Lecture 4 - Energy in Biological Systems Chemical Reactions and Glycolysis

Energy in Biological Systems

Concepts Related to Energy

• Biological systems rely fundamentally on energy for various processes. Energy is critical for chemical reactions that sustain life, metabolism, and the functioning of cells.

Types of Chemical Reactions

• Chemical reactions can be categorized into key types concerning energy transfer: metabolism encompasses both catabolism and anabolism, which involve the breakdown and synthesis of biomolecules, respectively.

Energy Transfer in the Environment

1. Plants as Energy Trappers

   • Plants capture radiant energy from sunlight through photosynthesis.

2. Storage of Energy

   • This energy is stored in the chemical bonds of biomolecules, primarily glucose.

3. Energy Use by Animals

   • Animals consume plants, utilizing this stored energy for their metabolic processes or retaining it for later use.

Why Do We Need Energy?

• Energy Capacity: Energy serves as the capacity to perform work essential for life.

• Types of Work: Living organisms require energy to perform:

  • Chemical work: Synthesis and breakdown of molecules.

  • Transport work: Movement of ions and molecules across membranes.

  • Mechanical work: Movement of cells, tissues, or organisms.

Types of Work in Biological Systems

Chemical Work

• Involves making or breaking chemical bonds, facilitating growth and homeostasis.

Transport Work

• Concerns ionic and molecular movement across membranes to create gradients vital for cellular functions.

Mechanical Work

• Encompasses movements at various scales:

  • Microscopic (e.g., organelles within cells) and macroscopic movements (e.g., muscle contraction).

Types of Energy and Their Relationship

Potential Energy

• This refers to stored energy that has the potential to do work.

Kinetic Energy

• Energy of motion; potential energy can convert to kinetic energy, as illustrated by a ball rolling down a ramp.

Energy Transfer Efficiency

• Energy transfers are not 100% efficient; a significant portion is lost as heat during metabolic processes.

• In human physical activities, about 70% of energy is lost in heat rather than being used for muscle contraction.

Thermodynamics Laws

1st Law of Thermodynamics

• States that energy in the universe is constant, and the human body must import energy from food sources.

2nd Law of Thermodynamics

• Indicates natural processes move towards disorder (entropy). To maintain order, living organisms need continuous energy input.

Chemical Reactions in Living Organisms

• Serve two primary purposes: transferring energy between molecules and utilizing potential energy stored in molecules. Larger biomolecules have more chemical bonds, which yield more energy when metabolized.

Activation Energy

• The energy required to initiate a chemical reaction. This "push" is necessary to break bonds and start the reaction process, similar to pushing a ball up a hill before its descent.

Types of Chemical Reactions

Key Features

• Reactants and Products: Every reaction involves reactants transforming into products, influenced by activation energy and net free energy change.

Exergonic Reactions

• Reactions that release energy, where the product has lower energy than the reactants (e.g., ATP hydrolysis).

Endergonic Reactions

• Reactions that consume energy, where products have more energy than reactants (e.g., synthesizing glycogen from glucose).

Coupled Reactions

• Energy released from exergonic reactions can drive endergonic reactions, evident in cellular metabolism.

Energy Storage and Transfer

• High-energy electron carriers like NADH, FADH2, and NADPH trap energy released from exergonic reactions and aid in activating endergonic reactions.

Enzymes

• Biological catalysts that increase reaction rates by positioning substrates optimally for reactions. Many enzymes are characterized by the suffix -ase.

Categories of Enzymatic Reactions

1. Oxidation-Reduction Reactions: Crucial for energy transfer; represented by the acronym "LEO says GER" (Lose Electrons = Oxidation; Gain Electrons = Reduction).

2. Hydrolysis-Dehydration Reactions: Involved in breaking down and synthesizing biomolecules (e.g., formation of sucrose from glucose and fructose).

3. Addition-Subtraction-Exchange Reactions: Involves removing, adding, or exchanging groups in substrates (e.g., kinases adding phosphate groups).

4. Ligation: Joins molecules together to form larger structures (e.g., formation of Acetyl CoA).

Metabolism

• Encompasses all the chemical reactions in organisms, categorized into anabolism (energy-consuming processes) and catabolism (energy-releasing processes).

Energy from Catabolic Processes

• Energy stored in the bonds of ATP and high-energy electron carriers results from catabolic breakdowns.

Energy for Anabolic Processes

• ATP and high-energy carriers provide the energy necessary for the formation of covalent bonds in biomolecules.

Structure of Metabolic Pathways

• Metabolic pathways proceed in a sequence, with each step facilitated by specific enzymes, leading to intermediates, some of which are key intermediates that participate in multiple pathways.

Regulation of Metabolic Pathways

Control Mechanisms

1. Enzyme Concentration: Adjusting enzyme levels alters reaction rates.

2. Enzyme Modulation: Certain molecules can enhance or inhibit enzyme activity, including hormones that trigger specific reactions.

3. Feedback Inhibition: Final products can inhibit earlier steps in the pathway to prevent overproduction.

4. Reversible Reactions: Some pathways can reverse direction depending on the cellular conditions.

5. Compartmentalization: Different enzymes may be localized in specific parts of the cell, separating pathways.

ATP Functionality

• ATP transfers energy necessary for various cellular functions, such as biosynthesis, muscle contraction, transport processes, and nerve conduction.

Aerobic vs. Anaerobic Processes

Aerobic

• Requires oxygen for the complete oxidation of substrates, producing high energy yield.

Anaerobic

• Occurs in the absence of oxygen, yielding lower energy outputs.

Glucose Catabolism

Predictable Outcomes

• Glucose catabolism follows glycolysis and the citric acid cycle, producing a modest amount of ATP, but more importantly, generating high-energy electron carriers (NADH and FADH2) crucial for the electron transport chain, where the majority of ATP is produced.

Glycolysis Overview

• Defined as the splitting of glucose into two pyruvate molecules, occurring in the cytosol through three main stages: trapping glucose, breaking it into two halves, and making pyruvate.

Detailed Stages of Glycolysis

Stage 1: Trapping Glucose

• Accomplished primarily through phosphorylation, utilizing ATP. This stage leads to the production of fructose 1,6-bisphosphate.

Stage 2: Breaking Glucose

• The enzyme cleaves fructose 1,6-bisphosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P). DHAP must convert to G3P to proceed.

Stage 3: Formation of Pyruvate

• Each G3P passes through stage three, producing NADH, ATP, and pyruvate.

Products of Glycolysis

• Final yield from one glucose molecule includes: 2 ATP consumed in stage 1, 4 ATP and 2 NADH produced, resulting in a net production of 2 ATP and 2 pyruvate.

Regulation of Glycolysis

• The process features several reversible reactions, but key irreversible reactions—particularly those catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase—are critical for regulation.

Importance of Phosphofructokinase

• Serves as the primary regulatory step, influenced by ATP levels; high ATP decreases activity, while low ATP increases it to facilitate energy production.

Comparison of Hexokinase and Phosphofructokinase

• The hexokinase step produces glucose 6-phosphate, which can enter alternative metabolic pathways, underscoring its less critical regulatory role compared to phosphofructokinase.

Rule of Thumb for Regulation

• The enzyme catalyzing the committed step of a metabolic process is typically the most crucial control point.