Photosynthesis: The Reaction

Recap……..

6 CO2 + 6 H20 C6H12O6 + 6 O2

http://www.youtube.com/watch?v=lqEDz2vfhpQ&feature=related

Leaf Structure• Most photosynthesis occurs in the palisade layer.• Gas exchange of CO2 and O2 occurs at openings called

stomata surrounded by guard cells on the lower leaf surface.

Palisade

Spongy

Chloroplast Structure• Inner membrane

called the thylakoid membrane.

• Thickened regions called thylakoids. A stack of thylakoids is called a granum. (Plural – grana)

• Stroma is a liquid surrounding the thylakoids.

Chloroplast Structure• Inner membrane

called the thylakoid membrane.

• Thickened regions called thylakoids. A stack of thylakoids is called a granum. (Plural – grana)

• Stroma is a liquid surrounding the thylakoids.

Pigments• Chlorophyll A is the most important

photosynthetic pigment. • Other pigments called antenna or

accessory pigments are also present in the leaf.o Chlorophyll Bo Carotenoids (orange / red)o Xanthophylls (yellow / brown)

• These pigments are embedded in the membranes of the chloroplast in groups called photosystems.

Photosynthesis Occurs in 2 Steps

• Light Reaction– Light reactions are the “photo” part of photosynthesis.

Light is absorbed by pigments– Energy from light drives the reaction

•Light Independent Reaction or Dark Reaction• The Calvin Cycle• the “synthesis” part of photosynthesis. Trapped energy

from the sun is converted to the chemical energy of sugars.

• Doesn’t need direct light energy, but it does need the high-energy products from the Light Reaction

Photon

Photon

Water-splittingphotosystem

NADPH-producingphotosystem

ATPmill

• Two types of photosystems cooperate in the light reactions

Light Reactions

• Light-dependent reactions occur on the thylakoid membranes.o Light and water are required for this

process.o Energy storage molecules are formed.

(ATP and NADPH)o Oxygen gas is made as a waste product.

Light Reaction

• Drawing first…….

Light Reaction

• Now words…….

STEP 1 - Light energy is absorbed by chlorophyll in the thylakoid membranes of the chloroplast

STEP 2 - In Photosystem II the electrons become energized or “excited”.

STEP 3 - The excited electrons are passed down a series of molecules called the electron transport chain.

STEP 4 - To replace the electrons that are lost, some are stolen from water . This breaks the water molecule apart like this: 2H2O 4H+ + O2

STEP 5 -As electrons move down the chain, their energy is used to make ATP (a high- energy molecule)

6. STEP 6 -The electrons now go to Photosystem I. Light energizes new electrons in PI and they combine with NADP+ to make NADPH (another high-energy molecule).

Reactants used during the light reaction:

1. Water2. Also used sunlight

The final products of the light reaction

1. ATP2. NADPH 3. O2

Review Light Reaction

• Light Reaction– Where does this happen:– Needs: – Makes:

That’s only HALF of the Photosynthesis Reaction……

lightH2O ---------------→ O2 + ATP + NADPH

chlorophyll

Dark Reactions

• Dark reactions (light-independent) occur in the stroma.• This reaction is ENZYMATIC not ELECTRONIC

• Doesn’t ACTUALLY happen in the dark….

o Carbon dioxide is “fixed” into the sugar glucose.

o ATP and NADPH molecules created during the light reactions power the production of this glucose.

Why fix CO2?

• By fixing CO2, energy is stored in organic compounds

Dark Reaction

• 3 steps:– Carboxylation– Reduction– Regeneration

*It takes 3 turns of the Calvin Cycle to release 1 PGAL*

* 2 PGAL = 1 Glucose*

Dark Reaction

• Write it:• Step 1: CO2 (1C) is added to RuBP (5C) making an

unstable 6C molecule that is quickly converted to 2 PGA (3C each) molecules. Catalyzed by rubisco

• Step 2: PGA (3C) is reduced to PGAL (3C) with the help of NADPH and ATP. 2 PGA (3C) 2 PGAL (3C)

• Step 3: PGAL is converted to RuBP with the help of ATP or “siphoned” off to create glucose (later)

Dark Reaction

• Step 4: REPEAT 2 more times

• At third turn: 1 PGAL is released. When 2 PGALs combine, glucose is created

Dark Reaction

• What we need from the Light Reaction:– CO2– NADPH– ATP

Dark Reaction

• Products:– SUGAR

• The Calvin cycle makes sugar from carbon dioxideo ATP generated by the light

reactions provides the energy for sugar synthesis

o 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 energyo Produce ATP & NADPH

AN OVERVIEW OF PHOTOSYNTHESIS

Dark Reaction

• Build it….

Photosynthesis

• Act it….

• 1,2,3 http://www.youtube.com/watch?v=yrQzEw9xY5k&feature=related

• Rap

http://www.youtube.com/watch?v=pdgkuT12e14&feature=related