Premed · Premed · General Biology 1
Lecture 12: Photosynthesis — Light Reactions
General Biology I — Molecular & Cellular
Learning Objectives
By the end of this lecture, students will be able to:
- Write the overall equation for photosynthesis and relate it to cellular respiration
- Describe the structure of a chloroplast and the location of the light reactions
- Explain how photosynthetic pigments absorb light and describe their absorption spectra
- Trace the flow of electrons through the light reactions (linear and cyclic electron flow)
- Explain how the proton gradient across the thylakoid membrane drives ATP synthesis
- Distinguish between the products of the light reactions and the Calvin cycle
Lecture Content
I. Overview of Photosynthesis
Photosynthesis is the process by which light energy is captured and converted into chemical energy stored in organic molecules. Its overall equation--6CO2 + 6H2O + light energy -> C6H12O6 + 6O2--is essentially the reverse of cellular respiration, and this complementarity is no coincidence: the two processes are the yin and yang of biological energy flow on Earth. Photosynthesis is carried out by plants, algae, and certain bacteria (most notably cyanobacteria and purple bacteria), and it proceeds in two interconnected stages. The light reactions (light-dependent reactions) take place in the thylakoid membranes of the chloroplast, where they capture light energy, produce ATP and NADPH, split water, and release oxygen. The Calvin cycle (light-independent reactions) occurs in the stroma and uses the ATP and NADPH generated by the light reactions to fix carbon dioxide into organic molecules.
II. Chloroplast Structure (Review)
The chloroplast is bounded by a smooth, permeable outer membrane and a more selective inner membrane. The fluid-filled interior, the stroma, contains the enzymes of the Calvin cycle, chloroplast DNA, and ribosomes. Suspended within the stroma are the thylakoids--flattened, disc-like membrane sacs where the light reactions occur. Thylakoids are stacked into columns called grana (singular: granum) and interconnected by stroma lamellae. The interior space of each thylakoid, the thylakoid lumen, is where protons accumulate during the light reactions to drive ATP synthesis.
III. Light and Pigments
Visible light is a narrow band of the electromagnetic spectrum, spanning wavelengths of approximately 380 to 750 nm. Light travels as discrete packets of energy called photons, and the energy of a photon is inversely proportional to its wavelength--shorter wavelengths (violet and blue) carry more energy than longer wavelengths (red).
Photosynthetic organisms capture light using pigments, each of which absorbs specific wavelengths while reflecting or transmitting others. Chlorophyll a is the primary pigment, absorbing strongly in the blue-violet (~430 nm) and red (~660 nm) regions while reflecting green light--which is why plants appear green. Chlorophyll b, an accessory pigment, absorbs blue (~455 nm) and orange-red (~640 nm) light, broadening the range of wavelengths that can drive photosynthesis. Carotenoids (including beta-carotene and xanthophylls) absorb blue-green light (~400-500 nm) and reflect yellow and orange; beyond their role in light harvesting, they serve as photoprotective agents, dissipating excess light energy as heat to prevent oxidative damage to the photosynthetic machinery.
The absorption spectrum of a pigment graphs the fraction of light absorbed at each wavelength. The action spectrum of photosynthesis graphs the rate of photosynthesis at each wavelength. The close correspondence between these two spectra--both peaking in the blue and red regions--provided early evidence that chlorophyll is the pigment driving the process.
<image>A two-panel figure. Panel A: The electromagnetic spectrum with visible light expanded (380-750 nm), showing violet, blue, green, yellow, orange, and red wavelengths. Below it, absorption spectra of chlorophyll a (peaks at ~430 nm and ~660 nm), chlorophyll b (peaks at ~455 nm and ~640 nm), and carotenoids (broad peak from ~400-500 nm) are plotted on the same axes. Panel B: An action spectrum of photosynthesis (rate of O2 evolution vs. wavelength) overlaid for comparison, showing peaks in the blue and red regions corresponding to chlorophyll absorption maxima, with a dip in the green region.</image>
IV. Photosystems
The pigment molecules of the thylakoid membrane are organized into large multiprotein complexes called photosystems, each consisting of two functional components. The antenna complex (or light-harvesting complex) contains hundreds of pigment molecules--chlorophylls a and b and carotenoids--bound to proteins. These pigments absorb photons and transfer the energy inward to the reaction center through a process called resonance energy transfer, much like a satellite dish focusing a signal. At the heart of the reaction center sits a special pair of chlorophyll a molecules. When this pair receives sufficient energy, one of its electrons is boosted to a higher energy level and transferred to a nearby molecule called the primary electron acceptor--this is the photochemical event that converts light energy into chemical energy.
Two photosystems cooperate in the light reactions. Photosystem II (PSII), whose reaction center chlorophyll absorbs maximally at 680 nm (hence P680), functions first in the electron flow despite its numbering (the names reflect the order of discovery, not function). Photosystem I (PSI), with its reaction center P700 (absorbing at 700 nm), acts second.
V. Linear (Noncyclic) Electron Flow
Linear electron flow is the primary pathway of the light reactions, producing both ATP and NADPH. Electrons travel from water through PSII, along an electron transport chain to PSI, and ultimately to NADP+.
The process begins when PSII absorbs light. P680 is excited to P680* and donates an electron to the primary electron acceptor pheophytin. The resulting P680+ is the strongest biological oxidant known, and it replenishes its lost electron by extracting electrons from water. The oxygen-evolving complex (OEC), a manganese-containing enzyme cluster, catalyzes the photolysis of water: 2H2O -> 4H+ + 4e- + O2. The oxygen released is a byproduct--yet it is the source of virtually all atmospheric O2 and the oxygen we breathe. The protons released into the thylakoid lumen contribute to the proton gradient.
From PSII, electrons pass through plastoquinone (PQ), a mobile carrier analogous to ubiquinone in mitochondria, to the cytochrome b6f complex, which pumps approximately 4 H+ into the thylakoid lumen per 2 electrons. Electrons then transfer to plastocyanin (PC), a small copper-containing protein, which delivers them to PSI.
At PSI, light energy once again boosts an electron to high energy. P700 donates the excited electron to its primary acceptor and is re-reduced by electrons arriving from plastocyanin. The high-energy electrons leaving PSI pass through ferredoxin to the enzyme NADP+ reductase, which catalyzes the reduction of NADP+ to NADPH on the stromal side of the membrane.
VI. Chemiosmosis in Chloroplasts
The light reactions generate a proton gradient across the thylakoid membrane through three contributions: the splitting of water releases H+ into the lumen, the cytochrome b6f complex actively pumps H+ into the lumen, and the consumption of H+ by NADP+ reductase on the stromal side further lowers stromal proton concentration. The resulting proton-motive force drives protons back through ATP synthase (the CF0-CF1 complex) embedded in the thylakoid membrane, producing ATP in the stroma--precisely where the Calvin cycle needs it.
This process is fundamentally analogous to chemiosmosis in mitochondria, but with an important spatial distinction. In mitochondria, protons are pumped from the matrix to the intermembrane space and flow back into the matrix through ATP synthase. In chloroplasts, protons accumulate in the thylakoid lumen and flow out to the stroma.
<image>A cross-sectional diagram of a thylakoid membrane showing the complete light reactions. From left to right: PSII (with the oxygen-evolving complex splitting water in the lumen, releasing O2 and H+), plastoquinone shuttling electrons and H+ across the membrane, the cytochrome b6f complex (pumping H+ into the lumen), plastocyanin carrying electrons on the lumen side, PSI (re-energizing electrons), ferredoxin, and NADP+ reductase (producing NADPH on the stroma side). ATP synthase is shown at the right, with H+ flowing from the lumen (high H+) to the stroma (low H+), driving ATP synthesis. The proton gradient is indicated with a concentration difference across the membrane.</image>
VII. Cyclic Electron Flow
In addition to the linear pathway, chloroplasts can carry out cyclic electron flow, an alternative route involving only PSI. In this pathway, electrons excited by PSI pass through ferredoxin but, instead of reducing NADP+, cycle back to the cytochrome b6f complex. The b6f complex pumps protons into the thylakoid lumen as before, driving ATP synthesis, but no NADPH is produced, no water is split, and no oxygen is released.
Why would a cell choose this less productive pathway? The Calvin cycle requires ATP and NADPH in a ratio of 3:2, but linear electron flow does not always produce them in exactly this proportion. Cyclic electron flow generates the additional ATP needed to balance the ratio. It may also be favored when NADP+ is limiting or under conditions of environmental stress. Evolutionary biologists consider cyclic electron flow the more ancient pathway, likely predating the evolution of PSII and the capacity for water splitting.
VIII. Summary of Light Reaction Products
The light reactions convert light energy into the chemical energy of ATP and NADPH while liberating oxygen as a byproduct of water splitting. These three products--ATP, NADPH, and the fixed electrons they carry--are the essential inputs for the Calvin cycle, which will use them to convert atmospheric CO2 into the organic molecules that feed virtually all life on Earth.
