Photosynthesis Chapter 9. Plant Kingdom -produce most of the food in the biosphere - Autotrophs:...

Photosynthesis

Chapter 9Chapter 9

Plant Kingdom

- produce most of the food in the biosphere

- Autotrophs: make their own food- Heterotrophs: obtain food from

other organisms- Plants are photoautotrophs:

convert light energy into chemical energy (food: glucose) using CO2 and H2O

(a) Plants

(b) Multicellular algae

(c) Unicellular protist 10 m

40 m(d) Cyanobacteria

1.5 m(e) Purple sulfurbacteria

Photoautotrophs

I. ChloroplastsI. Chloroplasts- site of photosynthesis in plantsA. Leaf

- main photosynthetic surface 1. Mesophyll

- middle tissue layer of leaf that contains chloroplasts

2. Stomata- pores in leaf that let in CO2

3. Vein- supply water

Guardcells

Stomatal pore

Epidermalcell

Surface view of a spiderwort(Tradescantia) leaf (LM)

(b)Stoma

Upperepidermis

Palisademesophyll

Spongymesophyll

Lowerepidermis

Guard cells

Phloem

Guard cells

Cutaway drawing of leaf tissues(a)

Vein Air spaces Guard cells

Transverse section of a lilac(Syringa) leaf (LM)

Leaf Structure

B.Structure of Chloroplasts

- 30-40/plant cell- probably evolved from free-living

cyanobacteria

1. Stroma- fluid within chloroplast

2. Thylakoids- interconnected, membranous sacs- stacked into grana- chlorophyll attached to membrane

Location of Photosynthesis in a Plant

Mesophyll cell

Mesophyll

Stomata

CO2 O2

Chloroplast

Outermembrane

Intermembranespace

Inner membrane

Thylakoid ThylakoidSpace

GranumStroma

Leaf cross section

A.Light Energy A.Light Energy - visible light only a small part of

the electromagnetic spectrum

Gammarays X-rays UV Infrared

Micro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

106 nm 103 m

380 450 500 550 600 650 700 750 nm

Visible light

Shorter wavelength

Higher energy

Longer wavelength

Lower energy

B. Photosynthetic Pigments

- Pigments are substances that absorb light energy

- Plants appear green because chlorophyll reflects green light

- Chlorophyll absorbs violet-blue and red light

LightReflectedLight

Chloroplast

Absorbed light

Granum

Transmittedlight

Why Leaves Are Green: Interaction of Light with Chloroplasts

1.Absorption Spectrum 1.Absorption Spectrum - shows wavelengths absorbed

by chlorophyll

400 500 600 700

Chlorophyll a

Chlorophyll b

Carotenoids

Wavelength of light (nm)

II. Photosynthesis

Overview II. Photosynthesis

Overview

6CO2 + 6H2O C6H12O6 + 6O2 Reactants Products

- not a single reaction, but a series of complex reactions in 2 main stages:

A. Light – Dependent Reaction

1. Light hits a chlorophyll molecule causing the electrons to move to a higher energy level.

2. This starts a series of reactions that converts light energy into ATP, which is the chemical energy source used in cells.

Light Dependent reactionLight Dependent reaction-This phase of photosynthesis

needs water also-enzymes remove electrons from

water and split oxygen and hydrogen.

-Oxygen is given off as by product.

-Hydrogen used to make sugars.

Main Point of Light Reaction:

- Light energy is used to produce ATP which is used in the Calvin Cycle to make sugar.

- Light energy is converted to chemical energy.

B.Calvin Cycle or Light Independent reaction

- occurs in the stroma

- Needs ATP produced by Light dependent reaction

- Carbon from carbon dioxide molecules combine with hydrogen to form sugars, a form of chemical energy

Calvin CycleCalvin Cycle- sugar factory in chloroplast

- takes CO2 with ATP from the light reaction to make sugar

- actual product is Glyceraldehyde-3-phosphate (G3P), a 3-C sugar which then turns into glucose, a 6 carbon sugar.

CALVINCYCLE

[CH2O](sugar)

ADP+ P i

LIGHT REACTIONS

Chloroplast

An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle

Think Pair ShareThink Pair ShareWhat does the Calvin cycle need

from the light reaction before it will work?

Why would you be dead if not for the Calvin cycle?

Structure of Chlorophyll Molecules in Chloroplasts of

Plants

Structure of Chlorophyll Molecules in Chloroplasts of

Plants

C CH2CH3

in chlorophyll a

in chlorophyll b

Porphyrin ring:Light-absorbing“head” of molecule;note magnesiumatom at center

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshown

1. Photon of light strikes a pigment molecule and energy is transferred through several pigment molecules.

Primary electionacceptor

Photon

Thylakoid

Light-harvestingcomplexes

Reactioncenter

PhotosystemSTROMA

Transferof energy Special

chlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

2. Energy is then funneled to Chlorophyll a (P680) in the reaction center of PS II.

H2O CO2

LIGHT REACTIONS

CALVIN CYCLE

[CH2O] (sugar)

Photosystem II (PS II)

Primary acceptor

3. Electron from “excited” Chlorophyll a is captured by Primary Acceptor Molecule.

H2O CO2

LIGHT REACTIONS

CALVIN CYCLE

[CH2O] (sugar)

Primary acceptor

4.An enzyme splits a H2O into 2 electrons, 2 H+ ions, and 1 Oxygen atom (O2 released).

H2O CO2

LIGHT REACTIONS

CALVIN CYCLE

[CH2O] (sugar)

Primary acceptor

O21⁄2

5. The electron from water replaces the “excited” electron lost by Chlorophyll a.

H2O CO2

LIGHT REACTIONS

CALVIN CYCLE

[CH2O] (sugar)

Primary acceptor

O21⁄2

6. As the “excited” electron falls back to its “normal” state, energy is harnessed to form ATP.

H2O CO2

LIGHT REACTIONS

CALVIN CYCLE

[CH2O] (sugar)

Primary acceptor

Cytochromecomplex

Electron transport chain

7. Light energy also excites an electron of Chlorophyll a (P700) in Photosystem I.

H2O CO2

LIGHT REACTIONS

CALVIN CYCLE

[CH2O] (sugar)

Primary acceptor

Cytochromecomplex

Primary acceptor

Photosystem I (PS I)

7. The Primary Acceptor Molecule of PS I captures that excited electron (replaced by the “spent” electron from the electron transport chain).

H2O CO2

LIGHT REACTIONS

CALVIN CYCLE

[CH2O] (sugar)

Primary acceptor

Cytochromecomplex

Primary acceptor

8. The excited electron of PS I passes through a shorter electron transport chain to NADP+ reducing it to NADPH.

LIGHT REACTIONS

CALVIN CYCLE

[CH2O] (sugar)

Primary acceptor

Cytochromecomplex

Primary acceptor

ElectronTransportchain

reductaseNADPH

+ 2 H+

A mechanical analogy for the light reactions

Millmakes

e–e–

Photosystem II Photosystem I

The Light Reactions and Chemiosmosis: The Organization of the Thylakoid

Membrane

The Light Reactions and Chemiosmosis: The Organization of the Thylakoid

Membrane

LIGHTREACTOR

CALVINCYCLE

[CH2O] (sugar)STROMA(Low H+ concentration)

Photosystem II

H2O CO2

Cytochromecomplex

Photosystem ILight

THYLAKOID SPACE(High H+ concentration)

STROMA(Low H+ concentration)

Thylakoidmembrane

ATPsynthase

reductase

NADPH + H+

NADP+ + 2H+

ToCalvincycle

2 H++2 H+

ATP Synthase, a Molecular Mill

STROMA

THYLAKOID SPACE

A rotor within the membrane spins clockwise whenH+ flows pastit down the H+

gradient.

A stator anchoredin the membraneholds the knobstationary.

A rod (or “stalk”)extending into the knob alsospins, activatingcatalytic sites inthe knob.

Three catalytic sites in the stationary knobjoin inorganic Phosphate to ADPto make ATP.

Comparison of chemiosmosis in mitochondria and

chloroplasts

Comparison of chemiosmosis in mitochondria and

chloroplastsKey

Higher [H+]

Lower [H+]

Mitochondrion Chloroplast

MITOCHONDRIONSTRUCTURE

Intermembrancespace

Membrance

Matrix

Electrontransport

H+ DiffusionThylakoidspace

Stroma

ATPSynthase

CHLOROPLASTSTRUCTURE

A. Carbon Fixation A. Carbon Fixation - CO2 attached to Ribulose

Bisphosphate (RuBP), a 5-C sugar

- The 6-C unstable intermediate molecule breaks down to 2 molecules of 3-Phosphoglycerate

The Calvin Cycle

LightH2O CO2

LIGHTREACTIONS

[CH2O] (sugar)

CALVINCYCLE

(Entering oneat a time)CO2

Phase 1: Carbon fixation

Rubisco

Short-livedintermediate

Ribulose bisphosphate(RuBP)

3-Phosphoglycerate6 ATP

CALVINCYCLE

B. Reduction B. Reduction - each 3-Phosphoglycerate gets a

phosphate from ATP to form 1,3-Bisphosphoglycerate

- 2 electrons from NADPH reduces 1,3-Bisphosphoglycerate to G3P

- one G3P molecule (out of every 6) is released: 2 G3P’s = 1 glucose

The Calvin Cycle

Rubisco

3-Phosphoglycerate

1,3-Bisphosphoglycerate

6 NADPH

6 NADP+

Glyceraldehyde-3-phosphate(G3P)

Phase 2:Reduction

CALVINCYCLE

G3P(a sugar)Output

Glucose andother organiccompounds

InputLightH2O CO2

LIGHTREACTIONS

[CH2O] (sugar)

CALVINCYCLE

C. Regeneration of RuBP C. Regeneration of RuBP - 5 G3P molecules are used to

make 3 RuBP molecules

- uses 3 ATP’s

- cycle continues

For each G3P molecule produced, 9 ATP’s and 6 NADPH’s used

The Calvin cycle

Rubisco

3-Phosphoglycerate

1,3-Bisphosphoglycerate

6 NADPH

6 NADP+

Glyceraldehyde-3-phosphate(G3P)

Phase 2:Reduction

3 ADP CALVINCYCLE

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

G3P(a sugar)Output

Glucose andother organiccompounds

LightH2O CO2

LIGHTREACTIONS

[CH2O] (sugar)

CALVINCYCLE

A Review of PhotosynthesisA Review of

Photosynthesis

Light reactions:• Are carried out by molecules in the thylakoid membranes• Convert light energy to the chemical energy of ATP and NADPH• Split H2O and release O2 to the atmosphere

Calvin cycle reactions:• Take place in the stroma• Use ATP and NADPH to convert CO2 to the sugar G3P• Return ADP, inorganic phosphate, and NADP+ to the light reactions

CO2H2O

Light reactions Calvin cycle

RuBP 3-Phosphoglycerate

Amino acidsFatty acids

Starch(storage)

Sucrose (export)

Photosystem IIElectron transport chain

Photosystem I

Chloroplast

V.Cyclic Electron FlowV.Cyclic Electron Flow- an alternate, “short circuit”, path

involving only PS I

- electron from Primary Electron Acceptor passes to Ferredoxin and then to Cytochrome Complex (electron transport chain)

- ATP produced but no NADPH

V.Cyclic Electron FlowV.Cyclic Electron Flow

Primaryacceptor

Cytochromecomplex

Primaryacceptor

reductase

ATPPhotosystem II(PS II)

Photosystem I(PS I)

Why? Why? - Noncyclic Electron Flow produces

ATP and NADPH in equal amounts but Calvin Cycle uses more ATP than NADPH

- Accumulated NADPH seems to cause a shift to Cyclic Electron Flow to correct the imbalance

VI. PhotorespirationVI. Photorespiration- C3 plants include 85% of all plants

- have a built-in inefficiency

- on hot, sunny days stomates close to conserve water

- closed stomates prevent CO2 from entering leaf

- O2 concentration in leaf increases while CO2 decreases

- Rubisco binds O2 to ribulose bisphosphate (instead of CO2) which splits forming a 2-carbon compound which is released as CO2

- Photo (light) & respiration (consumes O2 and produces CO2)

- Uses up ATP (rather than producing ATP)

~ 50% of fixed carbon lost from Calvin Cycle by photorespiration

- reduces photosynthetic output and crop yield

VII. C4 PlantsVII. C4 Plants- found in corn, sugarcane,

crabgrass, etc.

- an adaptation to hot, dry climate (avoids the problem of photorespiration)

- involves unique leaf anatomy

VII. C4 PlantsVII. C4 Plants

CorCornn

Sugar Sugar CaneCane

EndEnd

Photosynthesis Animation

Photosynthesis Chapter 9. Plant Kingdom -produce most of the food in the biosphere - Autotrophs:...

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