Aquaporin says it will be able to turn salt water into drinking water with a customized molecule that mimics the function of similar molecules found inside liver cells.

Yelp publishes restaurant and hotel reviews from unpaid contributors.

That pretty much underscores the differences between the green technology industry and the web. Every week, someone -- usually in a petulant voice -- asks me, "Where is the Google of green?" or "How come there is no Steve Jobs of green?"

My first reaction is, don't jinx it, people. The next thing you know, people will start obsequiously yammering about how their iShinglesolartile has fundamentally changed their outlook on roofing and building materials and life in general.  

Most people boil down the difference between the two industries to the money and capital required to gain entry into each. Green companies seek to replace gasoline with biofuels or batteries, or to produce electricity from solar panels instead of coal-burning power plants. In other words, most green companies need factories, and building factories requires lots of money.

But there are a lot of other, more important reasons, and they are:

1. Free. I use Google's multimillion-dollar data center a few hundred times a day. I've never paid them a cent. A home solar system costs $19,000 after rebates. If Google had to clamber around on your roof for three days and hand you a five-figure bill at the end before you conducted your first search, you'd probably switch to Yahoo.

2.  Safety. Twitter was a relatively unknown service in 2007 when the founders showed it off at the South by Southwest Conference. Two years later, nearly two billion tweets were being posted every quarter. Friends recommended it to friends.

Mark Shannon at the University of Illinois is developing a box that can take sewage streams and turn them into clean, drinkable water, methane, and minerals. With this box, your waste stream becomes not just a weapon in the war against climate change, it becomes a profit center. But are you going to start showering in recycled toilet water? It's like high-powered charging stations for electric cars. Everyone loves the idea, but they also worry about being electrocuted if someone drops a Super Big Gulp of Diet Mountain Dew somewhere in the vicinity of the dashboard.

3. Frivolity. When you boil it down, the Internet is really just a big entertainment network. Veterinarians aren't the ones clicking on cute kitten videos on YouTube. Video games constitute a multibillion dollar industry. People will spend money like thirteen-year-olds handed a stack of no-limit Visa cards to keep themselves amused.

When it comes to necessities, everyone gets adult all of the sudden, doing things like suing their utility. How many more times do you complain about your utility bill versus the cost of movies? You think Bruce Willis has to answer pointed questions in front of the CPUC about the plot inconsistencies in Die Hard 4? If people did what was good for them on an impulse, the racks near the cash register at Target wouldn't be filled with bubble gum tape.

4. Industrial stupidity. When scientists at Xerox showed executives the first personal computers and laser printers, the execs were only slightly intrigued. Microsoft, Compaq, Dell et al. snuck in and became giants.

No one sleeps now. Tesla Motors showed off its first prototype in 2006 and started selling cars in 2008. In response, Nissan, General Motors, Ford and others have cranked up efforts to deliver electrics and plug-ins for the mainstream market. Most of them will have cars out before Tesla's mainstream Model S arrives.

5. Difficulty. Part of the attraction of the web and successful ventures like Craigslist is that people believe they could have come up with it themselves. And they are right. Zappos? The difficult part was believing that someone would buy shoes without trying them on first. By using those services, you're also subconsciously voting for the American Dream. It explains why Tony Danza still pops up on TV: we think to ourselves, "if he can make it there, I can make it anywhere."

By contrast, Nanostellar engineers designer particles for removing particulates from diesel fuel. You can't really incubate that sort of thing with some college drop-outs and a foosball table.

6.  Replacement cycles. A web venture can become an overnight phenomenon through word-of-mouth marketing. And luckily, since people tend to get bored quickly, you'll likely try out the next overhyped web service or property right afterward. New insulating windows, on the other hand? You might get new ones every 30 years. LEDs last more than a decade.

7. Size of potential customer base. This one is huge. Google, Yahoo, Yelp and the rest have a potential customer base of more than 6 billion people. Basically, their potential market is most of the world's population. Not so for many green markets. The number of customers for smart grid technologies in the U.S. is close to 2,100: that's the number of electrical utilities in operation. Algae fuel? You probably won't buy it, at least not directly. Chevron, Valero and the handful of other large oil producers will buy it, mix it into gas, and sell it to you.

8. Age. If you're 28, you're probably already too old to start a social networking company. Because of time and the finicky customer base, a lot of company founders in green don't start until their 40s. Silver Spring Networks was founded by a father and son duo. Hycrete was founded by David Rosenberg, using his grandfather's recipe for waterproofing concrete.

But, again, that's a plus. Saul Griffith, a scientist/entrepreneur living a low-carbon lifestyle, noted recently that 2009 marked the first year that adult diapers outsold child ones, so the talent pool is huge.