Chapter 6: Photosynthesis in Higher Plants

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Photosynthesis Overview

Process and Importance

• Green plants are autotrophs; they make their own food via photosynthesis.


• All other organisms are heterotrophs, depending on plants for food.


• Photosynthesis uses sunlight energy to convert CO₂ and H₂O into organic compounds.


• It is the primary source of food and oxygen on Earth.

Memory Aids

• \"Auto\" means self; plants make their own food.


• Photosynthesis = Photo (light) + synthesis (putting together).

Quick Checkpoints

• What organisms are autotrophs?


• Why is photosynthesis vital for life?

Chemical Equation

Balanced Equation

• 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂


• Carbon dioxide and water convert into glucose and oxygen using sunlight.


• Chlorophyll and enzymes facilitate this reaction.

Memory Aids

• Remember: 6 CO2 + 6 H2O → glucose + 6 O2.


• Oxygen is a byproduct, not a reactant.

Quick Checkpoints

• Identify reactants and products in photosynthesis.


• What role does sunlight play?

Requirements for Photosynthesis

Essential Factors

• Chlorophyll (green pigment) is necessary.


• Light energy is required.


• Carbon dioxide must be available.


• Experiments show photosynthesis occurs only in green parts exposed to light.

Memory Aids

• Think: Chlorophyll + Light + CO2 = Photosynthesis.


• Green parts only perform photosynthesis.

Quick Checkpoints

• Why do only green parts photosynthesize?


• What happens if a leaf is covered and not exposed to light?

Early Experiments

Moll's Half-Leaf Experiment

• Part of leaf enclosed in KOH-soaked cotton absorbs CO₂; other half exposed to air.


• After light exposure, exposed half tests positive for starch; enclosed half negative.


• Conclusion: CO₂ is essential for photosynthesis.

Priestley's Experiment

• Burning candle or animal respiration produces \"foul air\" (phlogiston).


• Plants convert this into \"pure air\" (dephlogiston), restoring air quality.


• Demonstrated plants release oxygen needed for animal survival.

Priestley's experiment steps:

• Mouse and burning candle in closed jar consume oxygen; mouse dies, candle extinguishes.


• Adding a mint plant produces oxygen; mouse survives, candle burns.


• Shows plants release oxygen essential for respiration.

Memory Aids

• Moll: Half leaf + KOH = CO₂ needed.


• Priestley: Plants purify air by releasing oxygen.

Quick Checkpoints

• What does Moll's experiment prove about CO₂?


• How did Priestley demonstrate oxygen release by plants?

Early Research Milestones

Key Discoveries

• 1727: Stephen Hales - Plants need sunlight, air, and green leaves to grow.


• 1770-1773: Joseph Priestley - Introduced \"foul air\" and \"pure air\" concepts; oxygen discovery.


• 1779: Jan Ingenhousz - Plants release oxygen bubbles only in sunlight.


• 1845: Von Mayer - Plants convert sunlight into chemical energy.


• 1854: Julius von Sachs - Plants produce glucose stored as starch, first photosynthesis product.


• 1884-1858: Engelmann - Light color affects photosynthesis; studied with algae and bacteria.


• 1905: Blackmann - Identified limiting factors and dark phase in photosynthesis.


• 1931: Cornelius van Neil - Photosynthesis depends on light; hydrogen reduces CO₂ to sugar.

Memory Aids

• Hales to Neil: Sunlight, air, oxygen, energy, starch, light color, dark phase, hydrogen role.

Quick Checkpoints

• Who discovered oxygen release only in sunlight?


• What is the first product of photosynthesis?


• What did Blackmann identify about photosynthesis?

Modern Discoveries

Important Findings (1937-1966)

• 1937: Hill - Oxygen released during light reaction using chloroplasts.


• 1941: Ruben & Kamen - Oxygen originates from water, not CO₂.


• 1954: Arnon et al. - Photophosphorylation and radioactive carbon for carbon fixation study.


• 1954-35: Calvin - Traced CO₂ use in Calvin cycle; Nobel Prize awarded.


• 1957: Emerson - Discovered Emerson effect in photosynthesis.


• 1960: Hill & Bendall - Proposed Z-scheme with two photosystems.


• 1961: Peter Mitchell - Chemiosmotic theory explaining ATP synthesis.


• 1966: Hatch & Slack - Discovered C4 pathway for carbon fixation in tropical plants.

Memory Aids

• Hill to Hatch: Oxygen source, energy production, Calvin cycle, Emerson effect, Z-scheme, ATP, C4 pathway.

Quick Checkpoints

• What did Ruben and Kamen prove about oxygen source?


• What is the significance of the Calvin cycle?


• What does the Z-scheme explain?


• What is the C4 pathway?

Where Does Photosynthesis Take Place?

Chloroplasts and Their Structure

• Photosynthesis occurs mainly in green leaves and other green parts of plants.


• Mesophyll cells contain numerous chloroplasts, the green plastids where photosynthesis happens.


• Chloroplasts are discoid, ellipsoidal, or biconvex lens-shaped organelles, 4–10 μm long and 1–3 μm wide.


• Each chloroplast has a double membrane envelope acting as a selective barrier for metabolites.


• The inner space, called stroma, is a protein-rich matrix containing all enzymes needed for photosynthesis.


• Chloroplasts align along mesophyll cell walls to optimize light absorption.


• Membrane systems trap light energy and synthesize ATP and NADPH (light reactions).


• Stroma hosts enzymatic reactions incorporating CO₂ into sugars, forming starch (dark reactions).


• Thylakoids are flattened sacs stacked into grana, interconnected by stroma lamellae.


• Photosystems I and II are located in thylakoid membranes; PSI in stroma thylakoids, PSII in granal thylakoids.

Memory Aids

• Chloroplast = Green plastid + Double membrane + Stroma + Thylakoids.


• Light reactions = Thylakoid membranes; Dark reactions = Stroma enzymes.


• PSI in stroma thylakoids, PSII in granal thylakoids.

Quick Checkpoints

• Where in the plant cell does photosynthesis occur?


• What is the role of the chloroplast envelope?


• What are the main components of chloroplast involved in photosynthesis?


• Where do light and dark reactions take place within the chloroplast?


• How are photosystems distributed in thylakoid membranes?

Types of Photosynthetic Pigments

Overview

• Leaf color results from four pigments: Chlorophyll a (blue-green), Chlorophyll b (yellow-green), Xanthophylls (yellow), and Carotenoids (yellow to yellow-orange).

Memory Aids

• Four pigments: two chlorophylls + xanthophylls + carotenoids.

Quick Checkpoints

• What pigments contribute to leaf color?


• Which pigment is blue-green in chromatogram?

Chlorophylls

Key Points

• Green photosynthetic pigments; five types (a, b, c, d, e) in plants except bacteria.


• Only chlorophyll a and b occur in higher plant chloroplasts; chlorophyll b is ~25% of total chlorophyll.


• Chlorophyll a is universal and primary pigment; converts light energy to chemical/electrical energy.


• Other pigments absorb different wavelengths and transfer energy to chlorophyll a; called accessory pigments.


• Some chlorophyll a molecules act as reaction centers (P700, P680) for charge separation.


• Structure: porphyrin head (four pyrrole rings linked by methine bridges) with central magnesium atom, and phytol tail anchoring molecule in thylakoid membrane.


• Phytol formula: C20H40O; insoluble long carbon chain.

Memory Aids

• Chlorophyll a = primary, universal pigment.


• Phytol tail anchors chlorophyll in membrane.


• Reaction centers: P700 and P680.

Quick Checkpoints

• Which chlorophyll types are in higher plants?


• What is the role of chlorophyll a?


• Describe chlorophyll structure components.

Carotenoids

Key Points

• Yellow, brown to reddish pigments in chloroplasts and chromoplasts.


• Accessory pigments along with chlorophyll b; transfer energy to chlorophyll a.


• Two types: carotenes (orange hydrocarbons, C40H56) and xanthophylls (oxygenated derivatives).


• Carotenes include lycopene (red pigment in tomato, chili) and β-carotene (precursor of vitamin A).


• Xanthophylls include cryptoxanthin (C40H56O), lutein, and zeaxanthin (C40H56O2).


• Soluble in organic solvents; carotenes more soluble in carbon disulphide than xanthophylls.

Memory Aids

• Carotenes = orange, vitamin A precursors.


• Xanthophylls = yellow, oxygen-containing.

Quick Checkpoints

• What are the two types of carotenoids?


• Which carotenoid is vitamin A precursor?


• Where are carotenoids located?

Phycobilins

Key Points

• Accessory pigments in cyanobacteria, cryptomonads, and red algae.


• Water-soluble, protein-linked pigments in phycobilisomes attached to thylakoids.


• Open tetrapyrrole structure without magnesium or phytol tail.


• Two types: phycocyanin and phycoerythrin.

Memory Aids

• Phycobilins = water-soluble, in blue-green and red algae.


• Phycobilisomes = pigment granules on thylakoids.

Quick Checkpoints

• Which organisms have phycobilins?


• What distinguishes phycobilins structurally?

Action and Absorption Spectrum

Key Points

• Pigments absorb light at specific wavelengths; absorption spectrum shows energy absorbed vs wavelength.


• Chlorophyll a and b absorb mostly in blue and red regions, matching high photosynthesis rates.


• Action spectrum shows photosynthesis rate vs wavelength; peaks at blue (~430 nm) and red (~680 nm) light.


• Carotenoids absorb mainly in blue-green region (400-500 nm).


• Green-yellow light is least effective; explains green leaf color (reflection of green light).


• Accessory pigments broaden usable light spectrum and protect chlorophyll a from photo-oxidation.

Memory Aids

• Photosynthesis peaks at blue and red light.


• Green light is least absorbed, hence leaves appear green.


• Accessory pigments extend light absorption range.

Quick Checkpoints

• At which wavelengths is photosynthesis highest?


• Why do leaves appear green?


• Role of accessory pigments in light absorption?

Light Reaction

Overview

• Light reaction (photochemical phase) involves light absorption, water splitting, oxygen release, and formation of ATP and NADPH.


• Occurs in two photosystems: Photosystem I (PS I) and Photosystem II (PS II), each with distinct light harvesting complexes (LHC).


• LHCs contain pigment molecules (chlorophylls and carotenoids) that absorb light and transfer energy to the reaction center.


• Reaction centers have a special chlorophyll a molecule: P700 in PS I (absorbs at 700 nm) and P680 in PS II (absorbs at 680 nm).


• Energy transfer steps: photon hits pigment → energy passed among pigments → reaches reaction center → transferred to primary acceptor initiating chemical energy conversion.

Photosystem Details

• Photosystem I: More chlorophyll a, less chlorophyll b and carotenoids; involved in cyclic and non-cyclic photophosphorylation; core antenna and P700 bound to proteins Psa A and Psa B; contains Fe-S centers (ferredoxins).


• Photosystem II: Equal chlorophyll a and b, higher carotenoids; involved only in non-cyclic photophosphorylation; contains cytochrome b6f complex and plastocyanin; reaction center proteins D1 and D2 bind P680, additional pigments, and electron carriers.

Memory Aids

• PS I = P700, involved in both cyclic & non-cyclic pathways.


• PS II = P680, only non-cyclic pathway.


• LHC = antennae that capture and funnel light energy.

Quick Checkpoints

• What are the main functions of light reactions?


• Difference between P700 and P680?


• Role of light harvesting complexes?


• Which photosystem participates in cyclic photophosphorylation?

Emerson’s Enhancement Effect

Explanation

• Monochromatic light >680 nm causes a sharp drop in photosynthesis rate (red drop).


• Shorter wavelengths alone are also ineffective.


• Simultaneous exposure to both longer and shorter wavelengths increases photosynthesis rate.


• Effect explained by presence of two pigment systems and different harvesting molecules in photosynthetic units.

Memory Aids

• Red drop = reduced photosynthesis with >680 nm light alone.


• Enhancement effect = combined wavelengths boost photosynthesis.

Quick Checkpoints

• What causes the red drop in photosynthesis?


• How does simultaneous light of different wavelengths affect photosynthesis?


• What does Emerson’s effect reveal about photosystems?

Non-cyclic Photophosphorylation

Process Overview

• ATP synthesis by cells is called phosphorylation.


• Non-cyclic photophosphorylation involves both Photosystem II (PS II) and Photosystem I (PS I) working in series.


• Electrons excited in PS II (absorbing 680 nm light) move downhill through electron carriers to PS I.


• PS I (absorbing 700 nm light) excites electrons again, which reduce NADP⁺ to NADPH + H⁺.


• This electron flow produces both ATP and NADPH + H⁺.


• The electron transfer path forms a characteristic \"Z scheme\" on the redox potential scale.

Memory Aids

• \"Z scheme\" shape = electron flow from PS II to PS I.


• PS II absorbs 680 nm, PS I absorbs 700 nm light.


• ATP and NADPH produced together only in non-cyclic flow.

Quick Checkpoints

• What are the two photosystems involved?


• Which molecule is reduced to NADPH?


• What is the significance of the \"Z scheme\"?

Splitting of Water

Key Points

• Electrons lost from PS II are replaced by electrons from water splitting.


• Water splits into protons (H⁺), oxygen (O), and electrons.


• This process produces oxygen as a net product of photosynthesis.


• Reaction: 2H₂O ⟶ 4H⁺ + O₂ + 4e^-


• Water splitting complex is located on the inner side of the thylakoid membrane associated with PS II.

Memory Aids

• Water splitting replenishes electrons in PS II.


• Oxygen released is from water, not CO₂.

Quick Checkpoints

• What replaces electrons lost by PS II?


• Where is the water splitting complex located?


• What is the net product of water splitting?

Cyclic Photophosphorylation

Process Details

• Only Photosystem I (PS I) is involved.


• Electron expelled by excited P₇₀₀ returns to the same photocentre after passing through electron carriers.


• Occurs under low light, wavelengths >680 nm, or when CO₂ fixation is inhibited.


• Electron carriers involved: P₇₀₀ chlorophyll, plastoquinone (PQ), FeS complex, ferredoxin (Fd), cytochrome b₆f, plastocyanin.


• Produces ATP only; no NADPH or oxygen formed.

Memory Aids

• Cycle returns electrons to P₇₀₀.


• ATP produced without NADPH or O₂.


• Active when CO₂ fixation is low.

Quick Checkpoints

• Which photosystem is involved?


• What is the fate of the electron?


• What products are formed?

Chemiosmotic Hypothesis

Core Concepts

• Proposed by Peter Mitchell in 1961 to explain ATP synthesis.


• ATP synthesis linked to proton gradient across thylakoid membrane.


• Protons accumulate inside thylakoid lumen due to water splitting and electron transport.


• Electron transport moves protons from stroma to lumen, creating a proton gradient and pH difference.


• Protons flow back to stroma through ATP synthase (CF₀-CF₁ complex), driving ATP synthesis.


• ATP synthase has two parts: CF₀ (membrane channel) and CF₁ (catalytic unit facing stroma).


• ATP and NADPH produced are used for CO₂ fixation and sugar synthesis in stroma.

Memory Aids

• Proton gradient powers ATP synthase.


• CF₀ = proton channel; CF₁ = ATP synthesis.


• Energy from proton flow activates ATP production.

Quick Checkpoints

• Who proposed the chemiosmotic hypothesis?


• Where do protons accumulate during photosynthesis?


• What drives ATP synthesis?

Where are the ATP and NADPH Used?

Role in Photosynthesis

• Light reaction produces ATP, NADPH, and O\u0002; O\u0002 diffuses out of chloroplast.


• ATP and NADPH drive the biosynthetic (dark) phase for sugar synthesis.


• Dark phase depends on ATP, NADPH, CO\u00022, and H\u0002O, not directly on light.


• Occurs in chloroplast stroma where necessary enzymes are present.


• Discovered by F.F. Blackman; detailed by Calvin, Benson, and J. Bassham (Nobel Prize 1961).

Memory Aids

• \"ATP & NADPH fuel dark phase\"


• \"Dark phase = sugar synthesis in stroma\"


• \"Oxygen leaves, energy carriers stay\"

Quick Checkpoints

• What products of light reaction are used in the dark phase?


• Where in the chloroplast does the dark phase occur?


• Who established the dark reaction?

The Calvin Cycle

Phases and Process

• Fixes CO\u00022 using ATP and NADPH to form glucose.


• Primary CO\u00022 acceptor: Ribulose-1,5-bisphosphate (RuBP), a 5-carbon sugar.


• First stable product: 3-Phosphoglyceric acid (3-PGA).


• Divided into three phases: carboxylation, reduction, regeneration.


• Carboxylation: CO\u00022 + RuBP catalyzed by RuBisCO forms 3-PGA.


• Reduction: 3-PGA converted to triose phosphate using 2 ATP and 2 NADPH per CO\u00022.


• Regeneration: RuBP regenerated using 1 ATP to continue cycle.


• Six CO\u00022 molecules and six turns produce one glucose molecule.

Memory Aids

• \"RuBisCO fixes CO\u00022 to RuBP\"


• \"3 phases: Carboxylation, Reduction, Regeneration\"


• \"6 CO\u00022 = 1 glucose\"


• \"ATP & NADPH used in reduction and regeneration\"

Quick Checkpoints

• What is the first stable product of the Calvin cycle?


• Which enzyme catalyzes CO\u00022 fixation?


• How many ATP and NADPH molecules are used per CO\u00022 fixed?


• What is regenerated to keep the cycle going?

Net Reaction of C\u00023 Dark Fixation

Overall Equation

• 6 RuBP + 6 CO\u00022 + 18 ATP + 12 NADPH \u00021rarr; 6 RuBP + C\u00063H\u000612O\u00066 + 18 ADP + 18 Pi + 12 NADP\u0002 + 6 H\u0002O


• Shows ATP and NADPH consumption and glucose formation.

Memory Aids

• \"18 ATP and 12 NADPH per glucose\"


• \"6 CO\u00022 fixed per glucose\"

Quick Checkpoints

• How many ATP molecules are used to form one glucose?


• How many NADPH molecules are consumed per glucose?


• What are the products of the dark fixation reaction?

C4 Pathway

Overview and Anatomy

• Also called Hatch-Slack Pathway; found in tropical monocots and dicots like maize, sugarcane, Sorghum, Atriplex, Amaranthus.


• Called C4 plants because first stable product is a 4-carbon compound.


• Have Kranz anatomy: vascular bundles surrounded by large bundle sheath cells arranged in wreath-like layers.


• Mesophyll cells lack RuBisCO, contain PEP carboxylase (PEPcase) for initial CO2 fixation; chloroplasts lack starch.


• Bundle sheath cells have agranal chloroplasts, contain RuBisCO; protected from O2; perform Calvin cycle.

Biochemical Steps

• CO2 combines with phosphoenolpyruvate (PEP) via PEPcase to form oxaloacetate (OAA).


• OAA is reduced to malic acid or transaminated to aspartic acid.


• Malic/aspartic acid transported to bundle sheath cells.


• Decarboxylation releases CO2 and pyruvate in bundle sheath cells.


• CO2 enters Calvin cycle; pyruvate returns to mesophyll to regenerate PEP.


• Overall reaction: 6 PEP + 6 RuBP + 6 CO2 + 30 ATP + 12 NADPH → glucose + regenerates PEP and RuBP.

Memory Aids

• C4 = 4-carbon first product (oxaloacetate).


• Kranz = \"wreath\" anatomy around vascular bundles.


• PEPcase fixes CO2 in mesophyll; RuBisCO acts in bundle sheath.

Quick Checkpoints

• What enzyme fixes CO2 first in C4 plants?


• Where does the Calvin cycle occur in C4 plants?


• What is the first stable product of CO2 fixation?

Comparison C3 and C4 Plants

Key Differences

• C3 plants: simple leaf anatomy; C4 plants: Kranz anatomy with bundle sheath cells.


• Optimal temperature: C3 (20–25°C), C4 (30–45°C).


• CO2 compensation point: higher in C3 (25–100 ppm), lower in C4 (0–10 ppm).


• Photorespiration: high in C3, negligible in C4.


• Photosynthesis: C3 uses Calvin cycle in mesophyll; C4 separates initial CO2 fixation and Calvin cycle spatially.


• C4 plants more efficient in hot, dry, low CO2 environments.

Memory Aids

• C4 = 4-carbon compound, Kranz anatomy, low photorespiration.


• C3 = Calvin cycle only, higher photorespiration.

Quick Checkpoints

• Which plant type has Kranz anatomy?


• How does photorespiration differ between C3 and C4 plants?


• At what temperature do C4 plants perform best?

CAM Cycle

Mechanism and Adaptation

• Occurs in succulents of Crassulaceae family.


• Stomata open at night; CO2 fixed by PEP carboxylase into malic acid stored in vacuoles.


• Stomata closed during day; malic acid decarboxylated releasing CO2 for Calvin cycle.


• Both initial CO2 fixation and Calvin cycle occur in same cells but at different times (night and day).


• Adaptation reduces water loss while allowing photosynthesis.

Memory Aids

• CAM = Crassulacean Acid Metabolism, night CO2 fixation.


• Malic acid storage in vacuoles at night.


• Daytime CO2 release for Calvin cycle.

Quick Checkpoints

• When are stomata open in CAM plants?


• What is the storage form of CO2 at night?


• How does CAM reduce water loss?

Photorespiration

Process and Sites

• Occurs in chloroplast, peroxisome, and mitochondrion.


• RuBP carboxylase-oxygenase (RuBisCO) enzyme catalyses the reaction.


• At high temperatures, RuBisCO acts as oxygenase, fixing O₂ instead of CO₂.


• Produces 3-phosphoglyceric acid (3-PGA) and 2-phosphoglycolate.


• Photorespiration is cyclic and consumes energy without producing ATP, NADPH, or sugars.

Photorespiration in C₄ Plants

• C₄ plants avoid photorespiration by concentrating CO₂ in bundle sheath cells.


• C₄ acids from mesophyll release CO₂ in bundle sheath cells, increasing CO₂ concentration.


• High CO₂ ensures RuBisCO acts as carboxylase, reducing oxygenase activity.


• Results in higher productivity and heat tolerance in C₄ plants.

Memory Aids

• RuBisCO = RuBP carboxylase-oxygenase (dual function enzyme).


• Photorespiration wastes energy and releases CO₂.


• C₄ plants concentrate CO₂ to prevent photorespiration.

Quick Checkpoints

• What organelles are involved in photorespiration?


• How does RuBisCO function change with temperature?


• Why do C₄ plants not undergo photorespiration?

Photorespiratory Pathway Steps

• RuBisCO oxygenase activity in chloroplast produces phosphoglycolate.


• Phosphoglycolate converts to glycolate, transported to peroxisome.


• In peroxisome, glycolate converts to glyoxylate then glycine.


• Glycine moves to mitochondrion, breaks down into CO₂, NH₃, and serine, releasing NADH.


• Serine returns to peroxisome, then chloroplast.


• Serine converts to 3-phosphoglycerate in chloroplast, re-enters Calvin cycle.


• Cycle salvages carbon but releases CO₂ and consumes energy, reducing efficiency.

CO\b9 Concentration

Subtopics

• Increasing CO\b9 concentration up to 0.05% boosts photosynthesis in most C\b3 plants.


• CO\b9 compensation point: CO\b9 level where photosynthesis CO\b9 fixation equals CO\b9 release by respiration and photorespiration.


• Compensation point ranges: 25-100 ppm in C\b3 plants, 0-10 ppm in C\b4 plants.


• C\b4 plants saturate at ~360 \u0003bcL L\u0000b9\u0000b9; C\b3 plants saturate beyond 450 \u0003bcL L\u0000b9\u0000b9.

Memory Aids

• CO\b9 compensation point = balance of CO\b9 fixation and release.


• C\b4 plants saturate CO\b9 earlier than C\b3 plants.

Quick Checkpoints

• What is the CO\b9 compensation point?


• How do C\b3 and C\b4 plants differ in CO\b9 saturation?

Light

Subtopics

• Light compensation point: Light intensity where photosynthesis gas exchange is zero.


• Photosynthesis rate rises with light intensity until light saturation point.


• Light saturation occurs at ~10% full sunlight.


• Excess light breaks down chlorophyll, reducing photosynthesis.

Memory Aids

• Light compensation point = photosynthesis equals respiration.


• Light saturation point = maximum photosynthesis light intensity.

Quick Checkpoints

• Define light compensation and saturation points.


• What happens beyond light saturation?

Temperature

Subtopics

• Photosynthesis rate doubles with every 10\u0000b0C rise from minimum to optimum temperature.


• Above optimum, rate briefly rises then declines over time (time factor).


• Tropical plants have higher optimum temperatures than temperate plants.

Memory Aids

• Rate doubles per 10\u0000b0C rise until optimum.


• Time factor causes decline after optimum temperature.

Quick Checkpoints

• How does temperature affect photosynthesis rate?


• What is the time factor?

Oxygen

Subtopics

• High oxygen levels reduce photosynthesis rate.


• This reduction is called the Warburg effect.

Memory Aids

• Warburg effect = oxygen inhibits photosynthesis at high levels.

Quick Checkpoints

• What is the Warburg effect?


• How does oxygen concentration affect photosynthesis?

Water

Subtopics

• Water used in photosynthesis is minimal.


• Photosynthesis is highly sensitive to dehydration.

Memory Aids

• Small water use but critical for photosynthesis.


• Dehydration quickly reduces photosynthesis.

Quick Checkpoints

• How important is water for photosynthesis?


• What is the effect of dehydration?

Limiting Factor

Subtopics

• A limiting factor is deficient enough to restrict the process rate.


• Increasing a limiting factor increases photosynthesis rate.


• F.F. Blackman (1905) proposed the law of limiting factors.


• Based on Liebig's law of minimum: process rate limited by slowest factor.

Memory Aids

• Limiting factor = slowest step controls rate.


• Blackman's law explains factor limitation.

Quick Checkpoints

• Define limiting factor in photosynthesis.


• What does Blackman's law state?

Light Intensity Graph

Subtopics

• Region A: Low light, photosynthesis rate rises rapidly; light is limiting.


• Region B: Medium light, rate rises slower; other factors start limiting.


• Region C: High light, rate plateaus; other factors limit photosynthesis.


• Point E: Maximum photosynthesis rate.


• Point D: Light intensity where photosynthesis stops increasing.


• Photosynthesis depends on light intensity only up to a point.

Memory Aids

• Photosynthesis rate vs light intensity is a saturation curve.


• Beyond saturation, other factors limit the rate.

Quick Checkpoints

• What limits photosynthesis at low vs high light?


• What do points D and E represent?

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