PhotosynthesisCampbell & Reece Ch 10

Jeff Jewett

American College of Sofia

January 2010

Version 1.0

Image Credit: Jeff Jewett

Introduction

• Autotrophs – create own food, harvest energy Autotrophs – create own food, harvest energy from non-living parts of environment (also called from non-living parts of environment (also called “producers”)“producers”)

• Plants are Plants are PhotoautotrophsPhotoautotrophs (as opposed to (as opposed to chemoautotrophs…much less common)chemoautotrophs…much less common)

HeterotrophsHeterotrophs eat other organisms, or remains of eat other organisms, or remains of other organisms (also called “consumers”)other organisms (also called “consumers”)

All animals are heterotrophsAll animals are heterotrophs

Image Credit: Jeff Jewett

Slide used with permission from Stephen Taylor, Bandung International School

Photosynthesis as ReDox

• Respiration – sugar oxidized to CO2, releasing energy (exergonic). O2 accepts e-, forming H20. Mitochondria use energy to create ATP.

• Photosynthesis – CO2 reduced to sugar (endergonic), powered by light. H20 splits, releasing O2.

Structure of a Chloroplast

Function of photosynthesis

CR Figure 10.3

Overviewwww.wildlifecontrolsupplies.com

Image Credit: 1800sunstar.com

Image Credit: beaut.ie

• 1. Light-dependent (“Light”) reactions – light energy is captured and stored as

chemical energy (ATP, NADPH)– Occurs across Thylakoid Membrane of

chloroplast

• 2. Calvin Cycle (Light-Independent /“Dark” reactions)– chemical energy (ATP, NADPH) used to

make “food,” such as more complex compounds like glucose and starch

– Occurs in stroma of chloroplast

Image Credit: www.cavannapasta.com

CR Figure 10.5

Slide used with permission from Stephen Taylor, Bandung International School

Slide used with permission from Stephen Taylor, Bandung International School

Substances that absorb visible light are pigments

Slide used with permission from Stephen Taylor, Bandung International School

• Photons are only absorbed when their energy is exactly equal to difference between ground state and excited state of pigment

• This is why pigments only absorb some wavelengths, and have a distinct color

CR Figure 10.9

CR Figure 10.11

Light Dependent Reactions

• capture light energy, use it to power endergonic chemical reactions (thus storing it as chemical energy), ATP and NADPH are created

• Water is split (using light energy), releasing e- and H+.

• Light is used to reduce NADP+ NADPH• ADP ATP through photophosphorylation• Takes place across the thylakoid membrane of

chlorplast• Light reactions do not produce sugar!

Light Dependent Reactions – in Thylakoid

• Photosystems are light harvesting complexes (light antenna) surrounding a reaction center

Image Credit: Wikipedia

CR Figure 10.12

CR Figure 10.14

CR Figure 10.13

Use solar energy (light) to phosphorylate ADP ATP and reduce NADP+ NADPH

CR Figure 10.17

Animations

• Light Reactions (St. Olaf College)• Light Reactions (Vermont Forestry)• Light Reactions (Smith College)• Calvin Cycle (Smith College)• Calvin Cycle (McGraw Hill)• Photosynthesis (MasteringBio…slow to buffer)• Music Video – Light Reactions. Somewhat

cheesy, but an excellent summary.• Music Video – Calvin Cycle. Also cheesy.

(starts at 1:30 in)

CR Fig 10.16 page 198

Compare Chemiosmosis

COMPARISON OF ETC. in Respiration and Photosynthesis

ETC in Mitochondria – Aerobic Cellular Respiration

ETC in Chloroplasts (Light Reactions of Photosynthesis)

Electron Carriers

Electron Carriers: NADH NAD+, FADH2 FAD

Electron Carriers: NADP+ NADPH

Location inner membrane (of 2), folds called “cristae”

innermost membrane (of 3), called thylakoid membrane

Action of electron carrier

Oxidized, GIVING high-energy electrons to the ETCNADH NAD+ + 2e- (to ETC)

Reduced, RECEIVING high-energy electrons from the ETCNADP+ + 2e- (from ETC) NADPH

Proton pumps in ETC

Pump protons (H+) OUT of matrix to intermembrane space

Pump protons (H+) INTO inner thylakoid space from stroma

Water Water is produced, by reducing Oxygen and using electrons from ETCThis is the Oxygen-requiring step!

Water is split (photolysis), giving electrons to ETC. Water is oxidized.This is the Oxygen-producing step!

ATP Production

Protons flow IN through ATP Synthase (to mito. Matrix) , producing ATP (ADP + Phosphate ATP)

Protons flow OUT through ATP Synthase (to stroma), producing ATP (ADP + Phosphate ATP)

In/Out ATP produced; NAD+, FAD, and water as waste; Oxygen consumed

ATP and NADPH produced, oxygen is waste, water consumed,

Calvin Cycle – “Pour some sugar on me…”

• Why bother?• Uses ATP and NADPH from the light-dependent

reactions to convert CO2 into sugar (glucose, C6H12O6)

• Reduction of CO2 is endergonic process, powered by ATP and NADPH

• 3 phases1. Fixation of CO2

2. Reduction of CO2

3. Regeneration of RuBP

Calvin Cycle Phase 1:

Carbon Fixation

• Take CO2 from air (inorganic) organic molecules

• 1 C attaches to 5C Ribulose BisPhosphate (RuBP)

• Catalyzed by RuBisCo!

• Unstable 6C intermediate, splits into 2 phosphoglycerates (3C each)

Calvin Cycle Phase 2: Reduction / Energy

Input• Reduce

– Use 12 NADPH

• Phosphorylate!– Use 12 ATP

• Create 12 G3P (from 6 CO2)

• Use 2 G3P to make glucose, cellulose, everything else!

Calvin Cycle Phase 3:

Regenerate RuBP

• Out of 12 G3P: • 2 G3P

glucose• 10 G3P

regenerate RuBP

• Use 6 more ATP

• Reactions are complex

You’ve seen G-3-P before…Glycolysis! (CR Figure 9.9 page 168)

Calvin & Krebs ComparedCalvin Cycle (Photosynthesis)

Krebs Cycle (Respiration)

Entry C compound CO2 Pyruvate (3C)

Final C Compound G3P (3C)

Two used to make C6H12O6

CO2

What happens to Carbon Compound?

Reduction Oxidation

Cycle Regenerates Oxaloacetate Ribulose BisPhosphate

Action of Energy Carriers

NADPH NADP+ (Oxidized)

ATP ADP + Pi

NAD+ NADH

(Reduced)

ADP + Pi ATP

First Name Melvin Hans

Links

• Why Study Photosynthesis?

• Top 10 Reasons to love RuBisCo

• Biographies of Important People in Photosynthesis (many won Nobel Prizes)

Image source unknown. Question from the IBO.

Slide adapted with permission from Stephen Taylor, Bandung International School

Quiz Slide