Biology sem1- chap6

8/7/2019 Biology sem1- chap6

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OBJECTIVE

Define photosynthesis

Write overall chemical

equation for photosynthesis

Explain light absorption

spectrum

Name the photosynthetic

pigments


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Photosynthesis

The process whereby light energy is

converted to chemical energy that is stored

in glucose or other organic compounds.

In the presence of light, green plants produce

organic compounds and oxygen from carbon

dioxide and water.


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EQUATION FOR PHOTOSYNTHESIS

6 CO2 + 12 H2O + 18 ATP + 12 NADPH

C6H12O6 + 6O2 + 6H2O +18 ADP + 12NADP+ + 18 Pi

NADP : nicotinamide adenine dinucleotide phosphate


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PHOTOSYNTHETIC PIGMENTS

Photosynthesis occurs in thechloroplasts

Chlorophylls are the most

important pigments.

In the centre of the chlorophyll

ring is a magnesium atom.

At the peripheral location of the

ring is a long hydrocarbon tail

that can be associated with thehydrophobic region of the

thylakoid membrane.


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Chlorophylls absorb lights


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Chlorophylls

absorbblue and red lights

Accessory pigments

absorb light between theblue and

the red wavelengths

transfer the energy to the

chlorophylls.

Eg : Carotenoids such as betacarotene which absorbs light

in theblue and the

blue-green regions.


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ASORPTION SPECTRUM

A graph of a pigments light absorption

versus wavelength is called an absorption

spectrum.


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Chromatography technique to separate chlorophylls and

accessory

to separate mixtures into their components

For photosynthesis, a paper chromatography

is commonly used.


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Steps :

1. The chlorophyll mixture is dissolved in a suitable solvent.

2. Drops of the resultant solution are repeatedly placed on

top of each other to form a small concentrated spot near onend of a paper strip.

3. A line is drawn across the paper to mark the position of the

spot.

4. When the solvent front moves up the paper and about

1cmfrom the end, a line is drawn to mark the position of the

solvent front.


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Rf VALUE

The position of various pigments are

marked.

The Rf value of a solute / pigment iscalculated using the formula

Rf = distance moved by solute

distance moved by solvent front


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Structure of chloroplast


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Chloroplasts

Any green part of a plant has chloroplasts.

Leaves are the major site of photosynthesis

The color of a leaf comes from chlorophyll, the

green pigment in the chloroplasts.

Chlorophyll plays an important role in the absorptionof light energy during photosynthesis.


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Chloroplasts are found mainly in mesophyllcells

O2 exits and CO2 enters the leaf through

microscopic pores, stomata, in the leaf.Veins deliver water

from the roots and

carry off sugar from

mesophyll cells to

other plant areas.


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Each chloroplast has two membranes around a

central aqueous space, the stroma.

In the stroma are

membranous sacs,

the thylakoids.These have an internalaqueous space, thethylakoid lumen or

thylakoid space.Thylakoids may be stackedinto columns called grana.


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STAGES OF PHOTOSYNTHESIS

2 stages:

Lightdependent

reactions

Lightindependent

reactions


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THE LIGHT

DEPENDENTREACTION


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LIGHT REACTIONS

Light reactions

convert solar energy to

chemical energy

Light energy absorbed by

chlorophyll in the thylakoids

drives the transfer of electronsand hydrogen from water to

NADP+ (nicotinamide adenine

dinucleotide phosphate), forming

NADPH.


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The thylakoids convert light energyinto the chemical energy of ATP and

NADPH.

The light reaction also generates

ATP by photophosphorylation for

the Calvin cycle


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Wavelength

Light, like other formof electromagneticenergy, travels in

rhythmic waves.

The distance betweencrests of

electromagnetic wavesis called thewavelength

wavelength


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The entire range of electromagnetic radiation

is the electromagnetic spectrum.

The most important segment for life is a

narrow band between 380 to 750 nm, visible

light.


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While light travels as a wave, many of itsproperties are those of a discrete particle, the

photon.

Photon light particle

The amount of energy packaged in a photon is

inversely related to its wavelength.

While the sun radiates a full electromagneticspectrum, the atmosphere selectively screens

out most wavelengths, permitting only visible

light to pass in significant quantities.


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When light meets matter, it may be reflected,

transmitted, or absorbed.

Different pigments absorb photons of different

wavelengths.

A leaf looks greenbecause chlorophyll,

the dominant pigment,

absorbs red and blue

light, while transmittingand reflecting green

light.


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LIGHT ABSORPTION

Chlorophyll a

participates directly in the

light reactions

Accessory photosynthetic

pigments

absorb light and transfer

energy to chlorophyll a.


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When a molecule absorbs aphoton,

one of that molecules electrons is elevated to an orbital with

more potential energy. Photons are absorbed by clusters of pigment molecules

in the thylakoid membranes.


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The energy of the photon is converted to the potential

energy

electron raised from its ground state to an excited state.

Excited electrons are unstable.

They drop to their ground state,

releasing heat energy.


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In the thylakoid membrane

chlorophyll is organized along with proteins and smaller

organic molecules into photosystems.

A photosystem acts like a light-gathering antenna

complex

consist of chlorophyll a, chlorophyll b,and carotenoidmolecules.


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When antenna molecule absorbs a photon,

photon is transmitted from molecule to molecule until it

reaches a particularchlorophyll a molecule (reaction center).

At the reaction center,

primary electron acceptor removes an excited electron from

the reaction center.


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PHOTOACTIVATION OF

PHOTOSYSTEM I

AND

PHOTOSYSTEM II


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2 types.

I. Photosystem I

has a reaction center chlorophyll (P700), that has an

absorption peak at700

nm.II. Photosystem II

has a reaction center chlorophyll (P680) with an absorption

peak at 680nm.

These two photosystems work together to use

light energy to generate ATP and NADPH

PHOTOSYSTEM


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2 possible routes for electron flow ;

Cyclic and Non cyclic.

Fd


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Photophosphorylation

Cyclic

and

Non cyclic


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Non cyclic photophosporylation

1. Both photosystem 1 and II are used.

2. Non cyclic electron flow, produces both ATP and NADPH.


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5. Electrons are transport along the electron transport chain.

6. Electron flow provides energy for chemiosmotic synthesis of

ATP

7. At the same time, photosystem I (P700) absorb photons


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Water photolysis


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Water photolysis

Photolysis is a process of splitting water molecules using light energywith the release of electrons, protons and oxygen.

The proton (H+) are used to reduced NADP+.

Oxygen is given off or used in respiration.

2H2O 4H+ +4e- + O2

The important of photolysis = To replace electron in photosystem II(noncyclic photophosphorylation)


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Cyclicphotophosphorylation


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1. In cyclic photophosphorylation, photosystem I acts as its

own, without photosystem II

Fd

Cyclic photophosphorylation


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2. The reaction centre of photosystem I (P700) absorbs a

photon and become energised

Fd


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3. The electrons are emitted and reduces an oxidising agent,

ferredoxin (Fd)

Fd


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4. Ferredoxin passes its electron to the cytochrome complex

Fd


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5. The electron continues down a redox chain, pumping

protons as it goes.

Fd


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6. The P700 gets back an electron from the last reducing

agent at the end of the chain.

Fd


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Summary

1. Cyclic electron flow takes photons through

chlorophyll molecules,

2. Passing excited electrons through a redox chain

to produce ATP and some free energy as heat.

3. Electron deficit chlorophyll is restored and the

process repeats


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LIG

HTINDEPENDENT

REACTION


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Three kinds of CO2 fixation pathways

exists :

Calvin Cycle for C3 plants

Hatch-SlackPathway for C4plants

Crassulacean Acid

Metabolisme (CAM)


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CALVIN CYCLE

CALVIN CYCLE


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CALVIN CYCLE

IN C3 PLANTS


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CALVIN CYCLE?

The stage of photosynthesis where theCO2 and H2O are converted into a

carbohydrate

The carbohydrate produced andreleased from the Calvin cycle is

Phosphoglyceraldehyde (PGAL - a 3

carbon compound) - not glucose

The ATP andNADPH from the light

reaction are used to supply electrons

and reducing power for this reduction

reaction


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Occurs in the stroma of the

chloroplast and each stage is

mediated by an enzyme

Consist of three stages :

i. CO2 fixation

ii. CO2 reduction

iii. RuBP regeneration


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Stage 1 : CO2 fixation

Each CO2 molecule is

attached to a 5C sugar,ribulose bisphosphate

(RuBP)

Catalyzed by RuBP

carboxylase orrubisco

The 6C intermediate splitsinto half to form two

molecules of 3-

phosphoglycerate per CO2.

St 2 CO d ti


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Stage 2 : CO2 reduction

Occurs in two steps:i. Phosphorylation of 3-

phosphoglycerate by ATP toform a "bisphosphate

ii. Reduction of1,3-

bisphosphoglycerate byNADPH to form triosephosphate, a simple 3Ccarbohydrate

The NADP+and ADP formedin this process return to thethylakoids to regenerate

NADPH and ATP in the lightreactions.


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If our goal was to produce one

glycerate-3-phosphate (G3P)

net, we would start with 3 CO2(3C) and 3 RuBP (15C)

After fixation and reduction we

would have six molecules of

G3P (18C)

One of these six G3P (3C) is a net

gain of carbohydrate.

This molecule can exit the cycle

to be used by the plant cell.

The other five (15C) must remain

in the cycle to regenerate three

RuBP.

St 3 R BP ti


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Stage 3 : RuBP regeneration

Regeneration of the CO2 acceptor

(RuBP)

Involves a series of reactions

convert G3P to the 5C intermediate

Ru5P (ribulose5-phosphate),

phosphorylation of Ru5P to

regenerate RuBP (ribulose-

bisphosphate).

Requires ATP formed in the light

reactions

Overall


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Overall,

for every 3 turns of the cycle 1 molecule of product (G3P) is formed

(3CO2:1G3P).

G3P formed in the Calvin cycle can remain in the chloroplast where it is

converted to starch

The remaining 15 carbon atoms (5G3P) re-enter the cycle to produce three

molecules of RuBP

Photosynthesis


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Photosynthesis Structure of chloroplast

Light absorption Chlorophyll a and b

Accessory photosynthetic pigments

Light dependent reaction Location

Photoactivation Photosystem I and Photosystem II

definition differences

Photophosphorylation Cyclic

Non cyclic

Light independent reaction

Calvin cycle Hatch-Slack pathway

CAM

Photorespiration

Limiting factor of photosynthesis

Location, Products, Types of plant involved

Differences between Calvin cycle and Hatch-Slack

The function

The flow, The products and its significant, Differences


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HATCH-SLACK PATHWAYIN C4 PLANTS


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HATCH-SLACK PATHWAY

IN C4 PLANTS


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PHOTORESPIRATION

Light-dependent reactions released O2

O2 competes with CO2 for the active site of theenzyme RuBP carboxylase

If O2 is bound to RuBP, a faulty reactionmechanism occurs, producing one 3C molecule,

3-phosphoglycerate, and one 2C molecule,phosphoglycollate, which is lost from the Calvincycle, removing both carbon and energy from thecycle


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Plants have evolved a mechanism to return the lost

carbon, but this is complicated and 1 of every 4

carbon molecules to enter this pathway is lost ascarbon dioxide.

The recovery pathway uses oxygen and producescarbon dioxide, and so is known as

photorespiration.

Plants which are photorespiring can lose up to40% of the carbohydrate that they would

otherwise have produced under those conditions.

HATCH SLACK PATHWAY


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HATCH-SLACK PATHWAY In hot climates with high light

intensities, the rate of

photosynthesis is high,

increasing the oxygen

concentration within cells

and

causing a high rate ofphotorespiration

A group of plants has evolved

by using an alternative methodof fixing carbon dioxide which

does not rely on ribulose

1,3-bisphosphate

carboxylase

In these plants carbon


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t ese p a ts ca bo

fixation isseparated

spatially from

photochemical reactions.

Carbon fixation by

RUBISCO takes place in

bundle sheath cells, whereas

the oxygen is produced inmesophyllcells rich in

chloroplasts.

Separation of the light

reactions (photochemicalreactions) from the dark

reactions,

decreases photorespiration

increases photosynthesis.

Part I (in mesophyll cells)


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Part - I (in mesophyll cells)

First CO2 Fixation:

CO2 first combines with 3Cphosphoenol pyruvate (PEP) to form4C OAA (oxaloacetic acid).

Catalyzed by phosphoenol pyruvatecarboxylase.

As OAA is a dicarboxylic acid, this isalso known as the dicarboxylic acidpathway.

OAA may be converted into malate(4C) or aspartate (4C) and transportedto bundle sheath cells.

Part II (in bundle sheath cells)


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Part - II (in bundle sheath cells)

malate (4C) undergoes

decarboxylation to formCO2 and pyruvate (3C).

Second CO2 fixation :

CO2 combines withRUBP (5C) to form 2molecules of 3-

phosphoglycerate (3C)as in the Calvin cycle.

Further conversion of 3-phosphoglycerate (3C)to sugars is the same as

in the Calvin cycle.

The pyruvate produced in decarboxylation of malate is


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The pyruvate produced in decarboxylation of malate is

transported back to the mesophyll cells.

It was converted into PEP and again made available for the

C4pathway.


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DIFFERENCES BETWEEN C3 AND C4PLANTS

C3 C4


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C3 C4CO2 fixation Occurs once Occurs twice, first in mesophyll cells,

then in bundle sheath cells

CO2 acceptor RuBP, a 5C compound Mesophyll cells

PEP, a 3C

compound

Bundle sheath

cells

RuBP

CO2 fixing enzyme RuBP carboxylase PEP carboxylase

which is very

efficient

RuBP

carboxylase

First product of

photosynthesis

A C3 acid, G3P A C4 acid, Oxaloacetate

Photorespiration Occurs ; therefore O2is an inhibitor of

photosynthesis

Is inhibited by high CO2concentration. Therefore atmospheric

O2 not an inhibitor of photosynthesisEfficiency Less efficient

photosynthesis than C4plants. Yields usually

much lower

More efficient photosynthesis than

C3plants. Yields usually much higher

Photosynthesis


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y Structure of chloroplast







Non cyclic



CAM

Photorespiration




The function


CRASSULACEAN ACID


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CRASSULACEAN ACID

METABOLISM

CRASSULACEAN ACID


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CRASSULACEAN ACID

METABOLISM

A second strategy to minimize photorespiration is found in succulentplants, cacti, pineapples, and several other plant families.

These plants, known as CAM plants forcrassulacean acidmetabolism (CAM), open stomata during the night and

close them during the day.


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Temperatures aretypically lower atnight and humidity is

higher.

During the night, theseplants fix CO2 into avariety of organicacids in mesophyllcells.

During the day, the

light reactions supplyATP and NADPH tothe Calvin cycle andCO2 is released fromthe organic acids.


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Both C4

and CAM plants addCO2 into organic intermediates

before it enters the Calvincycle.

In C4plants, carbon fixation

and the Calvin cycle are

spatially separated.

In CAM plants, carbon fixationand the Calvin cycle aretemporally separated.

Both eventually use the Calvincycle to incorporate lightenergy into the production ofsugar.


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Less than 5% plants (e.g.cactus) have anotherbiochemical adaptation that

allows them to survive hot anddry environments CAMplants (crassulaceanacidmetabolism)

They utilize PEP carboxylase to

fix CO2, just like C4plants

Unlike C4plants, CAM plants

conduct the light dependentreactions and CO2 fixation at

different times of the day, ratherthan in different cells of the leaf


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CAM MECHANISM

During the night, stomata are

open

CO2 enters the leaf tissue

CO2 +PEP

Oxaloacetate(OAA) Malate

Malate is transported into the

vacuole

During the day, stomata are

closed

Malate is moved into the

chloroplasts

Malate CO2 +Pyruvate

CO2 enters Calvin cycle


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PHOTOSYNTHESIS IS THE BIOSPHERES


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PHOTOSYNTHESISIS THE BIOSPHERES

METABOLIC FOUNDATION

In photosynthesis, the energy that enters the chloroplasts as sunlightbecomes stored as chemical energy in organic compounds.

S

ugar made in the chloroplasts supplies the entire plant with chemicalenergy and carbon skeletons to synthesize all the major organicmolecules of cells.

About 50% of the organic material is consumed as fuel for cellular

respiration in plant mitochondria.

Carbohydrate in the form of the disaccharide sucrose travels via theveins to non photosynthetic cells.


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There, it provides fuel for respiration and

the raw materials for anabolic pathways includingsynthesis of proteins and lipids and building theextracellular polysaccharide cellulose.

Plants also store excess sugar by synthesizingstarch.

Some is stored as starch in chloroplasts or in

storage cells in roots, tubers, seeds, and fruits.


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Photosynthesis


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Non cyclic



CAM

Photorespiration




The function


LIMITING FACTORS OF


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LIMITING FACTORS OF

PHOTOSYNTHESIS

WAVELENGHT

LIGHT INTENSITY

TEMPERATURE

CARBON DIOXIDE

WAVELENGHT


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WAVELENGHT

WAVELENGHT


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WAVELENGHT

A pigment is anysubstance that absorbslight.

The color of thepigment comes fromthe wavelengths oflight reflected (in

other words, those notabsorbed).

Chl h ll h i


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Chlorophyll, the green pigment

common to all photosynthetic

cells, absorbs all wavelengths of

visible light except green, which

it reflects to be detected by our

eyes.

Black pigments absorb all of thewavelengths that strike them.

White pigments/lighter colors

reflect all or almost all of theenergy striking them.

LIGHT INTENSITY


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LIGHT INTENSITY

Plants need light energy to

make the chemical energy

needed to create

carbohydrates.

The greater the intensity

of light, the plant receives

more light energy. The

plant can photosynthesize

faster as a result.

As the light intensity decreases,


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g y ,the amount of light the plantreceives is less and therefore therate of photosynthesis

decreases.

Light is a limiting factor at lowlight intensities.

There comes a point though thatany extra light energy will notincrease the rate of the reaction.

This is because the enzymes

controlling the reaction areworking as fast as possible.

At this point light is no longer alimiting factor.

TEMPERATURE


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TEMPERATURE

When the temperaturerises the rate ofphotosynthesis risesalso.

This is because theparticles in thereaction move quickerand collide more.

There is an optimumtemperature however.


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At this point the rate of

photosynthesis progressesas fast as it can, limitedonly by the other factors.

Beyond this temperaturethe enzymes controllingthe reaction becomedenatured and the

reaction quickly comes toa halt.

CARBON DIOXIDE


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CARBON DIOXIDE

When the concentration of

carbon dioxide is low the

rate of photosynthesis is

also low.

This is because the plant

has to spend a certain

amount of time doing

nothing, waiting for more

carbon dioxide to arrive.

Increasing the concentration of


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Increasing the concentration ofcarbon dioxide increases therate of photosynthesis.

There is a point at which furtheraddition of carbon dioxide willnot increase the rate of

photosynthesis.

The enzymes controlling thereaction are working as fast as

possible, so the excess carbon

dioxide cannot be utilized.

Carbon dioxide is not thelimiting factor at this point.

PhotosynthesisSt t f hl l t


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Accessory photosynthetic pigments Light dependent reaction

Location




Non cyclic



CAM

Photorespiration




The function



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Thats all forthis topic.

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Biology sem1- chap6