copyright 2002 by harcourt college publishers, a division of thomson learning biology, sixth...

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Biology, Sixth Edition Chapter 8, Photosynthesis: Capturing Energy

Photosynthesis: Capturing Energy

Unit 3 ContinuedChapter 8

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Carbon Fixation

• Carbon fixation is the process of building complex carbon compounds from simple carbon compounds.

• Organisms that fix carbon are autotrophs – they use carbon dioxide as a carbon source, and combine it with water

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Name the type of organism that…

• Uses C02 as a carbon source• Cannot fix carbon• Uses light energy as an energy source• Uses organic compounds as sources of energy• Absorbs and converts light energy into

chemical energy• Uses light energy, but can’t do carbon fixation• Obtain energy from H2S, N02 or NH2• Use organic molecules as a source of energy

and carbon

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Photoautotrophs, Heterotrophs, and Chemoautotrophs

• Photoautotrophs: Organisms that gain energy from light in order to carry out carbon fixation• Photosynthetic plants, algae and bacteria• Use light energy to make ATP and carbohydrate

• Chemoautotrophs: Organisms that use chemical energy only to cause carbon fixation and to build structure• Certain bacteria

• Heterotrophs: Organisms that gain energy by eating other organisms, including autotrophs• Animals, nonphotosynthetic plants,

nonphotosynthetic unicellular organisms(such as protists), bacteria, and fungi

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What is the Electromagnetic Spectrum?

• Complete range of radiant energy

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• Order is ROYGBIV•Range is 380 – 760 nm•Violet has a shorter wavelength than red•Tiny packets of this energy is called a photon•Photons of light can energize an electron

Characteristics of Visible Light

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The Relationship Between Photon Energy Capture and

Electrons• Electrons can absorb photon energy.• When they do so, they hop to a higher

shell.• When they release energy, they release a

photon, and drop back to the lower shell.• The released photon has less energy than

the one that was absorbed• The released photon has a longer

wavelength• Light released this way is fluorescence.

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The Sun• The sun is a star• It has very high surface temperatures • It produces a vast amount of electromagnetic

radiation of widely varying frequencies• Gamma-rays, which have very short wavelengths • To the far-infrared (much longer )

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Absorption of Light Energy

• Light energy is absorbed by electrons in energy-absorbing atoms

• The energy causes electrons to change shells; the more energy absorbed, the further electrons move fromthe nucleus

• The energy may be shed as fluorescence ...

• Or transferred in the form of an electron to another molecule

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Leaf Structure

• Leaves have a layered organization

• The mesophyll tissue (middle layers of cells) is the main site of photosynthesis

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Biology, Sixth Edition Chapter 8, Photosynthesis: Capturing Energy The

Chloroplast

•The site of light harvesting & carbohydrate synthesis

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Parts of a Chloroplast• Thylakoid - Disc like

structure that contains the chlorophyll which absorbs the light

• Grana – stacks of thylakoids

• Purpose is to increase the amount of light that can be absorbed

• Stroma – middle fluid part that contains the enzymes needed for photosynthesis

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Chlorophyll

• Chlorophyll collects light energy (absorbs it) in a resonant porphyrin group that hangs out like a kite on the surface of the thylakoid

• Chlorophyll a initiates the light-dependent reactions• Bright green

• Chlorophyll b is an accessory pigment • Duller green

• Carotenoids are yellow and orange pigments that capture light energy and pass electrons to chlorophyll• Xanthophyll (yellow)• Carotene (orange)

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Chlorophyll Structure

• Note the double bonds and the Mg2+ ion

• The positive charges on the Mg2+ ion attract electrons

• The electrons become delocalized over the porphyrin ring (so they can reside there for some time, then can move easily on)

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Two types of Chlorophyll

1. Light antenna• These chlorophyll’s are the ones that

gather the light energy• When the light hits it, the chlorophyll

vibrates and the molecules move rapidly to the reaction center

2. Reaction Center• These chlorphylls are the ones that boost

the electrons to a higher energy state

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The Antenna-Complex

Energy Funnel

• About 200 chlorophylls ‘inhabit’ an antenna complex

• Electrons move from one chlorophyll to another, losing energy (depicted by a red-shift in the electrons in this diagram)

h

Reaction center

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Each photosynthetic Unit contains a slightly different type of antenna and

reaction center

There are 2 types of photosynthetic units, or

photosystems

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Photosystems

• Photosystem I• Also called P700• Absorbs light that

has a 700 nm wavelength

• Can operate independently so probably evolved first

• Photosystem II• Also called P680• Absorbs light that

has a 680 nm wavelength

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Overview of Photosynthesis

• Photosynthesis is a redox reaction:

• Carbon dioxide is reduced to sugar• Water is oxidized to molecular

oxygen

6 CO2 + 12 H2O C6H12O6 + 6O2 + 6H2O

C6H12O6 + 6O26 CO2 + 6 H2O

Reduction

Oxidation

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Function in the Chloroplast• The light-dependent reactions (the harvesting

of light) occur on thylakoid membranes• The carbon fixation reactions (formation of

carbohydrate) occur in the stroma

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Two Energy Carriers of Photosynthesis

• ATP, and…• NADPH:

• Much like NADH except that it bears a phosphate

• Phosphate is attached to the sugar group

• NADPH is characteristic of anabolic redox processes

H added

PO4

NADH

NADPH

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Two Different Photosynthesis Reaction Sets

1. Cyclic photophosphorylation• e- run in a cycle• (photophosphorylation of ADP to make ATP) by

chemiosmosis• No carbohydrate made• Uses only P700

2. Noncyclic photophosphorylation• e- derived from splitting of water• Releases molecular oxygen• Makes ATP• Makes carbohydrate b/c NADPH (terminal e- acceptor)

passes to the Calvin Cycle• Uses P700 and P680

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Summary Equation for light dependent reactions of

photosynthesis

12H20 + 12 NADP+ + 18 ADP + 18Pi

602 + 12 NADPH + 18ATP

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How Photosynthesis Starts• Sunlight hits chloroplast in a leaf

(Remember the sun is the ultimate source of energy on earth)

• Automatically, water goes through photolysis and splits• H20 02 + H+ + e-

• This provides O2 for the atmosphere and H+ ions and electron’s for photosystem II

• Antenna chlorophyll A associated with PII absorbs the photon

• This absorbed energy moves from one chlorphyll to another and eventually falls into the reaction center

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• In the reaction center, an electron gets energized and moves to a higher energy level

• Energized electrons get accepted to a primary electron acceptor and moved through the electron transport chain

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• The primary electron acceptor passes the electron to plastoquinone then to cytochrome complex than to plastocyanin. Each step of the electron transport chain creates ATP by chemiosmosis

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• Now the “tired” electron needs more energy, so it travels to Photosystem I and received more sunlight

• It gets energized and accepted by a primary electron acceptor

• Primary electron acceptor passes the electron to 6 different electron acceptors, with the final acceptor as ferredoxin

• Ferredoxin transfers the electron to NADP+ forming NADPH which is released into the stroma**Sometimes Ferredoxin transfers the electron back to PI, so

the light dependent reactions can be referred to as being cyclic

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Noncyclic Photophosphorylation

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Protons Build up Inside Thylakoids

• The activity of the ETC causes a gradient of protons across the thylakoid membrane

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The Chloroplast ATP Synthase

• Protons fall back through the chloroplast ATP synthase

• Makes ATP by combining ADP and phosphate in a process called chemiosmosis

• Much like the mitochondrial ATP synthase

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Biology, Sixth Edition Chapter 8, Photosynthesis: Capturing Energy The Electron Transport Chain and

Chemiosmosis

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Ok so now what

• All the light dependent phase did was transfer the energy from the sun into ATP and NADPH.

• The carbon fixation stage is next and will use this energy to make glucose!

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The Dark Reactions• So-called because they do not directly need

light• They occur in the stroma of the chloroplast• They fix carbon to make carbohydrate• They are the Calvin-Benson Cycle reactions

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Calvin Cycle

• Melvin Calvin discovered it and received the nobel prize in 1961

• Calvin Cycle means “fixing carbon” or putting together carbons

• (not as difficult and complex as the Krebs)

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Summary Equation for the Dark Phase (i.e. Calvin Cycle)

12 NADPH + 18 ATP + 6C02

C6H1206 + 12 NADP+ + 18 ADP + 18Pi + 6H20

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3 phases of Calvin Cycle

1. Carbon dioxide Uptake2. Carbon reduction3. RuBP regeneration

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Carbon Dioxide Uptake

• Carbon dioxide reacts with a 5C molecule called RuBP (ribulose biphosphate)

• This reaction is catalyzed by the enzyme Rubisco. (ribulose bisphosphate carboxylase) This enzyme is present in the chloroplast in large amounts

• The product is an unstable 6C molecule which immediately breaks down to 2 molecules of PGA (stands for phosphoglycerate). Each of these PGA molecules have 3C each.

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Carbon Reduction

• PGA is converted to G3P (glyceraldehyde 3 phosphate) – which is also nicknamed PGAL

• This reaction requires a lot of ATP and NADPH

• G3P is then converted to sugars like glucose! (and other carbohydrates)

• Not all G3P’s make sugar some go on to phase 3

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Regeneration of RuBP

• G3P converts to RP (ribulose phosphate)• RP then converts to RuBP using ATP• Now RuBP is available to join again with

CO2

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The Calvin Cycle

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Calvin Cycle = C3 Pathway

• Its named this way because the intermediate product G3P has 3 carbons

• This pathway is very inefficient; less than 1% of the light energy that reaches the chloroplast is found in glucose

• It’s all Rubisco’s fault!

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When RUBISCO Doesn’t Work• In high light and temperature:

• Photosynthesis is very active• Water is easily lost• Leaf stomata close (small pores on the

underside of the leaf) to protect against water loss

• Oxygen builds up• Ribulose bisphosphate carboxylase (=

RUBISCO) (it’s a carboxylase because it adds a -COOH) has mixed affinity for oxygen and CO2

• It binds to O2 when O2 is abundant

• G3P is NOT produced

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Photorespiration• And as a result, photorespiration occurs:• In the presence of oxygen and light,

carbohydrates are oxidized and therefore No carbohydrate is produced

• Instead, CO2 and H2O are produced

• In C3 plants, 50% of the glucose made may be reoxidized back into Co2 due to this

• So photorespiration reduces the efficiency of C3 plants because Rubisco can bind to O2 as much as it binds to C02

• In C4, Co2 is very concentrated, so photorespiration is limited

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Evolutionary “Work-Arounds” Avoid Photorespiration

1. The C4 pathway• Physically sequesters the carbon dioxide-

requiring RUBISCO (carbon fixation) away from high oxygen

• Uses compartmentation with biological membranes (in different cells)

• In crabgrass, corn, and sugar cane2. Crassulacean acid metabolism (CAM)

• Carries out C fixation separated in time from high temperature and high oxygen

• In desert plants: succulents and cacti (also lilies and orchids)

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C4 Pathway• CO2 is used by PEP

carboxylase to convert PEP (3C) to oxaloacetate (4C)

• Oxaloacetate is reduced to malate (4C) by NADPH

• Malate is transported via plasmodesmata to RUBISCO in the bundle sheath cell

• Malate is decarboxylated to yield CO2 (1C) and pyruvate (3C), which is transported to the mesophyll cell (C3 pathway)

• Pyruvate is phosphorylated to PEP by ATP

• PEP carboxylase has a higher affinity for C02 than Rubisco in C3. So PEP carboxylase is faster at obtaining Co2

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C3 vs. C4 Leaf Anatomy

• C3• Have Rubisco in

mesophyll cells• Bundle sheath cells

have few chloroplasts and no Rubisco; so they can’t fix C02

• C4• Have PEP carboxylase

in mesophyll cells• Bundle sheath cells

have Rubisco!• Bundle sheath cells

are close to mesophyll cells which permits C02 transfer from mesophyll to bundle sheath cells for Calvin Cycle

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C3 versus C4 Anatomy

• Bundle sheath cells are arranged differently in C3 and C4 plants

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Comparing C3 and C4 Pathways

• Extensive Photorespiration

• Does Calvin Cycle• RuBP accepts Co2• Enzyme Rubisco

• First product of C02 fixation is PGA (3C)

• Moderate affinity for C02• Occurs in mesophyll cells

• Minimal photorespiration• Does Calvin Cycle• PEP accepts C02• Enzymes Rubisco and

PEP carboxylase• First product of C02

fixation is Oxaloacetate (4c)

• High affinity for C02• Occurs in mesophyll and

bundle sheath cells

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The CAM pathway• Seen in xeric plants (very dry conditions; desert)• Separation of function by TIME instead of

location / compartmentation• PEP carboxylase works at night when the

stomata are open (they close during the day in desert plants)

• Oxaloacetate is made and malate derived from it is stored in the vacuole

• During the day, CO2 is derived from the malate and made available to the Calvin cycle.

• Not as efficient at supporting rapid growth as is seen in C4 plants like crabgrass and corn, is good adaptation for desert plants

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Calvin cycle is all-important!

• In C3, C4 and CAM plants, the Calvin Cycle always receives the carbon dioxide that is made into carbohydrate...

• . . . under all conditions!

• Scientists are very interested in getting C4 chemistry genetically introduced into plants to increase crop efficiency.

copyright 2002 by harcourt college publishers, a division of thomson learning biology, sixth...

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