Cellular Pathways

• Metabolic pathways occur in small steps, each catalyzed by a specific enzyme.

• Metabolic pathways are often compartmentalized and are highly regulated.

Cellular Respiration Overview

Obtaining Energy and Electrons from Glucose

When glucose burns, energy is released as heat and light:

C6H12O6 + 6 O2 6 CO2 + 6 H20 + energy

The same equation applies to the metabolism of glucose by cells, but the reaction is

accomplished in many separate steps so that the energy can be captured as ATP.

Energy Overview

• C6H12O6 + 6O2 ---------> 6CO2 + 6H2O + energy via heat and

ATP

• O atoms draw shared electron pairs to themselves (oxidation of C).

• Do the electrons get more/less stable?• Exergonic/endogonic?

C6H12O6 + 6CO2 6CO2 + 6H2O + ATP

• Breaking the bonds between the six C atoms of glucose, results in 6 CO2 molecules.

• Moving hydrogen atom electrons from glucose to oxygen, forms 6 H2O molecules.

• As much of the free energy released in the process as possible is trapped as ATP.

Obtaining Energy and Electrons from Glucose

• As a material is oxidized, the electrons it loses transfer to another material, which is thereby reduced.

• Such redox reactions transfer a lot of energy. Much of the energy liberated by the oxidation of the reducing agent is captured in the reduction of the oxidizing agent.

Redox Reactions

• The ultimate goal of cellular respiration is to capture as much of the available free energy in the form of ATP. This goal is accomplished through 2 different energy-transfer mechanisms:

1.substrate-level phosphorylation

2.oxidative phosphorylation

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(translation: sticking on a phosphate, which can later be used to make ATP from ADP)

1. Substrate-Level Phosphorylation (energy invested)

• ATP is formed directly• a phosphate-containing compound

transfers a PO4- directly to ADP, forming ATP

• 30.5 kJ/mol of potential energy is transferred.

• 2. Oxidative Phosphorylation

• ATP is formed indirectly

• involves sequential redox reactions, with O being the final electron acceptor

Energy TransferSubtrate-levelPhosphorylation• ATP is formed directly

in an enzyme-catalyzed reaction.

• Phosphate containing compound transfers a phosphate group directly to ADP forming ATP.

• 31KJ/Mol is transferred• 50KJ/Mol in living cells

Oxidative

Phosphorylation• Involves sequential redox

reactions.• O is the final electron

acceptor.• NAD+ removes 2H from

glucose and is reduced to NADH

• FAD is reduced to FADH2

• FADH2 and NADH move free energy from one

place to another.

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Energy Carriers

• NAD+ and FAD+ are low energy, oxidized coenzymes that act as electron acceptors.

• When an electron(s) is added to these molecules, they become reduced to NADH and FADH2.

• In this case, reducing a molecule gives it more energy.

• The coenzyme NAD is a key electron carrier in biological redox reactions. It exists in two forms, one oxidized (NAD+) and the other reduced (NADH + H+).

Mitochondria• Specialize in the production of ATP.• Only in eukaryotic cells.• Double membrane – smooth outer layer and folded inner

membrane (cristae).• Matrix is protein rich and fills the innermost space of the

mitochondria.• Intermembrane space (between the 2 membranes)• Contain their own DNA (mtDNA) which lead to the

endosymbiosis hypothesis.

Smooth

Highly folded

Folds of the inner membrane

Protein-rich liquid

Fluid-filled intermembrane

space

The Big Picture• The entire process occurs in 4 stages and in

3 different places within the cell:• Stage 1: Glycolysis - a 10 steps occurring in

the cytoplasm• Stage 2: Pyruvate Oxidation - 1 step

occuring in the mito. matrix• Stage 3: Krebs cycle - 8 steps occuring in the

mito. matrix• Stage 4: ETC and chemiosmosis - many

steps occuring in the mito. cristae

Overview

Energy Pathways

Glycolysis• There are 2 main phases of glycolysis:

Glycolysis I - activation phase, which uses ATP molecules

• Glycolysis II - oxidative and phosphorylation reactions, which not only reduce glucose to pyruvate but also produce ATP molecules

• C6H12O6 + 2ADP + 2Pi + 2 NAD+ -----> 2 pyruvate + 2ATP + 2(NADH + H+)

Stage 1: Glycolysis• Occurs in cytoplasm• Anerobic and does not

require oxygen• Glucose is split into two

3-C molecules called pyruvate (pyruvic acid).

• Transfers only 2.2% of free energy available in 1 mol of glucose to ATP.

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is not a strong enough oxidizer to strip electrons from C-H bonds in glucose at room or body temperature. Enzymes required.

Glycolysis: From Glucose to Pyruvate

• Glycolysis is a pathway of ten enzyme-catalyzed reactions located in the cytoplasm. It provides starting materials for both cellular respiration and fermentation.

Glycolysis: From Glucose to Pyruvate

• The energy-investing reactions of glycolysis use two ATPs per glucose molecule and eventually yield two glyceraldehyde 3-phosphate molecules. In the energy-harvesting reactions, two NADH molecules are produced, and four ATP molecules are generated by substrate-level phosphorylation. Two pyruvates are produced for each glucose molecule. Review

Glycolysis - Energy InStep 1: ATP

phosphorylates glucose

Step 2: rearrangement

Step 3: stick on another

phosphate (phosphorylation)

Step 4: split (F1,6BP)

into 2 things, most important one: G3P

Step 5: take 2nd thing and rearrange it to be G3P too!

Glycolysis - Energy OutStep 6: add a phosphate +

get some NADH Step 7: take off one

phosphate and put it on ADP

Step 8: rearrange to get

phosphate in a different place

Step 9: steal some H2O to

convert from 2PG to PEP

Step 10: take that last

phosphate off and in the process make ATP

Glycolysis - Energy Out

So…How much in?2 ATP, a glucose and 2 NAD+

How much out?4ATP – 2ATP = 2ATP

2NADH2

Pyruvate molecules (and a couple of water to boot!)

GLYCOLYSIS PRODUCTS• 2 ATPs are used in

steps 1 & 3 to prepare glucose for splitting.

• F 1,6-BP splits into DHAP and G3P.

• DHAP converts to G3P.

• 2 NADH are formed in step 6.

• 2 ATP are formed by substrate-level phosphorylation in both steps 7 and 10.

• 2 pyruvates are produced in step 10.

Pyruvate Oxidation

When O2 is available (aerobic respiration) Pyruvate “enters the matrix”

A CO2 is stripped off

An NAD+ is reduced by 2 H atoms

Coenzyme A is attached to make Acetyl-CoA

(basically, gets what’s left of

glucose ready for the Kreb’s Cycle)

Acetyl-CoA then moves on to the Kreb’s

Cycle

• A multi-enzyme complex catalyzes the following three changes:

• 1.the carboxyl group of pyruvate is removed as a CO2 molecule. This is a decarboxylation reaction catalyzed by the enzyme pyruvate decarboxylase.

• 2.The 2-C fragment is oxidized to form an acetate ion. Electrons from this reaction are picked up by NAD+ which is reduced to form NADH + H+

• 3.The acetyl group of the acetate ion is transferred to coenzyme A, forming acetyl CoA

Pyruvate Oxidation• Occurs in matrix of mitochondria.• 2 pyruvate + 2NAD+ + 2 CoA 2 acetyl-CoA + 2NADH + 2H+ + 2CO2

2 pyruvate + 2 NAD+ + 2 CoA ------> 2 acetyl-CoA + 2

NADH + 2 H+ + 2 CO2

Krebs Cycle• 8 steps that oxidize acetyl-CoA to CO2 and

H2O, forming a molecule of ATP. In addition, the cycle removes electrons, which are carried by 3 NADH and 1 FADH2 molecules to the ETC. The following is the overall chemical equation:

oxaloacetate + acetyl-CoA + ADP + Pi + 3NAD+ + FAD ---> CoA + ATP + 3 NADH + 3H+ + FADH2 + 2CO2+ oxaloacetate

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Production of Citrate

• The energy in acetyl CoA drives the reaction of acetate with oxaloacetate to produce citrate. The citric acid cycle is a series of reactions in which citrate is oxidized and oxaloacetate regenerated. It produces two CO2 , one FADH2, three NADH, and one ATP for each acetyl CoA.

Occurs twice for each molecule of glucose, 1 for each acetyl-CoA.

Animation: How the Krebs Cycle Works (Quiz 1)

What’s going on??1. In step 1, acetyl-CoA combines with oxaloacetate

to form citrate.2. In step 2, citrate is rearranged to isocitrate.3. NAD+ is reduced to NADH in steps 3, 4 and 8.4. FAD is reduced to FADH2 in step 6.5. ATP if formed in step 5 by substrate-level

phosphorylation. The phosphate group from succinyl-CoA is transferred to GDP, forming GTP, which then forms ATP.

6. In step 8, oxaloacetate is formed from malate, which is used as a reactant in step 1.

7. CO2 is released in steps 3 and 4.

Complete Overviewrespiration

The Electron Transport Chain

• NADH and FADH2 molecules derived from glycolysis and the Kreb’s Cycle each contain electrons they gained from their formation

• NADH (or FADH2) molecules carry these electrons to the inner membrane of the mitochondrion and electron transfer begins*NAD+ and FAD = electron carriers – takes H’s (and their e-) to electron transport chain!

REDOX   These electrons are passed through a series of electron carriers, one step at a time. As the electrons are passed along, one substance is oxidized, the other is reduced. (REDOX rxns) *What is the “driving force”?     At each step some energy is released.     This energy is used in certain places in the chain to pump protons (H+) from the matrix out into the inner membrane space.

ETC These electron carriers are collectively know as the

electron transport chain and are found embedded in the

inner mitochondrial membrane.

This chemical concentration gradient of protons creates

an electrical potential difference (a ‘chemical voltage’ of

sorts)

As protons re-enter the matrix trough the ATP synthase

particle (their pathway in), energy is released. This energy

is used to form ATP (ADP is phosphorylated to make ATP.

This process is referred to as the Chemiosmotic Theory

Electron Transport Chain (ETC)

• A series of electron acceptors (proteins) are embedded in the cristae.

• These proteins are arranged in order of increasing electronegativity.

• The weakest attractor of electrons (NADH dehydrogenase) is at the start of the chain and the strongest (cytochrome oxidase) is at the end.

• These proteins pass electrons from NADH and FADH2 to one another through a series of redox reactions.

• ETC protein complexes are alternately reduced and oxidized as they accept and donate electrons.

Energy Pumping• As the electrons pass from one molecule to

the next, it occupies a more stable position.• The free energy released is used to pump

protons (H+) to the intermembrane space.• 3 for every NADH and 2 for every FADH2.

• This creates an electrochemical gradient, creating potential difference (voltage) similar to a battery.

ETC

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• NADH and FADH2 transfer the electrons they got from glucose to the ETC.

The electrons move through a series of redox reactions.

Energy is released and used to pump protons (H+ ions) from the matrix into the intermembrane space.

• At the end of the chain, the electrons are so stable that only a oxygen is strong enough to oxidize the last protein complex.

• Oxygen strips 2 electrons from the final protein complex and forms water with 2 protons from the matrix.

Electron Transport Chain

ATP Synthase

• Protons enter the matrix through proton channels associated with ATP synthase (ATPase).

• For every H+ that passes through, enough free energy is released to create 1 ATP from the phosphorylation of ADP.

• Conditions must be aerobic because oxygen acts as the final electron and H+ acceptor (water is formed as a byproduct).

Oxidative phosphorylation

Chemiosmosis

Synthesis of ATP

• Protons begin to accumulate in the intermembrane space which creates an electrochemical gradient that stores free energy known as a proton-motive force (PMF). This gradient has 2 components:

• 1.an electrical component caused by a higher positive charge

• 2.a chemical component caused by a higher concentration of protons

• These protons are unable to diffuse through the phospholipid bilayer and are forced to pass through ATP synthase (ATPase).

• When protons move through ATPase, the free energy drives the synthesis of ATP from ADP and Pi found in the matrix (oxidative phosphorylation).

• ATP is then transported out into the cytoplasm by facilitated diffusion and can be used to drive endergonic processes such as movement and active transport.

The Respiratory Chain: Electrons, Proton Pumping,

and ATP • The chemiosmotic mechanism couples

proton transport to oxidative phosphorylation. As the electrons move along the respiratory chain, they lose energy, captured by proton pumps that actively transport H+ out of the mitochondrial matrix, establishing a gradient of proton concentration and electric charge—the proton-motive force.

ATPase - Chemiosmosis• The proton-motive

force causes protons to diffuse back into the mitochondrial interior through the membrane channel protein ATP synthase, which couples that diffusion to the production of ATP.

VCAC: Cellular Processes: ATP Synthase: The Movie

Review

1. Glycolysis: occurs in the cytoplasm: anaerobic

2. Pyruvate enters the mitochondrion, converts to Acetyl-CoA

3. Kreb’s Cycle: Acetyl-CoA broken down into CO2… drives production of NADH, FADH2 and ATP

4. Electron Transport Chain: NADH powers the electron transport system that chemiosmotically produces ATP

Aerobic Resp. Balance Sheet

ATP YIELD

• The theoretic yield of ATP per glucose molecule is about 36. However, it turns out that the actual yield is only 30 realistically. (Due to the fact that NADH produces an average of 2.5, not 3 and FAD produces an average of 1.5, not 2).

• Efficiency of energy conversion for aerobic respiration is much higher than the 2.2% of glycolysis. It is 32% efficient for aerobic. This allows multicellular organisms to exist. (compare to a car engine that is about 25% efficient!)

Metabolic Rate and BMR• Metabolic rate: the amount of energy

consumed by an organism in a given amount of time (increases when work is done).

• BMR (Basal Metabolic Rate): the minimum amount of energy needed to keep an organism alive. This is dependent on age, growth, development. This is a baseline for an organism's metabolic rate.

determining Calorie needs -

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Controlling Aerobic Respiration

• Regulated by feedback inhibition and product activation loops.

• Phosphofructokinase (PFK) is an allosteric enzyme that catalyzes the third reaction in glycolysis and is inhibited by ATP and stimulated by ADP.

• If citrate accumulates, some will enter the cytoplasm and inhibit PFK to slow down glycolysis.

• As citrate is used up, its concentration will decrease and the rate of glycolysis will increase.

Feedback inhibition

• A high concentration of NADH indicates that the ETCs are full of electrons and ATP production is high.

• NADH allosterically inhibits an enzyme that reduces the amount of acetyl-CoA that is shuttled to the Krebs cycle, reducing the amount of NADH produced.

Controlling Aerobic Respiration

Speed up:·n increase in ADP·adecrease of citrate·Both act on phosphofructokinase

Slow down:·n increase in ATP (acts on phosphofructokinase)·an increase in NADH (acts on pyruvate decarboxylase)TP