Cell Biology: Nucleus and Endoplasmic Reticulum

Nucleus

• The nucleus is the "brain" of the cell, controlling cell activities.
• The nuclear envelope is a double-layered membrane surrounding the nucleus, separating it from the cytoplasm.
• The outer layer of the nuclear envelope is studded with ribosomes, sites of protein synthesis.
• The inner layer of the nuclear envelope is lined with lamins, proteins crucial for cell division and interaction with chromatin.
• Nuclear pores are protein complexes allowing transport of ions, proteins, nucleotides, and other molecules between the nucleus and cytoplasm.
• The nucleolus is a structure within the nucleus where ribosome subunits are assembled.
• Chromatin is DNA complexed with histone proteins, appearing as euchromatin (loose, actively transcribed) or heterochromatin (tight, less active).
• The nucleus is responsible for DNA replication, transcription (DNA to RNA), and the synthesis of various RNA types, including tRNA, mRNA, and rRNA.

Endoplasmic Reticulum (ER)

• The rough ER is a network of membranes with ribosomes attached, facilitating protein synthesis.
• The smooth ER is a network of membranes lacking ribosomes, involved in lipid synthesis, detoxification, and calcium storage.
• The rough ER synthesizes proteins destined for lysosomes, membranes, or secretion.
• Rough ER processes proteins through folding and glycosylation (addition of sugars).
• The smooth ER synthesizes lipids, including fatty acids, phospholipids, and cholesterol.
• The smooth ER contains enzymes like CYP450 for detoxification.
• The smooth ER regulates calcium levels critical for various cellular processes and muscle contraction.
• The ER plays a key role in protein and lipid synthesis and processing, transporting these substances throughout the cell.

Golgi Apparatus

• The Golgi apparatus is a membrane-bound organelle that modifies, sorts, and packages proteins and lipids received from the ER.
• The cis Golgi is the receiving region, and the trans Golgi is the shipping region.
• The Golgi modifies proteins and lipids through glycosylation (addition of sugars), phosphorylation, and other post-translational modifications.
• N-type and O-type glycosylation modify proteins. O-type glycosylation occurs exclusively in the Golgi.
• The Golgi packages molecules into vesicles, directing them to their final destinations, such as lysosomes, cell membranes, or for secretion.

Cell Membrane

• The cell membrane is a selectively permeable barrier defining the boundary of the cell.
• The cell membrane is composed of a phospholipid bilayer, cholesterol, and proteins.
• The phospholipid bilayer has hydrophilic heads (water-loving) and hydrophobic tails (water-fearing).
• Cholesterol regulates membrane fluidity.
• Membrane proteins perform diverse functions, including transport, enzymatic reactions, and cell-cell recognition.

Lysosomes

• Lysosomes are membrane-bound organelles containing hydrolytic enzymes for breaking down macromolecules (proteins, nucleic acids, lipids, carbohydrates).
• Lysosomes degrade materials ingested by the cell via phagocytosis and endocytosis.
• Lysosomes contribute to autophagy (breakdown of cellular components) and autolysis (breakdown of damaged cells).

Peroxisomes

• Peroxisomes are organelles containing enzymes, like catalase and oxidase, crucial for breaking down harmful substances, including hydrogen peroxide.
• Peroxisomes participate in fatty acid metabolism, producing acetyl-CoA.
• Peroxisomes synthesize lipids, like plasmalogen, playing a vital role in brain tissue.
• Peroxisomes contribute to ethanol metabolism (to a lesser extent).

Mitochondria

• Mitochondria are the "powerhouses" of the cell, generating ATP through oxidative phosphorylation.
• Mitochondria have an outer membrane and an inner membrane folded into cristae.
• The mitochondrial matrix houses enzymes for crucial metabolic processes like the Krebs cycle, heme synthesis, urea cycle, and others.
• Mitochondria contain their own DNA (mtDNA), inherited maternally.

Ribosomes

• Ribosomes, composed of rRNA and proteins, are sites of protein synthesis (translation).
• Ribosomes can be free in the cytoplasm or bound to the rough ER.
• Free ribosomes synthesize proteins utilised within the cytosol.
• Bound ribosomes synthesize proteins destined for secretion or incorporation into membranes.

Cytoskeleton

• The cytoskeleton is a network of filaments maintaining cell shape and enabling movement.
• The cytoskeleton includes microfilaments (actin), intermediate filaments, and microtubules.
• Microfilaments participate in muscle contraction, cytokinesis, and cell movement.
• Intermediate filaments provide structural support.
• Microtubules facilitate intracellular transport via motor proteins like kinesin and dynein, and form cilia and flagella.
• Cilia and flagella enable cellular movement..