Photosynthesis in Higher Plants

Chapter at a glance

• Green plants are autotrophs that synthesise food through photosynthesis, the physico-chemical process using light energy to convert CO₂ and H₂O into carbohydrates while releasing O₂.
• Photosynthesis occurs in chloroplasts: light reactions (photochemical phase) take place in thylakoid membranes (grana and stroma lamellae) producing ATP, NADPH and O₂; dark reactions (Calvin cycle) occur in the stroma.
• Pigments are organised into two photosystems — PS I (reaction centre P700) and PS II (reaction centre P680) — within light-harvesting complexes (LHC).
• Light reactions follow the Z-scheme (non-cyclic photophosphorylation) or cyclic flow, with water splitting at PS II releasing O₂; ATP synthesis occurs via chemiosmosis across the thylakoid membrane.
• CO₂ fixation occurs via the C₃ pathway (first product PGA via RuBisCO) in most plants or the C₄ pathway (first product OAA via PEPcase, followed by Calvin cycle in bundle sheath cells) in plants with Kranz anatomy.
• Photorespiration occurs in C₃ plants when RuBisCO binds O₂ instead of CO₂, releasing CO₂ without ATP or sugar synthesis; it is negligible in C₄ plants.
• The overall process is represented by: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂ (light); O₂ released originates from water.
• Rate of photosynthesis is influenced by internal factors (chlorophyll, mesophyll, internal CO₂) and external factors (light, CO₂, temperature, water); Blackman’s Law of Limiting Factors applies.

Definitions and laws

• Photosynthesis: “a physico-chemical process by which [green plants] use light energy to drive the synthesis of organic compounds.”
• Light reaction (photochemical phase): “include light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH.”
• Dark reaction (carbon reaction): “not directly light driven but are dependent on the products of light reactions (ATP and NADPH).”
• Photophosphorylation: “the synthesis of ATP from ADP and inorganic phosphate in the presence of light.”
• Chemiosmotic hypothesis: ATP synthesis linked to development of a proton gradient across the thylakoid membrane; protons accumulate in the lumen.
• Law of Limiting Factors (Blackman, 1905): “If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed.”
• RuBisCO: “RuBP carboxylase-oxygenase.”
• Kranz anatomy: leaves with large bundle sheath cells surrounding vascular bundles, rich in chloroplasts, thick walls, no intercellular spaces.
• Photorespiration: pathway in which RuBP binds O₂ forming phosphoglycerate and phosphoglycolate; “there is neither synthesis of sugars, nor of ATP.”
• Overall equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂ (light); O₂ released is from water.

Important diagrams and activities

• Figure 11.1 (Priestley’s experiment): demonstrates plants restore to air what breathing animals and burning candles remove.
• Figure 11.2 (chloroplast ultrastructure): shows division of labour — thylakoid membranes for light reactions, stroma for Calvin cycle.
• Figure 11.3a–c (absorption and action spectra): shows chlorophyll a is the chief pigment; accessory pigments broaden wavelength range and protect chlorophyll a.
• Figure 11.4 (light harvesting complex): illustrates antenna pigments funnelling energy to reaction centre chlorophyll a.
• Figure 11.5 (Z-scheme): depicts non-cyclic electron transport from PS II to PS I to NADP⁺ with ATP and NADPH formation.
• Figure 11.6 (cyclic photophosphorylation): shows electron circulation within PS I producing only ATP.
• Figure 11.7 (chemiosmosis/ATP synthesis): explains proton gradient across thylakoid membrane driving ATP synthase.
• Figure 11.8 (Calvin cycle): shows carboxylation, reduction and regeneration phases leading to glucose.
• Figure 11.9 (Hatch-Slack/C₄ pathway): illustrates PEP carboxylation in mesophyll and Calvin cycle in bundle sheath cells.
• Figure 11.10 (light intensity vs photosynthesis rate): demonstrates light saturation and limiting factors.

Activities (must be describable): variegated leaf/black paper + starch test (chlorophyll and light required); KOH-enclosed leaf + starch test (CO₂ required); Priestley bell-jar (air restoration); Ingenhousz aquatic plant (O₂ bubbles in light from green parts); Engelmann’s Cladophora + bacteria (action spectrum peaks in blue-red); Sachs (glucose stored as starch in green parts/chloroplasts).

Common misconceptions and exam pitfalls

• Dark reactions do not occur only in darkness — they require ATP/NADPH from light reactions and stop when light products are exhausted.
• O₂ released comes from water (proved by radioisotope studies), not CO₂.
• RuBisCO functions as both carboxylase and oxygenase; photorespiration is distinct from cellular respiration and yields no ATP/sugar.
• C₄ plants still use the Calvin cycle but avoid photorespiration via Kranz anatomy and PEPcase; Calvin pathway is not absent in C₄ plants.
• Cyclic photophosphorylation occurs in stroma lamellae (lacks PS II and NADP reductase) and produces only ATP.
• Law of Limiting Factors: rate is governed by the factor nearest its minimum, not by all factors equally.
• Light saturation occurs at ~10% full sunlight; excess light damages chlorophyll.

Formula sheet