Cell Structure and Functions

Cell is the structural and functional unit of all the living organisms, which is capable of carrying out all life processes. Robert Hooke observed honeycomb like compartments under the microscope and named them ‘cells’.

There are two types of organisms based on the number of cells present in them:
  • Unicellular organisms: They are single celled. For example: Mycoplasma, Amoeba
  • Multicellular organisms. They possess two or more cells For example: Plant cell, animal cell
Similar functions in multicellular organisms are carried out by groups of specialised cells forming different tissues. Tissues combine to form organs. Cells occur in different shapes:
  • Usually cells are round, spherical, elongated, branched or spindle shaped.
  • Amoeba has no definite shape. It changes its shape by forming pseudopodia.
  • Similarly, WBC also changes its shape.
Cells occur in diverse sizes.
  • The smallest cell is 0.1 to 0.5 mm in bacteria.
  • The largest cell measuring 170 mm ×130 mm, is the egg of an ostrich.
Cells are enclosed by a plasma membrane. In plant cells, a cell wall is located outside the plasma membrane. Cytoplasm is the jelly-like substance present between the cell membrane and the nucleus.

Protoplasm includes the cytoplasm and the nucleus. In eukaryotes, the nucleus is separated from the cytoplasm by a nuclear membrane and it directs all the life processes of the cell. Within the nucleus, nucleolus is present. Nucleus contains the chromosomes, which carry genes. Mitochondria are the power house of the cells. Plant cell has a big central vacuole unlike a number of small vacuoles in animal cells. Plant cells possess plastids, which are of three types namely: chloroplasts, chromoplasts and leucoplasts. Prokaryotic cells lack membrane-bound organelles whereas eukaryotic cells possess membrane bound organelles.

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