Cracking the AP Biology Exam


Diversity of Organisms


Eubacteria lack distinct nuclei and other membranous organelles. They possess a cell wall made of peptidoglycan. Eubacteria are extremely diverse, especially in the way they obtain nutrients. Some are chemoautotrophs (nitrifying bacteria), some are photosynthetic autotrophs (Cyanobacteria), but most are heterotrophs. Within the heterotrophs, some are decomposers (breaking down organic material), and others are pathogens (disease-causing parasites). Some eubacteria have a flagellum.

Here’s a simple illustration of your basic prokaryote:

Cell Wall

The composition of bacterial cell walls isn’t that important for the AP exam. However, ETS might want you to know that peptidoglycan enables us to identify certain types of bacteria through “Gram staining.” A bacteria with a thick peptidoglycan cell wall will test Gram-positive, whereas one with a thin peptidoglycan layer will test Gram-negative. Chlamydiae and spirochetes are examples of Gram-positive bacteria; streptococci and staphylococci are Gram-negative.

Bacteria also vary in size and shape. They can be cocci (spherical), bacilli (rod-shaped), and spirilli (corkscrewed). Some bacteria also have flagella.

Oxygen Use

As far as respiration is concerned, most bacteria need oxygen. We call these bacteria obligate aerobes. Yet other bacteria cannot survive in the presence of oxygen. These bacteria are known as obligate anaerobes and live in places like the deep-sea floor, where sulfur-rich vents open up from the earth’s core. Still other bacteria are known as facultative anaerobes, which means that they can survive in the absence as well as in the presence of oxygen.


Bacteria reproduce asexually by binary fission. Binary fission occurs when the bacteria replicate their chromosomes and divide into two identical cells. While bacteria are not sexual, they do exchange genetic material. This exchange of genetic information is known as genetic recombination and happens in one of three ways: transformation, conjugation, or transduction.

1. Transformation occurs when a bacterium picks up naked DNA from the environment.

2. Conjugation occurs when two bacteria form little bridges called pili between one another and one transfers genetic material to another.

3. Transduction occurs with the intervention of a virus. The virus carries some DNA from one bacterium to another during the process of infection.

There are a few special types of bacteria you should know something about. One of them is called nitrogen-fixing bacteria. Plants need nitrogen to survive. However, in many places, the soil is relatively poor in nitrogen. You’ll recall that the atmosphere is very rich in nitrogen (some 78 percent of the air is nitrogen). Fortunately for plants living in nitrogen-poor soil, there are bacteria that are able to utilize or “fix” this atmospheric nitrogen.

The plants that have this special relationship with nitrogen-fixing bacteria are known as legumes. Pea plants and clover plants are examples of legumes. The nitrogen-fixing bacteria set up house in the root nodules of these plants, forming a mutualistic relationship: Both organisms are happy. The plants get their nitrogen, and the bacteria get their shelter. We’ll talk more about different types of symbiotic relationships later on. Nitrogen-fixing bacteria have symbiotic relationships with certain plants.

Now let’s move on to protista.