sptlt210Though it may seem a paradox, life is riddled with barriers. This is because it is sometimes necessary to create dead ends to keep things at a healthy distance. Obstructions of this kind exist at all levels of living matter. Specialized pores are found in membranes surrounding cells but also within cells, to ensure that only specific molecules are able to cross while the transit of others is barred. Aquaporin and sodium channels are two examples through which only water molecules or sodium ions fit, respectively. Another vital barrier is the one that keeps spermatozoa that belongs to one species from fertilizing eggs that belong to another - which would only bring about chaos. Though the mingling of germinal fluids and how life ensues have been discussed since the days of Aristotle, on the molecular level very little is known, still, on how species keep to themselves. A recent find in zebrafish has lifted a veil: scientists discovered a protein on the membranes of zebrafish eggs, which only allows access to zebrafish sperm. They called it Bouncer. Read more