Plant Metabolism

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 in cells

in which chlorophyll is embedded.

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

Carbon Dioxide

• Carbon dioxide (0.037% of air) reaches cholorplasts in the mesophyll cells by diffusing through the stomata into the leaf interior. 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 cytoplasm, vacuoles, and other 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

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 found in cyanobacteria and red algae), and several other types of chlorophyll (chlorophylls c, d, and e in algae and photosynthetic bacteria).

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

Fig. 10.4

Major Steps of Photosynthesis

• Light Dependent Reactions Water molecules split apart releasing

electrons and hydrogen ions (H+), and O2. Electrons from splitting water passed along

an electron transport chain. ATP produced. NADPH is produced.

Major Steps of Photosynthesis

• Light Independent Reactions Calvin Cycle- Takes place in the stroma of chloroplasts. - Carbon dioxide combines with RuBP and

then the resulting molecule enters the Calvin cycle and is converted to sugars (Glucose) .

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

Fig. 10.5

Fig. 10.6

Light Dependent Reactions - In Depth

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

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

Fig. 10.8

• Noncyclic photophosphorylation: The unidirectional flow of electrons from water through photosystem I (PS I) then photosystem II (PS II) and finally to NADP to produce NADPH. H+ are used to produce ATP via ATP synthase. Oxygen gas, produced from splitting water, is a byproduct of noncyclic photophosphorylation.

• Cyclic photophosphorylation: Only PS I is involved. Electrons boosted from PS I are shunted back into the reaction center via the electron transport system. ATP is produced from ADP and P, but no NADPH or oxygen is produced.

Fig. 10.9

Light Independent Reactions - In Depth

• Calvin Cycle Six molecules of CO2 combine with six molecules of

RuBP with the aid of rubisco (RuBP carboxylase/oxygenase).

Resulting complexes split into twelve 3PGA molecules. NADPH and ATP (from light dependent reactions) supply

energy and electrons that reduce the twelve 3PGA to 12 GA3P.

Ten of the twelve GA3P molecules are restructured into six RuBP molecules using another 6 ATPs. The remaining two GA3P are used in making glucose, starch and other cellular products (lipids & amino acids).

This is known as the C3 pathway (C3 plants).

The Calvin Cycle

Photorespiration

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

entering the leaf.- When carbon dioxide levels drop below about

50 parts per million, photorespiration is initiated.

Rubisco fixes oxygen instead of carbon dioxide allowing C3 plants to survive under hot, dry conditions. This helps to dissipate ATP and accumulated electrons from the light reactions thereby preventing photooxidative damage.

Light Independent Reactions - In Depth

• 4-Carbon Pathway Plants have Kranz Anatomy.

- 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

4-Carbon Pathway

• Plants w (has high affinity to CO2 and is not sensitive to oxygen).ith Kranz Anatomy produce oxaloacetic acid (4-carbon compound).

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

PEP carboxylase has high affinity to CO2 and is not sensitive to oxygen thus provides a major reduction in photorespiration.

Plants with Kranz anatomy are called C4 plants C4 plants have two advantages:

- high concentration of PEP carboxylase (with high affinity to CO2) in mesophyll cells thus converts CO2 at a lower concentration than Rubisco.- Optimum temperatures for C4 photosynthesis are much higher than C3 photosynthesis allowing C4 plants to thrive under temperatures that would adversely affect C3 plants.

CAM Photosynthesis

• 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

Respiration

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

• Aerobic respiration:

C6H12O6 + 6O2 6CO2 + 6H2O + energy

Respiration

• Fermentation

C6H12O6 2C2H5OH + 2CO2 + ATP

C6H12O6 2C3H6O3 + ATP

Anaerobic respiration and fermentation

Release less than 6% of the energy released

from a molecule of glucose by aerobic

Respiration.

Factors Affecting the Rate of Respiration

• Temperature• Water• Oxygen

Major Steps of Respiration

• Glycolysis Occurs in the cytoplasm. 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 Occurs in the mitochondria

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

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

• Electron Transport Oxidative Phosphorylation Chemiosmosis

Energy (ATP) resulting from aerobic respiration of a glucose molecule

Glycolysis (cytoplasm) ATP -2 +4

2 NADH (ea = 2 ATP when moved into mitoch.) +4

Pyruvic acid to acetyl CoA (mitoch. matrix)

2 NADH (ea = 3 ATP) +6

Citric Acid Cycle (mitoch. matrix)6 NADH (ea = 3 ATP)

+182 FADH2 (ea = 2 ATP)

+4 +2

TOTAL +36

• Efficiency of aerobic respiration:

The 36 ATP molecules produced represent 39% of the energy harnessed in the glucose molecule. The remaining 61% of the energy in glucose is lost as heat or is unavailable.

Aerobic respiration is still 18 times more efficient than anaerobic respiration.

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.

Review

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

• Respiration Glycolysis Electron Transport Chain

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