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© Cengage Learning 2015
6.5 How Do The Light-Independent
Reactions Work?
• The Calvin–Benson cycle:
– Build sugars in the stroma of chloroplasts
– Not powered by light energy
– Driving force is ATP and NADPH that formed in the light-
dependent reactions
– Uses carbon atoms from CO2 to make sugars
• Carbon fixation: carbon from an inorganic source gets incorporated into
an organic molecule
1
© Cengage Learning 2015
The Calvin-Benson Cycle
• Steps of the Calvin–Benson cycle:
1) The enzyme rubisco fixes carbon by attaching CO2 to RuBP
(ribulose bisphosphate)
2) Twelve PGA (phosphoglycerate) molecules form
2
© Cengage Learning 2015
The Calvin-Benson Cycle (cont’d.)
• Steps of the Calvin–Benson cycle (cont’d.):
3) Twelve PGAL (phosphoglyceraldehyde) molecules form
• Two PGAL may combine to form one six-carbon sugar (such as
glucose)
4) Ten remaining PGAL regenerate the starting compound,
RuBP
3
© Cengage Learning 2015
Adaptations to Climate
• Stomata are tiny gateways for gases
– Open stomata:
• Allow CO2 to diffuse from the air into photosynthetic tissues
• Allow O2 to diffuse out of these tissues into the air
– Closed stomata:
• Conserve water on hot, dry days
• Limit the availability of CO2 for the light-independent reactions; sugar
synthesis slows
4
© Cengage Learning 2015
Adaptations to Climate (cont’d.)
• C3 plants:
– Use only the Calvin–Benson cycle to fix
carbon
– When CO2 concentration declines, rubisco
uses oxygen as a substrate in photorespiration
– When stomata are closed during the day, C3
plants lose carbon instead of fixing it
– Extra energy is required to make sugars
– Contain high levels of rubisco
5
© Cengage Learning 2015
Adaptations to Climate (cont’d.)
6
glycolate
CO2
RuBP
sugar
PGA
Calvin–
Benson
Cycle
O2
When carbon
dioxide level
declines in the
leaves of a C3 plant,
rubisco uses oxygen
as a substrate in
photorespiration,
and sugar
production
becomes inefficient.
© Cengage Learning 2015
Adaptations to Climate (cont’d.)
• C4 plants:
– Close stomata on dry days, but their sugar production does not
decline
– Minimize photorespiration by fixing carbon twice in two cell
types
• Mesophyll cells
• Bundle-sheath cells
– Examples: corn, switchgrass, and bamboo
7
© Cengage Learning 2015
Adaptations to Climate (cont’d.)
• CAM plants:
– C4 plant that conserves water by fixing carbon twice, at
different times of day
– Conserve water even in desert regions with extremely high
daytime temperatures
– Open stomata at night when lower temperatures minimize
evaporative water loss
8
© Cengage Learning 2015
Adaptations to Climate (cont’d.)
9
mesophyll cell
bundle-sheath cell
In a C3 plant (left), chloroplasts – the sites of carbon fixation- occur mainly in mesophyll cells. In the C4
plant (right), carbon fixation occurs first in the mesophyll cells, then in bundle-sheath cells. C4 adaptations
maintain a high carbon dioxide to oxygen ratio near rubisco.
© Cengage Learning 2015
Adaptations to Climate (cont’d.)
10
Crabgrass “weeds”, 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.
© Cengage Learning 2015
6.6 Application: Green Energy
• With fossil fuel prices soaring, there is an increasing
demand for biofuels:
– Oils, gases, or alcohols made from organic matter that is not
fossilized
• Most materials we use for biofuel production today consist
of food crops:
– Mainly corn, soybeans, and sugarcane
11
© Cengage Learning 2015
Application: Green Energy (cont’d.)
• Fossil fuels:
– Petroleum, coal, and natural gas
– Formed from the remains of ancient swamp forests that
decayed and compacted over millions of years
– Consist of molecules originally assembled by ancient plants
12
© Cengage Learning 2015
Application: Green Energy (cont’d.)
• The process of using fossil fuels or biofuels are
fundamentally the same:
– Release energy by breaking the bonds of organic molecules
– Use oxygen to break those bonds
– Produce carbon dioxide
13
© Cengage Learning 2015
Application: Green Energy (cont’d.)
• Biofuels are a renewable source of energy
– Growing more plants creates more sources of biofuels
14
© Cengage Learning 2015
Application: Green Energy (cont’d.)
• Biofuels do not contribute to global climate change
– Growing plant matter for fuel recycles carbon that is already in
the atmosphere
15
© Cengage Learning 2015
Application: Green Energy (cont’d.)
• Corn and other food crops are rich in oils, starches, and
sugars that can be easily converted to biofuels
– Starch from corn kernels can be broken down to glucose, which
is converted to ethanol by bacteria or yeast
16
© Cengage Learning 2015
Application: Green Energy (cont’d.)
• Making biofuels from some plants require additional
steps, because these materials contain a higher
proportion of cellulose
– Researchers are currently working on cost-effective ways to
break down the abundant cellulose in fast-growing weeds
17
© Cengage Learning 2015
Application: Green Energy (cont’d.)
• According to tropical forest scientist Willie Smits, the
Arenga sugar palm has the potential to serve as the core
of a waste-free system
– Produces premium organic sugar and ethanol for fuel
– Provides food products and jobs to villagers
– Helps preserve the existing native rain forest
18

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Ch 6 6.5 and 6.6

  • 1. © Cengage Learning 2015 6.5 How Do The Light-Independent Reactions Work? • The Calvin–Benson cycle: – Build sugars in the stroma of chloroplasts – Not powered by light energy – Driving force is ATP and NADPH that formed in the light- dependent reactions – Uses carbon atoms from CO2 to make sugars • Carbon fixation: carbon from an inorganic source gets incorporated into an organic molecule 1
  • 2. © Cengage Learning 2015 The Calvin-Benson Cycle • Steps of the Calvin–Benson cycle: 1) The enzyme rubisco fixes carbon by attaching CO2 to RuBP (ribulose bisphosphate) 2) Twelve PGA (phosphoglycerate) molecules form 2
  • 3. © Cengage Learning 2015 The Calvin-Benson Cycle (cont’d.) • Steps of the Calvin–Benson cycle (cont’d.): 3) Twelve PGAL (phosphoglyceraldehyde) molecules form • Two PGAL may combine to form one six-carbon sugar (such as glucose) 4) Ten remaining PGAL regenerate the starting compound, RuBP 3
  • 4. © Cengage Learning 2015 Adaptations to Climate • Stomata are tiny gateways for gases – Open stomata: • Allow CO2 to diffuse from the air into photosynthetic tissues • Allow O2 to diffuse out of these tissues into the air – Closed stomata: • Conserve water on hot, dry days • Limit the availability of CO2 for the light-independent reactions; sugar synthesis slows 4
  • 5. © Cengage Learning 2015 Adaptations to Climate (cont’d.) • C3 plants: – Use only the Calvin–Benson cycle to fix carbon – When CO2 concentration declines, rubisco uses oxygen as a substrate in photorespiration – When stomata are closed during the day, C3 plants lose carbon instead of fixing it – Extra energy is required to make sugars – Contain high levels of rubisco 5
  • 6. © Cengage Learning 2015 Adaptations to Climate (cont’d.) 6 glycolate CO2 RuBP sugar PGA Calvin– Benson Cycle O2 When carbon dioxide level declines in the leaves of a C3 plant, rubisco uses oxygen as a substrate in photorespiration, and sugar production becomes inefficient.
  • 7. © Cengage Learning 2015 Adaptations to Climate (cont’d.) • C4 plants: – Close stomata on dry days, but their sugar production does not decline – Minimize photorespiration by fixing carbon twice in two cell types • Mesophyll cells • Bundle-sheath cells – Examples: corn, switchgrass, and bamboo 7
  • 8. © Cengage Learning 2015 Adaptations to Climate (cont’d.) • CAM plants: – C4 plant that conserves water by fixing carbon twice, at different times of day – Conserve water even in desert regions with extremely high daytime temperatures – Open stomata at night when lower temperatures minimize evaporative water loss 8
  • 9. © Cengage Learning 2015 Adaptations to Climate (cont’d.) 9 mesophyll cell bundle-sheath cell In a C3 plant (left), chloroplasts – the sites of carbon fixation- occur mainly in mesophyll cells. In the C4 plant (right), carbon fixation occurs first in the mesophyll cells, then in bundle-sheath cells. C4 adaptations maintain a high carbon dioxide to oxygen ratio near rubisco.
  • 10. © Cengage Learning 2015 Adaptations to Climate (cont’d.) 10 Crabgrass “weeds”, 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.
  • 11. © Cengage Learning 2015 6.6 Application: Green Energy • With fossil fuel prices soaring, there is an increasing demand for biofuels: – Oils, gases, or alcohols made from organic matter that is not fossilized • Most materials we use for biofuel production today consist of food crops: – Mainly corn, soybeans, and sugarcane 11
  • 12. © Cengage Learning 2015 Application: Green Energy (cont’d.) • Fossil fuels: – Petroleum, coal, and natural gas – Formed from the remains of ancient swamp forests that decayed and compacted over millions of years – Consist of molecules originally assembled by ancient plants 12
  • 13. © Cengage Learning 2015 Application: Green Energy (cont’d.) • The process of using fossil fuels or biofuels are fundamentally the same: – Release energy by breaking the bonds of organic molecules – Use oxygen to break those bonds – Produce carbon dioxide 13
  • 14. © Cengage Learning 2015 Application: Green Energy (cont’d.) • Biofuels are a renewable source of energy – Growing more plants creates more sources of biofuels 14
  • 15. © Cengage Learning 2015 Application: Green Energy (cont’d.) • Biofuels do not contribute to global climate change – Growing plant matter for fuel recycles carbon that is already in the atmosphere 15
  • 16. © Cengage Learning 2015 Application: Green Energy (cont’d.) • Corn and other food crops are rich in oils, starches, and sugars that can be easily converted to biofuels – Starch from corn kernels can be broken down to glucose, which is converted to ethanol by bacteria or yeast 16
  • 17. © Cengage Learning 2015 Application: Green Energy (cont’d.) • Making biofuels from some plants require additional steps, because these materials contain a higher proportion of cellulose – Researchers are currently working on cost-effective ways to break down the abundant cellulose in fast-growing weeds 17
  • 18. © Cengage Learning 2015 Application: Green Energy (cont’d.) • According to tropical forest scientist Willie Smits, the Arenga sugar palm has the potential to serve as the core of a waste-free system – Produces premium organic sugar and ethanol for fuel – Provides food products and jobs to villagers – Helps preserve the existing native rain forest 18

Editor's Notes

  1. Figure 6.9A Anatomical and biochemical specializations minimize photorespiration in C4 plants.
  2. Figure 6.9 Anatomical and biochemical specializations minimize photorespiration in C4 plants. Micrographs show leaf cross sections.
  3. Figure 6.10 - 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.