Energy and Cellular Respiration

Chapter 3 Overview: Energy and Cellular Respiration

• Ruby-throated hummingbirds need a lot of energy for movement and bodily functions, requiring insect and nectar consumption.
• Organisms need energy to survive, grow, reproduce, and perform daily activities.
• Energy is released from carbohydrates and other energy-rich organic molecules.
• Cellular respiration is the process that releases energy in most organisms.
• Anaerobic respiration and/or fermentation releases energy for species in low-oxygen environments.

Launch Activity: A Flutter of Activity

• The ruby-throated hummingbird is found throughout Canada and is one of the smallest birds there.
• Hummingbirds flap their wings 55-75 times per second, reaching speeds of 80 km/h or more and consume up to three times their body weight in a single day.
• Metabolic rate is the rate of chemical reactions maintaining cellular functions.
• The activity involves comparing the hummingbird's basal metabolic rate to that of other animals and humans.

Metabolism

• Metabolism refers to the chemical reactions transforming matter and energy in cells.
• Metabolic pathways are step-by-step sequences where substrates turn into products, catalyzed by enzymes.
• Catabolism breaks down energy-rich compounds, converting energy into a usable form with active transport and muscle contraction representing examples.
• Anabolism uses energy to build large molecules like proteins and fats.

Anabolic and Catabolic Pathways

• Anabolic reactions build complex molecules and require energy.
• Catabolic reactions break down complex molecules and release energy.

Energy Fundamentals

• Energy does work to change or move matter against opposing forces.
• Kinetic energy is the energy of motion while potential energy is stored energy.
• Thermal energy represents the kinetic energy of particles moving randomly.
• Heat transfers thermal energy due to temperature differences.
• Chemical energy is potential energy stored in a compound's bonds.

Bond Energy

• Energy is released when chemical bonds form, and the energy required to break a bond is equal to the energy released during its formation.
• Bond energy is the amount of energy needed to break or form a chemical bond.
• Free atoms possess more chemical energy than any compound.
• The amount of chemical potential energy is less than the amount of chemical potential energy possessed by the atoms it contains.
• Chemical reactions in living cells may result in the movement of compounds across cell membranes, muscle contractions, or light emission.

Thermodynamics

• Thermodynamics studies energy changes in activities.
• The first and second laws of thermodynamics describe how energy changes occur within systems and surroundings.
• Systems in thermodynamics can be organisms, cells, or sets of substrates and products while surroundings include everything outside the system.
• Biological systems are open systems and exchange matter/energy with their surroundings.

First Law of Thermodynamics

• The first law of thermodynamics, also called the law of conservation of energy, states that energy cannot be created or destroyed but can be transformed or transferred.
• Released energy in chemical reactions transforms into mechanical energy, heat, or other forms while thermal energy leaving a system must enter the surroundings.
• Energy cannot simply appear; chemical energy must be transformed into kinetic energy for movement.

Second Law of Thermodynamics

• The second law of thermodynamics states the universe's disorder (entropy) is continuously increasing where disorder is more likely than order.
• Organisms use matter and energy inputs to reduce randomness and stay alive.
• Energy that keeps organisms alive comes from the Sun, where plants transform light energy into chemical bonds of carbohydrates.

Free Energy

• Free energy represents the net effect of chemical bonding and heat and is the energy available to do work.
• For a molecule within a cell (constant pressure/volume), free energy (G) equals enthalpy (H) minus TS, where S is entropy and T is temperature where the equation is G = H-TS
• Chemical reactions can produce changes in free energy where change in free energy (ΔG) is ΔG = ΔH - TΔS

Endergonic and Exergonic Reactions

• ΔG is used to predict if a chemical reaction is spontaneous.
• Positive ΔG-products has more free energy than reactants where energy is required for the reaction to proceed.
• Endergonic reactions require an energy input or "inward energy."
• Negative ΔG-products has less free energy than reactants where reactions tends to proceed spontaneously.
• Exergonic reactions release excess energy as heat or "outward energy."
• Higher disorder (TΔS) than bond energies differences (ΔH) will proceeds spontaneously.

Thermodynamics and Metabolism

• Reactions that are spontaneous may still need energy to initiate.
• Activation energy destabilizes existing chemical bonds and initiates the reaction.
• Enzymes decrease the activation energy of each reaction.

ATP

• ATP, adenosine triphosphate, links catabolic reactions to anabolic to form the energy currency of the cell.
• ATP is produced by catabolic that forms the major source of energy for anabolic
• ATP hydrolysis releases energy as ATP becomes adenosine diphosphate (ADP) and inorganic phosphate (Pi) by undergoing repulsion between negatively charged phosphate groups.

Coupled Reactions, Electron Carriers

• Cells use ATP to drive endergonic reactions by coupling in exergonic fashion.
• Exergonic reactions supply the energy to synthesize ATP from ADP + Pi, then use the hydrolysis of ATP to provide energy for endergonic actions, to forming a cycle.
• Very few seconds worth of ATP for the cell and continually produces more from ADP + Pi.
• Redox reactions are coupled reactions key for energy flow.
• Compounds accept electrons (reduced) or lose electrons (oxidized) to carry energy with them, called reducing power.

Electron Carriers

• NAD+ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide), are important electron carriers where NAD+ and FAD are the oxidized forms, and NADH and FADH2 are the reduced forms to become reduced.
• Electron carriers pick up electrons from energy-rich compounds, then donate them to low-energy compounds, and get recycled.

Aerobic Respiration

• Cellular respiration includes the catabolic pathways breaking down energy-rich compounds for ATP production.
• Aerobic respiration uses oxygen and is made of multiple reactions in different parts of the cell, forming this overall reaction: C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l) + energy.

Overview of Aerobic Respiration Part 1

• Aerobic respiration is made of reaction pathways, being glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation.
• Glycolysis is not truly aerobic and occurs with/out oxygen, with its products are the starting materials for oxygen required pathways.
• Glycolysis occurs in the cytoplasm where a glucose molecule is broken down into 2 three-carbon compounds, converting 2 NAD+ molecules into 2 NADH.
• 2 ATP molecules are consumed and 4 molecules are created for a net 2 ATP molecules being produced overall.
• Substrate level phosphorylation phosphorylates transferring a phosphate group from a substrate molecule to ADP to form ATP.

Pyruvate

• When if oxygen is available, pyruvate from glycolysis transprots across outer and inner mitochondrial membranes into the mitochondrial matrix.
• The three-carbon pyruvate molecule then undergoes oxidation, producing one molecule of acetyl-coenzyme A with one carbon atom released in the form of carbon dioxide.

Acetyl-CoA and the Krebs Cycle

• Glycolysis produces Acetyl-CoA to enters the Krebs cycle, also known as the citric acid cycle and/or the tricarboxylic acid (TCA) cycle.
• Substrates, products, and enzymes for the krebs cycle are in the mitochondrial matrix where molecules of NADH and FADH2 diffuse to specific locations of of the inner membrane of the the mitochondrion and donate their electrons.
• A series of electron carriers embedded the inner mitochondrial membrane passes along those electrons and releases energy the process to pump hydrogen ions, H+, across the inner mitochondrial membrane.

Oxidative Phosphorylation

• Oxygen is required for oxidative phosphorylation to occur where energy from the flow of electrons from NADH and FADH2 phosphorylates adding Pi to ADP molecules to get ATP.
• The brreakdown of breakdown a molecule of glucose generates a maximum of 38 ATP molecules throughout the whole process.

Glycolysis

• Glycolysis involves the "splitting" of glucose into smaller molecules to convert it's molecules to two molecules of pyruvate.
• Glycolysis converts glucose in which aerobic respiration occurs into two molecules of pyruvate in two recurring steps for each molecule of glucose that enters glycolysis.

Glycolysis Steps

• In steps 1 and 3, two molecules of ATP is used to phosphorylate substrate molecules.
• In step 4, the six-carbon compound, fructose 1,6-bisphosphate, is split into two 3C compounds: dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P).
• In step 5, DHAP is converted into a second G3P molecule. Each of the two G3P molecules then proceeds through steps 6 to 10.
• In step 6, an inorganic phosphate group is added to G3P, and an NAD+ molecule is reduced to form NADH, resulting in 2 molecules of NAD+ to form 2 for each molecule of glucose enetering glycolysis.
• In step 7, ADP is converted to ATP by substrate level phosphorylation where 2 molecules of ADP are converted to 2 for every molecule of glucose entering glycolysis.
• In steps 8/9, the 3C substrate rearranges where ater molecules remove.
• In step 10, another molecule of ADP is converted to ATP by substrate level phosphorylation, resulting in two molecules to produce ATP for every glucose molecule that enters glycolysis.
• To summarize the products of the energy-yielding steps, two ATP are consuemd and and four ATP are produced for a net of 2, and two NAD+ come reduced to form to NADH molecules.

Pyruvate Oxidation

• When Oxygen is available, the pyruvate produced in the cytosol is transported into the mitochondrial matrix where its converted into a two carbon molecule where the reaction remains bound during all steps, called the Krebs Cycle.
• A reaction that converts pyruvate requires a complex that contains multiple copies of three different enzymes.
• Through oxidation, the reduction of NAD+ to produce NADH also occurs where two pyruvate molecules then undergo oxidation for two molecules of acetyl-CoA for linking to the Krebs cycle.

The Krebs Cycle

• The Krebs cycle oxidizes the original carbon atoms from glucose that entered glycolysis, named after Hans Adolf Krebs.
• Two carbon molecules will released from two acetyl CoA molecules during the Kreb cyle.
• Acetyl-CoA "delivers” carbons from glucose to the Krebs cycle by reacting with oxaloacetate to produce a six carbon molecule called citrate, a reaction that forms one ATP molecule.
• Once citrate is formed, the broken reactants yields 4 ATP molecules for an amount of 10 NADH to 2 molecules of FADH2.

Oxidative Phosphorylation, Chemiosmosis

• Glycolysis provides molecules of glucose and every cabon atom gets converted in Carbon Dioxide.
• The majority of the molecules are produced through NADH and FADH2 as reduced electron carriers.
• No oxygen has been used by this point. and Oxygen only reacts at the end of the part that gets converted to water.
• As electrons from NADH pass through the chain, that release of energy used to pump hydrogen ions occurs through only the second and third complex, creating a hydrogen ion gradient across the membrane.
• In chemiosmosis, Hydrogen from the reduced NADH and FADH2 creates gradients to power ATP creation.

Anaerobic Respiration and Fermentation

• Key for organisms that live in low to no oxygen to relese all other their food sources.
• Anaerobic respiration allows anoxic environments to not require oxygen in cells.
• Organisms function well with oxygen to carry various metabolic pathays in.

Fermentation, Yeast, Ethanol

• Many single celled organisms like yeasts and some bacteria only user glycolysis to generate ATP.
• Glycolysis cannot be fast enough in the cells to be delivered.
• Alcohol fermentation produces products for cells to released as a waste product.
• Glucose ferments in the body for many organisms and energy for reactions.