Cellular Respiration:Harvesting Chemical Energy

Principles of energy conservation The process of cellular respiration Related metabolic processes 6O2 +C6H 12O 6 6H2O + 6CO 2 + energy

Concept 9.1

Metabolic pathways that release energy are called catabolic pathways- fermentation and cellular respiration

Fermentation: partial degradation of sugars that occurs w/out the help of O2

Cellular respiration: O2 is consumed as a reactant along w/ the sugar- more efficient

Redox Reactions

Reduction: gain of electrons

Oxidation: loss of electrons

Cellular respiration is a redox process

Redox Reactions

Redox reactions release energy when electrons move closer to electronegative atoms

- the relocation of electrons releases the energy stored in food molecules, and this energy is used to synthesize ATP

Concept 9.1

There is a transfer of one or more e- from one reactant to another; the electron transfers are called oxidation-reduction reactions or redox rxns. - the loss of e- from one substance is called oxidation- the addition of e- to another substance is called reduction

Concept 9.1

Concept 9.1

Electrons “fall” from organic molecules to oxygen during cellular respiration

C6H12O6 + 6O2 6CO2 + 6H2O + Energy- by oxidizing glucose, cellular respiration takes energy out of storage and makes it available for ATP synthesis- carbohydrates and fats are reservoirs of electrons associated w/ hydrogen

Concept 9.1

The “fall” of electrons during respiration is stepwise, via NAD+ and an electron transport chain

Glucose is broken down over a series of steps that are each catalyzed by a specific enzyme

Hydrogen atoms are stripped from the glucose and usually passed to NAD+.- NAD+ is reduced in the rxn.

Concept 9.1

NAD+ is transformed to NADH- NADH will later be tapped to make ATP as the electrons continue their fall from NADH to oxygen

Respiration uses an electron transport chain to break the fall of electrons to oxygen into several energy-releasing steps instead of one explosive rxn.

Concept 9.1

Concept 9.1

Electrons removed from food are shuttled by NADH to the “top” end of the chain. At the “bottom”, oxygen captures the electrons along with H+ ions to form water

Food NADH ETC oxygen

Concept 9.1

Respiration consists of three stages:- glycolysis, the Krebs cycle, electron transport chain (ETC)

Glycolysis breaks down 1 glucose into 2 molecules of pyruvate- occurs in the cytosol

Krebs cycle breaks down pyruvate into CO2 - occurs in the mitochondrial matrix

Concept 9.1

ETC accepts electrons from the breakdown products of the first 2 stages

- the energy released at each step of the chain is used to make ATP (oxidative phosphorylation); through redox rxns.

oxidative phosphorylation accounts for 90% of generated ATP

Concept 9.1

Concept 9.1

Concept 9.1

Concept 9.1

Substrate-level phosphorylation: direct transfer of a phosphate to ADP by an enzyme

Each molecule of glucose is degraded into carbon dioxide, water and 38 molecules of ATP

Coenzymes are intermediate energy carriers Found in all cells Assist enzymes in energy transfer NAD and FAD Join with H+ and e- Many coenzymes are vitamins

Concept 9.1

Substrate-level phosphorylation: direct transfer of a phosphate to ADP by an enzyme

Each molecule of glucose is degraded into carbon dioxide, water and 38 molecules of ATP

Concept 9.1

Cellular Respiration

Glycolysis Krebs cycle Electron transport chain

Concept 9.1

Cellular respiration occurs in the mitochondria

Organic + O2 Carbon + H2O + Energy

compounds dioxide

C6H12O6 + 6O2 6CO2 + 6H2O + Energy

1 glucose = -686 kcals

Concept 9.2

Glycolysis means “splitting of sugar”- the 10 steps of glycolysis are broken down into two phases: energy investment and energy payoff- glucose (6C) 2 pyruvate (3C)

Energy investment phase: the cell spends 2 ATP to phosphorylate the fuel molecules

Concept 9.2

Energy payoff phase: 4 ATP are produced by substrate-level phosphorylation; 2 NAD+ are reduced to 2 NADH by the oxidation of food

Net energy yield: 2 ATP and 2 NADH

Glycolysis

Catabolic pathway “To split sugar” Glucose + 2ATP2 pyruvates + 4ATP +

2NADH Occurs in cytosol Anaerobic process 10 step pathway

Concept 9.3

If O2 is present, energy stored in NADH can be converted to ATP

Upon entering the mitochondrion, each pyruvate is first converted to a molecule of acetyl CoA (2C)

- another NAD+ is reduced to NADH

Concept 9.3

Intermediate Reaction(Oxidative Decarboxylation) Occurs if O2 is present

2 Pyruvate2 CO2 + 2 NADH + 2 Acetate Attaches acetate to CoA Acetyl CoA

Krebs Cycle

In mitochondrial matrix Oxidation of acetyl CoA to CO2

Products include: 2 ATP + 6 NADH + 2 FAD + 4 CO2

8 step pathway

Concept 9.3

Acetyl CoA will enter the Krebs cycle for further oxidation

Krebs cycle- 8 steps, each catalyzed by a specific enzyme

- Acetyl CoA (2C) enters, 2 CO2 (1C) leave, 3 NAD+ 3 NADH, 1 FAD 1 FADH2, 1 ADP 1 ATP

Concept 9.4

Cristae: inner membrane folding of the mitochondria

- increases surface area for more ETC’s

Electrons removed from food during gycolysis/Krebs are transferred by NADH to the first molecule of the ETC

The Electron Transport Chain

Embedded in the inner mitochondrial membrane

Most ETC molecules are proteins Primary ATP generating pathway Produces ATP via Substrate Level

Phosphorylation and Chemiosmosis

Concept 9.4

Most of the electron carriers in the ETC are proteins called cytochromes (cyt).

The process goes downhill with oxygen being the final e- acceptor

- for every 2 NADH, 1 O2 molecule is reduced into 2 molecules of water

Concept 9.4

FADH2 adds its e- at a lower energy level than NADH on the ETC.

-NADH = 3 ATP

- FADH2 = 2 ATP

ETC makes no ATP directly. It moves e- from food to oxygen breaking the energy drop to manageable amounts.

Concept 9.4

Inside the inner membrane are enzymes called ATP synthase.

- makes ATP from ADP and a phosphate

ATP synthase uses energy from the ion gradient to synthesize ATP.

- proton gradient

Concept 9.4

The ETC is an energy converter that uses the exergonic flow of e- to pump H+ ions across the membrane

- from the matrix to the inner membrane space.

ATP synthases are the only place that are freely permeable to H+

Chemiosmosis: the energy coupling mechanism Electron flow actively transports H+ from

the matrix into the intermembrane space Flow of H+ “down the gradient” thru the

ATP synthase complex generates ATP

Concept 9.4

H+ gradient across a membrane couples the redox rxns. of the ETC to ATP synthesis

- chemiosmosis: connection between the chemical rxn. Makes ATP and transport across a membrane

Concept 9.4

Chemiosmosis is also found in the chloroplasts

- ATP is generated during photosynthesis

- light drives both e- flow down the ETC and H+ gradient formation

Concept 9.4

H+ ions are pumped by members of the ETC. The resulting gradient is called a proton-motive force: the gradient has the capacity to do work

Concept 9.4

Concept 9.4

Energy flow during respiration:

Glucose NADH and FADH ETC proton-motive force ATP

38 ATP formed; 4 from substrate phosphorylation, 34 from oxidative phosphorylation

Concept 9.5

Alcohol fermentation: pyruvate ethanol

- CO2 is released to recycle NAD+

- 2 step process to regenerate NAD+

Concept 9.5

Concept 9.5

Lactic acid fermentation: pyruvate lactic acid

- human cells make ATP by (LAF) when oxygen is scarce

- lactate is carried away by blood to the liver; lactate is converted back to pyruvate by liver cells

Concept 9.5

Concept 9.5

w/out oxygen, the energy still stored in pyruvate is unavailable to cells

Facultative anaerobes: yeasts and bacteria that can make enough ATP to survive using either fermentation or respiration

Concept 9.5

Ancient prokaryotes probably used anaerobic fermentation before oxygen was present in the atmosphere

Also, glycolysis does not require mitochondria to occur

Related Metabolic Processes(The versatility of catabolism) Can use proteins, fats and

polysaccharides Monomers of fats and proteins enter

pathways at various points Can result in generation of energy or

biosynthesis of new molecules

Concept 9.6

Proteins must be broken down to amino acids

- various amino acids can be converted as intermediates of glycolysis and the Krebs cycle

Concept 9.6

Carbohydrates can be hydrolyzed to form glucose monomers to enter into glycolysis

Metabolism works on supply and demand!!!!

Concept 9.6

Carbs and fats can be converted to fats through intermediates of glycolysis and the Krebs cycle

We will store fat even if we have a fat free diet

Regulation of Cell Respiration

Feedback mechanism Allosteric enzyme-

phosphofructo- kinase

Citrate and ATP are the inhibitors

ADP and AMP are allosteric activators

Concept 9.6

Cellular respiration is controlled by feedback mechanisms

Feedback inhibition: end products inhibit the enzymes that catalyze the early steps of the process

Phosphofructokinase (enzyme for step 3 of glycolysis) is the pacemaker

Videos and Websites

http://vcell.ndsu.nodak.edu/animations/atpgradient/index.htm

http://vcell.ndsu.nodak.edu/animations/mito-pt/index.htm