PHOTOSYNTHESIS, RESPIRATION, AND TRANSLOCATION

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PHOTOSYNTHESIS Green plants convert radiant energy into chemical energy - utilizes chlorophyll of the chloroplasts

Molecular model of chlorophyll

PHOTOSYNTHESIS Principal Photosynthetic Process: Hydrogen + Carbon Dioxide → CH 2 O in presence of: Photosynthetically Active Radiation - PAR

Compensation Points Light: as PAR increases... photosynthetic CO 2 fixed equals respiration CO 2 released no net CO 2 movement until more PAR up to the Light Saturation Level

Compensation Points CO 2 : CO 2 fixed by photosynthesis equals CO 2 released by respiration no net CO 2 movement

Note: PAR level required for light saturation rises with increasing CO 2 Also: as PAR level increases, higher concentrations of CO 2 are required important differences in C 3 and C 4 plants

Chemical equation for photosynthesis (greatly simplified): 6 CO H 2 O + radiant energy w/ chlorophyll Yields: 6O 2 + C 6 H 12 O 6 (Glucose)

GLUCOSE ENERGY 1 mole Glucose (a 6-carbon sugar (C6)), has energy equal to ~ 686 kcals Written as: 686 kcal/mol

Light and Dark Reactions Two reactions in photosynthesis: Light Reactions - occur only in presence of light Dark Reactions - don’t require light; occur in light or complete darkness

Light reactions involve: photons electrons of the chlorophyll molecule water molecule NADP (nicotinamide adenine dinucleotide phosphate)

Visible Light

Light Reaction Process: 1) photons (light packets) energize electrons in chlorophyll molecule (z scheme) 2) energized chlorophyll splits water molecule 3) NADP captures H+ ion; holds it as NADP-H 4) ATP (adenosine triphosphate) formed by: a. light energy changed to chemical energy (NADPH) b. electron from H 2 O; energy released forms ATP Note: free O 2 is released in process

Structure of ATP

Dark Reactions (Calvin Cycle) Utilize: NADPH ATP CO 2 CO 2 combines w/ C 5 sugar Ribulose Diphosphate (RuDP) (catalyzed by RuDP-carboxylase, an enzyme)

Dark Reactions (Calvin Cycle) u n s t a b l e - immediately splits into two PGA molecules (Phosphoglyceric acid) Plants forming these PGA molecules are: C 3 Plants

Dark Reactions (Calvin Cycle) - H from NADPH transferred to PGA via ATP/NADPH energy - Phosphoglyceraldehyde (PGAL) is formed (a simple sugar) - PGAL combines into Glucose; however most PGAL is used to regenerate RuDP Special enzymes (RuDP-carboxylase) catalyze RuDP to combine with CO 2

Dark Reactions (Calvin Cycle) Takes: 18 molecules ATP + 12NADPH + 6CO 2 = C 6 H 12 O 6 also yields 6H 2 O, 18ADP, and 18P

Modified photosynthetic equation: 6CO H 2 O + radiant energy w/ chlorophyll → 6O 2 + 6H 2 O + C 6 H 12 O 6 shows that O 2 liberated in light reactions comes from H 2 O not CO 2 and that there are newly formed H 2 O molecules

C 3 and C 4 Plants Photosynthetic pathways are complicated Simply stated: C 3 plants are less efficient at photosynthesis Reduced efficiency due to an “energy robber”: Photorespiration

Occurs when C 3 plants oxygenase instead of carboxylase in the dark reaction; thus refer to enzyme as Rubisco for short Less efficient - can’t metabolize glycolate (C 2 ) produced; only passes one PGA to be reduced to PGAL Two carbon atoms are “lost” from cycle

C 4 Plants C 4 plants designed to: reduce O 2 concentrations increase CO 2 concentrations favor carboxylase reaction

C 4 Plants C 4 advantages: photosynthesize at lower CO 2 concentrations higher temperature optimums higher light saturation points rapid photosynthate movement

Rate of Photosynthesis

C 4 Plants Examples of C 4 plants: Corn* Sugarcane Sorghum Bermudagrass Sudangrass Note: C 4 weeds also - crabgrass, johnsongrass, shattercane, pigweed

C 3 Plants Examples of C 3 plants: Wheat Rice Soybeans Alfalfa Fescue Barley

CAM Plants CAM Plants - separate light and dark reactions according to: Time of Day CAM (Crassulacean Acid Metabolism) Plants include: Pineapple, Cacti, other succulents

CAM Plants Light reactions occur during daytime but Initial fixation of CO 2 occurs at night Allows stomata to remain closed during the day - conserve H 2 O

CAM Plants Also: 4-carbon Malic Acid “pool” accumulates overnight (lowers pH) During day stomata are closed Malic Acid releases CO 2 providing carbon source for dark reaction

CAM Plants

Environmental Factors Affecting Photosynthesis Light:intensity, quality, duration intensity – (see table 7-1; fig 7-7 p. 127) - etiolated vs. high light intensity - compensation point - saturation point quality - reds and blues; greens are reflected (fig. 7-6) duration - longer days = more photosynthesis

Light Spectrum

Light Quality - Chlorophyll

Light Quality - Photosynthesis

Environmental Factors Affecting Photosynthesis CO 2 : photosynthetic rate limited by small amounts of CO 2 increase by air movement; also CO 2 generators (greenhouse) Normal CO 2 content: ppm ( %)

Environmental Factors Affecting Photosynthesis CO 2 (cont) (see fig. 7-8) Recall CO 2 compensation point: CO 2 evolved in respiration = CO 2 consumed in photosynthesis

Environmental Factors Affecting Photosynthesis Temperature (Heat) 2x Photosynthetic Activity for each 10°C (18°F) increase in temperature Excess temp can lower photosynthesis and increase respiration

Environmental Factors Affecting Photosynthesis H 2 O content: wilted leaves - rate near zero due to reduced CO 2 by closed stomata water does not directly limit photosynthesis (only ~ 0.01 % of water absorbed by plants is used as H source)

Environmental Factors Affecting Photosynthesis but indirectly: low turgor - stomatal closing reduced leaf exposure enzymes affected excess soil moisture – anaerobic Lack of O 2 reduces respiration, uptake, etc.

RESPIRATION Release of energy stored in foods Controlled burning or “oxidation” at low temps by enzymes Respiration equation: C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O + energy (glucose) (oxygen)(carbon dioxide) (water)

RESPIRATION Modified Respiration Equation: Shows that H 2 O is an input as well as a product Specifies total net energy derived from one glucose molecule

Modified Respiration Equation: C 6 H 12 O 6 + 6O 2 + 6H 2 O→6CO H 2 O + 38ATP + heat

RESPIRATION Heat energy is of little value to plant (may be detrimental) ATP energy used for: Chemical reactions (energy req.) Assimilation (protoplasm) Maintenance (protoplasm) Synthesis (misc.) Accumulation (solutes) Conduction (foods) Motion (protoplasm, chromosomes)

Gas Exchange in Respiration Gas exchange is the opposite of photosynthesis Respiration takes in O 2 and releases CO 2 liberates more O 2 than needed for respiration requires more CO 2 than released by respiration

Gas Exchange in Compensation point (low light intensity): O 2 released in photosynthesis = CO 2 released in respiration

COMPARISON OF PHOTOSYNTHESIS AND RESPIRATION Under ideal photosynthetic conditions: Photosynthetic Rate ~ 10x Respiration Rate

COMPARISON OF PHOTOSYNTHESIS AND RESPIRATION Photosynthesis Cells w/chlorophyll In light Uses H 2 0 and CO 2 Releases O 2 Radiant energy to chemical energy Dry weight increases Food and energy produced Energy stored Respiration All living cells Light and dark Uses O 2 Forms CO 2 and H 2 0 Chemical energy to useful energy Dry weight decreases Food broken down Energy released

Factors Affecting Respiration Temperature - respiration increases as temperature increases Moisture - respiration increases as moisture decreases (stress) Injuries - respiration increases with injury Age of tissue - respiration greater in young tissue Kind of tissue - respiration greater in meristematic CO 2 /O 2 - respiration increases with high O 2 / low CO 2 Stored carbohydrates - respiration increases with increased stored energy

Respiration Problems/Hazards deterioration (fungi and bacteria) rot and decay loss of dry wt. loss of palatability high temperatures / high CO 2 (diseases; FIRE hazard)

ENERGY TRANSFER Glycolysis - sugar splitting Net production of: 2 ATP molecules 2 NADH molecules Forms: pyruvic acid

Aerobic Energy Transfer If O 2 and mitochondria are present: Krebs cycle - an energy converter converts glucose energy into usable energy via enzymes occurs in stroma of mitochondria “powerhouse”

Mitochondria Cristae

Electron Transport *must have O 2 present convert high energy from Krebs (NADH, FADH) into usable ATP occurs along cristae fingerlike projections in mitochondria where: cytochromes in enzymes transport electrons lowers and releases energy last cytochrome passes electrons to O 2 associates with 2 H+ protons forming H 2 O

ALTERNATE ENERGY TRANSFER If no O 2 and mitochondria present to respire alternative is: fermentation - e.g. fig. 7-14, p. 135 yeast (fungi) in beer, bread silage