Plant Bio Day 6

Photosynthesis • Chloroplast structure • Light reactions • Calvin cycle • Photorespiration • C4 & CAM

Photosynthesis Importance

* Source of O2 in atmosphere
* 50% terrestrial
* 50% marine phytoplankton & macroalgae

Photosynthesis involves _

Transfer of electrons

Reactants in electron transfer

6 CO2 & 12 H2O

Products: electron transfer

1 glucose, 6 water, 6 O2

water gets _

Oxidized (losses electrons) 6 H2O → 6 O2

CO2 gets _

Reduced (gains electrons) 6 CO2 → C6H12O6

Photosynthesis: 2 stages

1. Light reactions
2. Calvin cycle (dark rxns)

Light RXNS happen where

Thylakoid membrane

Calvin cycle happens where

Stroma

Light RXNs - Steps

1. Light hits chlorophyll molec
2. E-s bounce to higher energy level & OFF chlorophyll molec
3. Chl steals e-s from H2O (oxidized)
4. Water molec falls apart: photolysis → oxygen

* H2O → 2 H+ + 2e- + O


5. E-s and H+s from H2O transferred to NADP+ (gets reduced)

* NADP+ +2e- + H+ → NADPH (greater reducing power than H2O)


6. ADP + Pi makes ATP: Photophosphorylation

Light RXNs results

→ Light energy converted first to chemical energy of NADPH & ATP

→ O2 gas released (comes from water)

Light RXNs: Summary

H2O + Light + NADP+ +ADP + Pi → O2 + NADPH + ATP

NADP+ & NAD+

* Oxidizind agents
* Remove / accept e-s from other molecs 

NADPH & NADH

* Reducing agents
* Place e-s on other molecs

Chemical bonds in Photosynthesis

NADP+ & NADPH

Chemical bonds in Respiration

NAD+ & NADH

Chlorophyll a

* Absorbs mostly violet-blue & red
* Refelcts / transmits green light
* Accessory pigments broaden absorption

Photosynthesis = driven by what

Visible wavelengths

Calvin cycle makes what

* Make sugar (precursor)

Calcin Cycle Uses what and supplies what

* Use NADPH & ATP from light rxns
* Supply light rxns w NADP+ & ADP

Light Reactions Summary

H2O + Light + NADP+ + ADP + Pi → O2 + NADPH + ATP

Calvin Cycle: Summary

CO2 + NADPH + ATP → CH2O (Sugar) + NADP+ + ADP + Pi

What do Light RXNs do?

Split H2O & Release O2 to atmosphere

What do Dark RXNs do?

Return ADP, inorganic phosphate & NADP+

Rubisco

* Most abundant prot
* Most important prot
* Dual Nature!

Photorespiration

Rubisco attaches O2 to RuBP

Photorespiration = Metabolic pathway that

* Consumes O2
* Releases CO2
* Makes no ATP
* Wastes energy
* Decreases photosynthetic output

Photorespiration: Solution

 a new enzyme: PEP carboxylase (PEPC)

C4 photosynthesis seen where

Dry environments

Mesophyll cells

* O2 exits (light rxns)
* CO2 enters
* NO Rubisco

Rubisco: where

Only in bundle-sheath cells

* kept away from low \[CO2\]

C4: kind of seperation of steps

Spatial

CAM: Kind of seperation of steps

Temporal

Plants response to herbivores

Physical defences

Chemical defences

Recruit predatory animals

Organic substance made in one place & transported to another place, where it affects growth & other processes

Hormone

Important hormones

* Abscisic acid
* Auxins
* Cytokinis
* Ethylene

Hormone Action

Reception (hormone → receptor, conformation changes)

Transduction (production of relay molecules)

Response (cellular responses / stimuli)

Abscisic Acid, ABA

* Dry conditions: 
* ABA stimulates stomate closure
* Causes K+ to leave guard cells

Auxins: where

* Young leaves
* Shoot apical meristems

Auxins: major functions

* Stem elongation (phototopism (low conc. only))
* Lateral roots
* Enhances apical dominance (suppress lateral branches)

Greater cell elongation on DARK side

Phototropism

Auxin & Cell elongation 

Auxin increases activity of Protons (H+) pumps, & acidity

Acid-Growth Hypothesis

1. Auxin increases activity H+ pumps
2. Acidity increases in wall. Ion uptake increases
3. Expansins (enzymes) activated by low pH separate microfibrils from cross-linking polysaccharides 
4. Polysaccharides now more accessible to enzymes that loosen cell wall

Cytokinins: where

* Synthesized in roots, then transported
* Many other minor sites of production

Cytokinins: Major functions

* Cell division in roots & shoots
* Promote lateral buds (modify apical dominance) 
* Often act in opposition to auxins 

Apical Dominance

Lateral (axillary) branches

* Inhibited by auxins
* Promoted by cytokinins

If apply auxin to “stump” lateral branches will be inhibited

Ethylene (gas): Where

Most plant parts

Ethylene: Promotes

* Leaf abscission (shedding)
* Triple response in seedlings
* Fruit ripening
* Root hair production

Most commercially produced organic compound

Ethylene

Effect of ethylene on Fruit Ripening 

* Promotes fruit ripening & is produced during fruit ripening
* Autocatalytic: Promotes its own production
* Increases respiration