Cracking the AP Biology Exam
The Free-Response Questions
Let’s read an essay written by Joe Bloggs, an average student, which was given 4 points. (All of Joe’s grammar and spelling mistakes were retained.)
JOE BLOGGS’S ESSAY
Angiosperms are autotrophs which make their own food. They have chloroplasts within the leaves which captures sunlight. Angiosperms also get nutrients through their roots which absorb water and bring it up the leaf. Food, water and minerals are then transport in the plant by xylem and phloem. Vertebrates can not make their own food so they must get their nutrients from other organisms. Some vertebrates only eat plants while other vertebrates eat both plants and animals.
Two structures that angiosperms use to obtain nutrients from the environment are leaves and roots. Plants have leaves which are shaped to absorb the maximum amount of sunlight. Plants also have root hairs which increase the surface area so that more water and minerals can be taken up the leaf.
An example of a symbiotic relationship is the bacteria found in the root nodules of plants. Plants need nitrogen to make plant proteins but they can’t use the nitrogen from the atmosphere. Plants therefore work with nitrogen-fixing bacteria which convert nitrogen to nitrates. Both organisms benefit from this relationship.
Joe Bloggs would probably get a 4 for this essay. For part a, he clearly defined the term autotroph and received 1 point. However, he did not list the starting materials (CO2 + H2O) to make food in plants. He also failed to define heterotrophs. Consequently, he didn’t receive any points for his explanation of heterotrophs. Notice that for this paragraph, he wasn’t penalized for any grammar mistakes.
For part b, Joe received 2 points for discussing root hairs and their function. Unfortunately, his explanation of leaves was not detailed enough, so he did not receive any additional points.
For part c, Joe received 1 point for his example of a symbiotic relationship—nitrogen-fixing bacteria and plants. However, he did not specifically describe the benefits to each organism. Although he mentioned the benefits to plants, he neglected to discuss the benefits to bacteria. Had he completed his thought, he would have earned another point. Overall, although his essay was a little choppy, Joe was not penalized for it.
Now let’s read an essay written by Josephine Bloggs that was given 9 points.
JOSEPHINE BLOGGS’S ESSAY
Organisms have developed a variety of ways to obtain nutrients. Angiosperms, which are flowering plants, are autotrophs. This means that they are capable of making their own food via photosynthesis. The reaction for photosynthesis is:
Plants use carbon dioxide and water to produce glucose and ATP. Angiosperms also use their roots to absorb water and essential minerals from the soil.
Vertebrates, however, are heterotrophs. They are unable to make their own food. They must therefore rely on other organisms to obtain energy. They require organic compounds, such as proteins and carbohydrates, and break them down to simpler molecules that can be absorbed into the body. Vertebrates also require other essential substances such as vitamins, which serve as coenzymes in crucial biological reactions.
Two structural adaptations used by angiosperms to obtain nutrients are root hairs and stomates. Plants contain numerous root hairs which are tiny projections from a plant’s root that increase the surface area for the uptake of water and minerals. Plants also have stomates which regulate the intake of CO2. The carbon dioxide that enters the stomates can then be used to make glucose.
There are many examples of symbiotic relationships. A common example is nitrogen fixation. Certain plants, called legumes, require nitrogen to synthesize plant amino acids. These plants have nitrogen-fixing bacteria which live in their root nodules and convert nitrogen to nitrates. The plant uses the nitrates to make plant proteins. The bacteria, in turn, gets energy from the carbohydrates of the plant.
Another example of a symbiotic relationship is bacteria in the stomach of cows. Cows are herbivores that can not digest cellulose. They rely on bacteria in their stomach to digest some of the cellulose found in their stomach. Cows, therefore, get some of their energy from fermenting bacteria. A third example of symbiosis is the bacteria found in vertebrates, such as humans. Humans have bacteria that reside in their large intestine. These microorganisms break down undigested food particles and produce Vitamin K. The bacteria in turn have a place to live.
This essay received 9 points. Josephine clearly demonstrated a mastery of the material. In part a, she defined and discussed the terms autotroph and heterotroph. She also mentioned the important raw materials that are involved in each case. Notice that she presented and explained the equation for photosynthesis. She received 4 points—the maximum—for this section.
In part b, Josephine presented two structures and their function. Her coverage of the material was fairly thorough. She talked about two important structures: root hairs and stomates. She received 2 points, one for mentioning each structure, and 1 point for her discussion of root hairs. She didn’t mention the structure of stomates—that they’re pores in the epidermis—so she did not receive a point for the function of stomates.
For part c, Josephine presented three types of symbiotic relationships. The reader reviewed only the first two examples to assign points. Josephine received 2 points for her discussion of nitrogen fixation but only 1 point for her explanation of bacteria in the cow’s stomach. In this case, she forgot to mention the benefits to the bacteria.