PHOTOSYNTHESIS

Energy can be transformed from one form to another FREE ENERGY (available for work) vs. HEAT (not available for work)

THE SUN: MAIN SOURCE OF ENERGY FOR LIFE ON EARTH

THE BASICS OF PHOTOSYNTHESIS Almost all plants are photosynthetic autotrophs, as are some bacteria and protists Phototrophs generate their own organic matter through photosynthesis Sunlight energy is transformed to energy stored in the form of chemical bonds (c) Euglena (d) Cyanobacteria (b) Kelp (a) Mosses, ferns, and flowering plants

Food Chain

THE FOOD WEB

WHY ARE PLANTS GREEN? 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

Electromagnetic Spectrum and Visible Light Gamma rays Infrared & Microwaves X-rays UV Radio waves Visible light Wavelength (nm)

WHY ARE PLANTS GREEN? Different wavelengths of visible light are seen by the human eye as different colors. Gamma rays Micro- waves Radio waves X-rays UV Infrared Visible light Wavelength (nm)

The feathers of male cardinals are loaded with carotenoid pigments The feathers of male cardinals are loaded with carotenoid pigments. These pigments absorb some wavelengths of light and reflect others. Reflected light Sunlight minus absorbed wavelengths or colors equals the apparent color of an object.

Why are plants green? Reflected light Transmitted light

WHY ARE PLANTS GREEN? Plant Cells have Green Chloroplasts The thylakoid membrane of the chloroplast is impregnated with photosynthetic pigments (i.e., chlorophylls, carotenoids).

THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED Chloroplasts absorb light energy and convert it to chemical energy Reflected light Light Absorbed light Transmitted light Chloroplast

AN OVERVIEW OF PHOTOSYNTHESIS Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water Carbon dioxide Water Glucose Oxygen gas PHOTOSYNTHESIS

AN OVERVIEW OF PHOTOSYNTHESIS The light reactions convert solar energy to chemical energy Produce ATP & NADPH Light Chloroplast NADP ADP + P Calvin cycle Light reactions

AN OVERVIEW OF PHOTOSYNTHESIS The Calvin cycle or the dark reactions make 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 Light Chloroplast NADP ADP + P Calvin cycle Light reactions

Photosynthesis occurs in chloroplasts In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts Leaves generally have the most chloroplasts A chloroplast contains: stroma, a fluid grana, stacks of thylakoids The thylakoids contain chlorophyll Chlorophyll is the green pigment that captures light for photosynthesis

The location and structure of chloroplasts LEAF CROSS SECTION MESOPHYLL CELL LEAF Mesophyll CHLOROPLAST Intermembrane space Outer membrane Granum Inner membrane Grana Stroma Thylakoid compartment Stroma Thylakoid

Chloroplast Pigments Chloroplasts contain several pigments Chlorophyll a Chlorophyll b Carotenoids Figure 7.7

Chlorophyll a and b: primary pigment

Phytoerythrobilin: xanthophyll for bacteria

Β-carotene and lutein: accessory plant pigments

Different pigments absorb light differently

A light harvesting complex: LHCII

Chlorophyll and other pigments: antenna molecules:

The light-absorbing pigments of thylakoid or bacterial membranes are arranged in functional arrays called photosystems.

All the pigment molecules in a photosystem can absorb photons, but only a few chlorophyll molecules associated with the photochemical reaction center are specialized to transduce light into chemical energy.

The other pigment molecules in a photosystem are called light-harvesting or antenna molecules. They absorb light energy and transmit it rapidly and efficiently to the reaction center

Excitation of chlorophyll in a chloroplast Loss of energy due to heat causes the photons of light to be less energetic. But in nature, the structure of the light harvesting centers prevent the loss of energy as heat or even light. The proteins and other molecules allow the photochemical reactions to take place in a solid state manner e e Excited state 2 Heat Light Light (fluorescence) Photon Ground state Chlorophyll molecule (a) Absorption of a photon (b) fluorescence of isolated chlorophyll in solution

In plants, two photosynthetic reaction centers work in tandem Photosystem II and photosystem I work together to catalyze the light-driven movement of electrons from H2O to NADP+, essentially producing NADPH and ATP in the process The excited electrons are passed from the primary electron acceptor to electron transport chains Their energy ends up in ATP and NADPH

Two types of photosystems cooperate in the light reactions Photon ATP mill Photon Water-splitting photosystem NADPH-producing photosystem

Noncyclic Photophosphorylation Photosystem II regains electrons by splitting water, leaving O2 gas as a by-product Primary electron acceptor Electron transport chain Electron transport Photons PHOTOSYSTEM I PHOTOSYSTEM II Energy for synthesis of by chemiosmosis

Sample photosystem II: Synochoccus elongates cyanobacterium

Plants produce O2 gas by splitting H2O The O2 liberated by photosynthesis is made from the oxygen in water (H+ and e-)

The water-splitting complex (aka oxygen-evolving complex)

Plastocyanin and the cytochrome b6f complex connect the two photosystems

Plastocyanin and the cytochrome b6f complex connect the two photosystems

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

ELECTRON TRANSPORT CHAIN The production of ATP by chemiosmosis in photosynthesis Thylakoid compartment (high H+) Light Light Thylakoid membrane Antenna molecules Stroma (low H+) ELECTRON TRANSPORT CHAIN PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE

Bacteria: Cyclic Photophosphorylkation Process for ATP generation associated with some photosynthetic pacteria Reaction Center => 700 nm

Cyanobacteria utilize some electron carriers in both photophosphorylation and oxidative phosphorylation

How the Light Reactions Generate ATP and NADPH Primary electron acceptor NADP Energy to make Primary electron acceptor 3 2 Light Electron transport chain Light Primary electron acceptor Reaction- center chlorophyll 1 NADPH-producing photosystem Water-splitting photosystem 2 H + 1/2

Photosystem I: light to NADPH

Photosystem I: light to NADPH

NADPH and ATP produced by the light reactions is used to synthesize glucose

The 1st stage of CO2 assimilation in photosynthetic organisms: FIXATION

Rubisco: A photosynthetic VIP Rubisco, more properly known as ribulose 1,5-bisphosphate carboxylase, is the enzyme responsible for catalyzing the incorporation of 3 CO2 molecules into a single 3-phosphoglycerate molecule.

2nd stage of CO2 assimilation Conversion of 3-phosphoglycerate to glyceraldehyde 3-phosphate Essentially the reverse of the similar reaction in glycolysis, but NADPH is used instead of NADH.

3rd stage of CO2 assimilation Regeneration of ribulose 1,5-bisphosphate from triose phosphates

Overall: Photosynthesis uses light energy to make food molecules Chloroplast Photosystem II Electron transport chains Photosystem I CALVIN CYCLE Stroma Electrons Cellular respiration Cellulose Starch Other organic compounds LIGHT REACTIONS CALVIN CYCLE