Cell - The Basic Unit of Life
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| Cell - The Basic Unit of Life |
1. Cell - The Basic Unit of Life
Introduction
Cell is the basic unit of life. These are smallest structure capable of basic life process, such as taking in nutrients, expelling waste, and reproducing. All living things are composed of cells. Some organisms are unicellular such as bacteria and protozoa and the rest are multicellular. Different kinds of cells are organised into specialised groups called tissues such as tendons and bones. Different tissues types are assembled into organs, which are structures specialised to perform particular functions. Examples are heart, stomach and brain. Organs , in turn, are organised into systems such as the circulatory, digestive or nervous systems . All together, these assembled organ systems form Human body.
Important contributions
- Robert Hooke was the first to identify cells and gave them their name.
- Mathias schleiden , theodor Schwann and others formulated a cell theory.
- Virchow stated that new cells are formed due to the divisions of previously existing cells.
- Purkinje found that the protoplasm in a cell is instrumental for all the living activities of a plant.
- Robert Brown discovered nucleus.
- Strasburger and Weismann stated that the nucleus is concerned with the inheritance of characteristics of a living body
Activities with in cell
Metabolism is the sum total of biochemical reactions involved in the release and utilisation of energy within the organism is called Metabolism.
Anabolism consists of chemical reaction that forms complex substances from simpler substances resulting in the formation of more protoplasm and growth.
Catabolism consisting of chemical reactions that breaks down complex substances resulting in the release of energy.
Structure of cell
01) Cell wall: It is absent in animals but found in most of the plants. It is a semi rigid, permeable structure consisting of cellulose, lignin and other substances, gives shape to the cell.
02) Cell membrane: It is found both on plants and animals. It is a thin covering surrounding the cytoplasm. It is present just beneath the cell wall in plants. It is a semi permeable membrane which permits the passage of certain substances but prevents the passage of others.
03) Protoplasm: All components internal to cell. Cytoplasm is the portion of protoplasm without nucleus. It contains water (85 to 90 %), proteins (7-10%), fats (1-2%) and sugar and starches (1-2%) . Protoplasm contains a number of specialised structure called cell organelles and chemical compounds called cell inclusions.
04) Nucleus: The presence of a nucleus distinguishes the more complex eukaryotic cells of cells of plants and animals from simpler prokaryotic cells of bacteria that lack a nucleus. It is typically round and occupies about 10% of cells volume. The nucleus contains the nucleolus, which manufactures protein producing structures called ribosome. Genetic information in the form of DNA is stored in thread like structures called chromatin within nucleus. The chromatid is composed of nucleoproteins that are combinations of proteins and nucleic acids i.e. DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).The DNA in the nucleus also contains the instructions for regulating the amount and types of proteins made by the cell. These instructions are copied, or transcribed, into a type of ribonucleic acid (RNA) called messenger (mRNA). The mRNA is transported from the nucleus to ribosomes, where proteins are assembled.
05) Vacuole: It is found more common in plants than animals and separated from cytoplasm by a membrane tonoplast.
06) Plastids: These are found only in plant cells and three different types are found
- leucoplasts – found in sex cells, roots and underground cells
- Chromoplasts – It contains pigments that colour parts of plants
- Chloroplasts – That contain chlorophyll, green substance which converts carbon dioxide and water into sugar.
07) Centrosomes: It is a dense area of protoplasm which close to the nucleus. In the middle of the centrosomes are two small dot like, rod shaped or v shaped bodies called centrioles.
08) Endoplasmic reticulum (ER): It is an extensive network of tubes that manufacture, process, and transport materials within nucleated cells. The ER consists of a continuous membrane in the form of branching in the form of braching tubules and flattened sacs that extend throughout the cytoplasm and connect to the double membrane that surrounds the nucleus. There are two types of ER rough and smooth depending on the presence of ribosomes. Ribosomes contain RNA and are the sites of protein synthesis in the cells.
09) Mitochondria : These are the sites of energy release and called power houses of the cells.
10) Golgi apparatus : Also Golgi body or Golgi complex , network of stacked sacs found within nucleated cells that store, package and distribute the proteins and lipids made in ER. The Golgi apparatus in plants is known as dictyosome.
11) Lysosomes. These are vacuole like bodies that secrete enzymes to digest food substances. It is also involved in defence against bacteria and viruses, destroys old and worn out organelles and results in the death of the cell. So they are called suicide bags of the cell.
Cell division
The growth and development of all organisms depend upon the growth and division of cells. All these cell divisions starts from one cell called a fertilized egg cell or zygote. These are two types of cell divisions mitosis and meiosis. The two important events in cell divisions are nuclear division (Karyokinesis) followed by the division of cytoplasm(Cytokinesis).
Mitosis: This is the process in which a cell’s nucleus replicates and divides in preparation for the division of the cell. It results in 2 cells that genetically identical and occurs in 4 successive stages ; prophase, metaphase, anaphase and telophase. These four stages are preceded by a resting phase called interphase. In this phase, chromosomes undergo self duplication. Mitosis is vital for growth and replacement of damages or worn out cells and for asexual reproduction.
Meiosis: It is a type of cell division in which the cell’s genetic information, contained in chromosomes, is mixed and divided into sex cells (gametes) with half the normal number of chromosomes. The random sorting of chromosomes during meiosis assures that each new sex cell, and therefore each new offspring, has a unique genetic inheritance. Thus it results in the reduction of chromosomes in the daughter cells by half. So gametes are haploid and when they fuse together the produce a diploid zygote.Hence it is a source of new genetic variation.
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