6.1: Introduction

Chloroplasts

Credit: elvinsong [CC-BY-SA 3.0]

Chloroplasts arose through a second endosymbiotic event in plants and various protists. These light-harvesting organelles share a similarity in structure and genome to photoautotrophic cyanobacteria.

Light-Harvesting

The thylakoid membranes of chloroplasts and cyanobacteria provide additional surface area for energy capture of light to occur. The light-dependent reactions in chloroplasts utilize two protein complexes referred to as Photosystem I (PSI) and Photosystem II (PSII) located on the thylakoid membranes. At the center of each photosystem complexes are photopigments optimized to absorb specific wavelengths of light. When light is absorbed in a photosystem, an electron is excited and transferred to the electron transport chain. In PSII, the electron is regenerated by splitting of two water molecules into 4H⁺ + 4e^– + O₂. As the electrons move through the ETC, protons are pumped into the thylakoid space. The ETC leads to the reduction of a high energy electron carrier NADP⁺ to NADPH. Since this pathway uses consumes water in a chemical reaction, the apparent loss of water in the thylakoid space is referred to as chemiosmosis.

PSI is also known as the cyclic pathway since the excited electron runs through a closed circuit of the ETC to regenerate the lost electron. This closed circuit also generates a proton gradient through powering of a proton pump but does not lead to the reduction of NADPH. As with the ETC-powered proton pump in mitochondria, the proton gradient is used to power ATP-synthase in producing ATP molecules.

Light Independent Reactions

Credit: Mike Jones [CC-BY-SA 3.0]

The light-independent reactions are also known as the dark reactions or Calvin Cycle and utilize the ATP and NADPH from the light-dependent reactions to fix gaseous CO₂ into carbohydrate backbones. Photosynthesis is often simplified into 6CO₂ + 6H₂O + light –> C₆H₁₂O₆ + 6O₂ . However, the true product is 3-phosphoglycerate that can be used to generate longer carbohydrates like glucose. The starting point of carbon fixation is the carbohydrate Ribulose 1,5-bisphosphate. The enzyme Ribulose Bisphosphate Carboxylase (RuBisCO) captures a CO₂ molecule onto Ribulose 1,5-bisphosphate to generate 2 molecules of 3-phosphoglycerate which can enter the process of gluconeogenesis to generate glucose. ATP from the light reactions can then facilitate the conversion of 3-phosphoglycerate to 1,3 bisphosphoglycerate which can be reduced by NADPH to glyceraldehyde-3-phosphate (G3P). G3P can then be used to regenerate Ribulose 1,5-bisphosphate.

1: Carbon fixation by RuBisCO

2: Reduction by NADPH

3: Ribulose, 5-bisphosphate regeneration

The Great Oxygenation Event

Two estimates of the evolution of atmospheric O₂. The upper red and lower green lines represent the range of the estimates. Stage 1 (3.85–2.45 Ga) represents the primordial reducing atmosphere. Stage 2 (2.45–1.85 Ga) coincides with the emergence of oceanic cyanobacteria where O₂ was being absorbed by the oceans and sediment. O₂ escaped the oceans during Stage 3 (1.85–0.85 Ga). O₂sinks filled in Stage 4 (0.85–0.54 Ga ) and Stage 5 (0.54 Ga–present) leading to atmospheric accumulation.

Banded iron formations in 2.1 billion-year-old rock illustrate the oxidation of dissolved oceanic iron that precipitated in response to accumulating O₂ concentrations.

The Carbon Cycle illustrates carbon sequestration and release between various carbon sinks.

Projection of atmospheric CO₂ accumulation without reduction of fossil fuel reduction by NASA.