Regulation

Introduction

In the course description for AP^® Biology, the College Board lists regulation as one of the eight major themes for an integrated approach to advanced biology concepts. Use this set of three demonstrations to engage students in hands-on activities and review their understanding of this fundamental theme.

The set of three demonstrations includes:

1. Diffusion—Begin with the “textbook” dialysis tubing experiment but add a twist. In this demonstration, acids and bases cause an indicator to change color as they diffuse through the dialysis tubing.
2. Buffers—Continue the application of regulation with a study of buffers. In this demonstration, students observe the “reaction” of a buffer to acids and bases.
3. Organism Regulation—Use this unique set of cards to identify the different ways organisms regulate their bodies and the environment around them.

The series of demonstrations may be presented in a variety of ways. Each demonstration may be used to review a specific AP test topic, or all the demonstrations may be performed together to evaluate students’ ability to apply or relate concepts from different topics within the broader context of regulation. A student worksheet for each demonstration with comprehensive essay-type questions is included as an optional assessment tool for the instructor. 

Experiment Overview

Diffusion
The movement of water molecules and various solutes into and out of cells is vital for life processes. In this demonstration, challenge student understanding of this fundamental topic.

Buffers
Buffers provide an essential acid–base balancing act in consumer products, foods, lakes and streams, and even living cells. What are buffers made of and how do they work? This demonstration explores the properties of buffers.

Organism Regulation
The five kingdoms are filled with very different organisms with different needs and abilities. In this activity, flashcards are used to quiz students about their knowledge of the types of regulation required for life.

Materials

Safety Precautions

Hydrochloric acid and sodium hydroxide solutions are corrosive liquids. Avoid exposure to eyes and skin. Wear chemical splash goggles, chemical resistant gloves and a chemical-resistant apron. Wash your hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information. 

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All final solutions may be combined and disposed of down the drain with an excess of water according to Flinn Scientific Disposal Method #26b. The final indicator solutions in Buffers may be combined and disposed of down the drain with an excess of water according to Flinn Scientific Disposal Method #26b.

Prelab Preparation

Diffusion

1. Cut a six-inch length of dialysis tubing and place in distilled or deionized water to soften 15–30 minutes prior to beginning the lab.
2. Prepare 0.1 M hydrochloric acid solution by diluting 10 mL of 1 M HCl solution to 100 mL with DI water.
3. Prepare 0.1 M sodium hydroxide solution by diluting 10 mL of 1 M NaOH solution to 100 mL with DI water.
4. Prepare 0.004% bromthymol blue solution by diluting 1 mL of 0.04% bromthymol blue to 10 mL with DI water. Adjust the color of the bromthymol blue solution to green by adding either 0.1 M HCl or 0.1 M NaOH as needed to adjust the pH. If the indicator solution is yellow-green, add one drop of 0.1 M NaOH; if the indicator is blue-green, add one drop of 0.1 m HCl. Do not overshoot the pH range.

Buffers

1. Mix together 100 mL each of 0.2 M sodium phosphate monobasic (NaH₂PO₄) and 0.2 M sodium phosphate dibasic (Na₂HPO₄) solutions to prepare 200 mL of the phosphate buffer needed for the demonstration.

Organism Regulation

1. Select one card and consider the subcellular, cellular, organism and ecosystem regulations needed for that organism to carry on the functions of life.
2. Create one or more question for each card. Sample questions are provided.

Procedure

Diffusion

1. Label one beaker A and the second beaker B.
2. Pour 100 mL of 0.1 M HCl into beaker A and 100 mL of 0.1 M NaOH into beaker B.
3. Obtain the six-inch length of pre-soaked dialysis tubing. Keeping the tubing moist, tightly roll one end and clamp it closed with one of the dialysis tubing clamps, creating an open “bag.”
4. Open the free end of the dialysis bag by rubbing it between a thumb and forefinger. Use a graduated pipet to fill the bag with 10 mL of the 0.004% bromthymol blue indicator solution.
5. Roll and clamp the open end of the tubing closed without trapping a lot of air.
6. Rinse the bag with DI water and blot dry with a paper towel.
7. Discuss the properties of dialysis tubing and have students observe the color of the indicator solution in the bag. Instruct students to record observations on the Diffusion Worksheet. Ask the students to predict what will happen when the bag is placed into an acidic solution.
8. Place the filled dialysis tubing bag into beaker A.
9. Instruct students to closely observe the solutions in the beaker and inside the bag and to record their observations on the Diffusion Worksheet.
10. After about one minute, remove the dialysis bag from the beaker, rinse it well with DI water, and dry it off with paper towels.
11. Ask the students to predict what will happen when the bag is placed into a basic solution.
12. Place the filled dialysis tubing bag into beaker B.
13. Instruct students to closely observe the solutions in the beaker and inside the bag and to record their observations on the Diffusion Worksheet.
14. After about one minute, remove the dialysis bag from the beaker, rinse it with DI water, and dry it off with paper towels.
15. Ask the students to predict what will happen if the bag were placed back into the acidic solution.

Buffers

1. Set up four 150-mL beakers and label them 1–4.
2. Add 100 mL of DI water to each: beaker 1 and beaker 3.
3. Add 100 mL of the phosphate buffer to beaker 2 and beaker 4.
4. Add about 1 mL of bromthymol blue indicator to each beaker 1–4. Allow students to observe the color of the solution in all four beakers. Instruct students to record observations in the data table on the Buffer Worksheet. (The solutions should all be green. This is the “neutral” color of the indicator, corresponding to pH values between 6 and 7.6.)
5. Add 5 drops of 1 M HCl to beaker 1 and stir. Allow students to observe the color of the solution in the beaker and record their observations in the data table on the Buffer Worksheet. (Note the color change to yellow. This is the “acidic” color of the indicator.)
6. Add 5 drops of 1 M HCl to beaker 2 and stir. Allow students to observe the color of the solution in the beaker and record their observations in the data table on the Buffer Worksheet. (No color change—solution stays green.)
7. Add an extra 5 drops of 1 M HCl to beaker 2. Allow students to observe the color of the solution in the beaker. (Still no color change. Look frustrated!)
8. Use a graduated cylinder to add 3 mL of 1 M HCl to beaker 2. Allow students to observe the color of the solution in the beaker. (The frustration mounts as the solution remains green.)
9. Continue adding 1 M HCl in 3-mL increments until the color changes to yellow. Allow students to observe the color of the solution in the beaker and record their observations and the amount of acid necessary to produce this change in the buffer in the data table on the Buffer Worksheet. (This will probably take 2–3 more 3-mL portions of HCl, for a total of 9–12 mL.)
10. Discuss the behavior of the buffer with respect to excess strong acid. What will happen if strong base is added?
11. Add 5 drops of 1 M NaOH to beaker 3 and stir. Allow students to observe the color of the solution in the beaker and record their observations in the data table on the Buffer Worksheet. (Note the color change to blue. This is the “basic” color of the indicator.)
12. Add 5 drops of 1 M NaOH to beaker 4. Allow students to observe the color of the solution in the beaker and record their observations in the data table on the Buffer Worksheet. (No color change—solution stays green.)
13. Add an extra 5 drops of NaOH to beaker 4. Allow students to observe the color of the solution in the beaker. (Still no color change—but you are not surprised!)
14. Add 3 mL of 1 M NaOH to beaker 4. Allow students to observe the color of the solution in the beaker. (The solution remains green.)
15.  Continue adding 1 M NaOH in 3-mL increments until the color changes to blue. Allow students to observe the color of the solution in the beaker and record their observations and the amount of base necessary to produce this change in the buffer in the data table on the Buffer Worksheet. (This will probably take 2–3 more 3-mL portions of NaOH, for a total of 9–12 mL.)

Organism Regulation

1. Model the type of response required to answer an essay question about the organism on the AP^® Biology Exam. See the sample questions.
2. Organize the students into groups.
3. Randomly draw one question and organism card per group. Allow 5–10 minutes for brainstorming.
4. Call on each group to present their ideas. Allow constructive feedback from the remaining students.

Sample Questions

1. Name the method of photosynthesis for the pictured plant. Explain, in detail, the metabolic pathway and the ecosystem limitations created by the method of photosynthesis.
2. Living species are either osmoregulators or osmoconformers. For the organism provided, determine if it is an osmoregulators or an osmoconformer. Discuss the advantages and disadvantages of each type of homeostatic control.
3. Regulation does not just occur at a cellular level. Ecosystems and populations also face regulation. Discuss how an alteration in the number of predators affects both a population of prey and the habitat.
4. Many species can be cultured in a laboratory setting. The artificial environment must include the proper balance of nutrients, water, pH, light and temperature. For the species given, design an experiment to determine the optimal temperature for maximum growth and reproduction. Be sure to include specifics on how to measure the population of the species.
5. For the animal species given, describe how the animal regulates body temperature. Also explain how the animal copes with extremely high and extremely low temperatures.
6. Hormones are present in both plants and animals. Explain how hormones play a role in determining the height (size) of the species given.

Student Worksheet PDF

10975_Student1.pdf

Teacher Tips

• For best results in the Buffers demonstration, use either freshly distilled water or bottled water as the control in beakers 1 and 3. Distilled water, in particular, absorbs large quantities of carbon dioxide from the air during storage. The presence of dissolved CO₂ may make the water acidic enough to turn yellow with bromthymol blue indicator solution (step 4). In areas of the country where the water is not hard, tap water may be a suitable control.
• A video of this demonstration, Buffer Balancing Acts, presented by Irene Cesa, is available for viewing as part of the Flinn Scientific “Teaching Chemistry” eLearning Video Series. Please visit the eLearning website at http://elearning.flinnsci.com for viewing information. The video is part of the Buffers video package.

Sample Data

Diffusion

Buffers

Answers to Questions

Diffusion

1. Explain the observed indicator color change(s) in terms of the possible movement of specific ions in the acid, base and indicator solutions.

   The BTB indicator changes from its bluish-green color to yellow as H⁺ ions diffuse across the dialysis tubing membrane from the area of high concentration to the lower concentration area inside the dialysis tubing bag. The indicator molecules are too large to also diffuse thereby leaving the solution outside of the membrane colorless. When the bag is placed in the basic solution, OH^– ions follow the same process until a dynamic equilibrium is reached.

2. Describe how this demonstration illustrates diffusion.

   Diffusion is the net movement of molecules or ions from a region of high concentration to a region of low concentration. In this demonstration, the dialysis tubing provides a barrier that restricts the movement of large molecules or ions, such as the bromthymol blue indicator. Small H⁺ and OH^– ions, from of acid and base solutions, respectively, move through these microscopic holes to the region of low concentration. The movement causes the solution inside the dialysis tubing to change pH. The indicator changed color as the pH changed.

3. Concentration and temperature are two factors that affect diffusion. Describe two mechanisms or adaptations by which aquatic plants and animals that inhabit estuaries can regulate their exposure to large daily variations in saline concentration of the water.

   Estuary plants have evolved a variety of methods to deal with fluctuating salt concentrations. Many species have developed specialized root structures that filter out excess salt through an active transport mechanism. Other species actively excrete salt through their leaves.
   Estuary animals can either remove themselves from an unsuitable environment by leaving it (migration) or by burrowing into the bottom substrate. Both of these are considered behavioral regulation. Physiological regulation can be subdivided into osmoconforming and osmoregulating. Osmoconformers have internal saline levels that match that of the surrounding water. Osmoregulators have evolved specialized organs or tissues which actively pump salt in or out in order to maintain homeostasis. Many animals may alter how they cope with the variation in salt concentration throughout their lifetime. For example, larva tend to be osmoconformers that live in the “protective” estuary while the adult of the same species is a migratory osmoregulator.

Buffers

1. How much acid was added to beaker 2 compared to beaker 1 to obtain the same color change? Explain the different color changes in beaker 1 versus beaker 2. A lot more acid was needed to change the color of the solution in beaker 2 compared to beaker 1. Beaker 1 contained only distilled water, so when the hydrochloric acid was added the solution immediately became very acidic and turned yellow; the color bromthymol blue is an acid. In beaker 2 however, the presence of the phosphate buffer neutralized the acid until the buffer’s entire conjugate base had been used up. Thus it took longer for the solution to become acidic and for the color to change.
2. How much base was added to beaker 2 compared to beaker 1 to obtain the same color change? Explain the different color changes in beaker 3 versus beaker 4. Beaker 3 contained only distilled water, so when the sodium hydroxide was added the solution immediately became very basic and turned blue; the color of bromthymol blue is in a base. In beaker 4 however, the presence of the phosphate buffer neutralized the base until all of the buffer’s weak acid had been reacted. Thus, it took more base for the solution to change color.
3. Buffers resist changes in pH. How would a protein react to a change in pH? In your answer include information about the functional groups on the amino acids as well as structural changes to the protein. Amino acids are joined together via peptide linkages in proteins. The side chains (usually denoted by the letter R) in the structure of the 20 naturally occurring a.a. may contain acidic (CO₂H) or basic (—NH₂) side chains. Amine groups will accept protons (H⁺ ions), whereas carboxyl groups will donate H⁺ ions. The structure of the functional group plays a large role in how an amino acid, and therefore the entire protein, functions at a particular pH.
   Four amino acids are greatly altered by a change in pH. The pH of the cytosol in a typical human cell is 7.4. At this pH lysine and arginine gain protons from the solutions and gain an overall positive charge. At the same pH, 7.4, aspartic acid and glutamic acid donate protons and attain an overall negative charge. These charges affect the secondary, tertiary, and quaternary structure of any protein that contains any of these four amino acids.
   If the protein becomes contained within a lysosome with a very low pH the structure and function of the protein may be altered due to changes caused by the presence of excess protons. For example, protons will bind to the carboxyl group of aspartic acid and glutamic acid causing the amino acid side chains to become neutral. This change in overall charge in the amino acid then affects any ionic bonds that maintain the protein’s structure thereby inhibiting the protein’s ability to function. The reverse situation occurs when lysine and arginine are within a basic solution.

Organism Regulation

1. Name the method of photosynthesis for the pictured plant. Explain, in detail, the metabolic pathway and the ecosystem limitations created by the method of photosynthesis.
   1. C₃ Photosynthesis

      C₃ plants carry out both the light-dependent and Calvin cycle reactions simultaneously in the mesophyll cells during the day. An enzyme abbreviated rubisco (ribulose bisphosphate carboxylase oxygenase) fixes carbon dioxide coming through the open stomata by reacting it with a five-carbon molecule of ribulose bisphosphate (RuBP). The resulting sixcarbon molecule immediately splits in half to give two molecules of 3-phosphoglycerate. 3-phosphoglycerate is a threecarbon intermediate that undergoes further reduction to glyceraldehyde-3-phosphate (G3P) during the continuation of the Calvin cycle. The overall process is C₅ + C₁ → 2C₃.
      If the outside air is too hot and dry, C₃ plants must close their stomata to decrease dehydration due to transpiration. However, closing the stomata reduces the amount of CO₂ entering the leaf and increases the amount of O₂ build-up within the leaf. Consequently, C₃ plants tend to thrive in areas with moderate temperatures, moderate light, and moderate to high amounts of water.
      Most plants are C₃ plants. See the answers below for examples of C₄ and CAM plants.

   2. C₄ Photosynthesis

      C₄ plants carry out both the light-dependent and Calvin cycle reactions during the day but the processes occur in different areas of the plant. As in C₃ plants, the light-dependent reactions in a C₄ plant still take place in the mesophyll cells but the Calvin cycle occurs in tightly packed bundle-sheath cells which surround the veins of the leaf instead of within the mesophyll cells. Within the mesophyll cells, near the light-dependent reactions, the enzyme PEP carboxylase forms a four-carbon molecule called oxaloacetate by linking CO₂ to PEP (phosphoenolpyruvate). Under more intense light and high-temperature conditions, PEP carboxylase has a higher affinity for CO₂ than rubisco. The molecule with the captured CO₂ travels to the bundle-sheath cell where the CO₂ is released to be incorporated by rubisco and the Calvin cycle.
      C₄ plants thrive in areas with high temperatures, intense light, and moderate to high amounts of water. Because PEP carboxylase has a higher affinity for CO₂, the plant is able to close its stomata more often thereby halting water loss through transpiration.
      C₄ plants include corn, sorghum, crab grass, sugarcane and many annuals.

   3. CAM Photosynthesis

      CAM plants are succulent plants—plants that store water. Like C₃ plants, CAM plants carry out their light-dependent and Calvin cycle reactions simultaneously in the mesophyll cells, but rather than gather the CO₂ molecules as needed CAM plants build up and store CO₂ overnight. During the day the stomata are closed to conserve moisture. At night the stomata open and CO₂ enters the plant where it is captured and stored as intermediate organic molecules, similar to the C₄ plants. During the day, the captured CO₂ is gradually released into the same Calvin cycle reactions. CAM is an acronym for crassulacean acid metabolism. This process was first discovered in the Crassulaceae family of plants. CAM plants thrive in arid habitats. CAM plants include most cactus, pineapple, orchids, jade and sedum.

2. Living species are either osmoregulators or osmoconformers. For the organism provided, determine if it is an osmoregulator or an osmoconformer. Discuss the advantages and disadvantages of each type of homeostatic control. Describe one type of osmoregulation.
   1. Osmoconformers are all marine organisms that live in areas where the water has a stable composition. Rather than expend energy maintaining osmotic equilibrium with their surroundings, these organisms have concentrations of internal solutes that match their external surroundings. Osmoconformers have a much smaller habitat range than osmoregulators.
   2. Osmoregulators include all terrestrial, freshwater and certain marine species such as bony fishes, sharks, and all marine mammals. Even paramecia are osmoregulators. Osmoregulators must expend energy (from 5 to 30% of their resting metabolism) on maintaining homeostasis. In exchange, osmoregulators are able to inhabit a much larger range.
   1. Terrestrial organisms face dehydration (water loss) since they are not bathed in solute containing water. Layers of dead skin, scales, and exoskeletons are all ways that land animals prevent dehydration. Land plants have waxy cuticles and stomata that open and close to prevent dehydration. Plants and animals that live in arid conditions have further adaptations such as nocturnal behavior and the excretion of highly concentrated urine.
   2. Freshwater organisms constantly gain water by osmosis and lose salts by diffusion. In order to maintain homeostasis, freshwater animals excrete very dilute urine and actively transport chlorine across their gills, with sodium ions following passively.
   3. Marine osmoregulators typically use one of two strategies to maintain homeostasis—drinking copious amounts of seawater or urea build-up.
      1. Bony fishes drink copious amounts of seawater to replenish water lost by osmosis to the ocean. Choride ions are actively transported out of the gills with sodium ions following passively. A minimal amount of water is excreted by the kidneys along with excess calcium, magnesium, and sulfate ions.
      2. Sharks maintain osmotic balance by maintaining a high concentration of urea within their bodies. By keeping the urea level high a shark is hypertonic with the seawater even though the actual types of ions are vastly different. Since the shark is hypertonic to the surrounding seawater, water enters the shark through osmosis. Sharks do not drink seawater.
3. Regulation does not just occur at a cellular level. Ecosystems and populations also face regulation. Discuss how an alteration in the number of predators affects both a population of prey and the habitat.

   As the number of prey increases so does the number of predators, up to a certain maximum-carrying capacity. As the predators eat the prey, the prey’s competitors have an opportunity to increase in number. Changes in the relative sizes of each population also creates changes in the population of certain plants that are directly or indirectly consumed within this energy (food) chain. Changes in abiotic factors in the area may also occur as a result in the cycling of the populations. For example, more predators equates to a need for more nests or dens. Ultimately a change in the species of decomposer may also occur especially if the predator is an animal and the prey is a plant.

4. Many species can be cultured in a laboratory setting. The artificial environment must include the proper balance of nutrients, water, pH, light, and temperature. For the species given, design an experiment to determine the optimal temperature for maximum growth and reproduction. Be sure to include specifics on how to measure the population of the species.

   For all experiments with either microscopic or macroscopic organisms the laboratory procedure should include instruction on maintaining nutrients, pH, light intensity, and eliminating wastes. Temperatures may be maintained at the prescribed level using heating pads, incubators or room controls depending upon the species given to the student.

5. For the animal species given, describe how the animal regulates body temperature. Also explain how the animal copes with extremely high and extremely low temperatures.

   Thermoregulators, such as mammals, maintain their internal body temperature within a fairly narrow range. Internal heat is generated as a byproduct of metabolism. On a hot day an animal will cool its body temperature using either behavioral or physiological responses. Examples of behavior responses are seeking shade or wallowing in mud, water or sand. Examples of physiological responses are sweating, flushing and panting. Sweating is a complex process in which an elevation in the core body temperature causes a cascade of body changes. Veins and arteries near the body surface to dilate releasing heat to the skin. Glands in the skin secrete sweat which evaporates in the air becoming water vapor and therefore releasing heat to the environment.
   Most animals do not sweat. Primates and horses use sweating as a primary means of thermoregulation. A few other species, including dogs and cats, have sweat glands on the pads of their feet but they need to use panting for thermoregulation. Panting causes a reduction in body temperature because evaporation occurs in the lungs, pharynx and oral cavities.
   Layers of fat, hair, shivering, hibernation and vasodilation and vasoconstriction all contribute to the retention of heat by thermoregulators.

6. Hormones occur in both plants and animals. Explain how hormones play a role in determining the height (size) of the species given.

   Human growth hormone (GH) acts in many different manners in the body. It acts directly on cells. For example, GH stimulates the differentiation of chondrocyte cells in cartilage into collagen and proteoglycans. GH also acts on cells of other target organs causing the production of other hormones. One example of this occurs when GH targets specific cells within the liver. These cells produce insulin-like growth factors, or IGFs. The body produces two insulin-like growth factors termed IGF-1 and IGF-2. IGF-2 is produced in large amounts during fetal development. Production levels taper off during childhood and then remain steady throughout the rest of life. IGF-1 is produced after birth with peak production during adolescence. It is IGF-1 that is responsible for much of human growth. IGF-1 stimulates the proliferation of chondrocyte cells leading to bone growth. IGF-I also activates the differentiation and production of myoblasts.
   In general, plant hormones control growth and development by affecting the division, elongation, and differentiation of roots, stems and leaves. The three groups of plant hormones which trigger plant growth are the auxins, the cytokinins, and the gibberellins. Auxins stimulate cell elongation and cell division in the cambium. Auxins also inhibit the formation of lateral buds near its site of production—the apical meristem. Cytokinins also stimulate cell division and cell enlargement but instead of inhibiting lateral buds it stimulates the production of lateral buds. Gibberellins act on the stems causing cell division and elongation of stem cells.

Discussion

Diffusion
Regulation of the nutritional needs of the body involves many complex biofeedback systems. The intestines act as the gates for the regulation of molecules and ions into the body by both active and passive transport mechanisms. Active transport requires the expenditure of energy by the individual cells while passive transport mechanisms rely on the motion of the molecules and ions themselves. The primary type of passive transport is diffusion—defined as the net movement of a molecule or ion from a region where it is highly concentrated to a region where it is less concentrated.

Cells lining the intestines allow for the passive transportation of water and other small molecules directly past the cells and into the vast network of capillaries within the intestine. Cell membranes are said to be selectively permeable in that some types of molecules and ions can diffuse freely through while others cannot. In a classic oversimplification of the cell membrane we envision the membrane as being porous like a sieve. In this analogy it is easy to imagine that some molecules are small enough to fit through the pores while others are too large. Water molecules, dissolved gases (e.g., O₂, CO₂) and salt (which dissociates into sodium and chloride ions) are examples of substances that will diffuse freely through membranes.

Dialysis tubing provides a model for the cell membrane in this exercise. Dialysis tubing is made of cellulose perforated with microscopic pores. The pores are small enough that the tubing can be used to model the behavior of a cell membrane with respect to the sizes of molecules that will (or will not) diffuse through them. The use of an acid–base indicator solution allows for a quick visual identification of the change in pH that occurs when the H⁺ (acid) or OH^– (base) ions diffuse through the pores of the dialysis tubing (see Figure 1).

Buffers
The ability of buffers to resist changes in pH upon addition of strong acid or base can be traced to their chemical composition. All buffers contain a mixture of both a weak acid (HA) and its conjugate base (A^–). The buffer components HA and A^– are related to each other by means of the following ionization reaction that describes the behavior of the weak acid in water (Equation 1). Buffers control pH because the two buffer components are able to react with and therefore neutralize a small amount of strong acid or strong base added to the solution. The weak acid component HA reacts with any strong base, such as sodium hydroxide (NaOH), added to the solution to produce water and the conjugate base component A^– (Equation 2). The conjugate base component A^– reacts with an acid, such as hydrochloric acid (HCl), added to the solution to give its acid partner HA and chloride ions which are neutral—neither acid nor base (Equation 3). These complementary neutralization reactions can be visualized as a cyclic process (see Figure 2). Buffer activity will continue as long as both components remain present in solution. If one of the components A^– or HA is completely consumed, however, the buffer capacity will be exhausted and the buffer will no longer be effective. Organism Regulation
Many essay questions on the AP^® Biology Exam require students to link subcellular processes to an organism’s behaviors or responses to stimuli. Other frequently used question stems involve changing environmental conditions around an organism and how these conditions may either affect the organism’s cellular processes or alter its behavior. Allow students time to practice the initial brainstorming portion of the essay portion of the exam. Peer feedback will help guide students to a greater understanding of the major AP Biology theme of regulation.