BLG 143 Study Guide - Final Guide: Light-Independent Reactions, Carbon Fixation, Photorespiration
27 Apr 2018
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10.4 How Reduced Carbon Dioxide Produces Glucose
Calvin Cycle Fixes Carbon
Carbon fixation addition of carbon dioxide to an organic
compound; process converts/fixes CO2 to a biologically
useful form; once carbon atoms are fixed, they can be used
to build the molecules found in cells
Carbon fixation is a redox reaction, the carbon atom in CO2 is reduced.
This energy transformation of the light-dependent reactions and the carbon dioxide
reduction of the Calvin cycle are two separate but linked processes in photosynthesis.
ATP and NADPH are produced by I and II in the presence of light.
Reactions that produce sugar from carbon dioxide in the Calvin cycle are light independent.
These reactions require ATP and NADPH produced by the light dependent reactions.
Ribulose 1,5-biphosphate RuBP is the initial reactant with CO2 to produce 2 molecules of 3-
phosphoglycerate.
Ribulose-1,5-biphosphate carboxylase
➢ CO2 fixing enzyme
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➢ Found in all photosynthetic organisms that use the Calvin cycle to fix carbon and is
thought to be the most abundant enzyme on earth
➢ Inefficient cause although it catalyzes the addition of carbon dioxide to RuBP, it also
catalyzes the addition of oxygen to RuBP
Oxygen and carbon dioxide compete at the enzyme’s active sites, which slows the rate of
the carbon dioxide reduction.
When O2 and RuBP react in rubisco’s active site, one of the product undergoes
photorespiration, which undoes photosynthesis because it consumes energy and releases
fixed CO2.
When photorespiration occurs, the rate of photosynthesis declines drastically.
Carbon fixation is favoured over photorespiration when a cell’s CO2 [] is high and O2 [] is
low.
CO2 Enters Leaves through Stomata
The surface of a leaf is dotted with openings
bordered by 2 cells.
Guard cells paired cells; opening is called a
pore
Stoma entire structure
A strong concentration gradient favouring entry
of CO2 is maintained by the Calvin cycle, which constantly uses up the CO2 in chloroplasts.
C4 Photosynthesis
• Plants from hot, dry habitats
• C3 and C4 photosynthesis
• Spatially separates carbon fixation and the Calvin cycle
• During carbon fixation, C4 incorporate CO2 into 4-carbon organic acids instead of 3-
phosphoglycerate (performed by C3 plants)
• These plants, also in hot dry habitats, keep their stomata closed all day and open
them only at night
• C4 photosynthesis and CAM function as CO2 pumps; they minimize photorespiration
when stomata are closed and CO2 cannot diffuse in directly from the atmosphere
• C4 plants stockpile CO2 in cells when rubisco isn’t present, CAM plants store when
rubisco is inactive
• C4 plants, the reactions catalyzed by PEP carboxylase and rubisco are separated in
space; in CAM plants, the reactions are separated in time
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Carbon fixation addition of carbon dioxide to an organic compound; process converts/fixes co2 to a biologically useful form; once carbon atoms are fixed, they can be used to build the molecules found in cells. Carbon fixation is a redox reaction, the carbon atom in co2 is reduced. This energy transformation of the light-dependent reactions and the carbon dioxide reduction of the calvin cycle are two separate but linked processes in photosynthesis. Atp and nadph are produced by i and ii in the presence of light. Reactions that produce sugar from carbon dioxide in the calvin cycle are light independent. These reactions require atp and nadph produced by the light dependent reactions. Ribulose 1,5-biphosphate rubp is the initial reactant with co2 to produce 2 molecules of 3- phosphoglycerate. Found in all photosynthetic organisms that use the calvin cycle to fix carbon and is thought to be the most abundant enzyme on earth.
