Photosynthesis

Photosynthesis: Converting light energy into chemical energy

6CO2 + 12H2O + light energy C6H12O6 + 6O2 + 6H2O

Summary Formula:

6CO2 + 6H2O C6H12O6 + 6O2

Photosythesis provides Green Energy

One mature maple tree makes about 2 tons of sugar per season

Photoautotrophs capture sunlight and convert it to chemical energy

Photosynthesis is carried out by

•  Cyanobacteria •  7 groups of algae •  All land plants –

chloroplasts

In plants, photosynthesis occurs in chloroplasts

Mesophyll cells contain chloroplasts.

Outer membrane

Cutaway view of a chloroplast

Thylakoid membrane

Thylakoid lumen

Stromal lamellae

Granum

Stroma (space around thylakoids) • light-independent reactions

Thylakoids • Sites of light-

dependent reaction

Inner membrane

H2O

LIGHT REACTIONS

Chloroplast

Light

ATP

NADPH

O2

NADP+

CO2

ADP P + i CALVIN

CYCLE

[CH2O] (sugar)

Overview Photosynthesis

The light reaction occurs in the Thylakoids.

The light-independent Calvin cycle occurs in the Stroma.

Where does the oxygen come from?

Water is split during the light reaction

What is Light?

!  Light comes as electromagnetic waves. Visible light ranges from wavelengths of about 400 nm (violet) to 700 nm (red)

!  The shorter the wavelength, the greater the energy

!  Photons are discreet units of energy carried in light

Light is a form of electromagnetic radiation

Engelmann Experiment 1880: What part of light is necessary for Photosynthesis?

Strand of Spirogyra

Light A glass prism breaks up a beam of light into a spectrum of colors, which are cast across a microscope slide.

Bacteria

Chlorophyll Absorption Spectrum

!  Chlorophyll absorbs blue and red wavelengths most strongly

!  Chlorophyll a absorbs best at 425 & 680 nm

!  Chlorophyll b absorbs at 460 & 645 nm

!  Carotenoids absorb best at 450 & 490 nm

Action Spectrum

!  Chlorophylls and carotenoids work together to absorb photons during photosynthesis

Leaves are green because we see the reflected -not the absorbed- light

Structure of Chlorophyll

!  Chlorophylls are the major photosynthetic pigments in plants, green algae, and cyanobacteria

Light-absorbing head

in chlorophyll a in chlorophyll b

Hydrophobic side chain

The Light Reaction

Light

H2O CO2

O2

LIGHT REACTIONS

CALVIN CYCLE

ATP

NADPH

ADP NADP+!

[CH2O] (sugar)

Photon

Light-harvesting complexes

Photosystem

Reaction center

STROMA

Primary electron acceptor

e–

Transfer of energy

Special chlorophyll a molecules

Pigment molecules

THYLAKOID SPACE (INTERIOR OF THYLAKOID)

Thyl

akoi

d m

embr

ane

Photosystems

!  Photosystems are large complexes of light-absorbing pigments and proteins embedded in the thylakoid membranes to absorb light efficiently

Noncyclic Electron Flow

Ener

gy le

vel o

f ele

ctro

ns

Light energy

Photosystem II Primary acceptor

Plasto-quinone

pool

ATP synthase

Light energy

Plastocyanin

Photosystem I

Ferredoxin Primary acceptor

NADP+ reductase

To light-independent

reactions (Calvin cycle)

Cytochrome complex

ATP

Photosystem II

e–

e–

e–

e–

Mill makes

ATP

e–

e–

e–

Phot

on

Photosystem I

Phot

on

NADPH

Cyclic Electron Flow

ATP synthase To light-independent reactions

(Calvin cycle)

Plastocyanin

Ferredoxin

Cytochrome complex

Plasto-quinone

pool

Photosystem I Primary acceptor

Ferredoxin NADP+ reductase

Electron Transport Chain, Chemiosmosis and ATP synthesis

Light energy

Stroma (low proton concentration) Electron transfer

Antenna complex

Photosystem II

Thylakoid lumen (high proton concentration)

Thylakoid membrane

ATP synthase

To light-independent

reactions (Calvin cycle)

NADP+ reductase

Ferredoxin

Stator

Plastocyanin Water-splitting complex

Plastoquinone

Primary acceptor

Pigment molecules

Cytochrome complex

Light energy

Photosystem I

The Calvin Cycle (light-independent)

Light

H2O CO2

O2

LIGHT REACTIONS

CALVIN CYCLE

ATP

NADPH

ADP NADP+!

[CH2O] (sugar)

Calvin Cycle Summary

!  Carbon fixation – CO2 added to RuBP by Rubisco to produce two 3PGA molecules

!  Reduction – NADPH and ATP used to convert 3PGA into G3P, a higher energy molecule used to build sugars

!  Regeneration – remaining G3P molecules are used to recreate the starting material RuBP

Rubisco

Photosynthesis and Cellular Respiration both occur in Plant Cells

In hot and dry climates, plants convert to photorespiration

C4 plants separate carbon fixation and Calvin Cycle into different cells

Sugarcane

C4 plant

CALVIN CYCLE

3-C sugar

CO2

4-C compound

CO2 Mesophyll cell

Bundle-sheath cell

C4 plants

•  Include Corn, sugarcane, sorghum, and a number of other grasses

•  They initially fix carbon using PEP carboxylase in mesophyll cells

•  Oxaloacetate is produced (C4 compound), converted to malate, then transported to bundle-sheath cells

•  Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO2

•  Now Rubisco uses the released CO2, binds it to RuBP and the Calvin cycle can progress

CAM plants separate Carbon fixation and Calvin Cycle by time

Comparison C4 versus CAM plants

Sugarcane Pineapple

C4 CO2 CO2 CAM

Organic acid

Organic acid

Night

Day

CO2 CO2

Calvin Cycle

Calvin Cycle

Sugar Sugar

Bundle- sheath cell

(a) Spatial separation of steps

(b) Temporal separation of steps

Mesophyll cell

2

1 1

2

CAM plants

•  Many succulent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups

•  Stomata open during the night and close during the day – Reverse of that in most plants

•  Fix CO2 using PEP carboxylase during the night and store in vacuole

Adaptations for Photosynthesis in hot, dry climates

•  C3 – Plants that fix carbon using only C3

photosynthesis (the Calvin cycle)

•  C4 and CAM – Add CO2 to PEP to form 4 carbon molecule – Use PEP carboxylase – Greater affinity for CO2, no oxidase activity – C4 –two pathways occur in different cells – CAM – C4 pathway at night and the C3 pathway

during the day

Greenhouse Effect and Global Warming

Sunlight

ATMOSPHERE

Some heat energy escapes into space

Radiant heat trapped by CO2 and other gases

The Excess of CO2 in the atmosphere Is contributing to global warming