🫁 Cell Organelles

Mitochondria

• ‘Power house of cell’.
• Energy is generated as ATP. ATP is known as ‘Energy currency’.
• Mitochondria was firstly identified by Altman in 1886 as `Bioplast’ and suggested their association with respiration.
• The term ‘mitochondria’ has been given by C. Benda (1898).

• Matrix of Mitochondria is the site of aerobic respiration or Krebs cycle.
• Site of glycolysis: Hyaloplasm (not cytoplasm)
• Electron transport: Oxysome/F1 particle/Elementary particle.
• Enzymes confined in Peri-mitochondrial space and reactions on inner membrane.
• Mitochondria contain DNA, RNA (0.02% DNA, 3-4% RNA) & ribosomes.
• Hence called ‘semi-autonomous’ body of the cell because of capability of some protein synthesis.

Plastids

• According to Schimper (1885)
  • Green coloured plastids are called chloroplasts
  • Red, Yellow, Brown, Orange → Chromoplast (Chromo means Coloured)
  • Colourless: Leucoplasts. The main Function of leucoplasts is food storage.

Leucoplast

• Amyloplast: Storage of Starch
• Elaioplast: Storage of Oils
• Aleuronoplast or Proteinoplast: Storage of Protein

Chlorophyll

• Chloroplast contains pigment chlorophyll
  • Chlorophyll a: Blue black, C₅₅H₇₂O₅N₄Mg
  • Chlorophyll b: Green black, C₅₅H₇₀O₆N₄Mg
• Carotene: Yellowish Orange, C₄₀ H₅₆
• Xanthophyll: Yellow, C₄₀ H₅₆ O₂
• Chl. a & Chl. b in green plants: 65%
  • Xanthophyll → Yellow, 29%
  • Carotene → Red, 6%
  • Carotene & Xanthophyll are together called carotenoid pigment.
  • In chromoplast, only carotenoid pigments are found.

Plant pigments

• Plastid pigments e.g. chlorophyll, carotenoids; such pigments are soluble in organic solutions only. Found in leaves & skin of fruits.
• Sap pigments: Solution of salts & sugars found in vacuoles e.g. Anthocyanin (soluble in water) found in flower petals & beets.

Chloroplasts

Grana lamellae

• Thylakoids → Quantasomes contain pigment
• Lamellae means membranous tubes.
• Site of Light reaction: Grana/thylacoids/quantasomes → Chlorophyll
• Site of Dark reaction: Stroma/matrix → RuBISCO
• Chloroplast contains DNA (0.5%), RNA (3-4%) and ribosomes (70s). Therefore chloroplast is capable of some independent protein synthesis hence called semi-autonomous body of the cell.
• A plastid has two distinct regions viz. grana & stroma.
• Grans are stacks of membrane bound, flattened, discoid sacs containing chlorophyll molecules. Such molecules are responsible for the food production, hence called the kitchen of the cell.
• The homogenous matrix in which grana are embedded is known as stroma.

Endoplasmic Reticulum (ER)

• Endoplasmic Reticulum is a dense network of double membrane (unit membrane) structures running through the cytoplasm.
• It may be continuous, some parts are connected to the nuclear membrane while others are connected to the plasma membrane especially in animals.
• Its origin is from nuclear membranes.
• Ultrastructure of ER was first reported by Porter (1948).
• It is not a stable structure.
• It is capable of being broken down and reconstructed.
• It undergoes partial fragmentation at the time of cell division.
• Two types of ER
  • Rough ER (RER): When ribosomes are attached on it.
  • Smooth ER (SER): No ribosomes are attached on its surface.

Function

• ER forms endoskeleton to provide a particular shape to the cell (mechanical support), especially in animals.
• Membranes of ER provide the surface for the increased metabolic reaction.
• Intracellular transport of metabolic products or molecules (e.g. protein).
• It helps in the formation of cell plate and nuclear membrane during cell division.
• Synthesis of protein on ribosomes means Rough ER is associated with the protein synthesis.
• Smooth ER (SER) secretes lipids which along with proteins constitute cell membrane by a process called membrane biogenesis.
• Transmission of impulses in animals.
• SER plays a crucial role in detoxifying many poisons & drugs.

Ribosomes/RNA particles

• Consists of r-RNA (ribosomal RNA) 60-40% & Protein 40-60%.
• First observed by Claude (1943) but reported as ‘microsomes’ (ER fragments + ribosomes). Palade (1956) isolated the ribosomes and reported their detailed ultra-structure.
• The term ‘ribosomes’ by R.B. Robert (1958).

Types of RNA (non-genetic)**

• m-RNA (messenger): 5-10% of total RNA
• t-RNA (transfer) / soluble RNA (sRNA): 10-15% of total RNA, Smallest in size.
  • Clover leaf model.
  • m-RNA and t-RNA directly take part in protein synthesis.
• r-RNA: provides site for protein synthesis.
  • 80% of total RNA, Most stable RNA.
• Two major steps are involved in protein synthesis:
  • Transcription i.e. transfer of genetic information from DNA to mRNA.
  • Translation i.e. translation of the language of nucleic acids into that of proteins.

    

• A ribosome has two subunits, one is smaller and other is bigger.
• In bacterial cells & chloroplasts of higher plants, sedimentation coefficient (S) of complete ribosome is 70 S which has subunits 50 S and 30 S.
• In higher organisms sedimentation coefficient of ribosomes is 80 S which has subunits 60 S & 40 S.
  • Higher concentration of Mg⁺⁺ ion promotes association of subunits into complete ribosome.
  • Lower concentration of Mg⁺⁺ dissociates these subunits.
• There is no lipid content in ribosomes.

Golgibody/Dictyosome

• Discovered by Camillo Golgi (1898).
• Golgi complex or bodies are formed by stacks of flattened (saucer-shaped) membranes.
• Flattened sacks are called cisternae. Golgi bodies are usually called dictyosomes in plants.
• Its origin from ER.
• ‘Acrosomes’ are associated with Golgi complex.

Functions

• They store, modify, package and condense the protein synthesized in the ribosomes.
• They form the cell plate during cell division.
• They add sugars to some proteins and synthesize some polysaccharides for the cell membrane.
• They set aside digestive enzymes in tiny membrane bound vesicles which become ‘lysosomes’.

Lysosome

• Lysis means digestion; Soma means body.
• The saclike, small, spherical, single membrane bound vesicles containing digestive enzymes are called lysosomes.
• These enzymes are synthesized in RER which are brought to the ‘Golgi Complex’.
• Lysosomes are formed directly from Golgi complex & indirectly from ER.
• Lysosomes are discovered by De Duve (1955); mainly found in animals but also in some plants like Neurospora.
• In animals, epithelial cells of the intestine, kidney cells rich in lysosomes.

Functions

• Intracellular digestion. They help in breaking down (digesting) large molecules of cell.
• They work in defense against bacteria & virus.
• During starvation, lysosomes act on their own cellular organelles & digest them. This result in cell death hence are called ‘suicidal bag’ or demolition squads means cell autolysis or autophagy.

Spherosomes

• Single membrane bounded mainly in plants.
• Its function is fat metabolism.

Microsome

• Microsomes are the structures formed when cells are broken up in the laboratory.
• Differential centrifugation can be used to separate them from other cellular debris.
• They are used to imitate the activity of the endoplasmic reticulum in a test tube.
• They are also used to perform experiments that require protein synthesis on a membrane thus aiding in understanding the process of protein formation on the endoplasmic reticulum.

Vacuole

• Important structure in plant cell.
• They may occupy upto 90% of the cell volume.
• Single membrane bound and contain cell sap.
• Function: Osmo-regulation of molecules.
• Tonoplast is the membrane of vacuole.

Plasmodesmata

• Found in plants only.
• First observed and named by Strasburger (1903) in the form of cytoplasmic strands that connects the protoplast of adjacent cell.
• Origin: from ER
• Function: Provide metabolic contacts between cells e.g. companion cells.

Centrosome

• Present near nucleus, present in all animal cells and also in Chlamydomonas, some fungi, Gymnosperms etc.
• Two centrioles in one centrosome.
• At the time of mitosis, move to opposite pole and produce astral rays.
• The microtubules of the spindle originate from centrosome during mitosis in both plant and animal cells.

Ergastic Substances

• Non-living cell inclusions e.g. starch, sugar, organic acid, fats, oils, pigments etc.

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