In 1977, the physicist Edward Purcell calculated that if you push a bacteria and then let go, it will stop in about a millionth of a second. In that time, it will have traveled less than the width of a single atom. The same holds true for a sperm and many other microbes. It all has to do with being really small. Microscopic creatures inhabit a world alien to us, where making it through an inch of water is an incredible endeavor. But why does size matter so much for a swimmer? What makes the world of a sperm so fundamentally different from that of a sperm whale? To find out, we need to dive into the physics of fluids. Here's a way to think about it. Imagine you are swimming in a pool. It's you and a whole bunch of water molecules. Water molecules outnumber you a thousand trillion trillion to one. So, pushing past them with your gigantic body is easy, but if you were really small, say you were about the size of a water molecule, all of a sudden, it's like you're swimming in a pool of people. Rather than simply swishing by all the teeny, tiny molecules, now every single water molecule is like another person you have to push past to get anywhere. In 1883, the physicist Osborne Reynolds figured out that there is one simple number that can predict how a fluid will behave. It's called the Reynolds number, and it depends on simple properties like the size of the swimmer, its speed, the density of the fluid, and the stickiness, or the viscosity, of the fluid. What this means is that creatures of very different sizes inhabit vastly different worlds. For example, because of its huge size, a sperm whale inhabits the large Reynolds number world. If it flaps its tail once, it can coast ahead for an incredible distance. Meanwhile, sperm live in a low Reynolds number world. If a sperm were to stop flapping its tail, it wouldn't even coast past a single atom. To imagine what it would feel like to be a sperm, you need to bring yourself down to its Reynolds number. Picture yourself in a tub of molasses with your arms moving about as slow as the minute hand of a clock, and you'd have a pretty good idea of what a sperm is up against. So, how do microbes manage to get anywhere? Well, many don't bother swimming at all. They just let the food drift to them. This is somewhat like a lazy cow that waits for the grass under its mouth to grow back. But many microbes do swim, and this is where those incredible adaptations come in. One trick they can use is to deform the shape of their paddle. By cleverly flexing their paddle to create more drag on the power stroke than on the recovery stroke, single-celled organisms like paramecia manage to inch their way through the crowd of water molecules. But there's an even more ingenious solution arrived at by bacteria and sperm. Instead of wagging their paddles back and forth, they wind them like a cork screw. Just as a cork screw on a wine bottle converts winding motion into forward motion, these tiny creatures spin their helical tails to push themselves forward in a world where water feels as thick as cork. Other strategies are even stranger. Some bacteria take Batman's approach. They use grappling hooks to pull themselves along. They can even use this grappling hook like a sling shot and fling themselves forward. Others use chemical engineering. H. pylori lives only in the slimy, acidic mucus inside our stomachs. It releases a chemical that thins out the surrounding mucus, allowing it to glide through slime. Maybe it's no surprise that these guys are also responsible for stomach ulcers. So, when you look really closely at our bodies and the world around us, you can see all sorts of tiny creatures finding clever ways to get around in a sticky situation. Without these adaptations, bacteria would never find their hosts, and sperms would never make it to their eggs, which means you would never get stomach ulcers, but you would also never be born in the first place. (Pop)
Di tahun 1977, fisikawan Edward Purcell memperhitungkan jika Anda mendorong bakteri dan melepasnya, ia akan berhenti dalam sekitar seperjuta detik. Dalam waktu itu, ia akan berpindah jarak kurang dari lebar sebuah atom. Hal yang sama berlaku untuk sel sperma dan banyak mikroba lainnya. Semua itu ada hubungannya dengan ukuran mereka yang sangat kecil. Makhluk mikroskopis tinggal di dunia yang asing bagi kita, di mana menyelami satu inci air saja adalah pencapaian yang luar biasa. Namun, mengapa ukuran sangat penting bagi perenang? Apa yang membuat dunia sebuah sel sperma sangat jauh berbeda dengan paus sperma? Untuk mengetahuinya, kita harus menyelami dunia fisika fluida. Begini caranya. Bayangkan Anda berenang di kolam. Di sana ada Anda dan sekumpulan molekul air. Anda kalah jumlah dengan molekul air, seribu triliun triliun banding satu. Jadi, mudah saja bagi tubuh raksasa Anda untuk melaju melewati mereka. tetapi jika Anda berukuran sangat kecil, misalnya, sebesar molekul air, tiba-tiba terasa seperti sedang berenang dalam kolam yang ramai. Anda tidak lagi melewati molekul yang sangat kecil, sekarang tiap-tiap molekul air terasa seperti orang yang harus Anda lewati untuk berpindah tempat. Tahun 1883, fisikawan Osborne Reynolds menemukan bahwa ada satu angka sederhana yang dapat memprediksi perilaku fluida. Dinamakan angka Reynolds, dan angka tersebut bergantung pada ciri sederhana seperti ukuran perenang, kecepatan, densitas, dan viskositas cairan. Maksudnya, makhluk dengan berbagai ukuran hidup di dunia yang sangat berbeda. Contohnya, karena ukurannya yang besar, paus sperma hidup di dunia dengan angka Reynolds yang besar. Jika ekornya mengepak sekali, paus dapat berpindah tempat jauh sekali. Sementara sperma hidup di dunia dengan angka Reynolds rendah. Jika sperma berhenti mengepakkan ekornya, satu atom pun tidak akan ia lewati. Untuk membayangkan rasanya menjadi sperma, Anda harus menyamakan angka Reynolds Anda dengan mereka. Bayangkan Anda di bak penuh molase, tangan Anda bergerak lambat selambat jarum menit jam, Anda akan sedikit tahu apa yang dirasakan oleh sperma. Jadi, bagaimana mikroba bisa berpindah tempat? Kebanyakan tidak berenang sama sekali. Membiarkan makanan mendatangi mereka. Seperti sapi pemalas yang menunggu rumput di bawah mulutnya tumbuh kembali. Namun, banyak juga mikroba yang berenang, dan di sinilah adaptasi luar biasa itu dimulai. Satu trik yang mereka lakukan adalah mengubah bentuk ekor mereka. Dengan melenturkan ekor untuk menciptakan tarikan lebih pada langkah daya, single-celled organisms like paramecia dapat berenang melewati sekumpulan molekul air. Namun, ada cara yang lebih baik bagi bakteri dan sel sperma. Alih-alih mengepakkan ekornya terus-menerus, mereka memutarnya seperti kotrek. Seperti kotrek pada botol anggur yang mengubah gerak ulir menjadi gerak maju, makhluk mungil ini memutar ekor heliks mereka untuk bergerak maju di dunia di mana air terasa setebal sumbat botol. Cara lainnya lebih aneh. Beberapa bakteri memakai cara Batman. Mereka mamakai kait untuk menarik diri mereka sendiri. Mereka bahkan dapat memakai kait ini seperti ketapel dan meluncur maju. Lainnya menggunakan cara kimia. H. pylori hidup hanya di mukus berlendir dan asam di dalam perut kita. Ia mengeluarkan zat yang mengikis mukus di sekelilingnya, agar ia bisa bergerak. Tidak mengagetkan jika mereka juga bertanggung jawab atas sakit lambung. Jadi, saat Anda melihat tubuh dan dunia sekitar lebih dekat, Anda bisa melihat semua makhluk mungil mencari cara cerdas untuk lolos dari situasi sulit. Tanpa adaptasi semacam ini, bakteri tidak akan menemukan inangnya, dan sel sperma tidak akan bisa sampai ke telurnya, yang artinya perut Anda tidak akan pernah sakit, tapi Anda juga tidak akan lahir. (Tuing)