The process of photosynthesis

BEIRA HAILU bh.bio.pl@gmail.com 1

Physical nature of light
 To understand photosynthesis one should understand the
physical nature of light
 Light : is a form of radiant energy , a narrow band of energy with
in the continuous electromagnetic spectrum of radiation emitted
by the sun
 The term ‘light’ describes that portion of the electromagnetic
spectrum that cause the physiological sensation of vision in
human
 Light is defined by the range of wavelengths between 400-700
nm.

 Electromagnetic spectrum
 Visible radiation (light) 400-700 nm
 Infrared radiation - >700 nm
 Ultraviolet radiation 100-400 nm
 Colour is determined by wavelength of the radiation
 ATRIBUTES OF LIGHT
 Wave length
 Particle property
 Important in understanding the biological functions of light

Wave property
o Characterized by wave length or frequency
 Wavelength ()–the distance between successive crests
 Frequency ()- number of wave crests passing a point in
one second
 Frequency is related to wave length as:
Frequency =speed of light /wavelength
= = C/

 Particle property
 Light behaves as if its energy is divided in to particles
called photons when it is emitted
 Photons carry energy termed quantum and is related
to frequency and wave length.
 Thus,
Eq = hc/=h
h=planck’s constant=6.62*10-14 Js photon -1
 Quantum energy is inversely proportional to its
wavelength

Photons of violet end of the spectrum have highest energy
while photons of infrared have lowest energy
Eg.
 Light >1200 nm
 low energy content
 Too low to mediate chemical reaction
 Energy absorbed is converted to heat

 2oo-1200 nm
 Sufficient to produce a chemical change
 PAR is found with in this range
Photosynthetically active radiation
400 nm(blue end)-700 nm(red end)
Optimum wavelength for driving photosynthesis
 Other regions of the spectrum are absorbed by
molecule in the atmosphere

ABSORPTION SPECTRA
 Not all the wavelength of
light can be absorbed by the
plant pigment
 The chlorophyll can absorb
waves of certain length with
in the range of visible light
 Different chlorophylls show
different absorption peaks on
different region of the band

PHOTOSYNTHETIC MOLECULES
Plants posses pigment molecules that absorb
physiologically useful radiations
Called photoreceptors
 Process the energy and information content of light
into a form that can be used by the plant

Principal molecules
Chlorophyll
Chlorophyll a,b,c,d,e
Bacteriochlorophyll
a, b
Chlorobium 650,660
Carotenoids Phycoblins

Chlorophyll
 Is primarily responsible for harvesting light energy used in
photosynthesis
 Chlorophyll structure : has two parts
I. Porphyrin head
 Cyclic tetrapyrrole
 Made up of four nitrogen containing pyrrole rings arranged in
cyclic fashion
 Magnesium ion is chelated to the four nitrogen atoms in the
center of the ring
 Loss of Mg ion leads to formation of pheophytin

 Requires light for their synthesis
 Yellow appearance of etiolated leaves is due to lack
of light
 The reduction of proto chlorophyll to chlorophyll is
accomplished at the expense of light absorbed by
the protochlorophyll
 The reduction of the bond is catalysed by the enzyme
NADPH: protochlorophyll oxidoreductase
 Light sensitive part in angiosperm

II. Phytol tail
 Long, lipid-soluble hydrocarbon tail (20 C alcohol)
 Makes the molecule very hydrophobic
 Important for orientation and anchoring of
chlorophyll molecule in the chlorophyll membrane

Difference in chemical
structure
Ring II:
• Chll a: CH3
• Chll b: CHO
Ring I
• Chll a :CH2=CH
• Chll d: O-CHO
Chll c : lacks phytol tail

Carotenoids
 Comprises a family of orange and yellow pigments of most
photosynthetic organisms
 When chlorophyll pigments are degraded carotenoids account for
the brilliant orange and yellow colour
 Found in
 Carrot roots
 Tomato fruit
 Green leaves
 They are dominantly hydrocarbon s thus are lipid soluble and
located either in the chloroplast membrane or in
chromoplasts

Carotenoids
Hydrocarbon carotene
-red colour
-carotene -carotene Lycopene
-tomato
Oxygen derivatives
xanthophylls
- Yellow colour
Zeaxanthin Lutein Violaxanthin
??
Anthocyanins
(flvonoids)
Blue & Red

Significance
1. Protect against the photoxidation of chlorophyll
molecule by absorbing excess blue light
 Acts as preferred substrate in the photosynthesised oxidation
 Combine with oxygen (highly reactive form of O2 )to form violaxanthin
2. Absorb and transfer light energy to chlorophyll a

Phycoblins
• Blue green algae
Phycocynins (phycoerythroblin)
• Red algae
Phycoerythrin (phycocyanoblin)
• Blue green and red algae
Allophycocyanins (allophycocyanoblin)
• Regulates various aspects of growth and developments
Phytochromobilin

 All the study of these came from the study about
pigment–protein complex
 They are classified as accessory pigments
 The energy harvested by these pigments is transferred to
chlorophyll a similar to carotenoids before it is active in
photosynthesis

Site of photosynthesis
 The light–driven metabolism of CO2
 In plants photosynthesis takes place primarily in leaves
 The process occurs from start to completion in the
chloroplast
 Chloroplast is highly ordered complex structure that
floats free in the cytoplasm of green plants

Chemical composition of chloroplast
Protein 40-50%
Phospholipids 25-30 %
Chlorophylls 5-10%
Carotenoids 1-2%
RNA 5%
DNA as fragments

Chloroplast structure
 Chloroplast is composed of several compartments with its own
set of metabolic functions :
1. Outer envelop
 The ‘skin’ that holds every thing in.
 The external membrane , which is permeable to most
substances
 Smooth, composed of 2 lipid molecules
2. Inner envelop
 The inner membrane, impermeable to most molecules
 Contains transport proteins that control the movement of
substance in to and out of the chloroplast

3. Thylakoid
 System of internal membranes that contain the photosystems
and components of the electron transport chain
 Site of light reaction of photosynthesis
 Organized in to
 Compactly arranged regions -most important part
 Loosely arranged – grana amellas
 Thylakoid enclose a continuous fluid space known as the
lumen
 Contains ATP synthase , but ATP is not generated

4. Stroma
 Forms the matrix of the chloroplast- a protein filled gel that contains
soluble enzymes and metabolites
 Lamellae in this portion are loosely arranged called stroma lamella
 Consists of ribosomes serving as site of protein synthesis
 Site for dark reaction of photosynthesis
 The major protein in the stroma is the carboxilating enzyme RUBISCO

The photosynthetic process
 Photo= light , synthesis = putting together
 CO2 and water are combined using light energy from
sun light to form glucose
 An extremely complex process
 Oxygen is given off as waste product
 Source of oxygen in the atm
 Occurs in higher plants, algae, some bacteria

 Consists of two key process
1. Removal of H from water
2. Reduction of CO2 by these H atoms to form organic
molecules
 Photosynthesis is a two-way stage process in the
chloroplast
1. Light reaction (light dependent rxn) hill reaction
2. Dark reaction (light independent rxn)

Phases of photosynthesis
BEIRA HAILU bh.bio.pl@gmail.com
31

2. Dark reaction
• Occurs in the stroma
• Involves utilization of ATP &
NADPH
• Fixation of CO2 into carbohydrate
in the Calvin-Benson cycle
(reduction of CO2 into glucose)

NADPH
ATP
NADP+
ADP+
H2O
ADP
NADP+
CO2
GLUCOSE
Light reaction Dark Reaction
LIGHT

Events of over all photosynthetic

Light reaction (light dependent
rxn)
or
Hill reaction

hv
hv
2e PS II CYT PS I
NADPH +H+
NADP+ +2H+
H2O
1/2O2 + 2H+
Fig. Linear representation of light rxn

 Chloroplasts contain a system of thylakoid
membranes.
 Embeds six different complexes of integral
membrane proteins
1. Photosystem I
2. Photosystem II
3. Light harvesting complexes I
4. Light harvesting complexes II
5. Cytochrome b6 and f complex
6. ATP synthase

I. Photosystems
 They are multicellular complex
 Two photosystems
 PS I and PS II
 Each photosystem is consist of
a. Antennae
 Light harvesting system
 Chlorophyll a, b and carotenoids
 Light travels from antennae to inner antennae and
to reaction center

b. Reaction center
 Reaction center consists of special chlorophyll involved
in:
 Charge separation
 Electron transfer
 In PS II the reaction center chlorophyll is P680
 In PS II the reaction center chlorophyll is P700
 Subscripts – absorption maxima

PS I PS II
 12 protein molecules
 96 molecules of chll a
 2 molecules of rxn center chll
P700
 4 accessory molecules
 90 molecules that serve as
antenna pigments
 22 carotenoids molecule
 4 lipids molecules
 3 cluster of Fe4S4
 2 phylloquinones
 >20 different protein molecules
 50 chlorophyll a molecule
 2 molecules of the rxn center
chll P680
 2 accessory molecules close to
them
 2 molecules of pheophytin
 Antenna pigments
 Half dozen carotenoids
molecule
 2 molecules of plastoquinone

II. Light harvesting complex
 These are chlorophyll-protein complexes
 Function extended antenna systems for harvesting
additional light energy
 Important Role
 Dynamic regulation of energy distribution and
Electron transport

• Associated with PS I
• Small, has chll a/b ratio of
4/1
LHCI
• Associated with PS II
• Has chll a/b ratio of about ½
• Also contain the xanthophyll
LHCII

III. Cytochrome b/f complexes are uniformly
distributed through out both regions
IV. ATP synthase

PHOTOPHOSPHORYLATION
 Light-driven production of ATP by chloroplast:
a. Noncyclic Photophosphorylation
b. Cyclic Photophosphorylation
c. Pseudocyclic Photophosphorylation

Non-cyclic electron
transport
• Electron flow from water to NADP+ (Final
electron acceptor)
• ATP formation at one location only (Non-
cyclic Photophosphorylation)
• Both photosystems involve
• Water as primary electron source (oxidation of
water in the thylakoid lumen)
• NADP+ is reduced to form NADPH

Noncyclic Photophosphorylation
(Z-scheme)

 Oxidation of water as the primary source of electrons
 The reduction of the final electron acceptor NADP+
 Photophosphorylation (ATP synthesis)
 Electrons flow from water to NADP+
 Large vertical arrows represent the input of light energy into
the system
 NADP+ is reduced to NADPH on the stroma side of the
membrane

ATP synthase
Photosystem I
Cytochrome b6 /f complex
Photosystem II
Organization of the photosynthetic electron transport system in the
thylakoid membrane involves:
48BEIRA HAILU bh.bio.pl@gmail.com

Cyclic electron
transport
• Electrons from reduced feredoxin is
transferred back to plastoquinone
• Occurs when NADP+ is not available in its
oxidized form to trap electrons
• No oxidation of water is involved
• ATP formation at two locations (cyclic
Photophosphorylation)
• Only PS I involves
• Occurs when chlorophyll molecules are
exposed to light energy >680 nm

Pseudocyclic Photophosphorylation
 This path requires both photosystems
 the ferredoxin passes the electrons to molecular oxygen which act as
the electron accepter thereby forming hydrogen peroxide
 Is called Mehler reaction
 By the action of hydrogen peroxide the reduced oxygen is graded thus
giving rise to superoxide radical
 molecular hydrogen which reacts with superoxide radical and give rise
to very dangerous hydrogen peroxide

 There is no net oxygen exchange (take-up & evolved)
 So, here electrons come from water to oxygen and back to
water but the same electrons are not recycled like the cyclic
flow do and for this reason that is why is also not referred
to as cyclic flow.
 This flow takes place when oxygen concentrations are very
high or when carbon dioxide fixation is very low

Overall light reaction

ATP + NADPH NADP+
Triose
phosphate
CO2 +H2O (CH2O)2

The reactions catalyzing the reduction of CO2 to carbohydrate
are coupled to the consumption of NADPH & ATP by enzymes
in the stroma
Stroma reactions are long to be independent of light (dark
reactions)
But this reaction depend on the products of the photochemical
processes
• Directly regulated by light
• Properly referred to as carbon reactions of photosynthesis

Cyclic reactions that accomplish fixation and
reduction of CO2
 There are three types of photosynthesis
1. Calvin cycle (C3)
2. Hatch –slack cycle (C4)
3. Crassulacean acid metabolism (CAM)

I. The Calvin cycle
All photosynthetic eukaryotes reduce CO2 to carbohydrate via
the same basic mechanism:
 The photosynthetic carbon reduction (PCR) cycle
 Calvin cycle
 Reductive pentose phosphate (RPP) cycle
 C3 cycle
C3 photosynthesis is the typical photosynthesis that most plants
use
The other cycles are auxiliary to or dependent on the basic Calvin
cycle

Carboxylation
Reduction
Regeneration

The temporary chemical
(ATP) reducing (NADPH)
potentials that were
generated in the light
reactions are used to reduce
PGA to carbonyl (a
carbohydrate) called
glyceraldehyde-3-phosphate
This is a two step reaction
sequence
• PGA is phosphorylated with
ATP to 1,3-
bisphosphoglycerate (BPG)
• Reduction of BPG to
Glyceraldehyde-3-
phosphate (GAP, G-3P)
through the use of NADPH
generated by the light
reaction
2. Reduction

2PGA ATP
1,3-
bisphospho
glycerate
NADPH
Glyceraldeh
yde-3-
phosphate

3. Regeneration
 The continued uptake CO2 requires the availability
CO2 acceptor, ribulose -1,5 bisphosphate
 Regeneration of the CO2 acceptor RuBP fromG-3-P
 Three molecules of RuBP (15 C total) are formed by
reactions that reshuffle the carbons from the five
molecules of trios sugar

Trios
sugar
3 RuBP
6G-3-P

The reshuffling reaction consists
1. Conversion of one G3P to dihydroyaceton-3-phpsphate (DHAP)
2. DHAP undergoes aldol condensation with second molecule of G3P to give
fructose-1,6-bisphosphate
3. FBP is hydrolyzed to fructose -6-phosphate
4. F6P is transferred transketolase to a third G3P to give Erythrose-4-phosphate (E-
4-P) and xylulose-5- phosphate (X-5-P)
5. E-4-P combines vial aldolase with a fourth molecule of G3P to give a seven-
carbon sugar sedoheptulose-1,7-bisphosphate (SBP)
6.

6. SBP is then hydrolyzed to give sedoheptulose -7-phosphate (S-7-P)
7. S7P donates a two-carbon unit to the fifth(last) molecule G3P and
produce ribose-5-phosphate and xylulose-5-phosphate
8. The two xylulose-5-phosphate are converted to 2 molecules of
ribulose-5-phosphate (Ru-5-P) sugar by ribulose-5-phosphate
epimerase ; the third Ru-5-P is formed from ribose-5-phosphate by
ribose-5-phosphate isomerase
9. Phosphorylation of Ru-5-P with ATP to generate RUBP

Fig. C3 cycle

Summery
 Called C3 because the CO2 is first incorporated into a 3-carbon
compound.
 Stomata are open during the day.
 The net product is one molecule of trios sugar per 3CO2 taken
 9 ATP & 6 NADPH are consumed per 3CO2
 RUBISCO, the enzyme involved in photosynthesis, is also the enzyme
involved in the uptake of CO2.

 Adaptive Value:
 more efficient than C4 and CAM plants under cool and moist
conditions and under normal light because requires less
machinery (fewer enzymes and no specialized anatomy).
 Most plants are C3.

II. Hatch –slack cycle (C4)
There is difference in leaf anatomy between pants that have a
C4 carbon cycle(C4 plants) and those that photosynthesis
solely via Calvin photosynthetic cycle (3 plants)
The cross section of C3 leaf reveals one major cell type that
has chloroplast , the mesophyll .

 In contrast C4 leaf has two distinct chloroplast- containing
cell types:
 Mesophyll cells
 Bundle sheath cells
 Such distinction is called Kranz anatomy
 Both are connected by an extensive net work of
plasmodesmata , thus providing a pathway for the flow of
metabolites between the cell types

 The C4 cycle concentrates CO2 in bundle sheath cell
 The basic c4 cycle consists of four stages:
1. Fixation of CO2
 Carboxylation of phosphoenolpyruvate in the
mesophyll cells to form a C4 acid (malate or
asparate)
 Catalyzed by enzyme called phosphoenolpyruvate
carboxylase (PEP case)
2. Transport of the C4 acid (pyruvate or alanine) from
mesophyll cells to the bundle sheath cells

3. Decarboxylation
 C4 acid is decarboxylated with in the bundle sheath cell
 Generation of CO2
 CO2 released is reduced to carbohydrate via C3 cycle
4. Regeneration
 Transport of C3 acid (pyruvate) formed by decarboxylation
back to mesophyll cell
 Phosphorylation of pyruvate using ATP to generate CO2
acceptor PEP

Fig. Hatch-slack path way

Three variations of basic C4 cycle
Variation
1. In the c4 acid transported into the bundle sheath cell (asparate
or malate)
 The 3-carbon acid pyruvate or alanine returned to the
mesophyll cell
2. The nature of enzyme that catalyzes the decarboxylation step
 Thus their name is after the enzyme that catalyzes their
decarboxylation reaction

a. NADP-ME type
 This is NADP dependent
malic enzyme
 Found in the chloroplast of
bundle sheath
 Malate is transported bundle
sheath cell
 Pyruvate is transported to
mesophyll cell
 Example: corn, sugarcane,
sorghum

b. NAD-ME type
 NAD dependent malic enzyme
 Decarboxylation occurs in the
mitochondria
 Asparate is transported
bundle sheath cell
 Alanine is transported to
mesophyll cell
 Examples : millet, pigweed

c. PEP-CK type
 Phosphoenol-pyruvate
dependent carboxykinase
 Decarboxylation occurs in
the cytosol of chloroplast
 Asparate to bundle sheath
cell
 Alanine to mesophyll cell

Summery
Called C4 because the CO2 is first incorporated into a 4-carbon
compound.
Stomata are open during the day.
Uses PEP Carboxylase for the enzyme involved in the uptake of CO2
(HCO3 as substrate )
This enzyme allows CO2 to be taken into the plant very quickly, and
then it "delivers" the CO2 directly to RUBISCO for photosynthesis.
Photosynthesis takes place in inner cells (requires special anatomy
called Kranz Anatomy)
The concentration of CO2 in bundle sheath has an energy cost ; 5ATP
and 2NADPH per 1 CO2 consumed

 Adaptive Value:
 Photosynthesizes faster than C3 plants under high light intensity
and high temperatures because the CO2 is delivered directly to
RUBISCO, not allowing it to grab oxygen and undergo
photorespiration.
 Has better Water Use Efficiency because PEP Carboxylase brings in
CO2 faster and so does not need to keep stomata open as much (less
water lost by transpiration) for the same amount of CO2 gain for
photosynthesis.
 C4 plants include several thousand species in at least 19 plant families.

III. Crassulacean Acid Metabolism
 Called CAM after the plant family in which it was first found
(Crassulaceae) and because the CO2 is stored in the form of
an acid before use in photosynthesis
 The type of photosynthesis is similar to C4 cycle in many
respects but different in two important features:
1. Formation of c4 acid is both temporally and spatially separated (PEP
case and decarboxylase located in the cytosol function at different
time
2. A specialized anatomy is not needed

During Night
 Stomata open for uptake of CO2
 At night CO2 is captured by PEP carboxylase in the
cytosol
 Fixation of CO2 as malic acid temporally and is stored in
the vacuole
• Acidification of leaf when malic acid is stored in the vacuole

During Day
 Stomata are closed for reducing water loss
 Transportation of malate from vacuole to chloroplast
 Decarboxylation (deacidification) occurs , the released
CO2 is fixed by the Calvin cycle
 Refixation of internally released CO2 by C3 cycle
 Since stomata are closed ,internally released can not escape
from the leaf

86
HCO3
Phosphoeno
l pyruvate
Pi
OAA
malate
NADH
NAD+
Malate
CO2C3
cycle
Pyruvatestarch
Trios
phosphate
Chloroplast
NADP+ malic dehydogenase
PEP case

 Stomata open at night (when rates of water loss are usually
lower) and are usually closed during the day.
 The CO2 is converted to an acid and stored during the
night.
 During the day, the acid is broken down and the CO2 is
released to RUBISCO for photosynthesis
 CAM plants include many succulents such as cactuses and
agaves and also some orchids and bromeliads

 Adaptive Value:
 Better Water Use Efficiency than C3 plants under arid conditions
due to opening stomata at night when transpiration rates are lower
 When conditions are extremely arid, CAM plants can just leave
their stomata closed night and day.
 Oxygen given off in photosynthesis is used for respiration and CO2
given off in respiration is used for photosynthesis.
 CAM-idling does allow the plant to survive dry spells, and it allows
the plant to recover very quickly when water is available again
(unlike plants that drop their leaves and twigs and go dormant
during dry spells).

Photorespiration
 Many land plants take up oxygen and release CO2 in the
light.
 This process is called photorespiration
 However, it is normally masked by photosynthesis,
which is even faster.
 Photorespiration differs from true respiration.
 Plants do respire normally with mitochondria that
produces ATP and NADH, and occurs mostly in the dark.

 In contrast, photorespiration is wasteful and occurs mostly
in the light (produces no ATP)
 Photorespiration appears to serve no useful purpose.
 Its main effect is to reduce the apparent rate of
photosynthesis.
Phosphoglycolate +
phosphoglycerate

Not all plants photorespire
 Plants that photorespire
1. Typically show light saturation point (LSP)
 Point at which increasing light yields a constant
amount of photosynthesis
2. have higher light compensation point (LCP)
 Light at which the amount of photosynthesis just
equals the amount of respiration

 Oxygen inhibition of photosynthesis in plants that
photorespire is called Warburg effect
 Oxygen acts as antagonistic in photosynthesis and acts in a
competitive manner
 This is due to the fact that rubisco is not a substrate specific
enzyme
 i.e. also has an oxygenase function, thus binds oxygen to RuBP
although higher affinity for CO2
 Favoured by low CO2/O2 ratio

Involves three cellular organelles

Reading assignment
Factors
affecting the
process of
photosynthesis

The process of photosynthesis

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