Photosynthesis
Chapter 8
Photosynthesis
• Energy for the majority of life on Earth ultimately
comes from the sun through photosynthesis
• Photosynthesis is the process that converts
energy of sunlight the into chemical energy,
especially carbohydrates
• Carbon Fixation: Photosynthesis collects the
energy of sunlight and captures it into a chemical
form
• Photosynthesis uses CO2 to build carbohydrates
(C6H12O6)
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Photosynthesis
• Photoautotrophs: photosynthetic organisms that
can produce their own ‘food’
– these organisms fix the energy of sunlight into
carbohydrate form
– they then use the carbohydrates they produce for
cellular respiration
• Heterotrophs: organisms that require an
alternative food source
– these organisms consume photoautotrophs along with
the carbohydrates they produce
– they then use the carbohydrates they consume for
cellular respiration
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Photosynthesis
•
Oxygenic photosynthesis: produces O2
- carried out by cyanobacteria, 7 groups of algae, all
land plants
• Photosynthetic organisms serve as a food and
energy source for the majority of life on on earth
• Photosynthetic organisms directly or indirectly
provide energy for the majority of the life on earth
- most living things depend on the sun for energy
- a few exceptions include chemoautotrophs:
autotrophic organisms that utilize energy from other
sources, live independently from the sun
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Oxygenic Photosynthesis
• Energy Fixation
• Maintains O2 in the atmosphere
•
vital for aerobic respiration
• Removes CO2 from the atmosphere
• green house gases
• H2O production
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Photosynthesis
6CO2 + 12H2O
carbon dioxide
water
C6H12O6 + 6H2O + 6O2
glucose
water
oxygen
• Reactants: 6CO2 + 12H2O
• Products: C6H12O6 + 6H2O + 6O2
• Photosynthesis vs. Cellular Respiration:
6CO2 + 12H2O
C6H12O6 + 6O2
C6H12O6 + 6H2O + 6O2
6CO2 + 12H2O
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Photosynthesis vs. Cellular Respiration
• Photoautotrophs
• Chloroplasts
– grana and
• ETC
• Heterotrophs
• Mitochondria
thylakoids - mitochondrial membrane,
cisternae
– NADP reduced to NADPH • Krebs Cycle
- Carbohydrates
– ATP synthase, ATP
CO2
production
• Calvin Cycle
– CO2 reduced to
carbohydrates
oxidized to
• ETC
- NAD+ reduced to NADH
- ATP synthase, ATP
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production
Photosynthesis
Photosynthesis is divided into two reactions:
1. Light-dependent reactions
• capture energy from sunlight
• reduce NADP+ to NADPH
• produce ATP
• occur in chloroplasts
2. Carbon fixation reactions
• use ATP and NADPH to synthesize organic
molecules from CO2
• Occurs in stroma
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Photosynthesis Overview
• Photosynthesis takes place in chloroplasts
• Thylakoid membrane – internal
membrane arranged in flattened sacs
• contain chlorophyll and other pigments
• Grana – stacks of thylakoid membranes
• Stroma – semiliquid substance
surrounding thylakoid membranes
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Fig. 8.1-1
Fig. 8.1-2
Discovery of Photosynthesis
• The work of many scientists led to the
discovery of how photosynthesis works:
- Jan Baptista van Helmont (1580-1644)
- Joseph Priestly (1733-1804)
- Jan Ingen-Housz (1730-1799)
- F. F. Blackman (1866-1947)
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Discovery of Photosynthesis
• C. B. van Niel, 1930‘s
- proposed a general formula:
CO2+H2A + light energy
CH2O + H2O + 2A
- H2A is the electron donor
• van Niel identified water as the source of the
O2 released from photosynthesis
• Robin Hill confirmed van Niel’s proposal that
energy from the light reactions fuels
carbon fixation
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Energy from Light Reactions Fuels
Carbon Fixation
• Photon: a particle of light
- acts as a discrete bundle of energy
- energy content of a photon is inversely proportional
to the wavelength of the light
• Photoelectric effect: removal of an electron
from a molecule by light
- occurs when photons transfer energy to electrons
• Light energy is absorbed by pigments:
molecules that absorb visible light
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Energy from Light Reactions
Fuels Carbon Fixation
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Pigments
• Pigments: molecules that absorb visible
light
- chlorophyll a
- chlorophyll b
- carotenoids
• Each pigment has a characteristic
absorption spectrum: the range and
efficiency of photons it is capable of
absorbing.
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Pigments
• Chlorophyll a – primary pigment in plants
and cyanobacteria
- absorbs violet-blue and red light
• Chlorophyll b – secondary pigment
absorbing light wavelengths that chlorophyll
a does not absorb
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Pigments
• Structure of pigments:
• Porphyrin ring: complex ring structure with
alternating double and single bonds
- magnesium ion at the center of the ring
- photons excite electrons in the ring
- electrons are shuttled away from the ring
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Pigments
• Accessory pigments: secondary pigments
absorbing light wavelengths other than those
absorbed by chlorophyll a
- increase the range of light wavelengths that
can be used in photosynthesis
- include: chlorophyll b, carotenoids,
phycobiloproteins
- carotenoids also act as antioxidants
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Photosynthesis: Photosystems
• Photosystem: Enzyme complexes for
photosynthesis
– enzymes use light to oxidize H2O and reduce
CO2 to carbohydrates
• Located in the thylakoid membrane of
plants, algae and cyanobacteria or the
cytoplasmic membrane of photosynthetic
bacteria
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Photosystem Organization
• A photosystem consists of:
1. an antenna complex of hundreds of accessory
pigment molecules
2. a reaction center of one or more chlorophyll a
molecules
• Energy of electrons is transferred through
the antenna complex to the reaction center
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Photosystem Organization
• At the reaction center, the energy from the
antenna complex is transferred to chlorophyll a
• This energy causes an electron from chlorophyll
to become excited
- chlorophyll absorbs a photon, looses an electron
• The excited electron is transferred from
chlorophyll a to an electron acceptor, quinone
• Water donates an electron to chlorophyll a to
replace the excited electron
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Light-Dependent Reactions
Light-dependent reactions occur in 4 stages:
1. Primary Photoevent – a photon of light is
captured by a pigment molecule
2. Charge Separation – energy is transferred to
the reaction center; an excited electron is
transferred to an acceptor molecule
3. Electron Transport – electrons move through
carriers to reduce NADP+
4. Chemiosmosis – produces ATP (ATP synthase)
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Light-Dependent Reactions
• In sulfur bacteria, only one photosystem is
used for cyclic photophosphorylation
1. Hydrogen sulfide oxidized
- electrons joins a proton to produce hydrogen
- elemental S and protons produced as products
2. An electron is recycled to chlorophyll
- this process drives the chemiosmotic synthesis
of ATP
• This system only produces energy
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Light-Dependent Reactions
• In chloroplasts, two linked photosystems are
used in noncyclic photophosphorylation
1. Photosystem I
- reaction center pigment (P700) with a peak
absorption at 700nm
2. Photosystem II
- reaction center pigment (P680) has a peak
absorption at 680nm
• This system produces energy, O2, and
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NADPH for biosynthesis of carbohydrates
Light-Dependent Reactions
• Photosystem II acts first:
- accessory pigments shuttle energy to the P680
reaction center
- excited electrons from P680 are transferred to
the cytochrome/b6-f complex - connects
photosystems II and I
- electron lost from P680 is replaced by an
electron released from hydrolysis: the splitting
of water
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Light-Dependent Reactions
• The b6-f complex is a series of electron
carriers
- an electron transport chain
- electron carrier molecules are embedded in
the thylakoid membrane
- protons are pumped into the thylakoid space to
form a proton gradient
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Light-Dependent Reactions
• Photosystem I:
- receives energy from an antenna complex
- energy is shuttled to P700 reaction center
- excited electron is transferred to a membranebound electron carrier
- electrons are used to reduce NADP+ to
NADPH
- electrons lost from P700 are replaced from the
b6-f complex
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Fig. 8.13
Z Diagram of
Photosystems I and II
Fig. 8.13-1
Fig. 8.13-2
Fig. 8.13-3
Light-Dependent Reactions
• ATP is produced via chemiosmosis:
- ATP synthase is embedded in the thylakoid
membrane
- protons have accumulated in the thylakoid
space
- protons move into the stroma only through ATP
synthase
- ATP is produced from ADP + Pi
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Carbon Fixation Reactions
Calvin cycle
• biochemical pathway that allows for
carbon fixation
• incorporates CO2 into organic molecules
• occurs in the stroma
• uses ATP and NADPH as energy sources
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Carbon Fixation Reactions
To build carbohydrates, cells need:
1. Energy
- provided by ATP from light-dependent
reactions
2. Reduction Potential: hydrogen atoms
- provided by NADPH from photosystem I
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Carbon Fixation Reactions
The Calvin cycle has 3 phases:
1. Carbon Fixation
- RuBP + CO2
2 molecules PGA 3 phosphoglycerate
2. Reduction
- PGA is reduced to G3P glyceraldehyde-3-phosphate
3. Regeneration of RuBP
- G3P is used to regenerate RuBP
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Carbon Fixation Reactions
• Carbon fixation – the incorporation of CO2
into organic molecules
- occurs in the first step of the Calvin cycle:
ribulose-bis-phosphate + CO2
5 carbon molecule
2(PGA)
1 carbon
3
carbons
• Reaction is catalyzed by rubisco
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Fig. 8.16-1
Fig. 8.16-2
Fig. 8.16-3
Carbon Fixation Reactions
• Glucose is not a direct product of the Calvin
cycle
- 2 molecules of G3P leave the cycle
- each G3P contains 3 carbons
- 2 G3P are used to produce 1 glucose in
reactions in the cytoplasm
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Carbon Fixation Reactions
• During the Calvin cycle, energy is needed.
The energy is supplied from:
- 18 ATP molecules
- 12 NADPH molecules
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Carbon Fixation Reactions
• The energy cycle:
- photosynthesis uses the products of
respiration as starting substrates
- respiration uses the products of
photosynthesis as starting substrates
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Photorespiration
• Rubisco has 2 enzymatic activities:
1. Carboxylation – the addition of CO2 to
RuBP
- favored under normal conditions
2. Photorespiration – the oxidation of RuBP
by the addition of O2
- favored in hot conditions
- CO2 and O2 compete for the active site on
RuBP.
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Fig. 8.19-1
Fig. 8.19-2
Photorespiration
• Some plants can avoid photorespiration by
using an enzyme other than rubisco.
- PEP carboxylase
- adds CO2 to phosphoenolpyruvate (PEP)
- a 4 carbon compound is produced
- CO2 is later released from this 4-carbon
compound and used by rubisco in the Calvin
cycle
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Photorespiration
• C4 plants
- use PEP carboxylase to capture CO2
- CO2 is added to PEP in mesophyll cell
- the resulting 4-carbon compound is moved into
a bundle sheath cell where the CO2 is released
and used in the Calvin cycle
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Photorespiration
• CAM plants
- CO2 is captured at night when stomata are
open
- PEP carboxylase adds CO2 to PEP to produce
a 4 carbon compound
- this compound releases CO2 during the day
- CO2 is then used by rubisco in the Calvin cycle
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