Chapter 8: Photosynthesis: Capturing Energy

Photosynthesis and the Global Energy Cycle

Ecosystem Energy Dynamics: The relationship between photosynthesis and cellular respiration forms a continuous cycle based on the transfer of energy and matter. - In Chloroplasts: Photosynthesis converts energy from sunlight into chemical energy. The raw materials used are $CO_2$ and $H_2O$ . The outputs are organic molecules (such as glucose) and oxygen ( $O_2$ ). - In Mitochondria: Cellular respiration breaks down organic molecules using $O_2$ to produce ATP, which powers most cellular work. The byproducts of this process are $CO_2$ and $H_2O$ , which are then reused by chloroplasts. - Energy Loss: During these conversions, energy leaves the ecosystem as heat energy.

The Nature of Light and the Electromagnetic Spectrum

Composition of Light: Light is composed of particles of energy called photons.
Wave Properties: All energy in the electromagnetic spectrum travels as waves. - Wavelength-Energy Relationship: Shorter wavelengths possess more energy than longer wavelengths.
Visible Light: This is a small portion of the electromagnetic spectrum that the human eye can perceive. - Sunlight: This is a mixture of many different wavelengths.
The Electromagnetic Spectrum Segments (Ordered from longer to shorter wavelengths): - TV and Radio Waves: Longest wavelengths. - Microwaves - Infrared - Visible Light Spectrum: Ranges from approximately $380\,nm$ to $760\,nm$ . - Red: $760\,nm$ to $700\,nm$ . - Orange: Approximately $600\,nm$ . - Yellow: Approximately $580\,nm$ . - Green: Approximately $500\,nm$ . - Blue: Approximately $450\,nm$ . - Violet: $400\,nm$ to $380\,nm$ . - UV (Ultraviolet) - X-rays - Gamma Rays: Shortest wavelengths with the highest energy.

Chloroplast Structure and Leaf Anatomy

Primary Site of Photosynthesis: Chloroplasts are organelles enclosed by a double membrane. They are located primarily within the mesophyll cells inside the leaf.
Leaf Anatomy Components: - Stoma (Stomata): Pores on the leaf surface that allow for gas exchange ( $CO_2$ entering and $O_2$ exiting). - Mesophyll: The internal tissue of the leaf. - Palisade Mesophyll: Columns of cells located beneath the upper epidermis. - Spongy Mesophyll: Loosely packed cells that allow for gas circulation. - Vein: Responsible for the transport of water and nutrients.
Chloroplast Anatomy Internal Structures: - Membranes: Outer membrane and inner membrane separated by an intermembrane space. - Stroma: The fluid-filled region internal to the inner membrane and surrounding the thylakoids. This is the site of carbon fixation reactions. - Thylakoids: Flattened, sac-like membranes that contain chlorophyll. They are the site of light-dependent reactions. - Granum: A stack of thylakoids (plural: grana). - Thylakoid Lumen: The fluid-filled space inside the thylakoid. - Thylakoid Membrane: The membrane where the electron transport chain and ATP synthase are located.

The Chemistry and Physics of Chlorophyll

Chlorophyll Structure: The molecule consists of two main parts: - Porphyrin Ring: The light-absorbing portion of the molecule, containing a magnesium ( $Mg$ ) atom at its center. - Hydrocarbon Side Chain: A long tail that anchors the molecule in the thylakoid membrane.
Types of Chlorophyll: - Chlorophyll a: Contains a methyl group ( $-CH_3$ ). It is the primary photosynthetic pigment. - Chlorophyll b: Contains a carbonyl group ( $-CHO$ ). It acts as an accessory pigment.
Photon Absorption Mechanics: - When a photon is absorbed by an excitable electron, the electron moves from a low energy level (ground state) to a high energy level (excited state). - Two Potential Outcomes for the Excited Electron: 1. Return to Ground Level: The electron may drop back down, emitting a less energetic photon (fluorescence) and heat. 2. Electron Acceptance: The electron may be captured by an electron acceptor molecule, which is what occurs during photosynthesis.
Absorption Spectra: - Chlorophyll absorbs light most efficiently in the blue spectrum (approximately $400\,nm$ to $450\,nm$ ) and the red spectrum (approximately $650\,nm$ to $680\,nm$ ). - Green Light: Chlorophyll reflects or transmits green light, resulting in a low absorption and high transmittance rate for those wavelengths ( $500\,nm$ to $600\,nm$ ). A galvanometer would show high transmittance for green light and low transmittance for blue light passed through a chlorophyll solution.

The Two Phases of Photosynthesis

1. Light-Dependent Reactions: - Location: Thylakoids. - Process: Light energy is absorbed, and electrons are energized. - Products: ATP and NADPH are generated to be used in the next phase. Oxygen ( $O_2$ ) is released as a byproduct of water splitting.
2. Carbon Fixation Reactions (Calvin Cycle): - Location: Stroma. - Process: Compounds generated in phase one (ATP and NADPH) provide the energy and reducing power to form carbohydrates from $CO_2$ .

Photosystems and Light-Dependent Mechanics

Photosynthetic Units: Photosynthesis involves two types of units called Photosystems I and II.
Each Photosystem Includes: - Chlorophyll molecules. - Multiple antenna complexes that capture light energy and funnel it to the reaction center.
Reaction Centers: - Photosystem I (PS I): The reaction center is known as $P700$ , with an absorption peak at $700\,nm$ . - Photosystem II (PS II): The reaction center is known as $P680$ , with an absorption peak at $680\,nm$ .
The Electron Transport Chain (ETC): - Electrons energized by light pass through a series of redox reactions. - Noncyclic Electron Transport (Z-scheme): 1. Light hits PS II ( $P680$ ). Electrons are excited and passed to a primary electron acceptor. 2. These electrons are replaced by the photolysis of water: $H_2O \rightarrow \frac{1}{2}O_2 + 2H^+ + 2e^-$ . 3. Electrons move from PS II to PS I via an ETC involving Plastoquinone ( $P_q$ ), a Cytochrome complex, and Plastocyanin ( $P_c$ ). 4. This movement drives ATP production by chemiosmosis. 5. Light hits PS I ( $P700$ ), re-exciting the electrons. 6. The electrons pass through another ETC involving Ferredoxin and are finally accepted by $NADP^+$ (catalyzed by Ferredoxin- $NADP^+$ reductase) to form NADPH.

Cyclic vs. Noncyclic Electron Transport Table

Noncyclic Electron Transport: - Electron Source: $H_2O$ . - Oxygen Released: Yes (from $H_2O$ ). - Terminal Electron Acceptor: $NADP^+$ . - Energy Captured As: ATP (by chemiosmosis) and NADPH. - Photosystems Required: PS I ( $P700$ ) and PS II ( $P680$ ).
Cyclic Electron Transport: - Electron Source: None (electrons cycle through the system). - Oxygen Released: No. - Terminal Electron Acceptor: None (electrons cycle through the system). - Energy Captured As: ATP only (by chemiosmosis). - Photosystems Required: PS I ( $P700$ ) only.

Chemiosmosis and ATP Synthesis

Proton Gradient: As electrons move down the ETC, protons ( $H^+$ ) are pumped from the stroma into the thylakoid lumen.
Photolysis Contribution: The splitting of water also adds $H^+$ to the thylakoid lumen.
pH Change: The accumulation of $H^+$ creates a high concentration in the lumen, lowering the pH compared to the stroma.
ATP Production: Protons diffuse across the thylakoid membrane back into the stroma through special channels in the ATP synthase complex. This release of energy allows for the phosphorylation of ADP to ATP ( $ADP + P_i \rightarrow ATP$ ).

The Carbon Fixation Reactions: The Calvin Cycle

Definition: The process where the energy of ATP and NADPH is used to form organic molecules (sugars/glucose) from $CO_2$ .
Balanced Equation for Glucose Production: - $12\,NADPH + 18\,ATP + 6\,CO_2 + 12\,H^+ \rightarrow C_6H_{12}O_6 + 12\,NADP^+ + 18\,ADP + 18\,P_i + 6\,H_2O$
Three Phases of the Calvin Cycle: 1. $CO_2$ Uptake: $6\,CO_2$ molecules are captured by $6$ molecules of Ribulose bisphosphate (RuBP). This produces an unstable intermediate that immediately breaks into $12$ molecules of phosphoglycerate (PGA). This is catalyzed by the enzyme rubisco. 2. Carbon Reduction: PGA is phosphorylated by ATP and reduced by NADPH. A phosphate is removed, resulting in the formation of $12$ molecules of Glyceraldehyde-3-phosphate (G3P). Two ( $2$ ) G3P molecules exit the cycle to form glucose or other carbohydrates. 3. RuBP Regeneration: The remaining $10$ G3P molecules undergo a series of reactions to be rearranged back into $6$ molecules of RuBP, requiring an additional $6\,ATP$ .

Variations in Carbon Fixation Pathways

Dry Conditions and Photorespiration: When stomata close to conserve water, $CO_2$ is depleted and $O_2$ builds up. Rubisco begins to bind with $O_2$ instead of $CO_2$ , removing Calvin cycle intermediates.
C4 Pathway (Spatial Separation): - Plants store $CO_2$ in a $4$ -carbon molecule called oxaloacetate. - Process: Initial carbon fixation occurs in mesophyll cells (via PEP carboxylase), and the Calvin cycle proceeds in separate bundle sheath cells where $CO_2$ concentration is kept high.
CAM Pathway (Temporal Separation): - At Night: Plants open stomata and store $CO_2$ in oxaloacetate (which is converted to malic acid). - During Day: Stomata close to save water. The stored malic acid releases $CO_2$ to the Calvin cycle while light energy provides ATP and NADPH.

Comparison of Photosynthesis and Aerobic Respiration

Metabolic Reaction: - Photosynthesis: Anabolism. - Aerobic Respiration: Catabolism.
Raw Materials: - Photosynthesis: $CO_2, H_2O$ . - Aerobic Respiration: $C_6H_{12}O_6, O_2$ .
End Products: - Photosynthesis: $C_6H_{12}O_6, O_2$ . - Aerobic Respiration: $CO_2, H_2O$ .
Eukaryotic Cell Sites: - Photosynthesis: Chloroplasts. - Aerobic Respiration: Cytosol (Glycolysis) and Mitochondria.
ATP Production: - Photosynthesis: Photophosphorylation (chemiosmotic process). - Aerobic Respiration: Substrate-level phosphorylation and oxidative phosphorylation (chemiosmotic process).
Principal Electron Transfer Compound: - Photosynthesis: $NADP^+$ (reduced to NADPH during anabolism). - Aerobic Respiration: $NAD^+$ (reduced to NADH during catabolism).
ETC Location: - Photosynthesis: Thylakoid membrane. - Aerobic Respiration: Mitochondrial inner membrane (cristae).
Source of Electrons for ETC: - Photosynthesis: $H_2O$ (via photolysis). - Aerobic Respiration: Immediate source is NADH and $FADH_2$ ; ultimate source is glucose.
Terminal Electron Acceptor for ETC: - Photosynthesis: $NADP^+$ (forming NADPH). - Aerobic Respiration: $O_2$ (forming $H_2O$ ).