Plant Physiology Revision Notes for Biology NEET

Photosynthesis and respiration are central processes in plant physiology, enabling plants to produce and utilize energy essential for growth and development. This chapter explores the mechanisms of photosynthesis, the details of respiration, and the stages of plant growth and differentiation, all of which are vital concepts for the NEET Biology syllabus.

Photosynthesis: Autotrophic Nutrition and Site of Photosynthesis Plants are autotrophic organisms, meaning they prepare their own food by harnessing sunlight. Photosynthesis primarily takes place in the chloroplasts of mesophyll cells in leaves. These chloroplasts contain photosynthetic pigments that absorb light energy and convert it into chemical energy. Among the pigments, chlorophyll-a is the most crucial, directly participating in the reaction, while chlorophyll-b, carotenoids, and xanthophylls act as accessory pigments to maximize light absorption.

Photosynthetic Pigments: Types and Functions Different pigments absorb different wavelengths of light; this maximizes the use of available sunlight. The primary pigments include:

• Chlorophyll-a: Main pigment, bluish-green, absorbs violet-blue and red light.
• Chlorophyll-b: Accessory pigment, yellow-green, broadens absorption spectrum.
• Carotenoids (carotenes and xanthophylls): Absorb blue and green, provide photoprotection.

Phases of Photosynthesis: Photochemical and Biosynthetic Photosynthesis occurs in two main phases:

• Photochemical phase (Light Reaction): Takes place in thylakoids. Light energy splits water (photolysis), producing ATP and NADPH, and releases oxygen. Involves the electron transport chain and photophosphorylation.
• Biosynthetic phase (Dark Reaction or Calvin Cycle): Occurs in the stroma. Carbon dioxide is fixed and converted into glucose using ATP and NADPH generated in the light reaction.

Photophosphorylation: Cyclic and Non-Cyclic Pathways Photophosphorylation is the process of converting ADP to ATP using the energy of sunlight during photosynthesis. There are two types:

• Cyclic Photophosphorylation: Only Photosystem I involved; electrons cycle back. Only ATP is formed, no NADPH or oxygen produced.
• Non-cyclic Photophosphorylation: Both Photosystems I and II are involved; electrons do not return but reduce NADP to NADPH. Both ATP and NADPH are produced, and oxygen is released.

Chemiosmotic Hypothesis This hypothesis explains ATP synthesis in chloroplasts. As electrons move down the transport chain, protons are pumped into the thylakoid lumen, creating a proton gradient. The return flow of protons through ATP synthase leads to ATP production, similar to the process in mitochondria during respiration.

Photorespiration, C3 and C4 Pathways Photorespiration occurs in C3 plants when oxygen, instead of carbon dioxide, reacts with the enzyme RuBisCO, leading to loss of fixed carbon and reducing photosynthetic efficiency. C4 plants minimize this loss by using PEP carboxylase, which fixes CO₂ more effectively, especially under high light and temperature conditions. This adaptation allows C4 plants (e.g., maize, sugarcane) to outperform C3 plants (e.g., wheat, rice) in warm climates.

Factors Affecting Photosynthesis

• Light intensity and wavelength: Increases rate up to a saturation point.
• CO₂ concentration: Increases photosynthesis till the rate plateaus.
• Temperature: Each plant has an optimum; too high or low reduces rate.
• Water: Essential for photolysis; deficiency reduces photosynthetic efficiency.
• Oxygen: High oxygen increases photorespiration, decreasing net photosynthesis.

Respiration in Plants: Gas Exchange and Cellular Respiration Plants exchange gases mainly through stomata in leaves and lenticels in stems. Cellular respiration breaks down food to release energy. There are two main types:

• Aerobic Respiration: Complete oxidation of glucose using oxygen, producing carbon dioxide, water, and energy.
• Anaerobic Respiration (Fermentation): Occurs in the absence of oxygen, forming alcohol or lactic acid, releasing less energy.

Pathways of Cellular Respiration

• Glycolysis: Occurs in cytoplasm; breakdown of one glucose into two pyruvate, yielding 2 ATP and 2 NADH.
• Transition Step: Conversion of pyruvate to acetyl-CoA in mitochondria.
• TCA Cycle (Krebs Cycle): In mitochondrial matrix; acetyl-CoA is oxidized, producing CO₂, ATP, NADH, and FADH₂.
• Electron Transport System: Located in the inner mitochondrial membrane; NADH and FADH₂ donate electrons, generating a proton gradient that drives ATP synthesis. Oxygen acts as the final electron acceptor.

Energy Yield and Amphibolic Pathways Aerobic respiration of one glucose molecule generates up to 38 ATP (theoretical maximum), while anaerobic fermentation yields only 2 ATP. Respiration is an amphibolic pathway, meaning intermediates can be used in both catabolic and anabolic processes. For instance, some intermediates are used for synthesis of amino acids, nucleotides, or fatty acids.

Respiratory Quotient (RQ) RQ is the ratio of CO₂ evolved to O₂ consumed during respiration. For carbohydrates, the RQ is 1; for fats, it is less than 1; and for organic acids, it is greater than 1. This value helps indicate the substrate being respired.

Plant Growth and Development: Seed Germination and Growth Phases Seed germination marks the beginning of plant growth, requiring water, oxygen, and suitable temperature. The phases of growth include:

1. Cell division (meristematic phase): Rapid mitosis in meristems.
2. Cell Elongation: Cells grow in size and absorb nutrients.
3. Cell Maturation: Cells reach full form and specialized function.

Conditions and Rate of Growth The rate of plant growth is measured as an increase in mass or volume over time. It depends on environmental factors such as light, water, nutrients, and temperature. The optimal growth requires all conditions to be favorable.

Differentiation, Dedifferentiation, and Redifferentiation During plant development, cells become specialized (differentiation). Some differentiated cells can regain the ability to divide (dedifferentiation), as seen in the formation of cambium from parenchyma. If these dedifferentiated cells specialize again, it is called redifferentiation.

Developmental Sequence in a Plant Cell Development in plants follows this sequence: cell formation → elongation → differentiation → senescence (aging). Coordinated growth and development result in formation of tissues and organs.

Plant Growth Regulators Growth regulators are chemicals that influence various plant processes:

• Auxins: Promote cell elongation, root initiation, delay fruit ripening.
• Gibberellins: Stimulate stem elongation, seed germination, and flowering.
• Cytokinins: Promote cell division and delay senescence.
• Ethylene: Promotes fruit ripening and leaf abscission.
• Abscisic Acid (ABA): Inhibits growth, promotes dormancy and closure of stomata during water stress.

NEET Biology Notes – Plant Physiology: Key Points for Revision

Grasping Plant Physiology is crucial for scoring well in NEET Biology. These notes cover all essential subtopics like photosynthesis, respiration, and growth regulators, providing a concise summary to solidify your understanding and boost your confidence for quick revision.

Use these reliable revision pointers to clarify concepts such as chemiosmotic hypothesis and the differences between C3 and C4 pathways. The organized notes help you revise important facts and mechanisms efficiently, ensuring thorough preparation for exam day.