Cellular Energetics: Photosynthesis and Carbon Fixation

Photosynthesis

A process converting sunlight energy into chemical energy stored in carbohydrates and organic molecules.

Autotrophs

Organisms capable of surviving on CO2 as their principal carbon source.

Chemoautotrophs

Organisms utilizing chemical energy from inorganic molecules to convert CO2 into organic compounds.

Photoautotrophs

Organisms using sunlight energy to convert CO2 into organic compounds through photosynthesis.

Heterotrophs

Organisms relying on external sources for organic compounds.

Photosynthesis Stages

1. Light absorption, high-energy electron generation, and O2 formation; 2. Electron transport leading to NADPH reduction; 3. ATP synthesis; 4. CO2 conversion into carbohydrates.

Photosynthesis Equation

6 CO2 + 12 H2O → C6H12O6 + 6 H2O + 6 O2

Photosynthesis vs. Respiration

Photosynthesis oxidizes water to oxygen, while respiration reduces oxygen to form water.

Light-Dependent Reactions

Photosynthesis reactions where sunlight energy is converted to chemical energy in the form of ATP and NADPH.

Light-Independent Reactions

Photosynthesis reactions where carbon fixation occurs using ATP and NADPH energy.

Chloroplasts

Organelles where photosynthesis takes place, containing thylakoid membranes for light reactions and stroma for dark reactions.

Calvin Cycle

A series of chemical reactions in the stroma of chloroplasts that converts CO2 into carbohydrates using ATP and NADPH.

Photosynthetic Pigments

Molecules like chlorophyll that absorb specific wavelengths of light for photosynthesis.

Chlorophyll

Main pigment with a porphyrin ring that absorbs light and a phytol tail anchoring it to the thylakoid membrane.

Chlorophyll b

Type of chlorophyll with a formaldehyde (CHO) group instead of a CH3 group, absorbing violet-blue and red light most strongly.

Conjugated System

System in the porphyrin ring of chlorophyll containing alternating single and double bonded carbons that delocalize electrons.

Accessory Pigments

Pigments like carotenoids that absorb light in the blue-green region, enhancing light absorption in photosynthesis.

Photoelectron Transport

Primary event in photosynthesis where energized chlorophyll a molecules produce charge separation, initiating electron flow.

Light-Harvesting Complexes

Multiprotein complexes containing chlorophyll and other molecules that optimize light absorption and energy transfer.

Photosystems

Large pigment-protein complexes acting in series to raise electrons from water to NADP+, with PSII and PSI having distinct functions.

Z Scheme

Non-cyclic photophosphorylation where electrons move linearly from water to NADP+ through PSII and PSI, generating ATP and NADPH.

Cytochrome bf Complex

Complex that accepts electrons from QH2, transports protons into the lumen, and increases the proton-motive force.

Ferredoxin

Iron-sulfur protein that transfers electrons from P700 chlorophylls to ferredoxin-NADP+ reductase, aiding in NADP+ reduction.

PQH2

Plastoquinol that diffuses through the membrane, transferring electrons and translocating protons in the Q cycle.

Plastocyanin

Copper-containing protein that carries electrons from cytochrome b6f to PSI, facilitating electron transfer to the P700 reaction center.

Calvin cycle

Fixes CO2 into organic molecules in a series of reactions in the chloroplast stroma.

Photorespiration

Uptake of O2 and release of CO2, favored at low CO2 and high O2 levels.

C4 plants

Fix CO2 in outer mesophyll cells into four-carbon molecules for use in the Calvin cycle.

Melvin Calvin

Nobel Prize winner in 1961 for his work on the Calvin cycle.

Rubisco

Enzyme that catalyzes the condensation of RuBP and splitting of a six-carbon molecule in the Calvin cycle.

Glyceraldehyde 3-phosphate

Product of the Calvin cycle, formed from the reduction of 3-phosphoglycerate.

Ribulose bisphosphate carboxylase

Enzyme with a low turnover number that fixes ~3 molecules of CO2 per second.

Thioredoxin

Accepts electrons to reduce disulfide bridges in Calvin cycle enzymes, activating their enzymatic activity.

PEP carboxylase

Enzyme used in CAM plants to fix CO2 by attaching it to PEP to form malate.

CAM plants

Plants that carry out light reactions and CO2 fixation at different times of the day, keeping stomata closed during the day to reduce water loss.

Hatch-Slack pathway

Alternate pathway for carbon fixation used by grasses in tropical environments to prevent water loss.