Transcript Cellular compartmentalization

Cellular compartmentalization
Pages 497 - 512
Q1
• Name at least two of the three protein
complexes involved in the electron
transport chain?
Q2
• Which very important protein carries
electrons between Cytochrome b-c1
and cytochrome oxidase?
Q3
• How many protons are pumped across
the membrane by cytochrome oxidase
as part of the electron transport chain?
Q4
• What is the pH of the matrix of
mitochondria?
• A) 2
• B) 4
• C) 6
• D) 8
Q5
• Molecular oxygen is consumed at which
point in the oxidative phosphorylation
cycle?
• What is it converted to?
Cellular compartmentalization
• Normal situation is that the liver cell is a small
factory that is producing lots of different
biochemicals at one time.
• PROBLEM: If one takes a bunch of pure cells
(say from the liver) and grinds them up in a
blender. What will happen to the mixture?
• There resulting soup will begin a process of
self digestion.
• This experiment illustrates the importance of
cellular compartments
Procaryotes
• BACTERIA
– They have a single compartment
– They also have to perform many
biochemical reactions simultaneously
– How do they do it?
• COME BACK TO THIS LATER
Eukaryotes have
compartments
• A single eukaryotic cell is performing
many thousands of different reactions
simultaneously.
• In one location it may be making ATP
and at another it may be using ATP
• There needs to be some way to
segregate all the various reactions!
Solutions
• TWO BASIC WAYS
– 1) Aggregation of protein complexes
performing certain functions. e.g. DNA
replication machine
– 2) Segregation in to separate membrane enclosed compartments
Compartments
• Eukaryotes
use
15_02_cell_intestine.jpg
membrane
enclosed
compartments…
–
–
–
–
–
–
–
Nucleus
ER
Golgi
Lysozomes
Peroxisomes
Mitochondria
Chloroplasts
A suggested mechanism of how eukaryotes evolved the
nucleus and ER. It is thought that invagination of the plasma
membrane was the method and the need to increase the inner
membrane surface area was the driving force.
15_03_ER_evolved.jpg
We have already learnt that mitochondria and chloroplasts
were originally symbiotic bacteria that lived inside primitive
eukaryotic cells. They were able to give an evolutionary
jump to the eukaryotic cells.
Protein sorting
• A dividing cell MUST make sure that it gives
to each of its daughters at least some of each
organelle.
• Many organelles cannot be made from
scratch
• A cell that is deprived of (say mitochondria)
will die.
• Just prior to cell division the major organelles
are broken into small organettes and
dispersed to all sides of the cytoplasm
Organelles need
lipids to grow and
multiply. These lipids
are made by the ER
and then delivered to
the target organelle
by special
cytoplasmic proteins
Organelles are supplied
with proteins that are
made in the cytosol.
3 well characterized
methods of delivering
proteins in to organelles
are known:
Protein gates
Protein translocators
Transport vesicles
• How does the cell know where to deliver
a particular protein.
• Are there postman in the cell?
• YES there are, but it is more akin to the
self sorting warehouses that companies
like Amazon.com use
SIGNAL SEQUENCES - Proteins are tagged with
special sequences (signal sequences - 12 to 60 amino
acids long) that are like barcodes. These are read by
special courier protein systems within the cytoplasm and
then targeted to their destinations.
15_06_Signal_sequence.jpg
The nuclear pores are
like gates that span
across both nuclear
membranes.
The inside surface of the
nuclear membrane is
highly organized
The nuclear lamina
proteins maintain the
integrity and structure of
the nucleus.
The ER is continuous
with the nuclear
envelope
The nuclear pores are complex! They are made of about 100
different proteins. They open and close as needed. Material
may enter in either direction. They are very selective.
15_08_nuclear_pore.jpg
Cytosolic Nuclear transport receptors aid the passage of material
into the cell. The nuclear transport protein is able to open the port
just enough to allow the passage of the molecule.
15_09_pore_transport.jpg
• Proteins that enter the nucleus are able
to enter the nucleus in a folder
structure.
• Proteins which enter other organelles
are only able to do so in an unfolded
state
Here is an illustration of how proteins targeted to the
mitochondria are delivered.
First the protein must carry the appropriate signal sequence.
Then, it attaches to a receptor protein on the outer membrane.
This complex diffuses until it reaches a contact site, where it is
treaded through both channel proteins. Once inside the cell the
protein refolds and the signal sequence is cleaved off.
The endoplasmic reticulum (ER) can exist in two forms: Smooth
and Rough
• Ribosomes are closely associated with the rER and make
proteins which are threaded into it whilst being made.
o TWO PROTEINS
• Signal-recognition particle (SRP) with binds the signal
sequence
• SRP receptor on the ER membrane
15_12_pool_ribosomes.jpg
A number of mechanisms come together to deliver the protein to
its destination. The SRP actually acts as an inhibitor of further
protein synthesis by the ribosome, until it is dislodged by the
SRP receptor membrane protein.
15_14_enters_lumen.jpg
The protein carries its own code for insertion into the membrane.
These STOP-TRANSFER SEQUENCES are hydrophobic and
stall the passage of the peptide through the transloaction channel.
15_15_into_ER_membr.jpg
Other double-pass transmembrane proteins have multiple stop
and start transfer signals. These allow the protein to form loops
back and forth through the membrane. Once embedded they
cannot be dislodged.
15_17_Vesicles_bud.jpg