cell is derived from the Latin word “cellula” which means “a little room”.
4. Cells were first observed in cork by Robert Hooke in 1665.
5. Leeuwenhoek (1674), with the improved microscope, discovered the free living cells in pond water for the first time.
6. Cells exhibit variety of shapes and sizes.
7. Number of cells also varies from organism to organism.
8. Man is estimated to have about 100 trillion (10¹⁴) cells in number.
9. Some cells are big enough to be seen with the unaided eye. Hen’s egg is an example.
10. Some organisms are single-celled, while others contain large number of cells.
11. The single cell of unicellular organisms performs all the basic functions performed by a variety of cells in multicellular organisms.
12. Cells are enclosed by a plasma membrane composed of lipids and proteins.
13. The cell has three main parts, (i) the cell membrane, (ii) cytoplasm which contains smaller components called organelles, and (iii) the nucleus.
14. • Cell Membrane: The cell membrane also called the plasma membrane is an active part of the cell. It regulates the movement of materials between the ordered interior of the cell and the outer environment. The plasma membrane is porous and allows the movement of substances or materials both inward and outward.
    • Cell Wall: Plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall. The cell wall lies outside the plasma membrane. The plant cell wall is mainly composed of cellulose. Cellulose is a complex substance and provides structural strength to plants.
    • Nucleus: Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane. With a microscope of higher magnification, we can see a smaller spherical body in the nucleus. It is called the nucleolus.
    • In addition, nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of characters from the parents to the offspring.
    • Chromosomes contain information for inheritance of features from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules. Chromosomes are composed of DNA and protein. DNA molecules contain the information necessary for constructing and organising cells. Functional segments of DNA are called genes. In a cell which is not dividing, this DNA is present as part of chromatin material. Chromatin material is visible as entangled mass of thread like structures. Whenever the cell is about to divide, the chromatin material gets organised into chromosomes.
    • Gene: Gene is a unit of inheritance in living organisms. It controls the transfer of a hereditary characteristic from parents to offspring.
    • Cytoplasm : It is the jelly-like substance present between the cell membrane and the nucleus. Various other components, or organelles, of cells are present in the cytoplasm. These are mitochondria, golgi bodies, ribosomes, etc.
    • Vacuoles: Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small sized in animal cells while plant cells have very large vacuoles.
    • Most plant cells have large membranous organelles called plastids, which are of two types – chromoplasts and leucoplasts.
    • The Golgi apparatus consists of stacks of membrane-bound vesicles that function in the storage, modification and packaging of substances manufactured in the cell.
    • Chromoplasts that contain chlorophyll are called chloroplasts and they perform photosynthesis.The primary function of leucoplasts is storage.
    • The entire content of a living cell is known as protoplasm. It includes the cytoplasm and the nucleus. Protoplasm is called the living substance of the cell.
    • The nucleus of the bacterial cell is not well organised like the cells of multicellular organisms. There is no nuclear membrane. The cells having nuclear material without nuclear membrane are termed prokaryotic cells. The organisms with these kinds of cells are called prokaryotes (pro :primitive; karyon : nucleus). Examples are bacteria and blue green algae. Prokaryotic cells have no membrane-bound organelles, their chromosomes are composed of only nucleic acid, and they have only very small ribosomes as organelles.
    • The cells, like onion cells and cheek cells having well organised nucleus with a nuclear membrane are designated as eukaryotic cells. All organisms other than bacteria and blue green algae are called eukaryotes. (eu : true; karyon: nucleus).
15. Cell Division: Every living organism reproducing sexually is derived from a single cell, the zygote, which divides again and again to produce a large number of body cells. This division is accomplished in phases, the division of nucleus called mitisis and the cytoplasmic division called meiosis.
    • Mitosis
      • A parent cell will copy of all its internal components, divide them equally, and then split in half to form 2 daughter cells,
      • The daughter cells formed are identical to each other.
      • In single-celled, eukaryotic organisms this is the way that they form new individuals.
      • In multi-cellular organisms, mitosis is used for growth and repair of damaged tissue.
      • Mitosis is tightly controlled by the cell cycle to ensure that mitosis happens only when it is needed.
      • A normal cell completes the cycle with-in 24 hours.
      • Main steps during mitosis:
        1. Interphase :DNA is replicated along with organelles and other cellular components and the cell prepares for division. During the replication process, the DNA changes from one double helix (unreplicated) to two double helices (replicated).The two helices in the replicated chromosomes are joined to one another in the special region of the chromosome called the centromere – at this point, the two double helices are called are sister chromatids
        2. Prophase : (preparation phase) The DNA recoils and the chromosomes condense; the nuclear membrane disappears, and the mitotic spindles begin to form.
        3. Metaphase : (organizational phase). The chromosomes line up the middle of the cell with the help of spindle fibers attached to the centromere of each replicated chromosome.
        4. Telophase :The chromosomes, along with the cytoplasm and its organelles and membranes are divided into 2 portions
        5. Cytokinesis: The actual splitting of the daughter cells into two separate cells is called cytokinesis and occurs differently in both plant and animal cells as is illustrated below.
      • Animal cells: The cell pinches in on both sides to form a cleavage furrow. This will gradually move toward the center to split the parent cell into 2 daughter cells.
      • Plant cells: The cell forms a cell plate, which starts in the center and moves towards the outer edges to split the parent cell into two daughter cells.
    • Meiosis: Meiosis is used in sexual reproduction for the formation of gametes (egg and sperm cells). It creates genetic diversity. The gametes that are produced as an end result of meiosis are haploid cells, they contain half the genetic material of the parent cell. This will ensure that when gametes combine during fertilization that the new zygote will contain the normal amount of chromosomes instead of double the genetic material.
      • Just like in mitosis, during INTERPHASE, the DNA is replicated before the cell undergoes the division process.
        1. Prophase I : The chromosomes coil up and condense and the nuclear membrane disappears. The cell prepares for separation. During this process, the chromosomes of each homologous pair lie so close to one another that the arms can get tangled up. The lining up of homologous chromosomes in pairs is known as synapsis. This can lead to exchange of genetic material between the homologous pairs known as crossing-over. This will serve to create genetic diversity.
        2. Metaphase I : The homologous pairs line up in the center of the cell. During this time, the chromosomes obey the law of independent assortment – this says that each homologous pair arranges itself independently of the others.
        3. Anaphase I :The homologous pairs are separated and pulled opposite poles of the cell by the spindle fibers.
        4. Telophase I : The parent cell separates into 2 daughter cells with the division of the cytoplasm, organelles, and membranes. The new daughter cells now have the number of chromosomes of the parent cell but the chromosomes are still replicated. The two daughter cells are no longer identical to the parent cell or each other.
      • Meiosis II: This is essentially the same as mitosis – we have to split the replicated chromosomes so they are no longer in a replicated state. Remember that this is taking place now in both of the two new daughter cells we have just created.
        1. Prophase II: This stage resembles that of mitotic prophase – the chromosomes recoil and condense and any nuclear membrane that has formed will now disappear.
        2. Metaphase II: The chromosomes line up in the middles attached to the spindle fibers and prepare for separation – again, remember this is happening in two different cells simultaneously.
        3. Anaphase II: The sister chromatids are pulled apart to opposite ends of the cells by the spindle fibers.
        4. Telophase II: The cells split into 2 daughter cells, and cytokinesis occurs. The end result is 4 haploid daughter cells that are not identical to one another.