UNIT TWO PHOTOSYNTHESIS AND CARBOHYDRATE PRODUCTION

Photosynthesis and Carbohydrate Production The Photosynthetic Process Photosynthetic Reaction Factors Affecting Photosynthesis Source-Sink Relations Metabolism Carbohydrate Production

Photosynthesis and Carbohydrate 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, 1975 Question 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 Process Occurs in tissues within chloroplasts in plant cells Requirements are: – Green leafy tissue, healthy plants – Light, solar energy – CO 2 & H 2 O Net primary production from photosynthesis – 1.41 x metric tons of carbon per year – 3.9 billion metric tons of carbon per day – 8 trillion pounds/day

The Plant Cell

Photosynthetic Reaction – Generalized 6 CO H 2 O  C 6 H 12 O 6 + 6H 2 O + 6O 2 ^ green tissue ^ light

Photosynthetic Reaction – Generalized Resources used: – Water, nutrients CO 2, light energy Energy provided: – Chemical bond energy Products produced: – Sugars, lipids, amino acids (proteins), many others

Photosynthetic Reaction – Generalized Endothermic reaction: – 114 Kcal/mole CO 2 Total photosynthesis: – Total CO 2 uptake Net photosynthesis: (Measured CO 2 uptake)-(respiration losses) Photosynthesis beyond losses from respiration

Photosynthetic Reaction – Generalized Photosynthesis: H 2 O + CO 2  CH 2 O + O 2 Respiration: CH 2 O + O 2  H 2 O + CO 2 Note! These are almost the exact opposite reaction

There are two forms of chlorophyll

Chlorophyll is contained within specialized membranes of the chloroplast called thylacoids. Some thylacoids form stacked structure termed granum.

Chlorophyll molecules “capture” a photon of light which generates a high energy electron that is received by a primary acceptor of a “Photosystem”. There a two “photosystems” involved in the complete photsynthetic process. These are called Photosystem I and Photosystem II

Photosystem II captures a photon and generates 2 electrons by hydrolysis of water. As a result O 2 and 2 H + (hydrogen ions) are produced.

The electrons go to a primary acceptor where they are fed through a series of enzymatic (cytochromes) steps that use the electron’s energy to manufacture ATP. The electron is deposited to photosystem I. As photosystem I absorbs light it also kicks out a high energy electron.

The acceptor of photosystem I uses the electron energy to manufacture NADPH from NADP +, H + and e -

The capture chemical energy is used to “fix” CO 2 and generate carbohydrate in the Calvin Benson Cycle This reaction occurs in the chloroplast “matrix” (the space between the thylakoids)

To generate one sugar molecule it takes 6 “turns” of the cycle.

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

Factors Affecting Photosynthesis Light intensity – Sun leaves vs. shade leaves – Photosynthetic light response curve(Fig 7.7, p. 151) Dark respiration –Occurs at night, no photosynthesis Compensation point –When respiration = photosynthesis Saturation point –Light exists in surplus

Continued from previous slide Light Intensity - Maximum photosynthetic rate Determined by chloroplasts per leaf

Factors Affecting Photosynthesis Day length – Time of year – photo period CO 2 concentration – Hardly ever limiting Temperature – Temperature response curve Hot temps can be limiting Extreme cold also be limiting

Factors 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 Photosynthesis Source – S ink relationships – Sources Net exporters of photosynthetic products (leaves) – Sinks Net importers of photosynthetic products (roots, shoots)

Factors Affecting Photosynthesis Stages 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 Photosynthesis Plant metabolism – Anabolism Plant products (carbohydrates, oils, proteins) being produced or assembled energy storage – Catabolism Plant products (carbohydrates, oils, proteins) being broken down Energy release

Factors Affecting Photosynthesis Carbohydrate production via photosynthesis – Light reaction Splitting of H 2 O Release of H, O 2, e-, ATP

Factors Affecting Photosynthesis Carbohydrate production via photosynthesis C3 vs. C4 photosynthetic pathways Slightly different pathways C4 experience less net respiration losses, they recycle much of the respired CO -2 Examples: C4 – corn, sorghum, bermudagrass C3 – soybean, barley, rice, cotton

The End

The Plant Cell