WBR0847
| Author | [[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Serge Korjian)]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Biochemistry |
| Sub Category | SubCategory::General Principles |
| Prompt | [[Prompt::A researcher is studying the molecular mechanisms of cellular transport. He purifies and salt-washes organelles from squid axoplasm and observes their movement on purified microtubules using video-enhanced microscopy. Initially, the organelles exhibit a nearly undetectable level of movement despite an ATP rich microenvironment. However, addition of a high speed axoplasmic supernatant increases the movement frequency and speed more than 20-fold. The researcher also notices distinct movement of organelles from the negative pole to the positive pole of the microtubules. Which compound in the squid axoplasmic supernatant is responsible for the movement of vesicles?]] |
| Answer A | AnswerA::Vimentin |
| Answer A Explanation | AnswerAExp::Vimentin is a type III intermediate filament found in mesenchymal cells. |
| Answer B | AnswerB::Desmin |
| Answer B Explanation | AnswerBExp::Desmin is a type III intermediate filament found in muscle cells. |
| Answer C | AnswerC::Dynein |
| Answer C Explanation | AnswerCExp::Dynein is a protein that transports organelles from the positive pole to the negative pole of the microtubule. |
| Answer D | AnswerD::Kinesin |
| Answer D Explanation | AnswerDExp::Kinesin is a protein that transports organelles from the negative pole to the positive pole of the microtubule. |
| Answer E | AnswerE::Plasmin |
| Answer E Explanation | AnswerEExp::Plasmin is a protein that degrades fibrin mesh. It plays a role in the anticoagulation pathway. |
| Right Answer | RightAnswer::D |
| Explanation | [[Explanation::Transport of membrane-bound organelles (MBO) along the microtubules requires special proteins that can move organelles, like vesicles, from the positive pole of the microtubule to its negative pole and vice versa. Early experiments that studied the movement of vesicles showed that organelle transport requires not only ATP as a source of energy but also transport proteins, such as kinesin and dynein. Kinesin is capable of transporting organelles from the negative to positive pole of the microtubule; whereas dynein is capable of transporting organelles from the positive to negative pole. As organelles were initially purified and salt-washed, the transport proteins were lost, and despite an ATP rich microenvironment, no movement occurred. Addition of axoplasmic supernatant that contained the transport proteins led to eventual organelle transport along the microtubule. Educational Objective: Kinesin helps transport organelles from the (-) to (+) pole of microtubules. |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Kinesin, WBRKeyword::Dynein, WBRKeyword::Transport, WBRKeyword::Protein, WBRKeyword::Proteins, WBRKeyword::Organelles, WBRKeyword::Microtubule, WBRKeyword::Microtubules |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |