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CBSE Class 9 Biology Notes for Chapter 1 The Fundamental Unit of Life

The basic building blocks of living organisms are called cells.
There are different types of organisms based on the number of cells that they are made up of. Unicellular organisms are the organisms which have only one cell. For e.g. Amoeba. Multicellular organisms are the organisms made up of more than one cell. For e.g. Humans

  • Plasma membrane: it is also called the cell membrane. It is the outermost covering of the cell that separates the contents of the cell from its external environment. It is selectively permeable as it allows certain substances like water, oxygen, carbon dioxide, etc. to pass through it.


  • Cell wall: It is mostly found in plants and is mainly composed of cellulose. Because of these cell walls, plant cells can withstand greater changes in their surroundings ad compared to animal cells.


  • Nucleus: The nucleus contains chromosomes which can be seen when the cell is about to divide. It has a double layered covering called nuclear membrane. Organisms whose cells lack nuclear membrane are called prokaryotes and organisms whose cells have a nuclear membrane are called eukaryotes.


  • Cytoplasm: The fluid content inside the plasma membrane is called cytoplasm. It contains cell organelles.


  • Cell organelles:


  • Endoplasmic Reticulum(ER): It is a large network of membrane bound tubes and sheets. There are two types of ER- Rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). RER has ribosomes attached to it. These ribosomes are the sites of protein manufacture. The SER helps in the manufacture of fat molecules.
  •  Golgi Apparatus: It consists of a system of membrane bound vessels arranged almost parallel to each other in stacks called cisterns. Its functions include storage, modification and packaging of the products synthesised near the ER in its vesicles.
  • Lysosomes: These are the waste disposal system of the cell. Lysosomes help to keep the cell clean by digesting any foreign material as well as worn out cell organelles.
  • Mitochondria: Mitochondria are the power house of the cell. The energy required by the cell for various chemical activities is released by mitochondria in the form of ATP (Adenosine Triphosphate).
  • Plastids: Plastids are present in plant cells only. There are two types of plastids: chromoplasts and leucoplasts. Plastids containing the pigment chlorophyll are called chloroplasts. Leucoplasts are the white or colourless plastids which store materials like starch, oils and protein granules.
  • Vacuoles: These are the storage sacs for solid or liquid contents. Animal cells have small vacuoles while plant cells have large vacuoles.


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