UNIT 2: Metabolic Processes
Chapter 5: Photosynthesis: The Energy of Life pg. 210 - 240
5.4: Alternative Mechanisms of Carbon Fixation
pg. 231 – 234
Photosynthesis requires reactants; CO2 and H2O, to produce C6H12O6, H2O
and O2.
Atmospheric gases available to photosynthetic organisms are 0.04% CO2 and
21.0% O2.
Preventing Water Loss
Plants require water to perform photosynthesis; they acquire this water from
their environment, using their roots to absorb the water from the soil. Water
enters the cell creating the cytosol. Carbon dioxide also required for
photosynthesis enters the plant through the stomata of the leaf. The water in
the cytosol and carbon dioxide supply the chloroplast, where the process of
photosynthesis occurs.
Environmental factors change which may hinder water, carbon dioxide
concentrations in the plant and negatively influence photosynthesis. Plants
have special adaptations to limit the impact of these environmental factors.
Plants have a waxy water proof layer called a cuticle. This covering prevents
water lose by transpiration. This ensures that there is not a rapid loss of
water and photosynthesis may continue.
The exchange of gases with the atmosphere is controlled by the stomata of
the leaves. These stomata can open and close during the day or night. When
the stomata are open during the day, gas exchange takes place; carbon
dioxide enters the leaf and oxygen leaves entering the atmosphere. At night
the stomata close and photosynthesis stops.
Stomata – are small pores in the surface of a leaf that can be opened and
closed to control the exchange of gases between the atmosphere and the leaf
interior. (Stoma – singular)
Photorespiration: The Problem with Rubisco
Photorespiration – is the catalysis of O2 instead of CO2 by rubisco into
RuBP, which slows the Calvin cycle, consumes ATP, and results in a
release of carbon.
All metabolic reactions are catalyzed by enzymes. The rate or efficiency is
dependent on the availability of reactants, enzymes, temperature, pH, and
inhibitors.
Rubisco, is the most abundant enzyme on earth, is a very slow working
enzyme. The rate of photosynthesis occurs fast, the rate of this reaction is
supported by the abundance of rubisco.
Rubisco (Ribulose-1,5-bisphosphate carboxylase oxygenase) can also bind
with oxygen molecules and catalyzes RuBP with oxygen, but the product is
not used by the cell. This molecule must be converted back to useful
molecules and release RuBP. The process is long and occurs in the
chloroplast, peroxisomes and mitochondria. The final product released is
carbon dioxide. This process is called photorespiration.
C4 Plants
C4 Cycle – is an alternative form of carbon fixation that some plants use,
particularly in hot weather, to increase the concentration of CO2 available
for the Calvin cycle reactions.
C4 plants are plants that are found in hot, dry climates, and have internal leaf
structures and mode of carbon fixation that minimizes photorespiration. To
prevent photorespiration in these plants, the Calvin cycle is found in special
cells called bundle-heat cells, which are connected to mesophyll cells. This
structure reduces the exposure of rubisco to oxygen
In the C4 plant CO2 combines with phosphoenolpyruvate (PEP),
carboxylation reaction facilitated by PEP carboxylase, producing
oxaloacetate. The oxaloacetate is then reduced to malate. As the malate
passes into the bundle-sheath cell, it loses a CO2 molecule, decarboxylation
reaction, and becomes pyruvate. Pyruvate leaves the bundle-sheath cell
returning to the mesophyll cell, where it converted to PEP by ATP,
phosphorylation reaction.
The CO2 molecule released into the bundle-sheath cycle is now used in the
Calvin cycle (C3 cycle), with rubisco facilitating the reaction.
Oxygen is prevented from interacting with Rubisco, preventing
photorespiration, because oxygen is unable to reach the bundle-sheath cell.
This C4 cycle is energy consuming since ATP is used to convert pyruvate
into PEP.
Figure 3: C4 cycle, pg. 232
CAM Plants
Crassulacean Acid Metabolism (CAM) – is a metabolic pathway, used
mostly by succulent plants, in which the Calvin cycle and the C4 cycle are
separated in time for better efficiency of CO2 fixation.
Plants that run their C4 cycle in the same cells as their Calvin cycle (2
phases), unlike the C4 plant, that use mesophyll cells and bundle-sheath cells,
are called Crassulacean Acid Metabolism or Cam plants.
These types of plants live in hot and dry climates during the day and cool at
night, such as; cacti and succulent species. These plants open their stomata
at night to release oxygen that has built up during photosynthesis throughout
the day and take in carbon dioxide.
The carbon dioxide enters the C4 cycle right away, where it is fixed into
molecules of malate. The malate is stored as malic acid in the cell’s vacuoles
for use during the day.
During daylight, the C4 cycle stops, the stomata are closed and gas exchange
ceases. The surplus malic acid diffuses from the vacuoles into the cytosol
and is oxidized into pyruvate and carbon dioxide is released. The pyruvate is
recycled back to malate during the night. The carbon dioxide enters the
Calvin cycle, as Rubisco carboxylizes it into 3-phosphoglycerate.
Figure 4: C4 cycle and Cam plants, a) Some C4 plants, such as sugarcane, control the
location of the C4 and Calvin cycles. b) CAM plants, such as Pineapple, also use a C4
Cycle, carrying out the two cycles in the same cells but at different times. pg. 233