Science: Building Blocks of Life: The Structural and Functional Unit of Organism (For CBSE, ICSE, IAS, NET, NRA 2022)

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Cells are arranged variously to build the bodies of living beings. Every organism begins life as a single cell which is the fertilized egg. Cells divide to give more cells. Cells form tissues. Tissues make organs.

CELL – the Structural and Functional Unit of Organism

Robert Hooke discovered the cell in 1665.

M J Schleiden (1838) and T Schwann (1839) Cell Theory States That

  • Cell is the structural and functional unit of all living beings and bodies of all organisms are composed of cells.
  • All new cells arise by division of pre-existing cells.
  • The functions of an organism are an outcome of the combined activities and the interactions of the cells that make the organism.
  • A cell may be defined as the structural and functional unit of living organisms which is capable of independent existence.

All Cells Have Three Basic Parts

  • Cell membrane which limits the cell and gives it shape.
  • DNA which may be contained in a nucleus.
  • Fluid called cytoplasm filling in the space within the cell

Two Types of Cell

  • Prokaryotic cell
  • Eukaryotic cell

Prokaryotic Cell

These cells do not have a well-organized nucleus. The genetic material is a single molecule of DNA lying in the cytoplasm. Not only is the nuclear membrane absent, cell organelles like mitochondria, lysosomes, endoplasmic reticulum, chloroplast, nucleolus, etc. are also not present in prokaryotic cells.

Examples: Bacteria and blue-green algae.

Eukaryotic Cell

DNA is enclosed in a nuclear membrane forming a nucleus. The genetic material is made of two or more DNA molecules, which are present as a network of chromatin Fibres when the cell is not dividing. Membrane-bound organelles, such as mitochondria, endoplasmic reticulum, lysosome, chloroplast, nucleolus, etc. are present within the cytoplasm.

Examples: Cells of plants, fungi, protozoa and animals.

Eukaryotic Cell

Structure of a Eukaryotic Cell

Structure of a Yukaryotic Cell

Eukaryote-Specific Organelles

Eukaryote-Specific Organelles
NucleusStores genetic information; controls all cell activities
Endoplasmic reticulum (ER)Network of tubes and membranes that carry material through the cell and play a role in protein modification and lipid synthesis; has two parts: rough ER (contains ribosomes) and smooth ER (does not contain ribosomes)
Golgi bodyFlattened membrane discs that package and sort proteins
MitochondriaBreaks down sugar into energy for the cell
ChloroplastUses light energy to make food for plant cells
VacuoleStorage site for food, water, and waste; large central vacuole helps maintain the shape of plant cells
LysosomeBreaks down large molecules and digests old cell parts

Plant Cells Have Both Mitochondria and Chloroplasts

Some people believe that only animal cells have mitochondria, and plant cells have chloroplasts instead. However, plants need both chloroplasts and mitochondria to undergo photosynthesis and cellular respiration, respectively.


Protoplasm is the living substance of the cell. The nucleus and cytoplasm together form the protoplasm.

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