III. Light Dependent Reactions

A. Photo-oxidation of Chlorophyll

B. Chemiosmotic model of ATP production.IV. Light Independent

Reactions

A. Calvin Cycle

II. How plants harness sunlight

I. Anatomy of Photosynthesis

Non-cyclic

Photophosphorylation

pH = 4

pH = 8

+ charge

- charge

Cyclic photophosphorylation

Make extra ATP

Graph changes in pH of the thylakoid interior over one 24 hr period. Indicate when ATP production during the light reactions is zero.

6AM

pH

Night Day

7

14

1

Turn to your neighbor and…

Night

6 PM

4

9No ATP No ATP

The Big PictureWhat have we produced so far?

ATP NADPH CO2

provides the energy

provides high energy electron and H+ ions

provides the carbon and oxygen

Used to make energy rich sugar molecules during the light independent reactions!

The Calvin Cycle

(3) CO2 molecules enterRubisco attaches the CO2 to RuBP

CARBON FIXATION

6 ATP and 6 NADPH used

REDUCTION

1 G3P molecule produced

Regenerate RuBP

Use 3 more ATP

And boy can plants fix carbon!

What do plants do with the sugars they make?

most!

Fuel cellular respiration

Similarities between photosynthesis and cellular respiration…

Chemiosmosis Energy Coupling

Examples

Differences between cellular respiration and photosynthesis.

Photosynthesis

Cellular RespirationIn CR electron carriers bring electrons to the ETC, and O2 finally accepts them and is reduced to H2O.

In photosynthesis H2O brings electrons to the ETC and becomes O2, an electron carrier finally accepts the electrons.

Differences between cellular respiration and photosynthesis.During the Calvin cycle, CO2 is being reduced into sugar

CO2

During the Krebs cycle, sugars are oxidized and become CO2

Photosynthetic Adaptations

What problems may a plant experience when it is exposed to arid (dry) conditions?

Dehydration

Elevated O2 levels in the leaves

Response

Close stomata to prevent dehydration

Photorespiration Rubisco – an enzyme that catalyzes the addition of CO2 to RuBP.

O2 competes with CO2 for the active site on Rubisco.

PhotorespirationCalvin cycle sometimes called the C3 pathway

Rubisco

RuBp CO2

RuBp

Rubisco

P G A P G A

P G A

Later becomes glucose!

CO2CO

2

O2

Photorespiration – these CO2’s did not get incorporated into glucose this time!

Photorespiration can lead to lower photosynthetic output by siphoning carbon from the Calvin cycle!

They produce less sugar!!!

CO2 + RuBP2 molecules of 3-phosphoglycerate

CO2

Com

peti

ng

react

ion

s

rubisco

Turn to your neighbor and discuss the following questions…

1) What factor determines which reaction below occurs more often? a) The concentration

of rubisco

b) The relative concentration of CO2 and O2

c) The concentration of RuBp

2) This is most similar to which of the mechanisms of enzyme regulation we discussed earlier in the semester?

Adaptations to arid climates

A) C4 plants - spatial separation of steps

B) CAM plants - temporal separation of steps

Leaf anatomy of plants adapted for hot/arid conditions (C4 plants)… O2

C4 pathway

C3 pathway

Separate CO2 fixation and sugar making into two different cells

CAM Plants - steps performed at different times

• Close their stomata during the day to minimize water loss, and open stomata at night.

• Desert plants…cacti, succulent plants, pineapples…

• At night they store CO2 in organic acids.

Organic acid

CO2

CO2

Calvin Cycle

sugar• CO2 is released from organic acids in the day and used in the Calvin cycle.

The Silversword