The process of photosynthesis

BEIRA HAILU [email protected] 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 propertyo 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 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 BEIRA HAILU [email protected] 13


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


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



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


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



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 generatedBEIRA HAILU [email protected] 27

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 processPhoto= 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 [email protected]


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

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 II5. Cytochrome b6 and f complex6. 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 II12 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 BEIRA HAILU [email protected] 41


III. Cytochrome b/f complexes are

uniformly distributed through out both

regions

IV. ATP synthase


PHOTOPHOSPHORYLATIONLight-driven production of ATP by

chloroplast:

a. Noncyclic Photophosphorylation

b. Cyclic Photophosphorylation

c. Pseudocyclic Photophosphorylation




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 BEIRA HAILU [email protected] 47

Organization of the photosynthetic electron transport system in the thylakoid membrane involves:

48BEIRA HAILU [email protected]




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 BEIRA HAILU [email protected] 52

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 BEIRA HAILU [email protected] 57

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






2. Reduction


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 RUBPBEIRA HAILU [email protected] 68

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 cycleVariation

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


SummeryCalled 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 BEIRA HAILU [email protected] 81

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

Phosphoenol

pyruvate

Pi

OAA malate

NADHNAD+

Malate

CO2

C3 cycl

e

Pyruvate

starch

Trios phosphat

e

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 photorespirePlants 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


Science

The process of photosynthesis