I need help with this biology question please state why each

I need help with this biology question, please state why each wrong ending is false!

3. For each alternative ending to the following sentence, indicate why the ending is false:

“Proteins destined to enter the rough endoplasmic reticulum _______”

a. are transported into the ER after their synthesis is complete.

b. are synthesized on free ribosomes in the cytosol.

c. cross the membrane in a folded state.

d. must be proteins that will be secreted from the cell.

Solution

Most of the proteins are destined to enter in endoplasmic reticulum. Protein Inside ER undergo chemical modification of protein like- soluble secreted proteins, plasma membrane proteins,ER membrane proteins

A)-are transported into the ER after their synthesis is complete _______False,

because endoplasmic reticulum protein are once entered in ER lumen, they have retention KDEL- signal sequence on C-terminal of ER protein, after the chemical modification, modified ER protein retain inside ER lumen and localized to ER cell-membrane for proper function.

B) are synthesized on free ribosomes in the cytosol _______False,

because Mostly secreted proteins, lysosomal membrane protein have amino[C]-terminal signal sequence that will mark them to translocation into Lumen of the ER, but other protein like mitochondrial or chloroplast protein are synthesis on free ribosome and they transported to specific organelle through specific vesicles, they are not come under endomembrane system like other ER, Golgi apparatus, lysosomes and nucleus,

C) cross the membrane in a folded state _______False

because, after ER modification, protein transported to Golgi apparatus through vesicles for future modification, and they are not cross directly in folded state it’s depend on type of signal sequence, there are two types-1. protein insoluble or soluble protein, if the signal for insoluble protein,after the ER modification of this protein packed inside the vesicles and hydrophilic portion of protein outside the transport vesicles [COP1/COP2] and remain it hydrophobic environment in vesicles till the reach to cell membrane, after complete modification in golgi apparatus than insoluble protein or soluble protein undergo complete folded state inside the transported vesicles for proper function in targeted cell,

D) must be proteins that will be secreted from the cell____ False

because, all soluble secreted protein have specific signal, after the ER modification they target to specific organelle through transport vesicles, and these protein not enter into ER through this secretory pathway,

secretory pathway are -Rough ER à Golgià secretory vesicles-àcell exterior

Proteins translocated into ER during the synthesis on the membranes bound-ribosomes [cotranslational-translocation] and other way translation has completed on the free ribosomes in cytosol [posttranslational-translocation] because protein required for ER function, and they are come from post translation modification in complete folded state. But majority of the ER protein synthesis inside the ER they have retention signal sequence, for ER protein,