I saw this movie on TV in the US in about 1971 or 1972. I'm pretty sure that the movie was older than that, though. My impression now is that it could have been made in the 60s or late 50s, not sure whether it was B&W or colorized. I was only about 5 or 6 years old at the time, and I'm not completely sure.
The was a rocket expedition to another planet. I'm pretty sure it was in our solar system, possibly a 'counter-earth', or a wandering planet. The planet turned out to be a large living organism. They landed and sent a party into it. I remember the inside having a lot of vine- or tentacle-like things hanging from the ceilings. They may have been red and green in color. I remember that it was dangerous inside but I'm not sure what the danger was. As they were fleeing back to the ship, one of the party, I think an older scientist-type, gets injured and volunteers (or is forced) to be left behind so as not to slow down the escape. I remember feeling very sad when I saw that part.