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## Cell Biology ### **Totipotency** - The potential of a living nucleated cell to differentiate and divide to form a complete new organism. - A cell is totipotent because it has the entire genetic information of the organism in its nucleus. - Embryonic animal cells are totipotent and termed as ste...

## Cell Biology ### **Totipotency** - The potential of a living nucleated cell to differentiate and divide to form a complete new organism. - A cell is totipotent because it has the entire genetic information of the organism in its nucleus. - Embryonic animal cells are totipotent and termed as stem cells. Stem cells have great medical applications including cure for diseases. ### **Know the scientists** - **Matthias Schleiden** (1838) - German botanist. - Concluded that various tissues of plants are composed of different types of cells. - **Theodore Schwann** (1839) - British zoologist. - Proposed cells are bound by a thin membrane and the existence of a cell wall as a unique characteristic of plant cells. - Proposed both animals and plants are made up of cells and products of cells. ### **Postulates of modern cell theory** - All living organisms are made up of cells. - Living cells arise from pre-existing cells. - A cell is the structural and functional unit of life. - Total activities of cells are responsible for the activity of an organism. - Cells show transformation of energy. - Cells contain nucleic acids: DNA and RNA in the nucleus and cytoplasm. ### **Kinds of cells** - Living organisms are grouped into two main categories: - **Prokaryotes:** simple cellular organization - **Eukaryotes:** exhibit a high degree of organization. ### **Prokaryotic Cells** - **Chemically complex protective cell envelope:** - Cell envelope is three-layered structure with an outer glycocalyx, middle cell wall, and inner plasma membrane. - Glycocalyx or capsule (tough) is present as either a slime layer (loose sheath) or capsule. Bacteria are better observed when stained. - **Gram staining:** - The most followed staining method developed by Danish bacteriologist Hans Christian Gram. - The cell wall is made up of peptidoglycan (in Gram-positive bacteria) and murein (in Gram-negative bacteria). - Provides mechanical strength to the cell. - Cell membrane is a phospholipid bilayer. - All structures give protection to the cell and also help in inter-cellular transport. - **Motile bacteria:** - Possess either cilia or flagella. - Driven by rotatory movement produced by a basal body, which works as a motor. - **Other parts:** - Filament and hook. - **Surface projections:** - Tubular pili: help in inter-cellular communication. - Fimbriae: for clinging to support. - **Cell membrane:** - Shows infoldings called mesosomes, which help is cell wall formation, cellular respiration and DNA replication. - Some bacteria, especially photosynthetic cyanobacteria show longer extensions called chromatophores. - Carry photosynthetic pigments. - **Cytoplasm:** - Contains dense particles called ribosomes, helping in protein synthesis. - Ribosomes are described by their sedimentation rate in Svedberg units. - Bacterial ribosome are 70S (composed of a larger subunit 50S + smaller subunit 30S). - Chromosome (nucleoid region) - Ribosomes - Food granule - Flagellum - Pili - Capsule or slime layer - Cell wall - Plasma membrane - Cytosol - Plasmid (DNA) **Fig. 5.3 Prokaryotic cell**