Chapter 10 Notes — Photosynthesis (Campbell Biology, 12th Edition)
Chapter 10 Notes — Photosynthesis (Campbell Biology, 12th Edition)
Photosynthesis: overview and significance
Photosynthesis is the process that converts solar energy into chemical energy within chloroplasts.
It nourishes almost the entire living world directly or indirectly.
Overall concept: Photosynthesis feeds the biosphere.
Equation (overall, as presented):
The process consists of two main stages: the light reactions (photo part) and the Calvin cycle (synthesis part).
The light reactions convert light energy to chemical energy (ATP and NADPH); the Calvin cycle uses ATP and NADPH to reduce CO2 to carbohydrate.
Autotrophs, phototrophs, and the biosphere
Autotrophs are self-feeders that sustain themselves without eating other organisms; they are the producers of the biosphere and synthesize organic molecules from CO2 and inorganic molecules.
Almost all plants are photoautotrophs (use sunlight to make organic molecules); photosynthesis also occurs in algae, some protists, and some prokaryotes.
Heterotrophs obtain organic material from other organisms and are the consumers; some decompose dead material.
Chloroplasts: structure and the site of photosynthesis
Chloroplasts are the sites of photosynthesis in plants and are structurally similar to photosynthetic bacteria.
Most photosynthesis in plants occurs in the leaves, specifically in mesophyll cells.
Gas exchange: CO2 enters and O2 exits leaves via stomata; water transport from roots via veins.
Chloroplast organization:
Envelope with two membranes surrounding a fluid stroma.
Thylakoids: interconnected sacs forming a third membrane system; may be stacked into grana.
Chlorophyll resides in the thylakoid membranes.
Light reactions: energy capture and electron transport
Location: thylakoid membranes.
Main outputs: ATP and NADPH; by-product: O2 from water splitting.
Two photosystems cooperate: Photosystem II (PS II) and Photosystem I (PS I).
PS II reaction center chlorophyll a is P680; PS I reaction center chlorophyll a is P700.
Each photosystem associates with light-harvesting complexes that funnel energy to the reaction-center chlorophylls.
Water splitting (photolysis) provides electrons, protons, and O2:
Electron transport chain (ETC) components (linear flow):
PS II primary electron acceptor receives excited electrons from P680.
Electron transport chain includes plastoquinone (Pq), a cytochrome complex, and plastocyanin (Pc).
Electron flow oxidizes P680 to P680+; energy release pumps protons into the thylakoid space, creating a proton gradient.
The proton gradient drives ATP synthesis by chemiosmosis via ATP synthase.
Electrons are transferred from PS II to PS I via the electron transport chain.
Linear electron flow (the primary pathway):
1) Photon excites a pigment in the PS II light-harvesting complex; energy is transferred to P680.
2) An excited electron is transferred from P680 to the primary electron acceptor (P680+).
3) Water is split to replace electrons; electrons reduce P680+ back to P680; H+ released into the thylakoid space; O2 is produced from the oxygen atoms.
4) Electrons pass through the electron transport chain (Pq, cytochrome complex, Pc) to PS I; the flow pumps H+ into the thylakoid space.
5) Proton gradient drives ATP synthesis by chemiosmosis.
6) In PS I, P700 is excited and donates electrons to its primary electron acceptor, becoming P700+.
7) Electrons are transferred from PS II to PS I and then down a second electron transport chain to ferredoxin (Fd); this step does not pump protons or produce ATP in the described pathway.
8) NADP+ reductase catalyzes transfer of electrons from Fd to NADP+, forming NADPH; two electrons are needed to reduce NADP+ to NADPH; this also removes a proton from the stroma.The two routes of electron flow in the light reactions:
Linear electron flow: uses both photosystems; produces ATP and NADPH; fuels the Calvin cycle.
Cyclic electron flow: uses only PSI (photosystem I); generates extra ATP but no NADPH (per the provided notes, this pathway does not produce a proton gradient or ATP in the described step; traditionally cyclic flow increases ATP synthase activity without producing NADPH).
Connections to chemiosmosis: both chloroplasts and mitochondria generate ATP by chemiosmosis, driven by proton gradients across membranes; ATP synthase couples H+ diffusion to ATP formation.
Energy carriers produced in light reactions: ATP and NADPH; used by the Calvin cycle in the stroma.
The two photosystems and the light-harvesting apparatus
A photosystem consists of a reaction-center complex surrounded by light-harvesting complexes.
Reaction-center complex contains a special pair of chlorophyll a molecules and a primary electron acceptor.
Light-harvesting complexes: arrays of pigment molecules bound to proteins; transfer absorbed energy to the reaction-center chlorophyll a.
Primary electron acceptor in the reaction center accepts excited electrons, becoming reduced; initiates the electron transport chain.
PS II and PS I are numbered in order of discovery; PS II is P680 and PS I is P700 in absorbance terminology.
Diagrammatic integration of components: Photosystem II → electron transport chain (Pq, cytochrome complex, Pc) → plastocyanin → Photosystem I → ferredoxin → NADP+ reductase → NADPH.
The Calvin cycle (carbon fixation and sugar synthesis)
Location: stroma of chloroplasts.
Overall purpose: convert CO2 into carbohydrate using ATP and NADPH from light reactions.
Key features:
Carbon fixation begins by incorporating CO2 into a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP), catalyzed by Rubisco (RuBP carboxylase-oxygenase).
The resulting six-carbon intermediate splits into two molecules of 3-phosphoglycerate (3-PGA).
Reduction and phosphorylation convert 3-PGA into glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.
For every three CO2 molecules fixed, six molecules of G3P are formed; only one G3P is net gain for the synthesis of carbohydrates.
The remaining five G3P are rearranged to regenerate three molecules of RuBP, a process that consumes additional ATP.
Net inputs/outputs per G3P and per cycle:
Net synthesis of one G3P requires:
Carbon enters as CO2 and leaves as sugar (G3P). The cycle turns three times (three CO2 fixed) to yield one G3P.
Phases of the Calvin cycle:
Phase 1: Carbon fixation — CO2 + RuBP → 2 × 3-PGA (via Rubisco).
Phase 2: Reduction — 3-PGA is phosphorylated by ATP and reduced by NADPH to G3P; 3 CO2 fixed yield 6 × 3-PGA and 6 NADPH used; net gain after three turns is 1 G3P.
Phase 3: Regeneration — The remaining five G3P molecules are rearranged to regenerate RuBP; requires ATP (3 additional ATP per 3 CO2 fixed).
Outputs and downstream use:
G3P serves as a starting point for syntheses of glucose, sucrose, and other carbohydrates and structural molecules.
The Calvin cycle is anabolic: builds sugar using ATP and NADPH.
Connecting light reactions and the Calvin cycle
The light reactions supply the Calvin cycle with ATP and NADPH; the Calvin cycle returns ADP, Pi, and NADP+ to the light reactions.
The Calvin cycle operates in the stroma; the light reactions occur in the thylakoid membranes.
The energy and reducing power flow: Light reactions convert solar energy to ATP and NADPH; Calvin cycle uses those to fix carbon into sugars.
The Calvin cycle is the sugar-creating core following carbon fixation and reduction; the cycle regenerates CO2 acceptor RuBP for continued CO2 fixation.
Pigments and light capture
Light absorbers are pigments: chlorophyll a (main light-capturing pigment in reactions), chlorophyll b (accessory pigment), and carotenoids (accessory pigments; yellow/orange).
Absorption vs action spectra:
Absorption spectrum of chlorophyll a shows peaks in violet-blue and red light; green is least absorbed and thus reflected, giving leaves their green color.
Action spectrum for photosynthesis (effectiveness of light at different wavelengths) also shows violet-blue and red as most effective.
The action spectrum is broader than the chlorophyll a absorption spectrum, broadened by accessory pigments (chlorophyll b and carotenoids).
Carotenoids serve photoprotective roles, absorbing excess light that could damage chlorophyll or react with oxygen.
When a pigment absorbs light, electrons are excited from ground to excited states; excited electrons release energy as heat or light if not captured by the reaction center.
Basic energy conservation: the energy of a photon is inversely related to its wavelength; shorter wavelengths have higher energy per photon. A useful formalism is where h is Planck’s constant, c is the speed of light, and \lambda is wavelength.
The two photosystems in detail
Photosystem II (PS II): reaction-center chlorophyll a is P680; best absorbed at 680 nm.
Photosystem I (PS I): reaction-center chlorophyll a is P700; best absorbed at 700 nm.
A photosystem is a complex of pigment molecules bound to proteins; energy transfer from light-harvesting complexes funnels energy to the reaction-center chlorophyll a, enabling electron transfer to the primary electron acceptor.
In PS II, the primary electron acceptor accepts excited electrons from P680, which becomes P680+. The split of water replenishes electrons and releases O2.
In PS I, P700 becomes excited and donates an electron to its primary acceptor, becoming P700+. The electron travels through an electron transport chain to ferredoxin (Fd) and NADP+ reductase to form NADPH.
The two photosystems are connected by an electron transport chain that includes plastoquinone (Pq), a cytochrome complex, and plastocyanin (Pc).
NADP+ reductase catalyzes the transfer of electrons from ferredoxin to NADP+, producing NADPH; two electrons are required to reduce NADP+ to NADPH, and this process also consumes a proton from the stroma.
The energy captured by light drives the transfer of electrons and the pumping of protons across the thylakoid membrane, establishing a proton gradient used to synthesize ATP.
Cyclic and linear electron flow: routes of electron transport
Linear electron flow (the primary pathway): involves both PS II and PS I; produces ATP and NADPH, driven by light energy; supports the Calvin cycle.
Cyclic electron flow: involves only PS I; electrons cycle back from ferredoxin to the plastoquinone/cytochrome complex, increasing proton gradient and ATP production without making NADPH.
In linear flow, NADPH is produced to reduce carbon in the Calvin cycle, while ATP is used for phosphorylation steps in Calvin cycle; both are required in the correct stoichiometry.
The two pathways illustrate how chloroplasts balance the ATP/NADPH requirement for carbon fixation under varying environmental conditions.
The Calvin cycle in depth (carbon fixation, reduction, regeneration)
Carbon enters as CO2 and leaves as glyceraldehyde-3-phosphate (G3P).
For net synthesis of one G3P, CO2 must enter the cycle three times (fix three CO2 molecules).
The cycle has three phases:
Phase 1: Carbon fixation — CO2 binds RuBP (a five-carbon sugar) via rubisco to form an unstable six-carbon intermediate that splits into two 3-PGA molecules.
Phase 2: Reduction — 3-PGA is phosphorylated by ATP and reduced by NADPH to glyceraldehyde-3-phosphate (G3P). For every three CO2 fixed, six molecules of G3P are formed; only one G3P is net carbohydrate product.
Phase 3: Regeneration of RuBP — The remaining five G3P molecules are rearranged to regenerate three RuBP molecules. This regeneration consumes three additional ATP per turn.
Net stoichiometry per 3 CO2 fixed:
ATP:
NADPH:
Product: one net G3P (which can be used to synthesize glucose and other carbohydrates).
The Calvin cycle begins with carbon fixation and ends with regeneration of the CO2 acceptor RuBP, enabling continued carbon fixation.
The G3P produced in the Calvin cycle is a building block for glucose, sucrose, starch, and other organic molecules; the cycle is a hub for cellular carbon metabolism.
Carbon fixation adaptations: C3, C4, and CAM pathways
Many plants are C3 plants; the initial fixation of CO2 via rubisco yields a 3-carbon compound (3-PGA).
Photorespiration: when stomata close under hot, dry conditions, O2 builds up relative to CO2, leading rubisco to fix O2 instead of CO2 and produce a two-carbon compound; this is energetically wasteful for photosynthesis.
C4 plants minimize photorespiration by fixing CO2 into a four-carbon compound (via PEP carboxylase) in mesophyll cells; the four-carbon compound is transported to bundle-sheath cells where CO2 is released for use in the Calvin cycle.
C4 has evolved multiple times across many species and families (e.g., sugarcane, maize).
In hot, dry weather, stomata partially close to conserve water; CO2 availability falls, favoring C3 photorespiration unless C4 pathway is present.
Sugar production in C4 plants occurs through a three-step process: (1) production of four-carbon precursors via PEP carboxylase in mesophyll cells; (2) export to bundle-sheath cells; (3) release of CO2 in bundle-sheath cells for the Calvin cycle. Pyruvate returned to mesophyll cells, with ATP used to convert it back to PEP (ATP generated by cyclic electron flow).
CAM (Crassulacean Acid Metabolism) plants adapt to arid environments by temporal separation of carbon fixation and the Calvin cycle:
At night, CAM plants open stomata and fix CO2 into organic acids stored in vacuoles.
During the day, stomata close to conserve water; CO2 is released from stored organic acids and used in the Calvin cycle.
The C4 and CAM pathways illustrate alternative carbon fixation strategies that reduce water loss and enhance photosynthetic efficiency under hot, dry climates.
Global context:
CO2 levels have risen since the Industrial Revolution, affecting C3 and C4 plants differently; this has implications for plant communities and agriculture.
Agricultural interest includes engineering C4-like efficiency into C3 crops (e.g., rice) to increase yield under water- and resource-limited conditions.
Relevance, applications, and broader implications
CO2 from the atmosphere is captured by plants and builds biomass; carbon cycles through ecosystems and food webs.
The energy entering chloroplasts as sunlight is stored as chemical energy in organic compounds, with starch storage in chloroplasts, roots, tubers, seeds, and fruits as a way to store surplus sugar.
Practical and ethical considerations:
Climate change and rising CO2 influence on plant photosynthesis and water-use efficiency; potential shifts in plant community composition (C3 vs C4 balance).
Genetic modification efforts (e.g., rice engineered for C4-like efficiency) aim to increase crop yields under limited water and resources, with ecological and socio-economic considerations.
CAM plants demonstrate diverse strategies for water conservation in arid environments, informing agriculture and horticulture in water-scarce regions.
Quick reference: typical exam-style connections and questions
Light reactions location and outputs: thylakoid membranes; outputs are ATP and NADPH, with O2 as a by-product from water splitting.
Calvin cycle inputs and outputs: CO2, ATP, and NADPH are inputs; G3P is the export sugar; RuBP is regenerated; cycle uses 9 ATP and 6 NADPH per 1 G3P net gain (per 3 CO2 fixed).
How light and Calvin cycles connect: Light reactions provide ATP and NADPH; Calvin cycle consumes these to fix carbon; ADP, Pi, and NADP+ are shuttled back to the light reactions.
Carbon origin in plants: carbon in plant biomass ultimately comes from atmospheric CO2.
Which light is most effective for photosynthesis indoors: violet-blue and red light are most effective; green light is least useful because it is reflected.
Photoprotection and accessory pigments: carotenoids broaden the spectrum and help protect chlorophyll from light-induced damage.
Distinctions among mechanisms: oxidative phosphorylation (mitochondria) vs photophosphorylation (chloroplasts); chloroplasts convert light energy to ATP and NADPH, while mitochondria convert chemical energy in food to ATP.
Learning objectives recap
Describe the structure of a chloroplast and trace the movement of electrons in both linear and cyclic electron flow.
Explain the relationship between action spectra and absorption spectra and why accessory pigments broaden the spectrum of photosynthesis.
Describe the similarities and differences between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts.
Explain the role of ATP and NADPH in the Calvin cycle and why these are needed for sugar synthesis.
Describe two important photosynthetic adaptations that minimize photorespiration (C4 and CAM) and their ecological and agricultural implications.
Understand how environmental factors (CO2, temperature, water) influence photosynthesis and plant productivity.
Sources and visual references (from transcript cues)
Concept 10.1: Photosynthesis feeds the biosphere; autotrophs vs heterotrophs; role of chloroplasts.
Concept 10.2: Photosynthesis converts light energy to chemical energy; chloroplast structure and stromal/thylakoid compartments.
Concept 10.3: The light reactions convert solar energy to ATP and NADPH.
Concept 10.4: The Calvin cycle uses ATP and NADPH to reduce CO2 to sugar (G3P).
Concept 10.5: Alternative carbon fixation mechanisms in hot, arid climates (C4 and CAM).
Concept 10.6: Photosynthesis is essential for life on Earth: review and synthesis.
Additional context: Engelmann’s action spectrum demonstration; photoprotection by carotenoids; comparisons of chloroplast and mitochondrial chemiosmosis.
Note: All formulas and specific numerical values (e.g., 6 CO2, 12 H2O, 9 ATP, 6 NADPH, 3 CO2 per G3P) are captured from the transcript and presented here in LaTeX format for precise study and exam preparation.