Photosynthesis: Where it all begins!

Photosynthesis: Where it all begins!

AP Biology

Us versus Them

• Autotrophs make their own food (self-nourishing)

• Photoautotrophs use sunlight as the energy source

• Heterotrophs must feed on autotrophs, one another, or waste

Photosynthesis

• Is the main pathway by which carbon and energy enter the web of life.

• Where do we find carbon in living things?

An overview

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

Two divisions: Light dependent reactions, which yields ATP and H+

Light independent reactions, which uses the products of the light dependent reactions to make glucose

An Overview• Takes place in the chloroplasts• Two outer membranes surrounding a mostly fluid

interior called the stroma• Another folded membrane is stacked in the stroma. • The stacks of thylakoid discs are grana (granum)

Double membrane

Thylakoids

Stroma

Light Dependent Reactions

• Sunlight splits water molecules

• Oxygen diffuses away• Its electrons flow

through electron transfer chains

• This forms ATP• Coenzyme NADP

picks up the electrons and hydrogen

light reactions

During photosynthesis, CO2 will be reduced (gain electrons) to form glucose. The electrons needed to reduce CO2 are temporarily carried by NADPH.

Light | Pigments | Overview | Chloroplast | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return

light

ATP NADPH

Light Independent Reactions

• Occur in the stroma

• Does not require light

• ATP gives up energy

• Coenzyme NADPH gives up electrons and hydrogen

• CO2 is dismantled for its C and O atoms

• Glucose is made

ADP

NADP+


light-independent reactions(Calvin cycle)

C6H12O6

C02

H2O O2

light reactions

ATP NADPH

light

Not Really Glucose

• Glucose is quickly changed to sucrose, cellulose, or starch

Properties of Light• Light travels in waves• The distance between two crests is a

wavelength• The shorter the wavelength, the higher the

energy• All wavelengths combined appear as white

Photons

• The energy of light has a particle-like quality

• Energy, when absorbed, can be measured as packets called photons

• Each photon has a fixed amount of energy

Pigments • Absorb wavelengths of light• Most absorb only certain wavelengths• Reflect back or transmit the others• Chlorophyll looks green because it does

NOT absorb green wavelengths

Accessory Pigments

• Carotenoids• Phycobilin

– Phycoerythrin

– Phycocyanin

• Anthocyanins


A Closer Look

Produces ATP and NADPH

Photosystems

• In the thylakoid membrane, pigments are organized in clusters called photosystems

• Photons of light are absorbed by pigment, and the pigment’s electrons get “excited.”

Excited Electrons

• When electrons of an atom absorb energy, they move to a higher energy level

• Energy entering a pigment destabilizes the arrangement of electrons. (They jump around)

Excited Electrons

• The excited electrons quickly return to a lower energy level, stabilizes, and some energy is released in the form of light or heat.

(fluorescence.)• In photosynthesis, this releasing energy gets passed on

to another pigment in a random “walk” Some is lost as heat. The remaining energy matches to a wavelength that the photosystem’s reaction center can trap.

• The reaction center passes the energy in the form of excited electrons to an electron transport chain

Return

Thylakoidmembrane

A pigment molecule within the antenna absorbs a photon of light energy. The energy from that pigment molecule is passed to neighboring pigment molecules and eventually makes its way to pigment molecule called the reaction center. When the reaction center molecule becomes excited (energized), it loses an electron to an electron acceptor.

Reaction center

Electron acceptor

Light energy


Return

Antenna

Reaction center

Electron acceptor

Photosystem

Thylakoidmembrane

The antenna and electron acceptor are called a photosystem.

There are two kinds of photosystems in plants called photosystem I and photosystem II.

Photosystem I is sometimes called P700 and photosystem II is sometimes P680. The 680 and 700 designations refer to the wavelength of light that they absorb best.


The Photosystems• P700 is photosystem I, and can be cyclic.

P700 can cycle alone, or can receive electrons from P680 as part of a non-cyclic pathway.

• When it cycles alone, light energy excites electrons, boosting them to a higher energy level, and sending them through an electron transport chain. The end product of the electron transport chain is ATP.

Photosystems

• P680 is photosystem II and is non-cyclic. It receives energy from light, boosting electrons to a higher energy level and sending them through an electron transport chain to P700. The electrons it gives up from the pigments are replaced by the splitting of water (constant supply) p114

Photosystems

• Both electron transport chains use the energy from the “falling” electrons to pump H ions to the inside of the thylakoid membrane, resulting in a concentration gradient

• When H ions move through ATP synthase, the energy is used to attach ADP to P, making ATP, needed for the light indep.

How ATP is made

• Hydrogen ions from the splitting of water accumulate in the thylakoid membrane.

• Electron transport chains build up even more hydrogen ions in the thylakoid.

• Ions are pumped from the stroma into the thylakoid.

• Sets up a concentration gradient• When they flow into stroma, they are used to form

ATP from ADP and P (ATP synthase enzyme required)

Stroma

LightEnergy Chloroplast

Return

Photosystem II Photosystem I


The three blue circles represent the electron transport system. They are proteins embedded within the thylakoid membrane.The first protein receives the electron (and energy) from the electron acceptor.

H+

H+

H+ H+ H+

H+


As a result of gaining an electron (reduction), the first carrier of the electron transport system gains energy. It uses some of the energy to pump H+ into the thylakoid.

H+

H+

Thylakoids Stroma


H+

H+ H+ H+

H+


The carrier then passes the electron to the next carrier. Because it used some energy to pump H+, it has less energy (reducing capability) to pass to the next H+ pump.

H+

H+

Thylakoids Stroma


H+H+ H+ H+

H+


This carrier uses some of the remainder of the energy to pump more H+ into the thylakoid.

H+

H+

H+

Thylakoids Stroma


H+H+ H+ H+

H+


The electron is passed to the next carrier which also pumps H+.

H+

H+

H+

Thylakoids Stroma


H+H+ H+ H+

H+


The electron transport system functions to create a concentration gradient of H+inside the thylakoid.

H+

H+H+

Thylakoids Stroma


H+H+ H+ H+

H+


H+H+

H+

ATP

ADP + Pi

Thylakoids Stroma


The concentration gradient of H+ is used to synthesize ATP.

ATP is produced from ADP and Pi when hydrogen ions pass out of the thylakoid through ATP synthase.

H+H+ H+ H+

H+


H+H+

H+

Thylakoids Stroma


ATP

ADP + Pi

This method of synthesizing ATP by using a H+ gradient in the thylakoid is called photophosphorylation.

This method of making ATP

• Is called the chemiosmotic model for ATP production

• Will also happen in the mitochrondria

• In which place do you think more ATP will be made?

Light Independent Reactions

A Closer Look: takes the ATP and NADPH from light dependent and

makes glucose

A Cycle• Called the Calvin-

Benson cycle.• ATP drives the

reactions• NADPH delivers

hydrogen and electrons

• CO2 provides the carbon

Photophosphorylation

• Is a specific type of Chemiosmosis

• Chemiosmotic theory refers to the method of building up H+ concentration gradient to make ATP.

RuBPCarboxylase(rubisco)

6 C-C-C-C-C

6 CO2

6 C-C-C-C-C-C

CO2 Fixation


CO2 fixation refers to bonding CO2 to an organic molecule to make a larger molecule.

Each CO2 is bonded to ribulose biphosphate (RuBP).

C5 + CO2 C6

The enzyme that catalyzes this reaction is ribulose biphosphate carboxylase (rubisco).

C3 Photosynthesis


Each of these 6-carbon compounds splits to form two 3-carbon compounds called phosphoglycerate.

PGARuBP


6 C-C-C-C-C

6 CO2

6 C-C-C-C-C-C

12 C-C-C


The two molecules of PGA are reduced to form PGAL (phosphoglyceraldehyde).

PGA

PGAL

RuBP


12 C-C-C

6 C-C-C-C-C

6 CO2

6 C-C-C-C-C-C

12 C-C-C


PGA

PGAL

RuBP


12 C-C-C

6 C-C-C-C-C

6 CO2

6 C-C-C-C-C-C

12 C-C-C

12 ATP

12 NADPH

12 NADP+

12 ADP + P

PGA

PGAL

Glucose

RuBP


12 C-C-C

C-C-C-C-C-C

6 C-C-C-C-C

6 CO2

6 C-C-C-C-C-C

12 C-C-C

10 C-C-C

6 ADP + P

6 ATP

12 ATP

12 NADPH

12 NADP+

12 ADP + P


Two of the PGAL are used to form glucose phosphate, then glucose.

PGA

PGAL

Glucose

RuBP


12 C-C-C

C-C-C-C-C-C

6 C-C-C-C-C

6 CO2

6 C-C-C-C-C-C

12 C-C-C

10 C-C-C

6 ADP + P

6 ATP

12 ATP

12 NADPH

12 NADP+

12 ADP + P


The remaining 10 PGAL are rearranged to form 6 RuBP.

PGA

PGAL

Glucose

RuBP


12 C-C-C

C-C-C-C-C-C

6 C-C-C-C-C

6 CO2

6 C-C-C-C-C-C

12 C-C-C

10 C-C-C

6 ADP + P

6 ATP


12 ATP

12 NADPH

12 NADP+

12 ADP + P

This process requires energy in the form of ATP.

ADP

NADP+

C6H12O6

C02

H2O O2

Light reactions

ATP NADPH

light

PGA

PGAL

Glucose

RuBP


12 C-C-C

C-C-C-C-C-C

6 C-C-C-C-C

6 CO2

6 C-C-C-C-C-C

12 C-C-C

10 C-C-C 12 ATP

12 NADPH

12 NADP+

12 ADP + P

6 ADP + P6 ATP


H+H+ H+ H+

H+

Thylakoids Stroma


H+H+

H+

ATP

ADP + Pi

NADP+ + H+

NADPH

H2O 2e- + 2H+ + ½ O2


Light-independentreactions

Cyclic vs Non-cyclic

• Cyclic electron flow involves the P700 photosystem only

• Electrons are boosted in energy, passed thru an electron transport chain, producing ATP, and returned to the P700 photosystem

Cyclic vs Non-cyclic

• Noncyclic: electrons in P680 are boosted in energy, go thru the electron transport chain, producing ATP. The electrons still carry some energy, they move on to P700 and go through a second ETC, producing ATP AND NADPH

Electrons do not return to the P680. NADP is the final electron acceptor.

Non-cyclic

C3, CF4, and CAM Plants

• Stomata (openings for gas and water exchange) close in dry weather to conserve water

• But this means that CO2 in and O2 out is also stopped

C3, CF4, and CAM Plants

Halts the light independent reactions, but the light dependent reactions continue

O2 builds up and triggers an alternate pathway called photorespiration

Inefficient backdoor way to make small amounts of glucose

• This is what happens to most plants (C3.)

• Diagram on page 117

C4 plants have a better answer

• O2 also builds up here when stomata close, but an additional step keeps the CO2

concentration higher than the O2 concentration, so the inefficient photorespiration is not triggered.

• Involves two different cells

• Diagram page 117

CAM plants

• Like C4 plants, CAM plants use a C4 cycle and the Calvin-Benson cycle.

• But the cycles occur in the same cell, but one during day and the other at night.

Photosynthesis: Where it all begins!

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