Where It Starts – Photosynthesis

Lecture 8 - Autumn 2007

Sunlight as an Energy Source

• Photosynthesis runs on a fraction of the electromagnetic spectrum, or the full range of energy radiating from the sun

Visible Light

• Wavelengths humans perceive as different colors

• Violet (380 nm) to red (750 nm)

• Longer wavelengths, lower energy

Electromagnetic Spectrum

Shortest Gamma rays

wavelength X-rays

UV radiation

Visible light

Infrared radiation

Microwaves

Longest Radio waves

wavelength

Photons

• Packets of light energy

• Each type of photon has fixed amount of energy

• Photons having most energy travel as shortest wavelength (deep blue sea - blue light travels the deepest because it has the most

energy)

Pigments

• Light-absorbing molecules

• Absorb some wavelengths and transmit others

• Color you see are the wavelengths not absorbed

Variety of Pigments

Chlorophylls a and b

Carotenoids

Etc.

ChlorophyllsMain pigments in most photoautotrophs

- Organisms which make their own foodW

avel

eng

th a

bso

rpti

on

(%

)

Wavelength (nanometers)

chlorophyll b

chlorophyll a

Carotenoids• Found in all photoautotrophs

• Absorb blue-violet and blue-green that chlorophylls miss

• Reflect red, yellow, orange wavelengths

• Two types– Carotenes - pure

hydrocarbons

– Xanthophylls - contain oxygen

Xanthophylls

Yellow, brown, purple, or blue accessory pigments

Phycobilins & Anthocyanins

Red to purple pigments

• Phycobilins– Found in red algae and cyanobacteria

• Anthocyanins– Give many flowers their colors

T.E. Englemann’s Experiment - simple elegance - illuminate a long strand of photosynthetic algae with a spectrum of light, and watch were the mobile bacteria

accumulate!

1. Pigments absorb light energy, give up e- which enter electron transfer chains

2. Water molecules are split, ATP and NADH are formed, and oxygen is released (waste product - lucky for us!)

3. Pigments that gave up electrons get replacements

Light-Dependent Reactions

• Synthesis part of photosynthesis

• Can proceed in the dark

• Take place in the stroma (special

structures inside the chloroplast)

• Calvin-Benson cycle

• Synthesis part of photosynthesis

• Can proceed in the dark

• Take place in the stroma (special

structures inside the chloroplast)

• Calvin-Benson cycle

Light-Independent Reactions

Photosynthesis Equation- remember forever

Organelles of photosynthesis

Chloroplasts

leaf’s upper surface

vein stoma (gap) in lower epidermis

section from the leaf, showing its internal organization

photosynthetic cells

central vacuole

chloroplast

one photosynthetic cell inside the leaf

Inside the Chloroplast

• Two outer membranes enclose a semifluid interior, the stroma

• Thylakoid membrane inside the stroma

two outer membranes

thylakoidmembranesystem

stroma

chloroplastssee next slide

Inside the Chloroplast• Photosystems

are embedded in thylakoids, containing 200 to 300 pigments and other molecules that trap sun’s energy

• Two types of photosystems: I and II

electron transfer

chain

light harvesting complex

PHOTOSYSTEM IPHOTOSYSTEM II

thylakoidcompartment

thylakoidmembrane

• Photoautotrophs

– Carbon source is carbon dioxide

– Energy source is sunlight (mostly)

• Heterotrophs

– Get carbon and energy by eating autotrophs

(plants) and/or one another (cannibalism too)

Carbon and Energy Sources

Photoautotrophs

• Capture sunlight energy and use it to carry out photosynthesis

– Plants

– Some bacteria

– Many protistans

Linked Processes

Photosynthesis

• Energy-storing pathway

• Releases oxygen

• Requires carbon dioxide

Aerobic Respiration

• Energy-releasing pathway

• Requires oxygen

• Releases carbon dioxide

Photosystem Function: Harvester Pigments

• Most pigments in photosystem are harvester pigments– When excited by light energy, these

pigments transfer energy to adjacent pigment molecules

• Each transfer involves some energy loss - heat and atomic motion

photon a light-harvestingcomplex has aring of pigmentmolecules

A photosystem is surrounded by densely packed light harvesting complexes.

ATP and NADPH Formation

NADPH

NADPH + H+

thylakoidcompartment

thylakoidmembrane

stroma

ATPADP + Pi

H+

H+

H+H+

H+

H+

H+ H+

H+H+

H+

PHOTOSYSTEM IsunlightPHOTOSYSTEM II

LIGHT-HARVESTINGCOMPLEX

Using the Products of Photosynthesis

• Phosphorylated glucose is the building block for:

– Sucrose• The most easily transported plant carbohydrate

– Starch• The most common storage form

Summary of Photosynthesissunlight

Calvin-Benson

cycle

Light-DependentReactions

end products (e.g., sucrose, starch, cellulose)

ATP

12 PGAL

Light-Independent

Reactions

phosphorylated glucose

6H2O

6 RuBP

12H2O 6O2

NADPH NADP+

6CO2

ADP + Pi