1. Chapter 08 Photosynthesis
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2. 8.1 Overview of Photosynthesis-1
Photosynthesis converts solar energy into chemical energy of carbohydrates.
Organisms that carry on photosynthesis are called autotrophs.
Plants, algae, and cyanobacteria are organisms capable of photosynthesis
Heterotrophs are organisms that feed on other organisms.

3. 8.1 Overview of Photosynthesis-2
Autotrophs and heterotrophs use organic molecules produced by photosynthesis.
Pigments allow photosynthetic organisms to capture solar energy.
Most photosynthetic organisms contain the pigment chlorophyll.
Another common pigment group is the carotenoids.

4. Flowering Plants as Photosynthesizers
Photosynthesis occurs in the green parts of plants.
Particularly leaves, contain chlorophyll and other pigments
Leaves contain mesophyll tissue specialized for photosynthesis
Raw materials are water and CO2

5. 8.1 Overview of Photosynthesis-3
Water is taken up by roots and transported to leaves by veins.
Carbon dioxide enters through openings in the leaves called stomata.
Light energy is absorbed by chlorophyll and other pigments in the thylakoids of chloroplasts.

6. 8.1 Overview of Photosynthesis-4
Chloroplast structure
The chloroplast and its fluid-filled interior called stroma are surrounded by a double membrane.
Thylakoids are a different membrane system within the stroma that form flattened sacs.
Thylakoids are stacked together to form grana.
Thylakoid space is formed by a continuous connection between individual thylakoids.

7. 8.1 Overview of Photosynthesis-5
Figure 8.2(a)
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8. 8.1 Overview of Photosynthesis-6
Figure 8.2(b)
Chloroplast: © Science Source
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9. Photosynthetic Reaction
Glucose and oxygen are the products of photosynthesis
The oxygen given off comes from water
CO2 gains hydrogen atoms and becomes a carbohydrate
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10. Two Sets of Reactions-1 The two sets of reactions are called the:
Photosynthesis consists of two sets of reactions
Photo refers to capturing solar energy
Synthesis refers to producing a carbohydrate
The two sets of reactions are called the:
Light Reactions (light-dependent)
Calvin Reactions (light-independent)
Nicotinamide adenine dinucleotide phosphate (NADP+) links these reactions

11. Two Sets of Reactions-2 Figure 8.3 Jump to long image description
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12. 8.2 Plants as Solar Energy Converters-1
During the light reactions, different pigments within the thylakoid membranes absorb energy.
Solar energy can be described in terms of its wavelength and energy content.

13. 8.2 Plants as Solar Energy Converters-2
The electromagnetic spectrum extends from very short gamma rays to very long radio waves.
White or visible light is only a small portion of the spectrum.
Visible light is further divided into wavelengths between 380 and 750 nm.
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Figure 8.4
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14. Visible Light-1 Visible light contains various wavelengths
The colors of visible light range from:
Violet light
Shortest wavelength but high energy
Red light
Longest wavelength but lowest energy
Only about 42% of solar radiation that hits Earth’s atmosphere reaches the surface of Earth – most is in the visible-light range
Higher wavelengths are screened by the ozone layer

15. Visible Light-2
Most photosynthetic pigments in cells are chlorophylls a and b and the carotenoids
Can absorb specific various portions of visible light
The absorption spectrum shown in figure on the right
Figure 8.5
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16. Visible Light-3 Green light is reflected and only minimally absorbed
Leaves appear green
Other plant pigments become noticeable in the fall when chlorophyll breaks down and the other pigments are uncovered

17. Light Reactions Light Reactions Take place in thylakoid membrane
Light reactions consist of two pathways:
Noncyclic electron pathway
Cyclic electron pathway
Both pathways transform solar energy to chemical energy
Both pathways produce ATP
Only the noncyclic pathway produces NADPH

18. Noncyclic Electron Pathway-1
Noncyclic electron pathway, named because electron flow is traced from water to NADP+
Uses two photosystems (Photosystems I and II)
A photosystem consists of a pigment complex and electron acceptors within the thylakoid membrane
The pigment complex can be described as an “antenna” for gathering solar energy

19. Noncyclic Electron Pathway-2
Noncyclic Electron Pathway begins with photosystem II (PSII)
Pigment complex absorbs solar energy
Energy passes from one pigment to another until it is concentrated in reaction center
Chlorophyll a molecule
Electrons in the reaction center chlorophyll become energized
Escape from the reaction center and move to a nearby electron acceptor

20. Noncyclic Electron Pathway-3
Photosystem II would disintegrate without replacement electrons
Electrons provided by splitting water
Releases oxygen (O2) to atmosphere which benefits all organisms that use O2
Hydrogen ions (H+) stay in the thylakoid space
Contribute to formation of hydrogen ion gradient

21. Noncyclic Electron Pathway-4
In PSII, an electron acceptor receives energized electrons from the reaction center
It sends those electrons down an electron transport chain, (series of carriers that pass electrons from one to the other)
Energy is released to pump hydrogen ions (H+) into thylakoid space forming gradient
When hydrogen ions flow through ATP synthase it makes ATP

22. Noncyclic Electron Pathway-5
PSI comes next in noncyclic electron pathway
When the photosystem I complex absorbs solar energy, energized electrons leave reaction center and are captured by a different electron acceptor
Low energy PSII electrons used to replace those lost by PSI
Electron acceptor in photosystem I passes its electrons to NADP+ and it becomes NADPH

23. Noncyclic Electron Pathway-6
Figure 8.6
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24. The Organization of the Thylakoid Membrane-1
The following molecular complexes are present in the thylakoid Membrane:
PS II
Pigment complex and electron acceptors
Water is split to replace energized electrons
Oxygen (O2) is released
Electron transport chain
Carries electrons from PS II to PS I
Uses energy to pump H+ from the stroma into thylakoid space

25. The Organization of the Thylakoid Membrane-2
PS I
Pigment complex and electron acceptors
Adjacent to enzyme that reduces NADP+ to NADPH
ATP synthase complex
Has a channel for H+ flow
Flow drives ATP synthase to join ADP and P

26. The Organization of the ThylakoiMembrane-3
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Figure 8.7
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27. Cyclic Electron Pathway-1
Uses only photosystem I (PSI) and begins when PSI complex absorbs solar energy
Energized electrons escape from the reaction center and travel down electron transport chain
Released energy is stored in the form of a H+ gradient, which causes ATP production by ATP synthase
Spent electrons return to PSI (cyclic)
Pathway only results in ATP production

28. Cyclic Electron Pathway-2
Energized electrons leave the photosystem I reaction center and return to photosystem by an electron transport chain
ATP from cyclic electron transport used in Calvin cycle to make carbohydrates
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Figure 8.8
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29. ATP Production ATP Production
Thylakoid space acts as a reservoir for hydrogen ions (H+)
H+ from water being split within thylakoid space
Pumped in by electron transport chain
More H+ in thylakoid space than stroma
Electrochemical gradient
H+ can only flow through ATP synthase
Energy powers making ATP by chemiosmosis

30. 8.3 Plants as Carbon Dioxide Fixers-1
The Calvin Cycle (named after Melvin Calvin)
Series of reactions that use CO2 from the atmosphere to produce carbohydrate
Humans and most other organisms take in O2 and release CO2
Includes:
Carbon dioxide fixation
Carbon dioxide reduction
Ribulose-1,5-bisphosphate (RuBP) regeneration

31. 8.3 Plants as Carbon Dioxide Fixers-2
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Figure 8.9
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32. Fixation of Carbon Dioxide
Carbon dioxide fixation is the 1st step of the Calvin cycle
CO2 is attached to 5-carbon RuBP molecule
This reaction occurs three times
The result is a 6-carbon molecule that splits into two 3-carbon molecules 3-phoshoglycerate (3PG)
RuBP Carboxylase is the enzyme that makes this happen
Comparatively slow enzyme so there is a lot of it

33. Reduction of Carbon Dioxide-1
Each 3PG molecule undergoes reduction to G3P in two steps
Energy and electrons needed for this reaction are supplied by ATP and NADPH (from light reaction)

34. Reduction of Carbon Dioxide-2
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35. Regeneration of RuBP-1 Regeneration of RuBP
It takes three turns of the Calvin cycle to allow one G3P to exit
For every three turns of Calvin Cycle, five G3P (3-carbon molecule) are used
This re-forms three RuBP (5-carbon molecule)
5 X 3 (carbons in G3P) = 3 X 5 (carbons in RuBP)

36. Regeneration of RuBP-2 5×3 carbons in G3P =3×5 (carbons in RuBP)
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37. Importance of the Calvin Cycle
G3P (glyceraldehyde-3-phosphate) can be converted to many other molecules
These molecules meet the plant needs
The hydrocarbon skeleton of G3P can form:
Fatty acids and glycerol to make plant oil
Glucose phosphate (simple sugar)
Fructose (+ glucose = sucrose)
Starch and cellulose
Amino acids

38. 8.4 Alternate Pathways for Photosynthesis
C3 Photosynthesis
The leaves of C3 plants have a different structure and means of fixing CO2 than C4 plants
C3 plants such as wheat, rice, and oats have mesophyll cells of leaves in parallel layers
Bundle sheath cells around the plant veins do not contain chloroplasts
As a result, cells using Calvin cycle exposed to CO2

39. C3 Photosynthesis-1 RuBP carboxylase binds O2 as well as CO2
When bound to O2, the enzyme undergoes photorespiration
Wasteful reaction because it uses O2 and releases CO2, decreasing output of Calvin cycle
O2 concentration in leaf rises when weather is hot and dry, because plant keeps stomata closed to conserve water

40. tulips: © Evelyn Jo Johnson
C3 Photosynthesis-2
Figure 8.11(a)
Figure 8.10(a)
tulips: © Evelyn Jo Johnson
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41. C4 Photosynthesis-1
In C4 plants, such as sugarcane and corn, the mesophyll cells are arranged in concentric rings around the bundle sheath cells.
They also contain chloroplasts
In the mesophyll cells, CO2 is initially fixed into a 4-carbon molecule
The 4-carbon molecule is later broken down into a 3-carbon molecule and CO2
CO2 enters the Calvin cycle

42. C4 Photosynthesis-2 C4 Pathway
C4 plants tend to be found in hot, dry climates
In these climates, stomata tend to close to conserve water
Oxygen then builds-up in the leaves
But, RuBP carboxylase is not exposed to this O2 in C4 plants and photorespiration does not occur
Instead, in C4 plants, the CO2 is delivered to the Calvin cycle, which is located in bundle sheath cells that are sheltered from the air spaces of the leaf

43. corncob: © David Frazier/Corbis RF
C4 Photosynthesis-3
Figure 8.11(b)
Figure 8.10(b)
corncob: © David Frazier/Corbis RF
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44. C4 Photosynthesis-4
When the weather is moderate, C3 plants ordinarily have the advantage.
When the weather is hot and dry, C4 plants have the advantage, and can be expected to predominate.
In the early summer, C3 plants such as Kentucky bluegrass predominate in lawns in the cooler parts of the United States, but by midsummer, crabgrass, a C4 plant, begins to take over.

45. CAM Photosynthesis-1 CAM Pathway
This pathway is prevalent among most succulent plants that grow in deserts, including the cacti.
CAM plants partition carbon fixation according to time.
During the night, CAM plants fix CO2, forming C4 molecules.
The C4 molecules are stored in large vacuoles.
During daylight, C4 molecules release CO2 to the Calvin cycle.

46. Pineapple: © Pixtal/Age fotostock RF
CAM Photosynthesis-2
Figure 8.10(c)
Pineapple: © Pixtal/Age fotostock RF
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47. 8.5 Photosynthesis Versus Cellular Respiration-1
Both plant and animal cells carry out cellular respiration.
Occurs in mitochondria
Breaks glucose down
Utilizes O2 and gives off CO2
Plant cells photosynthesize, but animal cells do not.
Occurs in chloroplasts
Builds glucose
Utilizes CO2 and gives off O2
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48. 8.5 Photosynthesis Versus Cellular Respiration-2
Both processes utilize an electron transport chain and chemiosmosis for ATP production.
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49. 8.5 Photosynthesis Versus Cellular Respiration-3
Figure 8.12(a)
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50. 8.5 Photosynthesis Versus Cellular Respiration-4
Figure 8.12(b)
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51. 8.5 Photosynthesis Versus Cellular Respiration-5
Figure 8.12(c)
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