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)

Photosynthesis

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

- Takes place in cholorpolasts and other green parts of the organisms.

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

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, reducing

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), and other types of chlorophyll.

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

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-fixing reactions)

- 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.

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

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

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.

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 with Kranz Anatomy produce oxaloacetic acid (4-carbon compound). Phosphoenolpyruvate (PEP) and carbon dioxide

combined in mesophyll cells with the aid of PEP carboxylase.

- Provides a major reduction in photorespiration.

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.

C6H12O6 + 6O2 6CO2 + 6H2O + energy

Respiration

• Fermentation

C6H12O6 2C2H5OH + 2CO2 + ATP

C6H12O6 2C3H6O3 + ATP

Major Steps of Respiration

• Glycolysis Glucose molecule becomes a fructose molecule carrying

two phosphates. Fructose molecule is split into two GA3P molecules. Phosphates and hydrogens are removed

- Creating a net gain of 2 ATP and 2 NADH molecules from one glucose molecule

Major Steps of Respiration

• Aerobic Respiration Citric Acid (Krebs) Cycle

- 1 ATP, 3 NADH, 1 FADH2

• Electron Transport Oxidative Phosphorylation

- Energy contained in 1 NADH molecule is used to create 3 ATP molecules

- Energy contained in 1 FADH2 molecule is used to create 2 ATP molecules

• Total number of ATPs produced = 36

Assimilation and Digestion

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

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