• Post last modified:December 30, 2022
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Cell: The Unit of Life

A cell is a membrane-bound structural and functional unit of life that contains cytoplasm and organelles.

  • Robert Hooke was the first to observe cells in cork.
  • Anton von Leeuwenhoek first saw and described live cells. Robert brown later discovered the nucleus. 
  • Cell theory was given by Schleiden and Schwann in 1839.

Cell Theory

  1. All organisms are composed of one or more cells.
  2. Cells are the smallest living things, the basic units of organization of all organisms.
    Cells arise only by the division of the previously existing cells.

On the basis of nuclear organization, there are two cells types: 

  1. Prokaryotic cells include bacteria and a huge group of other micro-organisms without a nucleus and membrane-bound cell organelles.
  2. Eukaryotic cells contain plant cell and animal cells, they’ve unique nucleus and membrane-bound cell organelles.

Cell Structure

The significant parts of the cell are the nucleus, the management Centre, the cytoplasm, the distance within the cell, and the cell membrane, the protective skin of the cell.

Cell Components

Cell Membrane

A cell membrane is a selectively permeable arrangement that wraps the cell and protects the cell’s internal environment. Proteins can be incorporated or anchored on the cell membrane. 

  • Cell membranes provide a stable environment for cells, perform communication function among cells via the surface proteins, and selectively exchange material between a cell and its environment.
  • Cell Membrane Transport – The two main types of transport throughout the cell membrane: passive and active transport.
  • Passive transport requires no energy. Types of passive transport would be osmosis, diffusion, and facilitated diffusion.
  • Active transport requires energy from ATP.


It is a jelly-like semi-fluid general mass of protoplasm excluding the nucleus but, including all other components like cytoplasmic matrix, cell organelles and cell inclusions.


The nucleus includes the cell’s DNA. The nucleus communicates with nuclear pores and is shielded by a nuclear envelope.


The DNA is genetic material (Genetic material is that substance that controls the formation and expression of traits in an organism and can replicate and pass on from a cell to its daughter cell and from one generation to the next.), within the nucleus is packaged into chromosomes.

The nucleus controls the cell by acting and deciding on what the cell needs.

Cytoplasm & Cytoplasmic Organelles

The cell maintains life by assigning every liability to different specialized machines. These machines are called organelles. An organelle is a compartmentalized arrangement that performs a specialized function in a cell.

An animal cell includes a nucleus, ribosomes, mitochondria, rough endoplasmic reticulum, smooth endoplasmic reticulum, plasma membrane, Golgi apparatus, and lysosomes. The nucleus controls the cell function.


Ribosomes were discovered by Robinson and Brown (1953) in plant cells and by Palade (1955) in animal cells. 

  • Ribosomes make proteins for the cell.
  • Each ribosome consists of two protein subunits: the Large subunit and the Small subunit. 
  • The units cling to a strand of nucleic acid directions from the nucleus.
  • The ribosome reads the strand directions to make proteins for the cell to utilize in its normal activities.

Endoplasmic Reticulum

It is of two types: Rough Endoplasmic Reticulum and Smooth Endoplasmic Reticulum. 

  1. The Rough Endoplasmic Reticulum is located attached to the ribosomes outside of the nucleus. It provides the surface for protein synthesis.
  2. The Smooth Endoplasmic Reticulum isn’t attached to this nucleus and does not have attached ribosomes. Smooth ER synthesizes carbohydrates and lipids.

Golgi Apparatus

  • It was discovered by Camillo Golgi in 1898.
  • Most Golgi apparatus made up of flattened, folded sacs, ship bundles around the cell.
  • The functions of the Golgi apparatus are – process, package, transport, and release of secretory proteins.
  • Glycosidation of lipids and glycosylation of proteins to form glycolipids and glycoproteins.  


  • It was discovered by Christian de Duve in 1955.
  • Lysosomes store the hydrolyzing enzymes of the cell.
  • In injured and dead cells, the lysosome membrane rupture spontaneously, releasing the enzymes that lyse (dissolve) the weakened cells.
  • Therefore, the lysosomes are also called “suicide bags” of the cell.


Vacuoles are non-cytoplasmic areas present inside the cytoplasm formed by expansion and pinching off from the endoplasmic reticulum. 

Vacuoles are the following types:-

  1. Sap vacuoles are responsible for the colour of petals.
  2. Contractile vacuoles take part in osmoregulation and excretion.
  3. Food vacuoles contain digestive enzymes and help in the digestion of nutrients.


  • It was discovered by Kolliker in 1850. 
  • A mitochondrion has two membranes:
    1. The outer membrane is smooth and has porin protein which forms channels.
    2. The inner mitochondrial membrane is semipermeable and regulates the passage of materials into and out of the mitochondria.


Plastids are the largest cell organelles. These are involved in the formation and storage of soluble organic compounds.

Types of plastids:

  1. Chloroplasts: These are the Centre of photosynthesis.
  2. Chromoplasts: These are non-photosynthetic plastid and are variously coloured (except green). These store carotenoid pigments.
  3. Leucoplasts: These are colourless plastids found in storage organs.
    Types of leucoplast:-
    1. Aleuroplasts store proteins.
    2. Elaioplast store lipid or fat.
    3. Amyloplast store starch.


  • These are two cylindrical, rod-shaped, microtubular structures near the nucleus.
  • These polymerize for the formation of spindle fibres and astral rays during mitosis and meiosis.


It is a double membrane-bound dense protoplasmic body that controls cellular metabolism, encloses all the genetic information, and can transmit the same to the next generation.

The nucleus was discovered by Robert Brown. 

It can be differentiated into the following parts:

  • Nuclear envelop (consists of two unit membrane)
  • Nuclear matrix (the network of fibrils, function as a scaffold for chromatin)
  • Nucleoplasm (clear fluid containing substances for nucleotide synthesis). 
  • Nucleolus
  • Chromatin (a hereditary part of the nucleus and is differentiated into heterochromatin and euchromatin).


  • A cell is a building component of an organism that may function independently.
  • The cell maintains life by assigning every duty to separate specialized machines.
  • These machines are called organelles.
  • An organelle is a compartmentalized arrangement that performs a specialized function in a cell.