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Spines are common morphological features found in almost all major biological groups offering an opportunity to explore large-scale evolutionary convergence across disparate clades. As an example, opuntioid cacti have spines with barbed ornamentation that is remarkably similar in form and scale to that found on porcupine quills, suggesting specific biomechanical convergence across the animal and plant kingdoms. While the mechanics of porcupine quills as defensive mechanisms has been previously tested, the mechanics of cactus spines (which have evolved to fill a number of functions including defence, climbing and dispersal) has not been characterized. Here we study the puncturing and anchoring ability of six species of cactus, including both barbed and non-barbed spines. We found that barbed spines require less work to puncture a variety of targets than non-barbed spines. Barbed spines also require more work than non-barbed spines to withdraw from biological materials, owing to their barbs engaging with tissue fibres. These results closely match those found previously for barbed versus non-barbed porcupine quills, implying biomechanical convergence. The variation in performance of barbed versus non-barbed spines, as well as between barbed spines from different species, is probably tied to the diversity of ecological functions of cactus spines.

The Q-cycle mechanism of the bc1 complex explains how the electron transfer from ubihydroquinone (quinol, QH2) to cytochrome (cyt) c (or c2 in bacteria) is coupled to the pumping of protons across the membrane. The efficiency of proton pumping depends on the effectiveness of the bifurcated reaction at the Q(o)-site of the complex. This directs the two electrons from QH2 down two different pathways, one to the high potential chain for delivery to an electron acceptor, and the other across the membrane through a chain containing heme bL and bH to the Qi-site, to provide the vectorial charge transfer contributing to the proton gradient. In this review, we discuss problems associated with the turnover of the bc1 complex that center around rates calculated for the normal forward and reverse reactions, and for bypass (or short-circuit) reactions. Based on rate constants given by distances between redox centers in known structures, these appeared to preclude conventional electron transfer mechanisms involving an intermediate semiquinone (SQ) in the Q(o)-site reaction. However, previous research has strongly suggested that SQ is the reductant for O2 in generation of superoxide at the Q(o)-site, introducing an apparent paradox. A simple gating mechanism, in which an intermediate SQ mobile in the volume of the Q(o)-site is a necessary component, can readily account for the observed data through a coulombic interaction that prevents SQ anion from close approach to heme bL when the latter is reduced. This allows rapid and reversible QH2 oxidation, but prevents rapid bypass reactions. The mechanism is quite natural, and is well supported by experiments in which the role of a key residue, Glu-295, which facilitates proton transfer from the site through a rotational displacement, has been tested by mutation.