Lesson 10: Mitochondria + Chloroplasts

oxidative phosphorylation

• main way to produce ATP

• Need membrane-bound compartments  

• Takes place in the mitochondria in eukaryotic cells → depends on electron-transport process that drives the transport of protons (H+) across the inner mitochondrial membrane

• Mitochondria: electron-transport process (transport of H+ across inner mitochondrial membrane) 

• Chloroplasts: light-dependent reactions (transport of H+ across thylakoid membrane) 

membrane-based mechanisms to produce ATP

1. electron transport chain

2. ATP synthase

1. Set up electrochemical H+ gradient → energy released by electron transport is used to pump protons across the membrane

2. Use the gradient to generate ATP → energy stored in the protein gradient is harnessed by ATP synthetase to make ATP

Electron transport chain

A series of membrane-embedded electron carrier molecules that facilitate the movement of electrons from a higher to a lower energy level, as in oxidative phosphorylation and photosynthesis

• Electron transfers release energy that is used to pump protons (derived from water) across the membrane 

  • Generates an electrochemical proton gradient → ion gradient is a form of stored energy and can be harnessed to do work 

  • Can help ions flow back across their membrane down their electrochemical gradient 

ATP Synthase

a membrane-embedded protein complex that catalyzes the energy 

• Requires synthesis of ATP from ADP and phosphatase 

• Couples the movement of protons across the membrane to the production of ATP

chemiosmotic coupling mechanism

Mechanism that uses the energy stored in a transmembrane proton gradient to drive an energy-requiring process

• Ex: the synthesis of ATP by ATP synthase or the transport of a molecule across a membrane

• Cells can harness the energy of electron transfers

mitochondria

Membrane-enclosed organelle, about the size of a bacterium, that carries out oxidative phosphorylation and produces most of the ATP in eukaryotic cells

• primary function is the production of ATP

•  Can change their shape, location and number to suit a cell’s needs (highly mobile and plastic) 

  • Located where they are most needed in specialized cells (greatest demand for energy/ATP)

• Cylindrical shape (similar to bacteria)

• Carry out part of fatty acid metabolism through a process called beta-oxidation and act as calcium storage sites

mitochondria function

• Production of ATP

• Regeneration of NAD+

• Provision of precursors for biosynthesis of amino acids, nucleotides and fatty acids

• Participation in synthesis of heme and iron-sulffur clusters 

• Cell signaling

• Generation of reactive oxygen species

• Regulation of apoptosis

outer mitochondrial membrane (OMM)

contains large channel-forming proteins called porins 

• form water channels through the lipid bilayer

not selective

intermembrane space

contains several enzymes that use the ATP passing out of the matrix to phosphorylate other nucleotides 

• Contains proteins that are released during apoptosis

inner membrane (IMM)

has folds called mitochondrial cristae (a way to enormously increase the surface area to accommodate the proteins of the respiratory chain and the ATPases)

• Highly impermeable to the passage of ions and small molecules (very selective)

• High proportion of proteins: transporters, respiratory chain, ATP synthase, etc.  

• Folded into numerous cristae 

• Contains proteins that carry out oxidative phosphorylation (ex: electron-transport chain and ATP synthase)

mitochondrial matrix 

space contains a highly concentrated mixture of hundreds of enzymes (including those required for oxidation of pyruvate and fatty acids + for the citric acid cycle)

cellular respiration

1. glycolysis

2. pyruvate oxidation

3. krebs cycle/citric acid cycle

4. oxidative phosphorylation/respiratory chain 

• C6H12O6 + 6O2 → 6CO2 + 6H2O + 30-32 ATP + Heat

• Oxygen → carbon dioxide + water + energy 

glycolysis

• Occurs outside cytosol 

• Breaking of glucose into 2 molecules of 3C

• End product is pyruvate (intermediate molecule) 

• Breaking bonds releases energy → produces 2 molecules of ATP

Products

• 2 NADH

• 2 ATP

pyruvate oxidation

• mitochondrial matrix

• Produces Acetyl CoA → can enter next cycle 

• Releases CO2

• Point of no return (CO2 as a gas diffuses) 

products

• 2 NADH

krebs cycle

• mitochondrial matrix  

• Charge energetic carriers (ATP and NADH)

• Acetyl CoA enters cycle through glycolysis (sugar way) or through oxidizing fatty acids 

• Releases CO2

• 2 rounds

products

• 6 NADH

• 2 FADH2

• 2 ATP

• 2 CO2

oxidative phosphorylation

• Has all energy as ATP and NADH (carriers → donate electrons to pathway)

• NADH gives electrons to respiratory chain

• Electron movement allows pumping of protons which allows ATP synthesis due to ATP synthase

• Form water from “extra” reactive electrons → allows us to breathe 

• Coupled w/ATP synthase → formation of a large amount of ATP (30-32)

product

• 30-32 ATP

• NADH + FADH2 + O2 produces water (H2O)

anaerobic conditions (w/out oxygen)

• stops after glycolysis → fermentation → lactate acid 

  • BUT only provides 2 ATP 

Cyanobacterias

• origin of O2 actual atmosphere 

chloroplasts

Thylakoid membrane

• light-capturing systems, electron-transport chain and ATP synthase 

carbon fixation

process by which activated carriers (NADPH, ATP) can be used to convert CO2 into sugar inside a chloroplast

• Cells can store energy through high energy bonds, carriers (ATP, NADPH)

• Want to keep this energy → in high energy bonds

• Once you have bonds/energy, you can use it to form an organic molecule 

  • CO2 + H2O → C6H12O6

photosynthesis

Light energy + CO2 + H2O → sugars + O2 + heat energy

light-dependent reactions

• Thylakoid Membrane

• 2 photosystems

Steps

Photosystem II → plastoquinone (intermediate carrier) → hydrogen pump complex/cytochrome b6f → intermediate carrier → photosystem I→ intermediate carrier ferredoxin

light-dependent reactions: photosystem II

• Antenna complex (light-harvesting complex): contains chlorophyll a, chlorophyll b and carotenoid pigments that capture light energy 

• Reaction center (P680): special pair of chlorophyll a molecules absorb light at 680 nm

• Water-splitting complex (oxygen-evolving complex): contains manganese cluster that splits water (2H2O → O2 + 4H+ + 4e-)

• Primary electron acceptor: pheophytin

• Pump is 50% efficient

• Chlorophyll’s ability to harness energy derived from sunlight stems from its unique structure 

• Appropriate wavelength light → excited electron

light-dependent reactions: electron transport chain

• Plastoquinone (PQ): mobile electron carrier

• Cytochrome b6f complex: pumps protons into thylakoid lumen

light-dependent reactions: photosystem I

• Antenna complex: light-harvesting pigments

• Reaction center (P700): chlorophyll a pair that absorbs at 700 nm

• Primary electron acceptor: chlorophyll a molecule (Ao)

• Ferredoxin (Fd): iron-sulfur protein that receives electrons 

NADP+ Reductase

• Enzyme complex: reduces NADP+ to NADPH using electrons from ferredoxin

• Reaction: NADP+ + 2e- + h+ → NADPH

ATP Synthase (CF0CF1 complex)

• Uses the proton gradient across the thylakoid membrane to synthesize ATP

• Similar to mitochondrial ATP synthase but located in chloroplasts

light-independent reactions

• Calvin cycle in storm 

Key enzymes 

1. RuBisCO: most abundant enzyme on Earth, fixes CO2 to RuBP

2. Phosphoglycerate kinase: uses ATP

3. Glyceraldehyde-3-phosphate dehydrogenase: uses NADPH

4. Other enzymes: regenerate RuBP to continue the cycle

Steps

• Carbon fixation uses energy from carriers to → 2nd stage of photosynthesis

• CO2 + intermediate is catalyzed by rubisco to form 2 x 3C (3 PGAs) → can enter calvin cycle to produce glucose (final molecule)

• One PGA exits the cycle

• CO2 from the atmosphere enters the strom

• Enzyme RuBisCO catalyzes the attachment of CO2 to a 5-carbon sugar called RuBP

• This creates an unstable 6-carbon intermediate that immediately splits into 2 molecules of 3-phosphoglycerate (3-PGA) which are 3 carbon compounds