6. Photosynthesis

7/27/2019 6. Photosynthesis

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Lesson Objective: Give overall outline of photosynthetic

process that leads to the production of

glucose.

TOPIC 6: PHOTOSYNTHESIS

Green plants use sunlight as an energy source,

CO2 & water as raw materials for photosynthesis.

The light energy trapped by greens plant is

converted to chemical energy & stored in thebonds of organic molecules such as

carbohydrates.

O2

is released as a by product.

6CO2 + 12H20 C6H1206 + 6O2 + 6H2O

PHOTOSYNTHESIS

Light energy

Absorb by

chlorophyll


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1. Chlorophyll pigments inchloroplasts.

2. CO2

3. Water 4. Optimum temperature

(photosynthesis is abiochemical reaction

that involves manyenzyme).

5. Light

6.1: Photosynthesis requires:


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Lesson Objective: List and explain the photosynthetic

pigments involved in photosynthesis


Thylakoid membranes contain several kinds ofpigments.

Different pigments absorb light of differentwavelength.

Chlorophyll, the mainpigment of photo-

synthesis, absorb

light primarily in blue

& red regions of

visible spectrum.

Chlorophyll


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Chlorophyll a is the most abundant pigment in

plants. It absorbs light mainly in the blue-violet(430nm) & red (662nm) region.


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Chlorophyll b is absorbs light of about 453nm &

642nm. Chlorophyll b helps to increase therange of light a plant can utilize forphotosynthesis.

Carotenoid absorb light maximally in the blue-violet region (460-550nm).

The most common carotene is carotene, anorange pigment found for example in carrots.

Carotenoids act as accessory or antenna

pigments. These pigments pass the light energyto chlorophyll a in the reaction centre. Thecarotenoid also protect the chlorophyll fromoxidation.



The photosynthetic processinvolves 2 stages;

1. The light dependent reactionwhich requires light energy

& is biochemical in nature.

2. Light independent reaction

which consist of a seriesenzymatic biochemical

reactions that involve thebonding of CO2 tocomplex organiccompounds.

The mechanism of photosynthesis


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Lesson Objective: Explain the photoactivation of chlorophyll

resulting in the conversion of light energy

into ATP and reduced NADPH+.


These stages occur in the chloroplastthylakoids which contains complexphotosynthetic pigments.

The photosynthetic process involves thefollowing process;

i. Photoactivation

ii. Photolysis of wateriii. Photophosphorylation (produced ATP &

NADPH).

6.3: Light Dependent Reaction



= when a molecule ofchlorophyll

absorbs a photon, one of the

molecules electrons is elevated toan orbital where it has more

potential energy, the pigment

molecule is said to be in an excitedstate photo-activation process.

i. Photoactivation



The energy of an absorbed photon isconverted to the potential energy of

an electron raised from the ground

state to an exited state. The chlorophyll & accessory pigments

are group into 2 photosystem;

a) Photosystem I (PSI)

b) Photosystem II (PSII



Photosystem I has a reaction center

chlorophyll, the P700 center, that has an

absorption peak at 700nm.

Photosystem II has a reaction center with

a peak at 680nm.

These two photosystems work together to

use light energy to generate ATP and

NADPH.



The phosystems are embedded in thethylakoid membranes of the chloroplast.

A photosystem contains a complex of

about 200-300 pigment molecules(consisting of a cluster of a few

hundred chlorophyll a, chlorophyll b &

carotenoid molecules), primary electron

acceptor, electron transfer system &

enzymes.



Each photosystem has an

antenna of a few hundredpigment molecules.

When a photon strikes apigment molecule, theenergy is passed from

molecule to molecule until itreached the reactioncenter.

At the reaction center, theenergy drives an oxidation-

reduction reaction. Anexcited electron from thereaction-center chlorophyll iscaptured by a specialisedmolecule called the primaryelectron acceptor.



Photolysis of water is a process in which water is

split during the light reaction of photosynthesis.

This process occurs in the space of the thylakoid& catalysed by a water splitting enzyme.

The enzyme passes the electron from water to the

reaction center of PS II & forming O2, which is

liberated.

H2O O2 + 2H+ + 2e

H+ ions combine with NADP+ NADPH + H+

ii. Photolysis of Water The Hill

Reaction



=The process of generating ATP from

ADP + Piduring the light reactions ofphotosynthesis.

Light reaction consist of;

a. non-cyclic photophosphorylationb. cyclic photophosphorylation

iii. Photophosphorylation



Non-cyclic photophosphorylation involves 2 PS;

- PS I

- PS II

Light energy is absorbed by antenna pigments

of PS II.

The energy is transferred from one antenna

molecule to another molecules then transferredto the reaction center in PS680 in the PS II.

a. Non-cyclic Photophosphorylation


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When an e-reached the bottom of the e-

transport chain, it fills an e-hole in P700

(the chlorophyll a molecule in the reaction

center of PS I).

This replace the e- that light energy drives

from the chlorophyll to the primary e-

acceptor of PS I.

The primary e- acceptor of PS I passesthe photoexited e- to ferredoxin (Fd).


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An enzyme called NADP+ reductase then

transfers the e- from Fd to NADP+. This is theredox reaction that stores the high-energy e- in

NADPH, the molecules that will provide reducing

power for the synthesis of sugar in the Calvin

cycle.

2H+ + 2e- + NADP+ NADPH + H+

The H+ ions formed in water splitting is used in

formation of NADPH.


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Under certain condition, photoexcited e-take an alternative path called cyclic e-flow, which uses PS I but not PS II.

Light energy absorbed by antennamolecules is transferred to P700 in PS I.

Electron from P700 is photoexcited &

passed on to a primary e-

acceptor. Primary e- acceptor passes the e- to

ferredoxin (Fd).

b. Cyclic Photophosphorylation


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In cyclicphotophosphorylation, analternative route is used.The e- is passed from Fdto cytochrome complex.

Energy is released duringthe e- flow & this is used inchemiosmosis to generatemore ATP.

The e- then transferred toplastocyanin (Pc) & ispassed back to PS I.


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Lesson Objective: Describe the outline of Calvin cycle

involving the light-independent

fixation of CO2.


Consist of 4 main stages:

a. Carbon dioxide fixationb. Reduction phase

c. Regeneration of CO2 acceptor

d. Product synthesis phase

6.4. Light Independent Reaction/

Calvin Cycle


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CO2 from the atmosphere diffuse through thestomata into the intercellular spaces of the leaf.It then diffuse into the stroma in the chloroplastof the palisade & spongy mesophyll cells.

A five-carbon acceptor, ribulose biphosphate(RuBP) combines with molecule of CO2 to forman unstable six-carbon sugar.\

The process is catalysed by the enzyme RuBP

carboxylase. The six-carbon compound immediately splitsinto 2 molecules of glycerate-3-phosphate (PGA)(3-carbon compound).

a. Carbon dioxide Fixation


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Glycerate 3-phosphate receives an

additional phosphate group from ATP to

become glycerate 1,3-diphosphate.

Glycerate 1,3-diphosphate combines with

hydrogen atoms from reduces dinucleotide

phosphate (NADPH + H+) & is converted

into glyceraldehyde-3-phosphate(PGAL, triose phosphate).

b.Reduction Phase


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Some of the PGAL molecules are

rearranged in a series of complex

reactions to regenerate ribulosebiphosphate, this process

requires ATP.

c. Regeneration of CO2 acceptor


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The rest of the glyceraldehyde-3-

phosphate is used to assimilate

organic molecules such asglucose, amino acids, proteins &

lipids.

d. Product synthesis phase


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RuBP carboxylase (Rubisco), theenzyme that catalysed thecarboxylation of ribulose biphosphate,

can also catalyze the oxidation ofRuBP by molecular O2.

CO2 & O2 are alternative substrates

that compete with each other for thesame active sites on the enzyme.

PHOTORESPIRATION


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When the concentration of CO2 is high & that

of O2 is low, CO2 occupies the active sites,carboxylation is favored & carbohydrate

synthesis by the Calvin cycle proceeds.

But when the concentration of CO2

is low &

that of O2 is high, the site is occupied by O2,

oxidation is favored orphotorespiration occurs.

This occurs on hot dry days, which cause water

stress in plants. As a result of water stress, plants close their

stoma ( help the plant to conserve water)


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Once the stoma close, O2 produced during

photosynthesis accumulates in the chloroplast. So rubisco binds RuBP to O2 instead of CO2.

O2 + RuBP Phosphoglycolate +glycerate-3-phosphate

Two phosphoglycolate molecules then undergo

a series of reactions requiring O2 & ATP to

produce one molecule of glycerate-3-phosphate

& the release

(2 carbon compound)


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Photorespiration is wasteful because organiccarbon (phosphoglycolate) is converted into CO

2

with no net production of ATP or other energyrich metabolites.

Photorespiration can reduce the potentialphotosynthetic yield from between 30-40%.

This degradation process is calledphotorespiration because;

i. it occurs during the daylight

ii. it requires O2 (like aerobic respiration)iii. It produces CO2 & H2O (like aerobic

respiration)


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Eg. of C4 plants:

- maize plant (Zea mays)

- sugar cane (Saccharum officinale)

- sorghum (Sorghum bicolar)

- sunflower (Helianthus spp)

6.5: Hatch-Slack Pathway (C4 plants)


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In C4 plants, there are 2 distinct types of

photosynthetic cells:a. bundle-sheath cells

b. mesophyll cells.

Bundle sheath cells are arranged intotightly packed sheets around the veins ofthe leaf.

Between the bundle sheets & the leafsurface are the more loosely arrangedmesophyll cells.


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The

arrangement

pattern of

the cells

around thevascular

bundle is

known asKrantz

anatomy.


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CO2 in the atmosphere diffuse into the

mesophyll cells of C4 plants where they

combine with phosphoenolpyruvate

(PEP,3C acceptor) to produce

oxaloacetate (4C). This process is catalysed by the enzyme

phosphoenol-pyruvate carboxylase

(Pepco) which has higher affinity for CO2at low CO2 concentration.


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Oxaloacetate is reduced to malate (4C). Malate

is shunted through plasmodesmata into thebundle sheath cells.

Malate is then oxidised to pyruvate (3C) by theremoval of hydrogen & CO2.

This increases the concentration of CO2 in thebundle cells & the CO2 undergoes fixation as inthe normal C3 pathway.

High CO2

concentration in the bundle sheathcells inhibits photorespiration & as a result,production of carbohydrate (glucose) increases20-25%.


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Pyruvate diffuses into the mesophyll

cells & phosphorylated to regenerate

phosphoenol-pyruvate.

In general, C4 plants are found in hotclimates (tropical).

Under hot conditions, C4 plants attain

a higher photosynthetic ratecompared to C3 plants.


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Differences between C3 & C4 Plants

Aspect C3 plants C4 plants

Examples

CO2

fixation

Tomato,tobacco,

legume, wheat.

Once, only in

mesophyll cells.

Sugar cane,

Sorghum, maize.

Twice, first in

mesophyll cells &

then bundle sheath

cells.


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CO2

acceptor

RuBP ( 5C) PEP (3C) in

mesophyll cells. RuBP in bundle

sheath cells.

Enzyme Rubisco whichis inefficient at

low CO2

concentration.

Pepco which hashigh affinity for

CO2 at low

concentration.RuBP which is

efficient at high

CO2

concentration.


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First product

formed.

Glycerate 3-

phosphate

(a C3acid)

Oxaloacetate,

(a C4 acid).

Photo-

respiration

O2 acts as

competitive

inhibitor.

Photorespiration

occurs.

Photorespiration

is inhibited by

high

concentration of

CO2.


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Leaf

anatomy

Krantz

anatomy

absent.

Only

one type of

chloroplast

in mesophyll

cells.

Krantz anatomy

present with the

columnar palisade

cells arranged in a ring

around the bundle

sheath cells & the

vascular tissue.


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Leaf

anatomy

There are 2 different

forms of chloroplasts.

Mesophyll palisade cells

contain few small

chloroplasts, have many

large well-developed

grana but not starch

grains.

Bundle sheath cells have

many large chloroplasts with

fewer poorly developed grana& contain starch grains.


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E.g. of CAM plants are succulent plantssuch as the cacti & pineapples.

This succulent (Crassulaceae), avoidwater loss in their hot environment byclosing their stomata during the day &opening them at night.

CAM plants only contain mesophyll cells &no Krantz anatomy.

CAM Plants = Crassulacean Acid

Metabolism


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Photosynthetic pathway is the same as C4 plants butfixation of CO2 takes place at night when stomata are

open. Phosphoenolpyruvate (PEP) 3C

combines with CO2 to produce oxaloacetate

(OAA) 4C.

Malate is stored in the cell vacuole at nightto prevent pH changes in the cytoplasm.

During daytime, malate is oxidised producing pyruvate& CO2.

Concentration of CO2 increases in the mesophyll cell &

photorespiration is prevented. CO2 is used in Calvin Cycle producing organic

molecules.


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a) Light Intensity

At low light intensity there is no photosynthesis

6.6: FACTORS LIMITING THE RATE OF

PHOTOSYNTHESIS


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As in figure, a higher intensity of light increases

the rate of photosynthesis, unless influenced by

other limiting factors, such as very low CO2

concentration.

Under ideal conditions with no other limitingfactors, the rate of photosynthesis is directly

proportional to the intensity of light.

Increase in CO2 concentration increases the rate

of photosynthesis until saturation point is again

reached.


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At this point, CO2 concentration oranother new factor may limit the

photosynthetic process.

Light intensity only effects the light-dependent reactions & not the light-

independent reaction, which is

catalysed by specific enzymes.


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Visible light of differentwavelengths or differentcolour affect the rate ofphotosynthesis

differently. Red light of around 650-750nm & blue light of430-500nm are mosteffective for

photosynthesis whereasgreen light of 540nm isleast effective.

b) Waveleng th of Ligh t


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This is because red & blue lights areabsorbed by the chlorophyll whereas

green light is not absorbed at all.

The effects of different wavelengthsof light on photosynthesis can be

seen from the action spectrum below.

The action spectrum is correspondingto the absorption spectrum.


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c) Temperature

The rate of photosynthesis is temperature-

dependent because it is a series of reaction that

depend on enzymes.


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d) CO2Concentration

The rate of photos. is directly proportionalto the CO2 concentration, in conditionwhere the light intensity & temp. are not

limiting factors. The concentration of CO2 in the

atmosphere is only 0.035%. When thisvalue increases, the rate of photos. wouldincrease until a maximum is reached atapproximately 1.0%.


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A concentration of CO2 that exceeds 1.0%will stimulate the closing of the stoma &

will reduce the rate of photosynthesis.

Rateo

fphotosynthes

is

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6. Photosynthesis