Cell Structure and Function: The Cytoplasm, the Cell Organelles and Plastids (For CBSE, ICSE, IAS, NET, NRA 2022)

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The space between the plasma membrane and the nucleus is filled by an amorphous, translucent, homogenous, colloidal liquid called cytoplasm. Cytoplasm contains various cell organelles like; mitochondria and chloroplasts (that trap and release energy) , Golgi body, ribosomes and ER (that secretory or involved in synthesis and transport) , cilia and flagella (for motility) , lysosomes (suicidal bags) and the nucleus which controls all activities of the cell and carries the hereditary material.

Mitochondria and Chloroplast - The Energy Transformers

Mitochondria (found in plant and animal cells) are the energy releasers and the chloroplasts (found only in green plant cells) are the energy trappers.

Mitochondria (Singular = Mitochondrion)

Tiny thread like structure under light microscope (0.5 - 1.0 ) , Shaped may be granular, filamentous, rod-shaped, and spherical or thread like and each mitochondrion is covered by a double-membraned envelope; outer membrane is smooth and porous, while inner membrane is given out into folds called cristae. The cavity of the mitochondria is filled with matrix and this matrix contains lipids, proteins, circular DNA and ribosomes.

Image Showing Mitochondrion Structure

Functions: Mitochondria are miniature biochemical factories where food is oxidized, and energy is released. This energy is stored in the form of ATP. Hence, mitochondria are called powerhouse of the cell. Oxidizes pyruvic acid (breakdown product of glucose) to release energy which gets stored in the form of ATP for ready use. This process is also called cellular respiration. Synthesis of many amino acids occurs in mitochondria.


Plastids are cell organelles found only in plant cell. Plastids contain many membrane layers embedded in a material called the stroma. There are three types of plastids: chromoplasts (colored plastids) , chloroplasts (green) and leucoplasts (white or colorless plastids) .


Contain red, orange or yellow pigments (other than green) , non-photosynthetic and give color to fruits and flower.


Colorless plastids lack any pigment, non-photosynthetic and store nutrients like starch and oil drops abundant in seeds and starch grains.


They are green plastids, photosynthetic green pigment, i.e.. chlorophyll. They also contain various yellow or orange pigments in addition to chlorophyll. Each chloroplast is covered by a double layered unit membrane. The two membranes are separated by an intermembrane space. Inside the inner membrane, chloroplasts have a matrix or stoma and disc like bodies called grana.

Functions: Chloroplasts, the green plastids, help in photosynthesis and thus, help in the synthesis of food. These are called kitchen rooms of the cell.

Image Showing Structure of a Chloroplast

Chloroplast Versus Mitochondria

  • Mitochondria are involved in cellular respiration whereas chloroplasts are involved in photosynthesis. So, the chemical reactions for the processes occurring in them are different and reversed.
  • Chloroplasts contain pigments such as chlorophyll a, chlorophyll b and carotenoids. Mitochondria do not contain any such pigments.
  • Mitochondria are found in all animal and plant cells while Chloroplasts are found in only specific types of plant cells.

Similarities between Mitochondria and Chloroplasts

  • Mitochondria and chloroplast both have double membrane surrounding the organelles.
  • Their own circular DNA which codes for certain enzymes required for the chemical reactions that take place in these organelles.
  • Their own 70S ribosomes made up of 50S and 30S subunits to translate proteins.
  • Since chloroplasts and mitochondria contain their own DNA the hereditary molecule and also their own ribosomes, they are termed semi-autonomous only because they are incapable of independent existence though they have ribosomes and DNA.

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