Lecture 12Lecture 12

PHOTOSYNTHESIS AND CARBOHYDRATE PRODUCTION

Photosynthesis and Photosynthesis and Carbohydrate ProductionCarbohydrate Production

The Photosynthetic ProcessPhotosynthetic ReactionFactors Affecting PhotosynthesisSource-Sink RelationsMetabolismCarbohydrate Production

Photosynthesis and Photosynthesis and Carbohydrate ProductionCarbohydrate Production

“It is the business of agriculture to harvest the light energy that is used to produce food, fiber, and usable stuffs produced by agricultural plants and animals”

National Academy of Science, 1975Question of the Day – How does a corn kernel

weighing 0.15 g, become a mature plant weighing 6-7 Kg in 90 days?

The Photosynthetic ProcessThe Photosynthetic Process

Occurs in tissues within chloroplasts in plant cells Requirements are:

– Green leafy tissue, healthy plants– Light, solar energy

– CO2 & H2O

Net primary production from photosynthesis– 1.41 x 1011 metric tons of carbon per year– 3.9 billion metric tons of carbon per day– 8 trillion pounds/day

In Photosynthesis, always remember that light energy is In Photosynthesis, always remember that light energy is being converted to chemical energy or food.being converted to chemical energy or food.

The Plant Cell

Photosynthetic Reaction – Photosynthetic Reaction – GeneralizedGeneralized

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

^

green tissue

^

light

CO2 from the atmosphere around the leaf, CO2 from the atmosphere around the leaf, enters the stomate, travels to a photosynthetic enters the stomate, travels to a photosynthetic

cell where cell where glucoseglucose is produced is produced

Photosynthetic Reaction – Photosynthetic Reaction – GeneralizedGeneralized

Resources used:– Water, nutrients CO2, light energy

Energy provided:– Chemical bond energy

Products produced:– Sugars, lipids, amino acids (proteins) , many

others

Amino acids are the components of proteins and there Amino acids are the components of proteins and there are 20 in nature. Look carefully and you will see why are 20 in nature. Look carefully and you will see why

nitrogen (N) is so important in plants and animals.nitrogen (N) is so important in plants and animals.

Photosynthetic Reaction – Photosynthetic Reaction – GeneralizedGeneralized

Endothermic reaction:– 114 Kcal/mole CO2

Total photosynthesis:– Total CO2 uptake

Net photosynthesis:(Measured CO2 uptake)-(respiration losses)

Photosynthesis beyond losses from respiration

Photosynthetic Reaction – Photosynthetic Reaction – GeneralizedGeneralized

Photosynthesis: H2O + CO2 CH2O + O2

Respiration: CH2O + O2 H2O + CO2

Note! These are almost the exact opposite reaction

Factors Affecting PhotosynthesisFactors Affecting Photosynthesis

Light quality– Action spectrum of photosynthesis and growth– Function of chlorophyll in plant cells

Factors Affecting PhotosynthesisFactors Affecting PhotosynthesisLight intensity

– Sun leaves vs. shade leaves– Photosynthetic light response curve(Fig 7.7, p.

151)Dark respiration

– Occurs at night, no photosynthesisCompensation point

– When respiration = photosynthesisSaturation point

– Light exists in surplus

Continued from previous slideContinued from previous slide

Light Intensity -

Maximum photosynthetic rateDetermined by chloroplasts per leaf

Factors Affecting Factors Affecting PhotosynthesisPhotosynthesis

Day length– Time of year – photo period

CO2 concentration

– Hardly ever limiting Temperature

– Temperature response curveHot temps can be limitingExtreme cold also be limiting

Factors Affecting PhotosynthesisFactors Affecting Photosynthesis

Water– Water stress

guard cells become flaccid stomates close gas exchange stops factors affect water flow through xylem

– Drought incipient or prolonged

Factors Affecting PhotosynthesisFactors Affecting Photosynthesis

Source – Sink relationships– Sources

Net exporters of photosynthetic products (leaves)– Sinks

Net importers of photosynthetic products (roots, shoots)

Factors Affecting PhotosynthesisFactors Affecting PhotosynthesisStages of leaf development

– Heterotrophs (sinks)1 – 10% full size

– Autotrophs (sources)10 – 15% full size (unfolding)

– Exporters (sources)25% of full size (expanding)

– Senescing (source/sink transition)Aging, slowing down on photosynthesis

Factors Affecting PhotosynthesisFactors Affecting PhotosynthesisPlant metabolism

– AnabolismPlant products (carbohydrates, oils, proteins)

being produced or assembled energy storage

– CatabolismPlant products (carbohydrates, oils, proteins)

being broken downEnergy release

Relationship between portion of fullsunlight and rate of photosynthesis

0 0.2 0.4 0.6 0.8 1.0

Portion of full sunlight

50

100

Rat

e of

pho

tosy

nthe

sis

mg/

sq. d

m/h

r.

A

B

C

D

Examples of species which fitthe above curves:

A. Corn, sorghum, sugarcane,bermudagrass

B. Soybean, alfalfa, cotton

C. Orchard grass, red clover,tobacco

D. Oak, maple, and most houseplants

Factors Affecting PhotosynthesisFactors Affecting Photosynthesis

Carbohydrate production via photosynthesis– Light reaction

Splitting of H2O

Release of H, O2, e-, ATP

Factors Affecting Factors Affecting PhotosynthesisPhotosynthesis

Carbohydrate production via photosynthesisC3 vs. C4 photosynthetic pathways

Slightly different pathwaysC4 experience less net respiration losses, they recycle much of the respired CO-2

Examples:C4 – corn, sorghum, bermudagrassC3 – soybean, barley, rice, cotton

The EndThe End

The Plant Cell