Photosynthesis: Key Concepts in Plant Energy Conversion and Carbon Cycle

Autotrophs

Make their own food from inorganic sources (CO2, water).

Producers

Autotrophs that sustain ecosystems by producing organic molecules.

Photoautotrophs

Use sunlight to make food (plants, algae, cyanobacteria).

Chemoautotrophs

Use energy from inorganic compounds (e.g., some bacteria).

Heterotrophs

Consumers; depend on autotrophs for energy and carbon.

Carbon Cycle

Photosynthesis removes CO2 and produces O2; respiration returns CO2.

Stomata

Adjustable pores for gas exchange (CO2 in, O2 out).

Guard cells

Control stomatal opening/closing.

Photosynthetic cells

Mesophyll cells (palisade and spongy) contain chloroplasts.

Gas exchange

CO2 diffuses through stomata into mesophyll; O2 exits similarly.

Light reactions

Location: Thylakoid membranes; Inputs: Light, H2O, ADP, Pi, NADP+; Outputs: ATP, NADPH, O2.

Calvin cycle

Location: Stroma; Inputs: CO2, ATP, NADPH; Outputs: GA3P (3C sugar) → sucrose & starch.

Rubisco

Catalyzes first step: CO2 + RuBP → 2 × 3-PGA.

Ultimate electron source

Water.

Water-splitting reaction

2H2O → 4e- + 4H+ + O2 (O2 is by-product).

Photosynthesis is reductive

Adds electrons to CO2 to form C-H and C-C bonds.

Electrons needed for glucose

24 e- (from 12 H2O molecules).

Electron flow

Water → PSII (P680) → ETC → PSI (P700) → NADPH.

Energy source

Light (photons).

Photosynthetically Active Radiation (PAR)

400-700 nm; shorter wavelengths (blue) have higher energy.

Main pigments

Chlorophyll a (primary), chlorophyll b, carotenoids.

States of pigments

Ground (normal), Excited (after photon absorption), Oxidized (after electron donation).

Photosystem

Reaction center + light-harvesting complexes.

PSII (P680)

Splits water, provides electrons.

PSI (P700)

Boosts electrons to reduce NADP+ → NADPH.

ETC

Creates proton gradient for ATP synthase; ATP synthase uses Δ[H+] to regenerate ATP (chemiosmosis).

C3 plants

Fix CO2 into 3-PGA; prone to photorespiration when O2 builds up.

Photorespiration

Rubisco uses O2 instead of CO2; wastes energy.

C4 plants

Use PEP carboxylase; spatial separation of CO2 fixation and Calvin cycle.

CAM plants

Temporal separation; fix CO2 at night, Calvin cycle during day.

Atmospheric CO2

Fluctuates seasonally: Lower in northern summer (more photosynthesis).

Long-term trend of CO2

Rising due to fossil fuel combustion.