Comprehensive Notes on Photosynthesis
Photosynthesis: Comprehensive Study Notes
Definition and significance
Photosynthesis is the process by which plants, some bacteria, and some protists convert light energy from the sun into chemical energy (glucose).
This glucose can be fermented into pyruvate and then used in cellular respiration to generate ATP; oxygen is released as a by-product.
Overall word equation: \mathrm{CO2} + \mathrm{H2O} \rightarrow \mathrm{C6H{12}O6} + \mathrm{O2}.
The energy transfer from sunlight to chemical energy is mediated by chlorophyll, the green pigment of photosynthesis.
Chlorophyll and accessory pigments
Chlorophyll a is present in all photosynthetic organisms.
Accessory pigments absorb energy that chlorophyll a does not absorb; examples include:
Chlorophyll b (and c, d, e in algae and protists)
Xanthophylls
Carotenoids (e.g., beta-carotene)
Absorption characteristics of chlorophyll a:
Strong absorption in violet-blue and reddish-orange wavelengths
Weak absorption in green-yellow-orange (intermediate) wavelengths
Structural features of chlorophylls:
Lipid-soluble hydrocarbon tail (C${20}$H${39}$-)
Flat hydrophilic head with a central magnesium ion (Mg$^{2+}$)
Different chlorophylls have different side groups on the head
The tail and head are connected by an ester bond
Leaves and leaf structure
Leaves are the primary photosynthetic organs; not all plants have leaves.
Function: leaves act as solar collectors packed with photosynthetic cells.
Gas exchange pathway: raw materials (H$2$O and CO$2$) enter, products (sugars and O$_2$) exit.
Water transport: water enters roots and travels up to leaves via xylem vessels.
Gas exchange barriers and controls:
Guard cells regulate stomatal opening; stomata allow CO$2$ entry and O$2$ exit.
The cuticle (waxy layer) reduces water loss but blocks gas entry; CO$_2$ enters through stomata.
Oxygen leaving the leaf can also exit through stomata.
Water loss example: Cottonwood trees can lose about 100 gallons (~450 dm$^3$) of water per hour on hot desert days.
Structure of the chloroplast and photosynthetic membranes
Thylakoid: the structural unit of photosynthesis; contains photosynthetic pigments.
Grana: stacks of thylakoids; inter-stack regions are called stroma lamellae.
Stroma: the fluid-filled space surrounding thylakoids; site of the Calvin cycle.
Compartments and membranes:
Chloroplast has three membrane systems, forming three compartments: stroma, thylakoid lumen, and intermembrane space.
Photosynthetic reactions occur in two major locations:
Light-dependent reactions in the grana (thylakoid membranes)
Light-independent reactions (Calvin cycle) in the stroma
Stages of photosynthesis (overview)
Two-stage process:
Light-dependent reactions (in grana): require light to produce energy carriers ATP and NADPH and to split water.
Light-independent reactions (Calvin cycle) in the stroma: use ATP and NADPH to fix CO$_2$ into carbohydrates (reduction stage); initial product is glyceraldehyde-3-phosphate (GAP or GALP).
Core chemical ideas:
Photoactivation of chlorophyll a excites an electron, which is transferred to a primary electron acceptor.
Chlorophyll is oxidized (loses an electron).
Water splitting (photolysis) provides electrons and protons; energy transfer drives ATP and NADPH formation.
Reactions include condensation and phosphorylation (formation of water, attachment of phosphate groups) and redox reactions (electron transfer).
The light-dependent reactions
Photoexcitation and photoionisation:
Light absorbed by a chlorophyll molecule raises electrons to higher energy levels; sufficient energy leads to electron release (photoionisation).
Core photosystems: Photosystem II (PSII, P680) and Photosystem I (PSI, P700).
PSII is energized first, followed by PSI (the order is PSII then PSI; named by discovery order).
Electrons travel through an electron transport chain, enabling energy release for ATP formation.
Z-scheme (electron flow):
The energy changes form a Z-shaped path when drawn, hence the name Z scheme.
Key outcome: sufficient energy is released to synthesize ATP from ADP and phosphate.
Photolysis and electron transfer:
Water is split to release O$2$, H$^+$, and electrons: 2\mathrm{H2O} \rightarrow 4\mathrm{H^+} + \mathrm{O_2} + 4e^-.
NADP$^+$ reduction:
Electrons reduce NADP$^+$ to NADPH: \mathrm{NADP^+} + 2e^- + 2\mathrm{H^+} \rightarrow \mathrm{NADPH} + \mathrm{H^+}.
Energy carriers produced:
ATP via photophosphorylation
NADPH via reduction of NADP$^+$
The light-independent reactions (Calvin cycle)
Carbon fixation: CO$_2$ is captured and fixed into organic molecules.
The initial chemical step: CO$_2$ combines with RuBP (ribulose-1,5-bisphosphate) to form a six-carbon sugar that immediately splits into two molecules of glyceraldehyde-3-phosphate (GALP or GAP).
GALP as a key intermediate:
Each GALP is a 3-C molecule; two GALP molecules can be used to synthesize glucose and other carbohydrates, lipids, or amino acids.
Role of ATP and NADPH:
The energy and reducing power from ATP and NADPH (generated by the light-dependent reactions) are used to convert the 3-C molecules into carbohydrates.
Summary ofGALP production and fate:
From each CO$_2$ fixed, two molecules of GALP are produced.
Of the GALP produced, one molecule (per cycle) can be drawn off to form glucose and other storage molecules.
The rest of the GALP molecules are recycled to regenerate RuBP through a series of reactions powered by ATP.
Early Calvin cycle steps:
The first stable product is phosphoglycerate (PGA), a 3-carbon compound.
PGA is phosphorylated by ATP to form a high-energy glycerate diphosphate intermediate, which is then reduced by NADPH to GALP.
The Non-cyclic photophosphorylation (the Z scheme) - detailed flow
In PSII:
Photoionisation moves electrons to an electron acceptor.
Water is split (photolysis) to supply electrons; O$_2$ is released, and protons/electrons are produced.
The electron acceptor passes electrons to the electron transport chain; the final acceptor is PSI.
In PSI:
Absorbed light energy further excites electrons, enabling the reduction of NADP$^+$ to NADPH.
Net products:
ATP and NADPH are generated to power the Calvin cycle.
NADP$^+$ and NADPH forms:
The oxidised form: NADP$^+$
The reduced form: NADPH
Chemiosmosis and ATP synthesis in the chloroplast
Location: thylakoid membranes.
Proton motive force:
Electrons moving through the electron transport chain pump H$^+$ from the stroma into the thylakoid lumen, creating a higher H$^+$ concentration inside the thylakoid.
Proton gradient drives ATP synthesis:
H$^+$ ions diffuse back to the stroma through ATP synthase, generating ATP from ADP and Pi.
Concept: electrochemical gradient across the thylakoid membrane is the driving force of ATP production (chemiosmosis).
Cyclic phosphorylation
Purpose: to produce additional ATP without NADPH.
Mechanism:
Only PSI is involved; electrons excited in PSI are transferred to the electron transport chain and cycle back to PSI.
No NADPH is formed in cyclic phosphorylation; the cycle provides extra ATP to meet the energy demand of the Calvin cycle.
Net effect: more ATP is produced to satisfy the energy requirements of carbon fixation.
The light-independent reactions in depth (Calvin cycle)
Carbon fixation step (PGA formation):
Carbon dioxide combines with RuBP to form 2 molecules of PGA (3-PGA): \mathrm{CO_2} + \mathrm{RuBP} \rightarrow 2\mathrm{PGA}.
Reduction and phosphorylation steps:
PGA is phosphorylated by ATP to form glycerate phosphate intermediates (often described as 1,3-bisphosphoglycerate).
These are reduced by NADPH to form GALP (glyceraldehyde-3-phosphate, GAP).
Net stoichiometry (as described in the transcript):
From each 6 CO$_2$ fixed, 12 GALP molecules are produced.
Of the 12 GALP, 2 are removed to make glucose and other carbohydrates, lipids, or amino acids.
The remaining GALP molecules are converted back to RuBP through a series of reactions powered by ATP, allowing the cycle to continue.
First steps and products:
The first stable product is PGA (3-PGA).
ATP and NADPH from the light-dependent reactions power phosphorylation and reduction to GALP.
Regeneration of RuBP:
The majority of GALP is used to regenerate RuBP; this regeneration requires ATP.
Summary: integrated view of photosynthesis
Light-dependent reactions:
Occur in the grana/thylakoid membranes; produce ATP and NADPH; split water and release O$_2$.
Light-independent reactions (Calvin cycle):
Occur in the stroma; use ATP and NADPH to fix CO$_2$ into carbohydrate (GALP/G3P) and ultimately glucose.
Two main outputs and their roles:
ATP: energy currency for carbon fixation and RuBP regeneration.
NADPH: reducing power for converting 3-PGA to GALP.
Factors affecting the rate of photosynthesis
Limiting factors: light intensity, CO$_2$ concentration, and temperature.
Light intensity effects:
As light intensity increases, the rate of the light-dependent reactions increases, hence photosynthesis generally increases.
However, beyond a point, other factors become limiting and the rate no longer increases.
Wavelength dependence:
Chlorophyll a and the photosystems have specific absorption peaks: PSI around 700 nm and PSII around 680 nm.
Light with a high proportion of energy at these wavelengths yields higher photosynthesis rates.
Carbon dioxide concentration:
Higher CO$_2$ increases rate of carbon fixation in the Calvin cycle until another factor limits the rate.
Temperature:
Photosynthetic enzymes have optimum temperatures; rates rise with temperature up to the optimum and decline beyond it.
Practical connections and real-world relevance
Understanding photosynthesis helps explain plant growth, crop yields, and how environmental conditions influence plant productivity.
Water loss through stomata is a critical trade-off for CO$_2$ uptake; high transpiration occurs under hot, dry conditions, illustrated by the cottonwood example.
Ethical, philosophical, and practical implications
Efficient photosynthesis is foundational for food security and carbon cycling in ecosystems.
Engineering photosynthetic pathways or crops with improved light capture could have broad ecological and societal impacts.
Key terms and abbreviations to memorize
PGA: phosphoglycerate (3-PGA)
GALP/G3P: glyceraldehyde-3-phosphate
RuBP: ribulose-1,5-bisphosphate
CO$2$, H$2$O, O$_2$, NADP$^+$/NADPH, ATP, ADP, Pi (inorganic phosphate)
PSII (P680) and PSI (P700)
Important equations and numbers (LaTeX)
Word equation for photosynthesis: \mathrm{CO2} + \mathrm{H2O} \rightarrow \mathrm{C6H{12}O6} + \mathrm{O2}.
Water splitting (photolysis) in light-dependent reactions: 2\mathrm{H2O} \rightarrow 4\mathrm{H^+} + \mathrm{O2} + 4e^-.
NADP$^+$ reduction: \mathrm{NADP^+} + 2e^- + 2\mathrm{H^+} \rightarrow \mathrm{NADPH} + \mathrm{H^+}.
Photosystems order and energy (Z-scheme): PSII (P680) precedes PSI (P700); electron flow forms a Z-shaped energy diagram.
Calvin cycle fixation step (simplified): \mathrm{CO_2} + \mathrm{RuBP} \rightarrow 2\mathrm{PGA}.
Conversion of PGA to GALP (simplified): PGA is phosphorylated by ATP and reduced by NADPH to GALP, the 3-C sugar.
Stoichiometry note (textual): From 6 CO$_2$ fixed, 12 GALP are produced; 2 GALP used for glucose formation; remaining GALP regenerate RuBP via ATP.
Quick reference for exam prep
Identify where each stage occurs (light-dependent in grana; Calvin cycle in stroma).
Be able to explain why cyclic phosphorylation is used and when it operates.
Know the roles of PSII and PSI and what each contributes to ATP and NADPH production.
Be able to write and balance the key reactions: water photolysis, NADPH formation, CO$_2$ fixation, and GALP formation.
Understand the trade-off between stomatal opening for CO$_2$ uptake and water loss through transpiration.
End of notes: recall activity
Take practice quizzes on the photosynthesis process to reinforce understanding and recall of key terms, pathways, and equations.