CELLULAR ENERGETICS 2

Cellular Energetics

Overview of Cellular Energetics

• All cells require energy for their functions.

• Energy is stored in organic molecules, specifically carbohydrates, lipids, and proteins.

Autotrophs and Heterotrophs

• Autotrophs/Producers:

  • Organisms that produce their own carbohydrates via photosynthesis.

• Heterotrophs/Consumers:

  • Organisms that consume carbohydrates produced by autotrophs.

• Both groups break down carbohydrates to generate ATP, the primary energy molecule of cells.

Energy Processes

• Energy Influx:

  • The process of energy entering the organism is termed endergonic.

  • This involves anabolic reactions, where larger molecules are formed.

  • Example reaction:
    $ext{Light} + 6H<em>2O + 6CO</em>2 → C<em>6H</em>{12}O<em>6 + 6O</em>2$

• Energy Efflux:

  • The process of energy exiting the organism is termed exergonic.

  • This involves catabolic reactions, where large molecules are broken down.

  • Example reaction:
    $C<em>6H</em>{12}O<em>6 + 6O</em>2 → 6H<em>2O + 6CO</em>2 + ext{ATP} (+ ext{heat})$

Generating Heat

• One of the outputs of cellular respiration is heat.

Types of Organisms Based on Heat Generation

• Endotherms:

  • Generate heat mainly from within.

  • Must consume large amounts of food to maintain high metabolic rates.

• Ectotherms:

  • Generate heat from external sources.

  • Exhibit much lower metabolic rates.

• Homeotherms:

  • Organisms that maintain steady internal body temperatures.

• Poikilotherms:

  • Organisms with body temperatures that fluctuate widely with environmental temperatures.

• Example: Brown bears are categorized as endothermic poikilotherms; they generate substantial body heat but experience a drastic drop in temperature during hibernation.

Energy Utilization in Cells

• The breakdown of molecules involves the transfer of electrons from one molecule to another.

• As electrons move, they transport energy, either:

  • Stored in other molecular bonds,

  • Released as heat,

  • Harvested to produce ATP.

• Key Mechanism: Electrons travel as part of hydrogen atoms; thus, moving hydrogen effectively equates to moving electrons.

Oxidation and Reduction in Cellular Energetics

Definitions

• Oxidation:

  • Involves the addition of oxygen or removal of hydrogen.

  • Represents a loss of electrons, leading to energy release (exergonic process).

• Reduction:

  • Involves the removal of oxygen or addition of hydrogen.

  • Represents a gain in electrons, allowing energy storage (endergonic process).

• Example: In cellular respiration, glucose undergoes oxidation by losing electrons, while oxygen undergoes reduction by gaining electrons.

Electron Carriers in Cellular Respiration

• Importance: Electron carriers ferry electrons by shuttling hydrogen atoms across various pathways.

• Key Carriers:

  • NAD+ is reduced to NADH.

  • FAD2+ is reduced to FADH2.

Role in ATP Generation

• Electron carriers assist in creating an H+ gradient.

• As electrons traverse this gradient through the ATP synthase enzyme, ATP is synthesized:

  • A phosphate group is added to ADP, facilitating energy storage in ATP.

Adenosine Triphosphate (ATP)

Structure and Function

• ATP consists of an adenosine molecule bonded to three phosphate groups.

• The bond between the second and third phosphates is classified as high-energy.

• When this bond is cleaved, energy is released, forming ADP.

• The released phosphate can subsequently attach to another molecule, transferring energy through a mechanism known as phosphorylation.

  • This mechanism can be likened to the passing of a baton in a relay race.

  • For instance, in the sodium-potassium pump studied in Unit 2, phosphorylation occurs, resulting in shape changes that facilitate sodium transportation out of the cell.

Cellular Respiration Overview

• Structure of Mitochondria:

  • Mitochondria are double-membraned organelles involved in energy harvesting.

  • Outer Membrane: Smooth surface.

  • Inner Membrane: Highly folded (contains cristae), increasing surface area-to-volume ratio.

  • Intermembrane Space: Fluid-filled region between membranes.

  • Matrix: Inner fluid-filled area containing DNA, ribosomes, and enzymes (both free and membrane-bound).

• Mitochondrial Distribution:

  • Cells demanding high energy levels typically possess abundant mitochondria.

Pathways of Cellular Respiration

• Description: A catabolic pathway wherein glucose is broken down through multiple reactions with electrons shuttle by NADH along the Electron Transport Chain (ETC).

• Four Main Stages:

  1. Glycolysis

  2. Pyruvate Oxidation

  3. Krebs/Citric Acid Cycle

  4. Electron Transport Chain

• Oxygen Requirement: Stages 2, 3, and 4 necessitate the presence of oxygen.

Glycolysis

Occurrence and Principles

• Location: Cytoplasm (outside the mitochondria).

• Often referred to as “sugar splitting.”

• Anaerobic and Aerobic: Can occur with or without oxygen.

• Origin: Believed to stem from ancient metabolic processes (early prokaryotes) when oxygen was scarce.

• Molecular Breakdown: Partially oxidizes glucose ($C<em>6H</em>{12}O<em>6$) into two pyruvate molecules ($C</em>3H<em>6O</em>3$).

  • Net Gain: 2 ATP and 2 NADH are produced.

  • Two ATP investment: The process begins with an investment of 2 ATP to initiate conversion to pyruvate, accumulating to a net gain of 2 ATP (4 ATP produced - 2 ATP spent).

  • Other products include 2 H2O.

Detailed Steps of Glycolysis

Energy Investment Phase

• Glycolysis begins by investing ATP.

• Glucose is phosphorylated, rearranged, and split into two glyceraldehyde-3-phosphate (G3P) molecules (endergonic).

Energy Payoff Phase

• Here, ATP and NADH are harvested as G3P donates hydrogen to NAD+ (which is reduced to form NADH), leading to the pyruvate formation (exergonic).

• An intermediate molecule called phosphoenolpyruvate (PEP) donates a phosphate to ADP, yielding ATP via substrate-level phosphorylation.

Glycolysis and Fermentation

Fermentation Mechanisms

• Function: Fermentation sustains glycolysis by regenerating NAD+.

• Location: Occurs in the cytosol, requiring no oxygen (anaerobic).

• Products: Results in either ethanol and CO2 or lactate.

• Outputs: Yield ultimately 2 ATP from glycolysis.

• Performed by organisms categorized as obligate anaerobes or facultative anaerobes.

Types of Fermentation

Lactic Acid Fermentation

• Conversion of pyruvate to lactate.

• Found in fungi, certain bacteria, and human muscle cells.

• Useful in producing cheese, yogurt, and other products.

• Note: Lactate accumulation does NOT cause muscle fatigue; this remains a misconception.

• Under oxygen availability, lactate converts back to pyruvate in the liver, regenerating NAD+ to enable glycolysis restarting, which retains a net of 2 ATP.

Alcohol Fermentation

• Conversion of pyruvate to ethanol and CO2.

• Often conducted by bacteria and yeast, useful in brewing, winemaking, and baking.

• Over time, ethanol produced can kill the yeast and bacteria.

• NAD+ is reformed allowing glycolysis to recommence, maintaining a net gain of 2 ATP.

Aerobic Respiration

Pyruvate Oxidation

• When oxygen is available, pyruvate migrates into mitochondrial matrix.

• Converts to Acetyl CoA:

  • Process generates CO2 and NADH, and emits a 2-carbon sugar (two cycles for two pyruvates).

  • During the oxidation process, a carboxyl group is liberated from pyruvate, yielding CO2.

  • Coenzyme A (CoA) catalyzes the conversion of acetic acid to Acetyl CoA.

Krebs/Citric Acid Cycle

• Location: Takes place in the mitochondrial matrix.

• Involves the transformation of Acetyl CoA into citrate, which undergoes a series of reactions producing CO2, NADH, and FADH2.

• Net Yield:

  • 2 ATP (via substrate-level phosphorylation)

  • 6 NADH, 2 FADH2 (as electron carriers).

Significance of the Krebs Cycle

• Demonstrates the complete oxidation of glucose, represented as:
  $C<em>6H</em>{12}O<em>6 → CO</em>2$

• Results include a modest quantity of ATP through substrate-level phosphorylation and abundant electron carriers (NADH, FADH2).

• Notably, the process occurs twice due to the presence of two Acetyl CoAs.

Cycle Mechanism

• The Krebs cycle is cyclical because:

  • It commences with Acetyl CoA merging with oxaloacetate to produce citrate.

  • During the cycle, citrate is metabolized to regenerate oxaloacetate, allowing for another Acetyl CoA to enter.

Electron Transport Chain (ETC)

Overview

• Functionality occurs along the cristae within the inner mitochondrial membrane.

• Produces between 26-28 ATP through the creation of an H+ gradient.

• Specifically, hydrogen ions (H+) are translocated across the inner mitochondrial membrane.

• The end product results when H+ diffuse through ATP synthase, with oxygen serving as the final electron acceptor to form water.

• The extensive folding of the inner membrane enhances available space for enzymatic and reaction activities.

Building the Proton Gradient

• Electrons released from NADH and FADH2 are stripped of hydrogen.

• Chemical reaction breakdown:

  • $H → e^- + H^+$

• Electrons transition among electron carriers within the membrane.

• Protein pumps utilize energy to propel protons into the intermembrane space.

• The precursor materials include:

  • 10 NADH (4 from glycolysis, 6 from Krebs cycle).

  • 2 FADH2 (from Krebs cycle).

• The relationship between hydrogen ion concentrations and pH levels is established:

  • High H+ correlates with low pH (acidic condition).

  • Low H+ correlates with high pH (basic condition).

Electron Transport Dynamics

• Electrons stripped from NADH and FADH2 are passed to a series of electronegative carriers in the intermembrane space:

• Each succeeding carrier possesses greater electronegativity, drawing electrons more intently.

• Each electron transit equates to an oxidation step that releases energy (exergonic).

• This heat generation is especially essential for endothermic organisms.

• Oxygen acts as the final acceptor, culminating in water formation.

ATP Synthase Process

• H+ gradient formation instigates the diffusion through ATP synthase (facilitated diffusion due to concentration gradients).

• Mechanism is known as chemiosmosis; the ion diffusion across membranes.

• H+ flow prompts rotational movement within ATP synthase, activating the enzyme at its active site to join ADP and inorganic phosphate (P), yielding ATP through a process termed oxidative phosphorylation.

  • High H+ concentrations in the intermembrane space lead to a corresponding drop in concentration in the mitochondrial matrix, where ADP interacts with P to synthesize ATP.

Types of Fuel for Cellular Respiration

• Carbohydrates, fats, and proteins can serve as energy sources for cellular respiration.

• Monomer units can enter glycolysis or the citric acid cycle at varying points based on the type of fuel.

Photosynthesis Overview

General Function

• Photosynthesis transforms light energy into the chemical energy of food.

• Producers:

  • Photoautotrophs: Utilize light energy to fabricate organic molecules.

  • Chemoautotrophs: Create organic molecules using chemicals present in the environment.

• Consumers:

  • Heterotrophs derive energy by consuming organic molecules from other organisms.

Anatomy of Leaves and Chloroplasts

• Mesophyll: The central tissue in leaves where chloroplasts are predominantly located.

• Stomata: Pores in the leaf allowing CO2 intake and O2 expulsion.

• Thylakoids: Disc-shaped structures harboring chlorophyll, the site of the initial phase of photosynthesis.

• Grana: Thylakoid stacks that enhance surface area for absorption.

• Stroma: The fluid encasing thylakoids where subsequent reactions of photosynthesis take place, also housing ribosomes and chloroplast DNA.

Photosynthesis Equation

• Reaction format:
  $6CO<em>2 + 6H</em>2O + ext{Light Energy} → C<em>6H</em>{12}O<em>6 + 6O</em>2$

• It demonstrates a redox reaction, wherein water is dissociated, and electrons along with hydrogen ions (H+) are transferred to CO2, forming glucose.

• Key principle: OILRIG (Oxidation Is Loss, Reduction Is Gain).

• Reactants and Products:

  • Reactants: $6 ext{ CO}<em>2, 6 ext{ H}</em>2 ext{O}$

  • Products: $C<em>6H</em>{12}O<em>6, 6 ext{ O}</em>2$

• Endergonic Process: Requires energy input to commence.

Phases of Photosynthesis

1. Light Reactions:

   • Light energy is captured and transformed into ATP and NADPH, utilizing electrons sourced from H2O.

2. Calvin Cycle (Light-Independent Reactions):

   • Employs generated ATP to fuel chemical reactions, interconverting CO2 and the hydrogen from H2O to synthesize glucose.

Light Reactions Process Overview

• Chemical Reaction:
  $H<em>2O + ext{Light Energy} → ext{ATP} + ext{NADPH} + O</em>2$

Photosystem Functionality

Photosystems in Light Reactions

• Photosystem: A complex comprising pigment molecules intertwined with proteins and a primary electron acceptor.

  • Chlorophyll a: Absorbs blue-green light; pivotal in the light reaction for converting light into energy.

  • Chlorophyll b: Accompanies chlorophyll a, directing energy toward it.

  • Carotenoids: Absorb yellow and orange light, serving a protective function and broadening the absorption spectrum.

• Light absorption elevates electrons in the pigment molecules, necessitating two distinct photosystems:

  • Photosystem II (P680): Absorbs optimally at around 680 nm; initiates the light reaction and synthesizes ATP.

  • Photosystem I (P700): Absorbs best at 700 nm; generates NADPH.

Comparison: ETC in Photosynthesis vs. Cellular Respiration

• Similarities:

  • Both utilize excited electrons for vigorous hydrogen transportation.

  • Each process generates a proton gradient that powers ATP synthase.

• Differences:

  • Source of excited electrons varies (light photons in photosynthesis vs. NADH in cellular respiration).

  • Resulting products differ (ATP & NADPH from photosynthesis against NAD+, FAD, H2O, & ATP from respiration).

Photophosphorylation Mechanisms

1. Non-Cyclic Photophosphorylation:

   • Ideal scenario with electrons elevated in two steps at PSII and PSI:

     • PSII generates ATP.

     • PSI offers reducing power forming NADPH.

2. Cyclic Photophosphorylation:

   • Occurs when required ATP exceeds what was produced in light reactions; electrons cycle back to the ETC, resulting in additional ATP formation.

   • No NADPH is synthesized in this process.

Calvin Cycle Overview

• Equation: $ext{CO}<em>2 + ext{ATP} + ext{NADPH} → C</em>6H<em>{12}O</em>6 + ext{ADP} + ext{NADP}$

  • Takes place in the stroma of chloroplasts.

  • Functions as sugar-building reactions, using chemical energy to transform low-energy CO2 into high-energy glucose.

Phases of the Calvin Cycle

1. Carbon Fixation:

   • CO2 is fixed into a carbohydrate facilitated by the enzyme Rubisco.

2. Reduction:

   • ATP and NADPH contribute to the production of G3P (glyceraldehyde-3-phosphate), a 3-carbon sugar.

3. Regeneration:

   • G3P is utilized to regenerate RuBP which allows for continuous cycle operation.

   • The cycle must occur six times to synthesize one molecule of glucose.

Importance of G3P (Glyceraldehyde-3-Phosphate)

• Acts as the end product of the Calvin cycle, being a high-energy 3-carbon molecule.

• Acts as a precursor, allowing transformation into:

  • Glucose,

  • Carbohydrates,

  • Lipids,

  • Amino acids,

  • Nucleic acids.

C3, C4, and CAM Photosynthesis

C3 Photosynthesis

• The prevalent form of photosynthesis across 75% of plants, where the end product is a 3-carbon molecular structure (G3P).

• Plants close stomata on hot and dry days, limiting CO2 intake, which can cause Rubisco to bind with oxygen instead (photorespiration), leading to decreased sugar production.

C4 Photosynthesis

• Adaptation in plants to circumvent photorespiration by producing a 4-carbon molecule (e.g., corn, sugarcane).

• Operates efficiently in hot conditions as it enables CO2 fixation prior to entering the Calvin cycle, reducing water loss while maximizing sugar output.

CAM (Crassulacean Acid Metabolism)

• Adaptation in specific plants (like cacti) to optimize water usage:

  • Night: Stomata open, allowing CO2 to enter and be converted into organic acids stored in mesophyll cells.

  • Day: Stomata close, ATP and NADPH from light reactions allow CO2 to be released from stored organic acids to power the Calvin cycle.

  • This mechanism offers clear benefits in arid climates.

Conclusion

• Understanding cellular energetics, encompassing cellular respiration and photosynthesis, is crucial in appreciating the biochemical processes that sustain life. This complex web of reactions illustrates the intricate balance between energy acquisition, transformation, and use in biological systems.