capturing and releasing energy

Capturing and Releasing Energy

Chapter 5

5.1 Impacts/IssuesGreen Energy

We and most other organisms sustain ourselves by extracting energy stored in the organic products of photosynthesis

Photosynthesis• Metabolic pathway by which photoautotrophs

capture light energy and use it to make sugars from CO2 and water

Biofuels

Green Energy

Autotroph• Organism that makes its own food using carbon

from inorganic sources, such as CO2, and energy from the environment

Heterotroph• Organism that obtains energy and carbon from

organic compounds assembled by other organisms

Green Energy

Current biofuel research focuses on ways to break down abundant cellulose in fast growing weeds and agricultural wastes

Solar power

5.2 Capturing Rainbows

Energy radiating from the sun travels through space in waves and is organized in packets called photons

The spectrum of radiant energy from the sun includes visible light

Capturing Rainbows

Humans perceive different wavelengths of visible light as different colors

The shorter the wavelength, the greater the energy

Wavelength• Distance between the crests of two successive

waves of light

Capturing Rainbows

Photosynthetic species use pigments to harvest light energy for photosynthesis

Pigment• An organic molecule that can absorb light at

specific wavelengths

Chlorophyll a• Main photosynthetic pigment in plants

Wavelength and theElectromagnetic Spectrum

Some Photosynthetic Pigments

5.3 Storing Energy in Carbohydrates

Photosynthesis converts the energy of light into the energy of chemical bonds, which can power reactions of life and be stored for later use

Photosynthesis takes place in two stages• Light-dependent reactions• Light-independent reactions

The First Stage of Photosynthesis

Light-dependent reactions (“photo”)• Convert light energy to chemical energy of ATP

and NADPH, releasing oxygen• Occur at the thylakoid membrane in plant

chloroplasts

Photosystem• Cluster of pigments and proteins that converts

light energy to chemical energy in photosynthesis

Chloroplasts and the Thylakoid Membrane

Chloroplast• Organelle of photosynthesis in plants and some

protists

Thylakoid membrane• Chloroplast’s highly folded inner membrane system• Forms a continuous compartment in the stroma

The Second Stage of Photosynthesis

Light-independent reactions (“synthesis”)• ATP and NADPH drive synthesis of glucose and

other carbohydrates from water and CO2

• Occurs in the stroma

Stroma• Semifluid matrix between the thylakoid

membrane and the two outer membranes of a chloroplast

Fig. 5-3, p. 83

A Many photosynthetic cells in a leaf B Many chloroplasts in a photosynthetic

cell

C Many thylakoids in a chloroplast

A Leaf: Sites of Photosynthesis

Sites of photosynthesis

“green spots” arechloroplast

Summary: Photosynthesis

6CO2 + 6H2O → (light energy) → C6 H12O6 + 6O2

A Chloroplast

Fig. 5-4, p. 84Stepped Art

glucose

light water

light-dependent reactions

carbon dioxide, water

light- independent

reactions

NADPH, ATP

oxygen

NADP+, ADP

Chemical bookkeeping

5.4 The Light-Dependent Reactions

Chlorophylls and other pigments in the thylakoid membrane absorb light energy and pass it to photosystems, which then release electrons

Energized electrons leave photosystems and enter electron transfer chains in the membrane; hydrogen ion gradients drive ATP formation

Oxygen is released; electrons end up in NADPH

Light-Dependent Reactions

Steps in Light-Dependent Reactions

1. Light energy ejects electrons from a photosystem

2. Photosystem pulls replacement electrons from water, releasing O2

3. Electrons enter an electron transfer chain (ETC) in the thylakoid membrane

4. Electron energy is used to form a hydrogen-ion gradient across the thylakoid membrane

Steps in Light-Dependent Reactions

5. Another photosystem receives electrons from the ETC

6. Electrons move through a second ETC; NADPH is formed

7. Hydrogen ions flow across the thylakoid membrane through ATP synthase and power ATP formation in the stroma

Electron Transfer Phosphorylation

Electron transfer phosphorylation• Metabolic pathway in which electron flow through

electron transfer chains sets up a hydrogen ion gradient that drives ATP formation

Fig. 5-5, p. 85

to light-independent reactionslight energy light energy

41 5 7

2

thylakoid compartmentthylakoid membrane

The Light-Dependent Reactions of Photosynthesis

stroma3 6

Light-Dependent Reactions

5.5 The Light-Independent Reactions

Driven by the energy of ATP and electrons from NADPH, light-independent reactions use carbon and oxygen from CO2 to build sugars

Carbon Fixation

In the stroma of chloroplasts, the enzyme rubisco fixes carbon from CO2 in the Calvin–Benson cycle

Carbon fixation• Process by which carbon from an inorganic

source such as CO2 becomes incorporated into an organic molecule

Calvin-Benson Cycle

Calvin-Benson cycle• Light-independent reactions of photosynthesis• Cyclic pathway that forms glucose from CO2 • Uses energy from ATP and electrons from

NADPH

Rubisco• Enzyme that fixes carbon from CO2 to RuBP in

the Calvin-Benson cycle

Fig. 5-6, p. 86

chloroplast

CO2, H2Ostroma

PGA RuBPATP Calvin–

Benson CycleNADPH ATP

sugars

Light-Independent Reactions

Calvin-Benson cycle

Carbon-Fixing Adaptations

Several adaptations, such as a waterproof cuticle, allow plants to live where water is scarce

Stomata• Gaps that open between guard cells on plant

surfaces; allow gas exchange through the cuticle

C3 plants• Use only the Calvin-Benson cycle to fix carbon• Conserve water by closing stomata on dry days

Photorespiration

When stomata are closed, oxygen builds up and interferes with sugar production

Photorespiration• Reaction in which rubisco attaches O2 instead of

CO2 to RuBP

Fig. 5-7d, p. 87

5.6 Photosynthesis and Aerobic Respiration: A Global Connection

Earth’s atmosphere was permanently altered by the evolution of photosynthesis

Oxygen and the Atmosphere

Photoautotroph• Photosynthetic autotroph

Anaerobic• Occurring in the absence of oxygen

Aerobic• Involving or occurring in the presence of oxygen

Extracting Energy From Carbohydrates

Eukaryotic cells typically convert chemical energy of carbohydrates to chemical energy of ATP by oxygen-requiring aerobic respiration

Aerobic respiration• Aerobic pathway that breaks down carbohydrates

to produce ATP• Pathway finishes in mitochondria

Photosynthesis and Aerobic Respiration

An Overview of Aerobic Respiration

Aerobic respiration is divided into three steps1. Glycolysis2. Acetyl CoA formation and the Krebs cycle3. Electron transfer phosphorylation

In the first two stages, coenzymes pick up electrons

In the third stage, electron energy drives ATP synthesis

Aerobic Respiration Begins

Glycolysis• Reactions in which glucose or another sugar is

broken down into 2 pyruvates, netting 2 ATP

Pyruvate• Three-carbon product of glycolysis

Aerobic Respiration Continues

Krebs cycle• Cyclic pathway that, along with acetyl CoA

formation, breaks down pyruvate to CO2, netting 2 ATP and many reduced coenzymes

Acetyl CoA Formation and the Krebs Cycle

Fig. 5-10a, p. 90

Mitochondrion

outer membrane (next to cytoplasm)

inner membrane

inner mitochondrial compartment

outer mitochondrial compartment (in between the two membranes)

A An inner membrane divides a mitochondrion’s interior into an inner compartment and an outer compartment. The second and third stages of aerobic respiration take place at the inner mitochondrial membrane.

Fig. 5-10b, p. 90

Second Stage of Aerobic Respiration

2 pyruvate

outer membrane (next to cytoplasm)

inner membrane

6 CO 22 acetyl–CoA2 ATP

Breakdown of 2 pyruvate to 6 CO2 yields 2 ATP. Also, 10 coenzymes (8 NAD+, 2 FAD) combine with electrons and hydrogen ions, which they carry to the third and final stage of aerobic respiration.

Krebs Cycle

8 NADH

2 FADH2

B The second stage starts after membrane proteins transport pyruvate from the cytoplasm to the inner compartment. Six carbon atoms enter these reactions (in two molecules of pyruvate), and six leave (in six CO2). Two ATP form and ten coenzymes accept electrons and hydrogen ions.

The Krebs Cycle - details

Fig. 5-11, p. 91

Third Stage of Aerobic Respiration: Electron Transfer Phosphorylation

4

2

3 5

1

Stepped Art

Electron Transfer Phosphorylation

Summary: Aerobic Respiration

C6H12O6 (glucose) + 6O2 (oxygen) + 36 ADP →

6CO2 (carbon dioxide) + 6H2O (water) + 36 ATP

ATP C The third and final stage, electron transfer phosphorylation, occurs inside mitochondria. 10 NADH and 2 FADH2 donate electrons and hydrogen ions to electron transfer chains. Electron flow through the chains sets up hydrogen ion gradients that drive ATP formation. Oxygen accepts electrons at the end of the chains.

ATPATP

Electron Transfer Phosphorylation

H2Ooxygen32 ATP

Fig. 5-9, p. 89

glucose Aerobic RespirationCytoplasm

2 ATP ATP Glycolysis 4 ATP (2 net)

ATP A The first stage, glycolysis, occurs in the cell’s cytoplasm. Enzymes convert a glucose molecule to 2 pyruvate for a net yield of 2 ATP. 2 NAD + combine with electrons and hydrogen ions during the reactions, so 2 NADH also form.2 NADH 2 pyruvate

Mitochondrion

Krebs Cycle

6 CO2

B The second stage occurs in mitochondria. The 2 pyruvate are converted to a molecule that enters the Krebs cycle. CO2 forms and leaves the cell. 2 ATP, 8 NADH, and 2 FADH2 form during the reactions.

2 ATP ATP

8 NADH, 2 FADH2

Stepped Art

Summary: Aerobic Respiration

Overview of aerobic respiration

Where pathways start and finish

Third-stage reactions

Mitochondrial chemiosmosis

5.7 Fermentation

Fermentation• Anaerobic pathway that harvests energy from

carbohydrates • Alcoholic fermentation and lactate fermentation

In fermentation, ATP is formed by glycolysis only• Net yield of 2 ATP per glucose molecule• Coenzyme NAD+ is regenerated, which allows

glycolysis to continue• Fermentation pathways finish in the cytoplasm

Alcoholic Fermentation

Alcoholic fermentation• Anaerobic pathway that converts pyruvate to

ethanol and produces ATP• Examples: baking, wine production

Fig. 5-12b, p. 92

NADH NAD+

+

pyruvate ethanolacetaldehydecarbon dioxide

pyruvate lactate

NADH NAD+

Fermentation pathways

Lactate Fermentation

Lactate fermentation• Anaerobic pathway that converts pyruvate to

lactate and produces ATP• Examples: cheese, pickles

5.8 Alternative Energy Sources in the Body

Carbohydrates Fats Proteins

Energy from Carbohydrates

Glucose is absorbed from the intestines into the blood and broken down by glycolysis

Blood glucose levels are regulated by the pancreatic enzymes insulin and glucagon

Excess glucose intake stimulates storage as glycogen and fatty acids

Energy from Fats

The body stores most fats as triglycerides

When blood glucose falls, enzymes break triglycerides into glycerol and fatty acids• Glycerol enters glycolysis• Fatty acids enter the Krebs cycle as acetyl-CoA

Fatty acids yield more energy (ATP) than carbs

Energy from Proteins

Proteins enter the bloodstream as amino acids

Amino acids can be used for energy by removing the amino group (as ammonia) and converting the carbon backbone to acetyl-CoA, pyruvate, or an intermediate of the Krebs cycle

Food

Complex Carbohydrates

glucose, other simple sugars

Glycolysis

NADH pyruvate

Krebs Cycle

NADH, FADH2

Electron Transfer Phosphorylation

Fig. 5-14, p. 95

Fats

fatty acids glycerol

acetyl–CoA intermediate of glycolysis

Proteins

amino acids

acetyl–CoA

intermediate of Krebs cycle

Stepped Art

Alternative Energy Sources in the Body

5.9 Impacts/Issues Revisited

Human activities are disrupting the global cycling of carbon dioxide; we are adding more CO2 to the atmosphere than photoautotrophs are removing from it

The resulting imbalance fuels global warming

Fossil Fuel Emissions

Biofuels of the Future

Digging Into Data:Energy Efficiency of Biofuel Production