The Endomembrane System Flashcards
The Endomembrane System
An interconnected system of membrane-
enclosed compartments
Includes the plasma membrane, nuclear
envelope, endoplasmic reticulum, Golgi
apparatus and lysosomes
Membrane-bound droplets called vesicles
shuttle between the various components
In the cell, membrane components and
materials they contain are in constant motion
ER
A network of interconnected membranes
branching through the cytoplasm
The interior of the ER is called the lumen and is
distinct from the cytoplasm
The ER can enclose up to 10% of the interior
volume of the cell
Folding of the membrane gives a surface area
many times greater than that of the plasma
membrane
Two types of ER: smooth (ER) and rough (ER)
continued
‘Rough’ because of ribosomes attached to the outer surface of
the membrane
Ribosomes are involved in protein synthesis
The RER receives into its lumen certain newly synthesised
proteins, segregating them away from the cytoplasm
The RER can transport these proteins to other locations in the
cell (in vesicles)
Proteins can be chemically modified in the lumen of the RER
(post-translational modifications)
A protein enters the lumen of the RER through a pore as it is
synthesized. This is accomplished via a sequence of amino
acids on the protein, which acts as a RER localization signa
Post Translational Modifications
Post translational modifications could:
Alter the function of the protein, or
‘tag’ them for delivery to specific cellular
destinations
Proteins are shipped to cellular destinations
enclosed within vesicles that pinch off from the ER
Most membrane-bound proteins are made in the
RER
What happens to polypeptides after
translation?
Polypeptide folds as it emerges from the
ribosome.
The amino acid sequence (primary structure)
determines the pattern of folding (secondary
structure).
Amino acid sequence also contains a signal
sequence—an “address label” e.g. NLS
amino acid sequence gives a set of
instructions:
“Finish translation and send to an
organelle.”
OR
“Stop translation, go to the endoplasmic
reticulum (ER), finish synthesis there.”
What happens to polypeptides after
translation?
What happens to polypeptides after
translation?
Conformation of signal sequences allow
them to bind specific receptor proteins—
docking proteins—on outer membranes
of organelles.
Receptor forms a channel that the
protein passes through. May be
unfolded at this time by special proteins
called chaperonins
Signal Sequence
Signal sequence binds to a
signal receptor/receptor particle,
before translation is completed.
Polypeptide+SRP attaches to a
receptor on the ER, the growing
polypeptide chain passes
through the channel.
An enzyme in the ER lumen
then removes the signal
sequence.
Protein Modifications
- Proteolysis: cutting the polypeptide chain, by proteases.
- Glycosylation: addition of sugars to form glycoproteins.
- Phosphorylation: addition of phosphate groups by kinases. Charged
phosphate groups change the conformation of the protein
Glycosylation
- Proteins —> glycoproteins
- Occurs in ER and Golgi
apparatus - This sugar coating is essential
for directing proteins to
lysosomes - Essential for protein
conformation - Cell recognition
The smooth endoplasmic reticulum
(SER)
Lacks ribosomes
1. Site for synthesis of lipids, including
phospholipids and steroids
2. Chemically modifies small molecules e.g. adding
hydroxyl groups (-OH) to e.g. drugs & pesticides
making them more polar (water soluble) and
more easily removed (detoxification)
3. Site of glycogen metabolism in animal cells,
glucose-6-phosphatase
Glycogen
- Readily mobilized storage form of glucose.
- Large, branched polymer of glucose.
- Broken down to yield glucose molecules
when energy is needed - Especially important because glucose is
virtually the only fuel used by the brain. - Glucose can provide energy in the
absence of oxygen and can thus supply
energy for anaerobic activity. - Metabolised in SER
Glycogen Metabolism
- Glycogen phosphorylase targets the ‘free
end’ of Glycogen. - Produces Glucose-1-Phosphate.
- Phosphoglucomutase changes the
position of the phosphate group. - Produces Glucose-6-Phosphate.
- Glucose-6-phosphatase is a
transmembrane protein with 9 trans-
membrane a-helices in the SER (P
removed).
Glycogen Storage
Disease: GSD-1a patients
Lack functioning enzyme
needed to turn glycogen into
glucose in the liver.
* Glycogen builds up in the liver.
* Symptoms often appear in
babies
* Low blood sugar
(hypoglycemia) and a swollen
belly hepatomegaly (enlarged
liver)
Lysosomes
Primary lysosomes originate from the Golgi
Contain digestive enzymes to breakdown
macromolecules: proteins, polysaccharides, nucleic
acids and lipids
Lysosomes are about 1 mm in diameter and are
surrounded by a single membrane
Sites for the breakdown of food, other cells, or foreign
objects taken up by the cell by a process known as
phagocytosis
pH is more acidic (lower) in the interior of the lysosome
enhancing degradation reactions
Phagocytosis
a pocket forms in the plasma
membrane, deepens and encloses material from
outside the cell.
The pocket becomes a small vesicle called a
phagosome that breaks free of the plasma
membrane and moves into the cytoplasm.
The phagasome fuses with the primary lysosome to
form a secondary lysosome in which digestion
occurs
The products of digestion pass into the cytoplasm
and the ‘used’ secondary lysosome moves to the
plasma membrane, fuses with it and releases the
undigested material outside the cell
