PhotosynthesisBiology

1. Explain why photosynthesis is so important to energy and material flow for life on earth.

2. Know why plants tend to be green in appearance.3. Describe the organization of the chloroplast – the organelle of

photosynthesis in eukaryotes4. Understand that photosynthesis is a two fold process composed of

the light-dependent reactions and the light independent reactions 5. Explain where the light reactions and the CO2 fixation reactions

occur in the chloroplast.6. Define photosynthetic pigment composition and structure.7. Explain the absorption spectrum of chlorophyll and compare it to

the action spectrum of photosynthesis.

LEARNING OBJECTIVES

By the end of this lecture you will be able to:

• Almost all plants are photosynthetic autotrophs, as are some bacteria and protists

– Autotrophs generate their own organic matter through photosynthesis– Sunlight energy is transformed to energy stored in the form of chemical

bonds

(a) Mosses, ferns, andflowering plants

(b) Kelp

(c) Euglena (d) Cyanobacteria

PHOTOSYNTHESISPHOTOSYNTHESIS

Harvesting Light Energy

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

PHOTOSYNTHESISPHOTOSYNTHESIS

WHWHY ARY ARE PE PLANLANTS GTS GREREEN?EN?

It's not that easy bein' green Having to spend each day the color of the leaves When I think it could be nicer being red or yellow or gold Or something much more colorful like that…

Kermit the Frog

PHOTOSYNTHESISPHOTOSYNTHESIS

Electromagnetic Spectrum and Visible Light

Gammarays X-rays UV

Infrared & Microwaves Radio waves

Visible light

Wavelength (nm)

Different wavelengths of visible light are seen by the human eye as different colors.

WHYWHY ARE ARE PLA PLANTS NTS GREGREEN?EN?

Gammarays

X-rays UV Infrared Micro-waves

Radiowaves

Visible light

Wavelength (nm)

Sunlight minus absorbed Sunlight minus absorbed wavelengths or colors equals wavelengths or colors equals the apparent color of an the apparent color of an object.object.

The feathers of male cardinals are loaded with carotenoid pigments. These pigments absorb some wavelengths of light and reflect others.

Reflected light

Why are plants green?

Reflected lig

ht

Transmitted light

WHYWHY ARE ARE PLA PLANTS NTS GREGREEN? EN? Plant Cells have Green Chloroplasts

The thylakoid The thylakoid membrane membrane of the chloroplast is impregnated with photosynthetic pigments (i.e., chlorophylls, carotenoids).

Pigments

Pigments are lipids Plants use PIGMENTSPIGMENTS to

capture sun’s energy A pigment is a substance

that absorbs and reflects light.

The main pigment in plants is chlorophyll a Chlorophyll b Carotenoids

Chlorophyll molecules are embedded in the thylakoid membrane

Act like a light “antenna”

These molecules can absorb sunlight energy

LightReflected

light

Absorbedlight

Transmittedlight

Chloroplast

THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED

Chloroplast Chloroplast PigmentsPigments

Chloroplasts contain several pigments

– Chlorophyll a – Chlorophyll b – Carotenoids

Figure 7.7

Different pigments absorb light differently

Chloroplast PigmentsChloroplast Pigments

Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water

PHOTOSYNTHESISPHOTOSYNTHESIS

Carbondioxide

Water Glucose Oxygengas

PHOTOSYNTHESIS

carbon dioxide + water glucose + oxygen + water + energy

6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

– All green plant parts have chloroplasts and carry out photosynthesis

• The leaves have the most chloroplasts

• The green color comes from chlorophyll in the chloroplasts

• The pigments absorb visible light energy

• What wavelength is reflected?

PHOTOSYNTHESISPHOTOSYNTHESIS

internal leaf structure

chloroplasts

outer membrane

inner membrane

thylakoid

Chloroplasts: Sites of Photosynthesis In most plants,

photosynthesis occurs primarily in the chloroplasts in the leaves

A chloroplast contains: stroma, a fluid grana, stacks of

thylakoids The thylakoids contain

chlorophyll Chlorophyll is the green

pigment that captures light for photosynthesis

PHOTOSYNTHESISPHOTOSYNTHESIS

LEAF CROSS SECTION MESOPHYLL CELL

LEAF

Chloroplast

Mesophyll

CHLOROPLAST Intermembrane space

Outermembrane

Innermembrane

ThylakoidcompartmentThylakoidStroma

Granum

StromaGrana

Structure of Chloroplast

LightChloroplast

Lightreactions

Calvincycle

NADP

ADP+ P

The light reactions light reactions convert solar energy to chemical energy

Produce ATP & NADPH

The Calvin cycle Calvin cycle makes sugar from carbon dioxide

ATP generated by the light reactions provides the energy for sugar synthesis

The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose

PHOTOSYNTHESIS

Excitedstate

e

Heat

Light

Photon

Light(fluorescence)

Chlorophyllmolecule

Groundstate

2

(a) Absorption of a photon

(b) fluorescence of isolated chlorophyll in solution

Excitation of chlorophyll in a chloroplast

Loss of energy due to heat causes the photons of light to be less energetic.

Less energy translates into longer wavelength.

Transition toward the red end of the visible spectrum.

ELECTRONS AGAIN!

In the light reactions, electron In the light reactions, electron transport chains generate ATP, transport chains generate ATP, NADPH, & ONADPH, & O22

Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons

The excited electrons are passed from the primary electron acceptor to electron transport chains Their energy ends up

in ATP and NADPH

PhotosynthesisPhotosynthesis

Reaction-center

chlorophyll

Primaryelectronacceptor

Primaryelectronacceptor

Primaryelectronacceptor

1

How the Light Reactions Generate ATP and NADPH

PhotosynthesisPhotosynthesis

Primaryelectron acceptor

Primaryelectron acceptor

Electron transport chain

Electron transport

Photons

PHOTOSYSTEM I

PHOTOSYSTEM II

Energy forsynthesis of

by chemiosmosis

Plants produce OPlants produce O22 gas by splitting gas by splitting HH22OO

The O2 liberated by photosynthesis is made from the oxygen in water (H+ and e-)

Chemiosmosis powers ATP synthesis in the light reactions

The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane The flow of H+ back

through the membrane is harnessed by ATP synthase to make ATP

In the stroma, the H+ ions combine with NADP+ to form NADPH

PhotosynthesisPhotosynthesis

The production of ATP by chemiosmosis in photosynthesis

Thylakoidcompartment(high H+)

Thylakoidmembrane

Stroma(low H+)

Light

Antennamolecules

Light

ELECTRON TRANSPORT CHAIN

PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE

The Calvin Cycle

.

a.k.a. the “dark reactions”

WARNING: DOES NOT ONLY TAKE PLACE IN THE DARK!

Accomplishes “carbon fixation” taking carbon from inorganic molecules and putting it into organic molecules (CO2 sugars)

The Calvin Cycle

.

The quantities listed are for the production of one glucose molecule which is produced from six turns of the cycle

Chloroplast

Light

Stack ofthylakoids ADP

+ P

NADP

Stroma

Lightreactions

Calvincycle

Sugar used for

Cellular respiration Cellulose

Starch

Other organic compounds

PhotosynthesisPhotosynthesis

It's not that It's not that easy bein' easy bein' green… but it green… but it is essential for is essential for life on earth!life on earth!