1. CHAPTER 7 Photosynthesis: Using Light to Make Food
Modules 7.1 – 7.5

2. photosynthesis

3. Life in the Sun Light is central to the life of a plant
Photosynthesis is the most important chemical process on Earth
Primary Function: It provides food for virtually all organisms
Plant cells convert light into chemical signals that affect a plant’s life cycle
Seed germination, flower production

4. Light can influence the architecture of a plant
Plants that get adequate light are often bushy, with deep green leaves
Without enough light, plants become tall and spindly with small pale leaves
Too much sunlight can damage a plant
Chloroplasts and carotenoids help to prevent such damage
Too much UV = DNA damage, tumors

5. AN OVERVIEW OF PHOTOSYNTHESIS
Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water
Look at the formula: should look familiar (kind of)
Carbon dioxide
Water
Glucose
Oxygen gas
PHOTOSYNTHESIS

6. 7.1 Autotrophs are the producers of the biosphere
(TQ) Plants, some protists (algae/kelp), and some bacteria (even purple) are photosynthetic autotrophs. Animals and fungi are NOT photosynthetic
They are the ultimate producers of food consumed by virtually all organisms
Producers make organic (C6H12O6 or glucose) food material from very simple raw materials.
Raw materials (water and light, no C = inorganic)
New discovery: Animal incorporated chloroplasts.

7. On land, plants such as oak trees and cacti are the predominant producers
Figure 7.1A
Figure 7.1B

8. In aquatic environments, algae (kelp) and photosynthetic bacteria are the main food producers
Figure 7.1C
Figure 7.1D

9. 7.2 Photosynthesis occurs in chloroplasts
In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts.
Chloroplasts (concentrated in mesophyll tissue ) contains:
stroma, a fluid
grana, stacks of thylakoids
Stomata, small pores allowing gas exchange
The thylakoids contain chlorophyll
Chlorophyll is the green pigment that captures light for photosynthesis

10. The location and structure of chloroplasts
LEAF CROSS SECTION
MESOPHYLL CELL
LEAF
Mesophyll
CHLOROPLAST
Intermembrane space
Outer
membrane
Granum
Inner membrane
Grana
Stroma
Thylakoid compartment
Stroma
Figure 7.2
Thylakoid

11. 7.3 Plants produce O2 gas by splitting water
The O2 liberated by photosynthesis is made from the oxygen in water
Figure 7.3A

12. Hint: Make sure you know which reactants contribute to which products.
Experiment 1
Pink letters represent radioactive tracers. Good test application question.
Not labeled
Experiment 2
Labeled
Figure 7.3B
Reactants:
Hint: Make sure you know which reactants contribute to which products.
Products:
Figure 7.3C

13. 7.4 Photosynthesis is a redox process, as is cellular respiration
Water molecules are split apart and electrons and H+ ions are removed, leaving O2 gas
These electrons and H+ ions are transferred to CO2, producing sugar
Lose of electrons and the gaining of electrons
“LEO-GER”
Review definition of oxidation and reduction
Reduction
Oxidation
Figure 7.4A
Oxidation
Reduction
Figure 7.4B

14. Name a gas released as by-product of light dependent reactions of photosynthesis
Name the molecule that is the source of this gas
Why is oxygen removed from the molecule named in 2B?
1. O2
2. H20
3. H needed to build glucose molecules

15. Redox: cellular respiration vs. photosynthesis
(TQ) understand both types of redox reactions: CR and photosynthesis
The directions are reversed
Photosynthesis: water is oxidized to O2, CO2 is reduced to sugar
Goes “uphill,” electrons gaining energy from captured light energy by chlorophyll
Cell Resp.: sugar oxidized to CO2, O2 reduced to H2O
Goes “downhill,” electrons lose PE

16. 7.5 Overview: Photosynthesis occurs in two stages linked by ATP and NADPH
The complete process of photosynthesis consists of two linked sets of reactions:
the light reactions and the Calvin cycle
The light reactions convert light energy to chemical energy and produce O2
The Calvin cycle assembles sugar molecules from CO2 using the energy-carrying products of the light reactions

17. LIGHT REACTIONS (in grana) CALVIN CYCLE (in stroma)
An overview of photosynthesis
** Know Products and Reactants for both stages
H2O
CO2
Chloroplast
Light
NADP+
ADP + P
LIGHT REACTIONS (in grana)
CALVIN CYCLE (in stroma)
ATP
Electrons
NADPH O2
Sugar
Figure 7.5

18. Light reaction
Calvin cycle
Occurs in thylakoid
Light energy used to make ATP from ADP + P
Light energy used to make NADPH from NADP+. Temporary storage of energized electrons
No sugar made
Occurs in the stroma
Carbon fixation is the incorporation of carbon into organic compounds
Uses ATP as energy to run some steps of Calvin and NADPH electrons get oxidized.
Ultimately makes sugar
Can run during daytime

19. ?: Name two molecules that are produced during the light dependent reactions of photosynthesis and serve as temporary sites for energy storage? Ans: NADPH and ATP

20. 7.6 Visible radiation drives the light reactions
THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY
7.6 Visible radiation drives the light reactions
Certain wavelengths of visible light drive the light reactions of photosynthesis
Gamma rays
Micro- waves
Radio
waves
X-rays UV
Infrared
Visible light
Wavelength (nm)
Figure 7.6A

21. ?: Which has the most energy: one photon of green light, one photon of red light, or one photon of blue light?
Ans: Green light (transmitted and reflected by pigments = least likely absorbed)

22. Reflected light Light Chloroplast Absorbed light Transmitted light
Figure 7.6B

23. 7.7 Photosystems capture solar power
Each of the many light-harvesting photosystems consists of:
an “antenna” of chlorophyll and other pigment molecules that absorb light
a primary electron acceptor that receives excited electrons from the reaction-center chlorophyll

24. Primary electron acceptor
PHOTOSYSTEM
Photon
Reaction center
Pigment molecules of antenna
Figure 7.7C

25. Fluorescence of isolated chlorophyll in solution
Heat
Photon (fluorescence)
Photon
Chlorophyll molecule
Figure 7.7A

26. Primary electron acceptor
Excitation of chlorophyll in a chloroplast
Primary electron acceptor
Other compounds
Photon
Chlorophyll molecule
Figure 7.7B

27. Tracking the capture of solar power
Photon = light energy
(TQ) Pigment molecule absorbing light has an electron gain energy and move from ground to excited state.
Sometimes because the electron it is unstable it than loses energy and falls back to ground state or some energy released as heat
(TQ) Excited electron moved to neighboring molecule, called primary electron acceptor, and is reduced (while chlorophyll gets oxidized)
(TQ) Location of electron donation is called the reaction center, but only 1 chlorophyll a actually donates electrons. (TQ) Antenna molecules are other pigment molecules collecting light and funneling to reaction center
(TQ) Location is the thylakoid membrane

28. Photosystems
Photosystem 1 = chlorophyll a is P700 (wavelength red light)
Photosystem 2 = chlorophyll a is P680 (more orange than red wavelength)
(TQ) During the fall many trees break down chlorophyll in their leaves. The color of leaves is due to carotenoids

29. 7.8 In the light reactions, electron transport chains generate ATP, NADPH, and O2
Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons
The excited electrons are passed from the primary electron acceptor to electron transport chains (similar to those in cellular respiration
Their energy ends up in ATP and NADPH
Photosystem II – redox reactions shuttle down etc and electrons lose energy to form ATP (by chemiosmosis)
Photosytem I – NADP+ gets reduced to form NADPH

30. H+ stays in chloroplast, but O combines with another split O = O2
Big idea
Where do the electrons come from that keep the light reactions running?
NADPH requires 2 electrons from PS I, PS I gets electrons from PS II, PS II gets electrons from the splitting of water.
H+ stays in chloroplast, but O combines with another split O = O2
So what is the overall result of blocking reaction centers?
No donation of excited electrons to primary electron acceptor

31. Photosystem II regains electrons by splitting water, leaving O2 gas as a by-product
Primary electron acceptor
Electron transport
Primary electron acceptor
Electron transport chain
Photons
Energy for synthesis of
PHOTOSYSTEM I
PHOTOSYSTEM II
by chemiosmosis
Figure 7.8

32. 7.9 Chemiosmosis powers ATP synthesis in the light reactions
The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane
The flow of H+ back through the membrane is harnessed by ATP synthase (Look familiar?) to make ATP
In the stroma, the H+ ions combine with NADP+ to form NADPH (similar to role of O2 in Cell. Resp.)

33. ELECTRON TRANSPORT CHAIN
The production of ATP by chemiosmosis in photosynthesis (photophosphorylation)
Final electron acceptor is NADP+ (unlike O2 for Cell Resp.)
Thylakoid compartment (high H+)
Light
Light
Thylakoid membrane
Antenna molecules
Stroma (low H+)
ELECTRON TRANSPORT CHAIN
PHOTOSYSTEM II
PHOTOSYSTEM I
ATP SYNTHASE
Figure 7.9

34. 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle
THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS
7.10 ATP and NADPH power sugar synthesis in the Calvin cycle
The Calvin cycle occurs in the chloroplast’s stroma
This is where carbon fixation takes place and sugar is manufactured
INPUT
CALVIN CYCLE
Figure 7.10A
OUTPUT:

35. The Calvin cycle constructs G3P using
carbon from atmospheric CO2
electrons and H+ from NADPH
energy from ATP
Energy-rich sugar is then converted into glucose

36. Details of the Calvin cycle
INPUT: 3
In a reaction catalyzed by rubisco, 3 molecules of CO2 are fixed.
CO2
Step Carbon fixation. 1 1 3 P P 6 P
Enzyme rubisco combines 5C +1C to make 3C PGA
RuBP
3-PGA 6
ATP
3 ADP
Step Energy consumption and redox. 2
6 ADP + P 3
ATP
CALVIN CYCLE 2 6
ATP consumed, NADPH oxidized 4
NADPH
6 NADP+
Step Release of one molecule of G3P. 3 5 P 6 P
G3P
G3P
For every 6 PGA, 1 leaves, and 5 remain. For each 2 leaving = 1 glucose 3
Step Regeneration of RuBP. 4
Glucose and other compounds
OUTPUT: 1 P
5C G3P uses ATP to go back to 3C RuBP
G3P
Figure 7.10B

37. Rubisco most common enzyme in the world
Photorespiration may then occur
Not a good thing due to the breakdown of rubisco

38. 7.11 Review: Photosynthesis uses light energy to make food molecules
PHOTOSYNTHESIS REVIEWED AND EXTENDED
7.11 Review: Photosynthesis uses light energy to make food molecules
A summary of the chemical processes of photo-synthesis
Chloroplast
Light
Photosystem II Electron transport chains Photosystem I
CALVIN CYCLE
Stroma
Electrons
Cellular respiration
Cellulose
Starch
Other organic compounds
LIGHT REACTIONS
CALVIN CYCLE
Figure 7.11

39. Many plants make more sugar than they need
The excess is stored in roots, tuber, and fruits
These are a major source of food for animals

41. 7.12 C4 and CAM plants have special adaptations that save water
Most plants are C3 plants, which take CO2 directly from the air and use it in the Calvin cycle
In these types of plants, stomata on the leaf surface close when the weather is hot
This causes a drop in CO2 and an increase in O2 in the leaf
Photorespiration may then occur
Not a good thing due to the breakdown of rubisco

42. Photorespiration in a C3 plant
CALVIN CYCLE
2-C compound
Figure 7.12A

43. Some plants have special adaptations that enable them to save water
Special cells in C4 plants—corn and sugarcane—incorporate CO2 into a four-carbon molecule
This molecule can then donate CO2 to the Calvin cycle
4-C compound
CALVIN CYCLE
3-C sugar
Figure 7.12B

44. The CAM plants—pineapples, most cacti, and succulents—employ a different mechanism
They open their stomata at night and make a four-carbon compound
It is used as a CO2 source by the same cell during the day
4-C compound
Night
Day
CALVIN CYCLE
3-C sugar
Figure 7.12C

45. PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S ATMOSPHERE
7.13 Human activity is causing global warming; photosynthesis moderates it
Due to the increased burning of fossil fuels, atmospheric CO2 is increasing
CO2 warms Earth’s surface by trapping heat in the atmosphere
This is called the greenhouse effect

46. Sunlight
ATMOSPHERE
Radiant heat trapped by CO2 and other gases
Figure 7.13A & B

47. Because photosynthesis removes CO2 from the atmosphere, it moderates the greenhouse effect
Unfortunately, deforestation may cause a decline in global photosynthesis

48. 7.14 Talking About Science: Mario Molina talks about Earth’s protective ozone layer
Mario Molino received a Nobel Prize in 1995 for his work on the ozone layer
His research focuses on how certain pollutants (greenhouse gases) damage that layer
Figure 7.14A

49. The O2 in the atmosphere results from photosynthesis
Solar radiation converts O2 high in the atmosphere to ozone (O3)
Ozone shields organisms on the Earth’s surface from the damaging effects of UV radiation

50. International restrictions on these chemicals are allowing recovery
Industrial chemicals called CFCs have hastened ozone breakdown, causing dangerous thinning of the ozone layer
International restrictions on these chemicals are allowing recovery
Sunlight
Southern tip of South America
Antarctica
Figure 7.14B