CSIR NET Life Sciences and Biotechnology Unit 6 covers system physiology – plant. These topic-wise notes explain essential concepts in simple language for examination preparation.
Select a topic below to read its explanation, remember the key distinctions, and open the detailed notes for further study.
Unit 6 Topic-wise Notes
1. Photosynthetic electron transport
PSII oxidizes water and releases oxygen; electron transfer creates a proton gradient, and PSI supports NADPH formation.
For CSIR NET, begin by identifying the main components involved in photosynthetic electron transport and the order in which they interact. Explain what changes when one component is removed, inhibited, overexpressed or exposed to different conditions. This approach is especially useful for interpreting experiments rather than recalling isolated facts.
When solving a question, distinguish the underlying mechanism from its observable outcome. Compare the normal situation with the altered condition, and check whether the evidence supports a direct effect or only an association. The central distinction to retain is: oxygen evolved in photosynthesis comes from water.
Photosystem II extracts electrons from water and releases oxygen, while photosystem I supplies high-energy electrons for NADP+ reduction. Electron transfer through plastoquinone, cytochrome b6f and plastocyanin helps establish a proton gradient across the thylakoid membrane. ATP synthase uses that gradient to synthesize ATP. Cyclic electron flow around photosystem I increases ATP production without net NADPH or oxygen formation. If a treatment blocks electron flow after photosystem II, distinguish its effects on oxygen evolution, NADPH formation and proton-gradient generation.
Important: Oxygen evolved in photosynthesis comes from water.
Read Detailed Notes →2. C3, C4 and CAM pathways
C3 plants fix COâ‚‚ through Rubisco; C4 and CAM systems concentrate COâ‚‚ using spatial or temporal separation.
For CSIR NET, begin by identifying the main components involved in c3, c4 and cam pathways and the order in which they interact. Explain what changes when one component is removed, inhibited, overexpressed or exposed to different conditions. This approach is especially useful for interpreting experiments rather than recalling isolated facts.
When solving a question, distinguish the underlying mechanism from its observable outcome. Compare the normal situation with the altered condition, and check whether the evidence supports a direct effect or only an association. The central distinction to retain is: c4 separates fixation spatially; CAM commonly separates it by time.
Rubisco fixes CO2 but can also react with O2, creating photorespiratory costs. C4 plants spatially separate initial bicarbonate fixation by PEP carboxylase from Rubisco activity in bundle-sheath cells. CAM plants separate these processes mainly by time, storing organic acids at night and releasing CO2 during the day. Both adaptations can reduce photorespiration but require additional energy or storage capacity. Their relative advantage depends on temperature, water availability, light and CO2, not on a universal ranking of photosynthetic efficiency.
Important: C4 separates fixation spatially; CAM commonly separates it by time.
Read Detailed Notes →3. Photorespiration and respiration
Rubisco oxygenation initiates photorespiration across chloroplasts, peroxisomes and mitochondria.
For CSIR NET, begin by identifying the main components involved in photorespiration and respiration and the order in which they interact. Explain what changes when one component is removed, inhibited, overexpressed or exposed to different conditions. This approach is especially useful for interpreting experiments rather than recalling isolated facts.
When solving a question, distinguish the underlying mechanism from its observable outcome. Compare the normal situation with the altered condition, and check whether the evidence supports a direct effect or only an association. The central distinction to retain is: photorespiration tends to increase when COâ‚‚ relative to Oâ‚‚ falls.
Photorespiration recycles products of Rubisco oxygenation through chloroplasts, peroxisomes and mitochondria, with carbon and energy costs. Mitochondrial respiration oxidizes substrates to support ATP synthesis and redox balance in plant cells. Light does not switch mitochondrial respiration off; photosynthetic and respiratory fluxes can occur simultaneously. The alternative oxidase pathway can maintain electron flow with lower ATP conservation. When comparing gas-exchange measurements, distinguish gross photosynthesis, net CO2 assimilation and dark respiration.
Important: Photorespiration tends to increase when COâ‚‚ relative to Oâ‚‚ falls.
Read Detailed Notes →4. Nitrogen assimilation
Nitrate and nitrite reduction provide ammonium for amino-acid synthesis; GS–GOGAT assimilates nitrogen.
For CSIR NET, begin by identifying the main components involved in nitrogen assimilation and the order in which they interact. Explain what changes when one component is removed, inhibited, overexpressed or exposed to different conditions. This approach is especially useful for interpreting experiments rather than recalling isolated facts.
When solving a question, distinguish the underlying mechanism from its observable outcome. Compare the normal situation with the altered condition, and check whether the evidence supports a direct effect or only an association. The central distinction to retain is: nitrogenase fixes Nâ‚‚ and is oxygen-sensitive.
Nitrate reductase reduces nitrate to nitrite, and nitrite reductase generates ammonium for incorporation into amino acids. The GS–GOGAT cycle assimilates ammonium into glutamine and glutamate, linking nitrogen availability to carbon skeletons. Symbiotic nitrogen fixation by certain bacteria converts atmospheric N2 to ammonia using nitrogenase, which is oxygen-sensitive and ATP-demanding. Legume nodules regulate oxygen availability while sustaining respiration. Reduced plant growth under nitrogen limitation can reflect nitrogen assimilation, carbon balance or symbiosis, so identify the limiting process.
Important: Nitrogenase fixes Nâ‚‚ and is oxygen-sensitive.
Read Detailed Notes →5. Plant hormones
Auxin, cytokinin, gibberellin, ABA and ethylene regulate development and stress responses.
For CSIR NET, begin by identifying the main components involved in plant hormones and the order in which they interact. Explain what changes when one component is removed, inhibited, overexpressed or exposed to different conditions. This approach is especially useful for interpreting experiments rather than recalling isolated facts.
When solving a question, distinguish the underlying mechanism from its observable outcome. Compare the normal situation with the altered condition, and check whether the evidence supports a direct effect or only an association. The central distinction to retain is: hormone responses depend on tissue, concentration and context.
Auxin regulates cell expansion, tropisms and organ patterning through concentration-dependent transport and signaling. Cytokinins promote cell division and influence shoot development; gibberellins regulate elongation and germination; abscisic acid participates in drought responses and dormancy; ethylene controls aspects of ripening and stress responses. Hormonal effects depend on tissue, developmental stage and interactions with other hormones. A mutant with low growth may have defective biosynthesis, perception or downstream response; hormone application helps distinguish some of these possibilities.
Important: Hormone responses depend on tissue, concentration and context.
Read Detailed Notes →6. Light sensing
Phytochromes sense red/far-red light; cryptochromes and phototropins respond to blue light.
For CSIR NET, begin by identifying the main components involved in light sensing and the order in which they interact. Explain what changes when one component is removed, inhibited, overexpressed or exposed to different conditions. This approach is especially useful for interpreting experiments rather than recalling isolated facts.
When solving a question, distinguish the underlying mechanism from its observable outcome. Compare the normal situation with the altered condition, and check whether the evidence supports a direct effect or only an association. The central distinction to retain is: photoperiodism depends on light signaling and circadian timing.
Phytochromes sense red and far-red light and interconvert between signaling states. Cryptochromes and phototropins respond primarily to blue light and influence development, circadian timing or phototropism. Photoperiodic flowering depends on interactions among the circadian clock, light signals and mobile flowering regulators. Light intensity, wavelength and duration are different experimental variables. A red-light response reversed by far-red light suggests phytochrome involvement, but developmental context and the timing of irradiation remain important.
Important: Photoperiodism depends on light signaling and circadian timing.
Read Detailed Notes →7. Water and phloem transport
Water-potential gradients drive water movement; xylem transports water and phloem moves photoassimilates.
For CSIR NET, begin by identifying the main components involved in water and phloem transport and the order in which they interact. Explain what changes when one component is removed, inhibited, overexpressed or exposed to different conditions. This approach is especially useful for interpreting experiments rather than recalling isolated facts.
When solving a question, distinguish the underlying mechanism from its observable outcome. Compare the normal situation with the altered condition, and check whether the evidence supports a direct effect or only an association. The central distinction to retain is: phloem pressure flow links source and sink tissues.
Water movement in plants is driven by water-potential gradients that combine solute and pressure components. Transpiration creates tension in xylem, where cohesion and adhesion help maintain a continuous water column. Phloem translocation is commonly explained by pressure-flow from source tissues to sinks after sugar loading and water entry. Xylem and phloem differ in transported substances, cell types and driving forces. Stomatal closure can conserve water but reduce CO2 uptake, creating a trade-off between hydraulic safety and photosynthesis.
Important: Phloem pressure flow links source and sink tissues.
Read Detailed Notes →8. Stress and secondary metabolites
Terpenoids, phenolics and alkaloids contribute to defense and signaling; drought and pathogen stress trigger coordinated responses.
For CSIR NET, begin by identifying the main components involved in stress and secondary metabolites and the order in which they interact. Explain what changes when one component is removed, inhibited, overexpressed or exposed to different conditions. This approach is especially useful for interpreting experiments rather than recalling isolated facts.
When solving a question, distinguish the underlying mechanism from its observable outcome. Compare the normal situation with the altered condition, and check whether the evidence supports a direct effect or only an association. The central distinction to retain is: stress tolerance involves signaling networks, not one molecule.
Drought, salinity, temperature extremes and pathogen attack activate overlapping yet distinct stress pathways. Osmotic adjustment, antioxidant enzymes and protective proteins can improve stress tolerance, while prolonged stress may inhibit growth. Secondary metabolites include phenolics, terpenoids and alkaloids, with roles in defense, signaling and ecological interactions. Reactive oxygen species can act as signals at controlled levels but damage cellular components when excessive. Stress tolerance should be assessed by survival or performance under defined conditions, not by one molecular marker alone.
Important: Stress tolerance involves signaling networks, not one molecule.
Read Detailed Notes →Key Concepts and Quick Revision
- Photosynthetic electron transport: Oxygen evolved in photosynthesis comes from water.
- C3, C4 and CAM pathways: C4 separates fixation spatially; CAM commonly separates it by time.
- Photorespiration and respiration: Photorespiration tends to increase when COâ‚‚ relative to Oâ‚‚ falls.
- Nitrogen assimilation: Nitrogenase fixes Nâ‚‚ and is oxygen-sensitive.
- Plant hormones: Hormone responses depend on tissue, concentration and context.
- Light sensing: Photoperiodism depends on light signaling and circadian timing.
- Water and phloem transport: Phloem pressure flow links source and sink tissues.
- Stress and secondary metabolites: Stress tolerance involves signaling networks, not one molecule.
Practice and Revision Resources
Frequently Asked Questions
What topics are covered in Unit 6?
This unit covers system physiology – plant, including the key topics explained above.
How should I study this unit?
Read each explanation, revise the Remember points, and solve topic-wise questions and analytical problems.
Are these notes useful for Part C?
They establish the concepts needed for analytical questions; practice data interpretation and multi-step reasoning separately.
What is the best way to revise?
Use the quick revision notes and return to detailed explanations for difficult concepts.