Histology Wk 4

Question 1

Name some membranous and non membranous protein complexes

Answer 1

Membranous- mitochondria
Non membranous- ribosomes and proteasomes

Question 2

What are ribosomes

Answer 2

About 20 x 30 nm in size
They assemble proteins from a.a on molecules of tRNA in a sequence specified by mRNA

Question 3

What are the two subunits of ribosomes bound to a strand of mRNA

Answer 3

The core of the small ribosomal subunit is highly folded rRNA chain associated with more than 30 unique proteins

The core of the large subunit has three other rRNA molecules and nearly 50 other basic proteins

Question 4

What do the rRNA molecules do in the ribosomal subunits

Answer 4

Provide structural support
Catalyse the formation of the peptide bonds
The more peripheral proteins of the ribosome seem to function primarily to stabilise the catalytic RNA core

Question 5

What are polyribosomes or polysomes

Answer 5

During protein synthesis many ribosomes typically bind the same strand of mRNA to from larger complexes

Question 6

Why are polyribosomes intensely basophilic

Answer 6

Numerous phosphate groups of the constituent RNA molecules that act as poly anions

Question 7

What are free polyribosomes

Answer 7

Synthesise cytosolic and cytoskeletal proteins and proteins for the import into the nucleus, mitochondria and peroxisomes

Question 8

What are ER bound polyribosomes

Answer 8

Stored in lysosomes or eventually secreted from the cell are made on polysomes attached to the membranes of ER.

The proteins produced by these ribosomes are segregated during translation into the interior of the ER’s membrane cisternae

In both pathways misfolded proteins are conjugated to ubiquitin and targeted to proteasomal degradation

Question 9

What is the endoplasmic reticulum

Answer 9

Extends from the surface of the nucleus throughout most of the cytoplasm and encloses a series of inter communicating channels called cisternae

Question 10

What is the RER

Answer 10

Prominent in cells specialised for protein synthesis

Question 11

What is the function of the RER

Answer 11

Production of membrane associated proteins of many membranous organelles and proteins to be secreted by exocytosis

Includes the initial glycosylation of glycoproteins, certain postranslational modifications of newly formed polypeptides and the assemble of multi chain proteins

Question 12

What are the functions of the ER

Answer 12

Synthesis- provides a place for chemical reactions
A. Smooth ER is the site of lipid synthesis and carbohydrate metabolism
B. Rough ER synthesises proteins for secretion, incorporation into the plasma, membrane and as enzymes with lysosomes

Transport- moves molecules through the cisternae space from one part of the cell to another

Storage- stores newly synthesised molecules

Detoxification- smooth ER detoxifies drugs and alcohol

Question 13

Explain the movement of polypeptides into the RER

Answer 13

The newly translated amino terminus of a protein to be incorpo- rated into membranes or sequestered into vesicles contains 15-40 amino acids that include a specific sequence of hydrophobic residues comprising the signal sequence or signal peptide. This sequence is bound by a signal-recognition particle (SRP), which then recognizes and binds a receptor on the ER. Another recep- tor in the ER membrane binds a structural protein of the large
ribosomal subunit, more firmly attaching the ribosome to the ER.

The hydrophobic signal peptide is translocated through a protein pore (translocon) in the ER membrane, and the SRP is freed for reuse. The signal peptide is removed from the growing protein by a peptidase and translocation of the growing polypeptide contin- ues until it is completely segregated into the ER cisterna.

Question 14

What is the smooth endoplamsic reticulum (SER)

Answer 14

Regions of ER that lack bound polyribosomes make up the smooth endoplasmic reticulum (SER), which is continuous with RER but frequently less abundant

Question 15

What are the functions of the SER

Answer 15

Enzymes in the SER perform synthesis of phospholipids and steroids, major constituents of cellular membranes

Question 16

How are these lipids synthesised

Answer 16

These lipids are then transferred from the SER to other membranes by lateral diffusion into adjacent membranes, by phospholipid transfer proteins, or by vesicles which detach from the SER for movement along the cytoskeleton and fusion with other membranous organelles. In cells that secrete steroid hormones (eg, cells of the adrenal cor- tex), SER occupies a large portion of the cytoplasm

Question 17

How does detoxification occur

Answer 17

including those of the cytochrome P450 family, allow detoxification of potentially harmful exogenous molecules such as alcohol, barbiturates, and other drugs.

In liver cells, these enzymes also process endogenous molecules such as the components of bile.

Question 18

How is the SER involved in the sequestrian and controlled release of ca2+

Answer 18

which is part of the rapid response of cells to various stimuli.

This function is par- ticularly well developed in striated muscle cells, where the SER has an important role in the contraction process and assumes a specialized form called the sarcoplasmic reticulum

Question 19

What are eryhtroblasts

Answer 19

Cells that make few or no proteins for secretion have very little RER, with essentially all polyribosomes free in the cytoplasm

Question 20

What is an eosinophilic leukocyte

Answer 20

Cells that synthesize, segregate, and store various proteins in specific secretory granules or vesicles always have RER, a Golgi apparatus, and a supply of granules containing the proteins ready to be secreted.

Question 21

What is a plasma cell

Answer 21

Cells with extensive RER and a well-developed Golgi apparatus show few secretory granules because the proteins undergo exocytosis immediately after Golgi processing is complete. Many cells, especially those of epithelia, are polarized, meaning that the distribution of RER and secretory vesicles is different in various regions or poles of the cell

Question 22

What are epithelial cells

Answer 22

specialized for secretion have distinct polarity, with RER abundant at their basal ends and mature secretory gran- ules at the apical poles undergoing exocytosis into an enclosed extracellular compartment, the lumen of a gland.

Question 23

Functions of the Golgi in the RER

Answer 23

• New proteins are translocated into ER cisternae
• Preassembled mannose-rich oligosaccharides are added to specific asparagine residues (N-linked)
• Proteins are folded, guided by chaperones, with strict quality control
• Disulfide bonds are formed between specific cysteine residues

Question 24

What is the cis Golgi network

Answer 24

Vesicle movement from RER and forward through the CGN is
promoted by the coat protein COP-II
• Similarly, COP-I controls retrograde vesicle movements
• Mannose-6-phophate is added to future lysosomal enzymes
• N-linked oligosaccharides are trimmed and other sugars added

Question 25

What is the medial Golgi cisternae

Answer 25

New glycosylation occurs on –OH groups of some lipids
and serine and threonine residues (O-linked)
• N-linked oligosaccharides on proteins are modified further
• Glycoproteins and glycolipids are sorted into specific vesicles

Question 26

What is trans Golgi network

Answer 26

Sialic acid is added as the terminal sugar to certain
oligosaccharides
• Sulfation of tyrosine residues and some sugars occurs
• Specific vesicles with different destinations are separated and
sorted