1. Photosynthesis using light to make food
unit 5

2. Have you thanked a green plant today?

4. Energy for Life Processes
Review: All living things use energy - autotrophs / heterotrophs
Autotrophs- organisms that manufacture their own food.
Most autotrophs use PHOTOSYNTHESIS to obtain food.
Besides plants, algae and some bacteria are photosynthetic organisms

5. Energy from food monomers

6. Energy & Chemical Cycling

7. Organisms that photosynthesize

8. Overview - What is the general equation for photosynthesis?

9. Tracking atoms in photosynthesis
Reactants:
6 CO2
12 H2O
Products:
C6H12O6
6 H2O
6 O2
Figure 10.4 Tracking atoms through photosynthesis
Photosynthesis is a redox process in which
H2O is oxidized and CO2 is reduced

10. Today’s Focus:
Photosynthesis: process of converting solar energy into chemical energy
We can break down photosynthesis into two different stages:
Light-dependent reactions
Light-independent reactions
What are examples of transformations of energy we encounter every day?
Focus question: How do plants convert solar energy into chemical energy?

11. Photosynthesis 2-part overview

12. Not all plants photosynthesize: parasitic Dodder

13. Where in plants does photosynthesis occur?
All green parts 13

14. All green parts of a plant have chloroplasts!
Lots of plant cells

15. zoom into a plant cell inside a chloroplast
Chloroplast – is a light absorbing organelle
Features inside:
GRANA (granum – singular) - stacks of thylakoids. (thigh)
STROMA- solution that surrounds thylakoids
“Leaves have about 500,000 chloroplasts per millimeter squared of leaf surface”
Green color is due to chlorophyll pigment

16. Chloroplast: sites of photosynthesis

17. Stop and Jot
In a typical plant cell, what are the different membranes light has to cross to reach chlorophyll?
(cell wall, cell membrane, chloroplast membrane, and thylakoid membrane)

18. Beginning of photosynthesis story (Part I the light reactions):
Step 1. Light must be absorbed into the chloroplasts.
One cell in a plant leaf can have 50 or more chloroplasts.
BASICS OF LIGHT
Sunlight -> white light (really the whole visible spectrum) ROYGBIV
Wavelength
Pigment- compound that absorbs light
Absorbs, reflects, transmits.
Absorption spectra

20. Thylakoid
In membrane - lots of pigments: most imp. pigment -> chlorophylls
Accessory pigments - chlorophyll b, carotenoids, etc.
In leaves of plants the chlorophylls are most abundant pigment -> thus leaves green.
In Autumn - lose chlorophylls-> other pigments show through.
Non-photosynthetic parts of plant?

21. interacts with hydrophobic regions of proteins inside
Fig
CH3
in chlorophyll a
CHO
in chlorophyll b
Porphyrin ring:
light-absorbing
“head” of molecule;
note magnesium
atom at center
Figure Structure of chlorophyll molecules in chloroplasts of plants
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts; H atoms not
shown

22. Spectroscopy of Chlorophyll a and b

23. Visible Spectrum

24. i Light NADP+ ADP Light Reactions Chloroplast H2O + P Fig. 10-5-1
Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle
Chloroplast

25. i Light NADP+ ADP Light Reactions ATP NADPH Chloroplast O2 H2O + P
Fig
H2O
Light
NADP+
ADP + P i
Light
Reactions
ATP
Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle
NADPH
Chloroplast O2

26. i CO2 Light NADP+ ADP Calvin Cycle Light Reactions ATP NADPH
Fig
H2O
CO2
Light
NADP+
ADP + P i
Calvin
Cycle
Light
Reactions
ATP
Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle
NADPH
Chloroplast O2

27. i CO2 Light NADP+ ADP Calvin Cycle Light Reactions ATP NADPH
Fig
H2O
CO2
Light
NADP+
ADP + P i
Calvin
Cycle
Light
Reactions
ATP
Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle
NADPH
Chloroplast
[CH2O]
(sugar) O2

28. The Light Reactions

29. Photosystems: protein complex that contains pigments
There are two types of photosystems in the thylakoid membrane
Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm
The reaction-center chlorophyll a of PS II is called P680
Photosystem I (PS I) is best at absorbing a wavelength of 700 nm
The reaction-center chlorophyll a of PS I is called P700
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

30. Chlorophyll pigment is found in the thylakoid membrane
How does chloroplast structure relate to its function?
The stacks of thylakoids help to increase the surface area, allowing for more photosynthesis to occur. Having the thylakoids stacked also helps decrease the volume a chloroplast occupies, allowing numerous chloroplasts to be in a cell and also increase photosynthesis.
photosynthetic cells must be intricately set up in order to be able to harvest light energy & not let it dissipate in unproductive ways)

31. (INTERIOR OF THYLAKOID)
Fig
Photosystem
STROMA
Photon
Primary
electron
acceptor
Light-harvesting
complexes
Reaction-center
complex e–
Thylakoid membrane
Figure How a photosystem harvests light
Pigment
molecules
Transfer
of energy
Special pair of
chlorophyll a
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)

32. Electron Resonance Transfer (excitation energy)
What does the light energy end up doing in photosystem II?
A photon hits a pigment and its energy is passed among pigment molecules until it excites P680
An excited electron from P680 is transferred to the primary electron acceptor
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

33. Analyzing models
© 2010 Pearson Education, Inc.

34. Producing ATP and NADPH

35. ATP NADPH Mill makes ATP Photosystem II Photosystem I e– e– e– e– e–
Fig e–
ATP e– e–
NADPH e– e– e–
Mill
makes
ATP
Photon
Figure A mechanical analogy for the light reactions e–
Photon
Photosystem II
Photosystem I

36. Fig. 10-17 STROMA (low H+ concentration) Cytochrome complex
Photosystem II
Photosystem I
4 H+
Light
NADP+
reductase
Light Fd 3
NADP+ + H+ Pq
NADPH e– Pc e– 2
H2O 1
1/2 O2
THYLAKOID SPACE
(high H+ concentration)
+2 H+
4 H+ To
Calvin
Cycle
Figure The light reactions and chemiosmosis: the organization of the thylakoid membrane
Thylakoid
membrane
ATP
synthase
STROMA
(low H+ concentration)
ADP +
ATP P i H+

37. Summary of light reactions
Light reactions happen in the thylakoids
Split water
Release oxygen
Reduce NADP+ to NADPH
Generate ATP from ADP by photophosphorylation

38. Check For Understanding
Where does the original high energy electrons come from?
What is the final electron acceptor?

39. Relating cellular respiration to photosynthesis
Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP

40. Part II: The Calvin Cycle uses ATP and NADPH to convert CO2 to sugar
The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle
The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

41. The Calvin cycle has three phases:
Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)
For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2
The Calvin cycle has three phases:
Carbon fixation (catalyzed by rubisco)
Reduction
Regeneration of the CO2 acceptor (RuBP)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

42. Phase 1: Carbon fixation Ribulose bisphosphate
Fig
Input 3
(Entering one
at a time)
CO2
Phase 1: Carbon fixation
Rubisco 3 P P
Short-lived
intermediate 3 P P 6 P
Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate
Figure The Calvin cycle

43. Figure 10.18 The Calvin cycle
Input 3
(Entering one
at a time)
CO2
Phase 1: Carbon fixation
Rubisco 3 P P
Short-lived
intermediate 3 P P 6 P
Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate 6
ATP
6 ADP
Calvin
Cycle 6 P P
1,3-Bisphosphoglycerate 6
NADPH
6 NADP+ 6 P i
Figure The Calvin cycle 6 P
Glyceraldehyde-3-phosphate
(G3P)
Phase 2:
Reduction 1 P
Glucose and
other organic
compounds
Output
G3P
(a sugar)

44. Figure 10.18 The Calvin cycle
Input 3
(Entering one
at a time)
CO2
Phase 1: Carbon fixation
Rubisco 3 P P
Short-lived
intermediate 3 P P 6 P
Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate 6
ATP
6 ADP
3 ADP
Calvin
Cycle 6 3 P P
ATP
1,3-Bisphosphoglycerate 6
NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+ 6 P i
Figure The Calvin cycle 5 P
G3P 6 P
Glyceraldehyde-3-phosphate
(G3P)
Phase 2:
Reduction 1 P
Glucose and
other organic
compounds
Output
G3P
(a sugar)

45. Alternative mechanisms

46. Alternative Mechanisms
Stomata: absorb CO2 and release water
On a hot day, plants close their stomata (reduce the CO2 present)

47. Water Saving Adaptations: C4 and CAM Plants

48. C4

49. C4

50. CAM
Crassulacean acid metabolism – 4C compound

51. CAM Plants: Night and Day

54. Photorespiration CO2 becomes scarce Rubisco binds to O2 instead of CO2
ATP consumed rather than generated!
No sugar produced!
Consumes organic material from Calvin Cycle!
What’s the point?
In some case clear evidence that photorespiration protects plants from damaging products of light reaction which build up when CO2 is scarce

55. Atmospheric CO2 Concentrations