1. Photosynthesis
Chapter 6

2. Section 1: Light Reactions
All organisms use energy to carry out the functions of life
Organisms are classified by how they get energy
Autotrophs: use energy from sunlight
Photosynthesis: convert light energy into chemical energy in the form of organic compounds (carbohydrates)
Heterotrophs: get energy from food rather than sunlight

3. Light Reactions Cont.
Almost all organisms ultimately depend upon autotrophs to obtain the energy necessary to carry out the processes of life

4. Overview of Photosynthesis

5. Overview of Photosynthesis
Photosynthesis produces organic compounds from carbon dioxide and water
Oxygen and some organic compounds are used by the cell during cellular respiration
During Cellular respiration carbon dioxide and water are produced

6. Photosynthesis and Cellular Respiration
The reactants of photosynthesis are the products of cellular respiration

7. Photosynthesis Can be broken down into two stages:
Light reactions: light energy is converted to chemical energy, which is temporarily stored in ATP and the energy carrier molecule NADPH
Calvin cycle: organic compounds are formed using carbon dioxide and chemical energy stored in ATP and NADPH

8. Capturing Light Energy
Light reactions require light
Light is absorbed in chloroplasts
Each chloroplast contains
Inner and outer membranes
Thylakoids: membranes arranged in flattened sacs
Grana: stacks of thylakoids
Stroma: fluid inside inner membrane

9. The Chloroplast

10. Light and Pigments
Light from the sun appears white, but it is actually made up of a variety of colors
Light can be broken down into the visible spectrum

11. Light and Pigments
Light can be reflected, transmitted, or absorbed by an object
Pigments: compounds that absorb light
Most pigments absorb some colors more than others
Light that is reflected or transmitted has not been absorbed
Green leaves: all colors absorbed besides green

12. Chloroplast Pigments Most important pigments are chlorophylls
Several types of chlorophylls
Chlorophyll a: absorbs red light
Chlorophyll b: absorbs blue light
Note: neither absorbs green light well

13. Chloroplast Pigments Mainly chlorophyll a in light reactions
Chlorophyll b and carotenoids serve as accessory pigments
Accessory pigments allow more light to be captured

14. Light Energy  Chemical Energy
Light is absorbed and transformed into chemical energy
Chemical energy is temporarily stored in ATP and NADPH
Photosystems: clusters/groups of carotenoid and chlorophyll pigments
There are two photosystems
Photosystem I and Photosystem II

15. Photosystem I and II
Photosystems have similar pigments, different roles in th light reactions

16. Light Reactions
Light energy forces electrons to enter a higher energy level (excites them) in photosystem II
2. Electrons leave chlorophyll a molecules are accepted by the primary electron acceptor
3.Primary electron acceptor donates electrons to the electron transport chain (within the thylakoid membrane)
-moving from molecule to molecule they lose energy

17. Light Reactions
4. Light is absorbed by photosystem I and II. Electrons from chlorophyll molecules in photosystem II replace electrons that leave chlorophyll molecules in photosytem I -this needs to happen in order for photosynthesis to continue!

18. Light Reactions
5. Primary electron acceptor of photosystem I donates electrons to a different ETC This ETC brings electrons to the thylakoid membrane Electrons combine with a proton and NADP+ This creates NADPH from NADP+

19. Light Reactions

20. Photosystems

21. Replacing Electrons In LR
Electrons in PS II replace electrons in PS I
Replacement electrons for PS II come from water
Water is split into protons, electrons and oxygen
2H2O  4H e- + O2

22. Photosynthesis Video

23. Making ATP in LR Chemiosmosis: synthesizes ATP during light reactions
Relies on concentration gradient of protons across thylakoid membrane
Concentration of protons is greater inside thylakoid than in the stroma (outside thylakoid)
Potential energy from gradient harnessed by ATP synthase

24. Making ATP Continued
ATP Synthase makes ATP by adding a phosphate group to ADP
Energy for this reaction comes from movement of protons across thylakoid membrane
Potential energy converted to chemical energy

25. Chapter 6 Sect. 2 The Calvin Cycle
Series of enzyme-assisted chemical reactions that produces a 3 carbon sugar
Carbon dioxide molecules are “fixed” into organic compounds (sugars) in a process known as carbon fixation
3 Carbon dioxide molecules are needed to make each organic compound

26. The Calvin Cycle

27. The Calvin Cycle

28. The Calvin Cycle
1. CO2 diffuses into the stroma from the surrounding cytosol
An enzyme binds each CO2 with a 5 carbon molecule—ribulose bisphosphate (RuBP)
The resulting 6 carbon molecule is very unstable and immediately splits into two 3 carbon molecules—3-phosphoglycerate (3-PGA)

29. The Calvin Cycle
2. 3-PGA is converted into glyceraldehyde 3-phosphate (G3P) in a two part process
A. 3-PGA receives a phosphate croup from ATP
B. Then compound receives a proton (H+) from NADPH and releases a phosphate group
*The ADP and NADP+ are then used in the Light Rxns

30. The Calvin Cycle
3. One of the G3P molecules leaves the Calvin Cycle and is used to make carbohydrates where energy will be stored
4. Remaining G3P molecules are converted back into RuBP through addition of phosphate groups from ATP. The RuBP molecules enter the Calvin Cycle again

31. The Calvin Cycle

32. The Calvin Cycle The most common pathway for carbon fixation
Plant species that fix carbon exclusively with the Calvin Cycle are called C3 plants because of the 3 carbon compound initially formed in this cycle

33. Alternative Pathways
Plants that live in hot and dry conditions have evolved to fix carbon via alternative pathways
CO2 enters leaves through stomata
Plants living in hot and dry areas can lose a lot of water through these stomata
Water loss can be reduced through closing or partially opening stomata

34. Alternative Pathways C4 Pathway
Fixes carbon into four-carbon compounds
Plants that use this pathway are called C4 plants
C4 plants have partially closed stomata during the hot parts of the day
Carbon compounds can still be produced with limited CO2