Photosynthesis

Plants are the producers of FOOD• Green plants are autotrophic (prepare their own food).

• They 1st form simple carbohydrates (glucose) and later

they form complex carbohydrates (starch) in presence of

sunlight and chlorophyll, using H2O & CO2.

• This process is carried out in the presence of sunlight, thus

is known as Photo-synthesis.is known as Photo-synthesis.

• Due to the above factors green plants are known as

Producers.

• They utilize some part of the production in their biological

activity & rest is stored.

• This stored food is directly or indirectly used by other

living organisms.

Definition of PHOTOSYNTHESIS

• It is anabolic process by which plantsmanufacture carbohydrates, or foodmaterial with the help of H2O & CO2 inpresence of sunlight and chlorophyll.

• During this process, oxygen is liberated as aby-product.by-product.

• In photosynthesis, plant obtain H2O from thesoil & CO2 from air. The process isrepresented as follows:

•• 6CO6CO22 ++ 12 H12 H22O O CC66HH1212OO66 ++ 6O6O22 ++ 6H6H22OO

Discovery of photosynthesis and its history

• Aristotle: first to state that plants obtain their food byroots from soil.

• Stephen Hales(1727): leaves prepare food in presenceof light.

• J. Priestly(1772): Proved that green plants purifypolluted air (expted on rats and plants)polluted air (expted on rats and plants)

• J. Senebier(1788): first to state that green plants takeCO2 and liberate O2 in presence of sunlight. He alsoshowed the importance of H2O in photosynthesis.

• Blackman(1905): He proved that photosynthesis is notonly a photochemical process but it is also abiochemical process.

• Willstater and Stoll(1918): They explained thechemical composition and characteristics ofchlorophyll.

• Hill(1937): He stated that O2 is liberated duringphotosynthesis by chlorophyll, water andsunlight in the presence of H-acceptorcompound. He also stated that O2 is liberatedfrom H O and it is a light reaction.

2

from H2O and it is a light reaction.

• Robert Mayer(1948): He presented the law ofconservation of energy and made clear thatgreen plants absorb light energy and convert itinto chemical energy.

• Calvin and Benson (1967): They discovered dark

reaction of photosynthesis and discovered

pathway of Calvin Cycle stating that there are C-

atoms in first organic material, which was

identified as 3 PGA (Phosphoglyceric Acid)

• Hatch and Slack(1967): They discovered another• Hatch and Slack(1967): They discovered another

pathway of CO2 fixation in sugar cane, in which 4

C-compound (oxaloacetate) is the first stable

organic compound instead of 3 C-compound (PGA).

This pathway is found in members of grass family

(monocots).

Photosynthetic organ: The LEAF

- In plants the green parts

perform photosynthesis.

Leaves are most suitable

for this purpose.

- They arise at the nodes of

stem and appear green, stem and appear green,

flat and thin.

- They are exposed to sun

and specialized for the

process of photosynthesis

and transpiration.

Anatomy of a leaf• Anatomically, the leaf has outermost epidermal layers

on both the surfaces which are covered with waxycuticle.

• Between epidermal layers, palisade and spongyparenchyma are found which contains green plastidsknown as Chloroplasts.known as Chloroplasts.

• They together forms Mesophyll tissue.

• Mesophyll tissue = Palisade + Spongy (Parenchyma)

• The cells of palisade layer are elongated, with morechloroplasts, while spongy parenchyma containsalmost spherical cells with less number ofchloroplasts.

Photosynthetic Organ: The LEAF

Leaf Adaptations

1). They have broad, wide and flat surface to absorblight and CO2.

2). Leaves has minute pores known as stomata onboth surfaces to facilitate exchange of gasesbetween the leaf and the atmosphere.

3). Palisade tissues absorbs maximum sunlight as it is3). Palisade tissues absorbs maximum sunlight as it issituated just below the epidermis and alsobecause it contains abundant chloroplasts.

4). There is a continuous supply of water and mineralsin every cell through the extensive network ofveins within the leaf lamina. It also helps in thetranslocation of food to the other parts of plant.

5). The diffusion of CO2 becomes very easy in eachand every cell because of the presence of largeintercellular spaces inside the leaf.

6).- Each palisade cell has numerous chloroplasts.

- Water coming from the vein enters the cell by

osmosis.

- CO2 from the atmosphere also diffuses into- CO2 from the atmosphere also diffuses intothese cells.

- Chlorophyll pigments found inside chloroplast

absorb sunlight.

- The oxygen is released into the atm. throughthe stomata.

Chloroplasts

(Containing ChlorophyllChloroplast

(containing Chlorophyll)

Cellulose

Cell Wall

Cell

Membrane

Sunlight CO2

Vacuole

NucleusNucleus

Vacuole

Membrane

Cytoplasm

StarchH2O

Vacuole

Regulation of Stomata

Stomata

• The stomata are minute openings on leaf

surface.

• They are bordered by guard cells which

open or close according to their turgidity.open or close according to their turgidity.

• They are kidney shaped so when they are

turgid, the stomatal opening is wide

open.

Opening of Stomata

• During daytime the guard cells of stomata

photosynthesize actively.

• Thus, the sugar produced in the cell increases

osmotic pressure inside cell causing water to enter.

• It has been proved that due to photosynthesis guard• It has been proved that due to photosynthesis guard

cells indirectly absorb K+ ions from adjacent cells,

increasing osmotic pressure inside.

• This results in bulging out of the outer thin walls and

separation of guard cells from each other of the

inner thick walls resulting in the opening of stomata.

Closing of

Stomata

When watercontent in leafbecomes less theguard cells loseguard cells loseturgidity andbecome flaccid,closing thestomata.

Raw materials for Photosynthesis

• CO2 & H2O are the main raw materials for photo-synthesis.

• If these 2 substances are absent then plant willnot survive as it cannot prepares its food.

• In culture solutions sugars can replace CO2 but• In culture solutions sugars can replace CO2 butH2O cannot be replaced. But in natural conditionsthere is no substitute for CO2 .

• H2O serves as the only source to provide Hrequired for the reduction of CO2 to convert itinto Carbohydrates.

Factors essential for Photosynthesis

1. Chlorophyll1. Chlorophyll

2. Carbon dioxide2. Carbon dioxide

3. Water 3. Water

4. Sunlight4. Sunlight

1. Chlorophyll

• Green colour pigment. They are Chlorophyll a,b, c etc.

• These pigments are contained in cell organellescalled chloroplasts (a type of plastids).

• Chlorophyll is a highly complex, ring structure• Chlorophyll is a highly complex, ring structurewhich is composed of C, H, O, N & Mg.

• Chlorophyll is present in the palisade cell andspongy parenchyma of the leaves, in the guardcells of stomata and the green stem cells ofyoung plants.

• Besides chlorophyll, higher plants also contain other

photosynthetic pigments such as Carotene (orange),

Xanthophyll (yellow), and Phaeophytin (grey).

• Exposure of plant parts to light induces the formation of

chlorophyll.

• In some cases, even roots when exposed to sunlight become

green and occasionally form shoot buds.

• Lack of sunlight impairs chlorophyll synthesis and plants lose• Lack of sunlight impairs chlorophyll synthesis and plants lose

their green colour. This condition is called ETIOLATION.

• Chlorophyll is sensitive to only a part of the visible light.

• This pigment is destroyed when it is exposed to bright light for

a long period.

• The main function of this pigment is to get excited with light

energy which results in ejection of an energy rich electron.

Site of Photosynthesis

� Chloroplasts are the site of

Photosynthesis. Their number varies.

� Amt. of sunlight affects their size,

smaller in bright light and larger in

dark or dim light.

� The chloroplasts consist of

folded membranes known as the

LAMELLAE.LAMELLAE.

� When stacked at particular sites,

they form dense structures called

the GRANA.

�The GRANA are separated by a

watery matrix called the STROMA.

�The lamellae appear as sacs

known as THYLOKOIDS.

• The Stroma are the site of enzymes

responsible for the reduction of CO2

and formation of numerous starchand formation of numerous starch

grains which are formed by

photosynthesis.

2. Carbon dioxide

• The atmospheric CO2 enters the

plant body through minute openings

called the stomata which are present

in very large numbers on the upperin very large numbers on the upper

and lower surface of the leaves.

• CO2 simply diffuses into the cells.

3. Water• Its an important raw material for photosynthesis.

• Plant absorb H2O from the soil through their roots &

root hairs.

• The H2O is then translocated up to the leaves through

the stem by Xylem tissues present in roots & stems.

• Aquatic plants absorb H2O & minerals from their• Aquatic plants absorb H2O & minerals from their

general body surface.

• Minerals absorbed along with H2O contribute to the

process of photosynthesis, for example, Sulphates,

Nitrates and Phosphates are required by the plants for

converting the carbohydrates into proteins.

4. Sunlight

• During daytime, the sunlight

incident on leaves acts as the

source of energy.

• Light energy is trapped by

chlorophyll molecules in

chloroplasts and the complex

process of photosynthesis begins.

Mechanism of Photosynthesis

� It is very complex & a number of reactions are involved.

�2 phases ( Light Phase & Dark Phase)

�Light Phase is Light dependent (Light reaction / reaction / Photochemical Process)

�Dark Phase is Light independent (Dark reaction / Biosynthetic Phase)

Light Reaction

• Also known as Hill Reaction.

• These reactions take place in the GRANA of

Chloroplasts where chlorophyll containing

THYLAKOIDS are present.THYLAKOIDS are present.

• Light reaction has 3 main steps.

– 1). Absorption of Light

– 2). Splitting of Water

– 3). Photophosphorylation

1. Absorption of Light

• The chlorophyll molecules absorb the

incident sunlight and become excited.

• As a result electrons of its molecules reach

outer orbits.

• This is called activation of chlorophyll.

2. Splitting of Water• The energy of chlorophyll molecule is used in splitting the water molecule

into Hydrogen and Oxygen.

• 2 electrons are also liberated.

• This is called photolysis of water.

• The process is

4H2O 4OH- + 4H+

4OH- 2H2O + O2 + 4e-4OH 2H2O + O2 + 4e

• The O2 is evolved.

• The H-ions are used to reduce NADP forming NADPH (NicotinamideAdenosine Diphosphate).

H+ + e- + NADP NADPH

• The electrons are used for photo- phosphorylation.

3. Photophosphorylation

• It is the process where energy liberated by photolysis of H2O (in form of electrons) is used to add a molecule of phosphate to ADP (Adenosine Diphosphate) forming ATP (Adenosine Triphosphate).

ADP + Pi ATP

• (i) in P indicates inorganic Phosphate.• (i) in Pi indicates inorganic Phosphate.

• ATP molecule formed here is an energy rich molecule.

• All physiological processes of cells obtain energy by converting ATP back to ADP.

• This is again converted to ATP by Photo-phosphorylation.

Dark Reaction

• It takes place in STROMASTROMASTROMASTROMA of Chloroplast.

• These reactions are also known as Blackman’sBlackman’sReactionReaction oror CalvinCalvin CycleCycle.

• The term dark reaction means a reaction do notrequires light. i.e. it does not means that ittakes place in night. But it is light independenttakes place in night. But it is light independent& occurs along with light reactions.

• The absorbed CO2 is fixed by combining it withH from NADPH by utilizing ATP energy.

• The end product is glucose.

• The process takes place through intermediate

reaction where intermediate products are

converted ultimately leading to glucose.

• The most important of these compounds is RuBP

(Ribulose bi-phosphate).

• The glucose is converted to starch for storage.

END RESULTEND RESULTEND RESULTEND RESULTEND RESULTEND RESULTEND RESULTEND RESULT

• The overall process of photosynthesis can be

summed up by following reaction.

• 6666COCOCOCO2222

+ 12121212HHHH2222OOOO CCCC

6666HHHH

12121212OOOO

6666+ 6666OOOO

2222+ 6666HHHH

2222OOOO

Factors affecting Photosynthesis

1). Light: � Sun is the main source of energy.

� All living organisms are dependenton photosynthetic processes ofplants and it occurs in the presenceof light.

� Thus light is essential for� Thus light is essential forphotosynthesis and photosynthesisis maximum in red light followed byblue light and is least in green light.

� When the conc. of CO2 & Temp. arecontrolled at constant level, therate of photosynthesis is directlyproportional to light intensity up toa certain point.

• This graph focuses on the

maximum value at which

increasing light intensity

will increase the rate of

photosynthesis. Once light

intensity reaches

approximately 38%, the

rate of photosynthesis willrate of photosynthesis will

not increase anymore.

Values of light intensity

that are tested after 38%

will remain at one

constant peak value.

2). CO2). CO2). CO2). CO2222

::::

• It is a raw material used forphotosynthesis.

• Plants take CO2 fromatmosphere and transform itinto Hexose sugar (glucose).

• Photosynthesis is increased

by increasing CO to someby increasing CO2 to someextent.

• At very high light intensity,the rate of photosynthesisslows down because thepigment chlorophyll is

damaged by u-v rays.

• Sometimes photosynthesis is

limited by the concentration

of carbon dioxide in the air.

Even if there is plenty of

light, a plant cannot

photosynthesize if there is

not a sufficient amount of

carbon dioxide. If CO2 iscarbon dioxide. If CO2 is

added the graph will keep

going up until about .10%

CO2. At this point the graph

will plateau out.

3). H3). H22O:O:• Photosynthesis involves

many chemical reactions.

• All reactions takes place in asolution and H2O provides amedium for the reaction aswell as it is one of thereactant in the process.

• It splits and gives H+ & e- to• It splits and gives H+ & e- toNADP & liberates O2.

• If water is not supplied,wilting occurs and leads toStomatal closure whichwould affect photosynthesisby preventing the diffusionof CO2 into the leaves.

4). Temperature:

• Rate of photosynthesis increases with temperature

increases till 350 C (highest photosynthesis).

• Till this peak is reached photosynthesis rate is doubled

for every 100 C rise.

• The optimum temp for most of the plants are

between 25 C – 30 C.

The optimum temp for most of the plants are

between 25o C – 30o C.

• Above this temperature the rate of photosynthesis

falls due to inactivation of enzymes involved.

• However, when the temp. is too high the

photosynthetic enzyme are destroyed and so

photosynthesis stops altogether.

• If it gets too cold, the rate

of photosynthesis will

decrease. Plants cannot

photosynthesise if it gets

too hot.too hot.

Principle of Limiting Factors

• The main factors affecting the rate ofphotosynthesis are Light intensity, CO2

content and temperature.

• These factors are not independent.

• Thus, whichever factor is lowest in content• Thus, whichever factor is lowest in contentin the surroundings actually determines therate of photosynthesis .

• This is called the principle of limiting factors

(Blackman, 1905)

Internal Factors• 1). Chlorophyll:

– It is the most important pigment

– because it can absorb light energy

– which can be utilized for the formation of ATP andreduction of NADP.

• 2). Protoplasm:– Active protoplasm is essential for photosynthesis– Active protoplasm is essential for photosynthesis

– because chloroplast is the part of the protoplasm

– where the photosynthetic reactions takes place.

• 3). Leaf Anatomy:– Size, Structure, Position of stomata, number of chloroplasts

in the leaf all affect rate of photosynthesis.

– Thickness of cuticle and size of leaf also plays an importantrole.

Importance of Photosynthesis• 1). Source of Energy:

– Products of photosynthesis becomes the source of energyfor all living organisms for both plants and animals.

– The oxidation of food produced by plants generatesenergy for biological activities.

• 2). Production of Food:2). Production of Food:

– Only plants are capable of producing food which isdirectly or indirectly consumed by all animals andhumans.

– It also helps in the growth and development of plants.

– Plants transfers energy from one trophic level to theother trophic level.

• 3). Balancing of Atmospheric CO2 and O2:

– The combustion of Coal, oil, and other fuels releases CO2 in

the atmosphere.

– Expiration of living beings also releases CO2 in the

atmosphere.

– Photosynthesis requires CO2 which taken up from the

atmosphere.

– Consequently the amount of CO2 does not increase too much

in the atmosphere.2

in the atmosphere.

– CO2 is able to absorb solar heat. It retains in the atmosphere

and is not allowed to escape. This is known as Green House

Effect. It may lead to Global Warming.

– Photosynthesis enriches the atmosphere by supplying O2

which is used by living beings for respiration.

– Thus Green Plants maintain the balance between CO2 and O2.

Experiment:To prove that O2 is evolved during photosynthesis

Experiment:To demonstrate that STARCH is formed during photosynthesis

• Materials Required: 1. Leafy plant 2. Black paper or foil 3. Alcohol 4. Iodine solution 5. Benzol

• Procedure:

Take a healthy green leaf on a plant and cover a portion of it on both sides with twouniform pieces of black paper or foil, the previous evening so that the experiment isperformed with a starch-free leaf or keep a healthy, green pot plant in a dark roomfor 1 or 2 days so that its leaves become starch-free, and then cover a portion of a leafof this plant as above.

Fix the pieces of paper or foil properly with clips. To make sure that there is no starch,Fix the pieces of paper or foil properly with clips. To make sure that there is no starch,collect a few neighbouring leaves in the morning, decolourize them with alcohol anddip them into iodine solution. Note that they do not turn black, thus all the leaves arestarch-free. Now let the plant be exposed to light for some time, preferably till theevening and collect the leaf and decolourize it with alcohol. Place the blanched leaf iniodine solution for a minute.

• Observation:

The exposed portion turns blue or black, showing the presence of starch in it. Thescreened portion, on the other hand, turns yellowish brown as no starch formed in it,the yellow color is due to the action of iodine solution on protoplasm and cellulose. Itis better to place the iodine-treated leaf in benzol for a few minutes as benzolremoves the brownish colour from the protoplasm and the cell-walls, and then thebluish-black colour of starch becomes clear.

Carbon Cycle