Important Topic Biology Class 11 Notes And Solutions Types Of Cell – Unit 3
Biology Notes for Class 11: All living organisms are composed of one or many cells to perform their individual functions. A cell is the smallest unit of a life, which is able to control and perform several functions in all living organisms. Within a selective and retentive semipermeable membrane, it contains a complete set of different kinds of entities necessary to permit its own growth and reproduction from simple nutrients.
Different scientists proposed the definition of a cell in different ways like-
- Mathias Jacob Schleiden, a German botanist, (1804-1881) and Theodor Schwann, a German zoologist, (1810-1882), together postulated for the first time that the cell is the basic unit of structure and function in all life.
- G. Loewy and P. Siekevitz (1963) have defined a cell as a unit of biological activity delimited by a semipermeable membrane and capable of self-reproduction in a medium free of other living systems.
- Wilson and Morrison (1966) defined the cell as an integrated and continuously changing system.
- John Paul (1970) has defined the cell as the simplest integrated organization in living systems capable of independent survival.
All the above definitions have to define viruses as neither an organism nor a cell, though it contains a core of nucleic acid (DNA/RNA as genetic material) which is enclosed in an external mantle of protein. In a free state, viruses are quite inert. They become activated only when they infect a living host cell and in this process, only the nucleic acid core enters the host’s cell. The nucleic acid, which is the genetic substance, takes the metabolic machinery for its multiplication, ultimately host killing the host cell. Viruses are primitive and simpler units of life, which may be considered as cellular parasites that require the host system for its reproduction.
Broadly the cells are of two types; Prokaryotic cells and Eukaryotic cells. All unicellular organisms (single cell) including bacteria and archaea, which are composed of a single cell, are examples of prokaryotic cells. All multicellular organisms (many or more than one cell)protists, fungi, plants, and animals, including humans, are examples of Eukaryotic cells. Both prokaryotic cells and eukaryotic cells have cytoplasm, cell membrane, and genetic material.
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Eukaryotic cells (Eu= “true”, karyon= “nucleus”) are called so because they have a true nucleus. The nucleus, which houses DNA, is contained within a membrane and separated from other cellular structures. Prokaryotic cells (Pro = “before”, karyon = “nucleus”), have no true nucleus. DNA in a prokaryotic cell is not separated from the rest of the cell but coiled up in a region called the nucleoid.
As organized in the Three Domain System, prokaryotes include archaeans and bacteria. Eukaryotes include animals, plants, fungi and protists (ex. algae). Typically, eukaryotic cells are more complex and much larger than prokaryotic cells. On average, prokaryotic cells are about 10 times smaller in diameter than eukaryotic cells.
Here we compare two main processes; cellular reproduction and cellular respiration (production of energy) that sustain life in both eukaryotes and prokaryotes.
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Eukaryotic cells grow and multiply through a process called mitosis. In organisms that reproduce sexually, the reproductive cells are produced by a type of cell division called meiosis.
Most prokaryotes reproduce a sexually and some through a process called binary fission. During binary fission, the single DNA molecule replicates and the original cell is divided into two identical daughter cells. Some eukaryotic organisms also reproduce asexually through processes such as budding, regeneration, and parthenogenesis.
Both eukaryotic and prokaryotic organisms get the energy they need to grow and maintain normal cellular function through cellular respiration. Cellular respiration has three main stages: glycolysis, the citric acid cycle, and electron transport. In eukaryotes, most cellular respiration reactions take place within the mitochondria. In prokaryotes, they occur in the cytoplasm and/or within the cell membrane.
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