PhotosynthesisPhotosynthesis

Learning objectiveLearning objectiveTo be able to explain thatTo be able to explain that• light energy is absorbed by electrons in light energy is absorbed by electrons in chlorophyllchlorophyll•Some excited electrons gain enough energy to Some excited electrons gain enough energy to leave chlorophyllleave chlorophyll•The electrons are replaced by the photolysis of The electrons are replaced by the photolysis of waterwater• • the energy from these excited electrons is used to the energy from these excited electrons is used to generate ATP and reduced NADP;generate ATP and reduced NADP;

Adaptations in a leafAdaptations in a leafWrite down the adaptations in a leaf and how they facilitate Write down the adaptations in a leaf and how they facilitate

photosynthesisphotosynthesis• Thin – gases can diffuse in and out quicklyThin – gases can diffuse in and out quickly• Large surface area – maximise light absorptionLarge surface area – maximise light absorption• Thin layer of wax – to reduce evaporation from the upper Thin layer of wax – to reduce evaporation from the upper

surfacesurface• Rich network of vascular tissue – xylem to deliver water and Rich network of vascular tissue – xylem to deliver water and

minerals, phloem to remove products of photosynthesisminerals, phloem to remove products of photosynthesis• Cells in upper epidermis have no chloroplasts – more light gets Cells in upper epidermis have no chloroplasts – more light gets

through to pallisade layerthrough to pallisade layer• Tightly packed pallisade cells – larger surface area for Tightly packed pallisade cells – larger surface area for

photosynthesisphotosynthesis• Chloroplasts move within cytoplasm – have the chance of Chloroplasts move within cytoplasm – have the chance of

obtaining as much light as possibleobtaining as much light as possible• Stomata surrounded by guard cells – open in day to allow CO2 Stomata surrounded by guard cells – open in day to allow CO2

in, close at night to reduce water lossin, close at night to reduce water loss• Spongy mesophyll - creates air spaces making gas exchange Spongy mesophyll - creates air spaces making gas exchange

more efficient between the atmosphere and pallisade mesophyll.more efficient between the atmosphere and pallisade mesophyll.

Autotrophic nutritionAutotrophic nutrition• Autotrophic nutrition is making your own Autotrophic nutrition is making your own

foodfood• Autotrophs are able to use a source of Autotrophs are able to use a source of

energy to make complex organis molecules energy to make complex organis molecules from simple inorganic materialsfrom simple inorganic materials

• Chemoautotrophs harness energy released Chemoautotrophs harness energy released in exergonic chemical reactions to synthesis in exergonic chemical reactions to synthesis their own food (mainly bacteria)their own food (mainly bacteria)

• Photoautotrophs use light energy to Photoautotrophs use light energy to synthesise their own organic materials synthesise their own organic materials (green plants and algae)(green plants and algae)

Structure of cholroplastsStructure of cholroplasts

Phases of Photosynthesis

WaterLight dependant

reactionsOxygen

electrons energy

Carbon Dioxide

Light Independent

Reactions

Carbohydrate

Light dependant reactionLight dependant reaction

• Takes place in the thylakoidsTakes place in the thylakoids• Thylakoid membrane is a phospholipid Thylakoid membrane is a phospholipid

bilayer with chlorophyll molecules bilayer with chlorophyll molecules embedded in itembedded in it

• Chlorophyll molecules absorb light Chlorophyll molecules absorb light energy and then transfer it to other energy and then transfer it to other protein moleculesprotein molecules

• Light dependant reaction generates Light dependant reaction generates ATPATP

Chlorophyll Chlorophyll

• Chlorophyll has evolved to make use of Chlorophyll has evolved to make use of as much of the visible spectrum as as much of the visible spectrum as possiblepossible

• Chlorophyll is a mixture of compounds Chlorophyll is a mixture of compounds that fall into two basic types:that fall into two basic types:– ChlorophyllChlorophyll– CarotenoidsCarotenoids

• The two main chlorophylls are a and bThe two main chlorophylls are a and b

Absorption and Action Absorption and Action Spectrums Spectrums

• The action spectrum shows the rate of The action spectrum shows the rate of photosynthesis at different wavelengths.photosynthesis at different wavelengths.

• The absorption spectrum shows how strongly The absorption spectrum shows how strongly the pigments absorb at different wavelengths. the pigments absorb at different wavelengths.

Read the application box on page 6 and Read the application box on page 6 and answer questions 1 - 3answer questions 1 - 3

PhotosynthesisPhotosynthesis

Learning objectiveLearning objective

To be able to explain thatTo be able to explain that•In the light-dependent reaction light In the light-dependent reaction light energy excites electrons in energy excites electrons in chlorophyll;chlorophyll;

• • the energy from these excited the energy from these excited electrons is used to generate ATP electrons is used to generate ATP and reduced NADP;and reduced NADP;

Thylakoid membraneThylakoid membrane

• The first reactions of The first reactions of photosynthesis require photosynthesis require light energy, and are light energy, and are called light dependant called light dependant reactions which take reactions which take place in the thylakoid place in the thylakoid membrane.membrane.

• The thylakoid The thylakoid membranes have the membranes have the pigmentspigments arranged in arranged in funnel shaped funnel shaped ‘photosystems’‘photosystems’

Photosystems absorb some wavelengths of light and reflect others

There are two photosystems I and II that are in close proximity to one another in the thylakoid membrane

• Each photosystem holds Each photosystem holds the light absorbing the light absorbing pigments in the best pigments in the best position to maximize the position to maximize the absorbance of photons absorbance of photons of light.of light.

• Photosystem I contains Photosystem I contains chlorophyll a and chlorophyll a and absorbs best at 700 nm absorbs best at 700 nm (P700)(P700)

• Photosystem II contains Photosystem II contains chlorophyll b and chlorophyll b and absorbs best at 680 nm absorbs best at 680 nm (P680)(P680)

Chl. b 650

Chl. a 670

Chl. a 680

Chl. a 690

P700 Reaction centre I

Light reaction 1

About 300 light trapping chlorphyll

molecules

Photosystem I

Chl. b 650

Chl. a 670

Chl. a 680

P690Reaction centre II

Light reaction 2

About 300 light trapping chlorphyll

molecules

Photosystem II

Energy captured in the Light Energy captured in the Light Dependant reactionDependant reaction

Used for 2 things:Used for 2 things:• Making ATP from ADP and Pi is called Making ATP from ADP and Pi is called

phosphorylation. In the light dependant phosphorylation. In the light dependant reaction it is called PHOTOphosphorylation reaction it is called PHOTOphosphorylation because it involves lightbecause it involves light

• For splitting water into HFor splitting water into H++ and OH and OH–– ions –this is ions –this is called photolysiscalled photolysis

• For combining electrons, photons and NADP to For combining electrons, photons and NADP to produce reduced NADP (NADPH) powerful produce reduced NADP (NADPH) powerful reducing agent used in the light independent reducing agent used in the light independent reactions.reactions.

Electrons in chlorophyll gain energy from the sun and become excited

Energy used to add a phosphate group to ADP to form ATP

Photophosphorylation

Energy used to split water into protons, electrons and oxygen

Photolysis

Energy, protons and electrons used to reduce NADP

Reduced NADP Production

Oxygen released

ElectronsProtons

PhotophosphorylationPhotophosphorylation

• Excited electrons passed to electron Excited electrons passed to electron acceptoracceptor

• Electrons pass along chain of electron Electrons pass along chain of electron acceptors down energy gradientacceptors down energy gradient

• Electrons lose energy at each stage in Electrons lose energy at each stage in the chainthe chain

• Energy is used to add phosphate Energy is used to add phosphate molecule to ADP - molecule to ADP - photophosphorylationphotophosphorylation

PhotophosphorylationPhotophosphorylation

• Uses PSI and PSIIUses PSI and PSII

PSIIPSIIP680P680

LIGHTLIGHT

ee-- ADP + PADP + P

ATPATPPSIPSI

P700P700

LIGHTLIGHT

2e2e--

HH22OO ½ O½ O22 + 2H + 2H++

NADP + 2HNADP + 2H++

NADPH + HNADPH + H++

• Photosystem One - PS I: Its primary pigment is a Photosystem One - PS I: Its primary pigment is a molecule of chlorophyll a. What is it’s peak molecule of chlorophyll a. What is it’s peak absorbance?absorbance?

• Photosystem Two - PS II: Its primary pigment is a Photosystem Two - PS II: Its primary pigment is a molecule of chlorophyll b. What is it’s peak molecule of chlorophyll b. What is it’s peak absorbance?absorbance?

• Where precisely in the plant does the light Where precisely in the plant does the light dependant stage occur?dependant stage occur?

• Which two compounds produced in the light Which two compounds produced in the light dependant stage are used in the light dependant stage are used in the light independent stage?independent stage?

• Which of the light dependant reactions of Which of the light dependant reactions of photosynthesis are involved in producing these photosynthesis are involved in producing these compounds?compounds?

1. Photosystem One - PS I:1. Photosystem One - PS I:Its primary pigment is a molecule of Its primary pigment is a molecule of chlorophyll a. What is it’s peak absorbance?chlorophyll a. What is it’s peak absorbance?

• PSI absorption peak at 700nm. It is called PSI absorption peak at 700nm. It is called P700P700

2. Photosystem Two - PS II: Its primary pigment 2. Photosystem Two - PS II: Its primary pigment is a molecule of chlorophyll b. What is it’s is a molecule of chlorophyll b. What is it’s peak absorbance?peak absorbance?

• PSII absorption peak at 680nm. It is called PSII absorption peak at 680nm. It is called P680P680

3. Where precisely in the plant does the light 3. Where precisely in the plant does the light dependant stage occur?dependant stage occur?

In the thylakoid membranes of the chloroplastsIn the thylakoid membranes of the chloroplasts

4. Which two compounds produced in the 4. Which two compounds produced in the light dependant stage are used in the light light dependant stage are used in the light independent stage?independent stage?

ATP and NADPH + HATP and NADPH + H++

5. Which of the light dependant reactions of 5. Which of the light dependant reactions of photosynthesis are involved in producing photosynthesis are involved in producing these compounds?these compounds?

ATP is produced by both cyclic and non cyclic ATP is produced by both cyclic and non cyclic photophosphorylation NADPH is produced photophosphorylation NADPH is produced by non cyclic photophosphorylation by non cyclic photophosphorylation