Photosynthesis Flashcards
What are the two key outputs of the light-dependent reactions of photosynthesis, and what are their roles
ATP: energy currency used in the Calvin cycle to drive endergonic reactions
NADPH: reducing agent that donates electrons to reduce carbon compounds (e.g. 3-PGA to GAP)
How does photosynthesis support the global carbon cycle
Photosynthesis fixes inorganic carbon (CO₂) from the atmosphere into organic molecules (e.g., glucose), providing carbon skeletons for all living organisms and forming the base of food chains
Why are lithotrophs less widespread than phototrophs
Lithotrophs extract electrons from reduced inorganic compounds, but their habitats are limited to areas with abundant reduced substrates, such as hydrothermal vents - a rare and isolated niche
What were the ecological consequences of oxygenic photosynthesis on early Earth
Oxygen accumulation caused a mass extinction of obligate anaerobes (no oxidative protection)
Enabled aerobic respiration, which is much more efficient than anaerobic pathways
Facilitated the evolution of complex, multicellular life
Describe the molecular structure of chlorophyll and how it absorbs light
Tetrapyrrole ring: absorbs photons (π-electrons delocalised for resonance)
Central Mg²⁺ ion: helps stabilise the excited state
Hydrophobic tail (phytol): anchors chlorophyll in the thylakoid membrane
What happens when chlorophyll absorbs a photon
An electron is excited to a higher energy level. This excitation energy is either:
1. Lost as fluorescence (in isolated molecules)
2. Transferred to neighbouring pigments via resonance energy transfer
3. Used for charge separation in a reaction centre, launching the electron transport chain
What are photosystems and how do they function in light harvesting
Photosystems are protein-pigment complexes that include:
Antenna complex - captures and funnels excitation energy to reaction centre
Reaction centre - special chlorophyll that undergoes charge separation → electron is ejected to a carrier molecule
Why are two photosystems required in oxygenic photosynthesis
Photosystem II (PSII): has a high enough potential to oxidise water (P680⁺)
Photosystem I (PSI): provides the additional energy needed to reduce NADP⁺
This Z-scheme architecture bridges the energetic gap between water (high redox potential) and NADP⁺ (low redox potential)
What is “charge separation” in the context of the photosynthetic reaction centre
The process by which an excited electron is transferred away from chlorophyll, leaving behind a positively charged chlorophyll molecule. This electron is captured by an acceptor and passed along an electron transport chain
What is the electron flow pathway from PSII to NADPH
PSII → Pheophytin
Plastoquinone (PQ) → Cytochrome b6f
Plastocyanin (PC) → PSI
Ferredoxin (Fd) → Ferredoxin-NADP⁺ Reductase (FNR)
NADP⁺ → NADPH
How is ATP generated in photosynthesis
The proton gradient created by water splitting (in PSII) and the action of cytochrome b6f drives ATP synthase, which phosphorylates ADP to ATP
What is a phycobilisome and what is its function
A light-harvesting antenna complex found in cyanobacteria, composed of:
Phycobiliproteins - PE (phycoerythrin), PC (phycocyanin), APC (allophycocyanin)
Linker proteins - stabilise structure
Bilin chromophores - absorb blue/green light → suited for deep or shaded water
How are phycobilisomes different from plant antenna complexes
Plants use chlorophyll a/b + carotenoids in membrane-embedded antenna proteins
Cyanobacteria use phycobilisomes, which are externally attached to the membrane, maximising light capture in low-light aquatic environments
Describe the water-splitting reaction at Photosystem II
- P680 absorbs light → P680⁺ (oxidised)
- P680⁺ is so oxidising it can extract electrons from water
- At the Mn⁴CaO₅ cluster (oxygen-evolving complex), 2 H₂O molecules → 4 H⁺ + 4 e⁻ + O₂
- Electrons replenish the P680 and enter the ETC
What is the role of the manganese centre in PSII
Catalyses the oxidation of water in 4 sequential steps (S-state cycle) → allows safe accumulation and controlled release of electrons, protons, and O₂
Why can’t cytochrome b6f reduce NADP⁺ directly
Its redox potential is too low to reduce NADP⁺ efficiently.
PSII initiates electron flow
PSI provides a second energy boost so electrons can be transferred to NADP⁺ by ferredoxin-NADP⁺ reductase
What are the inputs and outputs of the Calvin cycle
Inputs per CO₂ fixed: 3 ATP + 2 NADPH
Output: 1 GAP (glyceraldehyde-3-phosphate) (used to make glucose and other carbs)
What happens after GAP is formed
Some GAP exits the cycle to form sugars, amino acids, lipids
Remaining GAP undergoes complex rearrangements to regenerate ribulose-5-phosphate, requiring enzymes like transketolase and aldolase
Outline the photosynthetic electron transport chain
PSII → Plastoquinone → Cytochrome b6f → Plastocyanin → PSI → Ferredoxin → FNR → NADPH
How are plant and cyanobacterial photosystems arranged differently
Plants: reaction centre is surrounded by antennae → absorb light from all angles
Cyanobacteria: antennae stacked on top of the complex
