Energy, Enzymes, and Metabolism

Potential vs. Kinetic Energy

Energy is the capacity to do work, and it exists in different forms. Two fundamental types are potential and kinetic energy.

• Potential Energy: Stored energy due to position or structure (e.g., a ball at the top of a hill, chemical bonds in glucose).

• Kinetic Energy: Energy of motion (e.g., a rolling ball, muscle movement, diffusion of molecules).

• Example: Food contains potential energy; when digested, it is converted to kinetic energy for movement.

The Laws of Thermodynamics

Thermodynamics governs how energy is transferred and transformed in biological systems.

• First Law (Law of Conservation of Energy): Energy cannot be created or destroyed, only transformed from one form to another.

• Second Law: Every energy transfer increases entropy (disorder/randomness) in the universe. Energy conversions are never 100% efficient; some energy is always lost as heat.

• Entropy: A measure of disorder. The universe naturally tends toward greater entropy unless energy is added.

Exergonic vs. Endergonic Reactions

Chemical reactions in cells can either release or require energy.

• Exergonic Reactions: Release energy (e.g., cellular respiration, breakdown of food).

• Endergonic Reactions: Require energy input (e.g., building proteins, photosynthesis).

• Hydrolysis: Breaking down molecules, often exergonic.

• Dehydration Synthesis: Building molecules, often endergonic.

• Free Energy (ΔG): Change in energy available to do work. Exergonic: ΔG < 0; Endergonic: ΔG > 0.

ATP – Chemical Makeup and Function

ATP (adenosine triphosphate) is the main energy currency of the cell.

• Structure: Adenine (base) + ribose (sugar) + 3 phosphate groups.

• Function: Stores and releases energy for cellular work (movement, transport, synthesis).

• Phosphorylation: Transfer of a phosphate group from ATP to another molecule, making it more reactive.

• Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Example: Muscle contraction, active transport.

Enzymes

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

• Substrate: The reactant an enzyme acts upon.

• Active Site: Region on the enzyme where the substrate binds.

• Factors Affecting Enzyme Activity: Temperature, pH, substrate concentration.

• Inhibitors: Molecules that decrease enzyme activity.

• Types of Inhibitors:

  • Competitive: Bind to the active site, competing with the substrate.

  • Noncompetitive: Bind elsewhere, changing the enzyme's shape and function.

• Activators: Molecules that increase enzyme activity.

Cellular Respiration

Equation for Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose.

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy (ATP)}$

Redox Reactions

Redox (reduction-oxidation) reactions transfer electrons between molecules.

• Oxidation: Loss of electrons (OIL: Oxidation Is Loss).

• Reduction: Gain of electrons (RIG: Reduction Is Gain).

• Reducing Agent: Donates electrons (gets oxidized).

• Oxidizing Agent: Accepts electrons (gets reduced).

Aerobic vs. Anaerobic Respiration

• Aerobic: Uses oxygen, produces more ATP (~30-32 ATP per glucose).

• Anaerobic: Does not use oxygen, produces less ATP (~2 ATP per glucose).

• Fermentation: Anaerobic process in muscles (lactic acid) and yeast (alcohol).

Stages of Aerobic Respiration

1. Glycolysis: Occurs in cytoplasm. Glucose → 2 pyruvate + 2 ATP.

2. Pyruvate Oxidation: Pyruvate → Acetyl-CoA + CO2.

3. Krebs Cycle (Citric Acid Cycle): Occurs in mitochondria. Acetyl-CoA → CO2 + electron carriers (NADH, FADH2) + 2 ATP.

4. Electron Transport Chain (ETC) & Chemiosmosis: Electron carriers donate electrons to ETC, which pumps H+ ions across the membrane. ATP synthase uses the gradient to make ATP (about 28 ATP).

Substrate-Level vs. Oxidative Phosphorylation

• Substrate-Level Phosphorylation: Direct transfer of phosphate to ADP to make ATP (occurs in glycolysis and Krebs cycle).

• Oxidative Phosphorylation: Uses ETC and chemiosmosis to produce most ATP.

Photosynthesis

Overview

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy (glucose).

Where is Chlorophyll Found?

• Located in the chloroplasts, mostly inside the thylakoid membranes of mesophyll cells in green leaf tissue.

Stomata

• Tiny openings on leaves that let CO2 in and O2 out.

• If stomata close (e.g., due to drought), photosynthesis slows.

Structure of Chloroplast

• Double Membrane: Outer and inner membranes.

• Thylakoids: Membranous sacs containing chlorophyll (site of light reactions).

• Grana: Stacks of thylakoids.

• Stroma: Fluid outside thylakoids (site of Calvin Cycle).

Photosynthetic Pigments

• Main Pigment: Chlorophyll a (absorbs red and blue light, reflects green).

• Accessory Pigments: Chlorophyll b, carotenoids (absorb additional wavelengths).

Photosynthesis Equation

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Light} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Light Reactions vs. Calvin Cycle

• Light Reactions: Occur in thylakoid membranes. Convert light energy to chemical energy (ATP, NADPH), release O2.

• Calvin Cycle: Occurs in stroma. Uses ATP and NADPH to fix CO2 into glucose.

Light Reactions

• Reactants: Light, H2O, NADP+, ADP.

• Products: O2, ATP, NADPH.

• Location: Thylakoid membrane.

• Types:

  • Noncyclic: Makes ATP and NADPH, releases O2.

  • Cyclic: Only makes ATP (when plant needs more ATP).

Calvin Cycle (Light-Independent Reactions)

• Reactants: CO2, ATP, NADPH.

• Products: Glucose (or G3P, a sugar).

• Phases:

  1. Carbon Fixation: CO2 joins RuBP (enzyme: Rubisco).

  2. Reduction: Uses ATP and NADPH to make G3P.

  3. Regeneration: Rebuilds RuBP to keep the cycle going.

• To make 1 glucose: 6 CO2, 18 ATP, 12 NADPH, 2 turns = 1 G3P; 2 G3P = 1 glucose.

Photosynthesis and Climate Change

• Plants take in CO2, lowering carbon in the air.

• Less CO2 = less greenhouse effect; helps slow global warming.

• Burning fuels adds CO2; photosynthesis removes CO2.

Comparison Table: Cellular Respiration vs. Photosynthesis