BIO 201: Botany
PHOTOSYNTHESIS

OVERVIEW

General reaction:  LIGHT ENERGY ---------> SUGAR

Chemical reaction:

                                                light

            CO₂ + H₂O  ---------------------------> C₆H₁₂O₆ + O₂

                                    chlorophyll, enzymes

For sulfur bacteria: H₂S used instead of H₂O
result in the production of elemental sulfur

CHARACTERISTICS OF LIGHT

light travels as a photon (physical) or quantum (energy)

each quantum has a different level of energy

energy measured through wavelength (see spectrum on p. 169)

            SHORT           HIGH               gamma

                                                            x-rays                                       violet  (~400 nm)

                                                            ultraviolet                                  blue

            wavelength                               visible ---------------------       green

                                    energy              infrared                                     yellow

                                                            microwave                                red (~750 nm)

            LONG             LOW               radiowave

FATE OF LIGHT

What might happen to a photon of light when it gets to a leaf?

            TRANSMITTED – goes through leaf

            REFLECTED – bounces off leaf surface

** these two are of no value to the plant

            ABSORBED – causes an effect in the leaf
type of response will depend on photon's energy level

PIGMENT COMPLEX

= Chlorophyll a and accessory pigments that facilitate the absorption of light energy

In eukaryotes there are two types based on the wavelength of best absorption
    Photosystem I (P700)
    Photosystem II (P680)

The photosystems include three groups of pigments:

1) CHLOROPHYLLS – all with greenish color
    absorb best in violet-blue and red wavelengths

    Chlorophyll a – photosynthetic organisms ex. bacteria
            ** most important light absorbing pigment **

    Chlorophyll b – many plants, esp. flowering plants

    Chlorophylls c & d – restricted to some algae

    Bacteriochlorophylls – found among some bacteria

2) CAROTENOIDS -
Carotenes – have an orangish coloration
Xanthophylls – have a yellowish color

3) WATER SOLUBLE PIGMENTS -
Phycoerythrin – has a reddish color
            found in some algae
Phycocyanin – has a bluish coloration
            found in some algae and the blue-greens

ANTHOCYANIN is not a photosynthetic pigment, but is a common water soluble pigment for UV protection in plants

Ability of pigments to absorb light related to wavelength (see p. 173)

THE PROCESS OF PHOTOSYNTHESIS

Two components:

a)      Light requiring phase – requires light and converts it into chemical bonds

b)      Non-light requiring phase – use of above energy to build sugar

LIGHT REQUIRING PHASE

OVERALL:   Light energy  --------->  Chemical Bond Energy

NECESSARY COMPONENTS:

1) pigment complex
molecules embedded in membranes of chloroplast
            a) chlorophyll a
            b) accessory pigments (direct quantum of light)
two Photosystems: P₇₀₀ & P₆₈₀ (best absorb wavelength)

2) series of acceptor molecules
cytochrome molecules direct energy to final acceptor
positioned in membranes of chloroplast (thylakoid)

3) final energy acceptor molecules
high energy molecules to pick up energy
            energy + ADP +  P_i  -----> ATP                      (note: P_i  = phosphate)
            energy + NADP+ + H+ ------> NADPH

PROCESS:

1) quantum of light absorbed by pigment complex

2) energy directed to chlorophyll a molecule

3) chlorophyll a molecule is "excited" and loses an e-

4) e- moves toward and is picked up by an acceptor molecule

5) the acceptor molecule - e- combination is unstable and e- is given off; the e- now has less energy due to interaction

6) the e- moves to another acceptor molecule to repeat process

7) the loss of energy from the e- during one of the interactions is enough energy for a  Phosphate (P_i) to be added to ADP ----> ATP  (occurs due to proton motive force – see p.175)

8) after several repetitions of process the e- comes to a final electron acceptor where it is stable and remains

This process of using light to add a  P_i  to ADP is = photophosphorylation

Two types:
1) CYCLIC PHOTOPHOSPHORYLATION (simpler system of prokaryotic organisms)
            pigment complex = Photosystem I (P₇₀₀)
            final acceptor = Photosystem I (P₇₀₀)

2) NONCYCLIC PHOTOPHOSPHORYLATION (see fig 10.8, p 174 for details)
pigment complex = Photosystem I (P₇₀₀) and Photosystem II (P₆₈₀)
            final acceptor of Photosystem I  =  NADP⁺
            final acceptor for Photosystem II  =  Photosystem I (P₇₀₀)
            Photosystem I restocked with electrons by Photosystem II
            Photosystem II restocked by splitting H₂O
                        H₂O ------> H⁺  +  O^- (eventually becomes O₂)

 (diagram of noncyclic photophosporylation)

Summary - light requiring phase (Noncyclic Photophosphorylation):

light

ADP  +  P_i  +  NADP⁺  +  H₂O ---------------------> ATP  +  O₂  +  NADPH

                                                        P₆₈₀    P₇₀₀

NONLIGHT REQUIRING PHASE

OVERALL:      Energy + CO₂ ---------> Sugar

NECESSARY COMPONENTS:
energy – from ATP and NADPH
enzymes
CO₂ - substrate for sugar production

PROCESS (CALVIN CYCLE): see p. 177 for diagram of the cycle

1) a CO₂ is added to Ribulose Biphosphate (RuBP) – a 6 carbon molecule
done with aid of enzyme Ribulose Biphosphate Caboxylase

2) this produces an unstable 6 carbon molecule which quickly breaks down into
2 Phosphoglycerate (PGA) – a 3 carbon molecule

3) energy of ATP (giving off the P_i) is used to convert PGA into Diphosphoglycerate (DPGA)

4) energy of NADPH (loosing an e^-) causes rearrangement to Glyceraldehyde Phosphate (PGAL)

5) pairs of PGAL can bind together to produce Glucose Phosphate, which loose the P_i, thus becoming Glucose

6) other PGAL molecules rearrange to produce RuBP; energy of ATP is needed

 (diagram of Calvin Cycle)

Summary – nonlight requiring phase (Calvin Cycle):

CO₂  +  ATP  +  NADPH  ------------------>  C₆H₁₂O₆  +  ADP  +  P_i  +  NADP⁺

                                                enzymes

HATCH-SLACK PATHWAY

Problems develop when there are low CO₂ levels
gas exchange stopped during hot part of day & CO₂ used up with O₂ produced

Normally:

                                      RuBP Carboxylase

            CO₂  + RuBP  ----------------------------->  2 Phosphoglycerate (PGA)

Low CO₂ and high O₂:

                                    RuBP Carboxylase

            O₂  + RuBP  ------------------------------>  1 Phosphoglycolate  +  1 PGA

            eventual breakdown of phosphoglycolate into CO₂

This second process = Photorespiration

Some plants have a second (added) pathway to reduce the problem of photorespiration
partitioning of the photosynthesis processes
1) uptake of CO₂ in area where [O₂] not important
2) release of CO₂ in area where [O₂] low

The Process:

See 178-179 + figs 10.11, 10.12

A. In cells where [O₂] may be high (mesophyll cells)

1)      Pyruvate is rearranged into Phosphoenolpyruvate (PEP) [3C] using energy of ATP

2)      PEP combines with CO₂ to produce Oxaloacetate (OAA) [4C]

3)      OAA converted to Malate (MAL) [4C] using energy of NADPH

Malate (aka Malic Acid) is then transported to the Bundle Sheath Cells

1)      Maltate broken down into CO₂ + Pyruvate [3C]

2)      The CO₂ is then used in the Calvin Cycle as usual where [O₂] is low

Pyruvate is transported to the mesophyll cells where the Cycle starts over

 (diagram of Hatch-Slack pathway)

Terminology for the two above processes:

1) Plants able to complete Hatch-Slack and Calvin Cycles = C₄ plants
first stable product is a 4-carbon molecule (Oxaloacetate)

2) Plants only able to complete Calvin Cycle = C₃ plants
first stable product is a 3-carbon molecule (Phosphoglycerate)

CRASSULACEAN ACID METABOLISM (CAM)

See pp. 179-180 + fig 10.13

Some C₄ plants have entire process occur in each cell – separation in time not space
Night – CO₂ fixed into Malate when stomates open
            malate stored in vacuoles
Day - Malate transfers CO₂ to RuBP

C₃ vs C₃ vs CAM PLANTS

C₃ plants:
plants of cooler climates or with growth during cooler seasons
require plenty of water, not too hot
have problems when 1) hot, 2) dry, 3) high light intensity
this is when O₂ and CO₂ compete with RuBP carboxylase
include wheat, rye, oats, rice, kentucky bluegrass
all plants which grow best in spring

C₄ plants:
plants adapted to tropics, hot summers in se USA, or deserts
better able to handle hot dry conditions
include maize, sugar cane, sorghum, crabgrass

CAM plants:
plants adapted to extremely dry areas – deserts
common to cacti, stonecrops, bromeliads (inc. pineapples)

Comparison Chart