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 also takes place in other green parts of the plants. There is a clear-cut division of labour within the 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.

Pigments Are Involved In Photosynthesis

Photosynthetic pigments are special molecules that absorb, transmit and reflect different colours of light from the visible spectrum of sunlight. Pigment appears in the colour that it reflects and uses the colour that 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 the second pyrrole ring. 
• Chl–b is an 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.

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-energised 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.

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-energised 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⁺.

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 ferredoxin (Fd) → cytochrome complex → plastocyanin (Pc).
• As electrons move through the chain, protons are pumped into the thylakoid space.
• The proton gradient powers ATP synthase to make ATP.
• Electrons return to PSI, completing the cycle.

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.

C₄-Pathway

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

Factors Affecting Photosynthesis

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

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

Steps of 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

Plant Hormones