Photosynthesis In Higher Plants Class 11th Notes - Free NCERT Class 11th Biology Chapter 13 Notes - Download PDF

Class 11 Biology chapter 13 notes will introduce you to photosynthesis in Higher plants. All animals on this planet earth ultimately depend on plants for their food requirements. Photosynthesis in Higher plants Class 11 notes will also investigate the steps involved in photosynthesis to prepare or synthesize food. CBSE Class 11 Biology chapter 13 notes inform that 'photosynthesis' is a physio-chemical process in which green plants use light energy from the sun to synthesis organic compounds.

This Photosynthesis in Higher plants Class 11 notes also tells you two major reasons highlighting the importance of photosynthesis, those are 1). They are the primary source of food on earth and 2)They also release oxygen into the atmosphere. Class 11 Biology chapter 13 notes mainly focus on the structure of photosynthetic machinery and various chemical reactions that are responsible for converting light energy into chemical energy. This Photosynthesis in Higher plants Class 11 notes will give you more knowledge on the subject and will help you pass your tests. Students can also get the Photosynthesis in Higher plants class 11 notes pdf download on the internet.

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• NCERT Solutions for Class 11 Biology Chapter 13 Photosynthesis in Higher plants
• NCERT Exemplar Class 11 Biology Chapter 13 Photosynthesis in Higher plants

NCERT Class 11 Biology Chapter 13 Photosynthesis in Higher plants Notes

Class 11 Photosynthesis in Higher plants notes informs that all animals on this planet earth including human beings depend on plants for their food. We need to know that magical mechanism with the help of which plants synthesize well. Green plants synthesise the food they need and all other organisms depend on them to fulfill their energy needs. The green plants make food that they need through photosynthesis and since they prepare their food on their own, therefore, called autotrophs. Only plants exhibit autotrophic nutrition.

Heterotrophs are species that rely on green plants for nutrition, including humans. Photosynthesis is a Physio-chemical process in which plants utilize the Sun's light energy to fuel the synthesis of organic molecules such as glucose. We must not forget that Ultimately, all living forms on earth depend on sunlight for energy. According to Photosynthesis in Higher plants, Class 11 notes Photosynthesis is important due to major two reasons 1). It's the primary source of all food on earth 2) Oxygen (important for aerobic life to exist on earth) is released into the atmosphere.

This chapter explores the photosynthetic machinery's structure as well as the numerous processes that turn sunlight's light energy into chemical energy.

What Do We Already Know?

We already know that 3 components which are chlorophyll that is the green coloured pigment of leaf, light, and carbon dioxide are required for photosynthesis to occur. In earlier classes, we also did some experiments where one leaf was partially covered with black paper, and then the other way posed to light. On testing leaves for the presence of starch, it is proved that photosynthesis occurred only in the green parts of the leaves of that plant that was exposed to sunlight thus showing the importance of sunlight.

Another experiment that can be carried out is, if a part of a leaf of a plant is enclosed in a test tube containing some KOH solution soaked in cotton (KOH absorbs CO₂), while the other half is exposed to air. The arrangement is then exposed to light for a period of time. Later, when the two halves of the leaf were tested for the presence of starch, it was discovered that the exposed part of the leaf that was exposed to CO₂ tested positive for starch while the half that was in the tube tested negative. This proves that CO₂ is necessary for photosynthesis.

Early Experiments

• In 1774 Joseph Priestly discovered oxygen and based on the experiments he hypothesised: Plants restore to the air whatever breathing animals and burning candles remove

An experiment performed by Joseph Priestly

• In 1854 Julius von Sachs determined that the green parts in plants are where glucose is made and glucose is generally saved as starch.
• T.W Engelmann (1843–1909), conducted a remarkable experiment. He separated light into its spectrum components using a prism and then lighted a green alga called Cladophora that was suspended in an anaerobic bacteria solution. The microorganisms were employed to find out where O2 evolved. He discovered that germs gathered mostly in the split spectrum's blue and red light regions. As a result, the first photosynthesis action spectrum was defined. It mimics the absorption spectra of chlorophyll a and b in appearance. The whole photosynthetic process for oxygen-evolving organisms was then represented by the empirical equation:

where [CH₂O] represents a carbohydrate (e.g., glucose, a six-carbon sugar).

• A milestone contribution to the understanding of photosynthesis was made by microbiologist Cornelius van Neill who based on his considers of purple and green microbes, illustrated that photosynthesis is a light-dependent response in which H₂ from a suitable hydrogen compound i.e. H₂O reduces carbon dioxide to carbohydrates.

that can be given as,

• In green plants, H₂O is the H₂ giver and is oxidized to O₂. A few life forms don't discharge O₂ during photosynthesis. When H₂S instep is the Hydrogen donor for purple and green sulfur microbes, the 'oxidation' item is sulphur/Sulphate. Hence it's concluded that O₂ evolved by plants(green) comes from H₂O and not from CO₂.

Thus, the equation of photosynthesis can be given as :

Site of Photosynthesis ( Where does it take place ):

Photoshthe takes place in the green leaves of plants, particularly in the chloroplast of their mesophyll

The membrane system is in charge of capturing solar energy as well as ATP and NADPH production; the reactions/ set of reactions involved are light-driven and are therefore called light reactions.

In the stroma, enzymes are responsible for taking CO₂ and leading to the synthesis of sugar, which in turn forms starch, the reactions are not directly light driven but dependent on the products of light reactions i.e. ATP and NADPH, and are called Dark reactions

How Many Types of Pigments Are Involved

There are four pigments in leaves named chlorophyll-a of bright or blue-green colour, chlorophyll b of yellow-green, Xanthophylls of yellow colour, and carotenoids of yellow - to yellow-orange colour.

Leaves showing different color pigments

We can conclude that chlorophyll is the chief pigment associated with photosynthesis.

Light Reactions

• It includes light absorption, water splitting, oxygen release, and the formation of high-energy chemicals i.e. ATP and NADPH.
• Several complexes viz LHC, Light-harvesting complexes in which pigments are there are in Photosystem 1 (PS1) and Photosystem 2 (PS2), these are named in a sequence of their discovery.
• Each photosystem has all the pigments except chlorophyll a and forms an LHS called antennae.
• The single-molecule chlorophyll-a forms the reaction centre.
• PS 1 has an absorption peak at 700 nm, which is referred to as P700, whereas PS 2 has an absorption peak at 680 nm, which is referred to as P680.

The Electron Transport

In PS 2 LHC receives the light of 680 nanometers that results in the excitation of electrons and thus they jump out of their orbit and are received by an electron acceptor that then passes it to the electron transport system. Electrons are not used up and are passed to pigments of PS1 with simultaneous excitation of electrons in PS1 because of absorption of light of 700 nanometers these electrons are passed to a molecule of energy NADP+ resulting in the formation of NADPH + H+

Electron transport showing Z scheme

This scheme is known as the Z scheme

Splitting of Water

• The electrons that were moved from Photosystem 2 are replaced by the splitting of H₂O. This creates Oxygen that is a product of photosynthesis.

2H₂O ⟶ 4 H+ + O₂ + 4e-

• The electrons needed to replace those removed from PS 1 are provided by PS 2.
• This splitting of H₂O is associated with PS 2.

Cyclic and Non-Cyclic Photophosphorylation

• Photophosphorylation is the synthesis of ATP from ADP and Pi in the presence of light. when PS 2 and then PS 1 act in series non-cyclic photophosphorylation occurs, both ATP and NADPH are synthesised here whereas;
• When only PS 1 is functional, the electron is circulated within PS 1 and phosphorylation occurs due to the cyclic flow of electrons. It takes place in stroma lamellae. It lacks NADP reductase enzyme, here synthesis of ATP takes place but not of NADPH + H+

Electron transport system in PS1

• It also occurs when only light off wavelength beyond 680 nanometers are available

Chemiosmotic Hypothesis

The ATP synthase enzyme is made up of two parts: the F0 is introduced into the thylakoid layer and produces a transmembrane channel that allows protons to diffuse across the layer quite easily. The other half, known as F1, is located on the stroma-facing side of the thylakoid membrane's exterior surface. The breakdown of the gradient provides enough energy to trigger a conformational shift in the ATP synthase's F1 moiety, causing the enzyme to synthesis multiple molecules of ATP. A membrane, a proton pump, a proton angle, and ATP synthase are all required for chemiosmosis.

Energy is used to pump protons over a membrane, to make a gradient or a high concentration of protons inside the thylakoid lumen. ATP synthase incorporates a channel that permits the release of protons back over the membrane, this releases sufficient energy to enact ATP synthase chemical that catalyses the arrangement of ATP. Along with the NADPH created by the development of electrons.

Where are the ATP and NADPH used?

These are utilized in a dark reaction called the biosynthetic phase of photosynthesis as the synthesis of food takes place here.

During World War 2nd, Melvin Calvin discovered that the first carbon dioxide fixation product was a three-carbon organic acid called 3 - phosphoglyceric acid or PGA hence it was called the Calvin cycle.

C3: First CO₂ fixation product is 3-C containing PGA

C4: First CO₂ fixation product is 4-C containing OAA

The Primary Acceptor of CO₂:

In C3 plants it's 5-C containing RuBP (ribulose bisphosphate) that combines with CO2 to produce sucrose.

The Calvin Cycle

i) Carboxylation: here carbon dioxide is utilized for decarboxylation of RuBP that results in two molecules of 3-PGA in the presence of enzyme name as RuBP carboxylase-oxygenase or RuBisCO having Infinity for both carbon dioxide and oxygen.

ii) Reduction: Two molecules of ATP are required for phosphorylation and two molecules of NADPH are required for the reduction of carbon dioxide. The fixation of six molecules of carbon dioxide and six turns of cycles are required.

iii) Regeneration: Regeneration of carbon dioxide acceptor molecule RuBP requires one ATP for phosphorylation to form RuBP

The C4 Pathway

These have two kinds of cells that are mesophyll and vascular bundles. The large cells around vascular bundles are called bundle sheath cells and leaves are set to have kranz anatomy. Bundle sheath cells may have several layers containing chloroplast and are impervious to gaseous exchange. The primary carbon dioxide acceptor is the three-carbon molecule phosphoenolpyruvate (PEP) is present in the mesophyll cells. The enzyme responsible is PEP carboxylase or PEP case, here RuBisCO enzyme is absent. Here C4 acid OAA is formed that then forms malic acid/aspartic acid in mesophyll itself and then transported to bundle sheath cells where C4 acids are broken down to release carbon dioxide and a three-carbon molecule. Again, it is transported to mesophyll, where it forms PEP again thus completing the cycle.

Hatch and slack cycle

The carbon dioxide released in the bundle sheath cells enters the C3/Calvin pathway that is common to all plants whether C3 or C4. The bundle sheath cells are rich in enzyme RuBisCO but lack PEPcase. As bundle sheath cells are impermeable to carbon dioxide gas its concentration increases.

Photorespiration

• RuBisCO, which is the most abundant enzyme in the world, is characterized by the fact that it can bind with O₂ as well as CO₂.

• In C3 plants some O₂ binds with RuBisCO resulting in the formation of phosphoglycerate and phosphoglycolate in a pathway called photorespiration. In this pathway, there is no synthesis of sugar and ATP/NADPH therefore it's called a wasteful process
• In C4 plants photorespiration does not occur, therefore, they have more productivity.

Factors Affecting Photosynthesis :

• The rate of Photosynthesis depends upon both internal as well as external factors.
• Internal factors like the number, size, age, Orientation of leaves, chloroplast etc., and Internal like CO₂ concentration, Temperature, water etc.
• When several factors affect any biochemical process, Blackman's(1905) Law of Limiting Factors come into effect i.e. In case a chemical process is influenced by more than one factor at that point its rate will be decided by the factor which is closest to its minimal value which is called Limiting factor. It's the figure which specifically influences the method if the amount is changed.

Light

Light is used as 10% of incident sunlight. At low intensity, it's directly proportional to CO₂ fixation. At higher intensity, other factors become limiting.

An increase in light beyond a limit causes the breakdown of chlorophyll and thus a decrease in Photosynthesis.

Carbon Dioxide Concentration

It's the major limiting factor as carbon dioxide is very low in the atmosphere that is 0.03% to 0.04%, increase in concentration up to 0.05% can cause an increase in the rate of photosynthesis

Temperature

The dark reactions being enzymatic are temperature controlled. C4 plants respond to higher temperatures and they also show higher rates of photosynthesis whereas C3 plants are found to show lower temperature optimum.

Water

Water stress causes the stomata to close hence reducing the carbon dioxide availability and it makes leaves wilt thereby reducing surface area.

Significance of NCERT Class 11 Biology Chapter 13 Notes:

Class 11 Biology chapter 13 notes will assist you in revising the chapter and gaining an understanding of the main subjects addressed. These class 11 Photosynthesis in Higher plants notes are also beneficial for covering the main themes of the CBSE Biology Syllabus in Class 11 as well as for competitive exams such as AIPMT, AIIMS, NEET, and others. The Class 11 Biology chapter 13 notes pdf download can be utilized for offline preparation.

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