Plant Bio Day 6

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

Photosynthesis Importance

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