Photosynthesis

Introduction to BiologyPpt from aurumscience.com

• How does a tree gain mass as it grows?• Law of Conservation of Mass: Mass cannot

be created or destroyed, it only changes form.

Van Helmont’s Experiment

• Jan Baptista van Helmont, a scientist from Belgium, conducted an experiment to determine the source of a tree’s mass.o He grew a Willow tree in a pot for 5 years and re-

measured the mass.o The Willow tree grew by 74kg, but the mass of the

soil changed very little.o Van Helmont concluded that the source of the plant’s

mass is water.

Woodward’s Experiment• John Woodward, a professor at Cambridge

university in the 1600s, decided to test this conclusion.o He measured the mass of water he added to the

plants.o He also measured the mass of the plants as they

grew.o After 77 days of plant growth, the plant increased in

mass by 1 gram. Over 76,000 grams of water had been added.

Priestley’s Experiment• Joseph Priestley believed that plants

changed the air somehow.• He placed a small mint plant in a jar with a

lit candle. o He closed the jar, the candle used up the oxygen, and

the flame extinguished.o After about a month, he was able to re-light the

candle, proving that the plant had changed the air by producing oxygen.

Priestley’s Second Experiment

• In his second experiment, Joseph Priestley kept a mouse in a closed jar of air until it collapsed.

• He then repeated the experiment, but included a large plant in the jar with the mouse. o The mouse survived!

The Answer• What are plants made of?

o Primarily carbohydrates such as cellulose, sucrose, fructose, etc.

o Carbohydrates are made of carbon, oxygen, and hydrogen.

• What would be the source of each of these elements for plants?

o Hydrogen: Watero Oxygen: Water o Carbon: ..?

Photosynthesis• Photo = “light”, Synthesis “to make”• Photosynthesis is using light energy to make

organic compounds such as sugars.

• Autotrophs are able to produce the molecules they need for life without eating anything.o Photoautotrophs use sunlight as their energy source.o Chemoautotrophs use non-living chemicals (like

Hydrogen sulfide gas) as their energy source

• Almost all plants are photoautotrophs.o Also includes algae, some protozoa, and some

bacteria.

LE 10-2

Plants

Unicellular protist

Multicellular algae Cyanobacteria

Purple sulfurbacteria

10 µm

1.5 µm

40 µm

• Heterotrophs obtain their organic material by eating other organisms

• Almost all heterotrophs, including humans, depend on photoautotrophs like plants for food and oxygen

• Energy from the sun travels to Earth in the form of light.

• Sunlight is a mixture of many different types of energy:o Ultraviolet: Invisible to us, causes sunburnso Visible Light: Wavelengths of light we can see, o Infrared: Energy in the form of heat

Energy in Sunlight

Energy• Our eyes see the different wavelengths of the

visible spectrum as different colors: red, orange, yellow, green, blue, indigo, and violet.

Pigments• Plants gather the sun’s energy

with light-absorbing molecules called pigments.

• The plants’ principal pigment is chlorophyll.o Chlorophyll is a green pigment.o Plants are green because chlorophyll

reflects green light and absorbs every other wavelength.

Pigments• There are two types of chlorophyll found in

plants, chlorophyll a and chlorophyll b.• Chlorophyll absorbs blue-violet and red light

very well, but not green.o Remember, green light is reflected, and not absorbed.

• A spectrophotometer measures a pigment’s ability to absorb various wavelengths

• This machine sends light through pigments and measures the fraction of light transmitted at each wavelength

Measuring Light Absorption

LE 10-8a

Whitelight

Refractingprism

Chlorophyllsolution

Photoelectrictube

Galvanometer

The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light.

Greenlight

Slit moves to pass light of selected wavelength

0 100

LE 10-8b

Whitelight

Refractingprism

Chlorophyllsolution

Photoelectrictube

The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light.

Bluelight

Slit moves to pass light of selected wavelength

0 100

• An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength

• The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis

LE 10-9a

Chlorophyll a

Chlorophyll b

Carotenoids

Wavelength of light (nm)

Absorption spectra

Ab

sorp

tio

n o

f lig

ht

by

chlo

rop

last

pig

men

ts

400 500 600 700

Pigments• Plant cells contain other pigments besides

chlorophyll that increase the wavelengths absorbed.o These are called carotenoids.

• During the summer, so much chlorophyll is produced that the green color overwhelms the other pigments.

• When temperatures drop, the plants stop producing chlorophyll, and the other pigments may be seen.

Chloroplasts• Photosynthesis takes place inside organelles

called chloroplasts.• Chloroplasts contain stacks called grana. • The grana contained stacked membranes

called thylakoids, which are interconnected.

Chloroplasts• Leaves are the major locations of

photosynthesis• Their green color is from chlorophyll, the

green pigment within chloroplasts• Light energy absorbed by chlorophyll drives

the reactions needed to produce sugars from carbon dioxide.

• The plant “breathes” through microscopic pores called stomata.o CO2 enters the leaf and O2 exits

Chloroplasts• Pigments are located in the thylakoid

membranes.• The fluid portion outside of the thylakoids is

known as the stroma.

Photosynthesis Equation

• Photosynthesis can be summarized in the following equation:

6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O

Carbon Water Sunlight Glucose Oxygen Water dioxide (Less)

LE 10-3

Leaf cross sectionVein

Mesophyll

Stomata CO2O2

Mesophyll cellChloroplast

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

Thylakoid

GranumStroma

1 µm

LE 10-4

Reactants:

Products:

6 CO2 12 H2O

C6H12O6 6 H2O 6 O2

Stages of Photosynthesis

• Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)

• The light reactions occur in the thylakoids of the chloroplast.o Splits water, releases O2, produces ATP and NADPH

• The Calvin cycle occurs in the stroma of the chloroplast.o Forms sugar from CO2 using ATP and NADPH

LE 10-5_1

H2O

LIGHTREACTIONS

Chloroplast

Light

LE 10-5_2

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

LE 10-5_3

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

NADP+

CO2

ADPP+ i

CALVINCYCLE

[CH2O](sugar)

ATP and NADPH• Chloroplasts are solar-powered chemical

factories• Their thylakoids transform light energy into

the chemical energy of ATP and NADPH.o These are small energy-containing molecules that can

be used to make glucose later.

LE 10-7

Chloroplast

Light

Reflected light

Absorbed light

Transmitted light

Granum

Absorption of Sunlight• When chlorophyll absorbs light, it goes from a

low-energy ground state to an high-energy excited state, which is unstable.

• When excited electrons fall back to the ground state, photons are given off causing fluorescence.

LE 10-11

Excitedstate

Heat

Photon(fluorescence)

GroundstateChlorophyll

molecule

Photon

Excitation of isolated chlorophyll molecule Fluorescence

En

erg

y o

f el

ectr

on

e–

The Photosystem• The basic unit of photosynthesis in the

thylakoid is called a photosystem. • A photosystem contains a reaction center

surrounded by light-harvesting complexes• The light-harvesting complexes (pigment

molecules) funnel the energy from photons of sunlight to the reaction center.

• The reaction center contains chlorophyll, which absorbs the energy from the photon.• This splits a water molecule into O2 , 2 H+ ions, and 2

electrons. • These electrons are energized and passed onto

another molecule called the primary electron acceptor.

LE 10-12

Thylakoid

Photon

Light-harvestingcomplexes

Photosystem

Reactioncenter

STROMA

Primary electronacceptor

e–

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Th

ylak

oid

mem

bra

ne

• There are two types of photosystems in the thylakoid membrane:• Photosystem II absorbs wavelengths of sunlight

680nm long.• Photosystem I then absorbs wavelengths of sunlight

700nm long.

• The two photosystems work together to use light energy to generate ATP and NADPH

LE 10-13_1

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2

En

erg

y o

f el

ectr

on

sO2

LE 10-13_2

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2

En

erg

y o

f el

ectr

on

sO2

e–

e–

+2 H+

H2O

O21/2

LE 10-13_3

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2

En

erg

y o

f el

ectr

on

sO2

e–

e–

+2 H+

H2O

O21/2

Pq

Cytochromecomplex

Electron transport chain

Pc

ATP

LE 10-13_4

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2

En

erg

y o

f el

ectr

on

s

O2

e–

e–

+2 H+

H2O

O21/2

Pq

Cytochromecomplex

Electron transport chain

Pc

ATP

P700

e–

Primaryacceptor

Photosystem I(PS I)

Light

LE 10-13_5

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADPCALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2E

ner

gy

of

elec

tro

ns

O2

e–

e–

+2 H+

H2O

O21/2

Pq

Cytochromecomplex

Electron transport chain

Pc

ATP

P700

e–

Primaryacceptor

Photosystem I(PS I)

e–e–

ElectronTransportchain

NADP+

reductase

Fd

NADP+

NADPH

+ H+

+ 2 H+

Light

LE 10-14

ATP

Photosystem II

e–

e–

e–e–

Millmakes

ATP

e–

e–

e–

Ph

oto

n

Photosystem I

Ph

oto

n

NADPH

LE 10-17

STROMA(Low H+ concentration)

Light

Photosystem IICytochrome

complex

2 H+

Light

Photosystem I

NADP+

reductase

Fd

PcPq

H2O O2

+2 H+

1/2

2 H+

NADP+ + 2H+

+ H+NADPH

ToCalvincycle

THYLAKOID SPACE(High H+ concentration)

STROMA(Low H+ concentration)

Thylakoidmembrane ATP

synthase

ATP

ADP+P

H+i

[CH2O] (sugar)O2

NADPH

ATP

ADPNADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

Light

Building Glucose• The Calvin cycle builds sugar from smaller

molecules by using ATP and NADPH• Carbon enters the cycle as CO2 and leaves as

a sugar named glyceraldehyde-3-phospate (G3P)o To make one G3P, the cycle must take place three

times, using up three molecules of CO2

• The Calvin cycle has three phases:o Three atoms of carbon from carbon dioxide

are added to the cycle using an enzyme called rubisco.• This creates a 6-carbon molecule

o ATP and NADPH is used to create two molecules of G3P • One leaves the cycle, one stays behind

o The original molecules in the cycle are then regenerated using more ATP

Play

LE 10-18_1

[CH2O] (sugar)O2

NADPH

ATP

ADPNADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

LightInput

3

CO2

(Entering oneat a time)

Rubisco

3 P P

Short-livedintermediate

Phase 1: Carbon fixation

6 P

3-Phosphoglycerate6 ATP

6 ADP

CALVINCYCLE

3 P P

Ribulose bisphosphate(RuBP)

LE 10-18_2

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

LightInput

CO2

(Entering oneat a time)

Rubisco

3 P P

Short-livedintermediate

Phase 1: Carbon fixation

6 P

3-Phosphoglycerate6 ATP

6 ADP

CALVINCYCLE

3

P P

Ribulose bisphosphate(RuBP)

3

6 NADP+

6

6 NADPH

P i

6 P

1,3-BisphosphoglycerateP

6 P

Glyceraldehyde-3-phosphate(G3P)

P1

G3P(a sugar)Output

Phase 2:Reduction

Glucose andother organiccompounds

LE 10-18_3

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

LightInput

CO2

(Entering oneat a time)

Rubisco

3 P P

Short-livedintermediate

Phase 1: Carbon fixation

6 P

3-Phosphoglycerate6 ATP

6 ADP

CALVINCYCLE

3

P P

Ribulose bisphosphate(RuBP)

3

6 NADP+

6

6 NADPH

P i

6 P

1,3-BisphosphoglycerateP

6 P

Glyceraldehyde-3-phosphate(G3P)

P1

G3P(a sugar)Output

Phase 2:Reduction

Glucose andother organiccompounds

3

3 ADP

ATP

Phase 3:Regeneration ofthe CO2 acceptor(RuBP) P5

G3P

Adaptations in Arid Environments

• Dehydration is a problem for plants, especially in hot, arid ecosystems.

• On hot, dry days, plants close their stomata, which conserves water but also limits photosynthesis.o Plants are unable to take in CO2 and remove O2.

• These conditions favor a seemingly wasteful process called photorespiration.

Photorespiration: An Evolutionary Relic?

• In photorespiration, O2 is added to the Calvin cycle instead of CO2

• This produces a molecule that must be sent to the mitochondria before it can be sent back and the Calvin cycle finished.o This uses more energy to produce G3P, and is much

less efficient for the plant.

• Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2

• In many plants, photorespiration is a problem because on a hot, dry day it can drain much of the plant’s ATP and NADPH.

C4 Plants• Some plants have an adaptation to manage

life in arid climates. These are called C4 plants.o Example: Sugar cane, corn

• These plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds and storing them in areas of the leaf less exposed to the dry air.

• These four-carbon compounds can be used to release carbon dioxide when the stomata are closed, allowing the Calvin cycle to continue like normal.

CAM Plants• CAM plants open their stomata at night,

incorporating CO2 into organic acids

• Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle

LE 10-20

Bundle-sheathcell

Mesophyllcell Organic acid

C4

CO2

CO2

CALVINCYCLE

Sugarcane Pineapple

Organic acidsrelease CO2 toCalvin cycle

CO2 incorporatedinto four-carbonorganic acids(carbon fixation)

Organic acid

CAMCO2

CO2

CALVINCYCLE

Sugar

Spatial separation of steps Temporal separation of steps

Sugar

Day

Night