1. Photosynthesis
AP Biology
Ms. Haut

2. Introduction
Photosynthesis is the process that converts solar energy into chemical energy
Directly or indirectly, photosynthesis nourishes almost the entire living world
Photosynthesis—process in which some of the solar energy is captured by plants (producers) and transformed into glucose molecules used by other organisms (consumers).
6CO2 + 6H2O C6H12O6 + 6O2
Light energy
enzymes

3. Glucose is the main source of energy for all life
Glucose is the main source of energy for all life. The energy is stored in the chemical bonds.
Cellular Respiration— process in which a cell breaks down the glucose so that energy can be released. This energy will enable a cell to carry out its activities.
C6H12O6 + 6O CO2 + 6H2O + energy
enzymes

4. Autotroph —organisms that synthesize organic molecules from inorganic materials (a.k.a. producers)
Photoautotrophs —use light as an energy source (plants, algae, some prokaryotes)
Chemoautotrophs —use the oxidation of inorganic substances (some bacteria)
Heterotroph —organisms that acquire organic molecules from compounds produced by other organisms (a.k.a. consumers)

6. Thylakoids trap sunlight
Sunlight = electromagnetic energy
Wavelike properties
Particlelike properties (photon)
Light may be reflected, transmitted, or absorbed when it contacts matter

7. Photosynthetic Pigments: The Light Receptors
Pigments are substances that absorb visible light
Different pigments absorb different wavelengths
Wavelengths that are not absorbed are reflected or transmitted
Leaves appear green because chlorophyll reflects and transmits green light

8. Photosynthetic Pigments
Pigments -substances that absorb light
(light receptors)
Wavelengths that are absorbed disappear
Wavelengths that are transmitted and reflected as the color you see
Chlorophyll —absorbs red and blue light but reflects green, hence the green color of leaves

9. Accessory Pigments
Absorb light of varying wavelengths and transfer the energy to chlorophyll a
Chlorophyll b -yellow-green pigment
Carotenoids -yellow and orange pigments

10. An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength
The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis
An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

11. Photosynthesis: redox process
Endergonic redox process; energy is required to reduce CO2
Light is the energy source that boosts potential energy of electrons (e-) as they are moved from water to glucose
When water is split, e- are transformed from the water to CO2, reducing it to sugar

12. oxidation
C6H12O6 + 6O CO2 + 6H2O + energy
reduction

13. The Two Stages of Photosynthesis: A Preview
Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)
Light reactions (in the thylakoids) split water, release O2, produce ATP, and form NADPH
Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH
The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules

14. Photosynthesis: 2 processes
Light reactions —convert light energy to chemical bond energy in ATP and NADPH
Occurs in thylakoids in chloroplasts
NADP+ reduced to NADPH—temporary energy storage (transferred from water)
Give off O2 as a by-product
Generates ATP by phosphorylating ADP

15. Photosynthesis: 2 processes
Calvin Cycle —carbon fixation reactions assimilate CO2 and then reduce it to a carbohydrate
Occur in the stroma of the chloroplast
Do not require light directly, but requires products of the light reactions
Incorporates into existing organic molecules and then reduces fixed carbon into carbohydrate
NADPH provides the reducing power
ATP provides chemical energy

16. Interdependent
Reactions
Light reactions produce: ATP and NADPH that are used by the Calvin cycle; O2 released
Calvin Cycle produces: ADP and NADP+ that are used by the light reactions; glucose produced

17. Photosystems: light-harvesting complexes in thylakoid membrane
Photosystem: assemblies of several hundred chlorophyll a, chlorophyll b, and carotenoid molecules in the thylakoid membrane
form a light gathering antennae that absorb photons and pass energy from molecule to molecule
Photosystem I —specialized chlorophyll a molecule, P700
Photosystem II —specialized chlorophyll a molecule, P680

19. Noncyclic Electron Flow
Light drives the light reactions to synthesize
NADPH and ATP
Includes cooperation of both photosystems, in
which e- pass continuously from water to
NADP+

20. When photosystem II absorbs light an e- is excited in the reaction center chlorophyll (P680) and gets captured by the primary e- acceptor.
This leaves a hole in the P680

21. To fill the hole left in P680, an enzyme extracts e- from water and supplies them to the reaction center
A water molecule is split into 2 H+ ions and an oxygen atom, which immediately combines with another oxygen to form O2

22. Each photoexcited e- passes from primary e- acceptor to photosystem I via an electron transport chain.
e- are transferred to plastoquinone (Pq) and plastocyanin (Pc) (e- carriers)

23. As e- cascade down the e- transport chain, energy is released and harnessed by the thylakoid membrane to produce ATP (PHOTOPHOSPHORYLATION)
This ATP is used to make glucose during Calvin cycle

24. When e- reach the bottom of e- transport chain, it fills the hole in the reaction center P700 of photosystem I.
Pre-existing hole was left by former e- that was excited

25. When photosystem I absorbs light an e- is excited in the reaction center chlorophyll (P700) and gets captured by the primary e- acceptor.
e- are transferred to ferredoxin (Fd) (e- carrier)
NADP+ reductase transfers e- from Fd to NADP+, storing energy in NADPH (reduction reaction)
NADPH provides reducing power for making glucose in Calvin cycle

26. Cyclic Electron Flow Only photosystem I is used Only ATP is produced
animation

27. Chemiosmosis
Energy released from e- transport chain is used to pump H+ ions (from the split water) from the stroma across the thylakoid membrane to the interior of the thylakoid.
Creates concentration gradient across thylakoid membrane
Process provides energy for chemisomostic production of ATP

28. The light reactions and chemiosmosis: the organization of the thylakoid membrane
REACTOR
NADP+
ADP
ATP
NADPH
CALVIN
CYCLE
[CH2O] (sugar)
STROMA
(Low H+ concentration)
Photosystem II
H2O
CO2
Cytochrome
complex O2 1
1⁄2 2
Photosystem I
Light
THYLAKOID SPACE
(High H+ concentration)
Thylakoid
membrane
synthase Pq Pc Fd
reductase
+ H+
NADP+ + 2H+ To
Calvin
cycle P 3 H+
2 H+
+2 H+
Figure 10.17

29. Calvin Cycle
Carbon enters the cycle in the form of CO2 and leaves in the form of sugar (glucose)
The cycle spends ATP as an energy source and consumes NADPH as a reducing agent for adding high energy e- to make sugar
For the net synthesis of this sugar, the cycle must take place 2 times

30. Calvin Cycle

31. Calvin Cycle

32. Calvin Cycle

33. Calvin Cycle
Carbon Fixation: 3 CO2 molecules bind to 3 5-Carbon sugars, ribulose bisphosphate (RuBP) using enzyme called RuBP carboxylase (rubisco)
Produces 6 molecules of a 3-carbon sugar, 3-phosphoglycerate

34. Calvin Cycle Carbon Fixation
Reduction: 6 ATP molecules transfer phosphate group to each molecule of 3-phos. to make 1,3-diphosphoglycerate
6 molecules of NADPH reduce each molecule of 1,3-diphosph. to make glyceraldehyde 3-phosphate (G3P)
One of the G3P exits the cycle to be used by the plant; the other 5 molecules are used to regenerate the CO2 acceptor, RuBP: 3 molecules of ATP are used to convert 5 molecules of G3P into RuBP

35. Calvin Cycle
3 more CO2 molecules enter the cycle, following the same chemical pathway to release another G3P from the cycle.
2 G3P molecules can be used to make glucose

37. Interdependent

38. Alternative mechanisms of carbon fixation have evolved in hot, arid climates
Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis
On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis
The closing of stomata reduces access to CO2 and causes O2 to build up
These conditions favor a seemingly wasteful process called photorespiration

39. Photorespiration: An Evolutionary Relic?
In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound
In photorespiration, rubisco adds O2 to the Calvin cycle instead of CO2
Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

40. Photorespiration: An Evolutionary Relic?
Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2
In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle

41. C4 Plants
C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells
These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle
Corn
Crab Grass

42. C4 Plants
PEP carboxylase-high affinity to CO2 and no affinity for O2, thus no photorespiration possible

43. CAM Plants
CAM plants open their stomata at night, incorporating CO2 into organic acids
Organic acids stored in vacuoles of mesophyll cells until morning, when stomata close
Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle

44. LE 10-20 Bundle- sheath cell Mesophyll Organic acid C4 CO2 CALVIN
CYCLE
Sugarcane
Pineapple
Organic acids
release CO2 to
Calvin cycle
CO2 incorporated
into four-carbon
organic acids
(carbon fixation)
CAM
Sugar
Spatial separation of steps
Temporal separation of steps
Day
Night

45. The CAM and C4 pathways:
Are similar in that CO2 is first incorporated into organic intermediates before it enters the Calvin cycle
Differ in that the initial steps of carbon fixation in C4 plants are structurally separate from the Calvin cycle; in CAM plants, the two steps occur at separate times
Regardless of whether the plant uses C3, C4, or CAM pathway, all plants use the Calvin Cycle to produce sugar from CO2