Photosynthesis

• Almost all plants are photosynthetic autotrophs, as are some bacteria and protists– Autotrophs generate their own organic matter

through photosynthesis– Sunlight is transformed to energy stored in the

form of chemical bonds

(a) Mosses, ferns, andflowering plants

(b) Kelp(c) Euglena (d) Cyanobacteria

THE BASICS OF PHOTOSYNTHESIS

Why is Photosynthesis important?

Makes organic molecules (glucose) out of inorganic materials (carbon dioxide and water).It begins all food chains/webs. Thus all life is supported by this process.It also makes oxygen gas!!

Photosynthesis-starts to ecological food webs!

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.

WHY ARE PLANTS GREEN?

Gammarays X-rays UV Infrared Micro-

wavesRadiowaves

Visible light

Wavelength (nm)

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

Why are plants green?

Reflected 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).

• Chloroplasts absorb light energy and convert it to chemical energy

LightReflected

light

Absorbedlight

Transmittedlight

Chloroplast

THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED

Plants use sunlight to turn water and carbon dioxide into glucose. Glucose is a kind of sugar. Plants use glucose as food for energy and as a building block for growing.Autotrophs make glucose and heterotrophs are consumers of it.

Photo-synthesismeans "putting together with light."

PHOTOSYNTHESIS• Absorbing Light Energy to make

chemical energy: glucose!– Pigments: Absorb different colors of

white light (ROY G BIV)• Main pigment: Chlorophyll a• Accessory pigments: Chlorophyll b and

Carotenoids (Carotene & Xanthophyll)• These pigments absorb all wavelengths

(light) BUT not green!• Maximum absorption of red & blue light.

Chloroplasts: Sites of Photosynthesis• Photosynthesis– Occurs in chloroplasts, organelles in certain

plants– 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 light energy

• In most plants, photosynthesis occurs primarily in the leaves, in the 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

Photosynthesis occurs in chloroplasts

• The location and structure of chloroplasts

LEAF CROSS SECTION MESOPHYLL CELLLEAF

Chloroplast

Mesophyll

CHLOROPLAST Intermembrane space

Outermembrane

Innermembrane

ThylakoidcompartmentThylakoidStroma

Granum

StromaGrana

• Chloroplasts contain several pigmentsChloroplast Pigments

– Chlorophyll a – Chlorophyll b – Carotenoids

Figure 7.7

Chlorophyll a & b• Chl a has a methyl

group • Chl b has a carbonyl

group

Porphyrin ring delocalized e-

Phytol tail

Different pigments absorb light differently

Excitedstate

e

Heat

Light

Photon

Light(fluorescence)

Chlorophyllmolecule

Groundstate

2

(a) Absorption of a photon

Excitation of chlorophyll in a chloroplast

e

Fall Colors

• During the fall, the green chlorophyll pigments are greatly reduced revealing the other pigments.

• Carotenoids are pigments that are either red or yellow.

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

AN OVERVIEW OF PHOTOSYNTHESIS

• The 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

LightChloroplast

Lightreactions

Calvincycle

NADP

ADP+ P

• The light reactions convert solar energy to chemical energy– Produce ATP &

NADPH

AN OVERVIEW OF PHOTOSYNTHESIS

• In plants and simple animals, waste products are removed by diffusion. Plants, for example, excrete O2, a product of photosynthesis.

Redox Reaction• The transfer of one or more electrons from one

reactant to another.

• Two types:1. Oxidation2. Reduction

Oxidation Reaction• The loss of electrons from a substance.• Or the gain of oxygen.

glucose

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

Oxidation

Reduction Reaction• The gain of electrons to a substance.• Or the addition of hydrogen.

glucose

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

Reduction

PHOTOSYNTHESIS• 2 Phases

– Light-dependent reaction– Light-independent reaction

• Light-dependent: converts light energy into chemical energy; produces ATP and NADPH molecules to be used to fuel light-independent reaction

• Light-independent: uses this ATP to make simple sugars/ glucose

PHOTOSYNTHESIS• Light-dependent reaction (LIGHT

Reaction)– Requires light– Occurs in chloroplast (in thylakoids)– Chlorophyll (thylakoid) traps energy from

light– Light excites electron (e-)

• Kicks e- out of chlorophyll to an electron transport chain

• Electron transport chain: series of proteins in thylakoid membrane

PHOTOSYNTHESIS• Light-dependent reaction (LIGHT

Reaction)– Energy lost along electron transport

chain– Lost energy used to recharge ATP from

ADP

– NADPH produced from e- transport chain• Stores energy until transfer to stroma• Plays important role in light-independent

reaction

– Total byproducts: ATP, NADP, O2

1. Light Reaction (Electron Flow)

• During the light reaction, there are two possible routes for electron flow.

A. Cyclic Electron FlowB. Noncyclic Electron Flow

Phot

on

Photon

Water-splittingphotosystem

NADPH-producingphotosystem

ATPmill

• Two types of photosystems cooperate in the light reactions

2 H + 1/2

Water-splittingphotosystem

Reaction-center

chlorophyll

Light

Primaryelectronacceptor

Energyto make

Electron transport chain

Primaryelectronacceptor

Primaryelectronacceptor

NADPH-producingphotosystem

Light

NADP

1

23

How the Light Reactions Generate ATP and NADPH

A. Cyclic Electron Flow

• Occurs in the thylakoid membrane.• Uses PS I only• P700 reaction center- chlorophyll a • Uses Electron Transport Chain (ETC)• Generates ATP only

ADP + ATPP

B. Noncyclic Electron Flow

• Occurs in the thylakoid membrane

• Uses PS II and PS I

• P680 rxn center (PSII) - chlorophyll a

• P700 rxn center (PS I) - chlorophyll a

• Uses Electron Transport Chain (ETC)

• Generates O2, ATP and NADPH

B. Noncyclic Electron Flow• ADP + ATP

• NADP+ + H NADPH

• Oxygen comes from the splitting of H2O, not CO2

H2O 1/2 O2 + 2H+

P(Reduced)

Noncyclic Photophosphorylation • Photosystem II regains electrons by splitting

water, leaving O2 gas as a by-product

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

Plants produce O2 gas by splitting H2O

Chemiosmosis• Powers ATP synthesis.• Located in the thylakoid membranes.• Uses ETC and ATP synthase (enzyme) to

make ATP.• Photophosphorylation: addition of

phosphate to ADP to make ATP.

ChemiosmosisH+ H+

ATP Synthase

H+ H+ H+ H+

H+ H+ high H+

concentration

H+ADP + P ATP

PS II PS IE

TC

low H+

concentration

H+

ThylakoidSpace

Thylakoid

SUN (Proton Pumping)

• 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

Chemiosmosis powers ATP synthesis in the light reactions

PHOTOSYNTHESIS• Light-independent reaction (Dark

Reaction)– Does not require light– Calvin Cycle

• Occurs in stroma of chloroplast• Requires CO2• Uses ATP and NADPH as fuel to run• Makes glucose sugar from CO2 and Hydrogen

Calvin Cycle• Carbon Fixation (light independent rxn).

• C3 plants (80% of plants on earth).

• Occurs in the stroma.

• Uses ATP and NADPH from light rxn.

• Uses CO2.

• To produce 1 glucose: it takes 2 turns and uses 18 ATP and 12 NADPH.

Chloroplast

GranumThylakoidStroma

Outer MembraneInner Membrane

• A Photosynthesis Road Map

Chloroplast

Light

Stack ofthylakoids ADP

+ P

NADP

Stroma

Lightreactions

Calvincycle

Sugar used for

Cellular respiration Cellulose Starch Other organic compounds

PHOTOSYNTHESIS• What affects photosynthesis?

– Light intensity: as light increases, rate of photosynthesis increases

PHOTOSYNTHESIS• What affects photosynthesis?

– Carbon Dioxide: As CO2 increases, rate of photosynthesis increases

PHOTOSYNTHESIS• What affects photosynthesis?

– Temperature: • Temperature Low = Rate of photosynthesis

low• Temperature Increases = Rate of

photosynthesis increases• If temperature too hot, rate drops

Concepts

• Photosynthesis: CO2 + Water --> Sugar + O2

– Photosynthesis is the production of sugar (stored energy) and oxygen using energy from the sun to combine carbon dioxide and water.

– CO2 is brought into plants and O2 is released from plants through pores (stomata) in their leaves and other tissues.

– RUBISCO is the enzyme plants use to undergo photosynthesis.

+ SolarEnergy

Stomata

Concepts

• Respiration: Sugar + O2 --> CO2 + Water + E– Respiration is the burning of sugar in the presence

of oxygen to release energy stored in the sugar and produces carbon dioxide and water as by-products.

• Photorespiration: Occurs under high light/heat when RUBISCO tends to react with O2 (undergoing respiration) rather than CO2 (undergoing photosynthesis). This slows rates of photosynthesis under high light/heat (this is not what the plant wants to happen).

Energy

Concepts

• Transpiration: Loss of water out of stomata (pores) of plants during gas exchange (O2 and CO2) while photosynthesizing and respiring.

• Water Use Efficiency (WUE): How good a plant is at bringing in CO2 without losing too much water. In other words it is the ratio of rate of photosynthesis (energy generation) to rate of transpiration (water lost).

Stoma

AP Biology

Leaf Structure

H2O

CO2

O2 H2O

phloem (sugar)xylem (water)

stomate guardcell

palisadeslayer

spongylayer

cuticleepidermis

O2 CO2

Transpiration

vascular bundle

Gas exchange

AP Biology

Controlling water loss from leaves Hot or dry days

stomates close to conserve water guard cells

gain H2O = stomates open lose H2O = stomates close

adaptation to living on land, but…

creates PROBLEMS!

AP Biology

When stomates close…

xylem (water)

phloem (sugars)

H2OO2 CO2

Closed stomates lead to… O2 build up from light reactions CO2 is depleted in Calvin cycle

causes problems in Calvin Cycle

AP Biology

Inefficiency of RuBisCo: CO2 vs O2 RuBisCo in Calvin cycle

carbon fixation enzyme normally bonds C to RuBP CO2 is the optimal substrate reduction of RuBP building sugars

when O2 concentration is high RuBisCo bonds O to RuBP O2 is a competitive substrate oxidation of RuBP breakdown sugars

photosynthesis

photorespiration

AP Biology

Calvin cycle when O2 is high

5CRuBP

3C2C

to mitochondria–––––––lost as CO2 without making ATP

photorespiration

O2

RuBisCo

AP Biology

Impact of Photorespiration Oxidation of RuBP

short circuit of Calvin cycle loss of carbons to CO2

can lose 50% of carbons fixed by Calvin cycle reduces production of photosynthesis

no ATP (energy) produced no C6H12O6 (food) produced

if photorespiration could be reduced, plant would become 50% more efficient strong selection pressure to evolve

alternative carbon fixation systems

AP Biology

Reducing photorespiration Separate carbon fixation from Calvin cycle

C4 plants separate carbon fixation from Calvin cycle by ANATOMY

different cells to fix carbon vs. where Calvin cycle occurs store carbon in 4C compounds

different enzyme to capture CO2 (fix carbon) PEP carboxylase

different leaf structure CAM plants

separate carbon fixation from Calvin cycle by TIME OF DAY fix carbon during night

store carbon in 4C compounds perform Calvin cycle during day

AP Biology

C4 plants A better way to capture CO2

1st step before Calvin cycle, fix carbon with enzymePEP carboxylase store as 4C compound

adaptation to hot, dry climates have to close stomates a lot different leaf anatomy

sugar cane, corn, other grasses…

sugar cane

corn

AP Biology

C4 leaf anatomyPEP (3C) + CO2 oxaloacetate (4C)

CO2

CO2

O2

light reactions

C4 anatomy

C3 anatomy PEP carboxylase enzyme

higher attraction for CO2 than O2

better than RuBisCo fixes CO2 in 4C compounds regenerates CO2 in inner cells for RuBisCo

keeping O2 away from RuBisCo

bundlesheathcell RuBisCo

PEPcarboxylase

stomate

AP Biology

CAM (Crassulacean Acid Metabolism) plants Adaptation to hot, dry climates

separate carbon fixation from Calvin cycle by TIME close stomates during day open stomates during night

at night: open stomates & fix carbonin 4C “storage” compounds

in day: release CO2 from 4C acids to Calvin cycle increases concentration of CO2 in cells

succulents, some cacti, pineapple

AP Biology

CAM plants

succulents

cacti

pineapple

AP Biology

C4 vs CAM Summary

C4 plants separate 2 steps of C fixation anatomically in 2 different cells

CAM plants separate 2 steps of C fixation temporally =2 different timesnight vs. day

solves CO2 / O2 gas exchange vs. H2O loss challenge

AP Biology

Why the C3 problem? Today it makes a difference

21% O2 vs. 0.03% CO2

photorespiration can drain away 50% of carbon fixed by Calvin cycle on a hot, dry day

strong selection pressure to evolve better way to fix carbon & minimize photorespiration

AP Biology

AP Biology

FACTORS NECESSARY FOR PHOTOSYNTHESIS

A number of factors affect the process of photosynthesis, as a result of which productivity is affected. These are

Carbon dioxide Water Chlorophyll Light

AP Biology

Principle of limiting factors The Principle of limiting factors also

states that when a biochemical process is affected by several factors, its rate is limited by that factor which is nearest its minimum value. That factor (known as limiting factor) directly affects the biochemical process if its quantity is changed.

AP Biology

CARBON DIOXIDE (CO2) Air contains 0.03% of CO2. It is released by

respiration, combustion of fossil fuels and microbial decomposition.

During early morning hours and evening hours, CO2 released in respiration is sufficient for photosynthesis. At this stage, there is no exchange of gases between the plant and the environment. This is called compensation point.

AP Biology

WE HAVE TEST ON NEXT THRUSDAY