Learning Objectives

1. Explain how autotrophs & heterotrophs obtain their energy.
   Autotrophs (“self feeders”) are organisms that CAN build their own energy-rich macromolecules
   • Photoautotrophs (like plants) use the energy of light to do this
   • Chemoautotrophs (like some bacteria) use the energy of inorganic molecules (like sulfur or ammonia)
   Autotrophs get their energy from sunlight.

   Because humans CANNOT make these molecules themselves, we are considered heterotrophs (“other feeders”)
   • Animals, fungi, and most bacteria are also heterotrophs
   Heterotrophs get their energy from ingesting organic molecules or eating autotrophs.

2. Write the overall chemical equation for photosynthesis.
   Energy + 6CO2 + 6H2O => C6H12O6 + 6O2

3. Identify which reaction words (anabolic / catabolic, endergonic / exergonic) accurately describe photosynthesis.
   Photosynthesis is an anabolic process. Anabolic processes build complex molecules from simpler ones. In photosynthesis, simple molecules of water and carbon dioxide are assembled into glucose, a complex sugar, along with the production of oxygen. This synthesis of glucose involves the formation of new chemical bonds, demonstrating the constructive nature of anabolism.

   Photosynthesis is an endergonic process. This means it requires an input of energy to proceed. The energy needed for photosynthesis comes from sunlight. In photosynthesis, light energy is captured by chlorophyll and used to convert carbon dioxide (CO2) and water (H2O) into glucose (C6H12O6) and oxygen (O2). The process stores energy in the chemical bonds of glucose, a form of potential energy that is higher than the energy of the reactants. Therefore, the reaction is non-spontaneous without the input of energy.

4. Identify the source of energy for photosynthesis.
   The source of energy for photosynthesis is sunlight.

5. Name the locations of the light reactions & the Calvin Cycle in a plant cell.
   The light-dependent reactions occur in photosystems that are embedded in the thylakoid membranes of the chloroplasts.

   The Calvin Cycle (a.k.a. light-independent reactions) occurs in the stroma of chloroplasts.

6. Describe the general functions of the light reactions & the Calvin Cycle in photosynthesis.
   The light reactions capture light energy from the sun and convert it into chemical energy in the form of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate, reduced form). These reactions take place in the thylakoid membranes of chloroplasts. Water molecules (H2O) are split into oxygen (O2), protons (H+), and electrons (e-). This process releases oxygen as a byproduct into the atmosphere. The movement of electrons through the electron transport chain in the thylakoid membrane creates a proton gradient across the membrane. ATP is synthesized as protons flow back across the membrane through ATP synthase, utilizing the energy stored in the gradient. The ATP and NADPH produced in the light reactions provide the energy and reducing power, respectively, needed for the synthesis of carbohydrates in the next phase of photosynthesis, the Calvin Cycle.

   The Calvin Cycle incorporates atmospheric carbon dioxide (CO2) into organic molecules, a process known as carbon fixation. This is the first step in forming glucose from CO2 and H2O. Using the ATP and NADPH generated during the light reactions, the Calvin Cycle chemically reduces the fixed carbon to produce the sugar G3P (glyceraldehyde-3-phosphate). Two molecules of G3P can be combined to form glucose or other carbohydrates. The cycle also regenerates ribulose-1,5-bisphosphate (RuBP), the molecule that accepts CO2 in the first step of the cycle, allowing the process to continue.

7. Explain how the wavelength of light relates to its color & energy level.
   The color of an object (like a leaf) is determined by which wavelengths of light are reflected off its surface

   Light energy is made of photons that travel in waves • Light with shorter wavelengths (like blue & purple) has more energy
   • Light with longer wavelengths (like red) has less energy

8. Explain why plants are green.
   To capture as much light as possible, plants use multiple pigment proteins that each absorb different wavelengths of light
   Chlorophyll a is the primary pigment protein in plants. Accessory pigments (like Chlorophyll b & β-carotene) absorb light & transfer that energy to Chlorophyll a
   Chlorophyll absorbs light most efficiently in the blue and red parts of the visible spectrum and reflects green light. This reflected green light is what gives plants their green color.

9. Name the structures that are found in a photosystem.
   Photosystems are complex structures found in the thylakoid membranes of chloroplasts in plants and in the membranes of cyanobacteria. They play a crucial role in the light-dependent reactions of photosynthesis by capturing light energy and converting it into chemical energy. A photosystem consists of several key components:
   Light-Harvesting Complex: This consists of hundreds of pigment molecules, including chlorophyll a, chlorophyll b (in plants), and various carotenoids. These pigments are structured to efficiently capture light of various wavelengths and funnel the energy towards the reaction center.
   Reaction Center: At the core of a photosystem is the reaction center, which contains a special pair of chlorophyll a molecules. Once the Light-Harvesting complex absorbs light energy and transfers it to the reaction center, the energy excites electrons in these chlorophyll molecules to a higher energy state.
   Primary Electron Acceptor: This molecule is located very close to the reaction center chlorophylls. It captures the high-energy electrons from the excited chlorophyll a molecules before they can return to their ground state. This step is crucial for the subsequent electron transport chain.
   Core Complex: The core complex includes proteins that bind the reaction center, the primary electron acceptor, and other components together. It ensures the proper alignment and functioning of the photosystem.
   Peripheral Proteins: These are various proteins attached to the photosystem that play roles in electron transport, proton translocation, and the stabilization of the complex.

10. Explain how photosystems trap light energy.
    Photosystems trap light energy through the following steps:
    Light Absorption: Pigment molecules in the antenna complex absorb light of various wavelengths.
    Energy Transfer: The absorbed light energy is transferred to a special chlorophyll a molecule in the reaction center.
    Electron Excitation: This energy excites an electron in the chlorophyll a to a higher energy state.
    Electron Transport: The excited electron is transferred to an electron acceptor, starting the electron transport chain, which generates ATP and NADPH.
    In essence, photosystems capture light energy and convert it into chemical energy, crucial for the light-dependent reactions of photosynthesis.

11. Identify the source of the excited electrons found in Photosystem II & Photosystem I.
    Each photosystem has a central reaction center with a Chlorophyll a molecule & a pair of special electrons
    During the light reactions, all pigments in the photosystem collect light energy
    • This energy is funneled toward the reaction center
    • Ultimately, it energizes Chlorophyll a’s special electrons
    • These energized special electrons then travel through Electron Transport Chain proteins and are ultimately donated to NAD+

12. Explain how the light reactions of photosynthesis generate oxygen.
    PSII absorbs light energy, which is used to excite electrons in the chlorophyll a molecules in the reaction center of PSII to a higher energy level. These high-energy electrons are then donated to the electron transport chain, creating a vacancy for electrons in the chlorophyll a molecules. To replace these electrons, molecules of water (H2O) are split in a process called photolysis, which occurs at the oxygen-evolving complex (OEC) of PSII.
    Water molecules are split into oxygen, protons, and electrons under the influence of light in Photosystem II. The oxygen is released as a byproduct.
    2H2O -> 4H+ + 4e- + O2

    The splitting of water molecules produces oxygen, protons, and electrons. The electrons replenish the electron supply in PSII, while the protons contribute to the formation of a proton gradient used for ATP synthesis. The oxygen molecules (O2) are released as a byproduct of this reaction and eventually exit the chloroplast and the plant, contributing to the oxygen in Earth's atmosphere.

13. Describe how the proton (H +) gradient across the thylakoid membrane is generated.
    The proton (H+) gradient across the thylakoid membrane in photosynthesis is created by the light-dependent reactions. When light excites Photosystem II, it splits water into oxygen, protons, and electrons, releasing protons into the thylakoid lumen. As electrons move through the electron transport chain, additional protons are pumped from the stroma into the thylakoid lumen. This movement creates a high concentration of protons inside the lumen, forming an electrochemical gradient. This gradient is used by ATP synthase to produce ATP, as protons flow back into the stroma, converting ADP and phosphate into ATP through a process known as chemiosmosis.

14. Explain how the H+ gradient is used to generate ATP.
    The H+ (proton) gradient across the thylakoid membrane is used to generate ATP through a process called chemiosmosis.
    As the light-dependent reactions create a high concentration of protons inside the thylakoid lumen, a proton gradient is established across the membrane. ATP synthase, a protein complex in the thylakoid membrane, allows protons to flow down their gradient from the lumen back into the stroma. This flow of protons through ATP synthase provides the energy needed to synthesize ATP from ADP and inorganic phosphate. This process efficiently converts the energy stored in the proton gradient into the chemical energy stored in ATP, which is then used for various cellular processes, including the Calvin cycle in photosynthesis.

15. List the products & reactants of the light reactions.
    Reactants:
    Sunlight
    H2O
    NADP+
    ADP + Pi

    Products:
    O2
    NADPH
    ATP

16. Explain the relationship between the products of photosynthesis’ light reactions & the reactants of the Calvin cycle.
    The ATP and NADPH generated by the light reactions are essential for the Calvin cycle to proceed. They link the energy captured from sunlight to the biochemical synthesis of glucose from CO2.
    The Calvin cycle, in turn, uses these products to produce glucose, which serves as an energy source for the plant and other organisms that consume it.
    Oxygen (O2) is released into the atmosphere as a byproduct of water splitting and is not used in the Calvin cycle.

17. Explain what happens in carbon fixation.
    In carbon fixation, atmospheric carbon dioxide (CO2) is captured and converted into organic molecules within the Calvin cycle. CO2 is attached to ribulose-1,5-bisphosphate (RuBP) by the enzyme Rubisco, forming an unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA). This process initiates the transformation of inorganic CO2 into organic compounds used by the plant.

18. Name the enzyme that performs carbon fixation.
    The enzyme that performs carbon fixation in the Calvin cycle is Rubisco. It catalyzes the attachment of CO2 to RuBP, starting the process of converting CO2 into glucose and other sugars.

19. Explain why 6 turns of the cycle is necessary to make 1 glucose.
    Six turns of the Calvin cycle are necessary to produce one glucose molecule because each turn fixes one molecule of CO2, and it takes two molecules of glyceraldehyde-3-phosphate (G3P), a three-carbon molecule, to form one six-carbon glucose molecule. Since one turn of the cycle produces one G3P molecule every three turns, six turns are required to generate the two G3P molecules needed for the synthesis of one glucose molecule.

20. Explain how photosynthesis & cellular respiration are related to one another.
    Photosynthesis and cellular respiration are complementary processes. Photosynthesis converts CO2 and water into glucose and oxygen using sunlight energy, essentially storing energy in chemical bonds of glucose. Cellular respiration, in turn, breaks down glucose in the presence of oxygen to produce CO2, water, and ATP, releasing the energy stored in glucose for cellular use. Thus, the products of photosynthesis (glucose and oxygen) are the reactants for cellular respiration, and the products of cellular respiration (CO2 and water) are reactants for photosynthesis, linking the energy flows of biological systems.