Reactions of Photosynthesis. Photosynthetic Equation Light

Reactions of Photosynthesis

Photosynthetic Equation

6CO2 + 6H2O C6H12O6 + 6O2 Carbon dioxide + water sugar + oxygen

Light

Two stages of photosynthesis

– 1. Light Reactions – aka light-dependent reactions - takes place within thylakoid membranes

• Makes ATP and NADPH for Light-Independent Reaction

– 2. Calvin Cycle – aka light-independent reactions – takes place in the stroma – a region outside thylakoid membrane

• Makes glucose

NADPH – “the hot coal carrier”• Sunlight excites electrons in chlorophyll and gain

energy. A special “carrier” is needed – an electron carrier is a compound that can accept a pair of high-energy electrons and transfer them along with most of their energy to another molecule

• NADP+ - electron carrier (nicotinamide adenine dinucleotide phosphate) that accepts and holds 2 high-energy electrons and a hydrogen H+ from water- converts NADP+ into NADPH

• NADPH – then carries electrons to chemical reactions elsewhere in the cell for building a variety of molecules

Summary: NADP+ accepts 2 electrons and H+ from water, turning it into NADPH, and can now move those electrons elsewhere in the cell to help build molecules.

Chloroplast

Light

O2

Sugars

CO2

Light Reactions

CalvinCycle

NADPH

ATP

ADP + PNADP+Chloroplast

Photosynthesis: An Overview

1. Light Reactions (aka Light-Dependent Reaction)

• Occurs in the thylakoids• Requires light to produce ATP and

NADPH (used for Calvin Cycle)• Produce oxygen gas and convert

ADP and NADP+ into energy carriers ATP and NADPH

• Converts light energy into chemical energy (ATP and NADPH)

What goes in:

•H2O

•Light

What comes out:

•ATP

•NADPH

•O2

Steps in Light Reactions

• Use light and H2O for making ATP and NADPH for Calvin Cycle

• 1) Chlorophyll in Water-Splitting Photosystem absorb light – light excites electrons and are passed along to the electron transport chain. Thylakoid membranes provide new electrons to chlorophyll to replace them. New electrons come from water molecules which are broken down into 2 electrons, 2 H+ ions, and 1 oxygen atom (oxygen eventually released)

Summary: Light absorbed by pigments = excited electrons. Electrons passed to chain. H2O breaks down and replenishes electrons. O2 released, H+ kept in thylakoid to fuel ATP formation.

Steps in Light Reactions(continued)

• 2) High-energy electrons move through electron transport chain from Water-Splitting Photosystem to NADPH-Producing Photosystem. Energy from electrons used in the chain to transport H+ ions from stroma to thylakoid to make ATP using ATP synthase protein

Summary: Electrons move down chain; move H+ into thylakoid to help fuel the process of making ATP using ATP Synthase protein

Steps in Light Reactions (continued)

• 3) Chlorophyll in uses energy from light to re-energize electrons. NADP+ then picks up these electrons and a H+ ion to become NADPH

Summary: Chorophyll uses sunlight to re-energize electrons; NADPH formed

Light Reactions – Step 1

Primaryelectronacceptor

Water-splittingphotosystem

Light

H2O

2 H

Reaction-centerchlorophyll

O21

2

2e–

2e–

1


H2O

O2



Light

H2O

2 H


O2

Electron transport chain

Energyto make ATP

1

2

2e–

2e–

1

2


H2O

O2



Light

H2O

2 HO2


Energyto make ATP


2e–

Light

NADPH-producingphotosystem


2e

NADPH

NADP

1

2

2e–

2e–

1

2

3

– –


Light Reactions - Overall

H2O

O2

Light

H2O

Thylakoidmembrane

2e–

O2

ATP

NADP

Light

Stroma

Inside thylakoid

Photosystem Photosystem


NADPH

P

H

ATPsynthase

To Calvin cycle

H

Electron flow

HH

H

H

H

1

2

Thylakoidmembrane

– –

Light Reactions

Documents

Reactions of Photosynthesis. Photosynthetic Equation Light