Lecture 9: Endoplasmic Reticulum and Golgi - Supply Chain of the Cell

Endomembrane is like:

• Train cars

• Hold and modify cargo that cannot move on its own

Cytoskeleton is like:

• Train tracks

• The pathway

Motor proteins are like:

• The train engine

Endomembrane system

• A series of enclosed membranes

  • Nuclear envelope

  • ER

  • Golgi Apparatus

  • Vesicles and Vacuoles

  • Cell Membrane

Movement of membranes throughout the endomembrane system occurs through the:

Cytoskeleton

ER is a major site of: 

Biological molecule synthesis 

ER parts:

• Cisternae: membranous tubules 

  • structure 

• Lumen: internal environment 

  • inside ER

• Enclosed in cystol 

  • which is enclosed in cell memb. 

ER synthesizes: 

• Lipids and membrane proteins 

  • Internally associated (chemically interact) together in the ER

• Proteins of the ER bud off as transport vesicles that integrate with the cell and other membranes

ER structurally and functionally divide into 2 zones: 

• Rough ER (ribosomes) 

• Smooth ER 

Rough ER

• Primary site of secretory protein synthesis

  • Ribosomes associated in membrane, bud off 

  • Synthesizes peptide hormones

  • Synthesizes glycoproteins 

• Some membrane lipid synthesis too 

Rough ER unique environment: 

• pH and other environmental factors facilitate coupled protein synthesis and folding

• Membrane bound enzymes facilitate bonding of carbs to proteins (glycoproteins)

Smooth ER

• Primary site of lipid synthesis 

  • Phospholipids (structural and signaling) 

  • Steroids (structural and signaling) 

Smooth ER Functions depending on cell/tissue/organ:

• Steroid or hormone synthesis 

  • glands and reproductive organs

• Xenobiotic (foreign compound) detoxification

  • liver and intestine

• Calcium storage and signaling 

  • muscle 

Golgi apparatus: 

• Receives transport vesicles from the ER 

  • Sorts, modifies, stores and ships molecules to and from other destinations 

  • Largest organelle in secretory cells 

Cisternal Maturation Model: 

• explains how golgi is organized with distinct directionality 

  • Cis face receives transport vesicles from ER with immature produces 

  • Trans face ships transport vesicles from the golgi with mature products

  • Cisternae and vesicles travel through this space fusing and separating 

  • (In reality, transport is multidirectional, doesn’t always follow this model) 

Modification of functional groups

• Happens as products move from cis to trans cisternae 

• Head groups of phospholipids and sphingolipids change 

• Carbohydrates of glycoproteins change

Transport vesicles leave the golgi:

• Receive modifications that target them to other organelles and membranes

• The tags are recognized by proteins that carry them along the cytoskeleton 

Phagocytosis:

• Selectivity is medium, uses general receptors and markers of the immune system 

• Particle sizes are large, engulfs items from macromolecs. to entire cells

Pinocytosis:

• Low selectivity. uses receptors and markers of the immune system 

• Medium particle size, engulfs any items small enough to fit in the forming pit 

Receptor-Mediated Endocytosis 

• High selectivity, uses solute-specific receptors 

• Small particle sizes, engulfs only items small enough to be identified by receptor 

• Solutes bound to receptors coat the vesicle 

Lysosomes

• Digest molecules in vacuoles 

• Made with enzymes and transporters that digest bio molecules and maintain acidic pH 

  • Derived from rough ER 

  • Matures through the golgi 

• Fuse with vesicles adn vacuoles to digest and process contents

  • Breaks down food, damaged organelles, microbes 

  • Allow molecules to be recycled 

When lysosomes break down:

• Extramembranous cytosol neutralizes acidity and renders lysosomal enzymes inactive