Energy Transformations

Kinetic and Potential Energy in Biological Systems

Energy transformations are fundamental to biological processes, enabling cells to perform work and maintain life. Energy exists in different forms, primarily as kinetic and potential energy.

• Kinetic Energy: The energy of motion. In biological contexts, this includes thermal energy, which is the energy of molecules moving.

• Potential Energy: Energy stored due to position or configuration. In cells, chemical energy is stored in chemical bonds.

Chemical Reactions Involve Energy Transformations

Bond Energy and Reaction Progress

Chemical reactions in cells involve changes in the energy stored in molecular bonds. The potential energy of a molecule depends on the configuration and position of its shared electrons.

• Weaker bonds with equally shared electrons have high potential energy.

• Stronger bonds with unequally shared electrons have low potential energy.

• For most reactions to proceed, bonds must be broken and new bonds formed.

How Enzymes Work

Enzyme Function and Catalysis

Enzymes are biological catalysts that accelerate chemical reactions by lowering activation energy. They bring reactants together in precise orientations and facilitate bond formation and breaking.

• Reactants must collide in a precise orientation and have enough kinetic energy to overcome repulsion between electrons.

• Most biological reactions occur fast enough only in the presence of an enzyme.

• Enzymes are protein catalysts that:

  • Bring reactants together in precise orientations.

  • Reduce the energy needed for reactions.

• Substrates bind via hydrogen bonding or other interactions with amino acids in the active site.

• Bonds break and form to generate products.

• Many enzymes undergo a conformational change during catalysis.

Enzyme Action

Stages of Enzyme-Catalyzed Reactions

Enzyme action typically proceeds through three main stages:

1. Initiation: Substrates are precisely oriented as they bind to the active site.

2. Transition State Facilitation: Interactions between the substrate and active site R-groups lower the activation energy.

3. Termination: Reaction products are released.

Enzyme Summary

Key Properties of Enzymes

• Bring reactant molecules (substrates) together.

• Allow chemical reactions to occur more quickly and require less energy.

• Are usually specific for a single type of reaction.

• Do not get "used up" in the reaction.

Regulation of Enzyme Activity

Mechanisms of Enzyme Regulation

Cells regulate enzyme activity to control metabolic pathways and respond to changing conditions.

• Competitive Inhibition: Occurs when a molecule competes with the substrate for the active site.

• Allosteric Regulation: Occurs when a molecule binds at a location other than the active site, causing the enzyme to change shape. This can activate or deactivate the enzyme.

• Feedback Inhibition: When an enzyme in a pathway is inhibited by the final product of that pathway, shutting down the pathway when products are abundant.

Protein Denaturation

Loss of Protein Structure and Function

Protein denaturation occurs when proteins unravel and lose their shape due to changes in temperature, pH, or other causes, resulting in loss of function.

• High fevers (above 104°F) in humans can cause protein denaturation.

Cellular Respiration

Purpose and Outcomes

Cellular respiration is the process by which cells harvest energy from high-energy molecules, such as glucose, to produce ATP.

• Used for biosynthesis (raw materials).

• Used as a fuel (cell respiration).

Why Do Cellular Respiration?

Importance of Energy in Cells

• Life requires energy.

• Metabolic pathways harvest energy from molecules like glucose.

• The energy is used to make ATP.

• ATP fuels work in cells.

ATP

The Energy Currency of the Cell

ATP (adenosine triphosphate) is the primary energy carrier in cells, enabling various cellular processes.

• Life requires energy.

• ATP is the "energy currency" of the cell.

• The energy helps cells perform:

  • Mechanical work

  • Transport work

  • Chemical work

(Aerobic) Cellular Respiration

Overview and Fuel Molecules

Aerobic cellular respiration is the breakdown of food molecules to provide energy (in the form of ATP) for cellular work. Glucose is a common fuel molecule.

• Cellular respiration provides energy for cells to do work.

• Glucose is a common fuel for cellular respiration.

Respiration and Cellular Respiration

Distinction and Relationship

• Lungs carry out breathing (respiration).

• Cells carry out cellular respiration.

Aerobic Cellular Respiration: Four Main Processes

Stages of Aerobic Respiration

Aerobic cellular respiration consists of four main processes:

1. Glycolysis: Six-carbon glucose is broken down into two three-carbon pyruvate molecules.

2. Pyruvate Processing: Each pyruvate is oxidized to form acetyl CoA.

3. Citric Acid Cycle: Each acetyl CoA is oxidized to CO2.

4. Electron Transport and Chemiosmosis (Oxidative Phosphorylation): Electrons move through a transport chain and their energy is used to set up a proton gradient, which is used to make ATP.

Glycolysis

Process and Products

Glycolysis is the breakdown of glucose into two molecules of pyruvic acid and occurs in the cytoplasm of cells.

• Energy investment phase: ATP is used to phosphorylate glucose.

• Energy payoff phase: NADH is made and ATP is produced by substrate-level phosphorylation.

• Net yield: 2 NADH, 2 ATP, and 2 pyruvic acids.

Electron Shuttle: NAD+ → NADH

Role in Cellular Respiration

• Nicotinamide Adenine Dinucleotide (NAD+): Electron carrier that undergoes redox reactions.

• Accepts electrons during glycolysis and other metabolic pathways, becoming NADH.

How is Glycolysis Regulated?

Feedback Inhibition and Enzyme Regulation

• High levels of ATP (a product of glycolysis) inhibit the third enzyme, phosphofructokinase.

• Phosphofructokinase has two binding sites for ATP.

Processing Pyruvate to Acetyl CoA

Transport and Conversion

Pyruvate is transported into mitochondria, where it is converted to acetyl CoA by a large enzyme complex.

• Occurs in the mitochondrial matrix in eukaryotes.

• Occurs in the cytosol in prokaryotes.

• Pyruvate undergoes a series of reactions catalyzed by pyruvate dehydrogenase.

• Mitochondria have inner and outer membranes; cristae are extensions of the inner membrane.

Citric Acid Cycle (Krebs Cycle)

Oxidation of Acetyl CoA

Each acetyl CoA from pyruvate processing is oxidized to CO2 in the citric acid cycle, generating electron carriers and ATP.

• Acetyl group is transferred from acetyl CoA to oxaloacetate to form citrate.

• Key reactions:

  • Reduction of NAD+ to NADH

  • Reduction of FAD to FADH2

  • Phosphorylation of ADP to form ATP

• The cycle runs twice for each glucose molecule (since two pyruvate are produced per glucose).

ATP Synthase and Chemiosmosis

Production of ATP

ATP synthase uses the proton gradient established by the electron transport chain to produce ATP from ADP and inorganic phosphate.

• Protons flow through ATP synthase embedded in the inner mitochondrial membrane.

• ATP is produced as protons move down their electrochemical gradient.

Aerobic vs. Anaerobic Respiration

Differences in Electron Acceptors

• Most eukaryotes and many prokaryotes use oxygen as the final electron acceptor for aerobic respiration.

• Some prokaryotes, especially those in oxygen-poor environments, use other molecules as electron acceptors (anaerobic respiration).

• Oxygen is the most effective electron acceptor.

Obtaining Food

Autotrophs and Heterotrophs

• Autotrophs: Organisms that produce their own food from inorganic materials (e.g., plants synthesizing food via photosynthesis).

• Heterotrophs: Organisms that obtain food by consuming other organisms.

Chloroplasts

Structure and Function

Chloroplasts are organelles in plant cells responsible for photosynthesis.

• Contain thylakoids, stroma, and photosynthetic pigments.

• Enzymes in the stroma convert CO2 to sugar.

Overall Equation of Photosynthesis

Summary of Photosynthetic Reaction

Photosynthesis converts carbon dioxide and water into glucose and oxygen using light energy.

• General equation:

• Light-dependent reactions capture light energy.

• Light-independent reactions (Calvin Cycle) use that energy to make sugar.