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NEET Biology
Plant Physiology

Plant Physiology

Plant Physiology is the branch of biology that deals with the functions and processes occurring in plants. It helps us understand how plants live, grow, develop, and respond to their environment. While plants may appear passive, they are constantly performing complex physiological activities such as:

1.0Photosynthesis In Higher Plants 

  • Green plants, in fact, have to make or rather synthesise the food they need and all other organisms depend on them for their needs. 
  • The green plants make or rather synthesise the food they need through photosynthesis and are therefore called autotrophs. 
  • Green plants carry out ‘photosynthesis’, a physico-chemical process by which they use light energy to drive the synthesis of organic compounds.

Where Does Photosynthesis Take Place?

Photosynthesis does take place in the green leaves of plants but it does so also in other green parts of the plants. There is a clear-cut division of labour within the chloroplast. 

Where Does Photosynthesis Take Place?


division of labour in chloroplast

Alignment of Chloroplasts

Usually the chloroplasts align themselves along the walls of the mesophyll cells, such that they get the optimum quantity of the incident light.

Conditions

Alignment of chloroplasts

High light intensity

Parallel to the incident light/Lateral walls (Parastrophe) 

Low light intensity 

Perpendicular to the incident light (Epistrophe)

Moderate light intensity 

Random (Apostrophe) 

Pigments Are Involved In Photosynthesis

Photosynthetic pigments are special molecules those absorb, transmit and reflect different colours of light from the visible spectrum of sunlight. Pigment appears in the colour which it reflects and uses the colour which it absorbs.

PAR (Photosynthetic Active Radiation) → 400-700nm 

Chlorophylls 

  • Chl – a, Chl – b, Chl – c, Chl – d, Chl – e 
  • Chl – a universal pigment, which is found in all O2 liberating photosynthetic organisms. Its colour is blue-green in the chromatogram. CH3 group in IInd pyrrole ring. 
  • Chl – b is accessory photosynthetic pigment found in euglenoids, green algae and higher plants. 
  • Its colour is yellowish green in the chromatogram. CHO group in IInd pyrrole ring.

Carotenoids

  • They are accessory pigments and make photosynthesis more efficient by absorbing different wavelengths of light. 
  • They protect chl-a from photooxidation and they also protect photosynthetic machinery by converting lethal nascent oxygen into unharmful molecular oxygen, thus called shield pigments.
  • They help in entomophily and zoochory.
  • Carotene acts as a precursor of vitamin-A.

 Carotenoids

Mechanism of Photosynthesis

Phycobillins

  • They are hot water-soluble pigments. They lack Mg and phytol tail. 
  • Phycocyanin – Blue 
  • Phycoerythrin – Red
  • Allophycocyanin – Light blue 
  • They occur exclusively in BGA and red algae as an accessory pigment.

Light Reaction

  • The light reaction happens in the thylakoid membranes of the chloroplasts. It's the part where sunlight energy is captured and used to make ATP and NADPH, which are needed for the next stage (the Calvin cycle).
  • Photons (light particles) hit chlorophyll molecules.
  • Energy excites electrons in Photosystem II (PSII).
  • Excited electrons move through an electron transport chain (ETC), releasing energy to pump protons (H⁺) into the thylakoid space.
  • This builds a proton gradient, and protons flow back through ATP synthase, making ATP (this process is called photophosphorylation).
  • Electrons reach Photosystem I (PSI), get re-energized by more light, and help reduce NADP⁺ into NADPH.
  • Meanwhile, water molecules are split (photolysis) to replace the lost electrons in PSII, releasing oxygen (O₂) as a byproduct.

Light Reaction

Non-Cyclic Photophosphorylation

  • It happens in the thylakoid membrane.
  • Both Photosystem II (PSII) and Photosystem I (PSI) are involved.
  • Electrons move in one direction — from water to NADP⁺ (no cycle).
  • Produces ATP, NADPH, and O₂.

Basic Steps:

  • Light hits PSII, exciting electrons in chlorophyll (P680).
  • Excited electrons are passed down the electron transport chain (ETC).
  • Energy from electrons pumps protons (H⁺) into the thylakoid space, generating a proton gradient → ATP synthesis.
  • Electrons reach PSI (P700) and are re-energized by another light photon.
  • Electrons are transferred to NADP⁺ to form NADPH.
  • Water is split at PSII to replace lost electrons, releasing O₂ and H⁺.

Non-Cyclic Photophosphorylation

Cyclic Photophosphorylation

  • It happens only in Photosystem I (PSI).
  • Electrons cycle back to the same photosystem — no NADPH formation, no oxygen release.
  • Main purpose: Produce extra ATP when the cell needs more energy.

Basic Steps:

  • Light excites electrons in PSI (P700).
  • Electrons are transferred to a primary electron acceptor.
  • Instead of reducing NADP⁺, electrons pass through an electron transport chain:
    Via ferrodoxin (Fd) → cytochrome complex → plastocyanin (Pc).
  • As electrons move through the chain, protons are pumped into the thylakoid space.
  • Proton gradient powers ATP synthase to make ATP.
  • Electrons return to PSI, completing the cycle.

Cyclic Photophosphorylation

C3-Pathway Or The Calvin Cycle

  • Happens in the stroma of the chloroplast.
  • Uses the ATP and NADPH produced in the light reaction.
  • Main goal: Convert CO₂ into sugar (glucose).
  • Called the C₃ pathway because the first stable product formed is a 3-carbon compound: 3-phosphoglyceric acid (3-PGA).

1. Carbon Fixation:

  • The enzyme RuBisCO fixes CO₂ onto a 5-carbon molecule called Ribulose-1,5-bisphosphate (RuBP).
  • Forms an unstable 6-carbon compound, which immediately splits into two 3-carbon molecules of 3-phosphoglycerate (3-PGA).

2. Reduction:

  • ATP adds energy to 3-PGA → forms 1,3-bisphosphoglycerate (1,3-BPG).
  • NADPH reduces 1,3-BPG → forms Glyceraldehyde-3-phosphate (G3P) (a 3-carbon sugar).

3. Regeneration of RuBP:

  • Some G3P molecules go on to form glucose and other carbohydrates.
  • Most G3P molecules are recycled back to regenerate RuBP, using more ATP.
  • This keeps the cycle going.

Regeneration of RuBP

C4-Pathway

  • Found in tropical and subtropical plants (like maize, sugarcane, sorghum).
  • It’s an adaptation to hot, dry climates.
  • Helps plants minimize photorespiration and increase photosynthesis efficiency.
  • Called C₄ because the first stable product is a 4-carbon compound: oxaloacetic acid (OAA).

digrammatic representation of the hatch and slack pathway

Factors Affecting Photosynthesis

  • The rate of photosynthesis is very important in determining the yield of plants including crop plants. 

Factors Affecting Photosynthesis

2.0Respiration in Plants

  • Respiration in plants is the process of breaking down food (glucose) to release energy (ATP) for all life activities.

Types of Respiration

Types

Oxygen used

End products

Energy

Aerobic Respiration

Yes

CO₂ + H₂O

High (36-38 ATP)

Anaerobic Respiration

No

Ethanol + CO₂ (in some plants) or Lactate (in animals)

Low (2 ATP)

Steps of Aerobic Respiration

Step

Location

What Happens

Glycolysis

Cytoplasm

Glucose → 2 Pyruvate + 2 ATP + 2 NADH

Link Reaction

Mitochondria (matrix)

Pyruvate → Acetyl-CoA + CO₂

Krebs Cycle (TCA Cycle)

Mitochondria (matrix)

Acetyl-CoA → CO₂ + ATP + NADH + FADH2

Electron Transport Chain (ETC)

Mitochondria (inner membrane)

NADH/FADH₂ → ATP (oxidative phosphorylation)

Aerobic Respiration

Respiratory Quotient (R.Q.) 

  • The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio. 
  • The respiratory quotient depends upon the type of respiratory substrate used during respiration

3.0Plant Growth and Development

  • Growth is regarded as one of the most fundamental and conspicuous characteristics of a living being. 
  • Growth can be defined as an irreversible permanent increase in size of an organ or its parts or even of an individual cell. 
  • Generally, growth is accompanied by metabolic processes (both anabolic and catabolic), that occur at the expense of energy. 

Types of Growth

Types of Growth

Plant Hormones


Auxins

Gibberellins

Cytokinins

Ethylene

Abscisic Acid

Plant Growth Regulators

Indole compounds IAA, IBA, NAA (Naphthalene Acetic Acid) 2, 4-D-(2, 4 DichloroPhenoxyacetic acid)

Terpenes, Gibberellic acid (GA3

Purine (adenine) derivatives. (N6-furfurly-amino purine, kinetin)

Gaseous (Ethylene-C2H4) [Ethephon compound is the source of ethylene]

Derivatives of carotenoids Stress Hormone

Discovery

F.W. went from tips of coleoptiles of oat seedings Auxin was first isolated from human urine.

From fungal pathogen Gibberella fujikuroi. E. Kurosawa (1926)

F. Skoog (By use of extracts of vascular tissues, yeast, coconut milk or DNA in plant tissue culture.) Miller et al – term kinetin

H.H. cousins (1910) – Volatile substance from ripened oranges

During mid 1960 three independent researchers found-Inhibitor-B abscission II and dormin. Later named-ABA

Table of Contents


  • 1.0Photosynthesis In Higher Plants 
  • 1.1Where Does Photosynthesis Take Place?
  • 1.2Alignment of Chloroplasts
  • 1.3Pigments Are Involved In Photosynthesis
  • 1.4Chlorophylls 
  • 1.5Carotenoids
  • 1.6Mechanism of Photosynthesis
  • 1.7Non-Cyclic Photophosphorylation
  • 1.8Cyclic Photophosphorylation
  • 1.9C
  • 1.10C
  • 1.11Factors Affecting Photosynthesis
  • 2.0Respiration in Plants
  • 2.1Types of Respiration
  • 2.2Steps of Aerobic Respiration
  • 2.3Respiratory Quotient (R.Q.) 
  • 3.0Plant Growth and Development
  • 3.1Types of Growth
  • 3.2Plant Hormones

Frequently Asked Questions

Stomata are pores on the surface of leaves that allow gas exchange (CO₂ in, O₂ out) and water vapor to exit (transpiration). The opening and closing of stomata are controlled by guard cells.

Both C₄ and CAM plants have adaptations that help minimize photorespiration: C₄ plants separate the fixation of CO₂ from the Calvin cycle by using a 4-carbon intermediate in the mesophyll cells. CAM plants fix CO₂ at night when oxygen levels are lower, thus preventing photorespiration during the day.

Auxins are hormones that promote cell elongation, root growth, and influence other processes like phototropism and gravitropism.

When plants face drought, they reduce water loss by closing their stomata. Some plants also produce abscisic acid (ABA), which helps them enter dormancy or reduce metabolic activity.

Soil pH affects nutrient availability to plants. Most plants prefer slightly acidic to neutral soil (pH 6-7). If the soil is too acidic or too alkaline, plants may not be able to absorb essential nutrients.

Seeds buried deep in soil may not get enough oxygen, so they temporarily use anaerobic respiration to release energy for germination.

Mitochondria are the site of aerobic respiration, where the Krebs cycle and oxidative phosphorylation occur, leading to the production of ATP.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP) The end products are carbon dioxide, water, and energy.

It yields very little energy (2 ATP), and its by-products like ethanol, can be toxic to cells.

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