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

Copyright © 2009 Pearson Education, Inc.

Chloroplast

Outer and innermembranes

IntermembranespaceGranumStroma Thylakoid

space

Thylakoid

Carbon dioxide

C6H12O6

Photosynthesis

H2OCO2 O2

Water

+ 66

Lightenergy

Oxygen gasGlucose

+ 6

Photosynthesis: a process that converts solar energy to chemical energy

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

CO2 O2Stoma

Vein

Chloroplasts

Leaves contain: Stomata = tiny pores in the leaf;

allow CO2 to enter and O2 to exit Veins deliver water & nutrients

absorbed by roots Chlorophyll= a light

absorbing pigment responsible for the

green color of plants located on thylakoid

membrane

Photosynthesis Occurs in Chloroplasts in Plant Cells

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 producing oxygen and CO2 is reduced producing 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 (shorter λ’s have greater energy)

– Light behaves as discrete packets of energy called photons (photons contain a 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 proteins that absorb specific wavelengths of light and transmit others

Various pigments are built into the thylakoid membrane

We see the color of the wavelengths that are transmitted (not absorbed)

– 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

Reactioncenter complex

e–

Light-harvestingcomplexes

Photosystem

Pigmentmolecules

Pair ofChlorophyll a molecules

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

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

A Photosystem consists of a light-harvesting complex surrounding a reaction center complex

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 in the Light Reactions

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++

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

– Energy from the pigments is passed to chl. a in the reaction center where it excites electrons of chl. a

– Excited e- from chl. a are transferred to the primary electron acceptor (reducing it)

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

Photosystems Capture Solar Power in the Light Reactions

The Light Reactions

– The primary e- acceptor passes electrons to an electron transport chain (ETC)

– To fill the electron void that chl. a (P680) now has, chl. a (P680) oxidizes H2O and takes electrons from water

– This is the step where water is oxidized and oxygen released

– The ETC is a bridge between photosystems II and I. The ETC also generates ATP.

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++

As the first protein of the ETC accepts electrons it pumps H+ into the thylakoid space. This generates a proton (H+) gradient.

H+ are concentrated in the thylakoid space and less concentrated in the stroma

Protons flow down their gradient through an enzyme called ATP synthase.

As H+ flow through the ATP synthase it phosphorylates ADP forming ATP

The phosphorylation of ADP is endergonic; the flow of protons down their gradient is exergonic.

The Light Reactions

Chemiosmosis is a mechanism where the cell couples exergonic and endergonic reactions

Ex. An ATP synthase couples the flow of H+ down their gradient (exergonic) to the phosphorylation of ADP (endergonic)

The chemiosmotic production of ATP in photosynthesis is called photophosphorylation

This step produces ATP in the stroma

Chemiosmosis Powers ATP Synthesis in the Light Reactions

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

– Electrons moving down the ETC are passed to P700 of Photosystem I, and ultimately to NADP+ by the enzyme NADP+ reductase

– This step produces NADPH in the stroma

+

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, a 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

Copyright © 2009 Pearson Education, Inc.

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

Chlorophyllmolecule

Excited state

Ground state

Heat

PhotonPhoton

(fluorescence)

e–