1. Anderson Spring 2017 College of the Redwoods
Photosynthesis
Anderson
Spring 2017
College of the Redwoods

2. How do plants get energy?
6CO2
6H2O
C6H12O6
6O2 +
carbon dioxide
water
sugar
oxygen
ATP
energy
Cellular Respiration
Photosynthesis
6CO2
6H2O
C6H12O6
6O2
light +
carbon dioxide
water
sugar
oxygen

3. Autotroph/Heterotroph
Autotroph – organism that produces its own food
auto = self, troph = feeder
Plants (and some bacteria and algae) are photoautotrophs - uses sunlight and carbon from CO2 to synthesize energy
Heterotroph – organisms that must obtain energy and carbon from food by consuming other organisms
hetero = other, troph = feeder
Dependent on autotrophs

4. Photosynthesis occurs in the mesophyll (middle layer)
Leaf Cross-Section
Photosynthesis occurs in the mesophyll (middle layer)
CO2 exchange happens through openings on bottom of leaf, called stomata

5. Chloroplasts Organelle in which photosynthesis takes place
Thylakoid – membrane-bound disc structure that contains chlorophyll (green)
Granum – stack of thylakoids
Stroma – space around granum

6. What is Light Energy?
Longer wavelengths carry less energy; shorter carry more
Sunlight emits broad range of electromagnetic radiation

7. Visible Light Spectrum
White light – light that contains all wavelengths of light
The colors we see are the light that’s NOT absorbed
When we see green, all light except for green is being absorbed

8. Chlorophyll Pigments Chlorophyll a – absorbs from violet and red range
Chlorophyll b – absorbs from blue and red-orange
Carotenoid – absorbs from blue-green (looks red, orange, yellow)
Not all plants are equal! (but all plant lives matter)

9. Photons Individual bundles of electromagnetic energy
Can provide energy to electrons (makes them move)
Basic unit of all light

10. Structure of Chlorophyll

11. Let’s tie all that nonsense together
Plants make their own food from sunlight energy – photoautotrophs
Sunlight contains tiny particles, called photons, that make up light
These photons have energy!
The photons in the light hit the chlorophyll pigments in plant leaves, green light is reflected, all other absorbed
A reaction occurs which is the start of photosynthesis

12. Stages of Photosynthesis
Stage 1 – Light Reactions
Energy from sun light splits water to produce oxygen
Reduces NADP+ to NADPH, which will then carry high energy electrons to Calvin Cycle
Stage 2: Calvin Cycle
Carbon dioxide and the high energy electrons from NADPH are used to create carbohydrates (glucose)
Called the “dark cycle” because sunlight is not necessary

13. Light-Dependent Reactions
Photosystem – grouping of pigment molecules (chlorophyll) and proteins
Exists in thylakoid membrane
Photon is “absorbed” by the chlorophyll and causes chlorophyll to become excited
The electron breaks free from chlorophyll – “donates” electron
The electron must be replaced – this is done by splitting water into oxygen and hydrogen ions (each water molecule releases 2 electrons)
H2O  ½ O2 + 2H+

14. Photosystems
In eukaryotes and some prokaryotes, there’s 2 photosystems:
Photosystem II and Photosystem I (named by discovery, not order of function)

15. Photosystem II
Light energy absorbed and enters reaction center – chlorophyll excited
Electron captured by acceptor
Water split, electrons replace donated ones from chlorophyll, O2 released as byproduct
Electrons are passed to photosystem I

16. Photosystem I
Once electrons re passed to photosystem I, electrons are captured by acceptor
Excited electrons oxidize NADP+ to NADPH (another energy storing molecule)

17. ATP Synthase
Electrochemical gradient of H+ ions created through electron transport chain
ATP synthase uses gradient to diffuse H+ out of thylakoid space

18. End Result of Light-Dependent
2 H2O  2 O2 + 2 H2 – oxygen is the byproduct 1 NADPH 1 ATP So where does the carbon come from to make the glucose?

19. Calvin Cycle
Reactions of photosynthesis that use energy stored from light- dependent reactions (ATP and NADPH) to form glucose
CO2 enters chloroplasts by diffusion (enters cell through stomata)
Calvin cycle occurs in stroma (space between membrane and thylakoids)
In addition to CO2, RuBisCO (an enzyme), and RuBP (5 carbon molecule) are needed

20. Calvin Cycle
Stage 1:
Carbon fixation – CO2 and RuBP (5 carbons) create 6- carbon compound, catalyzed by RuBisCO
The 6-carbon compound immediately breaks into 2 3-carbon compounds (3-PGA)

21. Calvin Cycle
Stage 2:
3-PGA (3-carbon molecule) reduced by NADPH and ATP
Results in G3P, NADP+ and ADP
1 ATP and 1 NADPH used for each 3-PGA molecule
1 G3P leaves cycle to make glucose

22. Calvin Cycle
Stage 3:
Remaining G3P molecules regenerate RuBP (5 3-carbon molecules rearrange to make 3 5-carbon molecules)
3 more ATP used, making 3 ADP
And the cycle is ready for the next CO2
Takes 6 turns to make 1 glucose (or 2 with 3 CO2 to start)

23. How do prokaryotes do it without chloroplasts?
Remember: prokaryotes lack membrane-bound organelles, such as chloroplasts
Prokaryotic photosynthetic autotropic organisms have infoldings of plasma membrane for chlorophyll attachment and photosynthesis

24. One organism’s waste is another organism’s energy
Carbon dioxide is “waste” for animals, plants need carbon dioxide for photosynthesis
Oxygen is “waste” for plants, animals need oxygen for cellular respiration
6CO2
6H2O
C6H12O6
6O2 +
carbon dioxide
water
sugar
oxygen
6CO2
6H2O
C6H12O6
6O2 +
carbon dioxide
water
sugar
oxygen

25. Plants Undergo Cellular Respiration Too!
Carbohydrates are energy storage molecules
Plants store their glucose as starch
The ATP from the light-reactions of photosynthesis is enough to fuel the Calvin cycle, not the whole plant cell
Plants have mitochondria too! Cellular respiration provides more ATP
When there’s little or no light, plants get their energy from cellular respiration (take in O2 and release CO2)

26. Do Plants Sleep?
During the night, when there’s no sunlight, plants consume oxygen to undergo cellular respiration (use their carbohydrate energy stores)
They will be using oxygen and releasing CO2, just like we do
But! Plants take up much more CO2 during photosynthesis than they release at night
So the next time a 5 year old asks you this question, you can tell them, “No, they don’t really sleep. They’re just doing other things at night that they don’t do during the day.”

27. Don’t listen to our President, global warming is real!