Mitochondria and Plastid

Mitochondria is a double membrane organelle found in the cytoplasm of eukaryotic cells. Mitochondria were first observed by Richard Altman (1894). And the term ‘mitochondria’ was coined by Carl Benda (1998). Mitochondria produces enzymes for the metabolic conversion of food to energy. Mitochondria are called the powerhouse of the cell. 

The term plastid was derived from the Greek word ‘plastikas’ meaning formed or moulded. This term was coined by Schimper in 1885. Plastids are the major organelles found in cells of plants and algae. They often contain pigments which are used in photosynthesis and the types of pigments can change or determine the colour of cells. 

Mitochondria

Morphology 

• Size: 0.05-1.0 
• Shape: bean-shaped 
• Number: depends on the type, size and functional state of a cell
• An average liver cell contains around 1500 numbers of mitochondria

Structure

1. Outer Membrane: Made up of simple phospholipid bilayer. Contains a large number of integral protein structures called porins which allow molecules to freely diffuse from one side of the membrane to the outer.
2. Inner Membrane: The inner membrane contains proteins that perform redox reactions in oxidative phosphorylation, ATP synthase and transport of proteins. 
3. The Inter Membrane Space: Also known as peri mitochondrial space, the space between the inner and outer membrane. 
4. Cristae: Tue folds in the inner membrane of mitochondria forming finger-like projections are called Cristae. This helps to synthesise ATP.
5. Matrix: This is the space enclosed by the inner membrane and contains 2/3rd of the total protein of mitochondria. The matrix contains enzymes, DNA molecules, ribosomes, tRNA, granules and fibrils. 

Functions

• Aerobic Respiration: Mitochondria use complex molecules and oxytocin to produce energy known as ATP

• Mitochondrial DNA contains plasma genes

• Synthesise Mt DNA, RNA, protein

• Synthesise 13 different polypeptides in humans

• Helps in synthesising oestrogen and progesterone-like hormones

• Responsible for cholesterol metabolism

• Helps in Hemel synthesis

Plastid

Plastids are the large cytoplasmic organelles found in plants and algae. They are broadly classified into two main types named chromoplast and leucoplast. 

Types

1. Chromoplast: A place for the pigments to be synthesised and stored in plants. The chloroplasts usually convert to chromoplast. Chromoplast causes the different colours of flowers in different plants. 
2. Gerontoplast: These are basically chloroplasts that are going through the ageing process. 
3. Leukoplast: These are the non pigmented organelles. Found in non-photosynthetic parts of the plants such as roots. They are used for synthesising amino acids and fatty acids. Leucoplasts are further subdivided into three different plastids. 

4. Amyloplast

5. Protrino plays

6. Elaioplast

7. Chloroplast: The most known plastid responsible for photosynthesis. Chloroplasts are filled with thylakoids, which is the place for photosynthesis. 

Morphology

• SHAPE: Varies in shape. Spheroid or ovoid in higher plants
• SIZE: in higher plants, 4-5 microns in length and thickness is 1-3 microns
• NUMBER: In higher plants, there are 20-40 chloroplasts per cell or up to 1000 chloroplasts

Structure

• Double membrane-bounded organelles and the membrane is called the chloroplast envelope
• In addition to the inner and outer membrane, chloroplasts have a third internal membrane called thylakoid membrane
• Thylakoid membrane forms a network of flattened discs called thylakoids which are frequently arranged in stacks called the Grana
• Grana is interconnected by tubules called stromal lamellae

Function

• Chloroplasts are the centre of synthesise and metabolism of carbohydrates
• During photosynthesis carbon dioxide and water are converted into organic substances like sugar, polysaccharides, fats, amino acids 
• Actively incorporates amino acids in the presence of ATP and is capable of a certain degree of protein synthesis
• The similarities with mitochondria are regarding the structure and some functions like containing enzymes for the Kreb’s cycle and for the synthesis of fatty acids

Conclusion

Mitochondria and plastid both are the cell organelles present in different types of cells. Mitochondria present in eukaryotes and plastids are found in plants or in some algae. Mitochondria usually cause the formation of energy by producing ATP and plastids are the organelles which help in photosynthesis. Mainly pigmentation of flowers is caused due to the presence of plastids in cells. Both these organelles have double membrane-bounded and they are capable of producing their replica by the process of cell division. The biosynthesis of plastid and mitochondria need participation from separate genomes. Proteins in mitochondria are encoded by a special genome. These organelles can be observed by light microscopy in the living cells.