Mitochondria and Plastids

1.0Mitochondria (Powerhouse of The Cell)

Mitochondria are double membrane bound organelles of eukaryotic cells.

Mitochondria are the sites where aerobic part of respiration called Kreb’s cycle is performed.

Occurrence

These are found in all eukaryotic cells except mature mammalian RBCs. These are absent in prokaryotic cells.

Shape

The shape may be fibrillar, spherical (in yeast), oval, sausage shaped or discoidal or cylindrical.

Origin

Mitochondria have a small and circular DNA, few RNA and 70S ribosome of their own and make some of their own proteins, showing that they were once aerobic bacteria. 

Structure

Mitochondria are semi-autonomous organelles bound by an envelope of two units membranes and filled with a fluid matrix. The outer membrane is smooth and has porous proteins which forms channels for the passage of molecules through it. It usually produces numerous infolds called cristae.

The cristae greatly increase the inner surface area of the mitochondria to hold a variety of enzymes. Cristae increases the surface area for ATP-generating chemical reaction. Cristae bear minute, regularly spaced tennis racket shaped particles known as F₁ particles or oxysomes. Oxysomes are concerned with ATP synthesis. Matrix contains various respiratory enzymes.

Function

Mitochondria are the main sites of cellular respiration. They bring about complete oxidation of food stuffs or respiratory substrates into carbon dioxide and water.

They are commonly known as 'power house of the cell' because they contain enzymes necessary for the complete oxidation of food and for release of high amount of energy in the form of ATP (Adenosine triphosphate) molecules. The body uses energy stored in the form of ATP for synthesis of new chemical compounds and for mechanical work. ATP is also known as energy currency of the cell.

2.0Plastids

Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell. Plastids often contain pigments used in photosynthesis and the types of pigments present can change or determine the cell's colour. Plastids are responsible for photosynthesis, storage of products like starch. They are double membranous cell organelles.

Origin: Like mitochondria, the chloroplast are self-duplicating organelles. They also have circular DNA and 70s ribosome of their own. They also have a prokaryotic origin.

Plastids and mitochondria are semiautonomous cell organelle as they are having their own DNA and ribosomes which are useful in protein synthesis. 

Plastids are of three types-

Chromoplasts

It is responsible for pigment synthesis and storage. Chromoplasts are red, yellow and orange in colour and are found in petals of flower and in fruits. 

Their colour is due to two pigments, carotene and xanthophyll.

Leucoplasts

Leucoplasts are colourless or white plastid. They occur in plant cell not exposed to light, such as roots and seeds. Leucoplasts are the centre of starch grain formation, and they are also involved in the storage of oil, starch and proteins.

Chloroplasts

Chloroplasts are probably the most important among the plastids since they are directly involved in photosynthesis. They are usually situated near the surface of the cell and occur in those parts that receive sufficient light. e.g. the mesophyll cells of leaves. The green colour of chloroplasts is caused by the green pigment chlorophyll.

Chloroplast mainly have pigment chlorophyll and also contain various yellow-orange pigment in addition to chlorophyll.

Structure of Chloroplast

Chloroplasts are usually disc-shaped and surrounded by a double membrane. Inside the inner membrane, there is a watery protein-rich ground substance called as stroma, in which, there is embedded a continuous membrane system, the granal network. This network forms a three-dimensional arrangement of membrane bound vesicles called thylakoids. The thylakoids usually lie in stacks called grana and contain the photosynthetic pigments-green chlorophyll a and b and the yellow to red carotenoids. The grana are interconnected by tubular membranes called lamellae.

Photoautotrophic bacteria possesses photosynthetic pigments inside small vesicles which may be attached to plasma membrane.

 Functions of Chloroplast

Chloroplasts are the sites of photosynthesis.  They contain enzymes and coenzymes necessary for the process of photosynthesis.

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