PhotosynthesisLecture 7Fall 2008

PhotosynthesisPhotosynthesis• The process by which light energy from the

sun is converted into chemical energy

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PhotosynthesisInputs• CO2

– Gas exchange occurs through stomata– Stomata (stoma) – small pore– CO2 in, O2 out

• H2O– enters roots from soil

• Energy from sunlight• Also need minerals from soil

– e.g. potassium (K), nitrogen (N), and phosphorous (P)

• Also need O2 from soil

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The Chloroplast3

Fig.10.3

The Chloroplast

Structure• Outer membrane• Inner membrane• Stroma

– Thick fluid inside inner membrane• Thylakoids

– Membrane bound sacs– Interconnected– Photosynthetic pigments

embedded in membranes– Thylakoid space

• Interior of the thylakoid

• Grana (granum)– Stacks of thylakoids– Large surface area

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Fig.10.3

Photosynthetic prokaryotes• Infoldings of plasma membrane allow for specialized

functions• Endosymbiosis of photosynthetic prokaryote led to chloroplast

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• Read Tracking Atoms through Photosynthesis: Scientific Inquiry, pgs. 187-188

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Photosynthesis & Cellular Respiration

Cellular respiration• Redox reactions move electrons (and hydrogen)

from glucose to oxygen• “fall” of electrons• Produces energy in the form of ATPPhotosynthesis• Redox reactions move electrons (and hydrogen)

from water to carbon dioxide to form glucose• Electrons moved “uphill”• Requires large initial investment of energy

(sunlight)• Produces energy in the form of glucose

molecules

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Photosynthesis Overview

Two metabolic stages of photosynthesis

Each process occurs in a specific area

• Light Reactions– In the thylakoids

• Calvin cycle– In the stroma

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Fig. 10.5

Photosynthesis Overview

• Light Reactions– Convert solar energy to

chemical energy• ATP & NADPH

– Water split

• Calvin cycle– Synthesizes sugar from

CO2

– Uses the ATP & NADPH produced in the light reactions

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Fig. 10.5

The Nature of Sunlight

Electromagnetic energy (electromagnetic radiation)

• Radiation = emission of energy in the form of electromagnetic waves or photons– Wavelength - distance

between the crests of two adjacent waves

– Photon - discrete packet of energy

• Electromagnetic spectrum– Range of wavelengths

of electromagnetic energy

•Gamma rays–Short waves–High energy

•Radio waves–Long waves–Low energy

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Fig. 10.6

The Nature of Sunlight

• Sunlight radiates the full spectrum

• Our atmosphere filters out much of the spectrum

• Visible light– Passes through

atmosphere– Light that humans can

see with our eyes (colors)– Wavelengths that powers

photosynthesis

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Fig. 10.6

The Nature of SunlightWhen light meets matter:• Reflected

– Wavelengths “bounce back” from matter• Transmitted

– Wavelengths pass through matter • Absorbed

– Wavelengths “disappear” into matter

Pigments• Chemical compounds that absorb certain wavelengths of

light– We only see “color” of wavelength that is reflected or transmitted– If a pigment absorbs all wavelengths, then we see black– If a pigment absorbs wavelengths from 380 to 550, what color

would we see?

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Which Wavelengths are Used in Photosynthesis: The Scientific Method at Work

Question:• Which wavelengths are used in photosynthesis?Observations:• Photosynthetic organisms use visible light from

the sun• Visible light comes in many wavelengths• By using a prism, light can be separated into its

wavelengths• Unicellular algae are photosynthetic organisms • Bacteria tend to gather in areas of high oxygen

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Which Wavelengths are Used in Photosynthesis: The Scientific Method at Work

Hypothesis:• Algae will photosynthesize when exposed to its

ideal wavelengthsPredictions:• If the algae photosynthesize in response to a

particular wavelength, then O2 will be released in that area

• If O2 is produced in an area, then aerobic bacteria will gather in that area

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Which Wavelengths are Used in Photosynthesis: The Scientific Method at Work

Methods:• Algae placed in strip

on microscope slide• Bacteria add to slide• Light shown through a

prism onto slide

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Which Wavelengths are Used in Photosynthesis: The Scientific Method at Work

Results?

Conclusions?

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Fig. 10.9

Why are Leaves Green?

• Inside chloroplasts are photosynthetic pigments– Pigments – chemical

compounds that absorb certain wavelengths of light

• Chlorophyll a absorbs blue-violet and red light

• Given that info – why are leaves green?

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Fig. 10.7

Photosynthetic Pigments in Chloroplasts

Chlorophyll a– Required for photosynthesis– Absorbs blue-violet and red light

Accessory pigments• Pigments other than chlorophyll a • Broadens the spectrum of light that can be

absorbed & used for photosynthesis• Used as a “sunscreen” – protection against

UV radiation• Coloration – attract pollinators to flowers,

attract fruit dispersers to fruit

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Photosynthetic Pigments in Chloroplasts

Accessory pigments• Chlorophyll b

– Present in plants, some algae

– Absorbs blue and orange light

• Carotenoids– Present in plants, algae,

cyanobacteria– Absorb blue-green light

• Chlorophyll c– Present in some algae

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Why would so many different pigments evolve?

Fig. 10.9

Photosystems

• The structure of thylakoids and position of pigments critical to function

• Pigments arranged into photosystems– Photosystem – reaction center plus light harvesting

complexes within the plasma membrane

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Fig. 7.10

PhotosystemsReaction center complex • Protein complex with:

– Special chlorophyll a molecule (2)– Primary electron acceptor

Light harvesting complex• Protein complex with many

photopigments (chl a, b, carotenoids)– Able to harvest light over broader

spectrum

How light is “harvested”• Photon absorbed by a pigment

molecule• Energy transferred from one

pigment molecule to another• Energy ultimately passed to chl

a in reaction center

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Fig. 10.12

Light EnergyWhat happens when energy from photon

arrives at the reaction center?• Electron from chl a is excited What is an “excited” electron?• Electron receives energy and move to

an ‘”excited” state– Higher orbital – more potential

energy• Unstable position, so electron falls

back down to ground stateProcess releases energy:• Heat• Light - florescenceIf there is a molecule to receive the

electron, it retains its high energy and does not fall to the ground state

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Fig. 10.11

Photosystems

What happens when energy arrives at the reaction center?

• Electron from chl a is excited• Electron passed to the primary

electron acceptor– Redox reaction

• Two paths for electrons, depending on photosystem type:– Creates NADPH– Gets passed to electron transport chain

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Photosystems

Two types:• Photosystem 2 or PSII

– Water-splitting photosystem

• Photosystem 1 or PS1 – NADPH-producing photosystem

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Fig. 10.13

PS1 (NADPH-producing photosystem)

• Electrons from reaction center chl a excited– P700

• Passed to primary electron acceptor

• Primary electron acceptor passes electrons ferredoxin (FD)

• FD transfers electron to NADP+– NADP+ reduced to NADPH– Requires NADP+ reductase– 2 electrons

• NADPH will take electrons to the Calvin cycle– Energy to produce sugar

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Fig. 10.13

PS1 (NADPH-producing photosystem)

Problem: If the electrons from the reaction center chl a get passed on to an electron acceptor, how do they get replaced?

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Fig. 10.13

PhotosystemsWhich is the stronger electron acceptor?1) the reaction center chl a in NADPH-producing

photosystem (PS1)Or2) the primary electron acceptor in the water-splitting

photosystem (PS2)

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Fig. 10.13

Photosystems

Electrons from water-splitting photosystem (PS2) “pulled” down electron transport chain by the reaction center chl a in the NADPH-producing photosystem (PS1)

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Fig. 10.13

PS2 - Water-splitting photosystem

• Electrons from reaction center chl a excited– P680

• Passed to primary electron acceptor

How do the electrons get replaced?

• Take electrons from H2O– Water-splitting step– Requires enzyme

– O2 forms & 4H+

What is the strongest electron acceptor (oxidizing agent) in these photosystems?

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Fig. 10.13

PS2 - Water-splitting photosystem

• Primary electron acceptor passes electron to electron transport chain

• Replaces electron in P700 chl• Entire process (PS2 – PS1) called linear

electron flow

Fig. 10.13

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Electron Transport ChainETC composed of many protein complexes embedded in

the thylakoid membrane• Plastoquinone (Pq)• Cytochrome complex• Plastocyanin (Pc)What benefit is gained from being in the thylakoid

membrane?

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Fig. 10.17

Electron Transport Chain

• Electrons provide by primary electron acceptor of PSII

• Electrons ”fall” down chain– Pulled by P700 chl

• Produces energy at each step

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Fig. 10.17

Electron Transport Chain

• Transfer of electrons activates transfer of H+ • H+ moved from stroma, across the thylakoid

membrane, and into the thylakoid space• Creates a concentration gradient of H+ across

the thylakoid membrane

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Fig. 10.17

ATP ProductionATP synthase

– Complex of proteins built into the inner membrane of the thylakoid

Chemiosmosis• Concentration gradient of H+ harnessed

to do cellular work • Proton-motive force

– The thylakoid membranes are not freely permeable to H+

– Path down concentration gradient is through ATP synthase

– As H+ travels through ATP synthase, it causes turbine-like structures to turn, activating enzymes

– Enzymes generate ATP fromADP + P = phosphorylation

– Photophosphorylation• Using energy from light to excite electrons that

go down the ETC

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Fig. 9.14

ATP Production

ATP synthase• H+ ions enter through half-channel

on stator• Enter binding sites on rotor

– Changes conformation, rotor spins

• One rotation, H+ exits through hal-channel

• Rotor spin causes rod to spin• Spinning rod activates catalytic

sites on knob• ATP produced from ADP + P

– Read Fig. 9.15 Inquiry

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Fig. 9.14

Cyclic Electron Flow

• FD passes electron to cytochrome complex

• Produces ATP

Fig. 10.15

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Light Reactions Summary

Light Reactions• Convert solar energy to

chemical energy– ATP & NADPH

• Requires H2O

• Produces O2 as “waste product”

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Fig. 10.5

The Calvin Cycle

Calvin cycle

• Synthesizes sugar from CO2

• Anabolic

Inputs• ATP• NADPH

• CO2

Output• Organic compound - G3P

(glyceraldehyde 3-phosphate)– Used to make glucose and

other compounds

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Fig. 10.5

The Calvin Cycle

3 Phases:

• Carbon Fixation

• Reduction

• Regeneration of RUBP

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The Calvin CycleCarbon fixation• CO2 enters one at a time• Attached to ribulose bisphosphate (5-carbon sugar) to

become 6-carbon intermediary– Enzyme: rubisco (RuBP carboxylase)– Most abundant enzyme

• Splits into 2 molecules of 3-phosphoglycerate

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Fig. 10.18

The Calvin Cycle

Reduction• 3-phosphoglycerate

phosphorylated by ATP– 1,3 bisphosphoglycerate

• 1,3 bisphosphoglycerate reduced by NADPH– G3P (glceraldehyd-3-

phosphate)– High potential energy

• One molecule of G3P is output

Fig. 10.18

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The Calvin Cycle

Regeneration of RuBP

• 5 molecules of G3P rearranged into 3 molecules of RuBP

• Requires ATP

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After the Calvin Cycle

What happens to the G3P produced in photosynthesis?

Transport to other cells1. Produces glucose & fructose2. They combine to form sucrose3. Sucrose transported to other cells• If growing cell

– Sucrose broken down to glucose & fructose & used in cellular respiration & growth

• If storage cell– Sucrose converted to starch

Starch production in photosynthetic cell• Starch broken down overnight to supply cellular

respiration

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