Photosynthesis is the process by which plants, algae, and some microorganisms use solar energy to convert carbon dioxide and water into glucose and oxygen. It occurs in two stages - the light reactions where solar energy is captured to make ATP and NADPH, and the carbon reactions where ATP and NADPH are used to incorporate CO2 into organic compounds to make glucose. The light reactions take place in the thylakoid membranes of chloroplasts and use two photosystems to transfer electrons and pump protons, generating a proton gradient used to make ATP. The carbon reactions occur in the chloroplast stroma and involve the Calvin cycle to reduce CO2 into glucose using the products of the light reactions.

1. Photosynthesis

2. Photosynthesis
• The process by which plants, algae, and some
microorganisms harness solar energy and convert
it into chemical energy.
• Endergonic reaction
• Redox reaction
• Only done by autotrophs
• Glucose used for: fuel own plant respiration
(50%), growth, make other important compounds
(amino acids, cellulose, starch, sucrose)
6CO2 + 6H2O C6H12O6 + 6O2

3. Photosynthesis Equation
Overall equation: 6CO2 + 6H2O  C6H12O6 + 6O2

4. Photosynthesis is a Redox Reaction
Redox-reactions

5. Photosynthesis/Respiration Cycle
Cellular
respiration
CO2 and H2O
ATP (available for
cellular tasks)
Heat energy

6. Atmospheric Oxygen

7. Light
• Light is the source of energy for
photosynthesis
– Made of photons—packets of kinetic energy
– Part of electromagnetic spectrum
– 3 types from the sun get to the earth
• Ultraviolet
• Visible
• Infrared

8. Light

9. Pigments
• Pigment—Substance that absorbs light energy
• Several types of pigments:
– Chlorophyll a—most abundant, green pigment,
absorb blue/red, reflect green
– Accessory Pigments:
• Chlorophyll b—absorb blue/red, reflect green
• Carotenes—absorb blue, reflect orange/red
• Xanthophylls—absorb purple/blue/ green, reflect
yellow

10. Pigments
Pigment Color Organisms
Major Pigment
Chlorophyll a
green (or yellow) plants, algae, bacteria
Accessory Pigment
Chlorophyll b
yellow plants, algae
Carotenoids (xanthophylls
and carotenes)
orange, red, yellow plants, algae, bacteria,
archaea

12. Structure of a leaf

13. Structure of a leaf

14. Chloroplasts
• Mainly found in cells in the LEAF
– Lots of surface area to absorb light
– Has abundant water
– Main site of gas exchange
• Exchange occurs through stomata surrounded by guard
cells
– Mainly located in mesophyll

15. Chloroplasts
Chloroplasts

16. Chloroplasts
• Stroma—inner fluid with DNA, ribosomes, fluid
• Grana—Stacks of thylakoid
• Thylakoid—Disks, membranes with
photosynthetic pigments
• Photosystem—in thylakoid membrane
– Chlorophyll a (approx. 300 molecules)
• Reaction Center
– Accessory pigments (approx. 50 molecules)
• Antenna pigment to funnel light to reaction center
– Proteins

17. Chloroplasts

18. Chloroplasts

19. Photosynthesis Overview
• Happens in 2 stages
– Light Reactions—convert solar energy into
chemical energy
• Occurs in thylakoid membrane
– Carbon Reactions—use ATP and NADPH to reduce
CO2 to glucose
• Occurs in the stroma

20. Photosynthesis Overview

21. Photosynthesis Overview

22. Light Reactions: Photosystems
What happens if you could capture this
energy?

23. Light Reactions: Photosystems
Plants capture this energy!

24. Light Reactions: Photosystems

25. The Light Reactions
• Photosystem II
– Pigment molecules absorb light and transfer to
reaction center (chlorophyll a)
– Water is split into 2H+ and ½ O2
– Water donates 2 electrons
– Energy “excites” 2 electrons to a higher energy orbital
– Chlorophyll a ejects “excited” electrons to first
electron transport chain (ETC)
– ETC makes a proton gradient from stroma into the
thylakoid space
– ATP synthase uses proton gradient to make ATP
(chemiosmotic phosphorylation)
• Used in carbon reactions

26. The Light Reactions—Photosystem II

27. ATP Generation—Photosystem II

28. The Light Reactions
• Photosystem I
– Pigment molecules absorb light and transfer to
reaction center (chlorophyll a)
– 2 electrons come from first ETC
– Energy “excites” 2 electrons to a higher energy
orbital
– Chlorophyll a ejects “excited” electrons to first
electron transport chain (ETC)
– Electrons are passed to NADP+ to reduce it to
NADPH (used in carbon reactions)

29. The Light Reactions—Photosystem I

30. NADPH Generation—Photosystem
I

31. Making ATP: Photophosphorylation
H+ gradient: as
electrons moved
within membrane,
H+ is pumped into
the thylakoid space

32. Making ATP: Photophosphorylation
ATP produced: ATP
synthase allows H+
to go down its
concentration
gradient, generates
ATP

33. ETC vs. Photophosphorylation
Energy source Final Electron Acceptor

34. The Carbon Reactions
• Also known as: Calvin Cycle, “Dark reactions”
• Occurs in the stroma
• Uses ATP and NADPH to make glucose from CO2
• Calvin Cycle:
– Step 1: Carbon fixation—incorporation of CO2 into an
organic molecule
• CO2 combines with RuBP, using enzyme called rubisco
– Step 2: PGAL Synthesis
– Step3: PGAL makes glucose
– Step 4: Regeneration of RuBP

35. Calvin Cycle

37. C3 Plants
• Calvin Cycle = C3 Pathway
• All plants use Calvin Cycle, but some plants ONLY
use C3 pathway
– 95% of plants are this way
• Inefficient—lose some energy to heat
– 30% on the best sunny day
– In Photorespiration rubisco uses O2 instead of CO2 as
a substrate
– Stomates open, O2 diffuses out, CO2 is used
– Hot dry climates, stomates cannot stay open—lost
water, O2 builds up, photorespiration takes over

38. C4 Plants
• C4—adaptation to help minimize
photorespiration (1% of plants)
• C4 Plants—Separate light reactions and Calvin
Cycle into different cells
– Light reactions and carbon fixation—mesophyll
– CO2 combines with 3 carbon molecule to make 4
carbon—C4
– C4—(malate) moves to bundle sheath cells, rest of
Calvin Cycle
• Bundle sheath cells NOT exposed to O2

39. C3 and C4 Plant Anatomy
C4 plantC3 plant
Vein
Stoma
Mesophyll
cell
Bundle-
sheath cell Mesophyll
cell
Stoma
Vein
Bundle-
sheath cell

40. CAM Plants
• Occurs in desert plants (3–4% of plants)
• Only open stomates at night to fix CO2, then
fix again during the day using Calvin Cycle
– Store night time CO2 as malate in vacuoles
– Stomates open, malate to chloroplast, release
CO2, used in Calvin Cycle
• Happens in same cells

41. Pathway
C3 plant C4 plant CAM plant

42. Global Warming
Green house effect: radiant heat trapped by CO2

43. Global Warming

44. Ozone Depletion
Ozone: O3 (O2 converted to O3) filters out UV rays

45. Ozone Depletion
Chlorofluorocarbons: release chlorine which destroys
ozone
Antarctica

46. Ozone
Depletion
Chlorofluorocarbons:
release chlorine which
destroys ozone
Antarctica