Photosynthesis in Higher Plants Class 11 Notes

Key Terminology & Glossary

Photosystem (PS)

Functional and structural units of protein complexes involved in photosynthesis, consisting of a Reaction Centre (chlorophyll a molecule) and Light Harvesting Complexes (LHC).

Photolysis

The enzymatic splitting of water molecules in the presence of light and manganese ($Mn^{2+}$) and chlorine ($Cl^-$) ions, releasing electrons, protons, and oxygen.

Chemiosmosis

The mechanism of ATP synthesis driven by a proton gradient generated across the thylakoid membrane.

Kranz Anatomy

A specialized leaf anatomy in $C_4$ plants where large bundle sheath cells containing dense chloroplasts surround the vascular bundles in a wreath-like arrangement.

RuBisCO

Ribulose-1,5-bisphosphate carboxylase-oxygenase; the most abundant enzyme on Earth, catalyzing the first step of carbon fixation in the Calvin Cycle.

Photorespiration

A wasteful pathway occurring in $C_3$ plants under high $O_2$ and low $CO_2$ conditions, where RuBisCO binds to $O_2$ instead of $CO_2$, wasting energy and losing carbon.

The Light Reactions: Photochemical Phase

The Light Reaction occurs within the thylakoid membranes (grana) of the chloroplast. It involves light absorption, water splitting, oxygen release, and the synthesis of high-energy chemical intermediates: ATP and NADPH.

1. Non-Cyclic Photophosphorylation (Z-Scheme)

• Photosystem II (PS II) absorbs light at $680\text{ nm}$ (P680). Excited electrons are ejected and accepted by a primary electron acceptor, passing down an Electron Transport System (ETS) consisting of plastoquinone, cytochrome $b_6f$ complex, and plastocyanin.
• This movement of electrons creates a proton gradient across the thylakoid membrane, driving ATP synthesis.
• Photosystem I (PS I) absorbs light at $700\text{ nm}$ (P700), taking up electrons from the ETS. These electrons are further excited and transferred to $NADP^+$ via ferredoxin to form NADPH.
• Splitting of Water: Occurs on the inner side of the thylakoid membrane, associated with PS II:

$2H_2O \rightarrow 4H^+ + O_2 + 4e^-$

2. Cyclic Photophosphorylation

• Occurs when only PS I is functional (usually when light wavelength is beyond $680\text{ nm}$ or in the stroma lamellae which lack PS II and $NADP^+$ reductase enzyme).
• Electrons circulate within PS I, and the flow only synthesizes ATP, with no formation of NADPH or $O_2$.

The Dark Reactions: The Calvin Cycle (C3 Pathway)

1. Carboxylation — The fixation of $CO_2$ into a stable organic intermediate. Ribulose-1,5-bisphosphate ($RuBP$) reacts with $CO_2$ in the presence of RuBisCO to form two molecules of 3-phosphoglyceric acid ($3-PGA$).
2. Reduction — A series of reactions using ATP and NADPH to reduce $3-PGA$ to triose phosphate (G3P). For every molecule of $CO_2$ fixed, 2 ATP and 2 NADPH are consumed.
3. Regeneration — The cycle must regenerate the primary $CO_2$ acceptor ($RuBP$) to keep the pathway moving continuously. This step requires 1 ATP molecule.

Comparison: C3 vs C4 Photosynthetic Pathways

Key Points & Quick Formula Checklist

• Photosynthetic pigments (Chlorophyll a, b, xanthophylls, carotenoids) can be separated using paper chromatography.
• Chlorophyll 'a' is the chief pigment of the reaction centre; others act as accessory pigments protecting it from photo-oxidation.
• Chemiosmotic ATP synthesis requires: a membrane, a proton pump, a proton gradient, and ATPase.
• Net equation for 1 molecule of Glucose in C3 Pathway: $6CO_2 + 18ATP + 12NADPH \rightarrow C_6H_{12}O_6 + 18ADP + 12NADP^+$.
• Net equation for 1 molecule of Glucose in C4 Pathway: $6CO_2 + 30ATP + 12NADPH \rightarrow C_6H_{12}O_6 + 30ADP + 12NADP^+$.
• Blackman's Law of Limiting Factors: If a chemical process is affected by more than one factor, its rate will be determined by the factor which is nearest to its minimal value.

CBSE Board Exam Strategies & Traps

• The ATP/NADPH Calculation Trap: Examiners frequently ask for the energetic cost of synthesizing 1 glucose molecule. Remember: $C_3$ pathway uses 18 ATP and 12 NADPH, while $C_4$ uses 30 ATP and 12 NADPH. Do not write 3 ATP and 2 NADPH (which is per single $CO_2$ fixed) unless specifically asked per turn of the cycle.
• Diagram Cues: Always label the locations in the Hatch-Slack pathway clearly. Ensure PEPcarboxylase is shown working in the mesophyll cells and RuBisCO is situated strictly inside the bundle-sheath cells.
• Blackman's Law: Be prepared to define and explain the curve of limiting factors. Carbon dioxide concentration is the major limiting factor for photosynthesis in nature.

Quick Revision Check

• Why is the C4 pathway more efficient than the C3 pathway despite requiring more ATP? The C4 pathway completely avoids photorespiration by concentrating CO2 around RuBisCO in the bundle-sheath cells, ensuring high photosynthetic yields even in high light and temperature.
• Where are PS II and PS I located on the thylakoid membrane? PS II is located in the appressed (stacked) regions of the grana thylakoids, while PS I is situated in the non-appressed regions of the grana and the stroma lamellae.
• What are the two essential components required for the breakdown of the proton gradient in Chemiosmosis? The enzyme ATPase (specifically its CF0 channel for proton diffusion and CF1 headpiece for ATP synthesis) and a proton accumulation within the thylakoid lumen.
• Under what environmental condition is CO2 not a limiting factor for C4 plants? At high light intensities, C4 plants show saturation at about 360 ppm of CO2, meaning current atmospheric levels are already optimal for them.

Frequently Asked Questions