Cellular Energy• In order to sustain life (steady state), cells constantly

expend energy in the form of ATP hydrolysis – the hydrolysis of ATP yields a molecule of ADP

(adenosine diphosphate) and a Phosphate group• ATP → ADP + P

• Following ATP hydrolysis, the cell synthesizes new molecules of ATP using ADP and P– ADP + P → ATP

– “recycling energy”– cells use the energy stored in the covalent bonds of

monosaccharides, fatty acids and amino acids as an energy source (fuel) to resynthesize ATP

Fuels for ATP Synthesis

Cellular Respiration

• Most cells of the body use glucose (in the presence of O2 that we inhale) as fuel source to synthesize molecules of ATP in a process called cellular respiration

• The products include molecules of carbon dioxide that we exhale, and water

• C6H12O6 + 6O2 + 38 ADP +38 P 6H2O + 6CO2 + 38 ATP

– note that the number of atoms in the reactants equal the number of atoms in the products

Cellular Respiration

Cellular Respiration

• The process of cellular respiration is divided into 3 sequential phases– Glycolysis

• occurs in the cytoplasm of a cell– Krebs cycle (Citric acid cycle)

• occurs in the mitochondrial matrix– The electron transport chain

• occurs across the inner mitochondrial membrane–between the mitochondrial matrix and the

intermembrane space

Cellular Respiration Overview

Cellular Respiration• During the phases of glycolysis and the Krebs cycle, a

single molecule of glucose is gradually broken apart in 20 sequential biochemical reactions into different carbohydrate intermediates – only 4 molecules of ATP are synthesized during

these phases by the energy released from the breaking of covalent bonds of the carbohydrate intermediates

• As the glucose molecule is broken apart, the 12 hydrogen atoms are gradually removed and used to create a large H+ gradient between the intermembrane space and the matrix of the mitochondria – the H+ gradient is used as an energy source by the

electron transport chain to synthesize the remaining 34 molecules of ATP

Glycolysis Overview

• Glycolysis = sugar breaking• The first 7 biochemical reactions of cellular respiration

are catalyzed by 7 different enzymes in the cytoplasm• Starts with 1 molecule of glucose (6 carbons) and

ends with 2 molecules of pyruvic acid (3 carbons x 2 = 6 carbons)

Glycolysis• During glycolysis:

– Energy is invested as 2 ATPs are hydrolyzed during steps 1 and 3

– One 6 carbon carbohydrate intermediate is split (lysis) into two 3 carbon carbohydrate intermediates during step 4

– each of the 3 carbon carbohydrate intermediates molecules lose a H (becoming oxidized) which is accepted by a coenzyme called NAD+ (becoming reduced to NADH) during step 5• NADH carries H to the mitochondrial matrix to

build the H+ gradient– 4 molecules of ATP are synthesized

• 2 during step 6• 2 during step 7

Glycolysis

Products of Glycolysis

• The products of glycolysis are: – 2 molecules of ATP

• 4 are synthesized, but 2 are hydrolyzed at the beginning of glycolysis

– 2 molecules of pyruvic acid (3 carbon molecule)– 2 molecules of NADH

• Only 2 of 38 molecules of ATP are synthesized during glycolysis

Pyruvic Acid

• The presence or absence of O2 in the cell determines how the cell will use the 2 molecules of pyruvic acid at the end of glycolysis

– If O2 is present pyruvic acid will be used in the process of aerobic respiration • 36 more molecules of ATP are synthesized as

cellular respiration continues through the Krebs cycle and the electron transport chain

– If O2 is absent the pyruvic acid will be used in the process of anaerobic respiration (fermentation)• produces no more ATP

Anaerobic Fermentation• In the absence of O2 (oxygen debt), the 2 molecules of

pyruvic acid from the end of glycolysis are converted into 2 molecules of lactic acid

• Anaerobic fermentation most commonly occurs in active skeletal muscle cells when they use O2 faster than it can be delivered by the respiratory and circulatory systems

• The accumulation of lactic acid causes:– muscle proteins to partially denature

• leading to muscle weakening (fatigue)– muscle soreness

Aerobic Respiration

• In the presence of O2, the 2 molecules of pyruvic acid from the end of glycolysis are transported into the mitochondrial matrix

• Each molecule of pyruvic acid is then converted into a molecule of Acetyl Coenzyme A (Acetyl CoA)– required step between glycolysis and the Krebs

cycle

2 Pyruvic Acid → 2 Acetyl CoA

During the conversion of each of the 2 molecules of pyruvic acid to Acetyl CoA the following occurs:

• Decarboxylation (CO2 is removed) of pyruvic acid making acetic acid (2 carbons)

• One H is removed from acetic acid (oxidized) by NAD+ (reduced to NADH) and further contributes to the H+ gradient

• Coenzyme A in the mitochondrial matrix is added to the acetic acid to make acetyl CoA

During this step the products are:– 2 molecules of CO2

– 2 molecules of NADH – 2 molecules of Acetyl CoA

• used in the Krebs Cycle

Krebs Cycle (Citric Acid Cycle)• A series of 10 biochemical reactions which begins and

ends (cyclic) with a molecule of citric acid • Each acetic acid (2 carbons) is combined with a

molecule of oxaloacetic acid in the mitochondrial matrix (4 carbons) to make citric acid (6 carbons) during step 1

• Each citric acid is then decarboxylated twice (steps 4 and 6) and oxidized (H is removed during steps 3, 5, 8 and 10) as it is converted into different carbohydrate intermediates– energy released from the hydrolysis of bonds is

used to synthesize ATP during step 7– NAD+ (steps 3, 5 and 10) and FAD (step 8) are

reduced to NADH and FADH2 and further contributes to the H+ gradient in the mitochondria

Krebs Cycle

Products of Krebs Cycle

• The reactions of the Krebs cycle produce:– 3 molecules of NADH – 1 molecule of FADH2

– 2 molecules of CO2

– 1 molecule of ATP– 1 molecule of oxaloacetic acid

• Since each molecule of glucose yields two molecules of acetyl CoA, 2 Krebs cycles yield:– 6 NADH – 2 FADH2

– 4 CO2

– 2 ATP– 2 oxaloacetic acid

• reenter the Krebs cycle of reactions after combined with another acetic acid

Products of Glycolysis → Krebs Cycle

• 4 ATP – 2 (net) from glycolysis– 2 from Krebs

• 10 NADH (10 H from original glucose molecule)– 2 from glycolysis– 2 from (2) Pyruvic Acid → (2)Acetyl CoA– 6 from Krebs

• 2 FADH2 (2 H from original glucose molecule)– 2 from Krebs

• 6 CO2 (6 C from original glucose molecule)– 2 from (2) Pyruvic Acid → (2)Acetyl CoA– 4 from Krebs

Glucose

Pyruvic Acid

Pyruvic Acid

ATP

Citric Acid

ATPADP

NADNADH

ADP

ADP ATP

ADP

ATP NADNADH

CO2

CO2

NAD

NADH

Acetyl CoA

Acetyl CoA

CoA

CoA

Acetate +

OAA

NADNADH

ADP ATP

ADP

ATP

OAA

NAD

NADH

NAD

NADH

NAD

NADH

FAD

FADH2

ADP

ATP

CO2

CO2

Citric Acid

Acetate +

OAA

GLYCOLYSIS KREBS (CITRIC ACID) CYCLE

CYTOPLASM MITOCHONDIAL MATRIX

2C3C

6C

2C3C

6C

6C

4C

4C

2C

2C

4C

OAA

NADH

NAD

NADH

NAD

NADH

FAD

FADH2

ADP

ATP

CO2

CO2

4C

Electron Transport Chain

• A group of 3 integral proteins and 2 membrane coenzymes associated with the inner mitochondrial membrane called the electron transport chain (ETC) creates a H+ gradient between the intermembrane space and the matrix of the mitochondria using the H of from NADH and FADH2 that accumulate in the mitochondrial matrix as products of glycolysis and the Krebs cycle

• 3 (integral) enzyme complexes of the ETC remove the H from NADH and FADH2 (oxidized back to NAD+ and FAD+) in the matrix

ETC and Oxidative Phosphorylation

FADH2 FAD+

NADH and the Electron Transport Chain

• NADH is oxidized by Enzyme complex 1 of the ETC• The H that is removed by Enzyme complex 1 is split

into an electron (e-) and a proton (H+) • The e- is passed from the beginning to the end of the

ETC and energizes the 1st, 2nd and 3rd Enzyme complexes along the way– Once energized, each enzyme complex pumps a

free proton from the mitochondrial matrix into the intermembrane space (3 total)

FADH2 and the Electron Transport Chain

• FADH2 is oxidized by Enzyme complex 2 of the ETC• The H that is removed by Enzyme complex 2 is split

into an electron (e-) and a proton (H+) • The e- is passed to the end of the ETC and energizes

the 2nd and 3rd Enzyme complexes along the way– Once energized, each enzyme complex pumps a

free proton from the mitochondrial matrix into the intermembrane space (2 total)

Proton (H+) gradient• The movement of protons from the matrix into the

intermembrane space creates a high H+ (pH = 7) concentration in the intermembrane space and a low H+ (pH = 8) concentration in the matrix – this proton gradient becomes the source of energy

used by the mitochondria to synthesize ATP, which is released as H+ diffuse from the intermembrane space back into the matrix

• The oxidation of NADH contributes more (pumps 3 H+) to the proton gradient than FADH2 (pumps 2 H+)

ATP Synthase

• ATP synthase is an integral membrane protein in the inner mitochondrial membrane that has 2 functions:– it acts as a H+ channel

• allows H+ to diffuse down its gradient from the intermembrane space into the matrix

– it acts as an enzyme• uses the energy that is released by the diffusion

of H+ to synthesize ATP from ADP and P• This type of ATP synthesis is called oxidative

phosphorylation because, this process requires the oxidation of NADH and FADH2

ATP Synthesis During Oxidative Phosphorylation

• Because the oxidation of NADH contributes more to the proton gradient than the oxidation of FADH2, more ATP is synthesized from NADH than FADH2

• On average, for each NADH that is oxidized 3 molecules of ATP are synthesized– 10 NADH x 3 ATP = 30 ATP

• On average, for each FADH2 that is oxidized 2 molecules of ATP are synthesized

– 2 FADH2 x 2 ATP = 4 ATP

• 34 of the 38 molecules of ATP are synthesized by oxidative phosphorylation

Formation of Water

• After the electrons arrive to the last enzyme complex of the ETC they are transferred to atoms of oxygen in the mitochondrial matrix– this makes oxygen especially negative

• In the matrix, these oxygen atom are combined with the H+ that have diffused back into the matrix from the intermembrane space to form water (H2O)

– the protons become “reunited” with their electron