The shapes and sizes of closed, pressurized random walks

Two-dimensional cell-like membranes acted on by osmotic pressure differentials are represented by closed, unrestricted random walks. The treatment omits excluded-volume effects, and the pressure that is imposed thus favors an oriented area, so that the shriveled configuration of a vesicle with excess external pressure is inaccessible in this model. Nevertheless, the approach has the decided advantage of yielding analytic expressions in a complete statistical analysis. Results are presented for the average square of the radius of gyration, the asphericity, and the probability distribution of the principal components of the radius of gyration tensor. The analysis is done in both the constant-pressure and constant-area ensembles.     

Kinesin walks hand-over-hand

Kinesin is a processive motor that takes 8.3-nm center-of-mass steps along microtubules for each adenosine triphosphate hydrolyzed. Whether kinesin moves by a "hand-over-hand" or an "inchworm" model has been controversial. We have labeled a single head of the kinesin dimer with a Cy3 fluorophore and localized the position of the dye to within 2 nm before and after a step. We observed that single kinesin heads take steps of 17.3 +/- 3.3 nm. A kinetic analysis of the dwell times between steps shows that the 17-nm steps alternate with 0-nm steps. These results strongly support a hand-over-hand mechanism, and not an inchworm mechanism. In addition, our results suggest that kinesin is bound by both heads to the microtubule while it waits for adenosine triphosphate in between steps.     

How bees remember flower shapes

Bees are able to learn to distinguish between flowers with different shapes or patterns. Some studies have suggested that bees remember only isolated features such as spatial frequency and line angles, rather than the photographic search images that are characteristic of vertebrates. New data indicate that this presumptive vertebrate-invertebrate dichotomy is false; bees can store flower patterns as a low-resolution eidetic image or photograph.     

Cerebellum shapes hippocampal spatial code

Spatial representation is an active process that requires complex multimodal integration from a large interacting network of cortical and subcortical structures. We sought to determine the role of cerebellar protein kinase C (PKC)-dependent plasticity in spatial navigation by recording the activity of hippocampal place cells in transgenic L7PKCI mice with selective disruption of PKC-dependent plasticity at parallel fiber-Purkinje cell synapses. Place cell properties were exclusively impaired when L7PKCI mice had to rely on self-motion cues. The behavioral consequence of such a deficit is evidenced here by selectively impaired navigation capabilities during a path integration task. Together, these results suggest that cerebellar PKC-dependent mechanisms are involved in processing self-motion signals essential to the shaping of hippocampal spatial representation.     

桃不同树形光合特性的研究

【目的】研究不同树形对桃树光合特性的影响。【方法】以6年生桃品种‘甜桃王’为试材,于2007年6~10月份,对开心形和纺锤形2种树形桃树的相关光合特性指标进行了测定,分析了不同树形光合特性的差异。【结果】(1)开心形和纺锤形桃树叶片光合速率的年变化均呈单峰型曲线,且峰值均出现在9月;同一月内,开心形桃树与纺锤形桃树的光合速率无显著差异。(2)开心形和纺锤形桃树的光合速率和光合有效辐射日变化均呈双峰曲线,峰值均出现在11:00和15:00;13:00时开心形桃树的光合速率显著高于纺锤形,其他时间二者间无显著差异;同一时间内纺锤形桃树的光合有效辐射始终显著高于开心形。(3)开心形和纺锤形桃树不同叶幕层的光合速率和光合有效辐射日变化均呈双峰曲线,且在同一时间内,两种树形上层、中层叶幕的光合速率均显著高于下层叶幕。(4)开心形和纺锤形桃树的冠层温度差异不显著,纺锤形树形冠层内的相对湿度均显著高于开心形。【结论】建议在桃树栽培中采用纺锤形树形,其可以在不影响对光能合理利用的条件下提高栽植密度、增加产量和经济效益。     

Myosin V walks by lever action and Brownian motion

Myosin V is a molecular motor that moves cargo along actin filaments. Its two heads, each attached to a long and relatively stiff neck, move alternately forward in a "hand-over-hand" fashion. To observe under a microscope how the necks move, we attached a micrometer-sized rod to one of the necks. The leading neck swings unidirectionally forward, whereas the trailing neck, once lifted, undergoes extensive Brownian rotation in all directions before landing on a site ahead of the leading head. The neck-neck joint is essentially free, and the neck motion supports a mechanism where the active swing of the leading neck biases the random motion of the lifted head to let it eventually land on a forward site.     

Myosin V walks hand-over-hand: single fluorophore imaging with 1.5-nm localization

Myosin V is a dimeric molecular motor that moves processively on actin, with the center of mass moving approximately 37 nanometers for each adenosine triphosphate hydrolyzed. We have labeled myosin V with a single fluorophore at different positions in the light-chain domain and measured the step size with a standard deviation of <1.5 nanometers, with 0.5-second temporal resolution, and observation times of minutes. The step size alternates between 37 + 2x nm and 37 - 2x, where x is the distance along the direction of motion between the dye and the midpoint between the two heads. These results strongly support a hand-over-hand model of motility, not an inchworm model.     

Domain shapes and patterns: the phenomenology of modulated phases

A wide variety of two- and three-dimensional physical-chemical systems display domain patterns in equilibrium. The phenomenology of these patterns, and of the shapes of their constituent domains, is reviewed here from a point of view that interprets these patterns as a manifestation of modulated phases. These phases are stabilized by competing interactions and are characterized by periodic spatial variations of the pertinent order parameter, the corresponding modulation period generally displaying a dependence on temperature and other external fields. This simple picture provides a unifying framework to account for striking and substantial similarities revealed in the prevalent "stripe" and "bubble" morphologies as well as in commonly observed, characteristic domain-shape instabilities. Several areas of particular current interest are discussed.     

Flexibility in algal endosymbioses shapes growth in reef corals

The relation between corals and their algal endosymbionts has been a key to the success of scleractinian (stony) corals as modern reef-builders, but little is known about early stages in the establishment of the symbiosis. Here, we show that initial uptake of zooxanthellae by juvenile corals during natural infection is nonspecific (a potentially adaptive trait); the association is flexible and characterized by a change in (dominant) zooxanthella strains over time; and growth rates of experimentally infected coral holobionts are partly contingent on the zooxanthella strain harbored, with clade C-infected juveniles growing two to three times as fast as those infected with clade D.     

Apple tree phytomass in plantings with different crown shapes

It is established that the studied crown shapes of apple trees grafted on M9 rootstock are small-sized. The total biomass of one tree is 34.3–45.0 kg, the economic phytomass (fruits) is 13.4–20.0 kg, and its variation depends to a greater degree on the biology of the variety and then on the crown shape.     

适用于农田水利建筑物的改良型数字混凝土模型

通过对传统数字混凝土建模方法的研究,提出了改良型数字混凝土建模方法,优化了数字混凝土界面厚度。对新建立的模型进行多工况分析,并将结果与传统模型的数值模拟结果以及试验结果相比较,通过分析得到以下主要结论:①当级配相同时,改变混凝土内部骨料分布会使彼此之间损伤形态产生很大差别,但是损伤单元数量相近;②在超过材料峰值强度之前,混凝土内部骨料位置的变化不会对其力学性能造成显著影响,而在达到失稳阶段时影响较显著;③改良后的模型能够使界面厚度明显降低,计算结果更加贴近试验结果,可以取代传统的数字混凝土模型在工程中的应用。     

增强实木地板物理形状稳定性探讨

实木地板物理形状不稳定的主要表现形式为涨变,裂缝和漆膜龟裂,出现这些现象的主要原因是地板生产工艺问题,安装不当导致二次吸湿等。应从生产工艺和安装施工两方面入手,增强实木地板物理形状稳定性。     

Observation of stable shapes and conformal diffusion in genus 2 vesicles

The observed equilibrium shapes of phospholipid vesicles of topological genus 2 (shapes with two holes) are found to be in agreement with theoretical predictions on the basis of a minimization of the elastic curvature energy for fluid membranes under the constraints of constant area, volume, and area difference (between the inner and outer layers of the membrane). For some particular geometrical characteristics, the shapes of the vesicles change continuously and randomly on a slow time scale (tens of seconds) and thus exhibit conformal diffusion. This phenomenon is a reflection of the conformal degeneracy of the elastic curvature energy. Its observation sets a limit (three constraints) on the number of physical constraints relevant to the determination of the shapes of vesicles.     

Natural selection shapes genome-wide patterns of copy-number polymorphism in Drosophila melanogaster

The role that natural selection plays in governing the locations and early evolution of copy-number mutations remains largely unexplored. We used high-density full-genome tiling arrays to create a fine-scale genomic map of copy-number polymorphisms (CNPs) in Drosophila melanogaster. We inferred a total of 2658 independent CNPs, 56% of which overlap genes. These include CNPs that are likely to be under positive selection, most notably high-frequency duplications encompassing toxin-response genes. The locations and frequencies of CNPs are strongly shaped by purifying selection, with deletions under stronger purifying selection than duplications. Among duplications, those overlapping exons or introns, as well as those falling on the X chromosome, seem to be subject to stronger purifying selection.