Unit 3: Energy, Enzymes, Cellular Respiration, and Photosynthesis

Unit 3: Energy and Metabolism

This unit covers the fundamental processes by which cells obtain, transform, and utilize energy. It includes the study of enzymes, cellular respiration, and photosynthesis, which are central to understanding metabolism in biological systems.

Chapter 8: Energy and Enzymes

Thermodynamic Terms: H, S, and G

• H (Enthalpy): The total heat content of a system. It reflects the energy stored in chemical bonds.

• S (Entropy): A measure of disorder or randomness in a system.

• G (Gibbs Free Energy): The energy available to do work in a system at constant temperature and pressure.

The change in these values is represented by the Greek letter delta ($\Delta$):

• $\Delta H$: Positive if heat is absorbed (endothermic), negative if released (exothermic).

• $\Delta S$: Positive if disorder increases, negative if disorder decreases.

• $\Delta G$: Determines spontaneity of a reaction. Negative $\Delta G$ means the reaction is spontaneous; positive $\Delta G$ means it is non-spontaneous.

The relationship is given by:

$\Delta G = \Delta H - T\Delta S$

Enzyme-Substrate Interaction

• Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy.

• The substrate binds to the enzyme's active site, forming an enzyme-substrate complex.

• The enzyme stabilizes the transition state, facilitating the conversion of substrate to product.

Example: The enzyme sucrase binds to sucrose and hydrolyzes it into glucose and fructose.

Uncatalyzed vs. Enzyme-Catalyzed Reactions

• Uncatalyzed Reaction: Proceeds slowly due to high activation energy.

• Enzyme-Catalyzed Reaction: Proceeds rapidly because the enzyme lowers the activation energy barrier.

Key Difference: Enzymes do not change the overall $\Delta G$ of the reaction; they only affect the rate.

Regulation of Enzyme Activity

• Allosteric Regulation: Molecules bind to sites other than the active site, changing enzyme activity.

• Covalent Modification: Addition or removal of chemical groups (e.g., phosphorylation).

• Competitive Inhibition: Inhibitor competes with substrate for the active site.

• Feedback Inhibition: End product of a pathway inhibits an upstream enzyme.

ATP: The Energy Currency of the Cell

• Adenosine Triphosphate (ATP): Stores and transfers energy within cells.

• Hydrolysis of ATP releases energy ($\Delta G$ is highly negative), which can be used to drive endergonic reactions.

• ATP's high-energy phosphate bonds make it especially effective for energy coupling.

Chapter 9: Cellular Respiration

Pathways and Locations

• Glycolysis: Occurs in the cytoplasm; breaks glucose into pyruvate, producing ATP and NADH.

• Pyruvate Processing: Occurs in the mitochondrial matrix; converts pyruvate to acetyl-CoA, producing NADH and CO2.

• Citric Acid Cycle (Krebs Cycle): Mitochondrial matrix; completes glucose oxidation, producing NADH, FADH2, ATP, and CO2.

• Electron Transport Chain (ETC) and Oxidative Phosphorylation: Inner mitochondrial membrane; uses NADH and FADH2 to generate a proton gradient, driving ATP synthesis and producing water.

Relevant Products: ATP, NADH, FADH2, CO2, H2O

Substrate-Level vs. Oxidative Phosphorylation

• Substrate-Level Phosphorylation: Direct transfer of a phosphate group to ADP from a phosphorylated intermediate. Occurs in glycolysis and the citric acid cycle.

• Oxidative (Chemiosmotic) Phosphorylation: ATP synthesis powered by the movement of protons across the mitochondrial membrane via ATP synthase, driven by the electron transport chain.

Key Regulatory Step in Glycolysis

• Phosphofructokinase (PFK): Catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate. This is the main regulatory (rate-limiting) step, inhibited by high levels of ATP (feedback inhibition).

Fermentation

• Purpose: Regenerates NAD+ from NADH, allowing glycolysis to continue in the absence of oxygen.

• Types:

  • Lactic Acid Fermentation: Occurs in animals (e.g., muscle is reduced to lactate.cells); pyruvate

  • Alcohol Fermentation: Occurs in yeast and some bacteria; pyruvate is converted to ethanol and CO2.

Chapter 10: Photosynthesis

Light-Dependent and Light-Independent Reactions

• Light-Dependent Reactions: Occur in the thylakoid membranes; use light energy to produce ATP and NADPH, and release O2 as a byproduct.

• Light-Independent Reactions (Calvin Cycle): Occur in the stroma; use ATP and NADPH to fix CO2 into carbohydrates.

• The two sets of reactions are linked: ATP and NADPH from the light-dependent reactions power the Calvin cycle.

Plant Pigments in Photosynthesis

• Main Pigment: Chlorophyll a absorbs light most efficiently in the blue and red wavelengths, reflecting green.

• Accessory pigments (e.g., chlorophyll b, carotenoids) broaden the spectrum of light absorption and protect the plant from excess light energy.

• Pigments capture light energy, exciting electrons that drive the light reactions.

Photosystems and Their Roles

• Photosystem II (PSII): Absorbs light, splits water molecules (photolysis), and generates ATP via the electron transport chain.

• Photosystem I (PSI): Absorbs light and uses electrons to reduce NADP+ to NADPH.

• Together, these systems convert light energy into chemical energy (ATP and NADPH).

The Calvin Cycle and Rubisco

• Three Main Steps:

  1. Carbon Fixation: CO2 is attached to ribulose-1,5-bisphosphate (RuBP) by the enzyme Rubisco.

  2. Reduction: ATP and NADPH are used to convert 3-phosphoglycerate into glyceraldehyde-3-phosphate (G3P).

  3. Regeneration: Some G3P is used to regenerate RuBP, enabling the cycle to continue.

• Rubisco: The enzyme that catalyzes the first step of carbon fixation; it is the most abundant enzyme on Earth.

Regulation of Photosynthesis

• Photosynthesis is regulated by light intensity, availability of water and CO2, and feedback from carbohydrate products.

• Enzyme activity in the Calvin cycle is also regulated by environmental conditions and the energy status of the cell.