Structure and function of Mitochondria

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Redox reaction

A reaction involving electron transfer where one molecule loses electrons (oxidized) and another gains electrons (reduced)

Oxidation

Loss of electrons, often loss of H or gain of O

Reduction

Gain of electrons, often gain of H

High-energy electrons

Electrons far from the nucleus, carrying high potential energy

Redox potential

A molecule’s ability to pull electrons

higher potential = stronger pull toward electrons

NADH redox potential

−320 mV meaning it pushes electrons away

Oxygen redox potential

+800 mV meaning O2 strongly pulls electrons

Glycolysis

Splitting of glucose into 2 pyruvate molecules occurs in the cytosol; no O2 required

Glycolysis net products

2 pyruvate, 2 ATP net, 2 NADH

Is glycolysis aerobic or anaerobic?

Anaerobic because it does not require O2

ATP production in glycolysis

Through substrate-level phosphorylation only

NADH fate from glycolysis (aerobic)

Electrons are shuttled into mitochondria for the ETC

NADH fate from glycolysis (anaerobic)

Used in fermentation to regenerate NAD+

Pyruvate fate with O2

Moves into mitochondrial matrix → PDH → acetyl-CoA

Pyruvate fate without O2

Undergoes fermentation to regenerate NAD+

Fermentation

Process that regenerates NAD+ by dumping electrons onto pyruvate, occurs without O2

Purpose of fermentation

Allow glycolysis to continue by regenerating NAD+

Products of fermentation (animal cells)

Lactic acid

Products of fermentation (yeast)

Ethanol + CO2

Why fermentation is inefficient

Only yields the 2 ATP from glycolysis (≈12.5% of full aerobic ATP)

Where pyruvate is oxidized

Mitochondrial matrix

Enzyme responsible

Pyruvate dehydrogenase complex (PDH)

Products of pyruvate oxidation

1 acetyl-CoA, 1 NADH, 1 CO2 per pyruvate

Purpose of PDH

Link glycolysis to Krebs cycle by producing acetyl-CoA

Acetyl-CoA from carbs

From pyruvate after glycolysis

Acetyl-CoA from fats

From β-oxidation which chops lipids into 2-carbon units, each → acetyl-CoA + NADH + FADH2

Where Krebs cycle occurs

Mitochondrial matrix

Input to Krebs cycle

Acetyl-CoA

CO2 released per acetyl-CoA

2 CO2 per turn

ATP made per turn

1 GTP converted to ATP

NADH per turn

3 NADH

FADH2 per turn

1 FADH2

Total per glucose (two turns)

6 NADH, 2 FADH2, 2 ATP, 4 CO2

Purpose of Krebs cycle

Extract high-energy electrons to NADH and FADH2

ETC location

Cristae membrane of mitochondria

ETC components

Complex I, II, III, IV, CoQ10, cytochrome C

Electron path (NADH)

Complex I → CoQ10 → Complex III → cytochrome C → Complex IV → O2

Electron path (FADH2)

Complex II → CoQ10 → Complex III → cytochrome C → Complex IV → O2

Protons pumped per NADH

10 H+

Protons pumped per FADH2

6 H+

Final electron acceptor

O2 which becomes H2O

Main outcome of ETC

Creates proton gradient (electrochemical gradient)

Redox centers

Iron-sulfur clusters + heme groups arranged in increasing redox potential

Chemiosmosis

Using the proton gradient to power ATP synthase

Chemiosmotic coupling

Two-stage process:

ETC pumps H+ into cristae lumen

H+ flows back through ATP synthase to make ATP

Why maintaining H+ gradient matters

Without it the ATP synthase turbine cannot spin → no ATP

ATP synthase

Molecular turbine in cristae membrane that uses H+ flow to convert ADP + Pi → ATP

Energy conversion in ATP synthase

Electrical → mechanical → chemical

Why ATP synthase is “reverse ion channel”

Ions flow through it passively but the flow drives work (spinning turbine)

ATP yield per glucose from Stage 2

28 ATP

Total ATP per glucose

32 ATP

Breakdown

4 ATP from Stage 1 + 28 ATP from Stage 2

Cristae

Folded inner membrane containing ETC + ATP synthase

Matrix

Fluid interior where PDH and Krebs cycle occur

Intermembrane space / cristae lumen

Space where protons accumulate during ETC

Mitochondrial genome role

Encodes subunits containing most redox centers in ETC

Anaerobic cellular respiration

Uses ETC but final electron acceptor is NOT O2, less efficient

Aerobic respiration

Uses O2 as final electron acceptor → most ATP produced

Fermentation vs anaerobic respiration

Fermentation has NO ETC, anaerobic respiration DOES have ETC

Energy storage forms

Glycogen (animals), starch (plants), triglycerides (fats)

Reason fats store more energy

Fatty acids generate lots of acetyl-CoA and electron carriers

Where ETC occurs in bacteria

Plasma membrane

Where ETC occurs in plant cells

Mitochondria (respiration) AND chloroplast thylakoid membrane (photosynthesis)

Where ETC occurs in animals

Mitochondria only

Similarity

All build proton gradients and make ATP using ATP synthase

Difference

Chloroplast ETC uses light → produces NADPH + ATP/ mitochondrial ETC uses electrons from food