Photosynthesis: Capturing Energy Chapter 8. Light Composed of photons – packets of energy Visible...
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Transcript of Photosynthesis: Capturing Energy Chapter 8. Light Composed of photons – packets of energy Visible...
Photosynthesis: Capturing Energy
Chapter 8
Light
• Composed of photons – packets of energy• Visible light is a small part of the
electromagnetic spectrum• All energy travels as waves• Wavelength is the distance from 1 wave
peak to the next• Shorter wavelengths have more energy
than longer wavelengths
• Visible light: 380-760 nm
• Energy from visible light is used in photosynthesis
• Why? • Longer
wavelengths don’t have enough energy; higher wavelengths have too much
TV andradio
waves
Micro-waves
Infrared
Visible
UV
X-rays
Gammarays
Colorspectrumof visiblelight
Red
Orange
Yellow
Green
Blue
Violet
760 nm
700 nm
600 nm
500 nm
400 nm380 nm
One wavelength
Longer wavelength
Electromagneticspectrum Shorter wavelength
Chloroplasts
• Organelles enclosed by a double membrane• Located mainly within mesophyll cells
inside a leaf• Each cell contains 20-100 chloroplasts• This portion of the leaf has many air spaces
and a high water vapor content
Palisademesophyll
Vein
Stoma
Spongy mesophyll
Outer membraneThylakoids
Intermembranespace
Thylakoidmembrane
StromaInnermembrane
Granum(stack ofthylakoids)
Thylakoidlumen
Chlorophyll
• Main photosynthetic pigment: • chlorophyll a (initiates the light-dependent reactions)
• Accessory pigments: chlorophyll b, carotenoids (these absorb different wavelengths of light and pass the energy on to chlorophyll a)
• These pigments are found in the thylakoid membranes of chloroplasts
• Pigments reflect the color of light we see; absorb the other colors
• Is green light used during photosynthesis? Why or why not?
Overview of photosynthesis
• Hydrogens from water reduce carbon dioxide; oxygen from water is oxidized
• Photosynthesis is a redox reaction:
6 CO2 + 6 H2O C6H12O6 + 6 O2
reduction
oxidation
Overview…
• Two phases:1st: Light-dependent (‘photo’)
Occurs in the thylakoids of the chloroplasts
H2O is split and molecular oxygen is releasedElectrons energized by light produce ATP and NADPH
which are both needed for the endergonic next phase
2nd: Carbon-fixation (‘synthesis’)Occurs in the stroma of the chloroplastsATP and NADPH provide the energy needed for the formation of carbohydrates
Overview…
Light reactions Carbon-fixation reactions
Light reactions
Calvin cycle
ATP
ADP
NADPH
NADP+
H20 02C02
carbohydrates
Light-dependent Reactions
• Occur in the thylakoids• Energy is absorbed from light and
converted to chemical energy stored in ATP and NADPH
• Oxygen is released• Photosystem I and II both involved – these
have similar pigments but different roles
Photosystems I and II
• Each system includes:• Chlorophyll a molecules and associated
proteins• Multiple antenna complexes• Photosystem I = P700 - wavelength absorbed• Photosystem II = P680 - wavelength absorbed
Primaryelectronacceptor
Photon
Photosystem
Chloroplast
Thylakoid
Antennacomplexes
Reactioncenter
e–
Light reactions: Step #1
• Light energy forces e- to a higher energy level in 2 chlorophyll a molecules of PS II (e- is excited)
• e- leave chlorophyll a (oxidation)
• a replacement e- is donated by H2O:• 2H2O 4 H+ + 4 e- + O2
• This is noncyclic electron transport
Light reactions: Step #2
• e- goes to a primary electron acceptor in the thylakoid membrane (reduction)
Light reactions: Step #3
• e- donated from the primary electron acceptor to a series of molecules in the thylakoid membrane – the electron transport chain
• e- lose energy as they move through the chain – this energy moves H+ into the thylakoid lumen
• this H+ gradient will be used to produce ATP from ADP and Pi using ATP synthase
Light reactions: Step #4
• Light is absorbed by PS I• e- leave chlorophyll a and go to another
primary electron acceptor • these e- are replaced by e- from the electron
transport chain• This is cyclic electron transport
Light reactions: Step #5
• e- from the primary electron acceptors in PS I go to another electron transport chain on the stroma side of the thylakoid membrane
• e- with H+ and NADP+ NADPH• ATP + NADPH made during the light
reactions are both needed to power the carbon-fixation reactions
Primaryelectronacceptor
Primaryelectronacceptor
NADPHNADP+
H2O
ATP
O2
ADP
1
2
Photosystem II(P680)
Productionof ATP by
chemiosmosis
H+
Ferredoxin
Plastiquinone
Cytochromecomplex
Plastocyanin
1/2 + 2 H+
Pi
A0
A1
FeSx
FeSB
FeSA
Photosystem I(P700)
(from medium)
Electrontransport
chain
Electrontransport
chain
ATP Synthesis and electron transport… the main ideas
• Electrons (e-) move down the electron transport chain and release energy as they go
• Protons (H+) move from the stroma to the thylakoid lumen, creating a proton gradient
• The greater concentration of H+ lowers the pH• The thylakoid membrane is impermeable to H+
except through ATP synthase• The flow of H+ through ATP synthase generates
ATP
Carbon-fixation reactions
• Also known as the Calvin Cycle or the light-independent reactions
• Occur in the stroma
• CO2 + chemical energy from ATP and NADPH are used to make organic compounds – carbon is ‘fixed’
Three phases of the Calvin Cycle
1. CO2 uptake
2. Carbon reduction
3. RuBP regeneration
Carbon fixation: Step #1
CO2 uptake:
CO2 + RuBP* unstable 6-C molecule, which splits 2 3-C molecules + 3 PGA**
*RuBP = ribulose biphosphate (5 C)
**PGA = 3 phosphoglycerate
Carbon fixation: Step #2
Carbon reduction:
2 molecules of 3-PGA are converted to 2 molecules of G3P* in a 2-part process, using the energy from ATP and the H+ from NADPH from the light reactions
* G3P = glyceraldehdye 3-phosphate
Carbon fixation: Step #3
RuBP regeneration:
One molecule of G3P leaves the Calvin cycle to be converted into carbohydrates such as glucose or starch
The other G3P molecule uses the energy from ATP and is converted back to RuBP
The RuBP is returned back to the Calvin cycle
CO 2 molecules arecaptured by RuBP,resulting in an unstableintermediate that isimmediately brokenapart into 2 PGA
PGA is phosphorylatedby ATP and reduced byNADPH. Removal of aphosphate results information of G3P.
Through a series ofreactions G3P isrearranged into newRuBP molecules oranother sugar
Glucose and othercarbohydrate synthesis
2 moleculesof glyceraldehyde-3-phosphate (G3P)
6 molecules ofribulose bisphosphate(RuBP)
CALVINCYCLE
Carbonreduction
phase
RuBPregeneration
phase
CO2 uptakephase
12 NADPH
12 ADP
12 molecules ofphosphoglycerate(PGA)
12 ATP
6 molecules of CO2
10 moleculesof G3P
6 molecules of ribulosephosphate (RP)
ATP
6 ADP
P
P
P
P
P
P P
P
12 moleculesof glyceraldehyde-3-phosphate (G3P)
1
32
12 NADP+
Adjustments based on weather…
• C3 plants – use the C3 pathway – the initial carbon fixation product is a 3-C sugar
• These plants must close their stomata during hot, dry weather to reduce water loss
• This reduces carb production
• Adaptations for hot, dry environments:• C4 plants – 1st fix CO2 into a 4-C compound• CAM plants – fix CO2 at night (cactus)
• Table 8-2 p. 167