Vegan and vegetarian menu items are taking the world of fast food by storm.

On Monday, Impossible Foods announced a $300 million series E funding round led by Temasek and Horizon Ventures. The news comes on the back of Beyond Meat's explosive public debut in early May, with shares soaring 163% on its first day of trading.

Restaurant chains are helping propel the companies' massive growth and fuel plant-based vegan menu items' mainstream acceptance.

Read more: Impossible Foods raises $300 million as the meat-substitute company struggles to meet skyrocketing demand

Impossible Whopper. Burger King

Chains including TGI Fridays, Carl's Jr., and Red Robin serve either the Impossible Burger or the Beyond Burger. Del Taco announced that its vegan Beyond Taco was on track to be one of the chain's best-ever new-product launches. And Burger King announced that it has partnered with Impossible Foods to roll out the Impossible Whopper nationally by the end of 2019.

Read more: Del Taco's vegan Beyond Meat taco is on track to become one of the chain's most successful new menu item launches ever

"There's momentum for very large operators to fulfill this meat-eater demand," David Lee, Impossible Foods' chief financial officer, told Business Insider.

Lee name-checked a Change.org petition titled "McDonald's Big Mistake," calling for the chain to add a plant-based burger to the menu.

"We believe that meat-eaters will reward companies that have a better product," Lee said. "We're betting on the meat eater to be able to discern what's craveable and makes them feel better about their choice."

Chick-fil-A and McDonald's are eyeing meat substitutes

Chick-fil-A is exploring vegan options. Hollis Johnson/Business Insider

Chains that have not yet added a vegan option to the menu are feeling the heat. Amanda Norris, the executive director of Chick-fil-A's menu, told Business Insider last week that the chain is in the early stages of exploring adding vegan options to the menu.

Norris anticipates that every chain is going to have a "point of view" on plant-based menu items. Whether a brand rolls out its own version of an Impossible Whopper or decides against doing so will be a decision that says something specific about the chain.

"We're certainly wanting to broaden our thinking and really start big in that funnel and come down," Norris said. "We think it is certainly beyond just no meat on salads or no meat in a wrap. It might be some kind of alternative protein on a sandwich."

Read more: Chick-fil-A is exploring vegan menu items as chains like Burger King and Chipotle double down on meat substitutes

McDonald's, another holdout, is under even greater pressure. The Change.org petition mentioned by Lee has been signed by more than 200,000 people as of Monday morning.

So far, McDonald's has avoided adding a meatless burger because of concerns regarding added complexity, CEO Steve Easterbrook told investors in April. However, with pressure building, some experts are wondering whether the chain might soon be changing its tune.

"I think it's just a matter of time before McDonald's has something in the US," Mark Kalinowski, president and CEO at Kalinowski Equity Research, recently told Business Insider.

"Now I don't know if that means 10 weeks from now or two decades from now," Kalinowski continued. "But McDonald's isn't dumb, and they have a lot of resources."

McDonald's McVegan. McDonald's Germany/Facebook

McDonald's recently launched the meatless Big Vegan TS in Germany, collaborating with Nestle to make the plant-based burger. The chain is also testing a vegan burger in South Australia, with the company attributing the decision to rising demand for meat-free options.

"Markets decide what's best for their customers and the launch of the Big Vegan TS in Germany is the latest example of that," the company said in an email to Business Insider.

Executives at Beyond Meat and Impossible Foods say that the fast-food industry takeover is far from over.

"I think it's a good bet that those in the business of serving consumer demand are recognizing that the consumer movement is already in place, already well underway," Lee said. "And so I think you'll see more and more great offerings in the market."

"It costs a lot of money to go out and buy a Tesla," Beyond Meat cofounder and CEO Ethan Brown told Business Insider the day of the company's public debut. "It takes $6 to buy the Beyond Burger and make a statement about what you believe in."