1. Modern Biology Chapter 6: Photosynthesis

3. Plant cell

4. 6-1: Capturing the Energy in Light

5. Energy for life processes
photosynthesis: process by which green plants convert solar energy into chemical energy
produces carbohydrates
produces oxygen

6. Energy for life processes
chloroplast structure
double membrane surrounds entire organelle
thylakoids: flattened sacs inside double membrane
grana: stacks of thylakoids
stroma: fluid surrounding thylakoids inside double membrane

8. Energy for life processes
sunlight
provides heat and energy to earth
white light from sun contains mixture of colors of light
wavelength of light determines its color
only a small portion of sun-light is visible to humans

9. The sun emits all visible wavelengths of light
Green plants absorb red, orange, blue and violet
They reflect yellow and green

11. Overview of photosynthesis
CO2
From Air
H2O
From Soil
Light energy
From Sun O2
To air
C6H12O6
To plant

12. Energy for life processes
pigment: colored substance that reflects or absorbs light

13. Energy for life processes
chlorophyll
type of pigment in thylakoid membranes
two types of chlorophyll
chlorophyll a absorbs light in red end of spectrum
chlorophyll b absorbs light in blue end of the spectrum (accessory pigment)
green light is not absorbed, but reflected giving the leaves the appearance of being green
by absorbing light pigments also absorb energy

15. Energy for life processes
Cartenoids: other accessory pigments
absorb different colors depending on chemical structure
become apparent when chlorophylls fade (fall colors)

17. The light reactions

18. The light reactions consist of three basic components
Photosystem 2
Photosystem 1
ATP synthase (chemiosmosis)

19. Photosystem 2 water-plastoquinone oxidoreductase
Uses the energy from sunlight to split the water molecule into three parts
2H2O  4 H+ + 4 e- + O2

20. Photosystem 1 plastocyanin: ferredoxin oxidoreductase
Uses the energy from sunlight to move the electrons onto NADP+ for transport to the next phase of the process

21. ATP synthase
Synthesizes ATP using a concentration gradient created by photosystem II

22. Light reactions
Light and the energy associated with it are absorbed into photosystem I and photosystem II

24. Light-dependent reactions a.k.a. light reactions
Electron transport occurs within membranes

25. Light-dependent reactions a.k.a. light reactions
photosystem II (PSII)
accessory pigments absorb light and acquire energy (E) (step 1)
energy is passed along membrane pigments until it reaches a specific pair of chlorophyll a molecules

26. Light-dependent reactions a.k.a. light reactions
photosystem II (PSII)
electron transport
E forces e- to increase E level (e- are said to be “excited”)
excited e- leave chlorophyll a
chlorophyll a is oxidized
PEA donates e-
e- reduces primary e- acceptor (PEA) (step 2)
e- transported down ETC (step 3)
each transfer, the e- loses some E
E is used to move p+ into thylakoid

28. Light-dependent reactions a.k.a. light reactions
photosystem I (PSI)
light absorbed by PSI (step 1b)
e- move from chlorophyll a to PEA (step 4)
e- lost are replaced by e- from PSII
PEA of PSI donates e- to NADP+ (step 5)
brings e- to edge of thylakoid membrane by stroma
e- combine with p+ and NADP+
NADP+ reduced to NADPH

29. Light-dependent reactions a.k.a. light reactions
replacing e- (step 6)
recall e- from chlorophyll in PSII replace e- that leave chlorophyll on PSI
e- from PSII need to be replaced or both ETCs cease

30. Light-dependent reactions a.k.a. light reactions
replacing e- (step 6)
replacement e- come from water
enzyme in thylakoid splits water molecule
2H2O  4 H+ + 4 e- + O2
p+ (H+) remain in thylakoid
O2 diffuses out and leaves the plant
replace e- lost by chlorophyll in PSII

31. Summary of Light Reactions
Summary: what is produced during the light reactions
p+ concentration gradient
NADPH

32. Summary of Light Reactions
Summary: what is produced during the light reactions
p+ concentration gradient
NADPH

33. Chemiosmosis
potential E from gradient is harnessed by ATP Synthase in thylakoid membrane
ATP Synthase serves two functions
Catalyzes ADP + (P)  ATP
Acts as carrier protein for p+
as H+ ions pass through ATP Synthase, ATP is produced

35. Section 6.2: The Calvin cycle

36. Stomata
Open
Closed

37. Section 6.2: The Calvin cycle
Light-independent reactions
Many names
Calvin (or Calvin-Benson) cycle (men who first described cycle)
Dark reactions (does not directly require light)
Carbon fixation (incorporation of C into organic substances)

38. Section 6.2: The Calvin cycle
sugars are long term energy storage (much more stable than ATP of NADP+)
requires carbon dioxide (CO2 ) and water (H2O)
CO2 enters plants through stomata (little tiny pores controlled by the plant)
H2O enters plant through osmosis, capillarity or stomata

39. Step 1
after diffusing into the stroma from the cytosol, CO2 joins with a 5-C sugar (RuBP) to produce 2 3-C molecules of PGA (process is known as carbon-fixation)

40. Step 2 PGA is converted into PGAL
2PGA + 2ATP + 2NADPH2PGAL + 2ADP + 2NADP+ +2 phosphate

41. Step 3 and 3B Most PGAL converted back into RuBP
2PGAL + ATP  RuBP + ADP + phosphate + some fixed C
Some PGAL leave Calvin cycle as fixed C (3B)

42. Balance Sheet Each turn of Calvin cycle results in fixation of 1 CO2
Three times around Calvin cycle results in 1 PGAL
each turn requires 3 ATP and 2 NADPH
2 ATP from step 2
1 ATP from step 3
3 turns requires 9 ATP and 6 NADPH

43. …
PGAL and other organic molecules like carbohydrates are formed and then used all over the cell for a variety of functions.
6CO2+ 6H2O + energy  C6H12O6 + 6O2

44. Alternative Pathways C4 pathway
when CO2 is low, enables plants to continue fixing carbon
grasses, corn
uses less water, but moves much more slowly

45. Alternative Pathways CAM pathway
when very hot and dry, enables plant to continue to fix carbon
cacti, pineapples
stomata open at night instead of during the day
uses less water, but moves much more slowly