PHOTOSYNTHESIS, RESPIRATION, AND TRANSLOCATION

 http://www.emc.maricopa.edu/faculty/far abee/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 → CH 2 O in presence of: Photosynthetically Active Radiation - PAR

Compensation Points

 Light: as PAR increases. . .

photosynthetic CO 2 equals fixed 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 2 + 6 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 2 + 12H 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

Photorespiration

   Occurs when C 3 plants

oxygenase

instead of carboxylase in the dark reaction; thus refer to enzyme as

Rubisco

for short Less efficient (C 2 can’t metabolize glycolate ) produced; only passes be reduced to PGAL

one

PGA to 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

reactions according to: light and dark

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: 300 - 350 ppm (0.030 - 0.035 %)

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

photosynthesis limit  (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

product H 2 O

is an

input

as well as a  Specifies

total net energy

one glucose molecule derived from

Modified Respiration Equation: C 6 H 12 O 6 + 6O 2 + 6H 2 O →6CO 2 + 12H 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 respiration than needed for



requires more CO 2 respiration than released by

Gas Exchange in Respiration

@ 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

increases - respiration increases as temperature

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

increased stored energy - respiration increases with

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 respire alternative is: to

fermentation

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