PHOTOSYNTHESIS, RESPIRATION, AND TRANSLOCATION

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPS.html

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 → CH2O

in presence of:

Photosynthetically Active Radiation - PAR

Compensation Points

Light: as PAR increases. . .

photosynthetic CO2 fixed equals

respiration CO2 released

no net CO2 movement until more PAR up to the Light Saturation Level

Compensation Points

CO2:

CO2 fixed by photosynthesis

equals

CO2 released by respiration

no net CO2 movement

Note: PAR level required for

light saturation rises with increasing CO2

Also: as PAR level increases, higher concentrations of CO2 are required

important differences in C3 and C4 plants

Chemical equation for photosynthesis (greatly simplified):

6 CO2 + 6 H2O + radiant energy

w/ chlorophyll

Yields:

6O2 + C6H12O6

(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 molecule3) NADP captures H+ ion; holds it as NADP-H4) ATP (adenosine triphosphate) formed by:

a. light energy changed to chemical energy (NADPH)

b. electron from H2O; energy released forms ATP

Note: free O2 is released in process

Structure of ATP

Dark Reactions (Calvin Cycle)

Utilize:• NADPH

• ATP

• CO2

CO2 combines w/ C5 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:

C3 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; howevermost PGAL is used to regenerate RuDP Special enzymes (RuDP-carboxylase) catalyze RuDP to combine with CO2

Dark Reactions (Calvin Cycle)

Takes:

18 molecules ATP

+ 12NADPH

+ 6CO2

= C6H12O6

also yields 6H2O, 18ADP, and 18P

Modified photosynthetic equation:

6CO2 + 12H2O + radiant energy

w/ chlorophyll

→ 6O2 + 6H2O + C6H12O6

shows that O2 liberated in light reactions

comes from H2O not CO2 and that there

are newly formed H2O molecules

C3 and C4 Plants

Photosynthetic pathways are complicated

Simply stated: C3 plants are less efficient at photosynthesis

Reduced efficiency due to an “energy robber”:

Photorespiration

Photorespiration

Occurs when C3 plants oxygenase instead of carboxylase in the dark reaction; thus refer to enzyme as Rubisco for short

Less efficient - can’t metabolize glycolate (C2) produced; only passes one PGA to be reduced to PGAL

Two carbon atoms are “lost” from cycle

C4 Plants

C4 plants designed to:

reduce O2 concentrations

increase CO2 concentrations favor carboxylase reaction

C4 Plants

C4 advantages:

photosynthesize at lower CO2 concentrations

higher temperature optimums higher light saturation points rapid photosynthate movement

Rate of Photosynthesis

C4 Plants

Examples of C4 plants: Corn* Sugarcane Sorghum Bermudagrass Sudangrass

Note: C4 weeds also - crabgrass, johnsongrass, shattercane, pigweed

C3 Plants

Examples of C3 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 CO2 occurs at night

Allows stomata to remain closed during the day - conserve H2O

CAM Plants

Also: 4-carbon Malic Acid “pool” accumulates

overnight (lowers pH) During day stomata are closed Malic Acid releases CO2 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

CO2: photosynthetic rate limited by small

amounts of CO2

increase by air movement; also CO2 generators (greenhouse)

Normal CO2 content: 300 - 350 ppm (0.030 - 0.035 %)

Environmental Factors Affecting Photosynthesis

CO2 (cont) (see fig. 7-8)

Recall CO2 compensation point:

CO2 evolved in respiration =

CO2 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

H2O content:

wilted leaves - rate near zero due to reduced CO2 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 O2 reduces respiration, uptake, etc.

RESPIRATION

Release of energy stored in foods Controlled burning or “oxidation” at

low temps by enzymes

Respiration equation:

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

(glucose) (oxygen) (carbon dioxide) (water)

RESPIRATION

Modified Respiration Equation:

Shows that H2O is an input as well as a product

Specifies total net energy derived from one glucose molecule

Modified Respiration Equation:

C6H12O6 + 6O2 + 6H2O→6CO2 + 12H2O + 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 O2 and releases CO2

liberates more O2 than needed for respiration

requires more CO2 than released by respiration

Gas Exchange in Respiration

@ Compensation point (low light intensity):

O2 released in photosynthesis = CO2 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 H20 and CO2

Releases O2

Radiant energy to chemical energy

Dry weight increases Food and energy produced Energy stored

Respiration All living cells Light and dark Uses O2

Forms CO2 and H20 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 CO2/O2 - respiration increases with high O2 / low CO2

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 CO2

(diseases; FIRE hazard)

ENERGY TRANSFER

Glycolysis - sugar splitting

Net production of: 2 ATP molecules 2 NADH molecules

Forms: pyruvic acid

Aerobic Energy Transfer

If O2 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 O2 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 O2 associates with 2 H+ protons forming H2O

ALTERNATE ENERGY TRANSFER

If no O2 and mitochondria present to respire alternative is:

fermentation - e.g. fig. 7-14, p. 135 yeast (fungi) in beer, bread silage