Bio102ReviewUnit3

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TYPES OF PHOTOSYNTHESIS

Concepts:

•

Photosynthesis:

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.

•

Stomata:

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).

•

Transpiration:

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.

•

Water 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.

•

Photorespiration:

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

photosynthesis..

The three

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.

C3

Photosynthesis : C3 plants.

•

Called

C3 because the CO2 is first incorporated into a 3-carbon compound.

•

Stomata

are open during the day.

•

RUBISCO,

the enzyme involved in photosynthesis, is also the enzyme involved in the

uptake of CO2.

•

Photosynthesis

takes place throughout the leaf.

•

Adaptive Value:

more 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.

C4

Photosynthesis : C4 plants.

•

Called

C4 because the CO2 is first incorporated into a 4-carbon compound.

•

Stomata

are open during the day.

•

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)

•

Adaptive Value:

◦

Photosynthesizes

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 for photosynthesis.

•

C4

plants include several thousand species in at least 19 plant families. Example:

fourwing saltbush pictured here, corn, and many of our summer annual plants.

CAM

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

•

Adaptive Value:

◦

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

Photo