By 6:45 Wednesday morning, University of Arizona student Katharine Gullotta was standing on a downtown sidewalk, Time magazine in hand, as one of about 75 people in line to sell their plasma.

"This is a really long line for this time of the morning," said Gullotta, who has been selling twice a week since June. "It shows the desperation."

The sellers Wednesday at ZLB Plasma, included parents, people working full time but struggling with gas prices and bills, and some who were recently laid off.

Gullotta, who does not have health insurance, began going to ZLB after an unexpected kidney infection left her with some medical bills. The 20-year-old biology and psychology major is putting herself through school and holds two jobs, one on campus and another as a bartender.

But food costs and tuition have gone up, and Gullotta said she needs extra money. She was raised by a single mother who died when she was 12, and she doesn't have any "magical family trust fund," though she often fantasizes about it, she said.

She now earns between $200 and $300 a month by selling plasma twice a week at ZLB.

Many of the Tucson residents in line to sell this week reported longer lines to make it into the center, which opens at 7 a.m.

Longtime neighborhood resident and local business owner Dot Kret said she, too, has noticed a huge difference in lineups. She walks by every morning between 6 and 7 a.m., and about two weeks ago counted more than 100 people in line.

"The long lines have only been in the last several months. I used to only see one or two people standing outside," she said. "I'd say it's the economy."

Some sellers say they feel like they are doing something positive — supplying plasma that will be used in products used to treat patients suffering from illnesses such as hemophilia and immune deficiencies.

But for many, it's a way to help make ends meet.

"This is my first time," a sleepy Mike O'Connor said Wednesday at 6:30 a.m. as he leaned against ZLB's outside wall. "I'll use the $40 to fill my tank with gas. I've been low on work."

O'Connor, 28, works as a house painter. A friend had recommended ZLB, where first- and second-time sellers receive $40. Payments after that range between $25 and $35 per visit, depending on body weight, and no one is allowed to give plasma more than twice a week.

Plasma donors must weigh at least 110 pounds and be in good health. It typically takes about 50 minutes to give plasma in a process that's similar to donating blood. The process is longer for first-time sellers who must fill out a medical questionnaire.

"I just didn't think there would be this many people here," O'Connor said.

Gas prices are what motivated 29-year-old Patrick Scott to start selling his plasma about a year ago. Scott has an Internet sales business — he sells items for people on eBay — but says the extra income helps. He goes to ZLB twice a week and said he earns close to $300 a month.

"It's only painful if they've got to re-stick you," he said.

Tucson's center is one of 60 ZLB has nationwide, said Christine Kuhinka, corporate communications manager for the Florida-based company.

She did not have statistics for the local center. Industrywide there's been an increase in what she refers to as plasma donations: There were 15 million donations in 2007, compared with 12 million in 2006.

But she said she can't necessarily link the growth in donations to an economic slump.

"We see it more as supply and demand. There is more demand for plasma products, and worldwide there have been new centers opening and expanding," Kuhinka said.

"I can't comment on individual donor decisions," she added. "It's understandable that people would want to help others and have an altruistic motive. We don't deny that people can earn money. It's a wonderful opportunity to not only give back and save lives but to supplement their income."

UA student Gullotta, who wants to one day earn a master's degree in public health, said that given the long line, she'd probably be at the center for about four hours on Wednesday. She anticipated receiving $25.

"You do what you have to do," she said.