Bio for NonMajors college course provided by Lumen Learning

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