4 - Eukaryotic Cell (endomembrane system) Flashcards
Endomembrane System
Unique to eukaryotic cells,
it is composed of the nuclear membrane, the endoplasmic reticulum (including both rough and smooth) the Golgi apparatus and the Lysosomes
Smooth Endoplasmic Reticulum
Most cell types have relatively little smooth ER (SER).
SER is involved in drug detoxification, carbohydrate metabolism and the
synthesis of fats, phospholipids and steroids (including the sex hormones)
SER is the location of what
phospholipid manufacture
role of SER
detoxifies lipid soluble drugs such as barbiturates by adding charged water-soluble groups such as sulphate or glucuronic acid.
SER in hepatocytes
responsible for the breakdown of stored glycogen and the release of glucose.
Sarcoplasmic Reticulum
Muscle cells have a highly specialized SER called the sarcoplasmic reticulum (SR) which sequesters calcium ions from the cytosol.
forms a network of tubular sacs in the cytoplasm of muscle cells and accumulates as an intercellular store of calcium, which regulates muscle contraction
Calcium pump
SR bound ATPase which pumps calcium ions into the SR lumen.
How is muscle contraction triggered
by a change in the action potential in the muscle cell plasma membrane which triggers the SR to release calcium ions into the muscle cytosol
Role of calcium
The calcium ions bind to a protein complex of tropomyosin and troponin which normally blocks the interaction between the myosin heads and actin.
Calcium ions attach this complex changing its conformation.
This allows the myosin heads to interact with the actin filaments.
Muscle contraction then takes place.
Rough Endoplasmic Reticulum Protein Synthesis in the ER
Protein synthesis of rough ER destined proteins begins in the cytosol.
Proteins synthesized in the rough ER includes: extracellular proteins, membrane proteins, lysosomal proteins and glycoproteins.
which proteins become attached to the RER
Only ribosome synthesizing proteins with a specific signal peptide sequence
The N-terminus of these proteins contains a signal peptide usually 20-30 amino acids long.
Protein Synthesis in the RER
A signal recognition particle (SRP) attaches to the signal peptide and
stops translation in the cytosol
The SRP docks to a SRP receptor on the ER membrane and translation starts again.
The hydrophobic signal peptide passes through the membrane and
loops back through the membrane and is cleaved off .
The rest of the peptide passes through the membrane and into the ER lumen
The signal sequence is cleaved off with the enzyme signal peptidase
Glycosylation
the addition of sugars or oligosaccharides
Glycosylation in the RER
An oligosaccharide is added in the RER.
It is composed of N-acetylglucosamine, mannose and glucose residues containing a total of 14 sugar residues is transferred to the proteins in the RER
Golgi Apparatus
The Golgi apparatus modifies and sorts the proteins which pass through it.
The Golgi complex also mediates the flow of proteins from the RER to various destinations within the endomembrane system.
lysosomal enzymes
Mannose residues of lysosome enzyme proteins are phosphorylated in the cis Golgi.
A mannose 6-phosphate receptor then binds these proteins in the trans
Golgi reticulum and directs their transfer to lysosomes
Glycosylation in the Golgi
Glycosylation is the principle modification, which occurs to the proteins as
they pass through the Golgi
Glycosylation of proteins is essential to their function, for example in the
case of mucin produced these are glycoproteins with long highly
oligosaccharide chains which are essential to produce a highly hydrated
gel like material
Lysosomes
Lysosomes are vesicular structures, limited by a single smooth membrane containing enzymes active at acid pHs
Lysosomes contain about 60 hydrolytic enzymes (manufactured in the ER) which will degrade almost all biomolecules
Carry out autophagy, phagocytosis and autolysis
Where do primary lysosomes originate from?
the trans face of the golgi
What happens when a lysosome fuses with a target?
hydrogen ions are pumped into this secondary lysosome to bring down the pH and activate the enzymes.
Autophagy
recycling of worn out organelles
Lysosomal Storage Diseases
• Partially degraded insoluble metabolites can accumulate within lysosomes if a particular lysosomal enzymes is defective.
• The resulting material results in enlarged lysosomes that compromise cell function in over 50 different lysosomal storage diseases.
E.g. tay-sachs disease
Tay-Sachs disease
- Hexosaminidase A enzyme deficiency results in the accumulation of the lipid ganglioside.
- The clinical symptoms are due to accumulation of ganglioside in nerve cells.
- Death usually occurs by 2-3 years of age
