Cell: The Unit of Life Class 11 Notes Biology Chapter 8

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Cell: The Unit of Life Class 11 Notes Biology Chapter 8

→ All organisms are composed of cells.

→ Some are composed of a single cell and called unicellular organisms while others, like us, are composed of many cells and called multicellular organisms.

→ The cell is the fundamental structural and functional unit of all living organisms.

→ Unicellular organisms explain that a cell is capable of independently existing and of performing the essential functions of life.

→ Robert Hooke was the first person to describe the cell in 1865 when he used a microscope built by him to examine a thin slice of cork.

→ In 1831, Robert Brown made an important discovery when he reported the presence of a small sphere in the cells of the orchid root. This rounded body which later came to be called the ‘nucleus’ was thought to be of common occurrence in the cells.

→ In 1838, Matthias Schleiden, a German Botanist, examined a large number of plants and observed that all plants are composed of different kinds of cells which form the different tissues of the plant.

→ Same time, Theodore Schwann (1839), a British Zoologist, also studied different types of animal cells. He observed the nuclei in these cells had a thin outer layer which is today known as the ‘plasma membrane’.

→ Schwann proposed the hypothesis that the bodies of animals and ( plants are composed of cells and products of cells.

→ Schleiden and Schwann combined their views and formulated the cell theory.

→ In 1855 when Rudolf Virchow first explained that cells divided and new cells are formed the pre-existing cells.

→ Cell theory as understood today is:

  1. all living organisms are composed of cells and products of cells,
  2. all cells arise from pre-existing cells.

→ A typical cell consists of a cell membrane, nucleus, and cytoplasm. Plant cells have a cell wall.

→ Cells are specialized to perform different functions and their shape and size may vary accordingly.

→ Cells are not only the building blocks of an organism but also the functional unit of life.

→ Cells vary in their shape, size, and activities/functions. Based on the presence or absence of a membrane-bound nucleus and other organelles, cells and hence organisms can be named eukaryotic or prokaryotic.

→ A typical eukaryotic cell consists of a cell membrane, nucleus, and cytoplasm.

→ Plant cells have a cell wall outside the cell membrane.

→ The plasma membrane is selectively permeable and facilitates the transport of several molecules. The endomembrane system includes ER, Golgi complex, lysosomes, and/vacuoles.

→ All the cell organelles perform different but specific functions. Centrosome and centriole form the basal body of cilia and flagella that facilitate locomotion.

→ In animal cells, centrioles also form spindle apparatus during cell division. The nucleus contains nucleoli and chromatin networks. It not only controls the activities of organelles but also plays a major role in heredity.

→ The endoplasmic reticulum contains tubules or cisternae. They are of two types: rough and smooth.

→ The Golgi body is a membranous organelle composed of flattened sacs. The secretions of cells are packed in them and transported from the cell.

→ Lysosomes are single membrane structures containing enzymes for the digestion of all types of macromolecules. Ribosomes are involved in protein synthesis.

→ Plastids are pigments containing organelles found in plant cells only. In-plant cells, chloroplasts are responsible for trapping light energy essential for photosynthesis.

→ The nucleus is enclosed by a nuclear envelope, a double membrane structure with nuclear pores. The inner membrane encloses the nucleoplasm and the chromatin material.

→ Cellular: A honeycomb-like structure with an empty compartment which Hooke called ‘cellular- a Latin word for a small room.

→ Plasma membrane: The nuclei in these cells had a thin outer layer which is today known as the ‘plasma membrane.

→ Organelles: Certain cellular functions are associated with different types of distinct structures called organelles present in the cytoplasm.

→ Peptidoglycan: The cell wall is rigid because of the presence of special macromolecules called peptidoglycan.

→ Polyribosomes: The ribosomes are the site of protein synthesis. Several ribosomes may attach to a single mRNA and form a chain called polyri¬bosomes or polysomes.

→ Pilin: The pills are elongate tubular structures made of a special protein called pinin.

→ Cell wall: The plant cells possess a conspicuous thick layer of cellulose covering the cell membrane called the cell wall.

→ Passive transport: Many molecules can move passively across the membrane without any requirement of energy called passive transport.

→ Endomembrane System: Each of the membranous organelles is distinct in terms of their structure and function but many of these are considered together as a part of the so-called Endomembrane system because their functions are coordinated.

→ Endoplasmic Reticulum (ER): The electron microscopic study of the variety of eukaryotic cells revealed the presence of a network or reticulum of the tiny tubular structure scattered in the cytoplasm and hence, called the Endoplasmic Reticulum.

→ Exocytosis: The hydrolytic enzymes present in the phagosomes partially digest the engulfed material a residual body is formed, which is usually eliminated from the cell by a process called exocytosis.

→ Turgor pressure: Vacuoles also exert a hydrostatic pressure called the turgor pressure that gives mechanical support to the cell.

→ Matrix: Each mitochondrion is a double membrane-bound structure with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments i.e. the outer compartment and the inner compartment called the matrix.

→ Hub: The central part of the centriole is also, proteinaceous called the hub.

→ Nucleus: The eukaryotic cells usually possess a large-sized, almost centrally located, and densely stained organelle containing the genetic material called the nucleus.

→ Histones: The biochemical analysis of the isolated chromatin has revealed that it contains DNA and some basic protein called histones.

→ Kinetochore: Every chromosome essentially has a primary constriction of the centromere on the sides of which disc-shaped structures called the kinetochore.

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