1. Photosynthesis

2. Energy for Life Processes
All organisms use energy to carry out the functions necessary to life
Autotrophs are organisms that manufacture their own food from inorganic substances & energy
Heterotrophs are organisms that cannot manufacture their own organic compounds from inorganic substances
Must consume autotrophs or other heterotrophs
Obtain energy indirectly

3. Autotrophs & Photosynthesis
Most autotrophs use photosynthesis to covert light energy from the sun into chemical energy
Plants, but also some algae & bacteria, are photosynthetic organisms
All life ultimately depends on autotrophs & the process of photosynthesis

4. What is Photosynthesis?
Conversion of light energy from sun into usable chemical energy
Chemical equation of photosynthesis
6 H2O + 6 CO2 + light energy  C6H12O6 + 6 O2
Reads: six molecules of water and 6 molecules of carbon dioxide in the presence of light are converted into one molecule of glucose and 6 molecules of oxygen

5. Biochemical Pathways
Photosynthesis is part of a biochemical pathway with cellular respiration
Biochemical pathway is where the product of one reaction is used in the next reaction

6. Light Reactions Begins with the absorption of light in chloroplasts
Structure of chloroplasts
Surrounded by pair of membranes
Inside inner membrane is another system of membranes—thylakoids, which are arranged as flattened sacs
Layered thylakoids form stacks called grana
Stroma is the solution surrounding the thylakoids

7. Light & Pigments
Light from sun appears white, but is actually made of variety of colors
Visible spectrum represents the colors white light separates into—ROY G BIV
Different colors of spectrum are different wavelengths
Pigment is a compound that absorbs light
Color that is not absorbed, is reflected

8. Chloroplast Pigments Pigments found in the thylakoid membranes
Most important are chlorophylls
Two common types are chlorophyll a & b
Chlorophyll a absorbs more red light than chlorophyll b
Chlorophyll b absorbs more blue light than chlorophyll a
Neither chlorophyll a nor b absorb much green light—reason why plants are green—color green is reflected

9. Chlorophyll a & Accessory Pigments
Chlorophyll a is directly involved in light reactions of photosynthesis
Accessory pigments assist chlorophyll a in capturing more light energy because they absorb colors of light that are not absorbed by chlorophyll a
Chlorophyll b
Yellow, orange, & brown carotenoids

10. Electron Transport
Photosystem represents a cluster of pigment molecules (chlorophylls & carotenoids) found in thylakoid membrane
2 Types of photosystems
Photosystem I and photosystem II
Similar in pigments they contain but differ in their roles in light reactions

11. Stage 1: Light Reaction
Light reactions begin with absorption of light by chlorophyll a & accessory pigments in thylakoids
By absorbing light, molecules acquire some energy
Energy passed quickly to other pigment molecules until reaches specific pair of chlorophyll a molecules

12. 5 Step Process of Light Reaction
1. Light excites electrons in chlorophyll a molecules of photosystem II
2. Excited electrons move to a primary electron acceptor
3. Electrons are transferred along series of molecules called electron transport chain
As electrons pass from molecule to molecule, lose most of their energy

13. 5 Step Process of Light Reaction
4. At same time light was absorbed by photosystem II, light is also absorbed by photosystem I.
Electrons move from pair of chlorophyll a molecules in photosystem I to another primary electron acceptor.
Electrons lost by chlorophyll a molecules are replaced by electrons that have passed through electron transport chain from photosystem II

14. 5 Step Process of Light Reaction
5. Electrons from photosystem I are transferred long 2nd electron transport chain.
Chain brings electrons to side of thylakoid membrane that faces the stroma.
There, NADP+ combines with H+ to form NADPH

15. Restoring Photosystem II
Photosynthesis would not occur if electrons from chlorophyll molecules in photosytem II did not replace electrons that leave chlorophyll molecules in photosystem I
Replacement of electrons is provided by water molecules
Enzyme inside thylakoid splits water molecules into p+, e-, & oxygen
2 H2O  4 H+ + 4 e- + O2
Oxygen is released out of the chloroplast & leaves the plant as a byproduct of photosynthesis

16. What is the difference between Photosystem I & II?
Photosystem I provides electrons used to make NADPH from NADP+
Photosystem II provides electrons to replace electrons lost by photosystem I & pumps protons into thylakoids

17. Light Reaction Animation

18. Chemiosmosis
Process that occurs during light reactions, which is responsible for making ATP
Concentration of protons is higher inside thylakoid than in stroma
Relies on concentration gradient of protons
Gradient represents PE of protons

19. Chemiosmosis Cont’d
ATP synthase makes ATP by adding a phosphate group to ADP
ATP synthase acts as both a carrier protein (allows protons to cross thylakoid membrane) & enzyme (speeds up reaction making of ATP from ADP)
Therefore, ATP synthase converts PE of proton concentration gradient into chemical energy stored in ATP
ATP & NADPH, together, provide energy for second stage of photosynthesis

20. Stage 2: Calvin Cycle
Uses energy stored in ATP & NADPH from light reactions to incorporate CO2 into organic compounds
Carbon fixation
Process occurs in stroma of chloroplast
Sometimes referred to as dark reactions

21. Steps of Calvin Cycle Step 1
CO2 diffuses into stroma from surrounding cytosol
RuBP, 5-carbon carbohydrate, combines with CO2 creating 6-carbon molecule
6 carbon molecule splits into PGA—2 three-carbon molecules

22. Steps of Calvin Cycle Step 2
Each PGA molecule receives a phosphate group from ATP
The compound then receives proton from NADPH & releases a phosphate group to produce PGAL
Byproducts of these reactions create ADP, NADP+ & phosphate to be used again in light reactions

23. Steps of Calvin Cycle Step 3 Most PGAL is converted back into RuBP
Allows the continuation of Calvin cycle
Some PGAL is used to make other organic compounds—carbohydrates, amino acids, & lipids

25. How much ATP & NADPH are required to make 1 molecule of PGAL from CO2?
Takes three turns Calvin cycle to produce each molecule of PGAL
Therefore, 3 turns of Calvin cycle uses 9 molecules of ATP & 6 molecules NADPH

26. Alternative Pathways
Majority of plants are C3 plants—fix carbon only through Calvin cycle
Some plants need to adjust to excessive water loss
Water loss occurs through stomata
Small pores located on undersurface of leaves
Also passageway for CO2 entering & O2 leaving

27. Alternative Pathways C4 pathway
Allow plants to fix CO2 into four carbon compounds instead of three carbon compounds
During hottest part of day, stomata are partially closed
Lose ~ ½ water C3 when producing same amount of carbohydrate
Corn, sugar cane, crabgrass

28. Alternative Pathways CAM Pathway
Open stomata at night & close them during day
Grow slower, but lose less water
Usually found in hot, dry climates
Examples: cacti, pineapples

29. Rate of Photosynthesis
Affected by plant’s environment
Light intensity
As light intensity increases, rate of photosynthesis initially increases, then plateaus
Higher light intensity excites more molecules of chlorophyll
Carbon dioxide
As CO2 increases, rate of photosynthesis increases until it plateaus
Temperature
Rate of photosynthesis increases as temp increases until reaches temperature threshold
Too hot—enzymes are ineffective & stomata close

30. Summary of Photosynthesis
Light reactions:
Trap solar energy & make ATP
Electrons convert NADP+ to NADPH
Oxygen is released into environment
Calvin cycle:
Fix carbon dioxide into glucose & other organic molecules