← Biology — Std 11
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Photosynthesis

Ch. 12Std 11

Easy Overview

Plants literally turn sunlight into food. Think about that for a moment — they take sunshine, water, and carbon dioxide, and they make sugar. We cannot do that. No machine can do it as efficiently. Every single bite of food you eat comes from photosynthesis, either directly (plants) or indirectly (animals that ate plants). And the oxygen you are breathing right now? That came from photosynthesis too. This chapter is about how plants pull off this incredible feat. Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy from the sun into chemical energy stored in glucose. The overall equation is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Carbon dioxide and water, using light energy, produce glucose and oxygen. The process happens in the chloroplasts. Chloroplasts have a double membrane. Inside, there is a fluid matrix called the stroma, and a system of interconnected membrane sacs called thylakoids. Thylakoids are often stacked into grana. The thylakoid membranes contain chlorophyll (the green pigment) and other photosynthetic pigments arranged in photosystems. The light-dependent reactions occur here — light energy is captured and converted into chemical energy in the form of ATP and NADPH. The stroma is where the light-independent reactions (the Calvin cycle) take place — ATP and NADPH are used to fix CO₂ into sugar. The light reactions begin when light strikes a chlorophyll molecule. The energy excites an electron to a higher energy level. This high-energy electron is captured by an electron transport chain, and the energy is used to make ATP (photophosphorylation) and NADPH. The electrons lost from chlorophyll are replaced by splitting water molecules (photolysis) — this releases oxygen as a byproduct. The Calvin cycle then uses the ATP and NADPH to reduce CO₂ to glucose. The key enzyme is RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) — the most abundant enzyme on Earth. It fixes CO₂ onto a 5-carbon molecule called RuBP, producing two molecules of 3-phosphoglycerate (3-PGA), which are then reduced to glyceraldehyde-3-phosphate (G3P) — the first sugar. Not all plants use basic C3 photosynthesis. C4 plants (maize, sugarcane) have an additional step that concentrates CO₂ in bundle sheath cells, reducing photorespiration (a wasteful process where RuBisCO fixes O₂ instead of CO₂). C4 plants are better adapted to hot, dry environments. CAM plants (cacti, succulents) open their stomata at night to take in CO₂ and store it, then use it during the day — this conserves water. The rate of photosynthesis is affected by several factors: light intensity, CO₂ concentration, temperature, and water availability. Blackman's law of limiting factors states that the rate is limited by the factor in shortest supply. Understanding photosynthesis is essential for agriculture, environmental science, and addressing climate change because plants are the primary entry point of energy into food webs and the main mechanism for removing CO₂ from the atmosphere.

What is Photosynthesis?

Photosynthesis is the process by which autotrophic organisms convert light energy from the sun into chemical energy stored in organic compounds (carbohydrates). The overall balanced equation: 6CO₂ + 12H₂O + light energy → C₆H₁₂O₆ + 6O₂ + 6H₂O. The simplified version: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The source of released oxygen was proven to be water, not CO₂, by isotope experiments using water labeled with ¹⁸O — when plants were given H₂¹⁸O, the oxygen released contained ¹⁸O; when given C¹⁸O₂, the oxygen did not contain ¹⁸O. This was shown by Ruben and Kamen in 1941. Photosynthesis occurs in the chloroplasts of plant cells, in the thylakoid membranes and stroma. It is the primary source of organic matter for almost all life on Earth (except chemosynthetic organisms), the source of atmospheric oxygen, and the mechanism by which CO₂ is removed from the atmosphere. Photosynthesis occurs in plants, algae, cyanobacteria (blue-green algae), and some protists. The efficiency of photosynthesis is about 3-6% of the sunlight that hits the leaf is converted to chemical energy — the rest is reflected, transmitted, or lost as heat. However, this is still much more efficient than any solar panel.

Chloroplast Structure

Chloroplasts are double-membraned organelles found in plant cells and some protists. The outer and inner membranes enclose the stroma — a fluid matrix containing enzymes for the Calvin cycle, starch grains, lipid droplets, chloroplast DNA (circular), and 70S ribosomes. Inside the stroma is the thylakoid system — a network of interconnected, flattened, disc-shaped sacs called thylakoids. The thylakoids are stacked into columns called grana (singular: granum). Each granum contains 10-100 thylakoids. The thylakoid membrane is the site of the light-dependent reactions — it contains the photosynthetic pigments (chlorophylls and carotenoids), photosystems (PS I and PS II), electron transport chain components, and ATP synthase. The space inside the thylakoid sac is the thylakoid lumen. The thylakoid membrane is highly folded, which greatly increases its surface area for capturing light and carrying out the light reactions. Chloroplasts are semi-autonomous — like mitochondria, they have their own DNA (circular, like bacteria) and 70S ribosomes. They also reproduce by binary fission. The endosymbiotic theory explains chloroplasts as descendants of photosynthetic cyanobacteria that were engulfed by an ancestral eukaryotic cell and established a symbiotic relationship. Evidence includes: double membrane (inner = bacterial, outer = host vesicle), circular DNA, 70S ribosomes, and autonomous division.

Photosynthetic Pigments

Photosynthetic pigments are molecules that absorb light energy and convert it to chemical energy. Different pigments absorb different wavelengths of light. Chlorophyll a (C₅₅H₇₂O₅N₄Mg) is the primary photosynthetic pigment — it is directly involved in converting light energy to chemical energy. It has a porphyrin ring (with a central magnesium atom) and a phytol tail. Chlorophyll a absorbs light most strongly in the blue-violet (430 nm) and red (662 nm) regions of the spectrum and reflects green light — that is why leaves look green. Chlorophyll b differs slightly (one functional group is different) and absorbs at slightly different wavelengths (blue at 455 nm, red at 644 nm). It acts as an accessory pigment — it absorbs light energy and transfers it to chlorophyll a, broadening the range of light that can be used for photosynthesis. Carotenoids (carotenes — orange, and xanthophylls — yellow) are accessory pigments that absorb blue light and transfer energy to chlorophyll a. They also have a protective function — they dissipate excess light energy as heat, preventing photo-oxidative damage to the chlorophyll (this is called the xanthophyll cycle). When leaves change color in autumn, chlorophyll breaks down first, revealing the yellow-orange carotenoids that were always present. The absorption spectrum shows the wavelengths of light absorbed by a pigment. The action spectrum shows the rate of photosynthesis at different wavelengths — it is similar to the absorption spectrum of chlorophyll but also shows contributions from accessory pigments, especially in the green region (where chlorophyll absorbs poorly but carotenoids provide some absorption).

Light Reaction — Introduction

The light reactions (light-dependent reactions) convert light energy into chemical energy stored in ATP and NADPH. They occur on the thylakoid membranes of chloroplasts. The two photosystems involved are Photosystem II (PS II) and Photosystem I (PS I). Each photosystem has a reaction center (a special pair of chlorophyll a molecules) surrounded by light-harvesting complexes (antenna molecules — chlorophyll b, carotenoids — that capture light energy and funnel it to the reaction center). PS II has a reaction center called P680 (because it absorbs best at 680 nm). PS I has a reaction center called P700 (absorbs best at 700 nm). The light reactions involve two types of photophosphorylation: non-cyclic and cyclic. In non-cyclic photophosphorylation (the main pathway), electrons flow from water through PS II and PS I to NADP⁺, producing ATP and NADPH and releasing O₂. In cyclic photophosphorylation (alternative pathway), electrons cycle through PS I only, producing ATP but no NADPH and no O₂. Both processes contribute to the ATP pool needed for the Calvin cycle.

Non-cyclic Photophosphorylation

Non-cyclic photophosphorylation is the main electron flow pathway during the light reactions. It involves both PS II and PS I. The process: (1) Light strikes PS II, exciting electrons in the reaction center P680 to a higher energy level. (2) The excited electrons are captured by the primary electron acceptor (pheophytin) and passed to an electron transport chain (ETC) consisting of plastoquinone (PQ), the cytochrome b₆f complex, and plastocyanin (PC). (3) As electrons pass through the ETC, energy is released and used to pump protons (H⁺) from the stroma into the thylakoid lumen, creating a proton gradient. (4) The electrons from PS II are replaced by splitting water molecules — this occurs at the oxygen-evolving complex (OEC) associated with PS II. The reaction: 2H₂O → 4H⁺ + 4e⁻ + O₂. This is photolysis of water. The protons stay in the lumen (contributing to the gradient); the oxygen is released as a byproduct. (5) The electrons reach PS I (after passing through the ETC and plastocyanin). (6) Light strikes PS I, re-exciting the electrons in P700 to an even higher energy level. (7) These high-energy electrons are captured by ferredoxin (Fd) and transferred to NADP⁺ reductase, which reduces NADP⁺ + H⁺ to NADPH. (8) The proton gradient (high H⁺ in lumen, low in stroma) drives ATP synthesis via ATP synthase (chemiosmosis). The products: ATP, NADPH, and O₂. This pathway is called 'non-cyclic' because electrons flow from water to NADP⁺ in a linear, one-way path.

Cyclic Photophosphorylation

Cyclic photophosphorylation involves only PS I (not PS II). Electrons from PS I are excited by light and passed to ferredoxin. Instead of going to NADP⁺ reductase, the electrons cycle back through the cytochrome b₆f complex and plastocyanin to return to PS I. As electrons pass through the cytochrome complex, protons are pumped into the thylakoid lumen, generating a proton gradient that drives ATP synthesis via ATP synthase. No NADPH is produced. No water is split, so no oxygen is released. This pathway is called 'cyclic' because the electrons return to their starting point (PS I). Cyclic photophosphorylation occurs when the cell needs more ATP than NADPH — for example, when the Calvin cycle is running and NADPH is already sufficient, but more ATP is needed to keep the cycle going (the Calvin cycle uses 3 ATP and 2 NADPH per CO₂ fixed, so the ATP:NADPH demand is 1.5:1, but non-cyclic photophosphorylation produces them in a roughly 1:1 ratio — cyclic phosphorylation makes up the difference). The relative rates of cyclic and non-cyclic photophosphorylation are dynamically regulated by the cell based on its needs. In some conditions (like when the leaf is under stress), cyclic photophosphorylation can be the dominant pathway.

Photolysis of Water and Oxygen Evolution

Photolysis (photo = light, lysis = splitting) is the splitting of water molecules using light energy. It occurs on the oxygen-evolving complex (OEC) located on the lumenal side of PS II in the thylakoid membrane. The OEC contains four manganese atoms (Mn₄) and one calcium ion (Ca²⁺) — these metal ions are essential for catalyzing the water-splitting reaction. The reaction: 2H₂O → 4H⁺ + 4e⁻ + O₂. The four electrons replace the four electrons lost from PS II (one at a time as PS II is excited by light). The four protons contribute to the proton gradient across the thylakoid membrane — they go into the thylakoid lumen, making it even more acidic. The oxygen is released as molecular O₂. This is the source of all the oxygen in our atmosphere. The OEC cycles through five different states (S₀ to S₄) as it accumulates the four oxidizing equivalents needed to split one water molecule (the Kok cycle or S-state cycle). After four photons hit PS II, the OEC is fully oxidized and can split two water molecules, releasing one O₂ molecule. The oxygen released comes exclusively from water — this was proven by the isotope experiments of Ruben and Kamen using water and CO₂ labeled with the heavy oxygen isotope ¹⁸O.

Chemiosmotic Hypothesis

The chemiosmotic hypothesis explains how ATP is synthesized during the light reactions. It was proposed by Peter Mitchell (who won the Nobel Prize in 1978). As electrons flow through the electron transport chain (from PS II through plastoquinone, the cytochrome b₆f complex, and plastocyanin to PS I), the energy released is used to actively pump protons (H⁺) from the stroma into the thylakoid lumen. This creates a high concentration of H⁺ in the lumen relative to the stroma — a steep electrochemical gradient (proton motive force). This gradient has two components: a pH difference (the lumen becomes acidic, pH ~4, while the stroma remains alkaline, pH ~8) and an electrical potential difference (the lumen is positive, the stroma is negative). The only way for protons to flow back into the stroma is through ATP synthase (also called CF₀-CF₁ complex in chloroplasts). ATP synthase is a large, multi-subunit enzyme complex embedded in the thylakoid membrane. CF₀ (the 'O' stands for oligomycin-sensitive) is the transmembrane proton channel. CF₁ is the catalytic head that projects into the stroma. As protons flow through CF₀ down their gradient, the energy released drives the rotation of CF₁, which catalyzes the phosphorylation of ADP to ATP. This is called photophosphorylation. The chemiosmotic mechanism is similar to how a hydroelectric dam works — water (protons) builds up behind the dam (the gradient), flows through turbines (ATP synthase), and generates power (ATP).

Dark Reaction — Calvin Cycle

The Calvin cycle (also called the light-independent reactions, C3 cycle, or reductive pentose phosphate pathway) occurs in the stroma of the chloroplast. It does not require light directly, but it needs the ATP and NADPH produced by the light reactions. The cycle was discovered by Melvin Calvin (Nobel Prize 1961) and has three phases. (1) Carbon fixation: RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the reaction of CO₂ with ribulose-1,5-bisphosphate (RuBP, a 5-carbon compound), producing an unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA, a 3-carbon compound). This is the first stable product of the Calvin cycle. (2) Reduction: 3-PGA is phosphorylated by ATP (producing 1,3-bisphosphoglycerate) and then reduced by NADPH (producing glyceraldehyde-3-phosphate or G3P, a 3-carbon sugar). The energy from ATP and the reducing power of NADPH are used to convert the carbon from the level of carboxylic acid to the level of aldehyde. (3) Regeneration: most of the G3P (5 out of 6 molecules) is used to regenerate RuBP through a series of reactions. This requires ATP. One G3P (3 carbons) is the net gain per cycle — three turns of the cycle fix 3 CO₂ molecules and produce one net G3P. Six turns fix 6 CO₂ to produce one glucose (C₆H₁₂O₆), which requires 18 ATP and 12 NADPH. The carbohydrates produced are used for energy, growth, and storage.

C3 Photosynthesis

C3 photosynthesis is the most common type of photosynthesis, used by about 85% of plants — including wheat, rice, soybean, cotton, potatoes, peanuts, tobacco, and most trees. The name comes from the first stable product after CO₂ fixation: 3-phosphoglycerate (3-PGA), a 3-carbon compound. The key enzyme is RuBisCO. C3 plants perform the Calvin cycle in the mesophyll cells of the leaves. They have a single type of photosynthetic cell. Advantages of C3: it is simpler and requires less ATP per CO₂ fixed (3 ATP and 2 NADPH per CO₂). Disadvantages: photorespiration. RuBisCO has both carboxylase activity (fixes CO₂) and oxygenase activity (fixes O₂). When the concentration of O₂ in the leaf is high (which happens when stomata close to conserve water in hot, dry conditions), RuBisCO fixes O₂ instead of CO₂, producing phosphoglycolate (a 2-carbon compound) instead of 3-PGA. Phosphoglycolate enters the photorespiratory pathway, which consumes ATP and NADPH and releases CO₂ without producing any sugar — it is essentially the opposite of photosynthesis. Photorespiration can reduce photosynthetic efficiency by up to 50% in C3 plants. This is why C3 plants are less productive in hot, dry climates. The optimum temperature for C3 photosynthesis is about 15-25°C.

C4 Photosynthesis — The Solution to Photorespiration

C4 plants (maize, sugarcane, sorghum, amaranth, many tropical grasses) have evolved a mechanism to concentrate CO₂ around RuBisCO, minimizing photorespiration. They have two types of photosynthetic cells: mesophyll cells (toward the leaf surface) and bundle sheath cells (around the veins, deeper in the leaf). This is called Kranz anatomy (German for 'wreath' or 'ring' — the bundle sheath cells form a ring around the veins). The C4 pathway: in mesophyll cells, CO₂ is initially fixed by PEP carboxylase (an enzyme with no oxygenase activity) using phosphoenolpyruvate (PEP) as the acceptor. This produces oxaloacetate (a 4-carbon compound — hence the name C4). Oxaloacetate is converted to malate or aspartate (also 4-carbon), which is transported to the bundle sheath cells. In the bundle sheath cells, the 4-carbon compound is decarboxylated to release CO₂, creating a high local concentration of CO₂ around RuBisCO. The released CO₂ then enters the normal Calvin cycle. The 3-carbon compound remaining (pyruvate) is transported back to the mesophyll cells and converted back to PEP (using ATP). C4 plants require more energy than C3 plants — 5 ATP per CO₂ fixed instead of 3 ATP. However, they are more water-efficient (they can keep their stomata more closed than C3 plants, reducing water loss while still fixing CO₂ because the initial fixation in mesophyll cells can operate at low CO₂ concentrations). C4 plants are adapted to hot, dry, and high-light environments with optimal temperatures of 30-45°C.

CAM Photosynthesis — Extreme Water Conservation

CAM (Crassulacean Acid Metabolism) plants are adapted to extremely dry conditions — deserts, semi-arid regions, and epiphytic environments. The name comes from the Crassulaceae family (stonecrops, jade plants), where this pathway was first discovered. CAM plants include cacti, succulents (Agave, Aloe), pineapple, some orchids, and ice plants. CAM plants separate CO₂ uptake and the Calvin cycle temporally (in time), unlike C4 plants which separate them spatially (in different cells). The CAM cycle: at night, the stomata open (when it is cooler and evaporation is lower), and CO₂ enters the leaf. PEP carboxylase fixes CO₂ into oxaloacetate, which is converted to malic acid. The malic acid is stored in the vacuole, making the leaf cells acidic at night (hence the name 'Crassulacean Acid Metabolism'). During the day, the stomata close (conserving water), and the malic acid is released from the vacuole and decarboxylated to release CO₂ inside the leaf. The CO₂ is then fixed by the Calvin cycle using the ATP and NADPH generated by the light reactions (which occur during the day). CAM plants have the highest water-use efficiency of any photosynthetic group — they can fix CO₂ while losing only 50-100 water molecules per CO₂, compared to 250-300 for C3 plants and 150-200 for C4 plants. The disadvantage is that CAM is energy-intensive and slow — CAM plants grow slowly compared to C3 and C4 plants.

Factors Affecting Photosynthesis

The rate of photosynthesis is affected by several environmental factors. Light intensity: at low light, the rate increases linearly with intensity. At high light, the rate plateaus (saturation point) because the light reactions are running at maximum speed. Very high light can cause photo-inhibition — damage to PS II from excess light energy. The light saturation point varies: shade plants saturate at lower light intensities than sun plants. CO₂ concentration: the rate increases with CO₂ concentration up to a saturation point. The current atmospheric CO₂ level (~0.04% or 400 ppm) is below saturation for C3 plants — this is why CO₂ enrichment in greenhouses increases crop yields. C4 plants reach saturation at lower CO₂ concentrations than C3 plants because the CO₂-concentrating mechanism already raises CO₂ levels around RuBisCO. Temperature: the rate increases with temperature up to an optimum (25-30°C for most C3 plants, 30-45°C for C4 plants) and then declines as enzymes denature (RuBisCO becomes less efficient and photorespiration increases at high temperatures). Water: water stress causes stomatal closure (to reduce water loss), which reduces CO₂ uptake and photosynthesis. Under severe drought, the leaf may wilt and the photosynthetic machinery may be damaged. Oxygen: high O₂ concentration increases photorespiration in C3 plants (the Warburg effect). Mineral nutrients: magnesium (for chlorophyll synthesis), nitrogen (for enzymes and chlorophyll), phosphorus (for ATP), iron and manganese (for electron transport), potassium (for stomatal opening). Blackman's law of limiting factors (1905): when a process is affected by multiple factors, the rate is limited by the factor in shortest supply. For example, if light is abundant but CO₂ is scarce, increasing light further will not increase photosynthesis — CO₂ is the limiting factor.

Photorespiration

Photorespiration is a wasteful process that occurs when RuBisCO acts as an oxygenase instead of a carboxylase. Instead of fixing CO₂ onto RuBP to make two molecules of 3-PGA, it fixes O₂ onto RuBP, producing one molecule of 3-PGA and one molecule of phosphoglycolate (2-carbon). The phosphoglycolate enters the photorespiratory pathway (also called the C2 cycle or glycolate pathway). This pathway involves three organelles: the chloroplast, peroxisome, and mitochondrion. The glycolate is converted to glycine (in peroxisomes), and two glycine molecules are converted to one serine + one CO₂ (in mitochondria, with the release of CO₂ and consumption of ATP and NAD(P)H). The serine is converted back to 3-PGA (in the peroxisome and chloroplast) and re-enters the Calvin cycle. The net result: no sugar is produced, CO₂ is released, and ATP and NADPH are consumed. Essentially, photorespiration undoes the work of photosynthesis. Photorespiration increases under conditions that favor the oxygenase activity of RuBisCO: high temperature (because O₂ becomes more soluble relative to CO₂ at higher temperatures, and RuBisCO's affinity for CO₂ decreases more than for O₂ with temperature increase), low CO₂ concentration (when stomata close to conserve water, CO₂ inside the leaf drops and O₂ builds up), and high O₂ concentration. In C3 plants, photorespiration can reduce photosynthetic efficiency by 20-50%. C4 and CAM plants minimize photorespiration through CO₂-concentrating mechanisms. There is ongoing research to engineer crops with reduced photorespiration to increase yields — this is a major goal of agricultural biotechnology.

Key Points

  • •Photosynthesis: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. Occurs in chloroplasts.
  • •Light reactions: thylakoid membranes, produce ATP and NADPH, release O₂ from water.
  • •Non-cyclic photophosphorylation: PS II + PS I, produces ATP + NADPH + O₂.
  • •Cyclic photophosphorylation: PS I only, produces ATP only — balances ATP:NADPH ratio.
  • •Photolysis: water splitting at OEC — 2H₂O → 4H⁺ + 4e⁻ + O₂.
  • •Chemiosmosis: proton gradient drives ATP synthesis via ATP synthase.
  • •Calvin cycle: stroma. Carbon fixation (RuBisCO + CO₂ → 3-PGA) → reduction (3-PGA → G3P) → regeneration (RuBP).
  • •RuBisCO: most abundant enzyme on Earth — fixes CO₂ but also O₂ (photorespiration).
  • •C3 plants: first product 3-PGA. 85% of plants. Photorespiration is a problem in heat/drought.
  • •C4 plants: spatially separate CO₂ fixation (mesophyll) and Calvin cycle (bundle sheath) — Kranz anatomy.
  • •C4 uses PEP carboxylase for initial fixation — no oxygenase activity. Minimizes photorespiration.
  • •CAM plants: temporally separate — fix CO₂ at night (as malic acid), Calvin cycle during day.
  • •Limiting factors: light, CO₂, temperature, water, minerals. Blackman's law of limiting factors.
  • •Photorespiration: RuBisCO fixes O₂ — consumes ATP/NADPH, releases CO₂, no sugar produced.
  • •Chlorophyll a: primary pigment. Accessory pigments: chlorophyll b, carotenoids (absorb and protect).
  • •Photosynthesis is the foundation of food webs and the source of atmospheric oxygen.

Practice Questions

  • Write the balanced equation for photosynthesis. Where does each stage occur in the chloroplast?
  • Explain the light reactions — non-cyclic and cyclic photophosphorylation. What are their products?
  • Describe the Calvin cycle with its three phases. What are the roles of ATP and NADPH?
  • Differentiate between C3, C4, and CAM plants with examples. What is Kranz anatomy?
  • What is photorespiration? Why is it wasteful? How do C4 plants avoid it?
  • Explain Blackman's law of limiting factors. How do light, CO₂, and temperature affect photosynthesis?
  • Describe the structure of a chloroplast with a labeled diagram. Explain the role of each part.
  • Explain the chemiosmotic hypothesis of ATP synthesis in photosynthesis. What is the role of the proton gradient?