Photosynthesis

Capturing sunlight to produce organic compounds

Overview Overall equation:

CO2 + H2O C6H12O6 + O2

2 Main Stages: I. Light Reactions: occurs in the membrane

and interior of thylakoids. II. Calvin Cycle (light-independent reactions):

occurs in stroma As in respiration, the products of earlier reactions and pathways

are used in later reactions and pathways.

Light Reactions “Big idea”:

energy from sunlight is used to excite e- of an electron transport chain (ETC).

ETC is used to generate ATP and NADPH.

These molecules “carry” energy to the next stage.

The second stage: Carbon fixation and the Calvin Cycle

“Big idea”: energy from ATP and NADPH is used

to build a simple organic compound that is a precursor of larger compounds, such as carbs, fats, proteins.

Carbon for organic compounds is acquired through carbon fixation (CO2 from atmosphere).

Leaf Cross-Section

Leaf Cross-Section

ChloroplastsChloroplasts

Photosynthetic Cell (3-D)

Inside a chloroplast

Chloroplast

Thylakoids: membrane-bound areas in the shape of flattened discs.

Stroma: Fluid that surrounds the thylakoids.

Thylakoid

sunlight

H2O O2

CO2

Carbohydrates

Light Absorption: How It Works Requires the use of pigments

(chlorophylls a and b and carotenoids) Pigments are clustered together in the thylakoid

membrane in groups of a few hundred A cluster of pigments = “photosystem”

Pigments absorb LIGHT energy and convert it to CHEMICAL energy

Light Absorption: How It Works Chlorophylls absorb violet, blue, and red

but reflect green

Light Absorption: How It Works Carotenoids absorb blue and some green

but reflect yellow, orange, and brown

Between the cholorphylls and the carotenoids, the majority of the visible spectrum is absorbed by the plant

Light Reactions: a closer look

Water is split to provide e- to ETC. H+ ions and oxygen are produced.

Light is absorbed by pigments in P.S. II. Each pigment absorbs a specific range of wavelengths. When light is absorbed by chlorophyll a, electrons become “excited” (gain energy).

The “excited” e- move to a primary e- acceptor and are then passed down the ETC.

Light is absorbed by pigments in P.S. II. Each pigment absorbs a specific range of wavelengths. When light is absorbed by chlorophyll a, electrons become “excited” (gain energy).

The “excited” e- move to a primary e- acceptor and are then passed down the ETC.

Light reactions (cont’d)

As e- move down the ETC they lose energy. The energy of the e- is used to pump H+ into the thylakoid.

When e- enter P.S. I, light is absorbed and e- become “excited” again.

Electrons move down another ETC and are taken by NADP+ to make NADPH. The e- in NADPH are still at a relatively high energy level. i.e.: the energy of sunlight is now stored as

chemical PE in an organic molecule.

The final step

Chemiosmosis: movement of H+ (protons) through ATP synthase transfers energy which is used to produce ATP. The high concentration of H+ was built by e-

moving down ETC = potential energy. The energy of the e- moving down the ETC came

from the sun. The energy of sunlight is now stored as chemical

PE in an organic molecule.

Chemiosmosis

•The high concentration of H+ was built by e- moving down ETC = potential energy.•The energy of the e- moving down the ETC came

from the sun.•The energy of sunlight is now stored as chemical

PE in an organic molecule.

Thylakoid

(stack = granum)

sunlight

StromaOverview of Photosynthesis

PhotosynthesisThe “Dark” Reactions

aka: Light-independent reactions

The Photosynthesis Equation

CO2 + H2O → C6H12O6 + O2

Which reactant was used in the light reactions (stages 1 and 2)? H2O (split to provide electrons for photosystem II)

Which product was produced in the light reactions? O2 (a byproduct of splitting water molecules)

CO2 and C6H12O6 were not involved in the light reactions- must be involved the third stage

Stage 2: Calvin Cycle During this stage, carbon is “fixed”. What does “carbon fixation” refer to?

Changing inorganic carbon (like CO2) to organic carbon (molecules with C bonded to other C)

Summary of the Calvin Cycle: CO2 molecules combine with an organic

compound. Energy from NADPH and ATP is used to make PGAL. PGAL is then used to build organic compounds (like glucose).

Stage 2: The Details1. A CO2 molecule combines with RuBP (5-C)

to create a 6-C molecule.

2. 6-C molecule immediately splits into 2 3-C molecules (PGA)

3. Energy from ATP and NADPH is used to change PGA into PGAL

4. Some PGAL is used to regenerate RuBP to keep Calvin cycle going; the rest is used to make organic compounds (like glucose)

Stage 2: The DetailsStage 2: The Details

6-C compound splits

2

PGA

2

PGAL

Stage 2: The Starting Materials Carbon fixation requires ATP, NADPH,

RuBP, and CO2

ATP and NADPH: made during the light reactions

RuBP: regenerated at the end of each cycle CO2: Some is created by the plant during cell

respiration and the rest is taken in from the atmosphere through openings in the plant leaves called stomata.

Leaf Cross-Section

Guard Cells and Stomata

•CO2 enters plant leaves through openings called stomata.

•Guard cells on either side of the stomata empty or fill with water to open and close the stomata.

Guard Cells and Stomata

The Problem with Stomata

When stomata open to allow CO2 in, it also allows H2O to escape.

So taking in CO2 comes at the expense of losing water. Not an issue for C3 plants (most plants are this

type) because they exist in temperate climates where water loss isn’t such a problem

Major problem for plants that exist in climates that are hot and/or dry

The solution: alternative carbon fixation

Alternative Carbon Fixation The C4 Pathway

C4 plants partially close their stomata during the hottest part of the day (reduces the water loss, but also the amount of CO2 coming in)

Contain enzymes that “fix” CO2 into 4-C compounds when CO2 level is low

Breakdown 4-C compounds later on to release CO2 (which can then be used in Calvin cycle)

C4 plants include corn, sugar cane, and crabgrass

Alternative Carbon Fixation

The CAM Pathway Plants living in the hottest and driest climates Open stomata at night and close during day (opposite

of other plants) Take in CO2 at night and “fix” into various organic

compounds CO2 released from these compounds during the day

and used in Calvin cycle CAM plants include cacti and pineapple