NEET Biology Important Chapter - Photosynthesis in Higher Plants

Photosynthesis in Higher Plants is a very important chapter from the NEET point of view and one must know all the basics in order to score maximum out of the chapter. Photosynthesis in Higher Plants notes for NEET carry all the important information of the chapter and, thus, can be really helpful for last-minute revision.

This article comprises the topics such as a short note on photosynthesis, where does it occur and what are the factors affecting it, how many types of pigments are involved in photosynthesis, what is a light reaction, electron transport, the use of ATP and NADPH, C₄ pathway and Photorespiration. 

Important Topics of Photosynthesis in Higher Plants

• Photosynthesis

• Light Reaction

• Electron Transport Chain

• C3 and C4 Pathways

• Photorespiration

Important Concepts of the Chapter

Photosynthesis is a physicochemical process in which organic molecules are synthesised using sunlight. Higher plants have more processes involved in photosynthesis, however, it is essentially the same. Oxygen is released into the atmosphere as a result of this process.

The chloroplast, which is present in the mesophyll cells of the leaves, is where photosynthesis takes place. Four major pigments are involved in carrying out the process of photosynthesis, namely:

• Chlorophyll a

• Chlorophyll b

• Xanthophylls

• Carotenoids

Photosynthesis in Higher Plants

Photosynthesis in Higher Plants can be carried out using two major reactions, namely, Light reaction and Dark reaction.

Light Reaction

• Light reaction, as the name suggests, occurs in the presence of light.

• The pigment gathers light and converts it to ATP, which is a source of energy.

• Light absorption, water splitting, oxygen release, and the synthesis of ATP and NADPH are the processes involved in the light reaction.

• Within two photosystems, PS-I and PS-II, protein-bound pigment molecules create light-harvesting complexes. Each photosystem does have a reaction centre made up of chlorophyll  ‘a’ molecules, as well as antennae holding colours.

• PS-I has a reaction centre of P-700 because chlorophyll ‘a’ has an absorption peak of 700 nm, while PS-II has a reaction centre of P-680 because chlorophyll a has an absorption peak of 680 nm.

Phosphorylation

Phosphorylation can be described as the process of the formation of ATP under sunlight. There exist two kinds of Phosphorylation, namely, Cyclic Phosphorylation, and Non-cyclic Phosphorylation.

1. Non-Cyclic Photophosphorylation

• PS-II catches the light with a wavelength of 680 nm, causing electron excitation. An electron acceptor accepts these energised electrons and transfers them to the electron transport system.

• The electron transport system then transmits the electrons to the PS-I. Parallelly, electrons at PS-I are stimulated by a wavelength of 700 nm.

• NADP⁺ is reduced to NADPH + H⁺ after receiving an electron from the electron acceptor.

• PS-II loses electrons that do not return to it, resulting in non-cyclic photophosphorylation.

• Both photosystems are engaged in this.

2. Cyclic Photophosphorylation

• In this process, it is just PS-I which is involved.

• The circulation of electrons is observed happening in the photosystem which apparently leads to the cyclic flow of electrons.

• Only the formation of ATP happens, no NADPH + H⁺ are produced.

Water Splitting

Photolysis is the light-dependent splitting of water. Manganese and chlorine play a key part in this process, which is linked to PS-II. The electrons that are lost from P680 are replaced by electrons that are created during this process. When light is absorbed by P680, a water molecule splits to release oxygen.

Dark Reaction

As the name says, Dark reactions are the ones that take place in the absence of light. This happens in the stroma of the chloroplasts. C₃ and C₄ cycles are the two cycles involved in this process. 

C₃ Cycle

The C₃ cycle, also called as Calvin Cycle is carried out in 3 steps, namely: carbon fixation, reduction and regeneration. 

• Ribulose-1, 5-bisphosphate mixes with carbon dioxide to form 3-phosphoglyceric acid, a three-carbon molecule. RuBisCO, an enzyme, is involved in the process.

• Glyceraldehyde-3-phosphate is formed when two molecules of ATP and one molecule of NADPH fix one molecule of carbon dioxide.

• Glyceraldehyde-3-phosphate molecules undertake a series of processes to create glucose, while RuBP regenerates to keep the cycle going.

C₄ Cycle

• Also called Hatch and Slack Pathway, it is a cyclic pathway.

• Mesophyll cells and Bundle Sheath cells contain the enzymes involved in the C₄ pathway.

• The leaves with this anatomy are referred to as having 'Kranz' anatomy. The word 'kranz' means 'wreath,' and it refers to the way the cells are arranged.

• Plants transform ambient carbon dioxide into a four-carbon-containing chemical molecule in this process.

• The principal carbon dioxide acceptor is phosphoenolpyruvate, which is found in the mesophyll cells. Phosphoenolpyruvate carboxylase catalyses the process.

• The mesophyll cells then produce aspartic and malic acids, which are then transferred to the bundle sheath cells. The C₄ acids break down here, releasing three-carbon molecules and CO₂.

• The three-carbon molecules return to mesophyll cells, where they are transformed to phosphoenolpyruvate and the cycle is completed.

• The Calvin cycle is completed when carbon dioxide enters the bundle sheath cells.

Photorespiration

• Krotkov coined the phrase.

• Decker and Tio found photorespiration and established that the C₂-cycle occurs during the day in C₃-plants and that Rubisco acts as an oxygenase in C₃-green cells when there is a higher concentration of O₂ and a lower concentration of CO₂.

• Light-dependent O₂ uptake and CO₂ release in a C₃ photosynthetic cell known as photorespiration.

• Because it is not linked to ATP synthesis (instead, ATP is consumed) like typical dark respiration, it is a detrimental or wasteful C₃ cycle function.

• It can be found in chloroplasts, peroxisomes, and mitochondria (three cell organelle reactions).

Factors Affecting Photosynthesis

Difference Between C₃ and C₄ Plants

Solved Examples from Chapter

1. Rate of photosynthesis is independent of

1. Intensity of light

2. Duration of light

3. Quality of light

4. Temperature

Ans: b. Duration of light

Photosynthesis is a natural process that occurs in most green autotrophic plants and certain other animals. It involves the plant absorbing carbon dioxide, water, and light energy before converting it to chemical energy and releasing oxygen into the atmosphere. 

Key point to remember: According to Blackman, the rate of photosynthesis is independent of the length of light and is reliant on the intensity, quality, and temperature of light.

2. Which of the following statements about dark reactions is correct?

1. They occur in darkness.

2. They are light dependent.

3. They are dependent upon the products synthesised during light reactions.

4. All of these.

Ans: c. They are dependent upon the products synthesised during light reactions.

The dark reaction is reliant on the products (ATP and NADPH) created by the light reaction, but it is not reliant on light directly.

Key point to remember: ATP and NADPH are created during the light reactions.

3. The loss of water in C₄ plants compared to C₃ plants for the same amount of CO₂ fixed is:

1. A Half

2. One-third

3. One-fourth

4. Double

Ans: b. One-third

When the same amount of Carbon dioxide is fixed in both plants, the loss of water in C₄ plants is really one-third of the volume of water lost by C₃ plants. The amount of water lost by C₄ plants is smaller than that of C₃ plants because C₄ plants have a unique Kranz anatomy that allows them to retain water for longer periods of time.

Key point to remember: The amount of water lost by C₄ plants is smaller than that of C₃ plants.

4. Photorespiration is favoured by

1. High oxygen and low carbon dioxide

2. High carbon dioxide and low oxygen

3. High temperature and low oxygen

4. High humidity and temperature

Ans: a. High oxygen and low carbon dioxide

Photorespiration is a respiratory process in which higher plants take in oxygen from the light and release carbon dioxide, which is in some ways the opposite of photosynthesis. Photorespiration is favoured by atmospheric conditions with high levels of oxygen and low levels of carbon dioxide, as well as lower humidity.

Key point to remember: Along with high levels of oxygen, low levels of carbon and humidity are required for photorespiration. 

Solved Problems of Previous Years’ Question from the Chapter

1. Which of the following statements is incorrect?

1. Grana lamellae have both PSI and PSII.

2. Cyclic photophosphorylation involves both PS I and PS II.

3. Both ATP and NADPH + H⁺ are non-cyclic synthesised during photophosphorylation.

4. Stroma lamellae have PS I only and lack NADP reductase.

Ans: b. Cyclic photophosphorylation involves both PS I and PS II.

PSI is engaged in photophosphorylation, both cyclic and non-cyclic. PS II primarily participates in non-cyclic photophosphorylation. PS II is found in the grana thylakoids' appressed (inner) portion. PS I is found in grana thylakoids and stroma thylakoids' non-appressed (outer) regions.

Key point to remember: PS II involves non-cyclic Phosphorylation. 

2. The oxygenation activity of RuBisCo enzyme in photorespiration leads to the formation of:

1. 1 molecule of the 3-C compound. 

2. 1 molecule of the 6-C compound.

3. 1 molecule of a 4-C compound and 1 molecule of the 2-C compound.

4. 2 molecules of the 3-C compound

Ans: a. 1 molecule of the 3-C compound

Photorespiration is a photosensitive technique. RuBP carboxylase acts as an oxygenase at high temperatures and oxidises ribulose 1, 5-biphosphate to form a 3-carbon phosphoglyceric acid and a 2 -carbon phosphoglycolate instead of fixing carbon dioxide (C₃ cycle).

Key point to remember: RuBP carboxylase acts as an oxygenase at high temperatures.

3. In the leaves of C₄-plants, malic acid formation during CO₂ fixation occurs in the cells of

1. Mesophyll

2. bundle sheath

3. Phloem

4. epidermis

Ans: a. Mesophyll

CO₂ fixation occurs in the mesophyll of the leaf, which contains chlorophyll pigments, in C₄ plants. Malic acid is generated here during CO₂ fixation. Then it's moved to the bundle sheath, where it'll be decarboxylated and released as CO₂. After that, the enzyme RuBisCo fixes the CO₂.

Key point to remember: Malic acid is generated in mesophyll during CO₂ fixation.

Practice Questions

1. Which sugar of the Calvin cycle utilises ATP?

1. Carboxylation

2. Reduction

3. Regeneration

4. Both 2 and 3

Ans: d. Both 2 and 3

2. Identify the statement that is true for C₄ plants but not for C₃ plants?

1. Initially fix carbon dioxide in mesophyll cells as 4-carbon long compounds. 

2. Initially fix carbon dioxide in bundle sheath cells as 3- carbon long compounds. 

3. Initially fix carbon dioxide in mesophyll cells as 3-carbon long compounds

4. Initially fix carbon dioxide in bundle sheath cells as 4- carbon long compounds.

Ans: a. Initially fix carbon dioxide in mesophyll cells as 4-carbon long compounds. 

3. Dark reactions of photosynthesis are actually not totally independent of light as:

1. The initial reactions occur in the presence of light.

2. The reaction will be inhibited by the presence of light.

3. They can occur during the day as well.

4. They utilise the product of the light reaction.

Ans: d. They utilise the product of the light reaction.

Conclusion

The Photosynthesis in Higher Plants notes PDF can help the student with the last minute revision. The article contains the important concepts of the chapter and ends with the practice questions. The sample questions and practice questions are added to provide an understanding of the type of questions that could be asked in the NEET examination.