1. Chapter 6 - Photosynthesis
6-1: The Light Reactions
6-2: The Calvin Cycle

2. 6-1: The Light Reactions
All organisms use ENERGY to carry out life processes
The energy comes directly or indirectly from the SUN

3. Obtaining Energy Organisms are classified by how they get energy
Autotrophs
Organisms that use energy from sunlight or from chemical bonds in inorganic substances to make organic substances
Use the process of photosynthesis
Convert light energy (sun) to chemical energy (carbohydrates)
Examples: Trees, algae

4. Obtaining Energy Heterotrophs
Animals or other organisms that must get energy from food, not the sun or inorganic substances
Examples:
Caterpillars
Birds
Humans

5. Biochemical Pathway
A series of linked reactions where the products of the 1st reaction are the reactants for the next reaction
Photosynthesis is a biochemical pathway with Cellular Respiration

6. Overview of Photosynthesis
Photosynthesis produces organic compounds from CO2 + H2O
Oxygen and some organic compounds are used in a process known as Cellular Respiration, which produces CO2 + H2O
Products of Photosynthesis are reactants in Cellular Respiration
Products of Cellular Respiration are reactants in Photosynthesis

7. Photo./C.R. Cycle

8. Two Stages of Photosynthesis
Light Reactions
Light energy is absorbed and converted + stored to chemical energy (ATP + NADPH)
Calvin Cycle
Organic compounds are made using CO2, ATP, + NADPH

9. Biochemical Equations
Photosynthetic Equation
6 CO2 + 6 H2O + nrg 1 C6H12O6 + 6 O2
Cellular Respiration Equation
1 C6H12O6 + 6 O2 6 CO2 + 6 H2O + nrg

10. Capturing Light Energy
Light Reactions – 1st rxns in Photosynthesis
Begins with the absorption of light
Occurs in the chloroplasts
Structure:
Surrounded by double membrane
Thylakoids – flattened sacs (inner membrane)
Grana – stacks of thylakoids
Stroma – fluid surrounding grana

11. Light travels from the Sun to Earth in waves of energy
LIGHT & PIGMENTS
Light travels from the Sun to Earth in waves of energy
ROYGBIV
When white light hits an object, its component colors can be:
Reflected
Transmitted
Absorbed

12. LIGHT & PIGMENTS
If an object contains a pigment (compound that absorbs light), the colors will react differently
The pigments will absorb certain colors, removing it from the visible spectrum

13. Chloroplast Pigments CHLOROPHYLLS
Most important pigments in Photosynthesis
Chlorophyll a
Absorbs more red light; less blue light
Involved directly in the Light Rxns
Chlorophyll b
Absorbs more blue light; less red light
An accessory pigment – helps capture light for chlorophyll a molecules
*Chlorophyll does not absorb any green light therefore most plants appear green in color*

14. Chlorophylls Carotenoids Fruits Flowers
Other pigments found in thylakoid membrane such as yellow, brown, + orange
Acts as an accessory pigment to help capture light for chlorophyll a
Most abundant in nonphotosynthetic plant parts:
Fruits
Flowers
Most of these colors can be seen in leaves when chlorophylls break down in the fall

15. Absorption Spectra

16. Converting Light Energy to Chemical Energy
Once the pigments capture the light energy, it must get converted into chemical energy
The energy is temporarily stored in ATP + NADPH; O2 is given off
(it’s importance will be discussed in Ch. 7)

17. Converting Light Energy to Chemical Energy
Photosystems (I+II)
Groups of chlorophyll + carotenoid pigments embedded in the thylakoid membrane
Trying to capture as much light as possible
Light reactions begin when accessory pigments absorb light energy from the sun
The captured energy must find its way to a specific pair of chlorophyll a molecules for photosynthesis to begin

18. Two Stages of the Light Reactions
Electron Transport Chain
Chemiosmosis

19. Electron Transport Chain (5 Steps)
Light energy forces electrons to enter a higher energy level in two chlorophyll a molecules of PSII (“excited electrons”)
Energy allows electrons to leave chlorophyll a molecules
Chlorophyll a molecules have been oxidized
REDOX must occur!
The acceptor of the electrons from chlorophyll a is a molecule in the thylakoid membrane called the primary electron acceptor

20. Electron Transport Chain (5 Steps)
The primary electron acceptor then donates the electrons to the first ETC
A series of molecules that as they pass the electrons from one to another, cause the movement of protons (H+) into the thylakoid
Light is absorbed by PSI causing electrons to become excited. Chlorophyll a loses those electrons to another primary electron acceptor, which then donates them to a second ETC

21. Electron Transport Chain (5 Steps)
As the electrons make it to the end of the second ETC, they are combined with a proton and NADP+
e- + H+ + NADP+ = NADPH

22. Replacing Electrons in Light Reactions
In order to keep the ETC going, water is split to provide PSII with electrons
As water splits, it releases O2 molecules to the environment
It also helps to build up a concentration gradient of protons (H+) inside the thylakoid
The purpose of the ETC is to make:
NADPH

23. Restoring PSII (cont.)
For every 2 water molecules that are split, 4 e- are available for PSII

24. ETC REVIEW

25. After the ETCs, Chemiosmosis takes place
ATP is main energy currency of cells
Chemiosmosis – synthesis of ATP
Relies on concentration gradient of protons (H+) across thylakoid membrane
Where does H+ come from?
Splitting of water
Pumped in from stroma to interior of thylakoid
Known as Proton Pump (needs nrg to work)

26. Proton pump

27. Chemiosmosis cont. ATP ATP synthase –
an enzyme that catalyzes the synthesis of ATP from ADP + Pi
Converts potential energy (protons) into chemical energy (ATP)
The purpose of Chemiosmosis is to make:
ATP

28. Let’s look at Chemiosmosis…

29. So what have we accomplish thus far?
Together, ATP + NADPH will provide the energy needed for the 2nd set of reactions for Photosynthesis to occur…..the Calvin Cycle
NADPH
ATP

30. Photosynthesis Video
Video Recap