### **1. Pigments, Photosystems, and Leaf Color** * **Light Perception:** Sunlight consists of colors in the visible spectrum (**ROY G BIV**). We only see **reflected light**, not absorbed light. * **Pigments:** * **Chlorophyll A:** Main photosynthetic pigment; absorbs violet, blue, and red light while **reflecting green light**. * **Chlorophyll B:** Accessory pigment; absorbs blue, orange, and red light while **reflecting green light**. * **Carotenoids:** Accessory pigments; absorb blue and green light while **reflecting yellow, orange, and red light**. * **Autumn Leaf Color Changes:** In spring, carotenoids are masked by abundant chlorophyll. In autumn, shorter days reduce sunlight, causing chlorophyll to break down and reveal carotenoids; thus, orange/yellow/red light is **reflected**. * **Photosystem Structure:** Photosystems are located on the **thylakoid membrane** and consist of proteins, chlorophyll A, chlorophyll B, and carotenoids. * **Antenna Complex:** Contains chlorophyll B, carotenoids, and proteins. It captures photons and passes solar energy around, but **does not donate electrons**. * **Reaction Center:** Contains a pair of **Chlorophyll A** molecules. It absorbs solar energy to excite low-energy electrons and **donates high-energy electrons** to the primary electron acceptor. * **Redox in Photosystems:** When the reaction center donates high-energy electrons, it becomes **oxidized**; the electron acceptor gains them and becomes **reduced**. The lost electrons are replenished by an **electron donor** that donates low-energy electrons and becomes oxidized. --- ### **2. Light-Dependent Reactions & Electron Transport Chain** * **Location:** Occurs on the **thylakoid membrane**. * **Photosystem II (PS II) vs. Photosystem I (PS I):** * **PS II:** Absorbs sunlight at **\\(680\text{ nm}\\)**. Its electron donor is **water (\\(\text{H}_2\text{O}\\))**, which is split/oxidized into \\(2\text{ e}^-\\), \\(2\text{ H}^+\\), and oxygen gas (\\(\text{O}_2\\)) inside the thylakoid space. PS II uses electron energy to drive **ATP synthesis**. * **PS I:** Absorbs sunlight at **\\(700\text{ nm}\\)**. Replaces its lost electrons directly from **Plastocyanin (PC)**. PS I uses re-excited electrons to make **NADPH**. * **Sequential Electron Order:** \\(\text{H}_2\text{O} \rightarrow \text{PS II (680 nm)} \rightarrow \text{Plastoquinone (PQ)} \rightarrow \text{B}_6\text{f complex} \rightarrow \text{Plastocyanin (PC)} \rightarrow \text{PS I (700 nm)} \rightarrow \text{Ferredoxin (FD)} \rightarrow \text{NADP}^+\text{ reductase} \rightarrow \text{NADP}^+ \rightarrow \text{NADPH}\\). * **Mobile Electron Carriers:** Light reactions use **3 mobile carriers**: * **Plastoquinone (PQ):** Accepts electrons from PS II and delivers them to the \\(\text{B}_6\text{f}\\) complex. * **Plastocyanin (PC):** Carries electrons with the **lowest energy**. * **Ferredoxin (FD):** Carries electrons with the **highest energy** because its electrons were re-excited by \\(700\text{ nm}\\) light at PS I. * **Proton Pump & Chemiosmosis:** * The **\\(\text{B}_6\text{f}\\) complex** is the single proton pump in photosynthesis. It uses electron energy to actively pump \\(\text{H}^+\\) ions from the stroma into the thylakoid space, making the thylakoid space **more acidic (lower pH, higher \\(\text{H}^+\\) concentration)**. * \\(\text{H}^+\\) ions rush back into the stroma through **ATP synthase** down their concentration gradient via **facilitated diffusion (passive transport)**, synthesizing ATP (chemiosmosis). * **Products & Energetics:** Produces **\\(\text{O}_2\\)** (in thylakoid space), **ATP**, and **NADPH** (both in stroma). Because ATP is produced, light-dependent reactions are **exergonic**. * **Final Electron Acceptor:** **\\(\text{NADP}^+\\)**, which is reduced to NADPH by the enzyme \\(\text{NADP}^+\\) reductase. --- ### **3. Photosynthetic ETC vs. Mitochondrial ETC** * **Mobile Carriers:** Photosynthesis uses **3** (PQ, PC, FD); mitochondria use **2** (Ubiquinone, Cytochrome C). * **Proton Pumps:** Photosynthesis uses **1** (\\(\text{B}_6\text{f}\\) complex); mitochondria use **3** (\\(\text{NADH}\\) dehydrogenase, \\(\text{BC}_1\\) complex, cytochrome oxidase complex). *(Do not confuse \\(\text{B}_6\text{f}\\) in thylakoids with \\(\text{BC}_1\\) in mitochondria!)*. --- ### **4. Light-Independent Reactions (Calvin Cycle / \\(C_3\\) Pathway)** * **Location & Energetics:** Takes place in the **stroma** of chloroplasts (or cytoplasm of cyanobacteria). It is an **endergonic** process (\\(\Delta G > 0\\), non-spontaneous) that consumes ATP and NADPH. * **Three Phases:** 1. **Carbon Fixation:** \\(\text{CO}_2\\) (\\(1\text{C}\\)) combines with **RuBP** (\\(5\text{C}\\) sugar), catalyzed by the enzyme **Rubisco**, to form two \\(3\text{C}\\) molecules of **3-phosphoglycerate (3-PG / PGA)**. *(Inorganic carbon becomes fixed into organic compounds)*. 2. **Reduction:** Uses \\(12\text{ ATP}\\) and \\(12\text{ NADPH}\\) to reduce \\(12\text{ PGA}\\) into \\(12\text{ G3P}\\) (glyceraldehyde 3-phosphate). **G3P is the direct product** of the Calvin cycle. Two G3P molecules leave the cycle to form **1 glucose molecule** (indirect product). 3. **Regeneration of RuBP:** Consumes \\(6\text{ ATP}\\) to regenerate RuBP from the remaining 10 G3P molecules so the cycle can continue. * **Energy Requirements per Glucose:** Requires **6 turns** of the cycle, consuming **\\(18\text{ ATP}\\)** (\\(12\\) in reduction, \\(6\\) in regeneration) and **\\(12\text{ NADPH}\\)** (used only in reduction). --- ### **5. Photorespiration & Plant Adaptations (\\(C_3\\), \\(C_4\\), and \\(CAM\\))** * **Photorespiration:** Under hot, dry conditions, plants close their **stomata** to prevent water loss, causing internal \\(\text{CO}_2\\) to drop and \\(\text{O}_2\\) to build up. **Rubisco** binds \\(\text{O}_2\\) instead of \\(\text{CO}_2\\) to RuBP, creating a **wasteful anabolic process** that consumes ATP and produces \\(\text{CO}_2\\) and \\(\text{H}_2\text{O}\\). * **\\(C_3\\) Plants:** Majority of plants. Have chloroplasts and Rubisco **only in mesophyll cells**. Run only the Calvin cycle and consume **\\(18\text{ ATP}\\)** per glucose. * **\\(C_4\\) Plants (e.g., corn, sugarcane):** * **Structural Difference:** Have chloroplasts in **both mesophyll cells and bundle-sheath cells**. * **Spatial Separation:** * **Mesophyll cells:** \\(\text{CO}_2 + \text{PEP} \rightarrow \text{Oxaloacetate } (4\text{C}) \rightarrow \text{Malate}\\) via **PEP carboxylase**. *(No Rubisco here)*. * **Bundle-sheath cells:** Malate is transported here and decarboxylated into \\(\text{CO}_2\\) and pyruvate. **Rubisco** uses this concentrated \\(\text{CO}_2\\) in the Calvin cycle (\\(C_3\\) pathway). * **Energy Cost:** Transporting each \\(\text{CO}_2\\) as malate costs \\(2\text{ ATP}\\) equivalents (\\(12\text{ ATP}\\) total for \\(6\text{ CO}_2\\)). Total energy per glucose = \\(18 + 12 =\\) **\\(30\text{ ATP}\\)**. * **\\(CAM\\) Plants (e.g., cacti, pineapples):** * **Temporal Separation:** Run both pathways in **mesophyll cells only** (where Rubisco is located). * **Night:** Stomata open at night to fix \\(\text{CO}_2\\) into malate (\\(C_4\\) pathway) and store it in central vacuoles. * **Day:** Stomata close; malate releases \\(\text{CO}_2\\) to drive the Calvin cycle (\\(C_3\\) pathway) during daylight. * **Energy Cost:** Consumes **\\(30\text{ ATP}\\)** per glucose. --- ### **6. Important Review Points for Exam 2** * **Final Electron Acceptors:** * Light-dependent reactions: **\\(\text{NADP}^+\\)** (reduced to \\(\text{NADPH}\\)). * Aerobic cellular respiration: **Oxygen (\\(\text{O}_2\\))** (reduced to \\(\text{H}_2\text{O}\\)). * Lactic acid fermentation: **Pyruvate** (reduced to lactate/lactic acid in skeletal muscles, causing muscle fatigue). * Ethanol/Alcohol fermentation: **Acetaldehyde** (reduced to ethanol, releasing \\(\text{CO}_2\\) in yeast). * Methanogens: **\\(\text{CO}_2\\)**. * **Glycolysis Energy & Enzymes:** * **Initial Phase:** Endergonic (uses \\(2\text{ ATP}\\)). * **Middle Phase:** Exergonic (produces \\(2\text{ ATP}\\) and \\(2\text{ NADH}\\); uses **kinase** and **dehydrogenase**). * **Final Phase:** Exergonic (produces \\(2\text{ ATP}\\)). * **Overall:** Exergonic (net yield of **\\(2\text{ ATP}\\)** per glucose). * **Krebs Cycle:** Exergonic; produces **\\(2\text{ ATP}\\)** per glucose via **substrate-level phosphorylation** (catalyzed by kinases). * **Isotope Labeling Experiment:** * Oxygen labeled in \\(\text{CO}_2\\) ends up in **glucose**. * Oxygen labeled in \\(\text{H}_2\text{O}\\) ends up in **oxygen gas (\\(\text{O}_2\\))**. * **Organism Interdependence:** Photoautotrophs can survive without heterotrophs, but heterotrophs cannot survive without photoautotrophs.

Evaporation

The process by which water changes from a liquid to a gas.

Condensation

The process by which water vapor cools and changes back into liquid water.

Precipitation

Any form of water, liquid or solid, falling from clouds to the ground.

Transpiration

The process by which water is absorbed by plants, moves through plants, and is released as vapor through pores in leaves.

Water Cycle

The continuous movement of water on, above, and below the surface of the Earth.

Groundwater

Water stored underground in aquifers or soil.

Runoff

Water that flows over the land surface and returns to bodies of water.

Infiltration

The process by which water on the ground surface enters the soil.

Aquifer

A geological formation that can store and transmit water.

Watershed

An area of land that separates waters flowing to different rivers, basins, or seas.