Cracking the AP Biology Exam


Animal Structure and Function


All cells need oxygen for aerobic respiration. For simple organisms, such as Platyhelminthes, no special structures are needed because the gases can easily diffuse across every cell membrane. In other multicellular organisms, however, the cells are not in direct contact with the environment. These organisms must find other ways of getting oxygen into their systems. For some animals, such as segmented worms, gas exchange occurs directly through their skin. Others, such as insects, have special tubes called tracheae. Air enters these tubes through tiny openings called spiracles. Among vertebrates, the respiratory structures you should be familiar with are lungs and gills (used by many aquatic creatures). Fish use counter current-exchange in order to transfer oxygen from the water to their blood.


Let’s talk about how air gets into the body. Air enters through the nose or mouth:

The nose cleans, warms, and moistens the incoming air and passes it through the pharynx (throat) and larynx (voice box). Next, air enters the trachea. A special flap called the epiglottis covers the trachea when you swallow, preventing food from going down the wrong pipe. The trachea also has cartilage rings to help keep the air passage open as air rushes in.

The trachea then branches into two bronchi: the left bronchus and the right bronchus. These two tubes service the lungs. In the lungs, the passageways break down into smaller tubes known as bronchioles. Each bronchiole ends in a tiny air sac known as an alveolus. These sacs enable the lungs to have an enormous surface area: about 100 square meters. Let’s take a look at one of these tiny air sacs.

You’ll notice that alongside the alveolus is a capillary. Oxygen and carbon dioxide diffuse across the membrane of both the alveolus and capillary. Every time you inhale, you send oxygen to the alveoli. Oxygen then diffuses into the capillaries. The capillaries, on the other hand, have a high concentration of carbon dioxide. Carbon dioxide then diffuses into the alveoli. When you exhale, you expel the carbon dioxide that diffused into your lungs. Gas exchange occurs via passive diffusion.

Transport of Oxygen

Oxygen is transported throughout the body by the iron-containing protein hemoglobin in red blood cells. Hemoglobin transports 97 percent of the oxygen while the other 3 percent is dissolved in the plasma (the fluid of the blood). The percent O2 saturation of hemoglobin is highest where the concentration of oxygen is greatest. Oxygen binds to hemoglobin in oxygen-rich blood leaving the lungs and dissociates from hemoglobin in oxygen-poor tissues.

Transport of Carbon Dioxide

We’ve just mentioned that carbon dioxide can leave the capillaries and enter the lungs. However, carbon dioxide can travel in many forms. Most of the carbon dioxide enters red blood cells and combines with water to eventually form bicarbonate ions (HCO3). Here’s a summary of the reaction:

Sometimes carbon dioxide combines with the amino group in hemoglobin and mixes with plasma, or is transported to the lungs.

Mechanics of Breathing

What happens to your body when you take a deep breath? Your diaphragm and intercostal muscles contract and your rib cage expands. This action increases the volume of the lungs, allowing air to rush in. This process of taking in oxygen is called inspiration. When you breathe out and let carbon dioxide out of your lungs, that’s called expiration. Your respiratory rate is controlled by chemoreceptors. As your blood pH decreases, chemoreceptors send nerve impulses to the diaphragm and intercostal muscles to increase your respiratory rate.


Directions: Each of the questions or incomplete statements below is followed by five suggested answers or completions. Select the answer that is best in each case. Answers can be found here.

1. All of the following cause oxygen and carbon dioxide to undergo exchange between the lungs and capillaries EXCEPT

(A) differences in partial pressure gradients in the alveolus wall and wall of surrounding capillaries

(B) the spongy, moist epithelium of the lungs

(C) the capillaries associated with the respiratory surfaces of the lungs.

(D) the thin epithelia of the alveoli

(E) the higher PO2 of blood entering the lungs

2. Underwater swimmers hyperventilate before going under water and take deep rapid breathes in order to do all of the following EXCEPT

(A) increase the oxygen content of the blood

(B) decrease the urge to breathe

(C) decrease the pH of the blood

(D) blow off carbon dioxide

(E) increase the amount of carbon dioxide of alveoli air

Directions: Each group of questions consists of five lettered headings followed by a list of numbered phrases or sentences. For each numbered phrase or sentence, select the one heading that is the most closely related to it and fill in the corresponding oval on the answer sheet. Each heading may be used once, more than once, or not at all in each group.

Questions 3–6 refer to the following:

(A) Diaphragm

(B) Rib cage

(C) Alveoli

(D) Bronchioles

(E) Trachea

3. Skeletal muscle that forms the bottom wall of chest cavity

4. Dead-end sacs that allow for gas exchange

5. Tiny air ducts covered by cilia and mucus

6. Surrounded by C-shaped rings of cartilage