Photosynthesis: Light Reactions, Calvin Cycle, and Plant Adaptations

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Autotrophs and Photosynthesis

Types of Autotrophs

Autotrophs are organisms capable of producing their own food from inorganic sources. They play a foundational role in ecosystems by converting energy from non-living sources into organic molecules.

Photoautotrophs: Use sunlight to synthesize food via photosynthesis (e.g., plants, algae, cyanobacteria).
Chemoautotrophs: Use chemical energy from inorganic substances (e.g., nitrifying bacteria).

Key idea: Autotrophs produce organic molecules (like glucose) from inorganic substances such as carbon dioxide and water.

Leaf Structure and Photosynthesis

Leaf Anatomy

Leaves are the primary site of photosynthesis in green plants, optimized for light absorption and gas exchange.

Chloroplasts: Organelles containing chlorophyll, the pigment that captures sunlight.
Mesophyll: Inner leaf tissue with two layers:
- Palisade mesophyll: Tightly packed cells, main site of photosynthesis.
- Spongy mesophyll: Loosely packed cells with air spaces for gas exchange.
Stomata: Pores controlling gas exchange (CO2 in, O2 out).
Veins: Transport system (xylem for water, phloem for sugars).

Electromagnetic Energy and Light

Electromagnetic Spectrum

Electromagnetic energy travels as waves and includes various forms of radiation. The spectrum is classified by wavelength and energy.

Type of Radiation	Wavelength (approx)	Energy Level	Notes
Gamma rays	~10-5 nm	Highest	Very high energy, dangerous
X-rays	~10-3 nm	Very high	Medical imaging
Ultraviolet (UV)	~1–400 nm	High	Can damage skin
Visible light	380–740 nm	Medium	What we can see
Infrared	~103 nm	Low	Heat energy
Microwaves	~106 nm	Lower	Cooking, communication
Radio waves	~103 m	Lowest	Radio, TV signals

Photon: A quantum of electromagnetic energy; no mass, travels at speed of light, energy depends on frequency.
Ozone (O3): Protects from UV radiation.

Visible Light and Photosynthesis

Plants absorb mainly blue-violet (380–500 nm) and red (650–740 nm) light.
Green light (500–550 nm) is reflected, making plants appear green.

Key idea: Only certain wavelengths of visible light are used efficiently for photosynthesis.

Color Vision in Organisms

Photoreceptor Types

Dichromatic: 2 types of cones (e.g., many mammals).
Trichromatic: 3 types of cones (e.g., humans: red, green, blue).
Tetrachromatic: 4 types of cones (e.g., many birds, some fish).

Key idea: More cone types allow detection of more colors.

Solar Radiation and Protection

Solar Wind and Radiation

Solar wind: Stream of charged particles from the Sun.
Radiation: Includes electromagnetic radiation (UV, X-rays, gamma rays).
High-energy radiation can damage cells and DNA, causing mutations or cancer.
Earth's ozone layer and magnetic field protect life from harmful radiation.

Photosynthesis: Light Reactions and Calvin Cycle

Photosystems

Photosystems are complexes in the thylakoid membrane that initiate light reactions.

Photosystem II (PSII): Absorbs light, splits water (photolysis), releases O2, starts electron transport.
Photosystem I (PSI): Absorbs light, re-energizes electrons, produces NADPH.

Linear Electron Flow

Linear electron flow is the main pathway in light reactions, moving electrons from water to NADPH and producing ATP.

Light excites electrons in PSII; water is split, releasing O2.
Electrons move through the electron transport chain, creating a proton gradient.
ATP is produced via chemiosmosis as protons flow through ATP synthase.
Electrons reach PSI, are re-energized, and reduce NADP+ to NADPH.

Final products: ATP, NADPH, O2.

Cyclic Electron Flow

Cyclic electron flow involves only PSI, producing ATP without NADPH or O2.

Electrons excited in PSI enter the electron transport chain.
ATP is produced as electrons cycle back to PSI.

Key idea: Cyclic flow supplements ATP production when NADPH is abundant.

Light Reactions vs Calvin Cycle

Feature	Light Reactions	Calvin Cycle (Dark Reactions)
Location	Thylakoid membranes	Stroma
Light needed?	Yes	No (depends on products of light reactions)
Main function	Convert light energy to chemical energy	Use chemical energy to make sugar
Inputs	Light, H2O, ADP + Pi, NADP+	CO2, ATP, NADPH
Outputs	O2, ATP, NADPH	G3P (sugar), ADP + Pi, NADP+, RuBP (recycled)
Energy role	Produces energy carriers	Uses energy carriers
Key process	Photolysis, electron transport chain	Carbon fixation, reduction, regeneration
Final result	Energy stored in ATP and NADPH	Sugar production (glucose via G3P)

Calvin Cycle (Dark Reactions)

The Calvin Cycle is the second stage of photosynthesis, occurring in the stroma and using ATP and NADPH to fix carbon dioxide into sugars.

Phase 1: Carbon Fixation
- CO2 attaches to RuBP (5-carbon sugar) via rubisco.
- Forms a 6-carbon intermediate, splits into two 3-PGA molecules.
Phase 2: Reduction
- ATP adds phosphate to 3-PGA, forming 1,3-bisphosphoglycerate.
- NADPH reduces it to G3P (glyceraldehyde-3-phosphate).
- For every 3 CO2, 6 G3P are produced; 1 exits, 5 regenerate RuBP.
Phase 3: Regeneration
- 5 G3P are rearranged using ATP to regenerate RuBP.

Inputs: CO2, ATP, NADPH, RuBP Outputs: G3P (sugar), ADP + Pi, NADP+, RuBP (recycled)

Key Molecules and Enzymes

RuBP: 5-carbon sugar, CO2 acceptor, recycled in Calvin cycle.
Rubisco: Enzyme catalyzing CO2 fixation; most abundant protein on Earth.
G3P: 3-carbon sugar, product of Calvin cycle, used to form glucose.

Chlorophyll A vs Chlorophyll B

Feature	Chlorophyll A	Chlorophyll B
Main role	Primary pigment (core photosynthesis)	Accessory pigment (supports A)
Function	Directly converts light energy	Transfers energy to A
Color	Blue-green	Yellow-green
Light absorption	Blue-violet, red	Blue, red-orange
Amount in plants	More abundant	Less abundant
Importance	Essential for photosynthesis	Enhances efficiency

Key idea: Chlorophyll A is the main worker; Chlorophyll B is the helper.

Action Spectrum of Photosynthesis

Engelmann's Experiment

Engelmann used algae and aerobic bacteria to determine which wavelengths drive photosynthesis.

Algae exposed to different colors of light.
Aerobic bacteria clustered where oxygen (from photosynthesis) was highest.
Results: Most oxygen (photosynthesis) in blue/violet and red light; least in green light.

Action spectrum: Photosynthesis is highest in blue and red wavelengths, lowest in green.

Light Aspects and Plant Responses

Light Properties

Quantity: Intensity or brightness (measured in lux).
Quality: Wavelength or color.
Duration: Length of exposure (photoperiod).

Lux: Unit of light intensity; more lux means more energy for photosynthesis.

Photoperiod and Plant Behavior

Photoperiod is the length of day and night, controlling flowering, growth, and seasonal responses.

Short-day plants: Flower when nights are long (e.g., rice).
Long-day plants: Flower when nights are short (e.g., spinach).
Day-neutral plants: Flowering not affected by day length (e.g., tomato).

C3, C4, and CAM Plant Pathways

Feature	C3	C4	CAM
First product	3-carbon (3-PGA)	4-carbon	4-carbon (stored at night)
CO2 timing	Day	Day	Night
Stomata open	Day	Day	Night
Best environment	Cool, moist	Hot, sunny	Hot, dry
Photorespiration	High	Low	Very low
Examples	Rice, wheat	Corn, sugarcane	Cactus, pineapple

C3 plants: Use Calvin cycle directly, produce 3-PGA, most common, best in cool/moist environments, susceptible to photorespiration.

Summary Equations

Photosynthesis Overall Equation

The general equation for photosynthesis is:

Calvin Cycle (Simplified)

For every 3 CO2:

Additional info:

Photons escaping the Sun take thousands to millions of years due to random walk; once at the surface, they reach Earth in ~8 minutes.
"Quantity of light" refers to intensity, not color; high intensity increases photosynthesis up to a limit.