Test Ch 15 Flashcards
Evidence for endosymbiotic evolution of mitochondria and chloroplasts
- Chloroplasts and mitochondria have:
-double membranes that come from being engulfed
-own circular DNA which is important because prokaryotes have circular DNA
-Ability to reproduce by dividing in two (fission)
-Strongest evidence: Proteins (including ribosomes and DNA Polymerase) more similar to prokaryotes than eukaryotes
Translation occurs on ribosomes:
-2 subunites each contain multiple polypeptides and rRNA
- Amino terminal end of new polypeptide made first
-ribosomes are found in cytosol and they are large protein complexes
What are some advantages and disadvantages to having compartments (membrane bound organelles) in the eukaryotic cell
Advantages:
-efficient specialization
-production of genetic material
-more complex cells
-since larger, less diffusion limitations
- protection of the cell
Disadvantages:
-Takes longer for things to happen
-increased energy demand
Ribosomes are found __
free in cytosol and bound to the ER
-ribosomes in cytosol stay in cytosol
-other ribosomes translated in ER
Mechanism for importing newly synthesized proteins into cell compartments
- Transport through nuclear pores
- Transport across membranes
- Transport by vesicles going from ER to Golgi
Proteins complete synthesizes on free ribosomes for 1 and 2 and Bound ribosomes for step 3.
Signal Sequence
A string of amino acids that determines where the protein will go.
-Signal seq. is like a zip code for your protein which tells you what part of town your mail goes to.
-If protein has no signal it stays in the cytosol
receptors recognize and bind to signal sequence
Mechanism #1: Through nuclear pores
-Nuclear Envelope Structure:
-Two lipid bilayers
-Contiguous with ER membrane
-Nuclear pores penetrate both lipid bilayers
Proteins into nucleus steps
-Translated on free ribosomes
in cytosol (not bound to ER)
-Protein folds (in lowest free energy form)
- If contain Nuclear Localization Signal sequence, Nuclear Import
Receptor binds to the signal
sequence - Nuclear Import Receptor
interacts with cytosolic fibrils
of nuclear pore - Active transport through pore
using GTP hydrolysis 12
Mechanism 2: Protein translocators: Mitochondria and Chloroplast
Translated on free
ribosomes,
- Protein does not fold (proteins going to mitochondria do not fold they things bound to it to help it remain unfolded)
- Contain signal sequence
for mitochondria - Binds Import Receptor
Protein on organelle outer
membrane - Translocated across both
membranes by Protein
Translocator. - Chaperone proteins help
pull the protein into matrix. - Signal sequence cleaved
off - Folds in matrix
- May move to other location
in organelle if additional
Mechanism 2: Translocator into ER
-All proteins are initially
translated by free ribosomes in cytosol.
If ER signal sequence present, the
ribosome becomes bound to ER to
complete translation
-Proteins synthesized
into ER are destined for:
ER, PM, Golgi, endosomes,
lysosome or secretion =
endomembrane system
Targeting a new protein to ER
-Translation begins on free
ribosomes
- ER signal sequence of protein
bound by Signal Recognition
particle (SRP) and ribosome,
slows translation. - SRP binds to SRP receptor in
ER membrane - Protein threaded through protein
translocator as translation
continues on ER bound
ribosomes
(every signal sequence has a receptor that recognizes it)
Synthesis of water-soluble protein in ER
- When translation complete,
signal peptidase removes
signal sequence (or cleaves signal) - Protein released inside ER
lumen
-Protein fold
The protein not embedded into the membrane can go into different locations
Synthesis of Multi-pass transmembrane protein in ER
- Internal hydrophobic
start-transfer signal
sequence and stop-
transfer sequences
moved from translocator
into membrane - Pairs of start and stop
transfer sequences
Everything has to have a signal sequence or else it remains in the cytosol
Mechanism #3: Transport Vesicles
.Proteins destined for Golgi,
plasma membrane, lysosomes
or secretion are first made in
ER then transported to
destination by transport
vesicles
Formation of Clathrin-coated vesicles to form transport vesicles
- Cargo protein binds to cargo
receptor in membrane - Binding induces adaptin to
bind to receptor - adaptin recruits clathrin
- As more clathrin binds, pulls
membrane outward - Dynamin pinches off
completed transport vesicle and uses GTP hydrolysis - Vesicle uncoats
-Vesicles move along
microtubules using motor
proteins
Docking and fusion of transport vesicles
Docking: The Rab
protein on the vesicle
and tethering protein
on the target
membrane direct
vesicles to correct
target compartments.
Membrane Fusion:
t-SNARE and v-SNARE
wind together to fuse the
membranes together
Rab is a recognition protein that is recognized by a tethering protein there are different rabs for different locations
Protein glycolysation begins in the ER
Oligosaccharide transferred
from dolichol lipid to
asparagine (ASN) on new
protein
Proteins are properly folded before leaving ER
Too many misfolded
proteins bind to sensors in the ER. Sensors trigger production of chaperone proteins and ER membrane to increase folding
capacity in the ER. (Unfolded Protein Response)
Golgi Apparatus
- Has a specific orientation has one side facing ER called cis golgi
-Golgi is a stack of membrane that aren’t interconnected
-Sugars (or oligosaccharides) that are initially added to proteins in the endoplasmic reticulum (ER) can indeed be further modified as the protein moves through the cisternae of the Golgi apparatus.
In the cis-Golgi (the side of the Golgi closest to the ER), early modifications of these sugar chains take place.
As the protein continues to move through the Golgi stack to the medial cisterna (the middle of the Golgi), additional sugar modifications can occur.
-Trans golgi proteins are sorted in different transport vesicles to different locations
-In Golgi glycosylation’s are added and modified and sorted to determine the location
Exocytosis or secretion
Constitutive Secretion (Unregulated):
Newly synthesized proteins and lipids are continuously transported in vesicles from the Golgi to the plasma membrane.
These vesicles fuse with the membrane, releasing their contents outside the cell without needing a signal.
Regulated Secretion:
Secretory vesicles storing proteins are kept in the cytosol until an extracellular signal (like a hormone or neurotransmitter) triggers their release.
Upon receiving the signal, the vesicles fuse with the plasma membrane, and the proteins are secreted outside the cell.
In short, one pathway releases proteins constantly (constitutive), while the other waits for a signal (regulated).
glycosylation occurs only on
Non cytosolic face
Summary of secretory pathway
Proteins are completed on ER bound ribosomes
Glycosylation begins and proteins folded in ER
Travel to cis Golgi by transport vesicles
Glycosylation modified in cis and medial Golgi
Proteins sorted into transport vesicles in Trans Golgi
Travel to PM
by Constitutive or Regulated Exocytosis
Secretory pathway also delivers new lipids to the compartments in
the pathway
Ribosomes made in cytosol stay in cytosol no need to travel
Summary of protein sorting
Signal sequence determines where protein ends up
Nucleus
- Free ribosomes
- Folds
- Nuclear localization signal binds to nuclear import receptor and transports through nuclear pore
Mitochondria and Chloroplasts
- Free ribosomes
- Unfolded
- Signal sequence binds to import receptor on organelle
- Passed through protein translocator
ER, golgi, lysosomes and PM
- ER bound ribosomes
- Moved through protein translocator while being translated
- Travel to golgi in transport vesicles
- Travel via transport vesicles to final destination 30
Endocytosis
the process of moving things from out to inside the cell
Types of endocytosis
Pinocytosis
- Cellular drinking
- All cells
- Clathrin coated pits/vesicles bring membrane and small material into cell
- Lead to endosomes then lysosomes
- Plasma membrane turnover
Phagocytosis
- Cellular eating
- Protozoa, phagocytes, macrophages
- Extension of plasma membrane engulfs large particles
- Phagosome to lysosome
Receptor Mediated (specific molecules brought into cell)
- Receptor on PM binds to specific molecule
- Clathrin coated pits/vesicles bring membrane and receptor and bound molecule into cell
- Lead to endosomes then lysosomes
- Receptor returned to PM from endosome
Receptor mediated endocytosis
for engulfing specific molecules
-bound to receptors on PM
Lysosomes: Site of intracellular degradation
The lysosome has a very acidic environment (pH ~5.0), maintained by an ATP-driven H+ pump. This pump uses energy (ATP) to transport protons (H+) into the lysosome, lowering the pH inside.
Acid hydrolases inside the lysosome (such as nucleases, proteases, glycosidases, lipases, phosphatases, sulfatases, and phospholipases) function optimally in this acidic environment to break down biomolecules like nucleic acids, proteins, lipids, and carbohydrates.
The surrounding cytosol has a neutral pH (~7.2).
Metabolite transporters in the lysosomal membrane help move degraded products back into the cytosol for reuse or disposal.
