Ans. 1. Most of the proteins meant to be translocated from cytoplasm (site of translation) to specific subcellular locations (plasma membrane, nucleus, mitochondria, etc.) bear a “signal recognition particle (SRP)” at their N-terminals. The signal peptide (also known as signal peptide, localization signals, leader peptide, etc.) mediate the sequential modifications and movement of newly synthesized from cytoplasm, to endoplasmic reticulum (ER) to Golgi bodies (GB) and to the final destination by being packed in transport vesicles released from GB. Case I: Non-functional SRP inhibits co-translational translocation of the nascent peptide into ER lumen and its subsequent modifications. Such unprocessed proteins generally mis-fold automatically in the cytoplasm. The misfolded protein is subsequently degraded and recycled by the cell. Case II: Mutation in SRP: The fate of the peptide depends on the extent and type of mutation in the SRP gene. If the mutation causes no significant alteration of co-translational translocation of the nascent peptide into ER lumen, the protein shall undergo normal processing and finally transported to its specified location. If mutation makes the SRP non-functional, the protein is degraded and recycled in the cytoplasm (see, case I). Ans. 2. Movement of protons from intermembrane space into mitochondrial matrix without involving ATP synthase produced no ATP. Such transport of protons into the matrix is called “proton leak” which may be mediated by uncouplers proteins or simply leaks across the membrane. The proton leaks do not form ATP and causes loss of energy as heat. A small fraction of protons naturally flow back into the matrix through the lipid bilayer because of relatively very large electrochemical gradient across it. Presence of perforation (say, large number of uncouplers) in the inner mitochondrial membrane reduces the strength of electrochemical gradient. Thus, relatively very small amount of ATP is produced during cellular respiration of one unit of energy molecule. Thus, limitation in ATP production rate also limits the ATP dependent movement of muscles. Solution Ans. 1. Most of the proteins meant to be translocated from cytoplasm (site of translation) to specific subcellular locations (plasma membrane, nucleus, mitochondria, etc.) bear a “signal recognition particle (SRP)” at their N-terminals. The signal peptide (also known as signal peptide, localization signals, leader peptide, etc.) mediate the sequential modifications and movement of newly synthesized from cytoplasm, to endoplasmic reticulum (ER) to Golgi bodies (GB) and to the final destination by being packed in transport vesicles released from GB. Case I: Non-functional SRP inhibits co-translational translocation of the nascent peptide into ER lumen and its subsequent modifications. Such unprocessed proteins generally mis-fold automatically in the cytoplasm. The misfolded protein is subsequently degraded and recycled by the cell. Case II: Mutati.

1. Ans. 1. Most of the proteins meant to be translocated from cytoplasm (site of translation) to
specific subcellular locations (plasma membrane, nucleus, mitochondria, etc.) bear a “signal
recognition particle (SRP)” at their N-terminals. The signal peptide (also known as signal
peptide, localization signals, leader peptide, etc.) mediate the sequential modifications and
movement of newly synthesized from cytoplasm, to endoplasmic reticulum (ER) to Golgi bodies
(GB) and to the final destination by being packed in transport vesicles released from GB.
Case I: Non-functional SRP inhibits co-translational translocation of the nascent peptide into ER
lumen and its subsequent modifications. Such unprocessed proteins generally mis-fold
automatically in the cytoplasm. The misfolded protein is subsequently degraded and recycled by
the cell.
Case II: Mutation in SRP: The fate of the peptide depends on the extent and type of mutation in
the SRP gene. If the mutation causes no significant alteration of co-translational translocation of
the nascent peptide into ER lumen, the protein shall undergo normal processing and finally
transported to its specified location. If mutation makes the SRP non-functional, the protein is
degraded and recycled in the cytoplasm (see, case I).
Ans. 2. Movement of protons from intermembrane space into mitochondrial matrix without
involving ATP synthase produced no ATP. Such transport of protons into the matrix is called
“proton leak” which may be mediated by uncouplers proteins or simply leaks across the
membrane. The proton leaks do not form ATP and causes loss of energy as heat. A small fraction
of protons naturally flow back into the matrix through the lipid bilayer because of relatively very
large electrochemical gradient across it.
Presence of perforation (say, large number of uncouplers) in the inner mitochondrial membrane
reduces the strength of electrochemical gradient. Thus, relatively very small amount of ATP is
produced during cellular respiration of one unit of energy molecule. Thus, limitation in ATP
production rate also limits the ATP dependent movement of muscles.
Solution
Ans. 1. Most of the proteins meant to be translocated from cytoplasm (site of translation) to
specific subcellular locations (plasma membrane, nucleus, mitochondria, etc.) bear a “signal
recognition particle (SRP)” at their N-terminals. The signal peptide (also known as signal
peptide, localization signals, leader peptide, etc.) mediate the sequential modifications and
movement of newly synthesized from cytoplasm, to endoplasmic reticulum (ER) to Golgi bodies
(GB) and to the final destination by being packed in transport vesicles released from GB.
Case I: Non-functional SRP inhibits co-translational translocation of the nascent peptide into ER

2. lumen and its subsequent modifications. Such unprocessed proteins generally mis-fold
automatically in the cytoplasm. The misfolded protein is subsequently degraded and recycled by
the cell.
Case II: Mutation in SRP: The fate of the peptide depends on the extent and type of mutation in
the SRP gene. If the mutation causes no significant alteration of co-translational translocation of
the nascent peptide into ER lumen, the protein shall undergo normal processing and finally
transported to its specified location. If mutation makes the SRP non-functional, the protein is
degraded and recycled in the cytoplasm (see, case I).
Ans. 2. Movement of protons from intermembrane space into mitochondrial matrix without
involving ATP synthase produced no ATP. Such transport of protons into the matrix is called
“proton leak” which may be mediated by uncouplers proteins or simply leaks across the
membrane. The proton leaks do not form ATP and causes loss of energy as heat. A small fraction
of protons naturally flow back into the matrix through the lipid bilayer because of relatively very
large electrochemical gradient across it.
Presence of perforation (say, large number of uncouplers) in the inner mitochondrial membrane
reduces the strength of electrochemical gradient. Thus, relatively very small amount of ATP is
produced during cellular respiration of one unit of energy molecule. Thus, limitation in ATP
production rate also limits the ATP dependent movement of muscles.