Chapter 15, Problem 9Q

A.

Summary Introduction

To examine: Whether the statement “Ribosomes are cytoplasmic structures that, during protein synthesis, become linked by an mRNA molecule to form polyribosomes” is correct or incorrect.

Introduction: The ribosome is a non-membranous structure found in the cytoplasm of the cell. It is the core organelle for the protein synthesis. It can be found free or attached with rough endoplasmic reticulum.

Answer to Problem 9Q

Correct answer: The given statement is correct.

Explanation of Solution

Ribosomes are cell organelles present in all living cells, which serve as the site of protein synthesis. They are found in both prokaryotic and eukaryotic cells and are attached to the membranes of ER in the eukaryotic cells. Ribosomes are linked with the mRNA strand and translate them to proteins. Groups of ribosomes attached with a single mRNA strand that synthesize proteins are called polyribosomes.

Hence, the given statement is correct.

B.

Summary Introduction

To examine: Whether the statement “The amino acid sequence Leu-His-Arg-Leu-Asp-Ala-Gln-Ser-Lys-Leu-Ser-Ser is a signal sequence that directs proteins to the ER” is correct or incorrect.

Introduction: The primary structure of proteins is the linear sequence of amino acids. It determines the shape and the functional ability of the protein. A small change in the amino acid sequence can alter a protein’s shape and its functions.

Answer to Problem 9Q

Correct answer: The given statement is incorrect.

Explanation of Solution

Amino acids are distinguished into hydrophobic and hydrophilic acids, based on their side chains. The amino acids that direct the proteins to the ER are hydrophobic amino acids which contain 8 or more amino acids. However, the sequence given here contains hydrophilic amino acids like histamine, arginine, aspartate, and lysine. Alanine, glycine, valine, leucine are examples of hydrophobic amino acids that can be used as a signal that directs proteins to the ER.

Hence, the given statement is incorrect.

C.

Summary Introduction

To examine: Whether the statement “All transport vesicles in the cell must have a v-SNARE protein in their membrane” is correct or incorrect.

Introduction: SNARE proteins are large proteins which mediate in the fusion of the vesicles with their target membrane-bound compartments. SNAREs on the vesicle are termed v-SNAREs and the SNAREs on the target membrane are termed t-SNAREs.

Answer to Problem 9Q

Correct answer: The given statement is correct.

Explanation of Solution

The transport vesicles must recognize its target and dock with its specific cell organelle. The vesicles cargo is recognized by tethering proteins. In addition to that, SNAREs also recognize it. V-SNARE is the SNARES on the vesicles and t-SNARE is the complementary SNAREs on the target membrane. v-SNARE interacts with the t-SNARES and docks the vesicles in its place. It is also involved in catalyzing the membrane fusion that is required for the transport vesicles to deliver the cargo.

Hence, the given statement is correct.

D.

Summary Introduction

To explain: Whether the statement “Transport vesicles deliver proteins and lipids to the cell surface” is correct or incorrect.

Introduction: Vesicular transport is needed to shuttle the proteins in and out of an organelle. Coated vesicles are the vesicles that bud from the membrane which have a unique protein coat on their cytosolic surface.

Answer to Problem 9Q

Correct answer: The given statement is correct.

Explanation of Solution

Transport vesicles deliver the proteins and lipids that are part of the vesicle membrane. The transport of a lipid or protein from the ER to the Golgi network and from there to other compartments is carried out by the transport vesicle. By continual budding and fusion, the vesicular transport provides routes of communication between the cell and its surroundings.

Hence, the given statement is correct.

E.

Summary Introduction

To examine: Whether the statement “If the delivery of prospective lysosomal proteins from the trans Golgi network to the late endosomes were blocked, lysososmal proteins would be secreted by the constitutive secretion pathways” is correct or incorrect.

Introduction: The lysosomes are the double membrane-bound organelle of the endomembrane system, which helps in the digestion of the cellular wastes. The lysosomes contain the hydrolytic enzymes, which are able to break the different kinds of biomolecules.

Answer to Problem 9Q

Correct answer: The given statement is correct.

Explanation of Solution

Lysosomes are the membrane-bound cell organelles which are present in the cytoplasm of the eukaryotic cells. The transport vesicles carry them from the trans Golgi network to the endosomes and if the process is interrupted, then the transport vesicles will transfer them to other cell surfaces by the constitutive secretion pathways.

Hence, the given statement is correct.

F.

Summary Introduction

To examine: Whether the statement “Lysosomes digest only substances that have been taken up by cells by endocytosis” is correct or incorrect.

Introduction: The movements of ions and molecules in and across the cell membrane are regulated by a set of transport mechanisms that are collectively termed as membrane transport mechanisms. Exocytosis and endocytosis are the two processes by which larger molecules are moved in and out of the cell.

Answer to Problem 9Q

Correct answer: The given statement is incorrect.

Explanation of Solution

Lysosomes digest the intracellular components such as the worn-out organelles and this mechanism is called autophagy. They can also fuse with the phagosomes to break down the phagocytised material.

Hence, the given statement is incorrect.

G.

Summary Introduction

To examine: Whether the statement “N-linked sugar chains are found on glycoproteins that face the cell surface, as well as on glycoproteins that face the lumen of the ER, trans Golgi network, and mitochondria” is correct or incorrect.

Introduction: The endomembrane system is a complex system that distributes cells into different compartment. The transport vesicle exchanges materials between different cellular compartments. For example, transport vesicle encloses the membrane proteins and other substances and carries to ER, Golgi apparatus, lysosomes, and other organelles.

Answer to Problem 9Q

Correct answer: The given statement is incorrect.

Explanation of Solution

Proteins are carried by the transport vesicles from the ER to the Golgi network and from the Golgi network to other cell compartments by the vesicular transport mechanism. Mitochondria, the power house of the cell, are not involved in the vesicular transport mechanism. Therefore, the N-linked glycoproteins which are exclusively gathered in the lumen of the ER are not transported to the mitochondria.

Hence, the given statement is incorrect.

H.

Summary Introduction

To examine: Whether the statement “Ribosomes bound to the outer nuclear membrane make proteins that are translocated co-translationally into the membrane” is correct or incorrect.

Introduction: The ribosomes are associated with the synthesis of proteins. The ribosomes are the small particles, which are comprised of proteins and RNA (ribonucleic acid). The ribosomes, which are attached to the endoplasmic reticulum carry out the process of translation and synthesize proteins.

Answer to Problem 9Q

Correct answer: The given statement is incorrect.

Explanation of Solution

The network of membrane-enclosed tubules and sacs extends from the nuclear membrane throughout the cytoplasm is called endoplasmic reticulum (ER). The outer nuclear membrane is continuous with the ER and all the proteins that are made up of ribosomes bound there end up in the ER lumen.

Hence, the given statement is incorrect.