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Chapter 5
Capturing and Releasing Energy
BIOLOGY: Today and Tomorrow, 4e
starr
evers starr
5.1 A Burning Concern
 Photosynthesizers remove CO2 from the atmosphere and lock
its carbon atoms in organic compounds
 When organisms break down organic compounds for energy,
carbon atoms (CO2) reenter the atmosphere
 When humans began burning forests to clear land, and fossil
fuels (coal, petroleum, natural gas) for energy, we put the
atmospheric carbon cycle out of balance
Products of fossil fuel combustion
Producers and Consumers
 Autotroph
 Organism that makes its own food using carbon from
inorganic sources (CO2) and energy from the environment
 Heterotroph
 Organism that obtains energy and carbon from organic
compounds assembled by other organisms
 Photosynthesis
 Metabolic pathway by which photoautotrophs capture light
energy and use it to make sugars from CO2 and water
5.2 Capturing Rainbows
 Energy radiating from the sun travels through space in waves
and is organized in packets called photons
 Humans perceive different wavelengths of visible light as
different colors
 Wavelength
 Distance between the crests of two successive waves
Wavelength and the Electromagnetic Spectrum
shortest wavelengths
(highest energy)
range of most radiation
reaching Earth’s surface
longest wavelengths
(lowest energy)
visible light
gamma
rays
ultraviolet
x-rays
radiation
400 nm
near-infrared
radiation
infrared
radiation
500 nm
microwaves
600 nm
radio waves
700 nm
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
Photosynthetic Pigments
5.3 Storing Energy in Sugar
 Photosynthesis converts light energy into energy of chemical
bonds, which power reactions and can be stored for later use
6CO2 + 6H2O
→ (light energy) → C
6
H12O6 + 6O2
The Chloroplast
 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
 Stroma
 Semifluid matrix between the thylakoid membrane and the
two outer membranes of a chloroplast
The site of photosynthesis
two outer membranes
of chloroplast
stroma
part of thylakoid
membrane system:
thylakoid
compartment,
cutaway view
Two Stages 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
 Light-independent reactions (“synthesis”)
 ATP and NADPH drive synthesis of glucose and other
carbohydrates from water and CO2
 Occurs in the stroma
Two Stages of Photosynthesis
ADP
NADP+
H2O
energy
ATP
A) Light-dependent reactions
O2
ATP
NADPH
CO2
NADPH
ADP
B) Light-independent
reactions (Calvin–Benson
cycle)
NADP +
glucose
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
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
8. ATP forms 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
Light-Dependent Reactions
1
light energy
3
photosystem
thylakoid
2
compartment H2O
electron
transfer
chain
4
5
light
energy
6
electron
transfer
chain
8
ATP
synthase
ADP,
phosphate
7
stroma
O2
5.5 The Light-Independent Reactions
 Calvin–Benson cycle
 Light-independent reactions of photosynthesis
 Cyclic carbon-fixing pathway that forms sugars from CO2
 Driven by energy of ATP and electrons from NADPH
 Carbon fixation
 Process by which carbon from an inorganic source such
as CO2 gets incorporated into an organic molecule
 In most plants, the enzyme rubisco fixes carbon by
attaching CO2 to RuBP
Light-Independent Reactions
chloroplast
stroma
CO2
PGA
RuBP
Calvin–
Benso
n Cycle
glucose
Adaptations to Hot and Dry Climates
 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; oxygen
builds up and interferes with sugar production
Stomata
A) Tiny pores called stomata are visible in this close-up of a leaf. Stomata close to conserve water on
hot, dry days, and this causes oxygen to accumulate inside the plant’s tissues. The buildup makes
sugar production inefficient in C3 plants.
Adaptations to Hot and Dry Climates
 Alternative light-independent reactions minimize binding of
oxygen to rubisco in some types of plants
 C4 plants
 Plants that minimize photorespiration by fixing carbon
twice, in two cell types
 CAM plants
 C4 plants that conserve water by fixing carbon twice, at
different times of day
C4 Plants
B) Crabgrass “weeds” overgrowing a lawn. Crabgrasses, which are C4 plants, thrive in hot, dry
summers, when they easily outcompete Kentucky bluegrass and other fine-leaved C3 grasses
commonly planted in residential lawns.
CAM Plants
C) The jade plant, Crassula argentea, and other CAM plants survive in hot deserts by opening stomata
to fix carbon only at night. They run the Calvin–Benson cycle during the day, when stomata are closed.
5.6 Photosynthesis and Aerobic Respiration:
A Global Connection
 Ancient organisms extracted energy and carbon from
molecules such as methane and hydrogen sulfide in Earth’s
early atmosphere
 Earth’s atmosphere was permanently altered by the evolution
of photosynthesis, when oxygen accumulated in the ocean
and the atmosphere
Then and Now
Oxygen and the Atmosphere
 Oxygen reacts with metals such as enzyme cofactors – free
radicals form and damage biological molecules, so they are
dangerous to life
 Anaerobic
 Occurring in the absence of oxygen
 Aerobic
 Involving or occurring in the presence of oxygen
Aerobic Respiration
 Aerobic respiration
 Aerobic pathway that
breaks down sugars to
produce ATP
 Requires oxygen
 Produces carbon dioxide
and water
Aerobic Respiration
 Glycolysis (stage 1)
 Reactions in which glucose or another sugar is broken
down into 2 pyruvates, netting 2 ATP
 Coenzymes (2 NADH) pick up electrons
 Krebs cycle (stage 2)
 Along with acetyl CoA formation, breaks down pyruvate to
CO2, netting 2 ATP and reduced coenzymes (8 NADH and
2 FADH2)
Oxidation of glucose to CO2
Aerobic Respiration
 Electron transfer phosphorylation (stage 3)
 NADH and FADH2 deliver electrons to the inner
mitochondrial membrane
 Electron flow through chains pumps H+ from inner to outer
compartment, forming a gradient
 O2 accepts electrons and H+, forming H2O
 H+ flows back into inner compartment through ATP
synthase, forming ATP from ADP and Pi
Cytoplasm
glucose
2
Glycolysis
2 NAD+
2 NADH
4
6 NADH
2 FADH2
(2 net)
2 pyruvate
2 NADH
2 NADH
Overview of Aerobic
Respiration
Mitochondrion
2 acetyl–CoA
Krebs
Cycle
2 CO2
4 CO2
2
32
Electron Transfer
Phosphorylation
oxygen H2O
Final Stage of Aerobic Respiration
2
5
electron
transfer chain
ADP, phosphate
1
inner membrane
3
O2
2H2O
4
outer membrane
cytoplasm
Summary: Aerobic Respiration
C6H12O6 (glucose) + 6O2 (oxygen) + 36 ADP
→
6CO2 (carbon dioxide) + 6H2O (water) + 36 ATP
3D ANIMATION: Cellular Respiration
3D ANIMATION: Photosynthesis Bio
Experience 3D
5.7 Fermentation
 Fermentation
 Anaerobic pathway that harvests energy from
carbohydrates
 Alcoholic fermentation
 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
 Anaerobic pathway that
converts pyruvate to
ethanol and produces ATP
 Examples: baking, wine
and beer production
Alcoholic Fermentation
NADH
NAD+
+
pyruvate
carbon
dioxide
acetaldehyde
ethanol
A) The last stages of alcoholic fermentation produce CO2, ethanol, and NAD+.
Alcoholic Fermentation
B) One product of alcoholic fermentation in Saccharomyces cells (ethanol) makes beer alcoholic;
another (CO2) makes it bubbly. Holes in bread are pockets where CO2 released by fermenting
Saccharomyces cells accumulated in the dough. The micrograph shows budding Saccharomyces cells.
Lactate Fermentation
 Lactate fermentation
 Anaerobic pathway that converts pyruvate to lactate and
produces ATP
 Examples: cheese, pickles
 Lactate production in muscles
 Skeletal muscles have two types of fibers: slow-twitch
(aerobic) and fast-twitch (anaerobic)
 Fast-twitch fibers have few mitochondria and rely on
lactate fermentation for quick energy
 Good for quick, strenuous activity such as sprinting or
weight-lifting
Lactate Fermentation
A) The last stage of lactate fermentation produces lactate and NAD+.
Lactate Fermentation
B) Lactate fermentation occurs in white muscle fibers, visible in this cross-section of
human thigh muscle. The red fibers, which make ATP by aerobic respiration, sustain
endurance activities.
Lactate Production in Muscles
C) Intense activity such as sprinting quickly depletes oxygen in muscles. Under anaerobic conditions,
ATP is produced mainly by lactate fermentation in white muscle fibers. Fermentation does not make
enough ATP to sustain this type of activity for long.
5.8 Alternative Energy Sources in Food
 In humans and other organisms, simple sugars from
carbohydrate breakdown, glycerol and fatty acids from fat
breakdown, and carbon backbones of amino acids from
protein breakdown may enter aerobic respiration at various
reaction steps
Complex Carbohydrates
 Complex carbohydrates are broken down into
monosaccharide subunits and broken down by glycolysis
 High concentration of ATP causes sugars to be diverted from
glycolysis and into a pathway that forms glycogen
 Between meals, the liver maintains blood glucose level by
converting stored glycogen to glucose
Fats
 Fats are broken down into glycerol and fatty acids
 Fatty acids enter the Krebs cycle as acetyl-CoA
 Glycerol enters glycolysis
 Fatty acids yield more energy (ATP) than carbs
 When blood glucose is high, acetyl–CoA is diverted from the
Krebs cycle and into a pathway that makes fatty acids
Proteins
 Proteins are broken down into 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
Alternative Energy Sources in the Body
starch (a complex carbohydrate)
glucose
Food
a fat
glycerol
head
fatty acids
fatty
acid
tails
Complex Carbohydrates
Fats
2
acetyl–CoA
glycerol
glucose, other simple sugars
3
Proteins
amino acids
1
PGAL
4
acetyl–CoA
Glycolysis
3
NADH pyruvate
Intermediate of
Krebs cycle
Krebs
Cycle
Alternative Energy
Sources in the Body
NADH, FADH2
Electron Transfer
Phosphorylation
Alternative Energy Sources in the Body
alanine (an amino acid)
pyruvate
5.9 A Burning Concern (revisited)
 Air and dust trapped in ancient Antarctic ice reveal the
composition of the atmosphere millions of years ago
 Due to human activities, the atmospheric CO2 level today is
higher than it has been for 15 million years
 The increase in CO2 is contributing to global climate change
Air bubbles trapped in Antarctic ice
Digging Into Data:
Energy Efficiency of Biofuel Production