Photosynthesis

The SunThe Ultimate Energy Source

For Life on Planet Earth

Pyramid of Biomass

The Sun

Photosynthesis (P/S)

Producers

Consumers

Autotrophs

Heterotrophs

Trophic Levels

The Equation

Note that there is carbon in CO2

This carbon came from CO2

Photosynthesis

6CO2 + 6H2 O C6 H12 O6 + 6O2Light

Chlorophyll

CO2: Carbon Dioxide

Air

78% N2

21% O2

.04% CO2

H2 O Exits

CO2 Enters

Open Stomate

Closed Stomate

2 Guard Cellssurrounding apore

TranspirationThe evaporative waterloss by a plant – primarilythrough open stomata

How Does Water Get In The Plant?

Water enters through the __________

Most water is lost from the plant through the _________

To reduce water loss leaves are covered with a waxy cuticle (plant “chapstick”)

Roots

Leaves (Actually the stomates on the leaf surface)

CO2 Enters

The Leaf is Covered with a Waxy Cuticle: “Plant Chapstick”

H2 O Exits

If water cannot get out of the leaf through the waxycuticle what cannot getinto the leaf for P/S?

Mesophyll cell or photosynthetic cell: Note the chloroplasts

Transpiration

The evaporative water loss by a plant, primarily through stomata

Degree of Stomatal Opening

Closed Partially FullyOpen Open

TranspirationRate

Note that this part of the graph shows that the more the stomates are open the greater the water loss via transpiration.

Photosynthesis and Transpiration

High Rates of P/S are associated with high transpiration rates

Degree of Stomatal Opening

Closed Partially FullyOpen Open

Rate ofP/S

Note that this part of the graph shows that the more the stomates are open the faster the rate of photosynthesis.

An Open Stomate

Guard cells

Pore

A Partially Closed Stomate

Notice the waxy cuticle on theleaf surface

Light:

Light Energy is inverselyproportional to wavelength

This means the smallerthe wavelength the higher the energy!

Chlorophyll: The Primary P/S Pigment

Pigments absorb light energy

The color you see is the color that is reflected

White versus Black

Why is chlorophyll green?

Would you expect green light to be an effective color of light for P/S?

It reflects green light

No. Green light is reflected. For light to be used on P/S it must be absorbed!

Structure of a Chloroplast

Chlorophyll Reflects Green Light

Chlorophyll absorbs blue and red light

Accessory pigments such as carotenoids absorb green light and pass the energy to chlorophyll

Absorption Spectra

Wavelength of Light (nm)

Amount Of LightAbsorbed

Notice that this part of the graphindicates not much green light isabsorbed. Thus green light isreflected.

This part of thegraph shows bluelight is absorbed

This part of the graph showsred light is absorbed

Accessory Pigments

Absorb colors of light that chlorophyll cannot (Yep – that means they absorb green light)

Pass the energy to chlorophyll

Example: Carotenoids -

Absorption Spectra

Wavelength of Light (nm)

Amount Of LightAbsorbed

Notice that carotenoids can absorb some colors ofgreen

Accessory PigmentsCarotenoids

Accessory Pigments

These quacking aspen trees are winterdeciduous. This means that they droptheir leaves in winter. Before they droptheir leaves, chlorophyll is reabsorbed, andtheir accessory pigments becomevisible. The visible accessory pigments areresponsible for “fall color”.

Stroma

Photosynthesisis a two stepprocess:

•The light reactions

• The dark rxnsor the CalvincycleNotice that the

light reactions produce oxygen

The darkreactions produceglucose

The light reactions require waterThe dark reactions requirecarbon dioxide

Photosynthesis is a Two Step Process

6CO2 + 6H2 O C6 H12 O6 + 6O2

The Light Reactions: Light Dependant

Photochemical

Convert light energy into chemical energy

The Light Independent or Dark Reactions

Temperature Dependant

Use the chemical energy created in the light reactions to convert CO2 to glucose

The Dark Reactions

The Light Reactions

The Light and Dark Reactions

The Light Reactions occur on the grana

The Dark Reactions take place in the stroma

The Light Reactions are Light Dependent

Light Intensity

Rate ofP/S

Light Saturation

Low Med High

Can you think of a habitator ecosystem where P/S might be limited by light?

When light saturation is reachedphotosynthesis is going as fastas it can. Turning up the lightbeyond this point has noeffect on the rate of P/S.

The Dark Reactions are Temperature Dependent

Temperature

Rate ofP/S

Low Med High Extreme

What is beginning to happen here?

Can you think of a habitat or ecosystem where P/S might be limited?

The temperature is so high, enzymes begin to denatureand the plant dies

This part of the graph saysthat if you increase the temperature you increase rate of photosynthesis

High Potential Energy Molecules

Does the formation of NADPH require or release energy?

NADP is made in the light reactions.

NADP+ + 2e- (2H) NADPH + H+

NADPH + H+ NADP+ + 2e-

Low P.E. High P.E.

High P.E. Low P.E.

It requires energy

Energy Required

Energy Released

ATP: The Universal Energy Molecule

Energy from the sun is used tomake ATP

ATP is cellulargasoline.

ATP is made in the light reactions

The Light Reactions

Photochemical: Light energy is converted to chemical energy in the form of two high potential energy molecules. (ATP and NADPH)

H2 O ½ O2

ATP

NADPHTwo electrons are transferredfrom water to NADP

2e-

When electrons are removedfrom water oxygen is produced

The Light Reactions

The Two High Potential Energy Molecules Produced are:

__________

__________

The Electron Source is?__________

When water gives up electrons what waste product is produced?

ATP

NADPH (NADPH2 )

Water

Oxygen

A model for thelight reactions

A Model For The Light Reactions

Low P.E.

High P.E.

The Light Independent or “Dark” Reactions

The Light Independent Reactions or the “Dark Reactions”

ATP ADP + P

6CO2 ---------------------------------------------------- C6H12O6

NADPH2 NADP + 2e-

What is the “fuel” that drives the dark reactions?__________

Where was this “fuel” produced?_______________________

High P.E Low P.E.

High P.E. Low P.E.

Chemical energy (ATP and NADPH2 )

The light reactions

C3 Photosynthesis: The Normal Pathway

Chloroplast (green): Any cell with chloroplasts is a photosynthetic cell.

Photosynthetic (P/S) cells)

(Air space between cells)

Carbon dioxide diffuses into the open stomates into intercellular air spaces. From the air spaces it thendiffuses into a P/S cell.

____ Turns to MakeOne Glucose

6

Diffusion is the driving force that gets CO2 into the leaf and into a P/S cell

C4 Photosynthesis: Kranz Anatomy

Photosynthetic (P/S) cells

Nonphotosynthetic (non P/S) cells(They lack chloroplasts)

Plants with Kranz Anatomy have the P/Scells in the center of the leaf around the leaf veins.

Fast Reaction

The C4 aciddrops off aCO2 for thedark reaction i.e.,the passenger hasbeen dropped offinto the P/S cell.

3C PEP is a taxi without a passengerIt will pick upa passengercalled CO2

(P/S Cell)

The C4 acid is analogousto a taxi witha passenger

The taxi called PEP will return to the non P/S cellto pick up another passenger (CO2 )

CO2 diffuses through the open stomate into a non- photosynthetic cell

CO2 is quickly grabbed by 3 carbon PEP and converted into a 4 carbon acid. This reaction occurs so fast that the concentration of CO2 leaf below the open stomate is so low that there is a very high diffusional pull for CO2 from the atmosphere into the leaf. This means that C4 plants suck in CO2 better than C3 plants.

Since C4 plants have a greater diffusional pull for CO2 , they can partially close their stomates to conserve water and still have fast rates of photosynthesis!!

Fast Reaction

Note that the dark reaction in thephotosynthetic cell is the exactsame dark reaction that occursin C3 plants. In C4 plants the only place this occurs is in cellsin the middle of the leaf thatsurround the leaf veins.

Tidestroma oblongifolia

The common name of this plant is “mouse ears”. It is the plant featured in the article High Efficiency Photosynthesis.

This is a C4 Plant with _______anatomy

Desert Holly – A C4 PlantDesert hollycan live in salty soils sinceit has salt glands

Kranz Anatomy – C4 Plants

Note that thechloroplastsare in thecenter of theleaf

CAM Photosynthesis: Cacti and Other Succulents

Stomata closed during the day and open at night

Take in CO2 at night and convert it into a 4C acid

The 4C acid is a storage form of CO2

It is analogous to filling up your pantry with CO2 at night

During the day, when the stomata are closed, the 4C acid releases CO2 to the Calvin-Benson Cycle (dark reaction)

This is analogous to using CO2 from the pantry for P/S since the stomates are closed and the plant can’t get CO2 from the air

What is the advantage of having the stomata open at night and closed during the day?

The stomates are only open when the temperature is low and humidity is high.This reduces the transpiration rate and allows the plant to conserve water.

Pancake Cactus – A Stem Succulent

GreenStem

CAM P/s

Question #28 in Worksheet II Would you expect plants in Death Valley to be actively photosynthesizing in August? Explain

There are 3 possible right answers:

If it is a C3 plant – No. The plant would be water stressed. When plants are water stressed the stomates are closed and CO2 cannot get into the leaf.

If it is a C4 plant – Maybe. C4 plants have a very high water use efficiency. They can partially close their stomates to reduce water loss and still do fairly fast photosynthesis.

If it is a CAM plant – Yes. CAM plants are the most water use efficient plants. In addition they are succulents and store water in their tissues. This stored water is used during periods of drought.