View source for 3.Membranous Cell organelles – structure and function

'''Membranous Cell Organelles – Structure and Function'''

# '''Mitochondria:''' 

#* 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).
# '''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.

#** '''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.

#** '''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.

#** '''6. Melanosomes:'''

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

§  They provide protection against UV radiation.

#** 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.