1. Photosynthesis Chapter 8 Adapted by G. Cornwall, Ph.D.
From Raven’s Biology, McGraw Hill Publishing

2. Photosynthesis Overview
Energy for all life on Earth ultimately comes from photosynthesis
6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
Oxygenic photosynthesis is carried out by
Cyanobacteria
7 groups of algae
All land plants – chloroplasts

3. Chloroplast Thylakoid membrane – internal membrane
Contains chlorophyll and other photosynthetic pigments
Pigments clustered into photosystems
Grana – stacks of flattened sacs of thylakoid membrane
Stroma lamella – connect grana
Stroma – semiliquid surrounding thylakoid membranes

4. Stages Light-dependent reactions
Require light
Capture energy from sunlight
Make ATP and reduce NADP+ to NADPH
Carbon fixation reactions or light-independent reactions
Does not require light
Use ATP and NADPH to synthesize organic molecules from CO2

6. Pigments Molecules that absorb light energy in the visible range
Light is a form of energy
Photon – particle of light
Acts as a discrete bundle of energy
Energy content of a photon is inversely proportional to the wavelength of the light
Photoelectric effect – removal of an electron from a molecule by light

7. Absorption spectrum
When a photon strikes a molecule, its energy is either
Lost as heat
Absorbed by the electrons of the molecule
Boosts electrons into higher energy level
Absorption spectrum – range and efficiency of photons molecule is capable of absorbing

8. Organisms have evolved a variety of different pigments
Only two general types are used in green plant photosynthesis
Chlorophylls
Carotenoids
In some organisms, other molecules also absorb light energy

9. Chlorophylls Chlorophyll a Chlorophyll b
Main pigment in plants and cyanobacteria
Only pigment that can act directly to convert light energy to chemical energy
Absorbs violet-blue and red light
Chlorophyll b
Accessory pigment or secondary pigment absorbing light wavelengths that chlorophyll a does not absorb

10. Structure of chlorophyll porphyrin ring
Complex ring structure with alternating double and single bonds
Magnesium ion at the center of the ring
Photons excite electrons in the ring
Electrons are shuttled away from the ring

11. Action spectrum
Relative effectiveness of different wavelengths of light in promoting photosynthesis
Corresponds to the absorption spectrum for chlorophylls

12. Carotenoids Phycobiloproteins
Carbon rings linked to chains with alternating single and double bonds
Can absorb photons with a wide range of energies
Also scavenge free radicals – antioxidant
Protective role
Phycobiloproteins
Important in low-light ocean areas

13. Photosystem Organization
Antenna complex
Hundreds of accessory pigment molecules
Gather photons and feed the captured light energy to the reaction center
Reaction center
1 or more chlorophyll a molecules
Passes excited electrons out of the photosystem

14. Antenna complex Also called light-harvesting complex
Captures photons from sunlight and channels them to the reaction center chlorophylls
In chloroplasts, light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins

15. Reaction center Transmembrane protein–pigment complex
When a chlorophyll in the reaction center absorbs a photon of light, an electron is excited to a higher energy level
Light-energized electron can be transferred to the primary electron acceptor, reducing it
Oxidized chlorophyll then fills its electron “hole” by oxidizing a donor molecule

16. Light-Dependent Reactions
Primary photoevent
Photon of light is captured by a pigment molecule
Charge separation
Energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule
Electron transport
Electrons move through carriers to reduce NADP+
Chemiosmosis
Produces ATP
Capture of light energy

17. Cyclic photophosphorylation
In sulfur bacteria, only one photosystem is used
Generates ATP via electron transport
Anoxygenic photosynthesis
Excited electron passed to electron transport chain
Generates a proton gradient for ATP synthesis

18. Chloroplasts have two connected photosystems
Oxygenic photosynthesis
Photosystem I (P700)
Functions like sulfur bacteria
Photosystem II (P680)
Can generate an oxidation potential high enough to oxidize water
Working together, the two photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH

20. The Two Photosystems in Plants Work Together
Photosystem II drives ATP production
Electron is passed to the reaction center of PS I (Replaces electron lost through excitation)
the energy released is used to synthesize ATP
Photosystem I drives NADPH production.
Electron is passed to NADP+  NADPH

21. Noncyclic photophosphorylation
Plants use photosystems II and I in series to produce both ATP and NADPH
Path of electrons not a circle
Photosystems replenished with electrons obtained by splitting water
Z diagram

22. Chemiosmosis Electrochemical gradient used to synthesize ATP
Chloroplast has ATP synthase enzymes in the thylakoid membrane
Allows protons back into stroma
Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions

23. Making more ATP It takes more energy to fix Carbon (1 ½ ATP / NADPH)
PS I can short circuit to make extra ATP via Cyclic phosphorylation.

24. Card Quiz A
What high energy molecule is the final product of photosynthesis?
Oxygen
ATP
NADPH
Glucose
Bloom’s level: Knowledge/Understanding

25. Card Quiz A Where is the chlorophyll located in a plant? Cristae
Outer membrane of the chloroplast
Thylakoid membrane
Stroma
Bloom’s level: Knowledge/Understanding

26. Card Quiz A A particle of light is a ____. Wave Photon Proton Newton
Bloom’s level: Knowledge/Understanding

27. Card Quiz A What happens at the reaction center of a photosystem?
Light is absorbed
An electron is energized
NADP+ is reduced
ATP is formed
Bloom’s level: Knowledge/Understanding

28. Card Quiz A Which of the following is made during the light reactions.
ADP and NADP+
Glucose and ATP
ATP and NADPH
NADPH and Carbon Dioxide
Bloom’s level: Knowledge/Understanding

29. Card Quiz Answers Green Red Blue
Bloom’s level: Knowledge/Understanding

30. Carbon Fixation – Calvin Cycle
To build carbohydrates cells use
Energy
ATP from light-dependent reactions
Cyclic and noncyclic photophosphorylation
Drives endergonic reaction
Reduction potential
NADPH from photosystem I
Source of protons and energetic electrons

31. Calvin cycle Named after Melvin Calvin (1911–1997)
Also called C3 photosynthesis
Key step is attachment of CO2 to RuBP to form PGA
Uses enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco

32. 3 phases Carbon fixation Reduction Regeneration of RuBP
RuBP + CO2 → PGA
Reduction
PGA is reduced to G3P
Regeneration of RuBP
PGA is used to regenerate RuBP
3 turns incorporate enough carbon to produce a new G3P
6 turns incorporate enough carbon for 1 glucose

34. Output of Calvin cycle
Glucose is not a direct product of the Calvin cycle
G3P is a 3 carbon sugar
Used to form sucrose
Major transport sugar in plants
Disaccharide made of fructose and glucose
Used to make starch
Insoluble glucose polymer
Stored for later use

35. Energy cycle
Photosynthesis uses the products of respiration as starting substrates
Respiration uses the products of photosynthesis as starting substrates
Production of glucose from G3P even uses part of the ancient glycolytic pathway, run in reverse
Principal proteins involved in electron transport and ATP production in plants are evolutionarily related to those in mitochondria

37. Photorespiration Rubisco has 2 enzymatic activities
Carboxylation
Addition of CO2 to RuBP
Favored under normal conditions
Photorespiration
Oxidation of RuBP by the addition of O2
Favored when stoma are closed in hot conditions
Creates low-CO2 and high-O2
CO2 and O2 compete for the active site on RuBP

38. Types of photosynthesisC3
Plants that fix carbon using only C3 photosynthesis (the Calvin cycle)
C4 and CAM
Add CO2 to PEP to form 4 carbon molecule
Use PEP carboxylase
Greater affinity for CO2, no oxidase activity
C4 – spatial solution
CAM – temporal solution

39. C4 plants Corn, sugarcane, sorghum, and a number of other grasses
Initially fix carbon using PEP carboxylase in mesophyll cells
Produces oxaloacetate, converted to malate, transported to bundle-sheath cells
Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO2
Carbon fixation then by rubisco and the Calvin cycle

40. C4 pathway, although it overcomes the problems of photorespiration, does have a cost
To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone
C4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C3 pathway alone

41. CAM plants
Many succulent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups
Stomata open during the night and close during the day
Reverse of that in most plants
Fix CO2 using PEP carboxylase during the night and store in vacuole

42. When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO2
High levels of CO2 drive the Calvin cycle and minimize photorespiration

43. Compare C4 and CAM Both use both C3 and C4 pathways
C4 – two pathways occur in different cells
CAM – C4 pathway at night and the C3 pathway during the day

44. Card Quiz B Which structure is responsible for gas exchange ? Matrix
Stroma
Stoma
Thylakoid
Bloom’s level: Knowledge/Understanding

45. Question 4
Making glucose via photosynthesis costs ____ATPs and ___NADPHs. However, 1 molecule of glucose is enough to make ___ ATPs.
18, 12, 36
12, 18, 36
12, 36, 18
36, 18, 12
Bloom’s level: Knowledge/Understanding

46. Question 9 What is the function of Rubisco?
Absorption of photon energy
Reduction of NADP+
Chemiosmosis
Carbon fixation
Bloom’s level: Knowledge/Understanding

47. Card Quiz B Where could a botanist expect to find C4 plants? Canada
Costa Rica
Tundra
Mount Everest
Bloom’s level: Application

48. Card Quiz B
The end product of photosynthesis is the starting material of cellular respiration.
This is true
This is false
Bloom’s level: Application

49. Card Quiz B
Which of the following would prevent the formation of phosphoglycerate?
Denaturation of the Rubisco enzyme
Non-functional b6-f complex
Anaerobic conditions
All of the above
Bloom’s level: Application