Photorespiration:

* Rubisco catalyze its oxygenation ability

omnipresent, even in anaerobic, autotrophic bacteria when exposed to oxygen

* Loss of CO2 from cells

* Competition: decrease the efficiency of photosynthesis * Interconnection: determined by the kinetic properties of rubisco, the concentration of substrates, and temperature

* C2 oxidative photosynthetic carbon cycle: act as a scavenger operation to recover fixed carbon lost during photorespiration

Three organellesCarbon flow 22C13C+CO

2

75%Nitrogen flow no changedOxygen flow 3 O2/2 RuBP

Malate-OAA shuttle supply NADH

Web Topics 8.6

[gas] µM = Pgas 106/ V0

In vitro vs. In vivo

Solubility of CO2 and O2 as a function of temperature

Pgas: partial pressure; : absorption coefficient

T tilt toward the C2 oxidative photosynthetic cycle

ROS

Photorespiration depends on the photosynthetic electron transport system

The biological function of photorespirationis under investigation

＊ a protective, to dissipate excess ATP and reducing

power, especially under high light intensity and low

[CO2]inter (e.g., water stress)

＊ mutants

lack glycerate kinase, not viable in normal air

＊ linked photorespiration to nitrate assimilation

a full understanding is still not at hand

CO2-concentrating mechanisms:

A. C4 photosynthetic carbon fixation (C4), in hot environment;B. Crassulacean acid metabolism (CAM), in desert environment;

C. CO2 pumps at the plasma membrane. In aquatic plants, such as unicellular cyanobacteria and algae.

In aquatic environment, [CO2] low rubisco specificity activity low

CO2-HCO3- pumps at the plasma membrane are induced,

to accumulate inorganic carbon

light energy provide ATP to uptake CO2 and HCO3-

carbonic anhydrase:

HCO3- + H+ → H2O + CO2 → Calvin cycle

[CO2] ↑ suppress photorespiration

0.03% CO2 / 21% O2

Cyanobacterial CO2 concentrating mechanism

— high homologous to the Rheus, a protein in erythrocytes

The C4 carbon cycleKranz (wreath) cells: present two distinct chloroplast-containing cells,

mesophyll and bundle sheath cells

spatial

OAA

Calvin cycle

sugarcane

Flaveria australasica

Poa sp

The C4 photosynthetic pathway: Hatch and Slack

Gramineae (corn, millet, sorghum, sugarcane);

Chenopodiaceae (Atriplex); Cyperaceae (sedges).

external

vascular

NADP-ME: in chloroplastNAD-ME: in mitochondriaPEP carboxykinase: in cytosol

plasmodesmataspecific translocators

Web Topic 8.7 (?)

Three variations of C4 metabolism

The form of transportation

The manner of decarboxylation

Aspartate aminotransferase

PEP carboxykinase

Alanine aminotransferase

(1) maize, crab grass, sugarcane, sorghum;

(2) pigweed, millet;

(3) guinea grass.

Kranz anatomy: mesophyll and bundle-sheath cells carbon concentrating mechanism / suppressed photorespiration

plasmodesmata

Photosynthetic Carbon assimilation reduction

Borszczowia aralocaspica

Bienertia cycloptera

Chloroplasts containing rubisco are near mitochondria with NAD-ME