Essential knowledge 2.A.1: All living systems require constant input of  free energy.
             a. Life requires a highly ordered system.
               Evidence of student learning is a demonstrated understanding of each of the following:
                      1. Order is maintained by constant free energy input into the system.
 
                      2. Loss of order or free energy flow results in death.
 
                      3. Increased disorder and entropy are offset by biological processes that maintain or increase order.
              b. Living systems do not violate the second law of thermodynamics, which states that entropy increases over time.
                    Evidence of student learning is a demonstrated understanding of each of the following:
                      1. Order is maintained by coupling cellular processes that increase entropy (and so have negative changes in free energy) with
                          those that decrease entropy (and so have positive changes in free energy).
 
                      2. Energy input must exceed free energy lost to entropy to maintain order and power cellular processes.

                      3. Energetically favorable exergonic reactions, such as ATP®ADP, that have a negative change in free energy can be used to
                           maintain or increase order in a system by being coupled with reactions that have a positive free energy change.

               c. Energy-related pathways in biological systems are sequential and may be entered at multiple points in the pathway. [See also 2.A.2]
                To foster student understanding of this concept, instructors can choose an illustrative example such as:
              •  Krebs cycle
                    •  Glycolysis
              •  Calvin cycle
              •  Fermentation  

               d. Organisms use free energy to maintain organization, grow and reproduce.
                Evidence of student learning is a demonstrated understanding of each of the following:

                      1. Organisms use various strategies to regulate body temperature and metabolism.
                     To foster student understanding of this concept, instructors can choose an illustrative example such as:
                    •  Endothermy (the use of thermal energy generated by metabolism to maintain homeostatic body temperatures)
                    •  Ectothermy (the use of external thermal energy to help regulate and maintain body temperature)
                    •  Elevated floral temperatures in some plant species

                      2. Reproduction and rearing of offspring require free energy beyond that used for maintenance and growth. Different
                            organisms use various reproductive strategies in response to energy availability.
                       To foster student understanding of this concept, instructors can choose an illustrative example such as:
                     •  Seasonal reproduction in animals and plants
                     •  Life-history strategy (biennial plants, reproductive diapause)

                       3. There is a relationship between metabolic rate per unit body mass and the size of multicellular organisms — generally, the smaller
                            the organism, the higher the metabolic rate.

                       4. Excess acquired free energy versus required free energy expenditure results in energy storage or growth.

                       5. Insufficient acquired free energy versus required free energy expenditure results in loss of mass and, ultimately, the death of an
                            organism.

          e. Changes in free energy availability can result in changes in population size.

          f. Changes in free energy availability can result in disruptions to an ecosystem.
           To foster student understanding of this concept, instructors can choose an illustrative example such as:
             •  Change in the producer level can affect the number and size of other trophic levels.
                    •  Change in energy resources levels such as sunlight can affect the number and size of the trophic levels.

Learning Objectives:
LO 2.1 The student is able to explain how biological systems use free energy based on empirical data that all organisms require constant energy input to maintain organization, to grow and to reproduce. [See SP 6.2]

LO 2.2 The student is able to justify a scientific claim that free energy is required for living systems to maintain organization, to grow or to reproduce, but that multiple strategies exist in different living systems. [See SP 6.1]

LO 2.3 The student is able to predict how changes in free energy availability affect organisms, populations and ecosystems. [See SP 6.4]  

      Essential knowledge 2.A.2: Organisms capture and store free energy for use in biological processes.

            a. Autotrophs capture free energy from physical sources in the environment.
             Evidence of student learning is a demonstrated understanding of each of the following:
                 1. Photosynthetic organisms capture free energy present in sunlight.

                 2. Chemosynthetic organisms capture free energy from small inorganic molecules present in their environment, and this process can occur
                      in the absence of oxygen.

           b. Heterotrophs capture free energy present in carbon compounds produced by other organisms.
             Evidence of student learning is a demonstrated understanding of each of the following:
                  1. Heterotrophs may metabolize carbohydrates, lipids and proteins by hydrolysis as sources of free energy.

                  2. Fermentation produces organic molecules, including alcohol and lactic acid, and it occurs in the absence of oxygen.  
     ✘ Specific steps, names of enzymes and intermediates of the pathways for these processes are beyond the scope of the course
                          and the AP Exam. 

        c. Different energy-capturing processes use different types of electron acceptors.
         To foster student understanding of this concept, instructors can choose an illustrative example such as:
           •  NADP+ in photosynthesis
           •  Oxygen in cellular respiration

        d. The light-dependent reactions of photosynthesis in eukaryotes involve a series of coordinated reaction pathways that capture free energy
             in light to yield ATP and NADPH, which power the production of organic molecules.
             Evidence of student learning is a demonstrated understanding of each of the following:

                    1. During photosynthesis, chlorophylls absorb free energy from light, boosting electrons to a higher energy level in Photosystems I
                        and Photosystem II.

                    2. Photosystems I and II are embedded in the internal membranes of chloroplasts (thylakoids) and are connected by the transfer of
                         higher free energy electrons through an electron transport chain (ETC). [See also 4.A.2]

                     3. When electrons are transferred between molecules in a sequence of reactions as they pass through the ETC, an
                          electrochemical gradient of hydrogen ions (protons) across the thykaloid membrane is established.

                     4. The formation of the proton gradient is a separate process, but it is linked to the synthesis of ATP from ADP and inorganic
                          phosphate via ATP synthase.

                      5. The energy captured in the light reactions as ATP and NADPH powers the production of carbohydrates from carbon dioxide in the
                          Calvin cycle, which occurs in the stroma of the chloroplast.
         ✘✘ Memorization of the steps in the Calvin cycle, the structure of the molecules and the names of enzymes (with the
                                exception of ATP synthase) are beyond the scope of the course and the AP Exam.

        e. Photosynthesis first evolved in prokaryotic organisms; scientific evidence supports that prokaryotic (bacterial) photosynthesis
             was responsible for the production of an oxygenated atmosphere; prokaryotic photosynthetic pathways were the foundation of eukaryotic
             photosynthesis.

        f. Cellular respiration in eukaryotes involves a series of coordinated enzyme-catalyzed reactions that harvest free energy from simple
             carbohydrates.
              Evidence of student learning is a demonstrated understanding of each of the following:

                         1. Glycolysis rearranges the bonds in glucose molecules, releasing free energy to form ATP from ADP and inorganic phosphate, and
                        resulting in the production of pyruvate.

                   2. Pyruvate is transported from the cytoplasm to the mitochondrion, where further oxidation occurs. [See also 4.A.2]

                   3. In the Krebs cycle, carbon dioxide is released from organic intermediates ATP is synthesized from ADP and inorganic
                       phosphate via substrate level phosphorylation and electrons are captured by coenzymes.

                   4. Electrons that are extracted in the series of Krebs cycle reactions are carried by NADH and FADH2 to the electron transport chain.  
                                                                                ✘✘ Memorization of the steps in glycolysis and the Krebs cycle, or of the structures of the molecules and the names of the
                         enzymes involved, are beyond the scope of the course and the AP Exam

          g. The electron transport chain captures free energy from electrons in a series of coupled reactions that establish an electrochemical gradient
               across membranes.

               Evidence of student learning is a demonstrated understanding of each of the following:
                       1. Electron transport chain reactions occur in chloroplasts (photosynthesis), mitochondria (cellular respiration) and prokaryotic
                            plasma membranes.

                       2. In cellular respiration, electrons delivered by NADH and FADH2 are passed to a series of electron acceptors as they move
                           toward the terminal electron acceptor, oxygen. In photosynthesis, the terminal electron acceptor is NADP+.

                       3. The passage of electrons is accompanied by the formation of a proton gradient across the inner mitochondrial membrane or
                           the thylakoid membrane of chloroplasts, with the membrane(s) separating a region of high proton concentration from a region
                           of low proton concentration. In prokaryotes, the passage of electrons is accompanied by the outward movement of protons across
                            the plasma membrane.

                       4. The flow of protons back through membrane-bound ATP synthase by chemiosmosis generates ATP from ADP and inorganic
                           phosphate.

                       5. In cellular respiration, decoupling oxidative phosphorylation from electron transport is involved in thermoregulation.  
                                                                                                  ✘✘ The names of the specific electron carriers in the ETC are beyondthe scope of the course and the AP Exam.

        Essential knowledge 4.B.1: Interactions between molecules affect their structure and function.

              a. Change in the structure of a molecular system may result in a change of the function of the system. [See also 3.D.3]

             b. The shape of enzymes, active sites and interaction with specific molecules are essential for basic functioning of the enzyme.

                  Evidence of student learning is a demonstrated understanding of each of the following:

                      1. For an enzyme-mediated chemical reaction to occur, the substrate must be complementary to the surface properties (shape and
                       charge) of the active site. In other words, the substrate must fit into the enzyme's active site.

                      2. Cofactors and coenzymes affect enzyme function; this interaction relates to a structural change that alters the activity rate of the
                      enzyme. The enzyme may only become active when all the appropriate cofactors or coenzymes are present and bind to the
                      appropriate sites on the enzyme.
       ✘✘ No specific cofactors or coenzymes are within the scope of the course and the AP Exam.

       Essential knowledge 4.B.2: Cooperative interactions within organisms promote efficiency in the use of energy and matter.

            a. Organisms have areas or compartments that perform a subset of functions related to energy and matter, and these parts contribute       to
                the whole. [See also 2.A.2, 4.A.2]
                 Evidence of student learning is a demonstrated understanding of each of the following:
                     1. At the cellular level, the plasma membrane, cytoplasm and, for eukaryotes, the organelles contribute to the overall specialization
                          and functioning of the cell.

                     2. Within multicellular organisms, specialization of organs contributes to the overall functioning of the organism.
                    To foster student understanding of this concept, instructors can choose an illustrative example such as:
                       •  Exchange of gases
                    •  Circulation of fluids
                       •  Digestion of food
                       •  Excretion of wastes
 
                    3. Interactions among cells of a population of unicellular organisms can be similar to those of multicellular organisms,
                         and these interactions lead to increased efficiency and utilization of energy and matter.      
                         To foster student understanding of this concept, instructors can choose an illustrative example such as:    
                                   •  Bacterial community in the rumen of animals
                        •  Bacterial community in and around deep sea vents