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Chapter 6: Energy and Metabolism

Two Major Categories of Energy
- Potential Energy
- Definition: Stored energy that has the potential to do work. It is energy possessed by an object due to its position or state.
- Example: Chemical energy stored in the bonds of molecules (e.g., glucose, ATP), gravitational energy, or the energy in a stretched spring.
- Kinetic Energy
- Definition: Energy of motion, actively performing work.
- Example: Movement of molecules or objects (e.g., flowing water, muscle contraction); thermal energy, which is the kinetic energy of molecules in a substance.
- Additional Types of Energy
- Free Energy
  - Definition: The amount of energy in a system that is available to do useful work at a constant temperature and pressure; denoted as G. The change in free energy, $\Delta G$ , determines the spontaneity of a reaction.
- Activation Energy
  - Definition: The minimum amount of energy required to initiate a chemical reaction by reaching its transition state. Enzymes work by lowering the activation energy without changing the overall $\Delta G$ of the reaction.
Two Laws of Thermodynamics
- First Law: Conservation of Energy
- Definition: Energy cannot be created or destroyed in an isolated system, only transformed from one form to another.
- Examples: In living systems, light energy is converted to chemical energy during photosynthesis, or chemical energy from food is converted to kinetic energy and heat during cellular respiration.
- Second Law: Entropy
- Definition: In every energy transformation, the total entropy (disorder or randomness) of a closed or isolated system will always increase. Systems tend naturally towards greater disorder.
- Implication: Energy conversions are inefficient, with some energy inevitably lost as unusable heat, thus increasing the entropy of the surroundings. Living organisms maintain their internal order by increasing the disorder of their environment.
Metabolism
- Anabolic Pathways
- Definition: Metabolic pathways that construct complex molecules from smaller units. These reactions require an input of energy (endergonic, \Delta G > 0).
- Example: Synthesis of glucose from carbon dioxide and water during photosynthesis; synthesis of proteins from amino acids.
- Catabolic Pathways
- Definition: Metabolic pathways that break down complex molecules into simpler units. These reactions release energy (exergonic, \Delta G < 0).
- Example: Cellular respiration, converting glucose into carbon dioxide and water to release energy for ATP synthesis; digestion of food.
Enzymes
- Substrate
- Definition: The specific reactant molecule that an enzyme acts upon, binding to its active site.
- Active Site
- Definition: A specific region on the enzyme, typically a pocket or groove, where substrate molecules bind and undergo a chemical reaction. The shape of the active site is complementary to the substrate, often described by the induced fit model.
- Factors Affecting Enzyme Activity
- pH
  - Enzymes have optimal pH levels at which their activity is highest. Deviations from this optimum can alter the ionization state of amino acid residues in the active site, disrupting ionic bonds and hydrogen bonds, which can lead to denaturation and loss of function.
- Temperature
  - Enzyme activity generally increases with temperature up to an optimum due to increased kinetic energy and collisions. However, excessively high temperatures can cause the enzyme's three-dimensional structure to denature, irrevocably altering the active site. Very low temperatures slow reaction rates without denaturing the enzyme.
- Salt Concentration
  - Extreme salt concentrations can interfere with the ionic bonds and hydrogen bonds essential for maintaining the enzyme's proper three-dimensional structure, leading to denaturation and loss of activity.
- Inducer
  - Molecules that enhance enzyme activity (also called activators) by binding to an enzyme, often at an allosteric site, causing a conformational change that increases the enzyme's affinity for its substrate or its catalytic rate.
- Coenzymes
  - Definition: Organic molecules, often derived from vitamins (e.g., NAD+, FAD), that bind to enzymes and assist during the catalysis of reactions, often by carrying electrons or specific functional groups.
- Cofactors
  - Definition: Inorganic ions (e.g., $Mg^{2+}$ , $Zn^{2+}$ , $Fe^{2+}$ ) that are necessary for the proper function of certain enzymes, often by stabilizing the enzyme structure or directly participating in the catalytic reaction.
- Enzyme Inhibition
- Competitive Inhibition
  - An inhibitor molecule structurally similar to the substrate competes directly with the substrate for binding to the enzyme's active site. This type of inhibition can often be overcome by increasing substrate concentration.
- Non-Competitive Inhibition
  - An inhibitor binds to a site on the enzyme other than the active site (an allosteric site), causing a conformational change that alters the shape of the active site and reduces its efficiency, regardless of substrate concentration.
- Feedback Inhibition
  - A regulatory mechanism where the end product of a metabolic pathway inhibits the activity of an enzyme earlier in the same pathway, thus controlling the rate of its own synthesis and preventing overproduction.
- Allosteric Inhibition
  - A form of non-competitive inhibition where the inhibitor binds to an allosteric site, causing a conformational change that decreases the enzyme's affinity for its substrate or reduces its maximal catalytic activity (Vmax).
Energy Currency: ATP
- Definition: Adenosine triphosphate, a nucleoside triphosphate that serves as the primary energy currency of the cell. Its hydrolysis, particularly the breaking of the terminal phosphate bond, releases a significant amount of free energy (approximately $7.3 \text{ kcal/mol}$ ) that powers most cellular processes, linking exergonic and endergonic reactions.

Chapter 6: Cellular Respiration

Equation of Cellular Respiration
- $\text{C}6\text{H}{12}\text{O}6 + 6\text{O}2 \rightarrow 6\text{CO}2 + 6\text{H}2\text{O} + \text{Energy (ATP + Heat)}$
Oxidation and Reduction
- Definitions:
- Oxidation: The loss of electrons from a substance (and often hydrogen atoms), increasing its oxidation state. "Oxidation Is Loss" (OIL).
- Reduction: The gain of electrons by a substance (and often hydrogen atoms), decreasing its oxidation state. "Reduction Is Gain" (RIG).
- Key Component: Electron carriers, such as NAD+ and FAD, facilitate the transfer of high-energy electrons from fuel molecules to other molecules, playing a crucial role in energy harvesting.
Electron Carriers
- Oxidized state:
- Examples: NAD+ (nicotinamide adenine dinucleotide), FAD+ (flavin adenine dinucleotide), NADP+ (nicotinamide adenine dinucleotide phosphate - primarily in anabolic reactions).
- Reduced state:
- Examples: NADH (carrying one H+ and two electrons), FADH2 (carrying two H+ and two electrons), NADPH. These molecules are vital for transporting energy in the form of electrons to the electron transport chain.
Glycolysis
- Location: Cytoplasm of the cell.
- Starting Materials: One molecule of hexose (e.g., glucose, a $C_6$ sugar).
- End Product: Two molecules of pyruvate (pyruvic acid, a $C_3$ compound).
- Process: Glucose is broken down into two pyruvate molecules through a series of 10 enzymatic steps, divided into an energy-investment phase (uses 2 ATP) and an energy-payoff phase (produces 4 ATP and 2 NADH).
- Net Gain of ATP: 2 ATP produced via substrate-level phosphorylation.
- Production of NADH: 2 NADH produced (these will later contribute to ATP production via oxidative phosphorylation).
- Anaerobic Process: Glycolysis does not require oxygen and is the metabolic pathway shared by both aerobic respiration and fermentation.
Pyruvate Oxidation and TCA Cycle
- Pyruvate Oxidation (Pyruvate Decarboxylation)
- Location: Mitochondrial matrix, after pyruvate is transported from the cytoplasm.
- Products (per one pyruvate):
  - One NADH and one Acetyl CoA (a $C_2$ compound) are produced.
  - One carbon atom is released as $CO_2$ (decarboxylation).
  - Since two pyruvate molecules result from one glucose, the total yield is 2 Acetyl CoA, 2 NADH, and 2 $CO_2$ .
TCA Cycle (Citric Acid Cycle / Krebs Cycle)
- Starting Material: Acetyl CoA (from pyruvate oxidation) combines with the 4-carbon compound oxaloacetate, forming citrate (a 6-carbon compound).
- Location: Mitochondrial matrix.
- Process: A series of eight enzymatic steps that completely oxidizes the Acetyl CoA, releasing carbons as $CO_2$ and producing electron carriers.
- Products (per one Acetyl CoA turn):
- Outputs: 3 NADH, 1 FADH2, and 1 GTP (which is readily converted to 1 ATP) are produced.
- Function: The primary function is to complete the breakdown of glucose derivatives and generate a large number of reduced electron carriers (NADH and FADH2) that will transfer their high-energy electrons to the electron transport chain (ETC).
Oxidative Phosphorylation
- Overall Function: The process responsible for the vast majority of ATP synthesis during aerobic respiration, utilizing the energy from electron transfer to pump protons and drive ATP production.
- Location: Inner mitochondrial membrane.
- Process: Comprises two main components:
1. Electron Transport Chain (ETC): Electrons from NADH and FADH2 are passed through a series of protein complexes (I, II, III, IV), called the electron transport chain. As electrons move down the chain, their energy is used to pump H+ ions (protons) from the mitochondrial matrix into the intermembrane space, creating a steep electrochemical proton gradient.
2. Chemiosmosis: Oxygen serves as the final electron acceptor at the end of the ETC, combining with electrons and H+ to form water, which is essential for continuous electron flow. The accumulated H+ ions in the intermembrane space flow back down their concentration gradient, through an enzyme complex called ATP synthase, embedded in the inner mitochondrial membrane. The force generated by this proton flow (proton-motive force) drives the rotation of ATP synthase, catalyzing the synthesis of ATP from ADP and inorganic phosphate (Pi).
- ATP Yield: Typically yields about 26-28 ATP molecules per glucose molecule.
Metabolism Without Oxygen: Fermentation
- Definition: An anaerobic process that allows cells to generate ATP (only from glycolysis) in the absence of oxygen. Its primary purpose is to regenerate NAD+ from NADH so that glycolysis can continue to produce ATP.
- Examples:
- Lactic acid fermentation: Pyruvate is converted to lactate, regenerating NAD+. Occurs in certain bacteria, fungi, and human muscle cells during intense exercise when oxygen supply is low.
- Ethanol fermentation: Pyruvate is converted to acetaldehyde, releasing $CO_2$ , and then acetaldehyde is reduced to ethanol, regenerating NAD+. Occurs in yeast and some bacteria, used in brewing and baking.
- Connection: Fermentation extracts much less energy from glucose (only 2 ATP per glucose) compared to aerobic respiration, but it is critical for survival in anaerobic conditions.

Chapter 8: Photosynthesis

Equation of Photosynthesis
- $6\text{CO}2 + 6\text{H}2\text{O} + \text{Light Energy} \rightarrow \text{C}6\text{H}{12}\text{O}6 + 6\text{O}2$
Two Stages of Photosynthesis
- Light-Dependent Reactions
- Location: Thylakoid membranes within the chloroplasts.
- Process: Chlorophyll and other pigments absorb light energy, which is used to split water molecules ( $H2O$ ), releasing oxygen ( $O2$ ), electrons, and protons ( $H+$ ). The energy is converted into chemical energy in the form of ATP and NADPH.
- Light-Independent Reactions (Calvin Cycle / Carbon Fixation Cycle)
- Location: Stroma of the chloroplasts.
- Uses NADPH and ATP produced during the light-dependent reactions as energy and reducing power.
- Process: Carbon dioxide ( $CO_2$ ) from the atmosphere is fixed (incorporated into organic molecules) and transformed into a 3-carbon sugar, glyceraldehyde-3-phosphate (G3P), which is then used to synthesize glucose and other organic compounds.
- Key Enzyme: Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the enzyme that catalyzes the initial step of carbon fixation, considered the most abundant protein on Earth.
Three Stages of the Calvin Cycle
- 1. Carbon Fixation
- Definition: Three molecules of $CO2$ join with three molecules of a 5-carbon sugar, RuBP (ribulose-1,5-bisphosphate), catalyzed by Rubisco. This forms an unstable 6-carbon intermediate that immediately splits into two molecules of 3-PGA (3-phosphoglycerate) for each $CO2$ fixed.
- 2. Reduction
- Definition: The 3-PGA molecules are phosphorylated by ATP and reduced by NADPH (derived from the light reactions) to form G3P (glyceraldehyde-3-phosphate). For every six G3P molecules produced, one molecule of G3P leaves the cycle to be used for the synthesis of glucose, sucrose, or other organic molecules.
- 3. Regeneration of RuBP
- Definition: The remaining five G3P molecules (from the six produced) are rearranged and converted back into three molecules of RuBP, a process that requires the expenditure of ATP. This regenerates the starting material for carbon fixation, allowing the cycle to continue.
Components of Light Reactions:
- Inputs: Light energy, Water ( $H_2O$ ), NADP+, ADP + Pi.
- Outputs: NADPH (a reduced electron carrier), ATP (chemical energy), Oxygen ( $O_2$ ) as a byproduct.
- Key Structures Involved: These components are embedded within the thylakoid membrane:
- Photosystems II (PSII) and I (PSI): Complexes of pigment molecules (like chlorophyll a and b) and proteins that absorb light energy and excite electrons.
- Cytochrome Complex: An electron transport chain component that pumps protons across the thylakoid membrane.
- ATP Synthase: An enzyme complex that utilizes the proton gradient to synthesize ATP through chemiosmosis.
- Thylakoid Membrane: The site where light-dependent reactions occur and the proton gradient is established.
Process of Light Reactions
- 1. Photon Absorption and Electron Excitation: Light energy is absorbed by pigments within Photosystem II (PSII), exciting an electron in chlorophyll to a higher energy level.
- 2. Water Splitting (Photolysis): To replace the lost electron in PSII, water molecules are split, releasing electrons (which replace those lost by chlorophyll), protons ( $H+$ ), and molecular oxygen ( $O_2$ ) as a waste product.
- 3. Electron Transport Flow and Proton Gradient: The excited electrons from PSII pass through an electron transport chain (including the cytochrome complex) to Photosystem I (PSI). As electrons transfer, energy is released and used to pump protons ( $H+$ ) from the stroma into the thylakoid lumen, creating a proton gradient.
- 4. Re-excitation and NADPH Formation: Electrons receive another boost of energy from light absorbed by PSI. These re-energized electrons are then passed to another electron carrier, NADP+, and reduced to NADPH by the enzyme NADP+ reductase.
- 5. ATP Formation (Chemiosmosis): The high concentration of protons in the thylakoid lumen flows back down its concentration gradient, through ATP synthase, into the stroma. This flow drives the phosphorylation of ADP to ATP (photophosphorylation), completing the conversion of light energy into chemical energy.