Objectives

• Contrast the roles of glycolysis and aerobic respiration in cellular respiration.

• Relate aerobic respiration to the structure of a mitochondrion.

• Summarize the events of the Krebs cycle.

• Summarize the events of the electron transport chain and H+/ ATP synthase pump.

• Calculate the efficiency of aerobic respiration.

Light energy

ECOSYSTEM

CO2 + H2O

Photosynthesis

in chloroplasts

Cellular respiration

in mitochondria

Organic

molecules+ O2

ATP

powers most cellular work

Heat

energy

PhotosynthesisCell Respiration Cycle

Life Is Work

• Living cells– Require

transfusions of energy (a.k.a., food) from outside sources to perform their many tasks

– The giant panda obtains energy for its cells by eating plants

Harvesting Chemical Energy

• Cellular respiration is the process by which cells break down organic compounds to produce ATP.

• Both autotrophs and heterotrophs use cellular respiration to make CO2 and water from organic compounds and O2.

• The products of cellular respiration are the reactants in photosynthesis; conversely, the products of photosynthesis are reactants in cellular respiration.

• Cellular respiration can be divided into two stages: glycolysis and aerobic respiration.

• An overview of cellular respiration…

Electrons

carried

via NADH

Glycolsis

Glucose Pyruvate

ATP

Substrate-level

phosphorylation

Electrons carried

via NADH and

FADH2

Citric

acid

cycle

Oxidative

phosphorylation:

electron transport

and

chemiosmosis

ATPATP

Substrate-level

phosphorylation

Oxidative

phosphorylation

Mitochondrion

Cytosol

AEROBIC RESPIRATIONAEROBIC RESPIRATION

Cellular Respiration

CELLULAR RESPIRATION (Figure 5.4, p. 133)

• ALL organisms do respiration!!!

• energy in glucose released to produce ATP

• two types of respiration– AEROBIC (requires oxygen) – ANAEROBIC (does not require oxygen)

• both types start with glycolysis

GLYCOLYSIS (Figure 5.6, p. 135)

• glycolysis is the first step of respiration• occurs in ALL cells!!!• occurs in cytoplasm and is anaerobic (no O2)

• glucose is converted to pyruvic acid– produces 2 ATP (~2% of energy in 1 glucose)– pyruvic acid then used in…

• fermentation or• aerobic respiration

Summary of Cellular Respiration

FERMENTATION (ANAEROBIC)

• occurs in cytoplasm– consumes pyruvic acid with no ATP production!!!

• LACTIC ACID FERMENTATION – creates lactic acid– example overworked muscles with low O2 – feels like cramp or soreness

• ALCOHOLIC FERMENTATION - produces ethyl alcohol and CO2

– used to make beer; wine; dough

• In alcohol fermentation– Pyruvate is

converted to ethanol in two steps, one of which releases CO2

• During lactic acid fermentation

– Pyruvate is reduced directly to NADH to form lactate as a waste product

2 ADP + 2 P1 2 ATP

GlycolysisGlucose

2 NAD+ 2 NADH2 Pyruvate

2 Acetaldehyde 2 Ethanol(a) Alcohol fermentation

2 ADP + 2 P1 2 ATP

GlycolysisGlucose

2 NAD+ 2 NADH

2 Lactate(b) Lactic acid fermentation

HH OH

CH3

C

O –

OCC O

CH3

HC O

CH3

O–

C OC OCH3O

C OC OHH

CH3

CO22

Overview of Aerobic Respiration • In eukaryotic cells, the processes of

aerobic respiration occur in the mitochondria. Aerobic respiration only occurs if oxygen is present in the cell.

• The Krebs cycle occurs in the mitochondrial matrix. The electron transport chain (which is associated with the production of ATPs) is located in the inner membrane.

Mitochondria, the super-energy harvesters

• Mitochondria

– are the sites of aerobic cellular respiration

– are found in nearly all eukaryotic cells

– are enclosed by two membranes• smooth outer membrane• an inner membrane folded into cristae (i.e.,

higher surface area for chemical reactions)

Mitochondrion

Intermembrane space

Outer

membrane

Free

ribosomes

in the

mitochondrial

matrix

Mitochondrial

DNA

Inner

membrane

Cristae

Matrix

100 µm

AEROBIC RESPIRATION (requires O2)

• Net reaction…

C6H12O6 + 6O2 6CO2 + 6H2O + ATPmitochondria

enzymes

The Krebs Cycle

• In the mitochondrial matrix, pyruvic acid produced in glycolysis reacts with coenzyme A to form acetyl CoA. Then, acetyl CoA enters the Krebs cycle.

• One glucose molecule is completely broken down in two turns of the Krebs cycle. These two turns produce four CO2 molecules, two ATP molecules, and hydrogen atoms* that are used to make six NADH* and two FADH2* molecules.

• * The energy released by the breakdown of glucose is about to be transferred to ATP!!!

Summary of Cellular Respiration

AEROBIC RESPIRATION (requires O2)

• occurs ONLY in mitochondria (Figure 5.10, p.138)• pyruvic acid from glycolysis is metabolized in

the KREB’S CYCLE– occurs in the matrix– two turns of cycle (1 glucose molecule) yields 4 CO2,

2 ATP, and several energy rich molecules (NADH and FADH2 )

– the energy rich molecules NADH and FADH2 are used to make a lot of ATP energy rich (using electron transport chains and ATP synthase)

KREB’S CYCLE (in the matrix)

• Enzymes in the Kreb’s (or citric acid) cycle…pyruvate

Summary of Kreb’s (Citric Acid) Cycle

X 2

X 2

X 2

X 2

X 2

diffuse out

used in e- transport chain

used in e- transport chain

Electron Transport Chain and ATP Synthesis

• High-energy electrons from from NADH and FADH2 are passed from molecule to molecule in the electron transport chain along the inner mitochondrial membrane.

• Hydrogen ions, H+, are also given up by NADH and FADH2.

• As the electrons move through the electron transport chain, they lose energy. This energy is used to pump protons from the matrix into the intermembrane space.

• The resulting concentration gradient of hydrogen ions drives ATP synthase and ATP production!

Electron Transport Chain, H+, & ATP synthesis• H+ move through ATP synthase to make ATP from ADP+Pi • Oxygen combines with the electrons and protons to form water.

Electron Transport Chain and ATP synthase

• The Importance of Oxygen– ATP can be synthesized using the diffusion of

H+ ions from the outer compartment to the inner compartment only if electrons continue to move along the electron transport chain.

– Oxygen is the final electron acceptor

– As a result, ATP can continue to be made through the ATP pump.

ATP Synthase

makes ATP !!!

An Accounting of ATP Production by Cellular Respiration

• During respiration, most energy flows in this sequence

glucose NADH e- transport chain H+ gradient ATP

Energy Summary of Cellular Respiration

AEROBIC RESPIRATION• Electron Transport Chains & ATP synthesis

– uses membrane bound proteins– O2 molecules are consumed

– this produces 32 more ATP• SO... total yield of ATP (1 glucose molecule) from

aerobic respiration AND glycolysis = 38 ATP• 19-times more ATP than from anaerobic

respiration alone !!! (~40% of the energy in a glucose molecule is converted to ATP)

• WHAT HAPPENS TO THE OTHER 60%?

Summary of Cellular Respiration

A Summary of Cellular Respiration

• Another Role of Cellular Respiration

– Providing cells with ATP is not the only important function of cellular respiration.

– Molecules formed at different steps in glycolysis and the Krebs cycle are often used by cells to make compounds that are missing in food.

• fatty acids, glycerol, amino acids

Comparing Aerobic and Anaerobic Respiration

• (REVIEW) Glycolysis

– Can produce ATP with or without oxygen, in aerobic or anaerobic conditions

– Glycolysis is often coupled with fermentation to produce a few more ATP, especially in prokaryotes

• Fermentation consists of

– Glycolysis plus either alcohol or lactic acid fermentation

Fermentation and Cellular Respiration Compared

• Both fermentation and cellular respiration

– Use glycolysis to oxidize glucose and other organic fuels to pyruvate

• Cellular respiration (i.e., complete oxidation) is more efficient than anaerobic respiration pathways

– Produces more 19x more ATP (38 versus 2 ATP)

• Pyruvate is a key juncture in catabolism

Glucose

CYTOSOL

Pyruvate

No O2 presentFermentation

O2 present Cellular respiration

Ethanolor

lactate

Acetyl CoA

MITOCHONDRION

Citricacidcycle

The Evolutionary Significance of Glycolysis

• Glycolysis

– Occurs in nearly all organisms

– Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere

– The pathways are similar (or conserved) across nearly all kingdoms (i.e., they are physiologically important and have not changed very much)