3.Membranous Cell organelles – structure and function

Membranous Cell Organelles – Structure and Function

1. Mitochondria:

1. • Elongated structures measuring 0.5–1 µm in diameter and up to 20 µm in length.
   • Double membrane-bound organelles found in most eukaryotic cells.
   • Mitochondria are often described as “the powerhouse of the cell” due to their role in ATP production.
   • Besides supplying cellular energy, mitochondria are involved in other essential tasks:
     • Signaling
     • Cellular differentiation and cell death
     • Maintaining control of the cell cycle and cell growth
     • Calcium storage and oxidation of fatty acids
   • Endosymbiotic Theory:
     • Mitochondria exhibit certain bacterial characteristics, leading to the hypothesis that they originated from an ancestral aerobic prokaryote.
     • The mitochondrial matrix contains a small circular DNA chromosome, ribosomes (for limited protein synthesis), mRNA, and transfer RNA.
     • Protein synthesis occurs within mitochondria.
     • The mitochondrial genome codes for only a fraction of mitochondrial proteins. These proteins have a small amino acid sequence that serves as a signal for their uptake across the mitochondrial membrane, facilitated by translocases.
   • Chemiosmotic Process (Theory):
     • Relates to the generation of ATP by the movement of hydrogen ions (protons) across a membrane during cellular respiration.
     • Hydrogen ions diffuse from areas of high proton concentration to areas of lower concentration, creating an electrochemical gradient that can be harnessed to produce ATP.
   • Mitochondria move through the cytoplasm along microtubules, and the number of mitochondria in a cell correlates with its energy needs.
   • Both mitochondrial membranes contain numerous proteins and have reduced fluidity:
     • The outer membrane contains transmembrane proteins called porins, forming channels for small molecules to enter the intermembrane space from the cytoplasm.
     • The inner membrane is folded into long infoldings called cristae, significantly increasing the membrane’s surface area. The number of cristae corresponds to the cell’s energy requirements.
     • Integral proteins in the impermeable lipid bilayer selectively allow small molecules required for ATP synthesis to pass through the inner membrane.
     • Matrix enzymes include those involved in oxidizing pyruvate and fatty acids to form acetyl CoA, as well as enzymes of the citric acid cycle that release CO₂ as a waste product. Small energy-rich molecules provide electrons for the electron transport chain (respiratory chain).
2. Rough and Smooth Endoplasmic Reticulum (ER):
   • Rough ER (RER):
     • Prominent in cells specialized for protein secretion.
     • Consists of saclike cisternae bounded by membranes continuous with the outer nuclear envelope membrane.
     • The surface of RER is abundant in ribosomes involved in protein synthesis, giving it a rough appearance.
     • RER manufactures lysosomal enzymes, secreted proteins, and integral membrane proteins.
   • Smooth ER (SER):
     • Lacks bound polyribosomes and is less abundant than RER in most cells.
     • Roles of SER include:
       • Synthesizing various phospholipid molecules constituting cellular membranes (directed to the membrane by vesicles or phospholipid transfer proteins).
       • Occupying a large portion of the cytoplasm in cells that synthesize steroid hormones (e.g., adrenal cortex cells). SER contains enzymes required for steroid syn
       • 3. Golgi Apparatus:

§  The Golgi apparatus packages proteins into membrane-bound vesicles inside the cell before sending them to their destination.

§  It plays a crucial role in processing proteins for secretion by attaching various sugar monomers (glycosylation) as proteins move through the apparatus.

§  Structure:

§  Two main networks:

§  Cis Golgi Network (CGN): A collection of fused, flattened membrane-enclosed disks known as cisternae.

§  Trans Golgi Network (TGN): The final cisternal structure, from which proteins are packaged into vesicles destined for lysosomes, secretory vesicles, or the cell surface.

§  The Golgi apparatus acts as a major collection and dispatch station for protein products received from the endoplasmic reticulum (ER).

§  Proteins synthesized in the ER are packaged into vesicles that fuse with the Golgi apparatus.

§  These cargo proteins are modified and either secreted via exocytosis or used within the cell.

§  Analogously, the Golgi can be thought of as a post office, packaging and labeling items for delivery to different parts of the cell or the extracellular space.

§  Additionally, the Golgi apparatus is involved in lipid transport and lysosome formation.

1. • • 4. Lysosomes:

§  Lysosomes are cellular organelles containing acid hydrolase enzymes that break down waste materials and cellular debris.

§  They can be described as the “stomach” of the cell.

§  Lysosomes digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria.

§  The lysosomal membrane allows digestive enzymes to work at their required pH.

§  The interior of lysosomes (pH 4.8) is acidic compared to the slightly basic cytosol (pH 7.2).

§  The lysosomal membrane protects the cytosol and the rest of the cell from degradative enzymes within the lysosome.

§  Lysosomal acid hydrolases do not function well or at all in the alkaline cytosol, preventing destruction of cytosolic molecules and organelles in case of enzyme leakage.

1. • • 5. Peroxisomes:

§  Peroxisomes are spherical, membrane-limited organelles approximately 0.5 µm in diameter.

§  They contain enzymes involved in lipid metabolism.

§  Peroxisomes oxidize specific organic substrates by removing hydrogen atoms, which are transferred to molecular oxygen.

§  Abundant in liver and kidney cells due to the presence of catalase enzyme (found in all peroxisomes), which aids in ethanol detoxification.

1. • • 6. Melanosomes:

§  Melanosomes are cell organelles containing pigment melanin derived from the amino acid tyrosine.

§  They provide protection against UV radiation.

1. • • thesis.
     • Detoxifying drugs and toxins (abundant in liver cells), where it processes oxidation, conjugation, and methylation to degrade certain hormones and neutralize harmful substances.
     • Storing and releasing calcium in a controlled manner, contributing to rapid cellular responses.