Cellular compartmentalization Pages

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

15_02_cell_intestine.jpg Eukaryotes use membrane enclosed compartments… –Nucleus –ER –Golgi –Lysozomes –Peroxisomes –Mitochondria –Chloroplasts Compartments

15_03_ER_evolved.jpg 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.

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 are supplied with proteins that are made in the cytosol. 3 well characterized methods of delivering proteins in to organelles are known: 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 Protein translocators Protein gates 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

15_06_Signal_sequence.jpg 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.

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

15_08_nuclear_pore.jpg 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_09_pore_transport.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.

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

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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.

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15_15_into_ER_membr.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.

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.

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