PACKET #34CHAPTER #10

Photorespiration

Introduction

In the 1960’s, it was discovered that illuminated plants consume and use O2 and produce CO2.

With low CO2 levels and high O2 levels, this photorespiration overwhelms photosynthetic CO2 fixation (light reactions + Calvin cycle).

Introduction II

In a very lengthy and costly process, O2 is converted into CO2 and 3-phosphoglyceraldehyde.

Photorespiration involves the use of three organelles Chloroplast Peroxisome Mitochondria

Photorespiration also requires the use of ATP and NADPH. Reducing the number of

those molecules readily available for the Calvin cycle (photosynthesis).

Introduction III

Introduction IV

Plants can be divided into three categories based on how they deal with photorespiration. C3 plants

No mechanism developed to decrease photorespiration. C4 plants CAM plants

Both C4 and CAM plants have mechanisms in place to decrease photorespiration.

Photorespiration & C3 Plants

Photorespiration in C3 Plants

On dry, hot days in the presence of light C3 plants close their stomata.

This causes the plant to use O2 retain as much H2O. O2 binds to rubisco and

starts the series of reactions.

H2O is retained for use in the light reactions to fill the ATP and NADPH used as a result of photorespiration.

C4 & CAM Plants Introduction

C4 and CAM plants have devised mechanisms that prevent/reduce the impact of photorespiration.

CONCENTRATING CO2

C4 Plants & Photorespiration

Introduction I

C4 plants occur largely in tropical regions because they grow faster under hot and sunny conditions. C3 plants live in cooler

climates where photorespiration is less of a burden and less ATP is required to fix carbon.

Introduction II

On a hot bright day, when photosynthesis has depleted the level of CO2 at the chloroplast and raised that of O2, the rate of photorespiration reaches the rate of photosynthesis.

However, C4 plants have leaves that are different anatomically to that of C3 plants and have devised a mechanism to reduce the impact of photorespiration.

Introduction III

Photorespiration is negligible in C4 plants because the concentration of carbon dioxide is always high in the bundle sheath cells. C4 plants

“concentrate” CO2.

C4 Plants Process I

Phosphoenol-pyruvate (PEP) Oxaloacetate PEP carboxylase adds

CO2 to PEP to produce Oxaloacetate

Occurs in the mesophyll cell.

C4 Plants Process II

OxaloacetateMalate Malate dehydrogenase

reduces oxaloacetate into malate. NADPH is used during

this step.

C4 Plants Process III

Malate is transported from mesophyll cell into the bundle sheath cell.

C4 Plants Process IV

Malate Pyruvate Malic enzyme converts

malate into pyruvate. Two byproducts are

made. CO2

• The CO2 is now considered to be concentrated.

NADPH• Both are used in the

Calvin Cycle that occurs within the bundle sheath cell.

C4 Plants Process V

Pyruvate leaves the bundle sheath cell and enters the mesophyll cell.

C4 Plants Process VI

PyruvatePEP Pyruvate-phosphate

dikinase converts pyruvate into PEP.

C4 Plants Process VII

Process is repeated to concentrate more CO2.

Additional Information on C4 Plants.

At lower light levels and temperature, C4 plants will utilize the traditional C3 pathway.

Examples of C4 plants Sugarcane Corn Crab grass.

STORING CO2

CAM Plants & Photorespiration

Introduction I

CAM is an acronym for crassulacean acid metabolism.

Examples Succulent plants

{family Crassulaceae} Family Cactaceae Family Lilaceae Family Orchidaceae Many others in 25

families.

Introduction II

CAM plants exhibit a pathway similar to C4 plants and allow them to live in highly xeric conditions. CAM plants store CO2

using a variation of the technique used by C4 plants.

CAM Plants—Night

CAM plants open their stomata at night. If these plants, living in

the xeric conditions opened their stomata during the daytime, would lose large amounts of H2O through osmosis and then evaporation.

PEP carboxylase fixes carbon at night in the mesophyll cells Stomata are open at night

Minimizes water loss and allows the entry of CO2

Calvin Cycle occurs during the daytime

CAM Plants—Night II

PEPoxaloacetatemalate PEP carboxylase and

malate dehydrogenase fixes CO2 at night in the mesophyll cells.

Malate is stored in vacuoles.

CAM Plants—Daytime I

During the daytime, while the stomata is closed, malate is converted into pyruvate in the bundle sheath cell. This allows the

production of CO2.

CO2 is used to drive the Calvin cycle in the bundle sheath cell.

Review

Review

Types of plants C3 C4

Concentrate CO2 in the bundle sheath cells.

CAM Store CO2 in the form of malate by having the stomata

only open at night.

Homework Assignment

What are some of the similarities, and differences, between C4 and CAM plants?