TYPES OF PHOTOSYNTHESIS
page introduces you to photosynthesis and explains the three types of photosynthesis
and their relevance for desert adaptation.
the bonding together of CO2 (carbon dioxide) with H2O
(water) to make CH2O (sugar) and O2
(oxygen), using the sun's energy. The sugar contains the stored energy and
serves as the raw material from which other compounds are made. Respiration
is the opposite of photosynthesis -- the stored energy in the sugar is released
in the presence of oxygen, and this reaction releases the CO2
and H2O originally jammed together by the sun's energy.
the "pores" in leaves (and stems) through which CO2
is taken in and O2 is released during photosynthesis.
Plants control when stomata are open or closed and the width of the opening
(formed by two guard cells that expand and contract to open and close the
space between them).
the water that evaporates out of stomata when they are open. This pulls more
water and nutrients up to the top of the plant, but causes the plant to lose
water and potentially dehydrate.
Use Efficiency (WUE):
How good the plant is at bringing in carbon dioxide for photosynthesis without
losing much water out of its stomata. More specifically, it is the ratio of
carbon dioxide intake to water lost through transpiration.
Under high light and high heat, the enzyme (RUBISCO) that grabs carbon dioxide
for photosynthesis may grab oxygen instead, causing repiration to occur instead
of photosynthesis, thus causing a slowing of the production of sugars from
types of photosynthesis are C3, C4,
and CAM. C3 photosynthesis is the typical photosynthesis
tha most plants use and that everyone learns about in school (it was all we
knew about until a few decades ago). C4 and CAM photosynthesis
are both adaptations to arid conditions because they result in better water
use efficiency. In addition, CAM plants can "idle," saving precious
energy and water during harsh times, and C4 plants can
photosynthesize faster under the desert's high heat and light conditions than
C3 plants because they use an extra biochemical pathway
and special anatomy to reduce photorespiration. Below are the details.
Photosynthesis : C3 plants.
- Called C3
because the CO2 is first incorporated into a 3-carbon
- Stomata are open during
- RUBISCO, the enzyme
involved in photosynthesis, is also the enzyme involved in the uptake of CO2.
- Photosynthesis takes
place throughout the leaf.
efficient than C4 and CAM plants under cool and moist
conditions and under normal light because requires less machinery (fewer enzymes
and no specialized anatomy)..
- Most plants are C3.
Photosynthesis : C4 plants.
- Called C4
because the CO2 is first incorporated into a 4-carbon
- Stomata are open during
- Uses PEP Carboxylase
for the enzyme involved in the uptake of CO2. This
enzyme allows CO2 to be taken into the plant very quickly,
and then it "delivers" the CO2 directly to
RUBISCO for photsynthesis.
- Photosynthesis takes
place in inner cells (requires special anatomy called Kranz Anatomy)
faster than C3 plants under high light intensity
and high temperatures because the CO2 is delivered
directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration.
- Has better Water
Use Efficiency because PEP Carboxylase brings in CO2
faster and so does not need to keep stomata open as much (less water lost
by transpiration) for the same amount of CO2 gain
plants include several thousand species in at least 19 plant families. Example:
fourwing saltbush pictured here, corn, and many of our summer annual plants.
Photosynthesis : CAM plants. CAM stands for Crassulacean Acid Metabolism
- Called CAM after the plant family in which it was first found (Crassulaceae)
and because the CO2
is stored in the form of an acid before use in photosynthesis.
- Stomata open at night (when evaporation rates are usually lower) and are
usually closed during the day. The CO2
is converted to an acid and stored during the night. During the day, the acid
is broken down and the CO2
is released to RUBISCO for photosynthesis
- Better Water Use
Efficiency than C3 plants under arid conditions due to opening stomata
at night when transpiration rates are lower (no sunlight, lower temperatures,
lower wind speeds, etc.).
- May CAM-idle. When
conditions are extremely arid, CAM plants can just leave their stomata
closed night and day. Oxygen given off in photosynthesis is used for respiration
and CO2 given off in respiration is used for photosynthesis.
This is a little like a perpetual energy machine, but there are costs
associated with running the machinery for respiration and photosynthesis
so the plant cannot CAM-idle forever. But CAM-idling does allow the plant
to survive dry spells, and it allows the plant to recover very quickly
when water is available again (unlike plants that drop their leaves and
twigs and go dormant during dry spells).
- CAM plants include many succulents such as cactuses
and agaves and also some orchids and bromeliads