Liverpool goalkeeper Jose Reina gave away a penalty and was beaten by Lucho Gonzalez's spot-kick on eight minutes.
Despite their error-strewn first-half performance, Dirk Kuyt pulled Liverpool level with a close-range header.
If Liverpool are to reach their third Champions League final in four years they will have to cut out the mistakes that littered their play in the first half.
On Pennant's dismissal, Kuyt told ITV1 afterwards: "He was unlucky. He tried to do his best and maybe it was a bit too much."
Kuyt added: "We are pleased because we played 30 minutes with 10 men.
"We didn't play well in the first half and were unlucky with the red card.
"The first half we had problems passing. We tried to do it better second half and we worked really hard. With a bit of luck we could have won.
"Porto is a really strong team and at least we didn't lose. If we can beat them at home we will be okay."
Manager Rafael Benitez was less forgiving of Pennant's indiscretion, and when asked if he deserved to go the Spaniard admitted: "I think so. It was hard, but I think so.
"Maybe he will learn from this."
He added: "I am really proud of the players in the second (half). These games we want to win but a point can be good.
"It was a night of suffering all the time - the only positive was the team playing with 10 players.
"The second half was much better."
