mitochondria and chloroplasts

both energy producing organelles

• Chloroplasts 

  • Fixation of co2 into sugars and other organic molecules 

  • Requires energy (from sunlight) 

  • Splitting of water and release of water  

• Mitochondria 

  • Release co2 

  • Consume oxygen 

mitochondria structure

• 2 different membranes 

  • Outer = porous 

  • Intermembrane proteins 

  • Nutrients and small organic molecules can move freely across 

  • Inner = highly folded 

  • Folds are called cristae 

  • Increases surface area 

• Matrix 

  • Where proteins are present (e.g. enzymes for citric acid cycle) 

chloroplast structure

• 3 membranes 

  • Inner 

  • Outer 

  • Thylakoid 

  • Into stacks of grana 

  • Large areas of membrane 

  • Where ATP synthase sits 

  • Where most photosynthetic machinery is 

  • e.g. photosystems 

mitochondria ATP synthesis

• Anabolic reactions of cells responsible for growth and repair processes 

• Catabolic reactions release energy needed to drive anabolic reactions 

• Must be an efficient linking or coupling of energy yielding to energy requiring processes 

• ATP is most commonly used as this energy intermediate 

• Energy currency of the cell 

• Transfers the energy captured during cellular respiration to the cellular sites that use energy 

• Cleavage of phosphate bonds provides energy 

• Cells obtain most of their energy from membrane bound mechanisms 

  • ATP synthase is found in the mitochondrial inner membrane, the chloroplast thylakoid membrane and the inner membrane of eubacteria 

  • Large multisubunit F-type ATPase made up of an F0 which in integral in the membrane and F1 which is peripheral 

structure and function of ATP synthase

• Large head is attached through a stalk to the transmembrane carrier for protons 

• As protons pass through the carrier it is thought that the stalk spins inducing the head to produce ATP 

• As protons move through ATP synthase, the stalk rotates 

  • Causes conformational change of shape (distorts F1) as gamma stalk is asymmetrical  

  • Provides energy for production of ATP from ADP and Pi 

  • From slides 

    • This rotation drives the conformational transitions of the catalytic subunits which, in turn, alters the nucleotide binding site affinities. As a consequence, conformational energy flows from the catalytic subunit into the bound ADP and Pi to promote their dehydration into ATP. 

more on ATP synthase

• The proton gradient is a form of stored energy 

  • Determines pH 

  • Across mitochondria  

    • Intermembrane space = pH 7 

    • Matrix space = pH 8 

• Can produce around 100 ATP per second 

• Around 3 protons are needed to synthesise 1 molecule of ATP 

• Can be reversed 

  • Use the hydrolysis of ATP to pump protons across membrane in the opposite direction  

• Location in mitochondria and chloroplasts 

how is the proton gradient across the mitochondrial membrane generated

• High energy electrons are passed along an electron transport chain 

  • An electron can bind and release a proton at each step in the chain 

  • When an electron is lost, the affinity for the proton is reduced and it is released 

  • This is oxidation 

• These electron transfers release large amounts of energy which is used to pump H+ across the membrane 

• Creates an electrochemical proton gradient 

NAD in mitochondrial membrane

• NAD + e- + H+ -> NADH 

  • Reduction 

  • Loses electron at NADH dehydrogenase 

    • oxidation 

  • Goes back to krebs  

  • Electron is transferred along electron transport chain 

  • Electron goes to final electron acceptor = water 

mobile electron carriers

• Ubiquinone carries electrons from the NADH dehydrogenase to the cytochrome b-c1 complex 

• Cytochrome c carries electrons from the cytochrome b-c1 complex to the cytochrome oxidase complex 

  • Has haem groups 

chemiosmotic coupling

• The linkage of electron transport, proton pumping and ATP synthesis 

• In mitochondria this is known as oxidative phosphorylation 

  • Consumption of oxygen 

how do electrons move along the electron transport chain?

• By a series of oxidation, reduction reactions 

• As one reactant is oxidised (loses electrons), another is reduced (gains electrons) 

• Reducing agents ranked according to electron transfer potential 

• NADH has high electron transfer potential (-ve value) 

• H₂O has low electron transfer potential (+ve value) 

• Standard redox potential E’₀ (measured in Volts) 

• ΔG₀’ = -nFΔE’₀ 

• Redox potential (electron affinity) increases along the mitochondrial electron transport chain 

  • Important as energy decreases down chain 

the nature of electron carrying groups

• Prosthetic groups are associated with these  

  • Metal associated with them have different redox potentials 

  • Have different electron affinities 

• NADH dehydrogenase 

  • Flavin nucleotides 

  • Fe-S 

• Cytochrome b-c1 complex 

  • Haem 

  • Fe-S 

• Cytochrome c 

  • Haem 

• Cytochrome oxidase complex 

  • Haem 

  • Cu_A 

  • Cu_B 

iron sulfur (Fe-S)

haem

the citric acid cycle

mitochondria structure

• Matrix 

  • Enzymes of citric acid cycle 

  • Mitochondrial DNA 

  • etc 

• Inner membrane 

  • Electron transfer proteins 

  • ATP synthase 

  • Transport proteins 

• Outer membrane 

  • Has large pores 

  • Lipid synthesis 

  • Conversion of lipid substrates into forms that can be metabolised in the matrix 

• Intermembrane space 

  • Several enzymes that use ATP passing out of the matrix phosphorylate other nucleotides 

• Cristae 

agents that interfere with oxidative phosphorylation

• Cyanide and carbon monoxide inhibit cytochrome oxidase 

• Block the passage of electrons to oxygen 

• ATP synthesis grinds to a halt 

uncoupled mitochondria generate heat

• In most newborn mammals including humans a type of adipose tissue (brown fat) uses fuel oxidation to produce heat and not ATP 

• This is achieved by a protein (thermogenin) which provides a path for protons to return to the matrix without passing through the F₀F₁ complex 

• The energy is dissipated as heat 

• Protons flow through mitochondrial membrane but not through ATP synthase 

• chloroplasts use energy from sunlight to fix carbon

features of photophosphorylation

• Unlike NADH, water is a poor donor of electrons 

• Requires energy input in the form of light to create a good electron donor 

how is the proton gradient across the thylakoid membrane generated

• Again electron transfer is coupled to proton pumping 

• Also protons released upon water oxidation contribute to the electrochemical proton gradient 

• Sunlight is absorbed by chlorophyll molecules and electrons interact with photons of light raising them to a higher energy level  

• The energy from hundreds of chlorophyll molecules (in the antenna complex) is channelled into a special pair of chlorophyll molecules in the reaction centre 

photosystems

• Reaction center chlorophyll

  • PS II: Uses P680 (absorbs light best at 680 nm)

  • PS I: Uses P700 (absorbs light best at 700 nm)

• Order in the light reactions

  • PS II: Acts first in the light-dependent reactions

  • PS I: Acts second

• Primary function

  • PS II: Splits water (photolysis) to release oxygen, protons (H⁺), and electrons

  • PS I: Produces NADPH by transferring electrons to NADP⁺

• Electron source

  • PS II: Electrons come from water (H₂O)

  • PS I: Electrons come from PS II via the electron transport chain

• Contribution to ATP formation

  • PS II: Helps create a proton gradient used for ATP synthesis

  • PS I: Does not directly contribute to the proton gradient (in non-cyclic photophosphorylation

the two photosystems work in series

mobile electron carriers

• Plastoquinone (closely resembles ubiquinone of mitochondria) 

• Plastocyanin (a small copper containing protein) 

• Ferredoxin (a small protein containing an iron-sulphur centre) 

redox potentials

light energy is converted to chemical energy

• ATP is generated by the proton gradient across the thylakoid membrane in the same way as in mitochondria 

• H+ generated by the splitting of water also contributes to the proton gradient 

• The high energy electrons are ultimately passed on to form the high energy compound NADPH 

  • ATP and NADPH are used for carbon fixation in the calvin cycle 

  • For every 3 carbon sugar produced 9 molecules of ATP and 6 of NADPH are required 

rubisco

• Ribulose bis phosphate carboxylase 

• Catalyses the initial reaction in carbon fixation 

• It is a sluggish enzyme, processing only about 3 molecules of substrate per second 

• Can make up to 50% of total chloroplast protein 

• Claimed to be the most abundant enzyme on earth 

• Rubisco fixes carbon 

carbon fixation

chloroplasts

• Thylakoid 

  • Photosystems 1 and 2 

  • ATP synthase 

  • NADP reductase 

• Stroma 

  • ATP synthesised 

  • NADPH synthesised 

  • Carbon fixation 

  • DNA 

similarities between processes in mitochondria and chloroplasts

• Both use proton gradients across membranes to produce ATP using ATP synthase 

• Electron transport along an electron transport chain drives proton pump 

• Similarities between some of the components of electron transport chain (cytochrome bc and b6f show sequence similarity and ubiquinone and plastoquinone resemble one another) 

differences between chloroplasts and mitochondria

• Chloroplasts 

  • Low energy electrons come from water but are excited to higher energy by light 

  • Ultimate electron acceptor is NADP+ 

  • Chemical bond energy and reducing power utilised in carbon fixation  

• Mitochondria 

  • High energy electrons come from NADH 

  • Ultimate electron acceptor is oxygen 

  • Chemical bond energy used in cellular processes