Photorespiration

AP BIO

Review• Stomates need

to be OPEN for gas exchange to occur in the leaf

• However, open stomates can lead to dehydration due to transpiration

Transpiration: the loss of water from a plant through stomates

Low water= closed stomates=little photosynthesis

Result: CO2 levels in plant DECREASE; O2 levels in plant INCREASEThis results in PHOTORESPIRATION! (This is a bad thing for the plant)

PhotorespirationPhotorespiration results from increased O2 levels in the plant

*RUBISCO (from Calvin Cycle)-prefers to bind to O2 instead of CO2 so the Calvin cycle will continue but without the necessary Carbon

-this uses ATP but produces no sugar

=decrease in photosynthetic output

O2

O2O2

O2

O2

O2CO2

C₃ PLANTSExamples:

-rice-wheat-soybeans

*create a 3 carbon compound in 1st step of Calvin

Cycle

*photorespiration is a problem for these

plants in dry/hot areas

How do plants adapt to Hot/ Dry Conditions?

Here are some different types of plants and the strategies they use to overcome the challenges of doing photosynthesis in hot/dry conditions

Photosynthesis Adaptations for Hot Climates

C4 Plants

Examples:-corn-sugarcane-grasses

*have two types of photosynthetic cells:-bundle sheath (around vein in picture on the left)-mesophyll cells

C4 Plants - continued

*Calvin Cycle happens in bundle sheath cells

*Calvin Cycle has a pre-step in the mesophyll cells

-PEP carboxylase binds to CO2 to form a 4-carbon compound

*PEP does NOT bind to O2

-4-carbon compound goes to bundle sheath cells and releases CO2 for Rubisco

RESULT: high CO2 concentration and no photorespiration

Photosynthesis Adaptations for Hot Climates (continued)

CAM Plants

Photosynthesis Adaptations for Hot Climates (continued)

Examples:-cacti, jade, pineapple

-Do gas exchange at night!

Open stomates at night and take in CO2 to store as organic acids

-During the day, stomates close to prevent water loss

-Stored organic acids are broken down to release CO2 into Calvin cycle so plant can make sugars

Before reading the next slides answer the following question.

• What are some factors that limit the rate of photosynthesis?– List as many as you can in your notes.

Limiting Factors for Photosynthesis

Light Intensity

*high light intensity increases rate of photosynthesis until it reaches saturation

Limiting Factors for Photosynthesis

Temperature

*temperature increase causes an increase in rate of photosynthesis until it reaches its ideal temperature, then

it decreases

Limiting Factors for Photosynthesis

CO2 and H2O Concentration

*high levels of both CO2 and H2O increase the rate of photosynthesis until it reaches saturation. Then the rate

levels off.

Concentration of CO2 and H2O

Rate

of P

hoto

synt

hesi

s

Limiting Factors for Photosynthesis

O2 Concentration

*high levels of O2 cause a decrease in the rate of photosynthesis, while lower levels increase the rate until it

levels off

Concentration of O2

Rate

of P

hoto

synt

hesi

s