Photosynthesis: Harvesting Light Energy

Created by C. Ippolito Jan 11, 2003updated Jan 17, 2006

Photosynthesis: Harvesting Light EnergyChapter 7

pages 157 - 179

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Organisms and Photosynthesis

Chapter 7.1 – 7.4

pages 157 - 162

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Updated 2/1/04Updated 2/1/04Created by C. Ippolito Jan 11, 2003Created by C. Ippolito Jan 11, 2003Updated Jan 17, 2006Updated Jan 17, 2006

Importance of Photosynthesis

sunlight (radiant energy) chemical energy removes carbon dioxide from atmosphere carbon dioxide and water to make glucose adds oxygen gas to atmosphere original source of fossil fuels

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Leaf Structure Cuticle – forms water

barrier Epidermis – protection Palisade – capture

sunlight Spongy – capture CO2 &

H2O Epidermis – protection Stomates – gas exchange Guard Cells – control

transpiration

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Chloroplast Structure

Thylakoids – folds of inner membrane

Grana – stacked sac of thylakoids

Stroma – space around grana Grana has chlorophyll

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Pigments Chemicals that absorb

light Phycocyanins - blues Chlorophylls - greens Xanthophylls - yellows Carotenes - oranges Phycoerythrins - reds

Each pigment has a specific absorption spectrum

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How Pigments Work Light hits free electrons moving them from

their ground state (low energy) to an excited state (high energy) absorbing energy.

Three possible outcomes:1. energy gained released as heat2. energy gained released as light (fluoresce)3. energy gained powers subsequent chemical

reaction Pigment captures light; excited electrons of

grana move energy captured into other reactions

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Homework

Check and Challenge Questions on page 162

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The Process of Photosynthesis

Chapter 7.5 – 7.8

Pages 164 – 170

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Discoveries Van Helmont – plants need waterwater for growth Priestly (1772)– plants release material

(oxygenoxygen) that keeps flame burningflame burning, mice alive

Lavoisier - proved that oxygenoxygen is removed from air when animals breathe or something is burned

Ingenhousz - discovered that plants need lightlight to correct bad air

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Discoveries

de Saussure (1804) – plant growth results from taking up waterwater and carbon carbon dioxidedioxide

Meyer (1845) – lightlight is absorbed and transformed into chemical energy

Engelmann - discovered which wavelengths (ccoolloouurrss) of lightlight are used by plants in photosynthesis

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Overview of Photosynthesis

Photosynthesis6CO2 + 6H2O C6H12O6 + 6O2

Light Reactions(grana)

Dark Reactions(stroma)

1. Requires lightlight2. Splits water (photolysis)3. Forms ATP and NADPH2

4. Releases oxygenoxygen

1. Does not require lightlight

2. Uses ATP and NADPH2

3. Takes in carbon dioxidecarbon dioxide4. Forms sugars

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Light Reactions

Photosystems – groups of ppiiggmmeenntt that absorb light energy and “channel” energy into one ppiiggmmeenntt molecule that passes it to cytochrome chains to form ATP.Photosystem I – P700 reaction centerPhotosystem II – P680 reaction center

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Photophosphorylation

light energy chemical energy of ATP

1. Cyclic Photophosphorylation waterwater or carbon dioxidecarbon dioxide not available Dark Reactions not involved Forms only ATP for immediate use by cell

2. Non Cyclic Photophosphorylation H2O and CO2 available products to Dark Reactions

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Cyclic Photophosphorylation Photosystem I

1. Excited e- moves down electron transport chain

2. ADP changed into ATP

3. Low energy e- returns to photosystem

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Non Cyclic Photophosphorylation

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Light Reactions in Grana

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Summary of Light Reaction Photosystem II

1. Photolysis replaces excited electron by spitting waterwater to release H+ and ½O2

2. Excited electron passes down cytochromes pumping H+ into grana

3. H+ diffuses out through ATP synthase to form ATP

4. Low energy electron passes to photosystem I

Photosystem I

1. Low energy electron replaces excited electron

2. Excited electron accepted by NAD+ and joined with H+ forming NADPH this keeps H+ outside low

3. NADPH and ATP go to the Dark Reactions

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Dark Reactions Consists of 3 pathways:

1. Carbon Fixation – adds six carbon dioxidecarbon dioxide from environment to six existing 5C sugars named RuBPRuBP (ribulose bisphosphate) to form six 6C sugars. This step uses energy from the ATPATP and NADPHNADPH formed

in Light Reactions 2. Calvin Cycle – the 6C sugars break into 12 PGAL

(3C) which regenerates the six 5C RuBPRuBPs and forms 2 PGALPGAL

3. Hexose Shunt – joins the 2 PGALPGAL to form glucose. Rubisco – enzyme that catalyzes initial reaction of

carbon fixation.

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Homework


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Photosynthesis and the Environment

Chapter 7.9 – 7.12

Pages 170 - 177

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Rate of Photosynthesis

Four environmental factors can act as limiting factors which affect the amount (rate of reaction) of photosynthesis:

1. Light Intensity

2. Temperature

3. Concentration of CO2

4. Concentration of O2

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As light intensity increases the rate of reaction increases and then levels off

Rate of photosynthesis is best at an optimum temperature between 15°C and 35°C

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Oxygen Affect on Photosynthesis

As concentration of O2 increases rate of reaction decreases

When there is an excess of O2 it will combine with Rubisco to form PGA and glycolate which will form CO2 in process called photorespirationphotorespiration

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Photorespiration

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Special Adaptations – C4 plants C4 plants evolved special

system to increase CO2 concentration to limit photorespiration

C4 plants can close their stomates in high light and temperature environments to prevent the rate of reaction from decreasing.

More efficient than C3 plants, fast growing.

Sugar, crabgrass, corn

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Special Adaptations – CAM plants

CAM (crassulacean acid metabolism) plants evolved special system to absorb CO2 at night when open stomates will not cause water loss

CAM plants store the CO2 in a 4C acid that releases the CO2 to Calvin cycle in day light.

Inefficient process plants very slow growing

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Chemoautotrophs

Bacteria use chemical energy and H2S (hydrogen sulfide) to form glucose

6H2S + 6CO2 → C6H12O6 + 6S2

Symbiotic with vent tube worms


Homework


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Photosynthesis: Harvesting Light Energy