By going through these CBSE Class 11 Biology Notes Chapter 8 Cell: The Unit of Life, students can recall all the concepts quickly.
Cell: The Unit of Life Notes Class 11 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:
- all living organisms are composed of cells and products of cells,
- 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.