Important Diagrams: Photosynthesis in Higher Plants | Biology Class 11 - NEET PDF Download

1. Joseph Priestley Experiment

Priestley’s experiment

2. Where does Photosynthesis takes place?

• Photosynthesis primarily occurs in the green leaves of plants, particularly in mesophyll cells, which contain numerous chloroplasts. These chloroplasts are arranged to optimize light absorption, aligning with their flat surfaces parallel to the walls of the cells for maximum light exposure.
• In addition to leaves, photosynthesis can take place in other green parts of plants, such as stems and unripe fruits.
• Chloroplasts contain a membranous system made up of grana, stroma lamellae, and matrix stroma. This structure is crucial for trapping light energy and synthesizing ATP and NADPH during the light-dependent reactions. The stroma is where enzymatic reactions convert these products into sugars, which can then be stored as starch.
• The light-driven reactions are referred to as "light reactions," while the subsequent reactions that synthesize sugars, dependent on ATP and NADPH, are called "dark reactions" or carbon reactions. It's important to note that dark reactions do not necessarily occur in darkness; they still rely on the products of light reactions.

Diagrammatic representation of an electron micrograph of a section of chloroplast

3. How many types of Pigments are involved in Photosynthesis?

(a) Graph showing the absorption spectrum of chlorophyll a, b and the carotenoids (b) Graph showing action spectrum of photosynthesis (c) Graph showing action spectrum of photosynthesis superimposed on absorption spectrum of chlorophyll a

4. What is Light Reaction?

The light reactions, also known as the photochemical phase, involve several key processes:

Light absorption, Water splitting, Oxygen release & Formation of high-energy chemical intermediates such as ATP and NADPH

Various protein complexes play a crucial role in these processes. The pigments responsible for light absorption are organized into two distinct photochemical light-harvesting complexes (LHC) within Photosystem I (PS I) and Photosystem II (PS II).

Photosystem I (PS I) and Photosystem II (PS II)

These complexes are named in the order of their discovery rather than the sequence in which they function during the light reaction.

The LHCs consist of numerous pigment molecules bound to proteins. Each photosystem, except for one molecule of chlorophyll a, has all the pigments forming a light-harvesting system, often referred to as antennae. These pigments enhance the efficiency of photosynthesis by absorbing different wavelengths of light.

The single chlorophyll a molecule in each photosystem forms the reaction center, which differs between the two photosystems:

• In Photosystem I (PS I), the reaction center chlorophyll a has an absorption peak at 700 nm, which is why it is called P700.
• In Photosystem II (PS II), the reaction center chlorophyll a has an absorption maximum at 680 nm, leading to its designation as P680.

The light harvesting complex

5. The Electron Transport

• In photosystem II, chlorophyll a absorbs red light at 680 nm, exciting electrons that jump to a higher energy level.
• These energized electrons are captured by an electron acceptor and passed through an electron transport chain made up of cytochromes, moving downhill in terms of redox potential.
• As the electrons travel through the chain, they are eventually transferred to the pigments of photosystem I (PS I).
• In PS I, chlorophyll a absorbs light at 700 nm, further exciting its electrons.
• These electrons are then transferred to another acceptor molecule with a higher redox potential, leading to their movement downhill to NADP.
• This process reduces NADP to NADPH + H, completing the electron transfer sequence.
• This entire pathway, from photosystem II to photosystem I and ultimately to NADP, is referred to as the Z scheme due to its characteristic shape when plotted on a redox potential scale.

Z scheme of light reaction

6. Cyclic and Non- Cyclic Phosphorylation

• Living organisms can extract energy from oxidizable substances and store it as bond energy, primarily in the form of ATP. The process of synthesizing ATP in cells (within mitochondria and chloroplasts) is known as phosphorylation. Specifically, photophosphorylation refers to the synthesis of ATP from ADP and inorganic phosphate in the presence of light.
• In the process of non-cyclic photophosphorylation, both photosystem II (PS II) and photosystem I (PS I) work in series, connected by an electron transport chain (as described in the Z scheme). This electron flow results in the synthesis of both ATP and NADPH + H+.
• When only PS I is active, a different process occurs called cyclic photophosphorylation. In this scenario, electrons are circulated within PS I, leading to ATP synthesis without the production of NADPH + H+. This process likely occurs in the stroma lamellae, which lack PS II and the NADP reductase enzyme. Here, excited electrons cycle back to the PS I complex via the electron transport chain.
• Cyclic photophosphorylation can also take place when only light wavelengths beyond 680 nm are available for excitation, resulting solely in the production of ATP.

Cyclic photophosphorylation

7. Chemiosmotic Hypothesis

ATP synthesis in chloroplasts occurs through the chemiosmotic hypothesis, which links ATP production to a proton gradient across the thylakoid membrane. Unlike respiration, where protons accumulate in the intermembrane space of mitochondria, in photosynthesis, protons accumulate in the lumen of the thylakoids.

The development of this proton gradient involves several steps:

1. Splitting of Water: Water molecules are split on the inner side of the thylakoid membrane, producing protons (hydrogen ions) that accumulate in the lumen.

2. Electron Transport: As electrons move through the photosystems, protons are transported across the membrane. The primary electron acceptor on the outer side transfers electrons to an H carrier, which removes protons from the stroma and releases them into the lumen when passing electrons to inner electron carriers.

3. NADP Reductase Activity: The NADP reductase enzyme, located on the stroma side, requires protons and electrons from PS I to reduce NADP+ to NADPH + H+. This reaction further decreases proton concentration in the stroma.

As a result, there is a decrease in proton concentration in the stroma and an accumulation in the lumen, creating a proton gradient and lowering the pH in the lumen.

The importance of this proton gradient lies in its role in ATP synthesis. When protons move back across the membrane into the stroma through the CF0 channel of ATP synthase, the energy released facilitates ATP production. ATP synthase consists of two parts: CF0, which is embedded in the thylakoid membrane and allows facilitated diffusion of protons, and CF1, which protrudes into the stroma. The movement of protons releases energy that causes conformational changes in CF1, leading to the synthesis of ATP.

Overall, chemiosmosis requires a membrane, a proton pump, a proton gradient, and ATP synthase. The ATP produced, along with NADPH, is immediately used in biosynthetic reactions in the stroma for fixing CO2 and synthesizing sugars.

ATP synthesis through chemiosmosis

8. The Calvin Cycle

8 The Calvin cycle proceeds in three stages : (1) carboxylation, during which CO2 combines with ribulose-1,5-bisphosphate; (2) reduction, during which carbohydrate is formed at the expense of the photochemically made ATP and NADPH; and (3) regeneration during which the CO2 acceptor ribulose1,5-bisphosphate is formed again so that the cycle continues

9. The C₄ Pathway

Plants that thrive in dry tropical regions utilize the C4 pathway for photosynthesis. While they produce C4 oxaloacetic acid as the initial product of CO2 fixation, they primarily rely on the C3 pathway, or Calvin cycle, for biosynthesis. So, what sets C4 plants apart from C3 plants? C4 plants possess distinct features that enhance their survival and productivity in challenging environments:

1. Special Leaf Anatomy

C4 plants exhibit a unique leaf structure known as Kranz anatomy. This involves the presence of large cells around the vascular bundles called bundle sheath cells, which contain numerous chloroplasts and have thick walls that prevent gas exchange. In contrast, C3 plants lack this specialized arrangement.

2. Tolerance to Higher Temperatures

C4 plants are better equipped to withstand higher temperatures compared to C3 plants. This adaptation allows them to thrive in hot and arid conditions.

3. Response to High Light Intensities

C4 plants can effectively utilize high light intensities, enhancing their photosynthetic efficiency. This is particularly beneficial in bright, sunny environments.

4. Absence of Photorespiration

C4 plants do not experience photorespiration, a wasteful process that occurs in C3 plants when oxygen competes with carbon dioxide for the active site of the enzyme RuBisCO. By avoiding photorespiration, C4 plants maximize their carbon fixation efficiency.

5. Greater Biomass Productivity

C4 plants generally produce more biomass compared to C3 plants. This increased productivity is a result of their efficient photosynthetic process and adaptations to their environment.

Understanding Kranz Anatomy

To better understand the differences between C3 and C4 plants, you can examine vertical sections of their leaves under a microscope.

• Mesophyll Cells: Observe the types of mesophyll cells present in both C3 and C4 plants. C4 plants typically have a different arrangement of mesophyll cells compared to C3 plants.
• Bundle Sheath Cells: Pay attention to the cells surrounding the vascular bundle sheath. In C4 plants, these cells are larger and contain a high number of chloroplasts. The thick walls of these cells prevent gaseous exchange, and the absence of intercellular spaces is a characteristic feature.

The Hatch and Slack Pathway

The Hatch and Slack Pathway is a cyclic process that describes how C4 plants efficiently fix carbon dioxide. The steps of this pathway are as follows:

• Primary CO2 Acceptor: The pathway begins with the fixation of CO2 by a 3-carbon molecule called phosphoenol pyruvate (PEP), which occurs in the mesophyll cells.
• PEP Carboxylase: The enzyme responsible for this fixation is PEP carboxylase (PEPcase). It is important to note that mesophyll cells do not contain the enzyme RuBisCO.
• Formation of C4 Acid: In the mesophyll cells, the C4 acid oxaloacetic acid (OAA) is formed and then converted into other 4-carbon compounds like malic acid or aspartic acid.
• Transport to Bundle Sheath Cells: These 4-carbon compounds are transported to the bundle sheath cells.
• Release of CO2: Inside the bundle sheath cells, the C4 acids are broken down to release CO2 and a 3-carbon molecule.
• Recycling of PEP: The 3-carbon molecule is transported back to the mesophyll cells, where it is converted back into PEP, completing the cycle.
• Calvin Pathway: The CO2 released in the bundle sheath cells enters the C3 or Calvin pathway, which is common to all plants. The bundle sheath cells are rich in the enzyme RuBisCO but lack PEPcase.

In summary, while both C3 and C4 plants use the Calvin pathway to produce sugars, the location and mechanisms differ. In C3 plants, the Calvin pathway occurs in all mesophyll cells, whereas in C4 plants, it takes place only in the bundle sheath cells. This adaptation allows C4 plants to efficiently fix carbon dioxide and minimize photorespiration, leading to greater productivity.

Diagrammatic representation of the Hatch and Slack Pathway

10. Factors affecting Photosynthesis (Light) 

Graph of light intensity on the rate of photosynthesis

Diagram Based Questions NEET

Q1: How many molecules of ATP and NADPH are required for every molecule of CO₂ fixed in the Calvin cycle? (NEET 2024)
(a) 2 molecules of ATP and 3 molecules of NADPH
(b) 2 molecules of ATP and 2 molecules of NADPH
(c) 3 molecules of ATP and 3 molecules of NADPH
(d) 3 molecules of ATP and 2 molecules of NADPH
Ans: (d)

The Calvin cycle, also known as the Calvin-Benson-Bassham cycle, is the set of chemical reactions that take place in chloroplasts during photosynthesis. The cycle is light-independent because it takes place after the energy has been captured from sunlight. The Calvin cycle uses ATP and NADPH as energy sources, and incorporates CO₂ into organic molecules, eventually producing glucose.

The cycle consists of three main stages:

• Carbon fixation
• Reduction phase
• Regeneration of the ribulose-1,5-bisphosphate (RuBP)

In the Calvin cycle:
Carbon Fixation: Each CO₂ molecule is attached to a five-carbon sugar named ribulose-1,5-bisphosphate (RuBP). This reaction is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). The product of this reaction is a six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).
Reduction Phase: Each 3-PGA is phosphorylated by ATP (consuming 2 ATP molecules total for two 3-PGA molecules) to form 1,3-bisphosphoglycerate. This molecule is then reduced by NADPH (using 2 NADPH molecules in total) to form glyceraldehyde-3-phosphate (G3P). Out of every six G3P formed, one exits the cycle to contribute towards forming glucose, while the rest are recycled to regenerate RuBP.
Regeneration of RuBP: For every three CO₂ molecules fixed, five molecules of G3P are used to regenerate three molecules of RuBP, requiring further ATP input (3 more ATP molecules).
Thus, for each incorporated CO₂ molecule, 3 ATP molecules and 2 NADPH molecules are required:
3 ATP molecules (2 for the reduction of two molecules of 3-PGA into 1,3-bisphosphoglycerate and then one for the regeneration phase of RuBP).
2 NADPH molecules are used to reduce two molecules of 1,3-bisphosphoglycerate to G3P.
Therefore, the correct answer to the question is Option D: 3 molecules of ATP and 2 molecules of NADPH are required for every molecule of CO₂ fixed in the Calvin cycle.

Q2: How many ATP and NADPH₂ are required for the synthesis of one molecule of Glucose during Calvin cycle? [NEET 2023]
(a) 12 ATP and 12 NADPH₂
(b) 18 ATP and 12 NADPH₂
(c) 12 ATP and 16 NADPH₂
(d) 18 ATP and 16 NADPH₂
Ans: (b)
For every CO₂ molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH₂ are required. To make one molecule of glucose, 6 turns of the cycle are required. Thus, ATP and NADPH₂ molecules required for synthesis of one molecule of glucose during Calvin cycle will be

Q3: Identify the correct statements regarding chemiosmotic hypothesis:       [NEET 2022 Phase 2]
(a) Splitting of the water molecule takes place on the inner side of the membrane.
(b) Protons accumulate within the lumen of the thylakoids.
(c) Primary acceptor of electron transfers the electrons to an electron carrier.
(d) NADP reductase enzyme is located on the stroma side of the membrane.
(e) Protons increase in number in stroma.
Choose the correct answer from the options given below:
(a) (b), (c) and (e)
(b) (a), (b) and (e)
(c) (a), (b) and (d)
(d) (b), (c) and (d)
Ans: (c)

• Primary acceptor of electron transfers its electron not to an electron carrier but to an H carrier.
• Protons increase in number in lumen of the thylakoid not in stroma.

Q4: Which one of the following is not true regarding the release of energy during ATP synthesis through chemiosmosis? It involves:
(a) Breakdown of proton gradient
(b) Breakdown of electron gradient
(c) Movement of protons across the membrane to the stroma
(d) Reduction of NADP to NADPH₂ on the stroma side of the membrane [NEET 2022 Phase 1]
Ans: (b)
Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATP synthase. Energy is used to pump protons across a membrane to create a gradient or a high concentration of protons within the thylakoid lumen.
The NADP reductase enzyme is located on the stroma side of the membrane. Along with the electrons that come from the acceptor of electrons of PS I, protons are necessary for reduction of NADP⁺ to NADPH + H⁺.
The process does not involve breaking of electron gradient.