Cell organelles part-3

 Mitochondria: These are the organelles that prepare energy and supply it to cells. These are also called the powerhouse of the cell. These are in the form of filaments, vesicles, or rods. although they vary in number, shape, and internal structure. These are double membrane-bounded organelles. The outer membrane is smooth while the inner membrane form infolds called cristae which enclose the matrix. It is a self-replicating organelle. The inner membrane has small knobs-like structures called F1 particles. The matrix of the mitochondria has a large number of enzymes, co-enzymes, and organic, and inorganic salts which help in several metabolic processes like Kerb's cycle, fatty acid metabolism, and aerobic respiration. The matrix also has its own circular DNA, all three types of RNA.  The mitochondria extract energy from organic food which is converted into ATP and provide to the cell on demand. The number of mitochondria varies from cell to cell and depends upon the physiological activity of the cell. The spent energy which is in the form of ADP is recycled to ATP by mitochondria. A crista is made up of lipoproteins. The mitochondria have their own DNA and ribosomes, so it is a self-replicating organelle. They divide by binary fission. The outer membrane is sieve-like due to the presence of special pores which are made up of special proteins called porins. These allow free passage of molecules while the inner membrane is selectively permeable. The inner membrane has also F0-F1 particles. These particles are actually ATP synthase. The inner mitochondrial membrane has also an electron transport chain which is used for the synthesis of ATP. All the mitochondria present in the zygote come from egg cells and not from sperm.  

Centrioles: These are non-membranous cell organelles found mainly in animal cells. These are also found in fungi. Centrioles are rod-like in shape and usually occur in pairs and these pairs are at right angles to each other. Each centriole is composed of nine triplets of microtubule which are circularly arranged around a central axis. The centriole gives rise to microtubules during cell division; this whole structure is called the mitotic apparatus. This help in the distribution of chromosomes. This organelle also gives rise to the basal bodies of cilia and flagella. 

Plastids:  Membrane-bound, mostly pigment-containing bodies in the cells are called Plastids. These are present in plant cells and algae, cyanobacteria. These are divided into meristematic cells and distributed into different parts of cells. Based on the division and presence of pigments, the plastids are divided into:

 Chloroplasts: are found in the green part of plants and act as the site of photosynthesis. It consists of three parts: envelope, stroma, and thylakoid. The envelope is also a double membrane structure. The outer membrane is exactly like the outer membrane of mitochondria as it has also porins. The inner membrane is semi-permeable and rich in proteins. Between the inner and outer membrane, there is inner membrane space. The groundmass of chloroplast is the stroma. The main difference between these two molecules is that chlorophyll has Mg++ while haem has Fe++ as the central atom. Under the light microscope, they appear to be heterogeneous structures with small granules known as grana embedded in the matrix. Stroma is a fluid that surrounds the thylakoids. It contains proteins, some ribosomes, and a small circular DNA. It is in this part of the chloroplast where CO2 is fixed to manufacture sugars. Some proteins are also synthesized in this part. Thylakoids are flattened vesicles that arrange themselves to form grana and intergrade. Chloroplasts vary in their shape and size with a diameter of about 4-6 nm. On average, there are 50 or more thylakoids piled to form one granum and there are about 40 grana in each chloroplast.  Small thylakoids form a stack which is called a granum. It is the photosynthetic part of a cell. Each granum is attached to another granum by the nonphotosynthetic part which is made up of a large thylakoid and called inter granum. The light-dependent reactions occur on the thylakoid membrane while light-independent reactions occur in the stroma of the chloroplast.

Leucoplasts: 

They are colorless. These are found in parenchyma cells of stems, roots, and seeds. They are triangular, tubular, or of some other shape. They are found in the underground parts of the plant and store food. If they store starch, then these are called amyloplast. If they store proteins, then these are called proteinoplast. If they store lipids, then these are called elaiopast.

Chromoplasts: They impart colors to plants other than green. They are present in the petals of the lower and ripened fruit. They help in the pollination and dispersal of seeds. 

Nucleus: The presence of a nucleus is discovered by Robert Brown in 1831 in the cell. It is slightly darker than the surrounding cytoplasm. It controls the life and activities of cells. The nucleus is covered by cell double membrane called nuclear envelope. These membranes are sperated by perinuclear space. These membranes are composed of lipid bilayer and proteins. The outer membrane is covered with ribosomes and is connected with ER. There are numerous pores in nuclear envelop called nuclear pores which are composed of special proteins called nucleoporin. Through these pores the exchange of material take place. In animal cell, it occupies the central space while in plant cells, it is pushed to the periphery due to large vacuole. In muscle fiber cells of animal , the nucleus is not found in central space. 

• The sieve tube cells in plants have no nucleus at all.
• The red blood cells of humans do have not a nucleus at all.
• Coenocytic cells have many nuclei. 

The fluid present in the nucleus is called nucleoplasm. It is transparent semifluid and it is the ground mass of the nucleus and consist of proteins, enzymes, free nucleotides and some metal ions like magnesium for the synthesis of DNA and RNA. It also contain some histone and non-histone proteins. So it is slightly different from the cytoplasm.

 Nucleolus: It is a non-membranous structure and may be one or two in a cell. It is a darkly stained area of the cell. Its peripheral area consists of ribosomal subunits and the central fimbrial area consists of rRNA and rDNA. So nucleolus is involved in the formation of ribosomes. 

 Chromosomes: In the nucleus, the chromatin material is present which is converted to chromosomes.  It consists of two arms called centrosomes and these two arms meet at a central point called centromere or primary constriction. Centromere also contains kinetochore fibers on which spindle fibers from centriole attach. The centrosomes attach on another point called secondary constriction or nuclear organizer. It gives rise to nucleoi during interphase.