Just two days into shopping period, as she walked into the main lecture room in Osborne Memorial Laboratories, Sally Tan ’10 was already noticing a difference between her high-school science classes and those at Yale.

“In genetics, I was surprised people had already bought the textbook and done the reading,” she said. “The pace is moving more quickly.”

For students used to small classes and individual attention from teachers, the college world of large lectures and separate laboratory courses can seem unfamiliar, some said. Pile on top of that the pressures from adjusting to college life in general, and even the best-prepared freshman can feel overwhelmed.

But although some incoming science majors are turned off from the discipline, many stick with it despite sometimes inauspicious beginnings — partly, professors said, because of recent changes in the science curriculum and new sources of support.

Nationwide, advanced high-school courses in all subjects, including the sciences, have become far more prevalent in recent years. According to a 2004 study by researchers at the University of California, Berkeley, that cited statistics from the College Board, the number of students taking Advanced Placement exams increased from 133,702 to 1,017,396 between 1981 and 2004. In addition to AP classes, students now frequently take courses at local colleges or participate in the International Baccalaureate program, an advanced curriculum found internationally.

But accelerated courses may not provide all the preparation they claim to, making the transition to college science that much harder, said William Segraves, associate dean for science education in Yale College.

“In many cases, Yale students weren’t challenged in the same ways by their high school courses, even ostensibly advanced courses,” he said in an e-mail.

Michael Fan ’10, who took AP exams — though not classes — in chemistry, physics and biology, said that even after just a week of classes, he has noticed college science focuses more on understanding than memorization.

“Science at Yale has been a lot harder,” he said. “In high school, teachers just expect you to remember things from the textbook. Especially in ‘Freshman Organic Chemistry,’ you have to understand the concepts behind the facts, the whys and the hows.”

Segraves, who is also a lecturer in the Molecular, Cellular and Developmental Biology Department, said he has personally observed the difficulties some freshmen face in the class “Biology of Reproduction,” which he taught from 1993 to 2005. Advanced courses in many high schools, he said, do not teach students what it means to understand science.

The structure of science courses at Yale may partly explain the focus on concepts. With about 10,000 undergraduate course registrations in the sciences each year, according to data from the Office of Institutional Research, small class sizes at the introductory level would be impractical. Separate sections taught by graduate students often revolve around problem sets, leaving lectures to focus on explaining concepts.

“In college it is more oriented to PowerPoint presentations of facts, whereas in high school it was more practicing the calculations and going over homework problems,” said Valerie Gordon ’09, who took “Freshman Organic Chemistry” last year.

Gordon said that learning the history of modern chemistry, rather than going over problems, made her love science even more than she had in high school. She is now taking biochemistry and neurobiology, and said she plans to major in molecular biophysics and biochemistry.

Besides sections, students can turn to professors’ office hours or to math and science tutors for extra attention in large lecture courses, Segraves said. Even if students are not struggling with material, they can still get more out of courses by meeting with faculty members, teaching fellows or tutors, he said. But he acknowledged it can be difficult for students to take that first step, since some do not know how to start if they do not have a specific question.

In high school, many students did not even need to approach teachers for extra help because it was built into the curriculum.

“In each class there was a lot of discussion and working in groups,” Tan said. “In high school the teachers are maybe more flexible. If students need help with a topic, they can slow down. [In college,] the teachers have the syllabus and stick to it.”

Perhaps the largest change between high school and college science is in laboratories. High-school labs, often just a 45-minute class period in length, are replaced by hours-long laboratories worth only half a credit for work that sometimes equals that demanded by the one-credit lectures.

“I think the biggest difference would be in the labs,” Gordon said. “In high school the lab report just consists of filling in papers they give you. When I got here, I was in lab once a week for four hours at a time, and you had to take meticulous notes and follow procedures and do lengthy write-ups.”

Segraves said the University has actually reduced the length of some labs in recent years, but that this can only be done up to a point without compromising the quality of education provided.

“Real science isn’t something that’s easily sandwiched into an 80-minute time slot,” he said.

Freshmen sometimes enter Yale planning on majoring in the sciences but change their minds after taking a large introductory science course. But Charles Bailyn, Director of Undergraduate Studies for astronomy, said fewer freshmen have been leaving science majors in recent years, a change he ascribed mostly to better placement procedures in chemistry and physics. In recent years, the Chemistry Department has been assigning preliminary placement to every incoming freshman.

“Getting into the right course makes a huge difference,” Bailyn said.

But college science is not limited to science majors. Bailyn, along with Douglas Kankel, director of undergraduate studies for the Molecular, Cellular and Developmental Biology Department, teaches a freshman seminar, “Current Topics in Science,” intended for nonmajors with some interest and talent in science. The seminar, offered for the past four years, was meant to bridge the gap, Bailyn said, between courses for majors and those just looking to fill a distributional requirement.