4 - Eukaryotic Cell (endomembrane system)

Question 1

Endomembrane System

Answer 1

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

Question 2

Smooth Endoplasmic Reticulum

Answer 2

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)

Question 3

SER is the location of what

Answer 3

phospholipid manufacture

Question 4

role of SER

Answer 4

detoxifies lipid soluble drugs such as barbiturates by adding charged water-soluble groups such as sulphate or glucuronic acid.

Question 5

SER in hepatocytes

Answer 5

responsible for the breakdown of stored glycogen and the release of glucose.

Question 6

Sarcoplasmic Reticulum

Answer 6

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

Question 7

Calcium pump

Answer 7

SR bound ATPase which pumps calcium ions into the SR lumen.

Question 8

How is muscle contraction triggered

Answer 8

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

Question 9

Role of calcium

Answer 9

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.

Question 10

Rough Endoplasmic Reticulum Protein Synthesis in the ER

Answer 10

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.

Question 11

which proteins become attached to the RER

Answer 11

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.

Question 12

Protein Synthesis in the RER

Answer 12

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

Question 13

Glycosylation

Answer 13

the addition of sugars or oligosaccharides

Question 14

Glycosylation in the RER

Answer 14

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

Question 15

Golgi Apparatus

Answer 15

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.

Question 16

lysosomal enzymes

Answer 16

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

Question 17

Glycosylation in the Golgi

Answer 17

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

Question 18

Lysosomes

Answer 18

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

Question 19

Where do primary lysosomes originate from?

Answer 19

the trans face of the golgi

Question 20

What happens when a lysosome fuses with a target?

Answer 20

hydrogen ions are pumped into this secondary lysosome to bring down the pH and activate the enzymes.

Question 21

Autophagy

Answer 21

recycling of worn out organelles

Question 22

Lysosomal Storage Diseases

Answer 22

• 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

Question 23

Tay-Sachs disease

Answer 23

• 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