Eukaryotic Organelles (Q/A)

What is the structure of a mitochondrion?

Inner and outer mitochondrial membrane with an inter-membranous space and in internal intercristal space.

About what proportion of cells are within the membrane bound organelles? Which, are the largest?

About half in the average cell is in membrane-bound organelles, varying a bit from cell-to-cell. Mitochondria are about 20% of the volume of a liver cell. The endoplasmic reticulum and the Golgi apparatus take up about 15% each. And the nucleus is about 6%. The rest are tiny proportions.

What marker on the membrane is most important in moving vesicles from the RER to the Golgi and from the cis Golgi through the cistern to the trans Golgi? What marker is present on vesicles moving in the other direction?

COPII (coat protein II) is present on things moving from the RER and from cis to trans faces. COPI is present on vesicles moving retrogradely

How do mitochondria generate ATP?

Electrons generated by the TCA cycle are carried by NADH to drive electron transporting units that move protons into the inter-membranous space. The high concentration of protons (acid) in this space is the engine that drives ATP synthase to create ATP from ADP.

What happens to proteins that are not folded properly?

Enzymes that recognize features of poorly folded proteins conjugate these proteins with ubiquitin. Such tagged proteins are delivered to proteasomes.

What is the major benefit of having membrane-bound organelles?

Increased surface area of membranes increase the surfaces on which processes can take place. This is important because membranes are very good at organizing functionally related proteins, especially enzyme systems.

What functions take place in the cis Golgi? The medial Golgi cisterns? The trans Golgi cisterns?

In the cis Golgi cisterns, N-linked oligosaccarides are trimmed and other sugars added. In the medial Golgi cisterns there is glycosylation of some lipids and serene and threonine residues. Also, there is further modification of N-linked oligosaccarides. In the trans Golgi cisterns, sialic acid is added to some carbohydrates along with sulfation of tyrosine and some sugars.

What are the three main categories of vesicles budding off of the trans-Golgi face?

Lysosomes, secretory vesicles, and vesicles fusing with the surface membrane.

What is the tag on enzymes destined for inclusion in the lysosomes?

Mannose 6-phosphate.

What are some benefits of having distinctive organelles in a eukaryotic cell?

Membrane-bound spaces permit the segregation of functions within the cell and the concentration of specific functions. organization of protein movement through the cell. Both facilitate increasing levels of complexity found in eukaryotes. Another key function is to facilitate degradation of worn-out cell components in the longer-lived eukaryotes. The same degradative system can be modified to process extracellular material.

Why do ribosomes represent an important target of antibiotics?

Prokaryotes (such as bacteria) have different ribosomal subunits which can be targeted by specific chemicals (killing the bacteria and not damaging the eukaryotic cell).

What is the function of autophagocytosis?

Recycling old, worn out organelles within the cell.

Which organelles are not membrane bound?

Ribosomes and proteasomes are macromolecular complexes.

What kinds of proteins are inserted into the endoplasmic reticulum?

Secreted proteins and proteins that become associated with the membranes of the cell surface or other organelles.

How does folding of proteins take place?

Some folding takes place simply based on amino acid sequence. However, complementary proteins (chaperone proteins) are often necessary to fold the protein properly.

What are the functions of the smooth endoplasmic reticulum?

Synthesis of membrane lipids (phospholipids, cholesterol and glycolipids), synthesis of steroid hormones, drug detoxification (cytochrome p450 enzymes) and sequestration of calcium.

What is the function of the Golgi?

The Golgi apparatus consists of stacks of flattened vesicles that have been likened to pita bread. Proteins from the ER are modified (e.g. glycosylation) and then sorted to their various specific final locations.

What is the driving force for ATP synthesis?

The electrochemical gradient of hydrogen ions in the intermembrane space, created by the electron transport system, is the driving force that activates ATP synthase.

Where do most of the reactions take place in a mitochondria?

The inner membrane.

Where are most of the proteins involved in ATP generation located?

The inner mitochondrial membrane (which is folded into christae) contains the proteins of the electron transport system and the enzyme ATP synthase.

What is the difference between the large and small ribosomal subunit (other than size)?

The small ribosomal subunit is involved in the initiation of protein synthesis (recognition of translation initiation complexes) and also in monitoring the complementarity of mRNA and tRNA during translation. The large ribosomal subunit contains A, P and E sites where tRNA bound amino acids are added, fused to the polypeptide chain, and then where the tRNA is ejected to be recycled. The large subunit contains RNA that functions as an enzyme to create the peptide bond in the elongating chain.

What is the role of the two components of the ribosomal complex?

The small subunit binds mRNA, which then binds to the large ribosomal unit. The large ribosomal unit has three sites (A-site, P-site and E-site) for binding of tRNAs. The large subunity also contains RNA that catalyzes the formation of the peptide bond between amino acids of the chain (an example of a non-protein enzyme).

What are the two models for movement of substrate through Golgi?

The vesicular transport model, in which "postmarked" vesicles from one Golgi compartment bud off and join the next, and the cisternal maturation model where layers of Golgi themselves move through the stacks along with their contents. There are also models of transient fusion between Golgi lamella, and various permutations and combinations of these theories of movement.

Can misfolded proteins be "fixed?

There are "chaperone proteins" that can aid in refolding by their interaction with the misfiled protein. This often does not work and the protein needs to be recycled.

How does protein know to go into the endoplasmic reticulum?

There is a signal recognition particle (SRP) that recognizes a signal sequence on the beginning of the developing protein. This particle binds to the endoplasmic reticulum and the forming protein is inserted through the membrane of the endoplasmic reticulum, where it folds.

What happens with deficiency of specific acid hydrolases in lysosomes?

There is accumulation of the product that can't be broken down (and often cell death).

What is the difference between free ribosomes, polyribosomes and rough endoplasmic reticulum?

There is no difference in the ribosome. When a string of free ribosomes line up along a messenger RNA molecule they become polyribosomes. When the protein that is being created has a signal sequence that is recognized by a signal recognition peptide, the ribosome becomes bound to a receptor on endoplasmic reticulum, and the developing protein is inserted through a protein translocator molecule in the endoplasmic reticulum membrane. Therefore, the ribosome appears directly bound to the endoplasmic reticulum, defining rough ER.

What is the function of a peroxisome?

They contain enzymes involved in the beta-oxidation of long-chain fatty acids, detoxification (including alcohol) and synthesis of plasmalogens (mostly in myelin of nerves). Oxydation results in generation of hydrogen peroxide.

What is the role of proteosomes and what would happen if these were not functional?

They degrade proteins attached to ubiquitin. This is used to recycle some normal proteins, but also to eliminate misfiled proteins. These proteins would accumulate in the cell if the system were not functional.

How are lysosomes related to Golgi?

Vesicles labeled with mannose-6-phosphate carry the critical enzymes (acid phosphatases) to the lysosomes as primary lysosomes. These fuse with late and assumes, phagosomes and autophagosomes to make secondary lysosomes.