Cellular Respiration: Obtaining Energy from Food

Chapter 6 Cellular Respiration: Obtaining Energy from Food

PowerPoint® Lectures for Campbell Essential Biology, Fifth Edition, and Campbell Essential Biology with Physiology, Fourth Edition – Eric J. Simon, Jean L. Dickey, and Jane B. Reece

• Animals depend on plants to convert the energy of sunlight to

– chemical energy of sugars and – other organic molecules we consume as food.

• Photosynthesis uses light energy from the sun to – power a chemical process and – make organic molecules.

• Plants and other autotrophs (self-feeders) – make their own organic matter from inorganic nutrients. • Heterotrophs (other-feeders) – include humans and other animals that cannot make organic molecules from inorganic ones.

• Autotrophs are producers because ecosystems depend upon them for food. • Heterotrophs are consumers because they eat plants or other animals.

• The ingredients for photosynthesis are carbon dioxide (CO2) and water (H2O).

– CO2 is obtained from the air by a plant’s leaves.

– H2O is obtained from the damp soil by a plant’s roots.

• Chloroplasts in the cells of leaves use light energy to rearrange the atoms of CO2 and H2O, which produces

– sugars (such as glucose), – other organic molecules, and – oxygen gas.

• Plant and animal cells perform cellular respiration, a chemical process that

– primarily occurs in mitochondria, – harvests energy stored in organic molecules, – uses oxygen, and – generates ATP.

• The waste products of cellular respiration are

– CO2 and H2O, – used in photosynthesis.

• Animals perform only cellular respiration. • Plants perform – photosynthesis and – cellular respiration.

• Plants usually make more organic molecules than they need for fuel. This surplus provides material that can be – used for the plant to grow or – stored as starch in potatoes.

Photosynthesis C H O 6 12 6 CO2

O2 H2O

Cellular respiration

ATP drives cellular work Heat energy exits ecosystem CELLULAR RESPIRATION: AEROBIC HARVEST OF FOOD ENERGY

• Cellular respiration is – the main way that chemical energy is harvested from food and converted to ATP and

– an aerobic process—it requires oxygen.

• Cellular respiration and breathing are closely related.

– Cellular respiration requires a cell to exchange gases with its surroundings. – Cells take in oxygen gas. – Cells release waste carbon dioxide gas. – Breathing exchanges these same gases between the blood and outside air.

O2 CO2

Breathing

Lungs

O 2 CO2

Muscle cells Cellular respiration The Simplified Equation for Cellular Respiration

• A common fuel molecule for cellular respiration is glucose.

• Cellular respiration can produce up to 32 ATP molecules for each glucose molecule consumed.

• The overall equation for what happens to glucose during cellular respiration is

– glucose & oxygen è CO2, H2O, & a release of energy.

C6H12O6 6 O2 6 CO2 6 H2O ATP

Glucose Oxygen Carbon Water Energy dioxide The Role of Oxygen in Cellular Respiration

• During cellular respiration, hydrogen and its bonding electrons change partners from sugar to oxygen, forming water as a product.

• Chemical reactions that transfer electrons from one substance to another are called – oxidation-reduction reactions or – redox reactions for short.

• The loss of electrons during a redox reaction is oxidation. • The acceptance of electrons during a redox reaction is reduction. • During cellular respiration – glucose is oxidized and – oxygen is reduced.

Oxidation Glucose loses electrons (and hydrogens)

C6H12O6 6 O2 6 CO2 6 H2O

Glucose Oxygen Carbon Water dioxide

Reduction Oxygen gains electrons (and hydrogens) Redox Reactions

• Why does electron transfer to oxygen release energy?

– When electrons move from glucose to oxygen, it is as though the electrons were falling. – This “fall” of electrons releases energy during cellular respiration.

• Cellular respiration is – a controlled fall of electrons and – a stepwise cascade much like going down a staircase.

• The path that electrons take on their way down from glucose to oxygen involves many steps. • The first step is an electron acceptor called NAD+. – NAD is made by cells from niacin, a B vitamin. – The transfer of electrons from organic fuel to NAD+ reduces it to NADH.

• The rest of the path consists of an electron transport chain, which – involves a series of redox reactions and – ultimately leads to the production of large amounts of ATP.

e- e- + Stepwise release NAD NADH of energy used to make

ATP 2 H+ 2 e-

2 e-

H+ 1 O 2 2 2

Hydrogen, electrons, and oxygen combine H2O to produce water Figure 6.5a Stepwise release ATP of energy used 2 H+ 2 e- to make ATP

Electron transport chain

2 e-

+ 1 2 H 2 O2

Hydrogen, electrons, and oxygen H O combine to produce water 2 An Overview of Cellular Respiration

• Cellular respiration is an example of a metabolic pathway, which is a series of chemical reactions in cells.

• All of the reactions involved in cellular respiration can be grouped into three main stages: 1. glycolysis, 2. the citric acid cycle, and 3. electron transport.

BioFlix Animation: Cellular Respiration

Cytoplasm

Animal cell Plant cell

Cytoplasm Mitochondrion

High-energy electrons via carrier molecules

Glycolysis Citric 2 Acid Electron Glucose Pyruvic Cycle Transport acid

ATP ATP ATP Figure 6.6a

Cytoplasm Mitochondrion

High-energy electrons via carrier molecules

Glycolysis Citric 2 Acid Electron Glucose Pyruvic Cycle Transport acid

ATP ATP ATP The Three Stages of Cellular Respiration

• With the big-picture view of cellular respiration in mind, let’s examine the process in more detail.

1. A six-carbon glucose molecule is split in half to form two molecules of pyruvic acid. 2. These two molecules then donate high energy electrons to NAD+, forming NADH.

INPUT OUTPUT – – NADH P P NAD+ 2 ATP 2 ADP

2 3 2 ATP P 2 ADP P 2 Pyruvic acid 1 P P P 2 3 Glucose 2 ADP + 2 ATP NAD P – – NADH

Energy investment phase Energy harvest phase Key Carbon atom P Phosphate group

– High-energy electron Figure 6.7a

INPUT OUTPUT

2 Pyruvic acid

Glucose Figure 6.7b-1

2 ATP 2 ADP

1 P

Energy investment phase Figure 6.7b-3

– – NADH P P + 2 ATP NAD 2 ADP

2 3 2 ATP P 2 ADP P

1 P P P 2 3

2 ADP + NAD P 2 ATP – – NADH

Energy investment phase Energy harvest phase Stage 1: Glycolysis

3. Glycolysis – uses two ATP molecules per glucose to split the six-carbon glucose and

– makes four additional ATP directly when enzymes transfer phosphate groups from fuel molecules to ADP.

• Thus, glycolysis produces a net of two molecules of ATP per glucose molecule.

Enzyme

P ADP ATP

P P Stage 2: The Citric Acid Cycle

• In the citric acid cycle, pyruvic acid from glycolysis is first “groomed.”

– Each pyruvic acid loses a carbon as CO2. – The remaining fuel molecule, with only two carbons left, is acetic acid.

• Oxidation of the fuel generates NADH.

• Finally, each acetic acid is attached to a molecule called coenzyme A to form acetyl CoA.

• The CoA escorts the acetic acid into the first reaction of the citric acid cycle.

• The CoA is then stripped and recycled.

INPUT OUTPUT 2 Oxidation of the fuel (from generates NADH (to citric glycolysis) acid cycle) – – NAD+ NADH CoA

1 Pyruvic acid Acetic 3 Acetic acid loses a carbon acid attaches to Acetyl CoA Pyruvic acid as CO2 coenzyme A CO2 Coenzyme A Figure 6.9a

INPUT OUTPUT (from (to citric glycolysis) acid cycle)

CoA

Acetyl CoA Pyruvic acid Figure 6.9b

2 Oxidation of the fuel generates NADH – – OUTPUT NAD+ NADH

1 Pyruvic acid Acetic 3 Acetic acid loses a carbon acid attaches to

as CO2 coenzyme A CO2 Coenzyme A Stage 2: The Citric Acid Cycle

• The citric acid cycle – extracts the energy of sugar by breaking the acetic

acid molecules all the way down to CO2, – uses some of this energy to make ATP, and

– forms NADH and FADH2.

Blast Animation: Harvesting Energy: Krebs Cycle

1 Acetic acid 2 2 CO2

ADP + P ATP 3 Citric Acid Cycle – – 3 NAD+ 3 NADH 4

– – FAD FADH2 5 6 Acceptor molecule Stage 3: Electron Transport

• Electron transport releases the energy your cells need to make the most of their ATP. • The molecules of the electron transport chain are built into the inner membranes of mitochondria. • The chain – functions as a chemical machine, which – uses energy released by the “fall” of electrons to pump hydrogen ions across the inner mitochondrial membrane, and – uses these ions to store potential energy.

• When the hydrogen ions flow back through the membrane, they release energy.

– The hydrogen ions flow through ATP synthase. – ATP synthase – takes the energy from this flow and – synthesizes ATP.

Space H+ between + + + + + H membranes H H H H H+ H+ Electron H+ + + carrier H+ H 3 H+ 5 H Protein complex

Inner mitochondrial

membrane – –

FADH2 FAD H+ Electron 2 1 O 2 + H O flow 2 2 H 2 6 – – 4 NADH NAD+ ADP P ATP 1 H+ H+ H+ H+ H+ Matrix Electron transport chain ATP synthase Figure 6.11a

Space H+ between + + + membranes H H+ H H+ H H+ H+ Electron H+ + H+ + carrier H 3 H+ 5 H Protein complex

Inner mitochondrial

membrane – –

FADH2 FAD H+ Electron 2

1 O H+ H O 6 flow 2 2 2 2 – – 4 NADH NAD+ ADP P ATP 1 H+ H+ H+ H+ H+ Matrix Electron transport chain ATP synthase Figure 6.11b

Space H

between + + + + H H membranes H H H+ H+ Electron H+ + carrier H+ H 3 H+ Protein complex

Inner mitochondrial

membrane – –

FADH2 FAD H+ Electron 2

1 O + flow 2 2 2 H

– – 4 NADH NAD+ 1 H+ H+ H+ H+ Matrix Electron transport chain Figure 6.11c H+

+ + H H+ H+ + H+ 5 H

1 O + H O 6 2 2 2 H 2 4 ADP P ATP

H+ H+

ATP synthase Stage 3: Electron Transport

• Cyanide is a deadly poison that – binds to one of the protein complexes in the electron transport chain,

– prevents the passage of electrons to oxygen, and – stops the production of ATP.

• Cellular respiration can generate up to 32 molecules of ATP per molecule of glucose.

Cytoplasm

Mitochondrion – – – – – – 6 NADH 2 NADH 2 NADH – –

2 FADH2

Glycolysis 2 2 Citric Electron Glucose Pyruvic Acetyl Acid Transport acid CoA Cycle Maximum per glucose:

2 2 About About ATP ATP 28 ATP 32 ATP

by direct by direct by ATP synthesis synthesis synthase Figure 6.12a

Glycolysis 2 2 Citric Electron Glucose Pyruvic Acetyl Acid Transport acid CoA Cycle

2 2 About ATP ATP 28 ATP

by direct by direct by ATP synthesis synthesis synthase The Results of Cellular Respiration

• In addition to glucose, cellular respiration can “burn”

– diverse types of carbohydrates, – fats, and – proteins.

Polysaccharides Fats Proteins

Sugars Glycerol Fatty acids Amino acids

Acetyl Citric Glycolysis Electron CoA Acid Cycle Transport

ATP FERMENTATION: ANAEROBIC HARVEST OF FOOD ENERGY

• Some of your cells can actually work for short periods without oxygen.

• Fermentation is the anaerobic (without oxygen) harvest of food energy.

• After functioning anaerobically for about 15 seconds, muscle cells begin to generate ATP by the process of fermentation.

• Fermentation relies on glycolysis to produce ATP. • Glycolysis – does not require oxygen and – produces two ATP molecules for each glucose broken down to pyruvic acid.

• Pyruvic acid, produced by glycolysis, – is reduced by NADH, – producing NAD+, which – keeps glycolysis going. • In human muscle cells, lactic acid is a by-product.

Animation: Fermentation Overview

INPUT OUTPUT 2 ADP 2 ATP 2 P

Glycolysis

– – – – 2 NAD+ 2 NADH 2 NADH 2 NAD+ 2 Pyruvic acid 2 H+ Glucose 2 Lactic acid Figure 6.14a

INPUT OUTPUT 2 ADP 2 ATP 2 P

Glycolysis

– – – – 2 NAD+ 2 NADH 2 NADH 2 NAD+ 2 Pyruvic acid 2 H+ Glucose 2 Lactic acid The Process of Science: What Causes Muscle Burn?

• Observation: Muscles produce lactic acid under anaerobic conditions. • Question: Does the buildup of lactic acid cause muscle fatigue?

• Hypothesis: The buildup of lactic acid would cause muscle activity to stop. • Experiment: Tested frog muscles under conditions when lactic acid – could and – could not diffuse away.

+ – + – Force Force measured measured

Frog muscle stimulated by electric current

Solution prevents Solution allows diffusion of lactic acid diffusion of lactic acid; muscle can work for twice as long The Process of Science: What Causes Muscle Burn?

• Results: When lactic acid could diffuse away, performance improved greatly.

• Conclusion: Lactic acid accumulation is the primary cause of failure in muscle tissue.

• However, recent evidence suggests that the role of lactic acid in muscle function remains unclear.

• Fermentation alone is able to sustain many types of microorganisms.

• The lactic acid produced by microbes using fermentation is used to produce

– cheese, sour cream, and yogurt, – soy sauce, pickles, and olives, and – sausage meat products.

• Yeast is a microscopic fungus that – uses a different type of fermentation and

– produces CO2 and ethyl alcohol instead of lactic acid. • This type of fermentation, called alcoholic fermentation, is used to produce – beer, – wine, and – breads.

INPUT OUTPUT 2 ADP 2 ATP + 2 P 2 CO2 released Glycolysis

– – – – 2 NAD+ 2 NADH 2 NADH 2 NAD+ 2 Ethyl alcohol 2 Pyruvic + 2 H+ Glucose acid Figure 6.16a

INPUT OUTPUT 2 ADP 2 ATP + 2 P 2 CO2 released Glycolysis

– – – – 2 NAD+ 2 NADH 2 NADH 2 NAD+ 2 Ethyl alcohol 2 Pyruvic +2 H+ Glucose acid Figure 6.16b Evolution Connection: Life before and after Oxygen

• Glycolysis could be used by ancient bacteria to make ATP

– when little oxygen was available, and – before organelles evolved. • Today, glycolysis – occurs in almost all organisms and – is a metabolic heirloom of the first stage in the breakdown of organic molecules.

2.1 O Earth First eukaryotic organisms 2.2 Atmospheric oxygen reaches 10% of modern levels 2.7 Atmospheric oxygen first appears

3.5 Oldest prokaryotic

Billions of years of ago Billions fossils

4.5 Origin of Earth Figure 6.17a 0 satmosphere ’ present in present in 2

2.1 O Earth First eukaryotic organisms 2.2 Atmospheric oxygen reaches 10% of modern levels 2.7 Atmospheric oxygen first appears

3.5 Oldest prokaryotic

Billions of years of ago Billions fossils

4.5 Origin of Earth Figure 6.17b Figure 6.UN03

Citric Electron Glycolysis Acid Cycle Transport

ATP ATP ATP Figure 6.UN04

Citric Electron Glycolysis Acid Cycle Transport

ATP ATP ATP Figure 6.UN05

Citric Electron Glycolysis Acid Cycle Transport

ATP ATP ATP Figure 6.UN06

Heat C6H12O6

Sunlight O2 ATP

Cellular Photosynthesis respiration

CO2 H2O Figure 6.UN07

C6H12O6 + 6 O2 6 CO2 + 6 H2O + Approx. 32 ATP Figure 6.UN08

Oxidation Glucose loses electrons (and hydrogens)

C6H12O6 CO2

Electrons ATP (and hydrogens)

O2 H2O Reduction Oxygen gains electrons (and hydrogens) Figure 6.UN09 Mitochondrion

– – – – – – 6 NADH 2 NADH 2 NADH – – 2 FADH2

Glycolysis 2 Citric 2 Acetyl Acid Electron Pyruvic Glucose CoA Transport acid Cycle

2 CO2 4 CO2 H2O About 2 by direct by direct 2 28 ATP by ATP ATP synthesis synthesis ATP synthase

About 32 ATP