Chapter 8.3The Reaction of Photosynthesis

The Chloroplast• Site of photosynthesis

The Chloroplast

• Inner membrane thylakoid– Highly folded so there is an increased surface area for

absorption of sunlight

• Stacks of thylakoids grana

• Clusters of chlorophyll and other pigments photosystems

• Liquid filling of chloroplast stroma

Electron Carriers

• A carrier molecule is a compound that can accept a pair of high-energy electrons and transfer them along with most of their energy to another molecule

• Main electron carrier of photosynthesis NADP+ Turns into NADPH when it is carrying

electrons

Photosystems

• There are two different groupings of photosystems in photosynthesis Light dependent reactions

• Happens on the thylakoid membrane• Has 2 photosystems (II and I)

Light independent reactions• Happens in the stroma• Also known as the Calvin cycle

Chloroplast

Light

O2

Sugars

CO2

Light-Dependent Reactions

CalvinCycle

NADPH

ATP

ADP + PNADP+Chloroplast

Section 8-3

Figure 8-7 Photosynthesis: An Overview

Photosynthesis

includes

of

take place intakes place in uses

to produce to produce

use

Light-dependentreactions

Calvin cycle

Thylakoidmembranes Stroma NADPHATPEnergy from

sunlight

ATP NADPH O2 Chloroplasts High-energysugars

Section 8-3

Concept Map

Light Dependent Reactions

• Part of photosynthesis that requires light

• Use light energy to produce ATP and NADPH, as well as oxygen

Light Dependent Reactions

• Pigments of photosystem II absorb light• Energy is absorbed by electrons• High energy electrons are transferred to NADP+

(electron carrier molecule)– These electrons are replaced by electrons produced

by the breakdown of water• Water is broken down into 2 electrons, 2 H+

ions, and 1 oxygen atom– Oxygen is released– Hydrogen is broken apart and released inside the

thylakoid

Light Dependent Reactions

• The high energy electrons are passed through the electron transport chain between photosystem II to photosystem I

– The energy from the electrons is used to transport H+ ions from the stroma into the inner thylakoid space

Light Dependent Reactions

• Light hits the electrons that are now in photosystem I to excite them again– These high energy electrons are picked up by

NADP+ – The NADP+ also picks up the H+ ions to

make NADPH

Light Dependent Reactions

• As the electrons move onto the NADP+, more H+ ions are pumped across the membrane– Eventually, the outside of the thylakoid

becomes negatively charged and the inside becomes positively charged• Difference in charge provides the energy to make

ATP

Light Dependent Reactions

• Thylakoid membrane contains an enzyme called ATP synthase H+ ions pass through it to synthesize ATP

– This energy will be used to make energy-containing carbohydrates

Video 3

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Video 3

Light-Dependent Reactions, Part 1

Video 4

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Video 4

Light-Dependent Reactions, Part 2

HydrogenIon Movement

Photosystem II

InnerThylakoidSpace

ThylakoidMembrane

Stroma

ATP synthase

Electron Transport Chain Photosystem I ATP Formation

Chloroplast

Section 8-3

Figure 8-10 Light-Dependent ReactionsLight Dependent Reactions

Light Dependent Reactions

Light Independent Reactions

• Also known as the Calvin cycle

• Plants use the energy that ATP and NADPH contain to build high-energy compounds that can be stored for a long time

• Does not require light

Light Independent Reactions

• Six carbon dioxide molecules enter the cycle from the atmosphere to produce 1 6-carbon sugar

• As the cycle continues, 12 3-carbon molecules are created– These 3-carbon molecules obtain energy from

ATP and NADPH to become high energy 3-carbon molecules.

Light Independent Reactions

• Two of the 12 3-carbon molecules are removed from the cycle

– Used to make sugars, lipids, amino acids, and other materials needed for plant metabolism and growth

– Rest of the 3-carbon molecules combine to make 6 5-carbon molecules

Factors that Affect Photosynthesis

• Presence of water Lack of water slows down photosynthesis– Plants also have evolved ways to help prevent water loss (waxy

covering on leaves)

• Temperature Enzymes function best at 0-35 degrees Celsius

• Intensity of Light Increasing light intensity increases the rate of photosynthesis– There is a certain amount of light intensity where the amount of

photosynthesis within a plant does not change

Video 5

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Video 5

Calvin Cycle

ChloropIast

CO2 Enters the Cycle

Energy Input

5-CarbonMoleculesRegenerated

Sugars and other compounds

6-Carbon SugarProduced

Section 8-3

Figure 8-11 Calvin Cycle