Overview: The Process That Feeds the Biosphere

• Photosynthesis is the process that converts solar energy into chemical energy

• Directly or indirectly, photosynthesis nourishes almost the entire living world

• Autotrophs sustain themselves without eating anything derived from other organisms

• Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules

• Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from H2O and CO2

Autotrophs

(a) Plants

(c) Unicellular protist10 µm

1.5 µm

40 µm(d) Cyanobacteria

(e) Purple sulfur bacteria

(b) Multicellular alga

Photosynthesis occurs in plants,

algae, certain other protists, and some prokaryotes

• Heterotrophs obtain their organic material from other organisms

• Heterotrophs are the consumers of the biosphere

• Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2

Heterotrophs

The Two Stages of Photosynthesis: A Preview

• Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)

• The “light” reactions are in the thylakoids

• The Calvin cycle is in the stroma

1 µm

Thylakoidspace

Chloroplast

GranumIntermembranespace

Innermembrane

Outermembrane

Stroma

Thylakoid

• The “light” reactions overview

– Capture light energy using chlorophyll

– Split H2O to electrons, protons and oxygen

– Release O2

– Reduce NADP+ to NADPH

– Generate ATP from ADP by photophosphorylation

• The Calvin cycle overview

- begins with carbon fixation, incorporating CO2 into organic molecules

- forms sugars from these organic molecules, using ATP and NADPH (from light reactions)

Light

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

Overview of Photosynthesis 1

Light

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

Overview of Photosynthesis 2

Light

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

CalvinCycle

CO2

Overview of Photosynthesis 3

Light

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

CalvinCycle

CO2

[CH2O]

(sugar)Overview of Photosynthesis 4

Chlorophyll is the light-capturing substance

• Anchored to thylakoid membranes by proteins

Surrounded by other pigment

molecules, enzymes and

electron carriers

A “photosystem”

(a) Excitation of isolated chlorophyll molecule

Heat

Excitedstate

(b) Fluorescence

Photon Groundstate

Photon(fluorescence)

En

erg

y o

f el

ectr

on

e–

Chlorophyllmolecule

If illuminated, a solution of chlorophyll will fluoresce, giving off light and heat

Illustration of important properties of chlorophyll

PhotosyntheticEnergy Capture

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

STROMA

e–

Pigments-chlorophylland accessory

Photon

Transferof energy

Special pair ofchlorophyll amolecules

Th

yla

koid

me

mb

ran

e

Photosystem

Primaryelectronacceptor

Reaction-centercomplex

Light-harvestingcomplexes

Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

4

Pq

Pc

Cytochromecomplex

Electron transport chain

5

ATP

Photosystem I(PS I)

Light

Primaryacceptor

e–

P700

6

Fd

Electron transport chain

NADP+

reductase

NADP+

+ H+

NADPH

8

7

e–

e–

6

“Z-Diagram” of photosynthesis

Photosystem II(PS II)

This illustrates “non-cyclic” electron flow- electrons enter with water and leave with NADPH

ATPPhotosystem II

Photosystem I

Primary acceptor

Pq

Cytochromecomplex

Fd

Pc

Primaryacceptor

Fd

NADP+

reductaseNADPH

NADP+

+ H+

Cyclic electron flow uses only photosystem I and

produces ATP, but not NADPH

• “Light” reactions of PSI and PSII generate ATP and “reducing power” in the form of NADPH

• Reducing power allows the conversion of carbon dioxide to carbohydrate

• CO2 to CH2O requires NADPH reducing power

Ribulose bisphosphate(RuBP)

3-Phosphoglycerate

Short-livedintermediate

Phase 1: Carbon fixation

(Entering oneat a time)

Rubisco

Input

CO2

P

3 6

3

3

P

PPP

ATP6

6 ADP

P P6

1,3-Bisphosphoglycerate

6

P

P6

66 NADP+

NADPH

i

Phase 2:Reduction

Glyceraldehyde-3-phosphate(G3P)

1 POutput G3P

(a sugar)

Glucose andother organiccompounds

CalvinCycle3

3 ADP

ATP

5 P

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

G3P

Carbon fixation conversion of carbon dioxide to carbohydrate

Rubisco is the key

enzyme of the cycle

Plants that use this cycle

exclusively are C3 plants

• Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis

• Plants have evolved two alternate types of carbon fixation

• C4 and Crassulean Acid Metabolism (CAM) fixation

LightReactions:

Photosystem II Electron transport chain

Photosystem I Electron transport chain

CO2

NADP+

ADP

P i+

RuBP 3-Phosphoglycerate

CalvinCycle

G3PATP

NADPHStarch(storage)

Sucrose (export)

Chloroplast

Light

H2O

O2

C3 Photosynthesis: a review

Note Card

What is the “reducing power” of NADPH and how does it link the light reactions with the Calvin

Cycle?