Introduction to Mitochondria

Learning Objectives

• Understand the 4 General Phases of Energy Formation in Cells:

  • Glycolysis: A metabolic pathway that converts glucose into pyruvate, yielding a small amount of ATP and NADH. This process occurs in the cytosol and is essential for both aerobic and anaerobic cellular respiration.

  • TCA Cycle (Citric Acid Cycle): Takes place in the mitochondrial matrix, where acetyl CoA is oxidized to produce CO2, ATP, NADH, and FADH2. This cycle plays a crucial role in aerobic respiration, providing high-energy electron carriers for the electron transport chain.

  • Electron Transport Chain (ETC): Located within the inner mitochondrial membrane, this series of proteins transfers electrons derived from NADH and FADH2, creating a proton gradient that drives ATP synthesis.

  • ATP Synthesis: The final stage where ATP synthase utilizes the proton gradient generated by the ETC to convert ADP and inorganic phosphate into ATP, the primary energy currency of the cell.

• Identify Components and Structure of Mitochondria:

  • Mitochondria are double-membraned organelles with specific structures crucial for energy metabolism, including the outer membrane, inner membrane, intermembrane space, and mitochondrial matrix.

• Explain Key Features of Mitochondrial Biology:

  • Mitochondria are involved in multiple cellular processes beyond ATP production, such as regulation of metabolism, programming cell death (apoptosis), and biosynthesis of certain molecules.

• Detail Steps of Glycolysis (2 Stages):

  • Investment Phase: Requires an initial investment of ATP to phosphorylate glucose and its intermediates.

  • Energy Harvesting Phase: Generates ATP and NADH as glucose is broken down into pyruvate.

• Role of Accessory Proteins in Cellular Energetics:

  • Accessory proteins play crucial roles in facilitating enzymatic reactions and maintaining the structural integrity of mitochondrial components.

• Differentiate between Glycolytic and Lipolytic Reactions for Energy Production:

  • Glycolytic reactions pertain to carbohydrate metabolism, while lipolytic reactions involve the breakdown of fatty acids for energy, showcasing metabolic flexibility.

Busy, Busy Cells…

• Cells as Dynamic Environments: Cells continuously synthesize and degrade biomolecules, highlighting the importance of metabolic regulation for maintaining homeostasis.

• Definition of Energy: The capacity to do work, which is essential for all biological processes and cellular activities.

• Key Forms of Energy:

  • Chemical Bonds: Energy stored in the bonds of molecules.

  • Chemical Gradients: Concentration differences across membranes can drive cellular processes.

  • Electrical Gradients: Differences in ion concentrations that impact membrane potentials.

Cellular Metabolism

• Metabolic Pathways: This interplay of reactions is critical for growth, reproduction, and maintaining cellular structure and function. Understanding metabolic pathways allows for insights into various diseases and metabolic disorders.

Cellular Currency: ATP

• Role of ATP: It serves as the energy currency of the cell, enabling work (e.g., biosynthesis, muscle contraction, and active transport).

• ATP Hydrolysis: The hydrolysis of ATP releases approximately -12 kCal/mol of energy, making it an efficient energy source for cellular reactions.

• Coupling Reactions: ATP hydrolysis is coupled to drive energetically unfavorable reactions, allowing cells to perform work efficiently.

• Mechanisms of ATP Production in Eukaryotic Cells:

  • Aerobic Oxidation: Most efficient energy production mechanism involving complete oxidation of glucose in the presence of oxygen.

  • Photosynthesis in Plants: Conversion of light energy to chemical energy stored in glucose.

Energy Production Stages

• Overview of Energy Production in Eukaryotes:

  • Cytosol: Site of glycolysis, where glucose is partially oxidized.

  • Mitochondrion: Sequenced stages from glycolysis to ATP synthesis highlight the complex accountability of each phase in overall energy yield with specific inputs (e.g., glucose, O2) and outputs (e.g., CO2, ATP).

Energetics: Step-Wise Reactions

• Metabolic Efficiency: The step-wise breakdown of substrates through a series of controlled reactions enables cells to maximize energy extraction while minimizing thermal energy loss to the environment.

Chemiosmotic Theory

• Proton Motive Force (PMF): Established by the ETC through the movement of electrons, leading to the translocation of protons across the inner membrane. This gradient is essential for ATP production and reflects the chemiosmotic coupling principle.

• it is generated after the food has been oxidized, involves energy carrier fed into the ETC

  

ATP Production Steps

• Four Key Steps in ATP Production:

  • I: Glycolysis

    • glucose —> pyruvate

  • II: Citric Acid Cycle

    • Pyruvate —> Acetyl CoA —> Oxidized

  • III: Electron Transport Chain

    • Electron —> Proton Motive Force

  • IV: ATP Synthesis

    • Proton Motive Force —> ATP

  • C6H12O6 +6O2 +30Pi2- +30ADP3- +30H+è 6CO2 +30ATP4- +36H2O

Glucose Sources

• Energy Sources in Organisms:

  • Major storage forms:

  • Plants = large, branched molecules called starch

  • Animals = large, branches molecules calles glycogen (liver and muscle)

Glycolysis Overview

• First Step in Cellular Respiration: Glycolysis signals the initiation of energy production, highlighting its importance in both aerobic and anaerobic pathways. Involves movement of electrons across membranes, oxidation of nutrients into CO2 and H2O, and phosphorylation of ADP by Pi into ATP

  • 6 carbon sugar

  • aka Hexose stage

  • 

Glycolysis Stages

• Hexose Stage: Focuses on initial enzyme actions, crucial for preparing glucose for further breakdown. Involves first 4 enzymes.

  • Energy investment: Enzyme 1 (hexokinase), 3 (phosphofructokinase-1)

  • Ends with enzyme 4: Aldolase, breaks down F1,6P into 2,3-carbon sugars

• Triose Stage: Describes energy production mechanisms, where the conversion of NAD+ to NADH marks a vital step in capturing energy.

  • 3 carbon sugar stage

  • enzymes 5 - 10

  • energy harvest begins:

    • NAD+ —> NADH

    • ADP + Pi —> ATP

  • Substrate level phosphorylation (vs oxidative phosphorylation)

Nicotinamide Adenine Dinucleotide (NAD+)

• Importance of NAD+: Serves as an essential electron carrier in the energy conversion processes within cells. Its role in redox reactions showcases its significance in maintaining cellular metabolic balance.

• commonly used temporary energy storage molecule in cells. Part of the energy harvest used in cells.

• has to be loaded and unloaded constantly. Electrons have to go somewhere.

  

Allosteric Regulation

• Regulatory Mechanisms in Glycolysis: Dynamic control of key metabolic enzymes enhances the ability of cells to adapt to changing energy demands and substrates availability.

• Cells have to control their metabolism

• Glycolysis can be sped up or tapped down through allosteric regulation sites on key enzymes.

Fermentation Overview

• Aerobic vs. Anaerobic Metabolism: Explores cellular strategies to generate energy in the absence of oxygen, particularly underscoring the role of pyruvate as a pivotal intermediate in metabolic pathways.

• Obligate aerobes need O2 to live. Faculatitive anaerobes can survive without 02. Pyruvate is the product of glycolysis in aerobically respiring organisms.

• In the absence of oxygen, some organism can still survive but pyruvate is not shunted to the mitochondria

• In yeast pyruvate is a waste product that is processed in two steps:

  • decarboxylation: pyruvate —> acetaldehyde + CO2

  • Reduction: acetaldehyde —> ethanol

• In human muscle lacking O2, they can ferment glucose for quick energy, but pyruvate is reduced into lactic acid and expelled.

• In BOTH, the electron sink is pyruvate rather than O2.

Fermentation Processes**:

• In Yeast: The two-step process showcases organisms' adaptations to anaerobic environments, converting pyruvate into ethanol and CO2.

• In Human Muscles: Describes a critical adaptation during intense exercise when oxygen becomes limiting, illustrating lactate's role in temporary energy production.

Glycolytic Pathway

• Summation of Reactions: Provides a comprehensive overview of the enzymatic steps transforming glucose into pyruvate, emphasizing structural changes and energy yields along the pathway.

Exercise and Lactate Production

• Cori Cycle: This cycle highlights the liver's role in regenerating glucose from lactate produced in muscles, emphasizing metabolic integration across organs.

• Vigorous exercise is notorious for lactate production. Lactate is secreted, taken to the liver. hepatic lactate dehydrogenase converts it to pyruvate and eventually back into glucose. That glucose can return to the bloodstream. —> Cori Cycle

Cellular Energy Harvest

• Energy Harvest:

  • Cytosol:

    • Phase I: ATP and NADH

  • Mitochondria:

    • Phase II: NADH, FADH2, GTP

    • Phase III: Proton Motive Force

    • Phase IV: ATP Synthase

Mitochondria: The Powerhouse of the Cell

• Double membrane organelles in cells

• stereotypical mitochondria in a cell - the size of an E. coli bacteria

Structure and Components of Mitochondria

• Outer Membrane = interacts with the cytosolic contents

• Inter membrane Space = thin space in between the membranes

• Inner Membrane = protein rich membrane, houses most of the ETC members

• Matrix = protein dense space, TCA members, PDH complex, etc.

• TEM Structure

TEM Structure

Cristae = inner membrane invaginations, protein-rich, housing ETC members, cristae increase the surface area of the inner membrane

• Cristae junction = specialized curvature region, curve because of the MICOS (mitochondrial contact site and cristae organizing system)

Mitochondrial Cristae

• In a 3D scan of a mitochondrium you see that there are many infoldings

• Mitochondria vary wildly in:

  • Size

  • Number of cristae

  • Number of copies of mtDNA

• Cristae surface area in liver cells = ~5x the outer membrane

• Cristae surface area in muscle cells = ~3x that of liver cells

• The more energy needed, the more cristae

Endosymbiosis Theory

• An early eukaryote encountered a bacteria and formed a symbiotic relationship

• it happened again in plants with cyanobacteria

Mitochondrial Biology

• Mitochondria have their own DNA, mtDNA that is covalent closed circular (ccc)

  • most genes were lost or transferred to the nucleus

  • mtDNA varies between species

• mitochondrial DNA encodes ETC members, mitochondrial ribosomal RNA, and mitochondrial tRNAs

• can and do make some of their own proteins, through their codon code can different from the host.

• mtDNA analysis shows the genes are more closely related to alphaproteobacterio than to human genes. Chloroplast DNA analysis shows the genes are more closely related to cyanobacteria than to the plants genes

• mtDNA is inherited maternally. Ofcourse there are exceptions:

  • typical human celll has 1000-2000 copied of mtDNA

  • human egg has 500,000 copies

  • human sperm cell has 100 copies

• mtDNA can be traced by maternal images, using the Y chromosome we can trace paternal lineages.

Mitochondrial Behavior

• Mitochondria behave independent of the host/parental cell.

• they engage in fusion and fission independently. they are extremely dynamic organelles

• Fusion/Fission:

  • mitochondria actively come together and break apart.

  • Important factors IDed including:

    • Mitofusion ½ (MFN1/2)

    • Optic Atrophy 1 (OPA1)

    • Dynamin-related protein 1 (DRP1)

      

Mitochondrial Health & Interactions:

• When cells are stressed their mitochondria will fragment and they will lose their networks. When cells are healthy their mitochondria will fuse and form networks.

• Mitochondria interact with other organelles:

  • ER, lysosomes, peroxisomes, plasma membrane, golgi endosomes, and lipid droplets

• ER contacts help regulate Ca2+ flux

  • more Ca2+ production = more energy production

• ER contains mitochondria associated membranes (MAM)