Chapter 13 Photosynthesis In Higher Plants

Notes For Class 11 Biology Download In PDF

POINTS TO REMEMBER

Photosynthesis : Photosynthesis is an enzyme regulated anabolic process

of manufacture of organic compounds inside the chlorophyll containing cells

from carbon dioxide and water with the help of sunlight as a source of energy.

Historical Perspective

Joseph Priestley (1770) : Showed that plants have the ability to take up CO 2

from atmosphere and release O . Jan Ingenhousz (1779) : Release of O 2 by plants was possible only in sunlight

and only by the green parts of plants.

Theodore de Saussure (1804) : Water is an essential requirement for photosynthesis

to occur.

Julius Von Sachs (1854) : Green parts in plant produce glucose which is

stored as starch.

T. W. Engelmann (1888) : The effect of different wavelength of light on

photosynthesis and plotted the first action spectrum of photosynthesis.

C. B.

Van Niel (1931) : Photosynthesis is essentially a light dependent reaction

in which hydrogen from an oxidisable compound reduces CO

to form

sugar. He gave a simplified chemical equation of photosynthesis.

Hill (1937) : Evolution of oxygen occurs in light reaction.

Calvin (1954-55) : Traced the pathway of carbon fixation.

Hatch and Slack (1965) : Discovered C 4 pathway of CO

fixation.

Site for photosynthesis : Photosynthesis takes place only in green parts of 2 the plant, mostly in leaves. Within a leaf, photosynthesis occurs in mesophyll

cells which contain the chloroplasts. Chloroplasts are the actual sites for photosynthesis.

The thylakoids in chloroplast contain most of pigments required for

capturing

solar energy

to initiate photosynthesis. The membrane system (grana)

2 is responsible for trapping the light energy and for the synthesis of ATP and

NADPH. Biosynthetic phase (dark reaction) is carried in stroma.

" Pigments involved in photosynthesis "

Chlorophyll a : (Bright or blue green in chromatograph). Major pigment,

act as reaction centre, involved in trapping and converting light into

chemical energy.

Chlorophyll b : (Yellow green)

Xanthophyll : (Yellow)

Carotenoids : (Yellow to yellow-orange)

In the blue and red regions of spectrum shows higher rate of photosynthesis.

Light Harvesting Complexes (LHC) : The light harvesting complexes are

made up of hundreds of pigment molecules bound to protein within the

photosystem I (PSI) and photosystem II (PSII). Each photosystem has all the

pigments except one molecule of chlorophyll ‘a’ forming a light harvesting system

(antennae). The reaction centre (chlorophyll a) is different

in both the photosystems.

Photosystem I (PSI) : Chlorophyll ‘a’ has an absorption peak at 700 nm

(P700).

Photosystem II (PSII) : Chlorophyll ‘a’ has absorption peak at 680 nm

(P680).

Process of photosynthesis : It includes two phases - Photochemical phase

and biosynthetic phase.

(i) Photochemical phase (Light reaction) : This phase includes - light

absorption, splitting of water, oxygen release and formation of ATP and

NADPH.

(ii) Biosynthetic phase (Dark reaction) : It is light independent phase,

synthesis of food material (sugars).

Photophosphorylation : The process of formation of high-energy

chemicals (ATP and NADPH).

Cyclic photophosphorylation : Two photosystems work in series ? First

PSII and then PSI. These two photosystems are connected through an electron

transport chain (Z. Scheme). Both ATP and NADPH + H

are synthesised by this

process. PSI and PSII are found in lamellae of grana, hence this process is

carried here.

Non-cyclic photophosphorylation : Only PSI works, the electron

circulates within the photosystem. It happens in the stroma lamellae (possible

location) because in this region PSII and NADP rectase enzyme are absent. Hence

only ATP molecules are synthesised.

The electron transport (Z-Scheme) : In PS II, reaction centre (chlo. a)

absorbs 680 nm wavelength of red light which make the electrons to become

excited. These electrons are taken up by the electron acceptor that passes them

to an electron transport system (ETS) consisting of cytochromes. The

movement of electron is down hill. Then, the electron pass to PSI and move

down hill further.

The splitting of water : It is linked to PS II. Water splits into H

, O and

electrons.

Chemiosmotic Hypothesis : Chemiosmotic hypothesis explain the

mechanism of ATP synthesis in chloroplast. In photosynthesis, ATP synthesis is

linked to development of a proton gradient across a membrane. The electrons

are accumulated inside of membrane of thylakoids (in lumen). ATPase has a

channel that allows diffusion of protons back across the membrane. This

releases energy to activate ATPase enzyme that catalyses the formation of ATP.

Plants : ATP and NADH, the products of light reaction are used in synthesis of

Biosynthetic phase in C

food. The first CO 2 3

fixation product in C

plant is 3-phosphoglyceric acid or

PGA. The CO 2 3

acceptor molecule is RuBP (ribulose bisphosphate). The cyclic

path of sugar formation is called Calvin cycle on the name of Melvin Calvin, the

discoverer of this pathway.

Calvin cycle proceeds in three stages :

(1) Carboxylation : CO

combines with ribulose 1, 5 bisphosphate to

form 3 PGA in the presence of RuBisCo enzyme. 2 (2) Reduction : Carbohydrate is formed at the expense of ATP and

NADPH.

(3) Regeneration : The CO

acceptor ribulose 1, 5-bisphosphate is formed

again . 2 6 turns of Calvin cycles and 18 ATP molecules are required to synthesize

one molecule of glucose.

The C 4 pathway : C

plants have special type of leaf anatomy, they tolerate

higher temperatures. In this pathway, oxaloacetic acid (OAA) is the first stable

product formed. It is 4 carbon atoms compound, hence called C 4 pathway (Hatch

and Slack Cycle).

The leaf has two types of cells : mesophyll cells and Bundle

sheath cells (Kranz anatomy). Initially CO 2 4

is taken up by phosphoenol pyruvate

(PEP) in mesophyll cells and changed to oxaloacetic acid (OAA) in the presence

of PEP carboxylase. Oxaloacetate is reduced to maltate/asparate that reach into

bundle sheath cells. and formation of pyruvate (3C). In high CO

The oxidation of maltate/asparate occurs with the release of O

concentration RuBisCo functions as

carboxylase and not as oxygenase, the photosynthetic losses are prevented. RuBP

operates now under Calvin cycle and pyruvate transported back to mesophyll

cells and changed into phosphoenol pyruvate to keep the cycle continue.

2 Photosrespiration : The light induced respiration in green plants is called

photorespiration. In C 3 plants some O 2 binds with RuBisCo and hence CO

fixation is decreased. In this process RuBP instead of being converted to 2

molecules of PGA binds with O

to form one molecule of PGA and

phosphoglycolate. 2 Law of Limiting Factors : If a chemical process is affected by more than

one factor, then its rate will be determined by the factor which is nearest to its

minimal value. It is the factor which directly affects the process if its quantity is

changed.

Factors affecting photosynthesis :

1. Light

2. Carbondioxide

3. Temperature

4. Water