PHOTOSYNTHESIS

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SUBTOPICS

16.5 Alternative mechanisms of carbon fixation : Hatch-Slack(C4) and Crassulacean Acid Metabolism (CAM) pathways

PREVIOUS LESSON

OBJECTIVES

At the end of this topic, students should beable to;

● Describe carbon fixation in C4 and CAM plants

PHOTORESPIRATION

Stomata on leaf surface of C3 plant close during hot day to conserve water.

This cause a drop in CO2 and an increase in O2 (by product of light dependent reaction) in the leaf.

Thus, cause photorespiration.

What is photorespiration?

PHOTORESPIRATION

“The process that reduces the efficiency of photosynthesis in C3 plants during hot day;

consumes oxygen and produces carbon dioxide through the degradation of Calvin cycle intermediates.”

(Solomon, 8th edition)

“A metabolic pathway that consumes oxygen and ATP, releases carbon dioxide and decreases

photosynthetic output.” (Campbell, 9th edition)

PHOTORESPIRATION

Why photorespiration lowers photosynthetic output?

Photorespiration decreases photosynthetic output by adding oxygen, instead of carbon dioxide to Calvin cycle. As the result, no sugar generated (no carbon

fixed) and oxygen is used rather than generated.

ALTERNATIVE MECHANISMS OF CARBON FIXATION

In some plant species, alternate modes of carbon fixation have evolved that minimize photorespiration and optimize the Calvin cycle even in hot, arid climates.

There two photosynthetic adaptations:C4 photosynthesis.Crassulacean Acid Metabolism (CAM).

HATCH-SLACK PATHWAY (C4)

C4 plant so named because they preface the Calvin cycle with an alternate mode of carbon fixation that

forms a four-carbon compound as its first product.

Campbell,9th Edition, page 246

LEAF ANATOMY OF C4 PLANT

In C4 plants, there are two types of photosynthetic cells:

Bundle sheath cells - arrange tightly packed sheath around the veins of leaf.

Mesophyll cells- loosely arranged between bundle sheath and leaf surface

This arrangement are called Kranz anatomy.

HATCH-SLACK PATHWAY (C4)

Involves enzyme called PEP carboxylase. Present in mesophyll cells.

PEP carboxylase has a much higher affinity for CO2 than does rubisco and no affinity to O2. PEP carboxylase can fix carbon

efficiently when rubisco cannot – during hot, dry and stomata are partially closed.

Cause CO2 concentration in leaf to decrease and O2 concentration to increase.

1. In mesophyll cells, the enzyme PEP carboxylase adds carbon dioxide to phosphoenolpyruvate (PEP).Forming four-carbon product

oxaloacetate (OAA).

2. OAA reduce to malate

3. Malate transported into the bundle-sheath cell via plasmodemata.

HATCH-SLACK PATHWAY (C4)

12

3

4

4. In bundle-sheath cell, malate release CO2 (decarboxylation) forming pyruvate CO2 enters Calvin cycle Pyruvate transported

back to mesophyll cell & phosphorylated to form PEP

Allowing the following cycle to continue

HATCH-SLACK PATHWAY (C4)

2

3

1

4

Crassulacean Acid Metabolism (CAM) MECHANISM

CAM plants open stomata during the night and close them during the day. To minimize photorespiration

Closing stomata during the day helps desert plants conserve water, But, this prevents CO

2 from

entering the leaves.

1. During the night, stomata

are open

CO2 enters the leaf tissue

CO2 fixed with PEP catalysed

by PEP carboxylase to form

oxaloacetate (OAA)

  OAA oxidise into malate

1. Malate is transported into the

vacuole

CAM PATHWAY

1

2

3

3.During day, stomata closed,

malate moved into chloroplast Malate undergo

decarboxylation release

CO2 and form pyruvate

back

CO2 then enters Calvin

cycle

CAM PATHWAY

1

2

3

CAM MECHANISM

C3, C4 and CAM plants

C3 Plants C4 Plants CAM Plants

WheatPaddy plantSoybean

CornSugar cane

CactusPineappleOrchid

QUESTIONS

1. Explain why photorespiration lowers photosynthetic output for plants.

2. Name the 2 important enzymes involved in C3 and C4 pathways, respectively. What important role do they play in fixing CO2?

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17.0 GASEOUS EXCHANGE AND ITS CONTROL