Lecture 5 hard

Smooth ER

• Located near the cell periphery, the smooth endoplasmic reticulum (SER) lacks ribosomes, distinguishing it from rough ER.
• Functions include lipid, steroid, and carbohydrate synthesis, along with metabolizing drugs and toxins.
• The type of cell influences the specific functions of the SER.
• In muscle cells, it's known as the sarcoplasmic reticulum, regulating calcium ion concentrations during muscle contraction and relaxation.
• In adrenal glands, SER secretes cholesterol, a precursor to steroid hormones.
• In liver cells, SER synthesizes and stores detoxifying enzymes to process harmful substances like alcohol and drugs.

Functions of ER

• Rough ER (RER) is primarily responsible for protein production, while SER synthesizes phospholipids and steroids.
• The ER detoxifies harmful exogenous molecules and regulates calcium ion sequestration and release.

Lysosomes

• Heterophagia involves breaking down foreign substances absorbed via endocytosis.
• Autophagy is the process of degrading internal materials, including nutrients and dysfunctional organelles.
• Autolysis occurs when lysosomes rupture, digesting the cell due to pathology or aging.
• Primary lysosomes form from the Golgi apparatus and contain inactive hydrolases for degrading large molecules.
• Secondary lysosomes form when primary lysosomes merge with endocytic vesicles, digesting various materials and recycling products to the cytoplasm.

Endoplasmic Reticulum Structure and Function

• The endoplasmic reticulum (ER) produces, processes, and secretes proteins, playing a vital role in every cell of humans and other organisms.
• Its structure consists of interconnected flattened sheets (cisternae) and tubules, facilitating efficient function.
• Rough ER is studded with ribosomes, while the smooth ER lacks them.

Rough ER

• Lumen of rough ER is narrower than that of smooth ER.
• Central to protein synthesis as ribosomes translate mRNA into proteins, which are then modified and transported.

Golgi Network

• Composed of two networks: cis and trans Golgi networks, each made of multiple cisternae.
• The cis Golgi network is adjacent to the ER, while the trans Golgi network is the farthest.
• The Golgi modifies proteins and lipids received from the ER, packaging them into vesicles for transport.

Protein Trafficking

• Newly synthesized proteins are transported from the ER to the Golgi for modification and sorting to their destinations.
• The Golgi creates polysaccharides and packages proteins and lipids within vesicles for secretion or internal use.

Lysosome Functionality

• Lysosomes are membrane-bound organelles responsible for cellular waste degradation and recycling.
• Their acidic internal environment (pH 4.5-5.5) contains hydrolytic enzymes, facilitating the breakdown of internal and external materials.
• They play a role in autophagy, cellular repair, and can trigger autolysis when a cell is irreparably damaged.

Ribosome Structure and Function

• Ribosomes are essential for protein synthesis, directing what type of proteins are produced based on location.
• Composed of two subunits (large and small), they come together to facilitate protein production when needed.
• The ribosomal process involves initiation, elongation, and termination of protein synthesis.

Golgi Apparatus Structure

• The Golgi apparatus, made up of stacked pouches called cisternae, operates closely with the ER.
• Composed of the cis face (closest to the ER), trans face (farthest from the ER), and medial section, it processes proteins and lipids.
• The endoplasmic reticulum-Golgi intermediate compartment (ERGIC) facilitates transport between the ER and Golgi.

Golgi Processing Steps

• Proteins or lipids leave the ER as vesicles, enter the Golgi at the cis face, and are modified as they move through its lumen.
• Modifications may include sugar replacements or the addition of fatty acids or phosphates.
• At the trans face, molecules are sorted and packaged into vesicles for delivery to specific cellular locations.