Summer
School
2015
L7 – Photosynthesis
Dr Agnieszka Adamczewska
Images from Wikimedia Commons
Major Concepts
After studying this lecture and the relevant references you should be
able to:
• Write, in words, the equation for photosynthesis
• Identify the structures in the chloroplast involved in photosynthesis
• Describe the processes that occur during light-dependent and lightindependent reactions
• Identify when water and carbon dioxide are used, and when oxygen
and glucose are made
• Name the main light harvesting pigments, electron transporting
complexes and ATP-synthesising complexes involved in
photosynthesis
• Explain “proton-motive force” and “chemiosmotic coupling” and how
they relate to photosynthesis
• Define the term “rubisco” and how it is used
• Define the role of ATP in photosynthesis
• Explain the difference between C3, C4 and CAM plants
Photoautotrophs
Autotrophy – produce organic molecules from CO2
and other inorganic raw materials
Autotrophs are the ultimate sources of organic
compounds for all non-autotrophic organisms
(heterotrophs)
Autotrophs are the producers of the biosphere
Photoautotrophs use light as the energy source
to synthesise organic substances
Photosynthetic organisms
A.P. Fri AM
Plants
C.L. & M.C. Thu PM
Algae
Eukaryotes
Cyanobacteria
Prokaryotes
Eucalyptus, Egeria and cyanobacteria images from Wikimedia Commons, Egeria and Spirogyra cells by Concepts in Biology students 2013
An overview of photosynthesis
Primary producers (green plants, photosynthetic
bacteria):
- are renewable food resources
- generate oxygen
- oxygen is a source of ozone (blocks UV
radiation and protects life)
- reduce carbon dioxide levels
Where does photosynthesis occur?
Photosynthetic eukaryotes use chloroplasts to
convert light energy into chemical energy
Leaf cross-section
Chloroplast
Images © McGraw-Hill Australia Pty Ltd
Leaf cell
Sun in, fuel out
“OILRIG”
CO2 + H2O + photons  CHO + O2
Water is oxidised
CO2 is reduced
Cytosol
Outer membrane
Light reactions
Thylakoid membrane
Inner membrane
Granum
ATP
Carbon fixation reactions
Chloroplast
NADPH
Stroma
Chloroplast image from Wikimedia Commons
Photosynthesis - two parts
Light dependent reactions - light-harvesting
complexes and photosystems that absorb light
energy and convert it by chemiosmosis into
electrical and then ATP energy, occurs on
thylakoid membranes
Dark (light independent) reactions - Absorbed ATP
energy is converted during a carbon fixation step
(Calvin cycle) into chemical energy (sugars) in the
stroma
Pigment molecules in thylakoid
membrane
Embedded in thylakoid membranes are protein
complexes:
• Light-harvesting complexes with pigment
molecules (chlorophyll a, b, carotenoids), which
absorb light mainly from the blue and red
regions of the spectrum
• Electron-transport complexes of Photosystem II
(PSII) and Photosystem I (PSI)
• ATP-synthesizing complexes
The solar spectrum
Electromagnetic radiation arrives from the sun
Photosynthetic
activity
400 nm
500 nm
600 nm
700 nm
Infrared
Chlorophyll a
Chlorophyll b
Carotenoids
“Action spectrum”
Only certain wavelengths are useful for
photosynthesis (practical 4)
Images from Wikimedia Commons
Pigments
Absorption
Ultraviolet
Photons excite electrons in pigments
Pigments absorb light
• Photon causes electron to be elevated to a higher energy
level
• When electron falls back down, releases heat and light
Chlorophyll a
“Accessory” pigments
Chlorophyll b
Chlorophyll c
Chlorophyll d
Chlorophyll f
Carotenoids
Images from Wikimedia Commons
Photosystems
Pigments are embedded into
large protein complexes
called photosystems
Thylakoid
membrane
Stroma
Photosystem
Thylakoid space
Images from Wikimedia Commons, RCSB PDB 2AXT
Photosystems
• Energy absorbed by pigments in antenna complex
• Energy is passed to the reaction centre
– A special protein called the electron acceptor
steals excited electrons as they fall
Reaction centre
Reaction centre has
Chlorophyll a
Electron acceptor
Stroma
Antenna
complex
Photosystem
Thylakoid
membrane
Thylakoid space
Light reactions
Light energy is converted into chemical energy
• Electrons from PSI received by NADP+  NADPH
• ETC brings electrons from PSII to PSI, PSII splits water
• ETC generates H+ gradient for ATP synthase
ADP + Pi
NADP+
ATP
NADPH
Thylakoid
membrane
Photosystem I
Electron transport chain
ATP synthase
Stroma
Photosystem II
½O2 + 2H+
H2O
Thylakoid space
Chloroplast image from Wikimedia Commons
Light dependent reactions
• PSI - photons strike pigment molecules, excite electrons - passed to
reaction centre with Chlorophyll a - passes excited electrons to electron
acceptor
• Electron acceptor passes electrons along protein complexes and final
electron acceptor is NADP+ makes NADPH; now PSI has “electron
holes” - need to be filled
• PSII - photon excites 2 electrons, move to electron carrier, ETC carries
them to PSI to fill “electron holes”
• PSII catalyses split of water into 2H+, O and 2 electrons that replace
those taken by electron acceptor
• ETC also pumps H+ into lumen to produce proton gradient inside
thylakoid membrane - this powers ATP synthase
• Products = NADPH (electron carrier) and ATP (energy)
NADP+ sounds like NAD+ ….
NADP+ = nicotinamide adenine
dicnucleotide phosphate
NADP+ + 2e- + H+  NADPH in
photosynthesis
NAD+ = nicotinamide adenine dinucleotide
NAD+ + 2e- + H+  NADH in respiration
Proton-motive force
Chemiosmosis
Cytosol
Outer membrane
Inner membrane
Outer membrane
Inner membrane
Stroma
ATP
ATP
Matrix
Intermembrane space
Thylakoid space
Matrix
Thylakoid membrane
Thylakoids increase surface area
Foldings of the thylakoid membrane greatly
increases the number of photosystems, electron
transport chain, and ATP synthase proteins
Stroma
Granum
Thylakoid membrane
Light reactions done …
CO2 + H2O + photons  CHO + O2
Light reactions
ATP
NADPH
Chloroplast image from Wikimedia Commons
Carbon fixation reactions in Calvin
cycle
Dark (light-independent) reactions:
• ATP and NADPH energy from light-dependent
reactions of photosynthesis drives the fixation of
atmospheric carbon (CO2), produce carbohydrates
• The reaction is catalysed by ribulose bisphosphate
carboxylase-oxygenase (Rubisco), which forms up
to 50% of chloroplast protein and is the most
abundant protein on earth!
Carbon fixation reactions
Energy (ATP & NADPH) is used to ‘fix’ inorganic
carbon (CO2) in the Calvin cycle in the chloroplast
stroma
stroma
3× CO2
3× Ribulose bisphosphate
RuBP
Rubisco
6x 3C molecules
ATP
ADP
ATP
For every 3x CO2
get 1x 3C sugar
ADP
NADPH
5x 3C molecules
CO2 + H2O + photons  CHO + O2
NADP+
1× G3P (triose sugar)
Chloroplast image from Wikimedia Commons
Synthesising carbohydrates
More complex CHOs can be made from the triose
E.g. glucose made by ‘reversing’ first half of
glycolysis
Cellulose
Cytosol
Starch
Sucrose
Remember glycolysis? Reverse it!
Glucose
Triose
Carbon fixation in Calvin cycle
by ribulose bisphosphate (RuBP)
• ATP and NADPH energy from light-dependent reactions of
photosynthesis drives the fixation of atmospheric carbon (CO2)
• Carbon fixation means the attachment of CO2 to a 5C-sugar,
ribulose bisphosphate (RuBP) (carboxylation reaction), forming
a short-lived 6C intermediate
• The reaction is catalysed by ribulose bisphosphate
carboxylase-oxygenase (Rubisco), which forms up to 50% of
chloroplast protein and is the most abundant protein on earth!
• The 6C intermediate splits into two molecules of (3C)
phosphoglyceric acid, which is converted (reduced) to
glyceraldehyde 3-phosphate using ATP and NADPH energy
• Glyceraldehyde 3-phosphate can follow one of three paths:
- regenerated into new RuBP, ready to accept another CO2
- exported to cytoplasm, rearranged into fructose and sucrose
(transport carbohydrates)
- rearranged and used for synthesis of starch (storage)
Photosynthesis
CO2 + H2O + photons  CHO + O2
Cytosol
Outer membrane
Light reactions
Thylakoid membrane
Inner membrane
Granum
ATP
Carbon fixation reactions
sugar
Chloroplast
NADPH
Stroma
Chloroplast image from Wikimedia Commons
The problem with RuBisCO
Ribulose-bis-phosphate Carboxylase Oxygenase
Adds CO2
5 C sugar with
2 Phosphate
Adds O2
Called
photorespiration
The binding site for CO2 can also bind O2 – i.e.
lack of specificity for CO2
Carbons:
5+1=
5+0=
3+3
2+3
+
+
Much more CO2 is fixed than O2 but wasteful to have oxygenation
When did RuBisCO evolve?
• [O2] in the atmosphere was low
• RubisCO very conserved e.g. algae and higher
plants
Photorespiration can
reduce the net
photosynthetic carbon
fixation by 25-50%
BUT: some plants
have improved their
CO2 fixing ability …
26
How does CO2
enter a plants?
+O2
27
CO2 diffuses to the chloroplasts
C4 plants
phosphoenolpyruvate
First stable product is…. C4
compound - C3 + CO2  C4
(malic/aspartic acid) .
Kranz
anatomy
28
C4 pathway of photosynthesis
• Photorespiration (a reverse of Rubisco reaction in Calvin cycle)
can reduce the net photosynthetic carbon fixation by 25-50%
• Some plants (eg. tropical grasses) have evolved a more
efficient carbon fixation based on 4C compounds
• C4 plants have special bundle sheath cells surrounding the
vascular bundle in addition to mesophyll cells; the two cell
types have chloroplasts of unique structure and function
• Mesophyll cell cytoplasm contains additional carboxylation
enzyme, phosphoenolpyruvate (PEP) carboxylase, which binds
CO2 and forms a C4 acid (oxaloacetate, malate, aspartate).
• C4 acid enters the chloroplast in bundle sheath cells and is
decarboxylated, releasing CO2 for fixation in Calvin cycle
• C4 photosynthesis allows concentration and storage of CO2,
inhibiting photorespiration. Additionally it minimizes the
opening of stomata and thus reduces transpiration water loss
Photosynthesis
CO2 + H2O + photons  CHO + O2
Cytosol
Outer membrane
Light reactions
Thylakoid membrane
Inner membrane
Granum
ATP
Carbon fixation reactions
sugar
Chloroplast
NADPH
Stroma
Chloroplast image from Wikimedia Commons
• Read:
• Knox et al. (2010) Biology: An Australian Focus.
4th Edition.
- Chapter 6 Harvesting Energy (from page 124)