1. Cellular Energy
Photosynthesis

2. How do plants make energy & food?
Plants use the energy from the sun
to make ATP energy
to make sugars
glucose, sucrose, cellulose, starch, & more
sun
ATP
sugars

3. Overview of Photosynthesis
use sun’s energy to make ATP
use CO2 & water to make sugar
Occurs in chloroplasts
makes a waste product
oxygen (O2)
 glucose + oxygen
carbon
dioxide
sun
energy
+ water +
6CO2
6H2O
C6H12O6
6O2
sun
energy  +

4. Overview of Photosynthesis
Summarize the two main phases of photosynthesis, the light reactions and the Calvin cycle.
1. What is needed?
2. What is produced?

5. Overview of Photosynthesis
Photosynthesis involves 2 energy conversions:
Conversion of light energy into chemical energy (The Light Reactions)
Storage of chemical energy in the form of sugars (The Calvin Cycle)

6. Chloroplasts – organelles of photosynthesis
sun
Leaves
Chloroplasts in cell
Chloroplasts
contain
Chlorophyll
Pigments in chloroplast absorb sunlight. (All colors?)

7. Chloroplasts- organelles of photosynthesis
Granum: a stack of thylakoids
Stroma: Fluid around Grana
Thylakoids:
Membranes containing pigments

8. Chloroplasts- organelles of photosynthesis
Pigments – light absorbing substances. Found in thylakoid membranes of chloroplasts. Absorb waves of visible light, reflect the color seen by your eye
Chlorophyll a and b – Major pigments involved in photosynthesis. Green pigments, absorb all colors except green.
Accessory pigments – absorb green. Visible in autumn, when chlorophyll breaks down.

9. Photosynthesis: 2 Stages
Light Reactions: in thylakoid membranes
Sunlight  ATP + NADPH
Calvin Cycle: in stroma
NADPH + ATP  Sugar

10. The Light Reactions
Converts visible light into chemical energy carried by electrons in high-energy molecules (ATP and NADPH)
Splits an H2O. Uses H, releases O.
Next, we zoom in to look more closely at the thylakoids!

11. 1. Light strikes the first photosystem (PSI), causing it to transfer excited e- to the primary electron acceptor. These e- are replaced by splitting H2O, which releases O2 as a product.
2. The excited e- travel down and e-transport chain. This process pumps H+ ions across the membrane into the thylakoid.
3. Light-excited e- in PSII are transferred to NADP+. These e- are replaced by those coming from e- transport chain.
4. The “backflow” of hydrogen ions out of the thylakoid pass through ATP synthase, powering ATP production. 2 1 4 3

12. The Light Reactions – Another View

13. The Light Reactions - Summary
Light absorbed into thylakoid membrane by pigments in photosystems PSI and PSII (see figure 4.10)
Light energy is transferred to the reaction center within the photosystem.
H2O splits apart at PSII (2H2O  4H+ + 4e- + O2)
Oxygen diffuses out of the plant
Protons (H+): Transported to the thylakoid
Electrons (e-): electron transport chain to PSI

14. The Light Reactions – Summary cont.
Formation of NADPH (use e- from H2O splitting)
Electrons reach PSI
Used to join NADP+ with H+  NADPH
This high-energy molecule will be used later for the Calvin cycle.
Build-up of H+ ions
Energy electrons received from reaction center is used in active transport to pump protons into thylakoid
H+ build up inside thylakoid, result in potential energy difference (like a battery)

15. Review: Light Reaction
H2O + sunlight  O2 + ATP + NADPH
To see this in action, check out the YouTube video: Light Dependent Reactions
2 Unstable compounds formed. Must be converted to C6H12O6 for storage!

16. The Calvin Cycle (Dark Reactions)
Occurs in stroma of chloroplast.
Energy from ATP and NADPH molecules converted to chemical bonds within glucose for storage.

17. 1. Carbon Fixation:
CO2 taken in through leaves. Combines with RuBP, a 5-Carbon sugar-phosphate, by enzyme rubisco.
CO2 gas is “fixed” into an organic molecule.
6-Carbon sugar f ormed, immediately splits into two 3-C PGA molecules
The Calvin Cycle
4. Remaining 5 PGAL rearranged, turned back into 3 RuBP molecules. Calvin cycle complete, can start over again. 1 4 2
2. ATP and NADPH used to rearrange 6 PGA molecules into 6 G3P (PGAL).
3. One PGAL released, remaining 5 stay in Calvin cycle.
2 PGAL  1 Glucose 3

18. Calvin Cycle - Summary Any Questions??
ATP + NADPH  PGAL or Glucose
Carbon fixation by rubisco
Rearrange molecules to produce a PGAL and get back to starting molecule (RuBP)
YouTube video: The Calvin Cycle by Prentice Hall
Any Questions??

19. How are products of photosynthesis used?
PGAL produced in Calvin cycle can be used in many ways:
Synthesized into larger carbohydrates: glucose, sucrose
Modified to make amino acids, glycerol
 sugars used by leaf cell, or transported to other cells in plant.
 This occurs in chloroplasts, cell, or other organisms if plant is eaten.

20. Rate of Photosynthesis
Measured by CO2 consumed per unit time or by O2 produced.
Bromthymol Blue indicator. Turns yellow in the presence of CO2.
An acid-base indicator: H2O + CO2  H2CO3 (l)
In water, CO2 creates carbonic acid.

21. Rate affected by several factors
Light intensity
Temperature
Concentration of CO2 and O2
Photoinhibition: A reduction in the rate of photosynthesis.

22. Photorespiration O2 enters Calvin Cycle
Enzyme Rubisco fixes CO2 in Calvin Cycle. Due to shape, rubisco can bind with either O2 or CO2.
CO2 binds to rubisco  2 PGA molecules
O2 binds to rubisco  1 PGA and 1 2-C acid (glycolate).
Plant loses fixed carbons instead of gaining.
Benefits unknown.
Uses O2 and liberates CO2, which is why it is called photorespiration.

23. Photorespiration Effects of greenhouse gasses? Atmospheric Conditions
Photosynthesis
Photorespiration
High CO2, Low O2
Favored
Inhibited
Low CO2, High O2

24. Photorespiration Weather and Photosyntheis
In hot, dry weather, plants close leaf openings called stomates. Helps reduce water loss. No CO2 enters. Photorespiration is favored.
Light reactions release oxygen
high light intensities and
high temperatures (above ~ 30°C)
favor photorespiration.

25. Photorespiration: C3, C4 and CAM plants
All plants carry out photosynthesis by adding CO2 to a 5-carbon sugar.
Reaction is catalyzed by the enzyme RUBISCO.
Results in production of PGA (starting molecule for glucose)
The process is called the Calvin cycle and the pathway is called the C3 pathway.
PGA has 3 carbons…. C3

26. Photorespiration - C3 Plants
C3 Plants – Plants that use only the Calvin Cycle to fix CO2.
Make up 90% of plants on Earth
Wheat, rice, soy
Most vulnerable to high O2 concentrations.
Some plants have evolved strategies for increasing photosynthesis, and reducing photorespiration: C4 and CAM plants.

27. C4 Plants Limit Photorespiration
The C4 cycle: Structural changes in leaf anatomy
- C4 and C3 pathways are separated in different parts of the leaf
- RUBISCO sequestered where the CO2 level is high, O2 level low.

28. C4 Plants Limit Photorespiration
C4 plants are well adapted to habitats with
(1) high daytime temperatures and
(2)intense sunlight.
Some examples:
crabgrass
corn (maize)
sugarcane

29. CAM Plants Limit Photorespiration
How are
C4 and
CAM
Different?

30. CAM Plants Limit Photorespiration
CAM plants separate their C3 and C4 cycles by time.
*Note: C4 plants separate C3 and C4 cycles by location. CAM plants separate cycles by time of day.
At night,
CAM plants take in CO2 through their stomata
CO2 is fixed into a 4-carbon molecule that accumulates in the central vacuole of the cells.
In the morning,
the stomata close conserving moisture
The accumulated 4-C molecule leaves the vacuole and is broken down to release CO2.
The CO2 is taken up into the Calvin (C3) cycle.

31. CAM Plants Limit Photorespiration
CAM plants well adapted to conditions of
(1) high daytime temperatures
(2) intense sunlight, and
(3) low soil moisture.
Examples: cacti, aloe, pineapple, bryophyllum
CAM – least efficient system. These plants grow slowly.

32. Evolution of C4 and CAM Plants
When photosynthetic organisms first evolved, the atmosphere contained no oxygen.
Rubisco did not need to differentiate.
More photosynthesis occurring  more O2 in the air.
Plants adapted by developing other pathways to fix carbon
C4 cells in C3 plants

33. Photosynthesis Summary