Plant Metabolism

Chapter 10

Copyright © McGraw-Hill Companies PermissionRequired for Reproduction or Display

Outline

• Photosynthesis Major Steps of Photosynthesis Light-Dependent Reactions Light-Independent Reactions C4 Photosynthesis CAM Photosynthesis

• Respiration Glycolysis Electron Transport Chain

Enzymes and Energy Transfer

• Enzymes regulate most metabolic activities. Anabolism - Storing Energy.

- Photosynthesis reactions Catabolism - Consuming Stored Energy.

- Respiration reactions• Oxidation-Reduction Reactions

Oxidation - Loss of electron(s). Reduction - Gain of electron(s)

- Usually coupled

Photosynthesis

• Energy for most cellular activity involves adenosine triphosphate (ATP). Plants make ATP using light as an energy

source.- Take place in cholorpolasts and other

green parts of the organisms.

6CO2+12H2O + light C6H12O6+6O2+6H2O

Carbon Dioxide

• Carbon dioxide (0.035% of air) reaches cholorplasts in the mesophyll cells by diffusing through the stomata into the leaf interior. If not replenished, CO2 would be used up in 22

years. Use of fossil fuels, deforestation, and other human activities have added excess carbon dioxide to the atmosphere.

- May enhance photosynthesis. Plants may counter-balance by developing

fewer stomata.

Water

• Less than 1% of all the water absorbed by plants is used in photosynthesis. Most of the remainder is transpired or

incorporated into plant materials.• If water is in short supply, stomata usually

close and thus reduce the supply of carbon dioxide available for photosynthesis.

Light

• About 40% of the radiant energy received on earth is in the form of visible light. Leaves commonly absorb about 80% of

the visible light reaching them.- Light intensity varies with time of day,

season, altitude, latitude, and atmospheric composition.

Considerable variation in the light intensities necessary for optimal photosynthetic rates.

Light Wavelengths

Effects of Light and Temperature on Photosynthesis

Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display

Chlorophyll

• Several different types of chlorophyll. Most plants contain both chlorophyll a

(blue-green) and chlorophyll b (yellow-green).

- Other pigments include carotenoids (yellow and orange) phycobilins (blue or red), and several other types of chlorophyll.

- About 250-400 pigment molecules group as a photosynthetic unit.

Grass green in color: C55H72MgN4O5

Chlorophyll b: bluish green in color : C55H70MgN4O6

3a:1b

Major Steps of Photosynthesis

• Light Dependent Reactions Water molecules split apart. Electrons passed along electron transport. ATP produced. NADPH produced.

Major Steps of Photosynthesis

• Light Independent Reactions Calvin Cycle

- Carbon dioxide combined with RuBP and then combined molecules are converted to sugars (Glucose).

Energy furnished by ATP and NADPH from Light-Dependent Reactions.

Light Dependent Reactions - In Depth

• Each pigment has its own distinctive pattern of light absorption.

When pigment absorbs light: electrons are excited. When this occurs, energy may beimmediately emmitted as light:flourescence or emmitted after a delay : phosphorescence ormay be converted to heat.

Light Dependent Reactions - In Depth

• Two types of photosynthetic units present in most chloroplasts make up photosystems. Photosystems I and II

- Both can produce ATP.- Only organisms with both photosystem I

and photosystem II can produce NADPH and oxygen as a consequence of electron flow.

Photosystems

Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display

Mitchell Theory or Chemiosmosis

Light Independent Reactions - In Depth

• Calvin Cycle Six molecules of CO2 combine with six

molecules of RuBP with the aid of rubisco. Resulting complexes split into twelve

3PGA molecules. NADPH and ATP supply energy and

electrons that reduce 3PGA to 12 GA3P. Ten of the twelve GA3P molecules are

restructured into six RuBP molecules.

The Calvin Cycle

Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display

Photorespiration

• Stomata usually close on hot, dry days. Closed stomata prevent carbon dioxide

from entering the leaf. Then CO2 decrease as relative O2 increase.

- When carbon dioxide levels drop below about 50 parts per million, photorespiration is initiated.

Rubisco fixes oxygen instead of carbon dioxide.

Light Independent Reactions - In Depth

• 4-Carbon Pathway Sugar cane, corn, sorghum and at least

1000 species of plants have Kranz Anatomy. Two forms of chloroplasts:

- Large chloroplast with few to no grana in the bundle sheath cells surrounding the veins.

- Smaller chloroplasts with well-developed grana in the mesophyll cells.

Corn (Zea Mays) Cross-Section

Fig. 10.2a

4-Carbon Pathway

• Plants with Kranz Anatomy produce oxaloacetic acid (4-carbon compound). Phosphoenolpyruvat

e (PEP) and carbon dioxide combined in mesophyll cells with the aid of PEP carboxylase.

- Provides a major reduction in photorespiration.

CAM Photosynthesis

• Seen in plants of about 30 families including cacti orchids etc.

• Similar to C4 photosynthesis in that 4-carbon compounds are produced during the light-independent reactions. However, in CAM, the organic acids

accumulate at night and break down during the day, releasing carbon dioxide.

- Allows plants to function well under limited water supplies, as well as high light intensity.

CAM Photosynthesis

Fig. 10.22

Comparision

Leaf Anatomy No bundle sheath

Krantz Anatomy

No palliside cells, large vauoles

Carboxylation Enzymes

RuBisco PEP +RuBisco Drakness PEP +Light RuBisco

Transpiration ratio (gm H2O/ gm dry weight increase)

450-950 250-350 50-55

Optimum P.S. Temperature

15-25 oC 30-40 oC 35 oC

C3 C4CAM

CO2 Compensation point

30-70 ppm 0-10 ppm 0-5 ppm in dark

Dry matter prod. 22 tons/ha/year 39 tons/ha/year Low; highly variable

C4 vs. CAM

• C4 Stores CO2 in 4

carbon acid Separates CO2

fixation from sugar synthesis in SPACE(Bundle sheath vs mesophyll space)

Adaptations to TROPICAL environments

• CAM Stores CO2 in 4

carbon acid Separates CO2

fixation from sugar synthesis in TIME

Adaptations to ARID environments

Other Processes in Chloroplasts

• 1) Sulphates : reduced to sulphides

these then used for biosynthesis of

S-containing amino acids

• 2) Nitrites converted to ammonia. This is then converted to amino groups which are integral part of several important amino acids such as glutamine and aspartic acid.

Respiration

• Respiration is essentially the release of energy from glucose molecules that are broken down to individual carbon dioxide molecules.

C6H12O6 + 6O2 6CO2 + 6H2O + energy

Respiration

• Fermentation

C6H12O6 2C2H5OH + 2CO2 + ATP

C6H12O6 2C3H6O3 + ATP

Factors Affecting the Rate of Respiration

• Temperature• Water• Oxygen

Major Steps of Respiration

• Glycolysis Glucose molecule becomes a fructose

molecule carrying two phosphates. Fructose molecule is split into two GA3P

molecules. Some hydrogen, energy, and water are

removed, leaving pyruvic acid.

Major Steps of Respiration

• Aerobic Respiration Citric Acid (Krebs) Cycle

O.A. + acetyl CoA + ADP+P+3NAD + FAD

O.A. + CoA+ATP+3NADH+H+ + FADH2+2CO2

• Electron Transport Oxidative Phosphorylation Chemiosmosis

Assimilation and Digestion

• Assimilation is the process of using organic matter produced through photosynthesis to build protoplasm and cell walls.

• Digestion is the conversion of starch and other insoluble carbohydrates to soluble forms. Nearly always hydrolysis.

Endosperm starch grainsof rice

Starch Soluble forms

DIGESTION

Conversion of PSic products to structural formssuch as protoplasm or cellwall is called ASSIMILATION

Rs• Releases energy from

sugar molecules• Releases carbon dioxide

and water• Decreases weight• Occurs in either light or

darkness• Occurs in all living cells• Utilizes oxygen (aerobic

respiration)• Produces ATP with energy

released from sugar

Ps•Stores energy in sugar •Uses carbon dioxide and water•Increases weight•Occurs only in light•Occurs only in cells containing chlorphyll•Produces oxygen in green organisms•Produces ATP with light energy

Review

• Photosynthesis Major Steps of Photosynthesis Light-Dependent Reactions Light-Independent Reactions C4 Photosynthesis CAM Photosynthesis

• Respiration Glycolysis Electron Transport Chain

Copyright © McGraw-Hill Companies PermissionRequired for Reproduction or Display