Unit 1-2 Flashcards
Membranes divide
the cytoplasm of eukaryotic cells into distinct compartments.
The endomembrane system includes organelles such as (5):
(1) endoplasmic reticulum
(2) golgi complex
(3) endosomes
(4) lysosomes
(5) vacuoles
Organelles of the endomembrane system are part of
an integrated network in which materials are shuttled back and forth.
Materials are shuttled between organelles in
transport vesicles.
Upon reaching their destination, transport vesicles:
fuse with the membrane of the acceptor compartment.
The endoplasmic reticulum (ER) is
a network of interconnected internal membranes that extends from the nuclear membrane throughout the cytoplasm
The ER membrane forms a continuous sheet enclosing a single internal space: the __
ER lumen
The ER is the
largest organelle of most eukaryotic cells:
the ER membrane is __ of all cell membranes and the ER lumen is __ of the total cell volume
The ER is the largest organelle of most eukaryotic cells: the ER membrane is ~50% of all cell membranes and the ER lumen ~10% of the total cell volume
Three distinct types of ER perform different functions within the cell:
(1) The rough ER
(2) The smooth ER
(3) Transitional ER
Ribosomes bound to the rough ER synthesize :
virtually all proteins to be secreted from the cell and proteins for most of the cell’s organelles including:
Ribosomes bound to the rough ER synthesize virtually all proteins to be secreted from the cell and proteins for most of the cell’s organelles including (5):
a) the ER itself
b) the Golgi apparatus
c) lysosomes
d) secretory vesicles
e) the plasma membrane
Many proteins are glycosylated within the ER by:
covalent attachment of sugars (glucose and mannose to their polypeptide chains)
Proteins synthesized on free ribosomes in the cytosol either remain in the cytosol or are transported to:
a) the nucleus b) mitochondria c) chloroplasts d) peroxisomes
The synthesis of lipids, including fatty acids, cholesterol and phospholipids, occurs in
the smooth ER
The smooth ER makes a major contribution to
the lipid composition of membranes of all organelles by producing most of their lipids
After their synthesis in the smooth ER, lipids are
transported from the ER to their ultimate destinations either in vesicles or by carrier proteins
SER has developed specialized functions in specific cells: In endocrine cells:
synthesis of steroid hormones
SER has developed specialized functions in specific cells: In liver cells:
detoxification of various organic compounds (home of the P450 enzymes)
SER has developed specialized functions in specific cells: in muscle cells:
sequestration of calcium ion from cytoplasm of muscle cells
Modifying the lipid composition of membranes: Three distinct mechanisms:
- Enzymatic modification
(head group) - Modification during
vesicle formation - Modification by
phospholipid transfer proteins
The rough ER forms
oriented stacks of flattened cisternae
The smooth ER forms a
fine network of tubules connected to the RER
Transitional ER
A region of the ER where
secretory vesicles exit the ER en route to the Golgi apparatus
Both proteins and lipids are exported from the ER in
transport vesicles
Both proteins and lipids are exported from the ER in transport vesicles
These vesicles bud from:
the transitional ER
Both proteins and lipids are exported from the ER in transport vesicles
These vesicles bud from the transitional ER
After budding, these vesicles carry their cargo first to __ and then to __
(1)ER-golgi intermediate compartment
(2) golgi appartus
One of the main functions of the ER once a protein has entered this organelle is called
quality control, a system that monitors proper folding of a protein
quality control
- After initial glycosylation in the ER, glucose residues are removed leaving a single terminal glucose
- The protein then associates with chaperones including calnexin
- The terminal glucose is removed and if the protein is not folded correctly….
- ….the enzyme UGGT adds the glucose back….
- ….and refolding is attempted again.
- If the protein then folds it can be
transported to the next
compartment. - If it remains unfolded after several
attempts, more sugars (mannose)
are removed…. - ….and the protein is dislocated from
the ER to the cytoplasm to be degraded by the proteasome (ER- associated degradation (ERAD)).
If too many unfolded proteins accumulate in the ER, then
the unfolded protein response (UPR) is activated:
If too many unfolded proteins accumulate in the ER, then the unfolded protein response (UPR) is activated (3):
- Stops translation
- Degrades misfolded proteins
- Produces more chaperones
Prolonged UPR leads to
apoptosis
Morphologically the Golgi is composed of
flattened membrane-
enclosed sacs (cisternae) and associated vesicles
The Golgi apparatus is usually located
near the cell nucleus
Functions of the Golgi (3):
- A factory in which proteins received from the ER:
a. are further glycosylated
b. are sorted for transport to their eventual destinations - Some lipids, including glycolipids and sphingomyelin, are synthesized within the Golgi complex
- In plant cells, the Golgi serves as the site at which the complex polysaccharides of the cell wall, including hemicelluloses and pectins, are synthesized
Functions of the Golgi
1. A factory in which proteins received from the ER:
a. are further glycosylated
b. are sorted for transport to their eventual destinations (3):
- lysosomes
- the plasma membrane - extracellular medium
__are synthesized within the Golgi complex
Some lipids, including glycolipids and sphingomyelin
. In plant cells, the Golgi serves as the site at which the
complex polysaccharides of the cell wall, including hemicelluloses and pectins, are synthesized
The Golgi consists of an ordered series
of compartments
Each Golgi stack has two distinct faces:
- en entry, or cis,
face - anexit,ortrans,
face
The cis face of the Golgi is adjacent to
the ER
the trans face of the golgi points toward:
the plasma membrane
Five functionally distinct compartments of the golgi:
- the cis Golgi network
- the cis compartment of the Golgi stack
- the medial compartment of the Golgi stack 4. the trans compartment of the Golgi stack 5. the trans Golgi network
The movement of proteins within the Golgi
- Proteins and lipids enter the cis Golgi network in transport vesicles from the ER
- The proteins and lipids then progress to the cis, medial and trans compartments (cisterna) of the Golgi stack
- The proteins and lipids then move to the trans Golgi network, which acts as a sorting and distribution center, directing molecular traffic of transport vesicles to lysosomes, the plasma membrane and cell exterior
The movement of proteins within the Golgi
Vesicular transport:
cargo is shuttled from the CGN to the TGN in vesicles
The movement of proteins within the Golgi:Cisternal maturation:
each cisterna matures as it moves from the cis face to the trans face, mediated by vesicles traveling from trans face to cis face
