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CHAPTER 6

Cellular Respiration and Fermentation

The main energy-yielding processes of living cells

• Fermentation: Simple definition: Energy extraction from glucose in the absence of O2 More general: Energy extraction by the partial oxidation of organic molecules; the electron

acceptor is an organic molecule (often the same) • Respiration: Simple definition: Energy extraction from glucose through complete oxidation by O2 More general: Oxidation of organic molecules by an inorganic electron acceptor (O2, Fe,

NO3-, S, SO4

2-, CO2, …..)

• Photosynthesis: Simple definition: Light-driven synthesis of glucose from CO2 and H2O More general: Light-driven synthesis of ATP (and then organic molecules) using simple

molecules as electron donor (H2O, H2S, …)

http://en.wikipedia.org/wiki/File:Geologic_Clock_with_events_and_periods.svg

Primitive forms of fermentation and respiration

(not clear what came first)

Primitive forms of photo-synthesis

Origin of O2-releasing photosynthesis

Origin of O2-consuming respiration

Evolution of energy metabolism

• Photosynthesis – Uses light energy to synthesize organic

molecules from carbon dioxide and water – Occurs in chloroplasts and in some bacteria

Copyright © 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings

• Respiration – Is the most efficient chemical process to extract

energy from organic molecules – Occurs in mitochondria and in some bacteria – Uses oxygen = is an aerobic process

Current state: O2-producing photosynthesis and O2-using respiration dominate energy metabolism and form a global cycle

21%

0.035%

• Autotrophs (=“self-feeders”) – Can make organic matter from inorganic nutrients (plants, some microbes), in

most cases by photosynthesis – Are the producers in an ecosystem

• Heterotrophs (=“other-feeders”)

– Cannot make organic molecules from inorganic ones (animals, fungi, many microbes)

– Depend on autotrophs for their organic fuel and material for growth and repair

– Are the consumers in the ecosystem

Organisms according to their ability to perform photosynthesis

• Respiration (O2-driven respiration)

• Fermentation

• Photosynthesis

We now discuss energy metabolism in the order used in the textbook

Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Nature of Cellular Respiration

• A common fuel molecule for cellular respiration is glucose.

Cellular respiration consists of many chemical steps

The process produces up to 32 ATP molecules for each glucose molecule consumed


Role of oxygen in cellular respiration

• During respiration, hydrogen and its electrons move from sugar carbon to oxygen, forming water.

H+ + e-


Redox reactions

• Chemical reactions that transfer electrons from one substance to another are called oxidation-reduction reactions (= “redox” reactions).

• Oxidation = loss of electrons • Reduction = acceptance of electrons

Why does electron transfer to oxygen release energy? • Oxygen attracts electrons more than almost any other atom (“electronegativity”) • Electronegativity is the power of an atom in a molecule to attract electrons to itself

Oxygen pulls away electrons from carbon

Fully reduced carbon atom

Partially reduced carbon atom

Fully oxidized carbon atom

Oxidation state (number of electrons “owned” by the red carbon)

In an oxidizing milieu (like on earth), increasing the oxidation state of carbon releases energy

The “oxidation state” of carbon is an easy way to compare the energy contents of biomolecules

• A rapid electron fall generates an explosive release of energy in the form of heat or light

• It would be difficult to capture the burst of energy released from such an explosive reaction

Uncontrolled redox reactions


Controlled redox reactions: Harvesting the energy in small portions • Cellular respiration is a controlled “fall” of electrons energy is harvested in little

portions in a cascade, the electron transport chain • Electrons (in hydrogen atoms) are extracted from the food molecules by the reaction RH2 + NAD+ → NADH + H+ + R (RH2 are the food molecules; NAD+ is a small electron transporter molecule)

a) NAD+ (nicotinamide adenine dinucleotide) b) NADH • Electron transport chain

– Is a group of membrane proteins – electrons travel from step to step of the protein cascade – Each step release of energy ATP formation – Final step: electron and proton bind to oxygen H2O

• NADH – NAD+ is a nucleotide (base + sugar + phosphate) – It is the universal electron transporter in

catabolic (= degradative) metabolic pathways – NAD+ + H+ + 2e- NADH


•Stages of respiration: 1. Partial breakdown of glucose; partial oxidation of carbon: C6 C3 2. Complete breakdown of C3, complete oxidation of carbon: C3 C2 C1 3. Diffusion of H (= H+ and e-) in the form of NADH to the electron transport chain 4. Movement of electrons down the electron transport chain to the final acceptor (O2).

The Metabolic Pathway of Cellular Respiration

1 2

3 3

4


Stage 1: Glycolysis

•Location: in the cytosol (not mitochondria!) •C6 (glucose) 2C3 (pyruvate) •The process generates some ATP and NADH, but much less than in the following stages

C6

2 C3


• Glycolysis makes some ATP directly: enzymes transfer phosphate groups from a substrate (fuel molecules) to ADP

• This is called “substrate level phosphorylation”


Stage 2: The linking reaction and the Citric Acid Cycle

• Stage 2a: The linking reaction (links glycolysis and Citric Acid Cycle): C3 (pyruvate) C2 (“active acetic acid”) • This linking reaction takes place in the mitochondrial matrix and releases CO2 and NADH

C3 C2 (“active acetic

acid”) C1


• Stage 2b: The Citric Acid Cycle (also takes place in the mitochondrial matrix) • It is a catalytic cycle that breaks does the C2 compound (active acetic acid) into C1

(=CO2). The H atoms are captured mainly as NADH • The main function of the Citric Acid Cycle is to form NADH (and the related

FADH2) The cycle also makes a small amount of ATP by substrate-level phosphorylation.

C2 C1


Stage 3: The Electron Transport Chain • Location: inner membranes of mitochondria.

– The chain is a machine that uses energy released by the “fall” of electrons to pump hydrogen ions across the inner mitochondrial membrane.

– The resulting gradient of H+ ions across the membrane contains potential energy – The H+ ions flow back through a channel formed by the ATP-synthesizing enzyme

•ATP synthesis is similar to a hydropower plant - Water flows through a turbine - H+ ions flow through ATP synthase.

ATP synthase

Inner mitochondrial membrane

ATP H+ gradient

ATP synthase

Animation video, ~ 3 minutes http://www.youtube.com/watch?v=PjdPTY1wHdQ

•ATP synthase – a nanomachine driven by the gradient of protons

http://vcell.ndsu.edu/animations/etc/atpsynthase.htm http://en.wikipedia.org/wiki/ATP_synthase


Adding up the ATP from respiration

• A summary of ATP yield during cellular respiration

C6 C3 C2 C1

Newest estimate is 32 molecules of ATP per molecule of glucose

28 X X

32


The oxygen balance of respiration

The free oxygen becomes part of water

The bound oxygen becomes part of CO2

The familiar summary equation of respiration:


6xO

But then the numbers do not match up:

12xO

12xO 6xO

The corrected summary equation of respiration •The summary equation of respiration given in the textbook is

6(CH2O) + 6O2 6H2O + 6CO2

•When we highlight the origins and fates of the oxygen atoms, we get: 6(CH2O) + 6O26H2O + 6CO2

• However … the colors do not add up!!

•Correctly it should be:

6(CH2O) + 6H2O + 6O2 12H2O + 6CO2

These water molecules come in during the Citric Acid Cycle


The versatility of cellular respiration

• Cellular respiration can “burn” other kinds of molecules besides glucose: – Diverse types of carbohydrates – Fats – Proteins


Relationship between cellular respiration and breathing

• Cellular respiration and breathing are closely related.

– Cellular respiration requires a cell to exchange gases with its surroundings.

– Breathing exchanges these gases between the blood and outside air

Breathing is respiration on the level of the whole organisms

Copyright © 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings

Overview

• Respiration

• Fermentation

• Photosynthesis

Simplified definition of fermentation: Fermentation = breakdown of glucose in the absence of oxygen Fermentation does not break down molecules completely

Glucose Lactate Glucose Ethanol + CO2 Since it does not use a highly electronegative electron recipient, the energy yield of fermentation is low.

Principle of Fermentation: Anaerobic Harvest of Energy

C3 C6

C2 C6 C1

Relationship between fermentation and respiration

Fermentation Respiration

Glucose Glycolysis

• The C3 compound (pyruvic acid) resulting from glycolysis can have different fates

C3

C6

C2 C2

C2

C3

Fermentation: In the absence of sufficient oxygen

Respiration: In the presence of oxygen

Glucose Glycolysis

• The fate of pyruvate depends on the physiological situation

• Many of our cells use fermentation temporarily: • Especially muscles, when work demand is higher than oxygen supply.

Fermentation in yeast

Fermentation in animals

Respiration in yeast and animals

Glucose Glycolysis


The electron acceptor in fermentation

• In glycolysis, NAD+ is the primary electron acceptor NADH • In aerobic conditions, this NADH gives its electron to O2 (via the electron transport

chain) • In anaerobic conditions (= no O2), this NADH cannot give its electron to O2, so

NADH gives its electron to a C3 (pyruvic acid) reduced C3 (lactic acid) • This regenerates the NAD+ and glycolysis can continue (but the Citric Acid Cycle

stops since it depends on pyruvate)

Anaerobic conditions


Fermentation is the oldest biotech industry

• Fermentation products: kimchi, cheese, sour cream, yogurt

• Soy source (soybean), pickle cucumbers, pepperoni, salami

Industrial application of fermentation

• Yeast cells carry out a slightly different type of fermentation pathway.

• This pathway produces CO2 and ethyl alcohol.

Summary – Cellular respiration and fermentation •Fermentation: Partial oxidation of organic molecules that also serve as electron acceptors.

•Small differences in electronegativities modest ATP yields. •Respiration: Oxidation of organic molecules (typically glucose) by an inorganic electron acceptor (typically oxygen).

•Large differences in electronegativities large ATP yields. •Most cells switch between fermentation and respiration depending on the availability of O2.

•High oxygen respiration dominates; low oxygen fermentation is activated.

•The initial stages of fermentation and respiration are identical and are called glycolysis. •Simplified glycolysis summary: C6 2C3 + 2 ATP + 2 NADH. •The ATP is generated by substrate-level phosphorylation.

•Respiration: The NADH generated by glycolysis is regenerated to NAD in mitochondria (when oxygen is available). This allows the C3 to be further oxidized.

•C3 is fully oxidized to water in the citric acid cycle, which generates the majority of NADH. •All NADH loses its electrons to the respiratory chain, which transfers the electron to oxygen ( H2O) and creates an electrochemical gradient across the inner mitochondrial membrane. •The electrochemical gradient acts like the water pressure in a hydroelectric power station, driving synthesis of ATP with the ATP synthase, which acts like a turbine and generator.

•Fermentation: The NADH generated by glycolysis is regenerated to NAD by reducing the product (C3) of glycolysis.

•Two major versions of this final step lead to either lactic acid or ethanol + CO2. •Since fermentation yields much less ATP per glucose molecule than respiration, much more glucose is needed by cells to produce enough ATP in the absence of oxygen.

요약- 세포호흡과 발효 • 발효: 전자 수용체 역할도 하는 유기분자의 불완전 산화

• 전기음성도의 차이가 작음 → 적은 양의 ATP 생산.

• 호흡: 무기 전자 수용체 (일반적으로 산소)에 의한 유기분자 (일반적으로 당)의 산화. • 전기음성도의 차이가 큼 → 많은 양의 ATP 생산.

• 대부분의 세포들은 산소 이용에 따라 발효와 호흡을 바꾸며 이용한다. • 높은 산소 농도 → 호흡이 대부분; 낮은 산소 농도 → 발효가 활성화.

• 발효와 호흡의 첫 번째 단계는 동일하며, 이를 해당작용 (glycolysis)라 부른다.

• 단순화 시킨 해당작용 공식: C6 → 2C3 + 2ATP + 2NADH. • ATP는 기질 수준의 인산화에 의해 생산된다.

• 호흡: 해당작용에 의해 생산된 NADH는 미토콘드리에에서 NAD로 재생된다 (산소가 이용가능 할 때). 이것은 C3를 더욱 산화되게끔 이끈다.

• C3는 시트르산 회로_Citric acid cycle에서 물로 완전히 산화되어, NADH의 대부분을 생산한다. • 모든 NADH는 호흡연쇄 (respiratory chain)에서 전자를 산소로 전달 (→H₂O)하여 미토콘드리아 내막 사이에 전기화학적 기울기를 만든다. • 전기화학적 기울기는 수력발전소에서 수압과 같은 역할을 하며, 이는 터빈과 발전기 역할과 비슷한 ATP synthase와 함께 ATP를 생산한다.

• 발효: 해당과정에서 생산된 NADH는 해당과정의 환원 생산물 (C3)에 의해 NAD로 재생된다.

• 두 종류의 마지막 과정은 각각 젖산 또는 에탄올 + CO₂를 만든다. • 발효는 호흡에 비해 당에 대한 ATP 생산량이 적기 때문에, 산소 없이 충분한 ATP를 생산하기 위해서 세포는 더 많은 양의 당이 필요하다.

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