CC BY Leslie Orzetti-Gollhofer

1. Photosynthesis

2. Photosynthesis
• Process by which plants, algae, and some
microorganisms use solar energy and CO2 and
convert it into chemical energy
• Endergonic
• Redox reaction
• 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 is a Redox Reaction
Cellular Respiration
Photosynthesis

4. Atmospheric Oxygen

5. 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

6. The Electromagnetic Spectrum
and Visible Light

7. 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

8. 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

9. Absorption Spectrum of Pigments

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

11. Chloroplast

12. 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
– Most abundant in mesophyll

13. Structure of a leaf

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

15. Photosynthesis Overview

16. 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) passes to Photosystem I
– 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

17. Light Reactions: Photosystems

18. The Light Reactions
• Photosystem I –
– Pigment molecules absorb light and transfer to
reaction center (chlorophyll a)
– 2 electrons come from first ETC (Photosystem II)
– 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)

19. ATP Generation – Photosystem I

20. The Light Reactions
• Final Outputs of Light Reactions
– Oxygen (emitted from plant)
– NADPH (used in Calvin Cycle)
– ATP (used in Calvin Cycle)

21. The Calvin Cycle
• Also known as: Carbon reactions, 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
• Makes PGAL (2 3 carbon molecules)
– Step 2: Reduction of PGAL from step 1 to G3P
• Uses NADPH and ATP from light reactions
– Step 3: G3P converted glucose
– Step 4: Regeneration of RuBP

22. The Calvin Cycle

23. The Calvin Cycle
• Calvin Cycle must “turn” 6 times to make 1
glucose molecule
– 1 turn for each CO2 fixed
– Calvin Cycle Uses:
• 12 ATP
• 12 NADPH
• Another 6 ATP to regenerate RuBP
• Final Outputs of Calvin Cycle
– C6H12O6
– O2

24. C-3 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

25. 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 02

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

27. 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

28. C3 plant C4 plant CAM plant

29. Global Climate Change
Green house effect: radiant heat trapped by CO2

30. Global Climate Change

31. Global Climate Change

32. Global Climate Change

33. Global Climate Change
• What would you do to curb climate change?