AP Biology notes for chapter 6 photosynthesis and respiration

1. Pathways that Harvest and
Store Chemical Energy
6

2. Beginning of
Student Led
Notes

3. Key Concepts
• 6.1 ATP, Reduced Coenzymes, and Chemiosmosis
Play Important Roles in Biological Energy
Metabolism
• 6.2 Carbohydrate Catabolism in the Presence of
Oxygen Releases a Large Amount of Energy
• 6.3 Carbohydrate Catabolism in the Absence of
Oxygen Releases a Small Amount of Energy

4. • 6.4 Catabolic and Anabolic Pathways Are Integrated
• 6.5 During Photosynthesis, Light Energy Is Converted to
Chemical Energy
• 6.6 Photosynthetic Organisms Use Chemical Energy to
Convert CO2 to Carbohydrates

5. Concept 6.1 ATP, Reduced Coenzymes, and
Chemiosmosis Play Important Roles in
Biological Energy Metabolism
–Energy is stored in chemical bonds and can
be released and transformed by metabolic
pathways.
–Chemical energy available to do work is
termed free energy (G).

6. –In cells, energy-transforming reactions are often
coupled:
–An energy-releasing (exergonic) reaction is coupled
to an energy-requiring (endergonic) reaction.
Energy

7. – Adenosine triphosphate (ATP) is a kind of “energy
currency” in cells.
– Energy released by exergonic reactions is stored in
the bonds of ATP.
– When ATP is hydrolyzed, free energy is released to
drive endergonic reactions.

8. Figure 6.1 The Concept of Coupling Reactions

9. Figure 6.2 ATP

10. – Hydrolysis of ATP is exergonic:
ΔG is about –7.3 kcal
energyfreePADPOHATP i  2

11. – Free energy of the bond between phosphate
groups is much higher than the energy of the
O—H bond that forms after hydrolysis.
text art pg 102 here
(1st one, in left-hand column)

12. –Phosphate groups are negatively charged, so
energy is required to get them near enough
to each other to make the covalent bonds in
the ATP molecule.
–ATP can be formed by substrate-level
phosphorylation or oxidative
phosphorylation.

13. –Energy can also be transferred by the
transfer of electrons in oxidation–reduction,
or redox reactions.
•Reduction is the gain of one or more
electrons.
–• Oxidation is the loss of one or more
electrons.
–Redox video
Redox Reactions

14. –Oxidation and reduction always occur
together.

15. – Transfers of hydrogen atoms involve transfers of electrons (H
= H+ + e–).
– When a molecule loses a hydrogen atom, it becomes
oxidized.
– The more reduced a molecule is, the more energy is stored
in its bonds.
– Energy is transferred in a redox reaction.
– Energy in the reducing agent is transferred to the reduced
product.

16. Figure 6.3 Oxidation, Reduction, and Energy

17. – Coenzyme NAD+ is a key electron carrier in redox
reactions.
– NAD+ (oxidized form)
– NADH (reduced form)

18. Figure 6.4 A NAD+/NADH Is an Electron Carrier in Redox Reactions

19. – Reduction of NAD+ is highly endergonic:
– Oxidation of NADH is highly exergonic:
NADHeHNAD  
2
OHNADOHNADH 222
1  

20. Figure 6.4 B NAD+/NADH Is an Electron Carrier in Redox Reactions

21. • In cells, energy is released in catabolism by
oxidation and trapped by reduction of coenzymes
such as NADH.
– • Energy for anabolic processes is supplied by ATP.
– Oxidative phosphorylation transfers energy from
NADH to ATP.

22. – Oxidative phosphorylation couples oxidation of
NADH:
– with production of ATP:
energyeHNADNADH  
2
ATPPADPenergy i 

23. –The coupling is chemiosmosis—diffusion of
protons across a membrane, which drives
the synthesis of ATP.
–Chemiosmosis converts potential energy of a
proton gradient across a membrane into the
chemical energy in ATP.

24. Figure 6.5 A Chemiosmosis

25. – ATP synthase—membrane protein with two
subunits:
– F0 is the H+ channel; potential energy of the
proton gradient drives the H+ through.
– F1 has active sites for ATP synthesis.

26. Figure 6.5 B Chemiosmosis

27. –Chemiosmosis can be demonstrated
experimentally.
–A proton gradient can be introduced
artificially in chloroplasts or mitochondria in
a test tube.
–ATP is synthesized if ATP synthase, ADP, and
inorganic phosphate are present.

28. Concept 6.1 ATP, Reduced Coenzymes, and
Chemiosmosis Play Important Roles in
Biological Energy Metabolism
– Cellular respiration is a major catabolic pathway. Glucose is
oxidized:
– Photosynthesis is a major anabolic pathway. Light energy is
converted to chemical energy:
tecarbohydraOenergylightOHCO  222 666
energychemicalOHCOOtecarbohydra  222 666

29. Figure 6.7 ATP, Reduced Coenzymes, and Metabolism

30. stop

31. Cellular Respiration

32. – A lot of energy is released when reduced molecules
with many C—C and C—H bonds are fully oxidized
to CO2.
– Oxidation occurs in a series of small steps in three
pathways:
– 1. glycolysis
– 2. pyruvate oxidation
– 3. citric acid cycle (Krebs Cycle)

33. Figure 6.8 Energy Metabolism Occurs in Small Steps

34. Figure 6.9 Energy-Releasing Metabolic Pathways

35. Glycolysis
– Glycolysis: ten reactions.
– Takes place in the cytosol.
– Final products:
– 2 molecules of pyruvate (pyruvic acid)
– 2 molecules of ATP
– 2 molecules of NADH (electron acceptor to be used
later)

36. Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 1)

37. Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 2)

38. Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 3)

39. – Examples of reaction types common in metabolic
pathways:
– Step 6: Oxidation–reduction-Electrons are
transferred and/or accepted.
– Step 7: Substrate-level phosphorylation- a
phosphate group along with its energy is added to
ADP to create ATP

40. – Pyruvate Oxidation:
– Products: CO2 and acetate; acetate is then bound to
coenzyme A (CoA)
What happens to the pyruvate made in glycolysis?

41. Citric Acid Cycle (Krebs Cycle)
– Citric Acid Cycle: 8 reactions, operates twice for every
glucose molecule that enters glycolysis. (Because two
pyruvates are made…)
– Starts with Acetyl CoA; acetyl group is oxidized to two CO2.
– Oxaloacetate is regenerated in the last step.

42. Figure 6.11 The Citric Acid Cycle

43. Electron Transport Chain
– Electron transport/ATP Synthesis:
– NADH is reoxidized to NAD+ and O2 is reduced to H2O in a
series of steps.
– Respiratory chain—series of redox carrier proteins
embedded in the inner mitochondrial membrane.
– Electron transport—electrons from the oxidation of NADH
and FADH2 pass from one carrier to the next in the chain.

44. Figure 6.12 Electron Transport and ATP Synthesis in Mitochondria

45. – The oxidation reactions are exergonic; the energy is
used to actively transport H+ ions out of the
mitochondrial matrix, setting up a proton gradient.
– ATP synthase in the membrane uses the H+ gradient
to synthesize ATP by chemiosmosis.

46. – About 32 molecules of ATP are produced for each
fully oxidized glucose.
– The role of O2: most of the ATP produced is formed
by oxidative phosphorylation, which is due to the
reoxidation of NADH.

47. –Oxidative phosphorylation differs from
substrate level phosphorylation in that
the energy from the phosphate does
NOT accompany the phosphate group.
Instead, it gives up electrons to
generate ATP during each step of the
process going down the chain.

48. Electron Transport Chain-Recap
–Oxidative phosphorylation is the
process of producing ATP from NADH
and FADH2.
–Electrons from these enzymes pass
along an electron transport chain
and are used to phosphorylate or
put phosphate on ADP.

49. – The chain consists of several carrier proteins.
– Some of the carrier proteins are called
cytochromes which are common in all living
things and can be used to determine
evolutionary relationships.
– The final electron acceptor is oxygen which
forms water as a waste product.

50. – Under anaerobic conditions, NADH is reoxidized by
fermentation.
– There are many different types of fermentation, but
all operate to regenerate NAD+.
– The overall yield of ATP is only two—the ATP made
in glycolysis.
Fermentation

51. –Lactic acid fermentation:
– End product is lactic acid (lactate).
– NADH is used to reduce pyruvate to lactic
acid, thus regenerating NAD+.

52. Figure 6.13 A Fermentation

53. – Alcoholic fermentation:
– End product is ethyl alcohol (ethanol).
– Pyruvate is converted to acetaldehyde, and CO2 is
released. NADH is used to reduce acetaldehyde to
ethanol, regenerating NAD+ for glycolysis.

54. Figure 6.13 B Fermentation

55. Figure 6.14 Relationships among the Major Metabolic Pathways of the Cell

56. stop

57. Photosynthesis

58. Concept 6.5 During Photosynthesis, Light Energy
Is Converted to Chemical Energy
– Photosynthesis involves two pathways:
– Light reactions convert light energy into chemical
energy (in ATP and the reduced electron carrier
NADPH).
– Carbon-fixation reactions use the ATP and NADPH,
along with CO2, to produce carbohydrates.

59. Figure 6.15 An Overview of Photosynthesis

60. – Light is a form of electromagnetic radiation, which
travels as a wave but also behaves as particles
(photons).
– Photons can be absorbed by a molecule, adding
energy to the molecule—it moves to an excited
state.

61. Figure 6.16 The Electromagnetic Spectrum

62. – Pigments: molecules that absorb wavelengths in the visible
spectrum.
– Chlorophyll absorbs blue and red light; the remaining light is
mostly green.
– Absorption spectrum—plot of light energy absorbed against
wavelength.
– Action spectrum—plot of the biological activity of an
organism against the wavelengths to which it is exposed

63. Figure 6.17 Absorption and Action Spectra

64. – In plants, two chlorophylls absorb light energy
chlorophyll a and chlorophyll b.
– Accessory pigments—absorb wavelengths between
red and blue and transfer some of that energy to
the chlorophylls.

65. Figure 6.18 The Molecular Structure of Chlorophyll (Part 1)

66. Figure 6.18 The Molecular Structure of Chlorophyll (Part 2)

67. – The pigments are arranged into light-harvesting
complexes, or antenna systems.
– A photosystem spans the thylakoid membrane in
the chloroplast; it consists of antenna systems and
a reaction center.

68. – When chlorophyll (Chl) absorbs light, it enters an
excited state (Chl*), then rapidly returns to ground
state, releasing an excited electron.
– Chl* gives the excited electron to an acceptor and
becomes oxidized to Chl+.
– The acceptor molecule is reduced.

 acceptorChlacceptorChl*

69. – The electron acceptor is first in an electron
transport system in the thylakoid membrane.
– Final electron acceptor is NADP+, which gets
reduced:
– ATP is produced chemiosmotically during electron
transport (photophosphorylation).
NADPHeHNADP  
2

70. Figure 6.19 Noncyclic Electron Transport Uses Two Photosystems

71. – Two photosystems:
• Photosystem I absorbs light energy at 700 nm,
passes an excited electron to NADP+, reducing it to
NADPH.
– • Photosystem II absorbs light energy at
680 nm, produces ATP, and oxidizes water
molecules.

72. – Photosystem II
– When Chl* gives up an electron, it is unstable and
grabs an electron from another molecule, H2O,
which splits the H—O—H bonds.
22
1
2 2*2 OHChlOHChl  

73. – Photosystem I
– When Chl* gives up an electron, it grabs another
electron from the end of the transport system of
Photosystem II. This electron ends up reducing
NADP+ to NADPH.

74. – ATP is needed for carbon-fixation pathways.
– Cyclic electron transport uses only photosystem I
and produces ATP; an electron is passed from an
excited chlorophyll and recycles back to the same
chlorophyll.

75. Figure 6.20 Cyclic Electron Transport Traps Light Energy as ATP

76. – The Calvin cycle: CO2 fixation. It occurs in the
stroma of the chloroplast.
– Each reaction is catalyzed by a specific enzyme.

77. Figure 6.21 The Calvin Cycle

78. – 1. Fixation of CO2:
– CO2 is added to ribulose 1,5-bisphosphate (RuBP).
– Ribulose bisphosphate carboxylase/oxygenase
(rubisco) catalyzes the reaction.
– A 6-carbon molecule results, which quickly breaks
into two 3-carbon molecules: 3-phosphoglycerate
(3PG).

79. Figure 6.22 RuBP Is the Carbon Dioxide Acceptor

80. – 2. 3PG is reduced to form glyceraldehyde 3-
phosphate (G3P).

81. – 3. The CO2 acceptor, RuBP, is regenerated from
G3P.
– Some of the extra G3P is exported to the cytosol
and is converted to hexoses (glucose and fructose).
– When glucose accumulates, it is linked to form
starch, a storage carbohydrate.

82. – The C—H bonds generated by the Calvin cycle
provide almost all the energy for life on Earth.
– Photosynthetic organisms (autotrophs) use most of
this energy to support their own growth and
reproduction.
– Heterotrophs cannot photosynthesize and depend
on autotrophs for chemical energy.