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Chapter_12_Solutions - Chapter 12 Intracellular...

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DEFINITIONS 12–1 Transmembrane transport 12–2 Cytoplasm 12–3 Signal patch 12–4 Cytosol 12–5 Gated transport 12–6 Organelle 12–7 Signal sequence TRUE/FALSE 12–8 False. Lipid bilayers by themselves are impermeable to hydrophilic molecules, but biological membranes, which contain proteins in addition to the bilayer, are not. Cellular membranes contain various transport proteins that make them selectively permeable, allowing certain small molecules and particular proteins to cross. It is this selective permeability that establishes the unique chemical identity of each compartment. 12–9 False. The interior of the nucleus and the cytosol communicate through the nuclear pore complexes, which allow free passage of ions and small mol- ecules. The cytoplasm and the nucleus are said to be topologically equiva- lent because the outer and inner nuclear membranes are continuous with one another, so that the flow of material between the nucleus and cytosol occurs without crossing a lipid bilayer. By contrast, the lumen of the ER and the outside of the cell are each separated from the cytosol by a layer of mem- brane and thus are topologically distinct from the cytosol, but they are topo- logically equivalent to each other. 12–10 True. Ribosomes all begin translating mRNAs in the cytosol. The mRNAs for certain proteins encode a signal sequence for the ER membrane. After this sequence has been synthesized, it directs the nascent protein, along with the ribosome and the mRNA, to the ER membrane. Ribosomes translating mRNAs that do not encode such a sequence remain free in the cytosol. 12–11 True. Stretches of amino acids, typically 15–60 residues long, serve as sorting signals for most proteins in the cell. Signal sequences that specify particular cellular destinations—import into ER, import into nucleus, etc.—have char- acteristic features that allow their interaction with appropriate sorting receptors, which guide the proteins to their correct compartment. THE COMPARTMENTALIZATION OF CELLS In This Chapter THE A253 COMPARTMENTALIZATION OF CELLS THE TRANSPORT OF A257 MOLECULES BETWEEN THE NUCLEUS AND THE CYTOSOL THE TRANSPORT OF A265 PROTEINS INTO MITOCHONDRIA AND CHLOROPLASTS PEROXISOMES A270 THE ENDOPLASMIC A273 RETICULUM Chapter 12 A253 12 Intracellular Compartments and Protein Sorting
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A254 Chapter 12: Intracellular Compartments and Protein Sorting THOUGHT PROBLEMS 12–12 In terms of its functional importance to a cell, the plasma membrane is any- thing but minor. It is the boundary that separates the cell from the outside world, it controls selective entry and exit of molecules, and it is the principal site at which intercellular communications are received. Only in terms of its surface area and mass is it a minor component, accounting for 2–5% of all the membranes in a cell.
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