Class 11 Photosynthesis in Higher Plants

Photosynthesis is one of the most fundamental processes carried out by plants for their survival and growth. With the help of this process, the plants are able to prepare their own food and give out oxygen to the environment. Since photosynthesis in plants is an inevitable phenomenon of life, it is essentially included as an important topic in Biology. The NCERT biology Class 11 syllabus has a very interesting chapter on Photosynthesis in Higher Plants exploring the salient features of this process. This blog brings you comprehensive study notes on this chapter to help you master this topic in a better way.

What is Photosynthesis?

Plants are the food source for almost all living beings, either directly or indirectly. Even the smallest of living organisms obtain their food from plants, big or small. But why are plants so important? The answer to that question is photosynthesis.

Photosynthesis is a physicochemical process that green plants use to convert sunlight’s energy into organic food compounds. Only green plants can perform photosynthesis because of the presence of a pigment called chlorophyll (which is also responsible for the green color of most plants).

The process of photosynthesis in higher plants is extremely important for two reasons:

It Converts Sunlight into Food

The food that plants produce is consumed by herbivores, which are, in turn, consumed by carnivores and omnivores. This keeps the food chain progressing. Animals and humans would not be able to survive without the supply of food from plants.

It Releases Oxygen

Oxygen is required for almost every bodily function in humans and animals. Plants release oxygen into the atmosphere. Oxygen is a by-product of the process of photosynthesis in higher plants. Plants use carbon dioxide for the process of photosynthesis, thereby effectively reducing carbon dioxide levels in the atmosphere. 

Factors Affecting Photosynthesis

The rate of photosynthesis is directly responsible for the yield of all plants, including food crops. There are both internal and external factors that determine the rate of photosynthesis. Internal factors include the plant’s size and age, the number and orientation of the leaves, the number of mesophyll cells and chloroplasts, the amount of chlorophyll, and the amount of CO₂ within the plant.  The external factors include the amount of sunlight, CO₂, and water available and the external temperature. 

The Law of Limiting Factors

According to Blackman’s Law of Limiting Factors, if a procedure is conditioned by multiple factors, then the rate of the procedure will be limited by the speed of the slowest factor. The effect of different factors as per the process of photosynthesis in higher plants is as follows:

• Light: The quality, intensity, and duration of light affect the rate of photosynthesis. Light saturation occurs at 10 percent. An increase in incident light beyond the optimal point causes a breakdown of chlorophyll and reduces the rate of photosynthesis in higher plants. There is additionally a direct connection between the incident light and carbon dioxide fixation rates at low light intensities. As the light intensity rises, other factors become more limiting, and the rate does not increase further. 

• Carbon Dioxide: This is the major limiting factor. In our surroundings, the concentration of carbon dioxide is between 0.03 and 0.04 per cent, which is very low. An increase in CO₂ by just 0.05 per cent may lead to an increase in fixation rates. Beyond this, it can become harmful
• Temperature: The reactions in the plants which take place in the dark are controlled by temperature. Reactions happening in the presence of light are affected by temperature on a lesser magnitude. The optimal temperature for photosynthesis also relies on the surroundings where the plant is growing. Tropical plants require more heat and temperature to begin the photosynthetic process
• Water: Fewer amounts of water cause stress to the plant and the stomata closes, reducing CO₂ intake. Water scarcity causes leaves to wilt, thereby reducing the expanse of leaves and the sunlight intake

Where Does Photosynthesis Happen?

In the next section of Photosynthesis in higher plants, you will get to know about where photosynthesis happens. The entire plant is not involved in the process of photosynthesis. Photosynthesis mostly takes place in the green parts of the plant that have chloroplasts. Mesophyll cells in the leaves have a large number of chloroplasts in them. This is because the chloroplasts can find the optimum quantity of sunlight they need for photosynthesis at the edges of the mesophyll cells of the leaves. 

Within the chloroplast, there is a clear division of labor. Each component of the chloroplast has its own function. The main components in the chloroplast are the inner and outer membranes, the stroma lamellae, the grana, and the fluid stroma. The membranes are responsible for trapping sunlight energy. The membrane also synthesizes ATP (Adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide hydrogen phosphate). These reactions involve light energy directly and are called light reactions. 

The stroma is responsible for the reactions that convert carbon dioxide into sugar, which forms starch. These are called dark reactions because they don’t directly utilize sunlight. They require by-products, ATP, and NADPH. 

Important Pigments Involved in Photosynthesis

The pigments involved in the process of photosynthesis in higher plants is chlorophyll a (bright or blue-green), chlorophyll b (yellow-green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange). Different pigments absorb light at different wavelengths. There are certain wavelengths at which photosynthesis occurs best. Chlorophyll absorbs these wavelengths (if the light is bright) better than all the other pigments. Therefore, it is the most crucial pigment in the process of photosynthesis.

The process of photosynthesis in higher plants includes several steps, the first of which is light reactions. These include light absorption, water splitting, and the formation of ATP and NADPH. 

Photosynthesis in Higher Plants: Light Reactions

Different wavelengths of light are absorbed by different pigments to make photosynthesis work. Light-harvesting complexes (LHC) are responsible for absorbing light, and they exist within two Photosystems I and II. According to the chapter on Photosynthesis in higher plants, each photosystem has a chlorophyll molecule at the centre, forming a reaction centre. There are antennae to absorb light better. In PS I the reaction centre has an absorption peak of 700 nm (therefore called P700), and in PS II, the absorption peak is at 680 nm (therefore called P680). 

The absorption of red light at the 680 nm wavelength causes electrons to orbit away from the atomic nucleus. These electrons are transported by electron acceptors and thrown into a system of cytochromes. The electrons are passed on to the pigments of PS I. In PS I, red light at the 700 nm wavelength causes electrons to separate. These are also transported but to a molecule of NADP. The addition of electrons to NADP forms NADPH. 

Photosynthesis in Higher Plants: Splitting Of Water

The splitting of water by PS II creates H+, [O], and electrons. The formula is:

Cyclic and Noncyclic Photophosphorylation

According to the chapter, photosynthesis in higher plants, Photophosphorylation is the process of synthesizing ATP from ADP and inorganic phosphates in the presence of light. In Noncyclic Photophosphorylation, both photosystems work and produce ATP and NADPH. In Cyclic Photophosphorylation, only PS I is involved, and it occurs when light beyond 680 nm is available for exciting the electrons. In the cyclic process, only ATP is formed.

Photosynthesis in Higher Plants: Dark Reactions

These do not require light directly. This is the biosynthetic phase that uses ATP and NADPH. The Calvin Cycle involves three steps:

• Carboxylation – This is when CO₂ combines and turns into an organic compound. For the carboxylation of RuBP, CO₂ is often used. Two molecules of 3-PGA are formed. The catalytic enzyme is called RuBisCO
• Reduction – A series of reactions form glucose. Two molecules, each of ATP and NADPH, reduce each CO₂ molecule fixed.
• Regeneration – RuBP has to be regenerated to keep the process going. One ATP is required for phosphorylation to form RuBP.

Here is a table representing the inputs and outputs of the process of photosynthesis:

There are some tropical plants that demonstrate a different type of photosynthetic process known as the C4 pathway. The enzymes are found in the mesophyll cells (Phosphoenolpyruvate, which is the primary carbon dioxide acceptor here) and bundle sheath cells. Plants convert the atmospheric CO₂ into a 4-carbon compound. Aspartic and malic acids are formed in the mesophyll, then carried to the bundle sheath cells where they are broken down to form three-carbon molecules and CO₂. 

The carbon molecules move back to the mesophyll and are converted to phosphoenolpyruvate. CO₂ enters the bundle sheath cells, and the Calvin cycle is completed. Photorespiration occurs when plants take in CO₂ and release oxygen. ATP and NADPH are not involved. 

MCQs

• Which of the following is correct for photosynthesis? 

1. Biological oxidation process
2. Photochemical catabolic process
3. Photo-oxidative metabolism 
4. Biological photo metabolism

• The maximum evolution of oxygen is by the greatest producers of organic matter.

1. Great land area
2. Crops
3. Phytoplankton of sea
4. Forests

• Photosynthesis is: 

1. Oxidative, exergonic, catabolic
2. Reductive, endergonic, anabolic
3. Reductive, exergonic, anabolic
4. Reductive, endergonic, catabolic

• The electron transport chain of the photosynthetic process is:

1. In the stroma of the chloroplast
2. Bound to the thylakoid membranes
3. Present in the outer membrane of the chloroplast
4. Present in mitochondria

• Calvin’s cycle is found in:

1. Only C₃ plants
2. Only photophilous plants
3. All C₄ plants
4. All photosynthetic plants

• C₄ plants are found among: 

1. Gramineae only
2. Monocots only
3. Dicots only
4. Monocots as well as dicots

• In chemosynthesis ofNO2bacteria, the carbohydrates are formed by

1. NO₂ and H₂O
2. NH3 and CO₂
3. CO₂, H₂O and SO₂
4. Hydrocarbons

• What effect would occur on photosynthesis, if the amount of oxygen in the atmosphere decreases?

1. Increase in C₃ cycle and decrease in C₄ cycle
2. Increase in C₄ cycle and decrease in C₃ cycle
3. Increase in C₃ cycle and no change in C₄ cycle
4. Increase in C₄ cycle and no change in C₃ cycle

• The first product of CO₂ fixation in the Hatch and Slack (C₄ ) cycle in plants is: 

1. Formation of oxaloacetate by carboxylation of phosphoenolpyruvate (PEP) in bundle sheath cells
2. Formation of phosphoglyceric acid in mesophyll cells
3. Formation of bundle sheath cells
4. Formation of oxaloacetate by carboxylation of phosphoenolpyruvate (PEP) in the mesophyll cells

• If half of the leaves of a plant are removed, the rate of photosynthesis would be

1. More and the photosynthetic yield is also more
2. More but the photosynthetic yield is less
3. Less and the photosynthetic yield is also less
4. Less but the photosynthetic yield is more

Answers: 4, 3, 2, 2, 4, 4,2, 3, 4, 2

Photosynthesis in Higher Plants Class 11 PPT

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Thus, we hope that this study’s notes on Photosynthesis in Higher Plants clear all your doubts on this topic. If you aim to pursue a degree in Biology after the 12th and are confused about the right course, reach out to our experts at Leverage Edu and we will help you in exploring the best course and university as per your interests and preferences! Sign up for a free career counseling session with us today!