AN OVERVIEW OF PHOTOSYNTHESIS

Copyright © 2009 Pearson Education, Inc.

Carbon dioxide

C6H12O6

Photosynthesis

H2OCO2 O2

Water

+ 66

Lightenergy

Oxygen gasGlucose

+ 6

Plants use water and atmospheric carbon dioxide to produce a simple sugar and release oxygen

– Photosynthesis: a process that converts solar energy to chemical energy

Autotrophs Are the Producers of The Biosphere

Autotrophs make their own food without using organic molecules derived from any other living thing

– Photoautotrophs : Autotrophs that use the energy of light to produce organic molecules

– Most plants, algae and other protists, and some prokaryotes are photoautotrophs

– The ability to photosynthesize is directly related to the structure of chloroplasts

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CO2 O2Stoma

Vein

Chloroplasts

Chlorophyll light absorbing pigment in

chloroplasts responsible for the green

color of plants located on thylakoid

membrane Stomata tiny pores in the leaf; allow

CO2 to enter and O2 to exit Veins in the leaf deliver

water absorbed by roots

Photosynthesis Occurs in Chloroplasts in Plant Cells

Chloroplast

Outer and innermembranes

IntermembranespaceGranumStroma Thylakoid

space

Thylakoid

Photosynthesis is a Redox Process, as is Cellular Respiration

Photosynthesis is a redox (oxidation-reduction) process

A loss of electrons = oxidation

A gain of electrons = reduction *(OIL RIG)

In photosynthesis: Water is oxidized and CO2 is reduced

The oxidation of water produces oxygen and the reduction of CO2 produces sugar

6 CO2 + 6 H2O C6H12O6 + 6 O2

Reduction

Oxidation

Photosynthesis is a Redox Process, as is Cellular Respiration

H2O

ADP

P

LIGHTREACTIONS

(in thylakoids)

Light

Chloroplast

NADPH

ATP

O2

CALVINCYCLE

(in stroma)

Sugar

CO2

NADP+

Visible Radiation Drives the Light Reactions

Sunlight is a type of electromagnetic energy (radiation)

Visible light is a small part of the electromagnetic spectrum

Light exhibits the properties of both waves and particles

– One wavelength = distance between the crests of two adjacent waves (the shorter the wavelength, the greater the energy)

– Light behaves as discrete packets of energy called photons (fixed quantity of light energy)

Wavelength (nm)

10–5 nm

Increasing energy

Visible light

650nm

10–3 nm 1 nm 103 nm 106 nm 1 m 103 m

380 400 500 600 700 750

Radiowaves

Micro-waves

InfraredX-rays UVGammarays

Pigments are molecules that absorb specific wavelengths of light and transmit others

Many pigments are built into the thylakoid membrane

We see the color of the wavelengths that are transmitted/reflected

– Ex. chlorophyll transmits green

Visible Radiation Drives the Light Reactions

Chloroplasts contain several different pigments and all absorb light of different wavelengths

– Chlorophyll a: absorbs blue violet and red light and reflects green

– Chlorophyll b: absorbs blue and orange and reflects yellow-green

– The carotenoids: absorb mainly blue-green light and reflect yellow and orange

Visible Radiation Drives the Light Reactions

Chlorophyllmolecule

Excited state

Ground state

Heat

PhotonPhoton

(fluorescence)

e–

Reactioncenter complex

e–

Light-harvestingcomplexes

Photosystem

Pigmentmolecules

Pair ofChlorophyll a molecules

Solar energy could be released as heat or light but instead is conserved and passed from one molecule to another

All of the components to accomplish this are organized in thylakoid membranes in clusters called photosystems

Photosystems = light-harvesting complexes surrounding a reaction center complex

Photosystems Capture Solar Power

When light strikes the photosystem energy is passed from molecule to molecule within the photosystem

– Energy from the pigment molecules, passes to the reaction center where it excites an e- of the chl. a molecules

– Excited e- are transferred to the primary electron acceptor (becomes reduced)

– This solar-powered transfer of an electron from the reaction center pigment to the primary electron acceptor is the first step of the light reactions

Photosystems Capture Solar Power

Two types of photosystems exist: photosystem I and photosystem II

– Each type of photosystem has a characteristic reaction center

– Photosystem II: functions first; is called P680 (chl a best absorbs light w/ a wavelength of 680, red)

– Photosystem I: functions next; is called P700 (chl a absorbs wavelength of 700, also red)

Photosystems Capture Solar Power

Two Photosystems Connected by an Electron Transport Chain Generate ATP and NADPH

During the light reactions, light energy is transformed into the chemical energy of ATP and NADPH

– Electrons removed from water pass from photosystem II to photosystem I and are accepted by NADP+

– The bridge between photosystems II and I is an electron transport chain that provides energy for the synthesis of ATP

– NADPH, ATP, and O2 are the products of the light reactions

NADPH

Photosystem II

e–

Millmakes

ATP Ph

oto

n

Photosystem I

ATP

e–

e–

e–

e–

e–

e–

Ph

oto

n

Stroma

O2

H2O 12 H+

NADP+ NADPHPhoton

Photosystem II

Electron transport chainProvides energy forsynthesis of

by chemiosmosis

+ 2

Primaryacceptor

1

Thylakoidmem-brane

P680

2

4

3Thylakoidspace

e–e–

5

Primaryacceptor

P700

6

Photon

Photosystem IATP

H++

Chemiosmosis Powers ATP Synthesis in the Light Reactions

In Chemiosmosis, the potential energy of a H+ gradient is used to synthesize ATP

The H+ gradient is generated by the ETC

ATP synthase couples the flow of H+ to the phosphorylation of ADP

– The chemiosmotic production of ATP in photosynthesis is called photophosphorylation

+

O2

H2O12 H+

NADP+ H+ NADPH

+ 2

H+

H+

H+ H+

H+

H+

H+

H+

H+H+

H+

H+

H+ H+

Photosystem II Photosystem IElectrontransport

chain

ATP synthase

LightLight

Stroma (low H+

concentration)

Thylakoid space(high H+ concentration)

ADP + P ATP

THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS

CO2

ATPNADPH

Input

CALVINCYCLE

G3P

Output:

The Calvin cycle makes sugar within a chloroplast

Atmospheric CO2, ATP, and NADPH are required to produce sugar

Using these three ingredients, an energy-rich, three-carbon sugar called glyceraldehyde-3-phosphate (G3P) is produced

A plant cell may then use G3P to make glucose and other organic molecules

ATP and NADPH Power Sugar Synthesis in the Calvin Cycle

The Calvin Cycle Has Three Phases:

1. Carbon Fixation-Atmospheric carbon in the form of CO2 is incorporated into a molecule of ribulose bisphosphate (RuBP) via rubisco

2. Reduction Phase – NADPH reduces 3PGA to G3P (requires 3 molec’s. of CO2 for 1 G3P)

3. Regeneration of Starting Material –RuBP is regenerated and the cycle starts again

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ATP and NADPH Power Sugar Synthesis in the Calvin Cycle

NADPH

ATP

RuBP

3

P

G3P

P

Input:CO2

1

Rubisco

3 P

Step Carbon fixation

3-PGA6 P

CALVINCYCLE

6

6

6

6

P

Step Reduction

2

2

G3P5 P

3

3

G3P1 P

Glucoseand othercompounds

Output:

Step Release of one

molecule of G3P

1

Step Regeneration of RuBP4

4ATP3

3 ADP

NADP+

6 ADP +

PHOTOSYNTHESIS REVIEWED AND EXTENDED

Copyright © 2009 Pearson Education, Inc.

NADP+

NADPH

ATP

CO2

+

H2O

ADPP

Electrontransport

chainsThylakoidmembranes

LightChloroplast

O2

CALVINCYCLE

(in stroma)

Sugars

Photosystem II

Photosystem I

LIGHT REACTIONS

RuBP

3-PGA

CALVIN CYCLE

Stroma

G3P Cellularrespiration

Cellulose

Starch

Other organiccompounds

EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants

In hot climates, plant stomata close to reduce water loss so oxygen builds up

– Rubisco adds oxygen instead of carbon dioxide to RuBP in a process called photorespiration

– Photorespiration consumes oxygen, produces CO2, and produces no sugar, or ATP

Copyright © 2009 Pearson Education, Inc.

EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants

Some plants have evolved a means of carbon fixation that saves water during photosynthesis

– C4 plants partially shut stomata when hot and dry to conserve water

– This reduces CO2 levels, so contain PEP carboxylase to bind CO2 at low levels (carbon fixation); They’re called C4 plants because they first fix CO2 into a four-carbon compound.

– This allows plant to still make sugar by photosynthesis

Copyright © 2009 Pearson Education, Inc.

EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants

Another adaptation to hot and dry environments has evolved

– CAM open their stomata at night thus admitting CO2 in w/o loss of H2O

– CO2 enters, and is fixed into a four-carbon compound, (carbon fixation)

– It is released into the Calvin cycle during the day

Copyright © 2009 Pearson Education, Inc.

Mesophyllcell

CO2

CALVINCYCLE

CO2

Bundle-sheathcell 3-C sugar

C4 plant

4-C compound

CO2

CALVINCYCLE

CO2

3-C sugar

CAM plant

4-C compound

Night

Day