Energy, Enzymes, and Metabolism

Potential vs. Kinetic Energy

Energy exists in different forms, primarily as potential energy (stored energy) and kinetic energy (energy of motion). Understanding these forms is essential for studying biological processes.

• Potential Energy: Energy stored due to position or structure. Example: A ball at the top of a hill, food before digestion, or chemical bonds in glucose.

• Kinetic Energy: Energy of movement. Example: A ball rolling down a hill, muscle contraction, molecules diffusing.

• Mnemonic: Potential = parked, Kinetic = in motion.

The Laws of Thermodynamics

The laws of thermodynamics govern energy transformations in biological systems.

• First Law (Conservation of Energy): Energy cannot be created or destroyed, only transformed. Example: Food energy → movement + heat.

• Second Law (Entropy): Every energy transfer increases entropy (disorder/randomness). Entropy is a measure of disorder; the universe naturally becomes more disordered unless energy is added.

Exergonic vs. Endergonic Reactions

Chemical reactions in cells can be classified as exergonic (energy releasing) or endergonic (energy requiring).

• Exergonic: Releases energy. Example: Cellular respiration, breakdown of food.

• Endergonic: Requires energy input. Example: Building proteins, photosynthesis.

• Equation: (Gibbs free energy)

  • Exergonic: (energy released)

  • Endergonic: (energy absorbed)

ATP – Chemical Makeup and Function

ATP (Adenosine Triphosphate) is the main energy currency of the cell, powering cellular work.

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

• Function: Energy released when a phosphate group is removed (hydrolysis).

• Equation:

• Example: ATP powers muscle contraction, transport, and synthesis.

Enzymes

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

• Substrate: The molecule an enzyme acts upon.

• Factors affecting enzyme activity:

  • Temperature

  • pH

  • Substrate concentration

• Inhibitors: Molecules that slow or stop enzyme activity.

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

  • Noncompetitive inhibitors: Bind elsewhere, changing enzyme shape.

• Activators: Help enzymes work faster/better.

Cellular Respiration

Equation for Cellular Respiration

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

• Equation:

Redox Reactions

Redox (reduction-oxidation) reactions transfer electrons between molecules, crucial for energy production.

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

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

• Reducing agent: Gives electrons (gets oxidized)

• Oxidizing agent: Receives electrons (gets reduced)

Aerobic vs. Anaerobic Respiration

Cells can produce energy with or without oxygen.

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

• Anaerobic: No oxygen, less ATP (only 2 ATP per glucose)

• Examples: Lactic acid fermentation (muscles), alcoholic fermentation (yeast)

Stages of Aerobic Respiration

• Glycolysis: Occurs in cytoplasm; glucose → pyruvate + 2 ATP

• Pyruvate oxidation: Pyruvate → Acetyl-CoA

• Krebs Cycle (Citric Acid Cycle): Makes electron carriers (NADH, FADH2), 2 ATP

• ETC & Chemiosmosis: Makes most ATP (28+)

Substrate-Level vs. Oxidative Phosphorylation

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP (4 total: 2 in glycolysis, 2 in Krebs)

• Oxidative phosphorylation: Uses electron transport chain and chemiosmosis to make ATP

Photosynthesis

Where is Chlorophyll Found?

Chlorophyll is the pigment responsible for capturing light energy for photosynthesis.

• Located in the thylakoid membranes of mesophyll cells (green leaf tissue)

Stomata

Stomata are tiny openings on leaves that regulate gas exchange.

• Allow CO2 in and O2 out

• If stomata close (dry), photosynthesis slows

Structure of Chloroplast

• Double membrane

• Thylakoids: Stacks with chlorophyll (site of light reactions)

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

Photosynthetic Pigments

Pigments absorb light for photosynthesis.

• Main pigment: Chlorophyll

• Absorbs red & blue light, reflects green

• Shorter wavelength = more energy

Photosynthesis Equation

• Equation:

Light Reactions

Light reactions convert solar energy to chemical energy (ATP and NADPH).

• Reactants: Light + H2O + NADP+ + ADP

• Products: ATP + NADPH

• Location: Thylakoid membrane

• Types:

  • Noncyclic (normal): Makes ATP & NADPH, releases O2

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

Calvin Cycle (Dark Reactions)

The Calvin Cycle uses ATP and NADPH to fix carbon dioxide and produce glucose.

• Reactants: CO2 + ATP + NADPH

• Products: Glucose (or G3P, a sugar)

• Phases:

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

  2. Reduction: Uses ATP & NADPH to make G3P

  3. Regeneration: Rebuilds RuBP to keep cycle going

• Summary: 6 CO2 + 18 ATP + 12 NADPH → 2 turns = 1 G3P; 2 G3P = 1 glucose

Photosynthesis and Climate Change

Photosynthesis helps regulate atmospheric CO2 and impacts 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: Aerobic vs. Anaerobic Respiration

Comparison Table: Light Reactions vs. Calvin Cycle

Additional info: Some explanations and examples have been expanded for clarity and completeness.