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<jats:title>Electricity from thermal sources</jats:title> <jats:p> It is desirable to harvest as much energy as possible from processes that produce useful amounts of heat and convert it from waste into electrical power. Thermoelectrics and thermophotovoltaics can harness and convert heat waste but tend to operate at high temperatures. Davids <jats:italic>et al.</jats:italic> designed and fabricated a complementary metal-oxide semiconductor infrared photonic device that can harvest and recover energy from low-temperature thermal sources (see the Perspective by Raman). Using a new conversion mechanism, they experimentally demonstrate large thermal-to-electrical power generation in a bipolar grating-coupled tunneling device, rivaling the best thermoelectric devices. The device design could be used for energy harvesting of waste heat and the development of compact thermal batteries. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6484" page="1341" related-article-type="in-this-issue" vol="367" xlink:href="10.1126/science.aba2089">1341</jats:related-article> ; see also p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6484" page="1301" related-article-type="in-this-issue" vol="367" xlink:href="10.1126/science.aba8976">1301</jats:related-article> </jats:p>

<jats:title>Choosing a partner that fits</jats:title> <jats:p> G protein–coupled receptors (GPCRs) are responsible for transducing diverse signals from outside to inside cells. This process requires specificity both in ligand binding to GPCRs and in coupling between GPCRs and their intracellular partners, G proteins. Qiao <jats:italic>et al.</jats:italic> determined the structure of the human glucagon receptor (GCGR), a type B GPCR, bound to glucagon and one of two heterotrimeric G proteins, G <jats:sub>s</jats:sub> or G <jats:sub>i1</jats:sub> . GCGR signals mainly through G <jats:sub>s</jats:sub> , and the structures provide a basis for this specificity. Conformational changes in GCGR, relative to the inactive state, create a binding cavity for the G proteins. The pocket is opened sufficiently to accommodate a bulky binding motif in G <jats:sub>s</jats:sub> . G <jats:sub>i1</jats:sub> can still bind but the pocket does not close around it, so there is a smaller interaction interface. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6484" page="1346" related-article-type="in-this-issue" vol="367" xlink:href="10.1126/science.aaz5346">1346</jats:related-article> </jats:p>

<jats:title>Mapping perovskite trap states</jats:title> <jats:p> The high efficiency of hybrid inorganic-organic perovskite solar cells is mainly limited by defects that trap the charge carriers and lead to unproductive recombination. Ni <jats:italic>et al.</jats:italic> used drive-level capacitance profiling to map the spatial and energetic distribution of trap states in both polycrystalline and single-crystal perovskite solar cells. The interface trap densities were up to five orders of magnitude higher than the bulk trap densities. Deep traps were mainly located at the interface of perovskites and hole-transport layers, where processing created a high density of nanocrystals. These results should aid efforts aimed at avoiding trap-state formation or passivating such defects. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6484" page="1352" related-article-type="in-this-issue" vol="367" xlink:href="10.1126/science.aba0893">1352</jats:related-article> </jats:p>

<jats:title>Erosion-vegetation interactions</jats:title> <jats:p> The impact of vegetation on erosion rates is hard to gauge. Although vegetation can hold soils in place mechanically, root systems can also loosen soils or even help to fracture rock. These processes can increase erosion, especially because areas of heavy vegetation tend to be in areas with high precipitation rates. Starke <jats:italic>et al.</jats:italic> tackled this issue using a large set of observations that span 3500 km of the Andes mountain range. They found a complex set of interactions where increasing vegetation decreases erosion in more arid regions but can accelerate erosion in vegetation dense regions. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6484" page="1358" related-article-type="in-this-issue" vol="367" xlink:href="10.1126/science.aaz0840">1358</jats:related-article> </jats:p>