notes on photosynthesis 11th class chapter.
Answers
Answer:
photosynthesis is a mode in which the plants make their own food
Explanation:
Photosynthesis is a Physic o – chemical process, uses light energy to synthesis organic compounds (sugar).
Importance of photosynthesis:
· Primary source of food
· Release O2 to atmosphere
Early Discoveries
· Joseph Priestly: Candle with bell jar and mouse experiment – He concluded that air is necessary for the growth of a plant. He discovered the fact that plants restore oxygen in the air.
· Jan Ingenhousz: Experiment with aquatic plant in light and dark – He concluded that sunlight is essential for plant processes that purify the air.
· Julius Von Sachs: Green parts of plant make glucose and store as starch.
· T.W. Engelmann: Spilt light using prism into 7 colours (VIBGYOR) - Green algae Cladophora placed in a suspension of aerobic bacteria - Bacteria were used to detect the sites of O2 evolutions.
· Cornelius van Niel: He did experiment with purple and green bacteria and demonstrated photosynthesis is a light dependent process with hydrogen from H2O reduces CO2 to carbohydrates. He concluded that oxygen comes from H2O, and not from CO2. Finally, the correct equation for photosynthesis was discovered.
6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
Site of Photosynthesis
o Green leaves, green stems and floral parts (sepal)
o Chloroplast - found in mesophyll cells of leaves
o In chloroplast – the membrane system is responsible for trapping the light energy and also for the synthesis of ATP and NADPH. Where stroma has enzymes for the reduction of CO2 in to carbohydrates (sugars )
Pigments Involved in Photosynthesis
4 types of pigments may be present in leaves:
Chlorophyll a
Chlorophyll b
Xanthophylls
Carotenoids
An absorption spectrum is the graph plotted against the fraction of light absorbed by the pigment.
An action spectrum is the rate of a physiological activity plotted against the wavelength of light.
· Photosystems are pigments that are organized in the thyllakoid membrane in to two different photosystems (PS 1 & PS 11 )
· Each PS has one specific chlorophyll – a, and many other accessory pigments bound by proteins.
· Chlorophyll – a forms the reaction centre (actual reaction takes place) other pigments form the light harvesting complex (LHC) called antennae.
· PS 1 reaction centre is p700 (chlorophyll –a absorbs light at 700 nm)
· PS 11 reaction centre is p680 ( chlorophyll –a absorbs light at 680 nm)
Light Reaction (Photochemical Phase)
This phase directly depends on light. The pigments absorb light energy and produce ATP.
Includes:
Light absorption
Water splitting
Oxygen release
Formation of ATP and NADPH, which is then used in the biosynthetic phase
Pigment molecules bound to the proteins form LHC (light harvesting complexes). LHC are located within two photosystems − PSI and PSII
Each photosystem has two parts:
Reaction centre − consisting of chlorophyll a molecule
Antennae − consisting of accessory pigments, which increase the efficiency of photosynthesis by absorbing different wavelengths of light
Reaction centre is different in both photosystems:
PSI − P700; since chlorophyll a has absorption peak at 700 nm here
PSII − P680; since chlorophyll a has absorption peak at 680 nm here.
Photo-Phosphorylation
The process of formation of ATP in chloroplast in the presence of sunlight
Photo-phosphorylation is of two types:
Non-cyclic photo-phosphorylation
Cyclic photo-phosphorylation
Non-Cyclic Photo-Phosphorylation
PSII absorbs 680 nm wavelength of red light, causing electrons to become excited and these electrons are then accepted by an electron acceptor, which sends them to an electron transport system.
Electron transport system transfers the electrons to PSI.
Electrons in PSI are simultaneously excited on receiving a wavelength of 700 nm.
From the electron acceptor, electrons are transferred to the molecule of NADP+.
Addition of these electrons reduces the NADP+ to NADPH+ H+.
Since the electrons lost by PSII do not come back to it, this process of formation of ATP is called non-cyclic photo-phosphorylation.
Cyclic Photo-Phosphorylation
In this scheme, only PSI is functional. Hence, the electrons are circulated within the photosystem.
This results in a cyclic flow of electrons.
This scheme could possibly be occurring in stroma lamellae because it lacks both PSII and NADP reductase enzyme.
This cyclic flow results only in the synthesis of ATP, and not of NADPH + H+.
Splitting Of Water
Water splitting complex is associated with PSII.
Manganese, chlorine, etc., play an important role.
The light-dependent splitting of water is called photolysis
2H2O → 4H+ + O2 + 4e−
Electrons formed are used for replacing the electrons lost from P680.
P680 absorbs light and becomes as a strong oxidizing agent and splits a molecule of water to release oxygen. Oxygen is liberated as a by-product of photosynthesis.
Protons are used for the formation of reducing power NADP to NADPH+.