Photosynthesis: Capturing Energy

Chapter 8

Light

• Composed of photons – packets of energy• Visible light is a small part of the

electromagnetic spectrum• All energy travels as waves• Wavelength is the distance from 1 wave

peak to the next• Shorter wavelengths have more energy

than longer wavelengths

• Visible light: 380-760 nm

• Energy from visible light is used in photosynthesis

• Why? • Longer

wavelengths don’t have enough energy; higher wavelengths have too much

TV andradio

waves

Micro-waves

Infrared

Visible

UV

X-rays

Gammarays

Colorspectrumof visiblelight

Red

Orange

Yellow

Green

Blue

Violet

760 nm

700 nm

600 nm

500 nm

400 nm380 nm

One wavelength

Longer wavelength

Electromagneticspectrum Shorter wavelength

Chloroplasts

• Organelles enclosed by a double membrane• Located mainly within mesophyll cells

inside a leaf• Each cell contains 20-100 chloroplasts• This portion of the leaf has many air spaces

and a high water vapor content

Palisademesophyll

Vein

Stoma

Spongy mesophyll

Outer membraneThylakoids

Intermembranespace

Thylakoidmembrane

StromaInnermembrane

Granum(stack ofthylakoids)

Thylakoidlumen

Chlorophyll

• Main photosynthetic pigment: • chlorophyll a (initiates the light-dependent reactions)

• Accessory pigments: chlorophyll b, carotenoids (these absorb different wavelengths of light and pass the energy on to chlorophyll a)

• These pigments are found in the thylakoid membranes of chloroplasts

• Pigments reflect the color of light we see; absorb the other colors

• Is green light used during photosynthesis? Why or why not?

Overview of photosynthesis

• Hydrogens from water reduce carbon dioxide; oxygen from water is oxidized

• Photosynthesis is a redox reaction:

6 CO2 + 6 H2O C6H12O6 + 6 O2

reduction

oxidation

Overview…

• Two phases:1st: Light-dependent (‘photo’)

Occurs in the thylakoids of the chloroplasts

H2O is split and molecular oxygen is releasedElectrons energized by light produce ATP and NADPH

which are both needed for the endergonic next phase

2nd: Carbon-fixation (‘synthesis’)Occurs in the stroma of the chloroplastsATP and NADPH provide the energy needed for the formation of carbohydrates

Overview…

Light reactions Carbon-fixation reactions

Light reactions

Calvin cycle

ATP

ADP

NADPH

NADP+

H20 02C02

carbohydrates

Light-dependent Reactions

• Occur in the thylakoids• Energy is absorbed from light and

converted to chemical energy stored in ATP and NADPH

• Oxygen is released• Photosystem I and II both involved – these

have similar pigments but different roles

Photosystems I and II

• Each system includes:• Chlorophyll a molecules and associated

proteins• Multiple antenna complexes• Photosystem I = P700 - wavelength absorbed• Photosystem II = P680 - wavelength absorbed

Primaryelectronacceptor

Photon

Photosystem

Chloroplast

Thylakoid

Antennacomplexes

Reactioncenter

e–

Light reactions: Step #1

• Light energy forces e- to a higher energy level in 2 chlorophyll a molecules of PS II (e- is excited)

• e- leave chlorophyll a (oxidation)

• a replacement e- is donated by H2O:• 2H2O 4 H+ + 4 e- + O2

• This is noncyclic electron transport

Light reactions: Step #2

• e- goes to a primary electron acceptor in the thylakoid membrane (reduction)

Light reactions: Step #3

• e- donated from the primary electron acceptor to a series of molecules in the thylakoid membrane – the electron transport chain

• e- lose energy as they move through the chain – this energy moves H+ into the thylakoid lumen

• this H+ gradient will be used to produce ATP from ADP and Pi using ATP synthase

Light reactions: Step #4

• Light is absorbed by PS I• e- leave chlorophyll a and go to another

primary electron acceptor • these e- are replaced by e- from the electron

transport chain• This is cyclic electron transport

Light reactions: Step #5

• e- from the primary electron acceptors in PS I go to another electron transport chain on the stroma side of the thylakoid membrane

• e- with H+ and NADP+ NADPH• ATP + NADPH made during the light

reactions are both needed to power the carbon-fixation reactions

Primaryelectronacceptor

Primaryelectronacceptor

NADPHNADP+

H2O

ATP

O2

ADP

1

2

Photosystem II(P680)

Productionof ATP by

chemiosmosis

H+

Ferredoxin

Plastiquinone

Cytochromecomplex

Plastocyanin

1/2 + 2 H+

Pi

A0

A1

FeSx

FeSB

FeSA

Photosystem I(P700)

(from medium)

Electrontransport

chain

Electrontransport

chain

ATP Synthesis and electron transport… the main ideas

• Electrons (e-) move down the electron transport chain and release energy as they go

• Protons (H+) move from the stroma to the thylakoid lumen, creating a proton gradient

• The greater concentration of H+ lowers the pH• The thylakoid membrane is impermeable to H+

except through ATP synthase• The flow of H+ through ATP synthase generates

ATP

Carbon-fixation reactions

• Also known as the Calvin Cycle or the light-independent reactions

• Occur in the stroma

• CO2 + chemical energy from ATP and NADPH are used to make organic compounds – carbon is ‘fixed’

Three phases of the Calvin Cycle

1. CO2 uptake

2. Carbon reduction

3. RuBP regeneration

Carbon fixation: Step #1

CO2 uptake:

CO2 + RuBP* unstable 6-C molecule, which splits 2 3-C molecules + 3 PGA**

*RuBP = ribulose biphosphate (5 C)

**PGA = 3 phosphoglycerate

Carbon fixation: Step #2

Carbon reduction:

2 molecules of 3-PGA are converted to 2 molecules of G3P* in a 2-part process, using the energy from ATP and the H+ from NADPH from the light reactions

* G3P = glyceraldehdye 3-phosphate

Carbon fixation: Step #3

RuBP regeneration:

One molecule of G3P leaves the Calvin cycle to be converted into carbohydrates such as glucose or starch

The other G3P molecule uses the energy from ATP and is converted back to RuBP

The RuBP is returned back to the Calvin cycle

CO 2 molecules arecaptured by RuBP,resulting in an unstableintermediate that isimmediately brokenapart into 2 PGA

PGA is phosphorylatedby ATP and reduced byNADPH. Removal of aphosphate results information of G3P.

Through a series ofreactions G3P isrearranged into newRuBP molecules oranother sugar

Glucose and othercarbohydrate synthesis

2 moleculesof glyceraldehyde-3-phosphate (G3P)

6 molecules ofribulose bisphosphate(RuBP)

CALVINCYCLE

Carbonreduction

phase

RuBPregeneration

phase

CO2 uptakephase

12 NADPH

12 ADP

12 molecules ofphosphoglycerate(PGA)

12 ATP

6 molecules of CO2

10 moleculesof G3P

6 molecules of ribulosephosphate (RP)

ATP

6 ADP

P

P

P

P

P

P P

P

12 moleculesof glyceraldehyde-3-phosphate (G3P)

1

32

12 NADP+

Adjustments based on weather…

• C3 plants – use the C3 pathway – the initial carbon fixation product is a 3-C sugar

• These plants must close their stomata during hot, dry weather to reduce water loss

• This reduces carb production

• Adaptations for hot, dry environments:• C4 plants – 1st fix CO2 into a 4-C compound• CAM plants – fix CO2 at night (cactus)

• Table 8-2 p. 167