Modern Biology Chapter 6: Photosynthesis

Plant cell

6-1: Capturing the Energy in Light

Energy for life processes photosynthesis: process by which green plants convert solar energy into chemical energy produces carbohydrates produces oxygen

Energy for life processes chloroplast structure double membrane surrounds entire organelle thylakoids: flattened sacs inside double membrane grana: stacks of thylakoids stroma: fluid surrounding thylakoids inside double membrane

Energy for life processes sunlight provides heat and energy to earth white light from sun contains mixture of colors of light wavelength of light determines its color only a small portion of sun-light is visible to humans

The sun emits all visible wavelengths of light Green plants absorb red, orange, blue and violet They reflect yellow and green

Overview of photosynthesis CO2 From Air H2O From Soil Light energy From Sun O2 To air C6H12O6 To plant

Energy for life processes pigment: colored substance that reflects or absorbs light

Energy for life processes chlorophyll type of pigment in thylakoid membranes two types of chlorophyll chlorophyll a absorbs light in red end of spectrum chlorophyll b absorbs light in blue end of the spectrum (accessory pigment) green light is not absorbed, but reflected giving the leaves the appearance of being green by absorbing light pigments also absorb energy

Energy for life processes Cartenoids: other accessory pigments absorb different colors depending on chemical structure become apparent when chlorophylls fade (fall colors)

The light reactions

The light reactions consist of three basic components Photosystem 2 Photosystem 1 ATP synthase (chemiosmosis)

Photosystem 2 water-plastoquinone oxidoreductase Uses the energy from sunlight to split the water molecule into three parts 2H2O  4 H+ + 4 e- + O2

Photosystem 1 plastocyanin: ferredoxin oxidoreductase Uses the energy from sunlight to move the electrons onto NADP+ for transport to the next phase of the process

ATP synthase Synthesizes ATP using a concentration gradient created by photosystem II

Light reactions Light and the energy associated with it are absorbed into photosystem I and photosystem II

Light-dependent reactions a.k.a. light reactions Electron transport occurs within membranes

Light-dependent reactions a.k.a. light reactions photosystem II (PSII) accessory pigments absorb light and acquire energy (E) (step 1) energy is passed along membrane pigments until it reaches a specific pair of chlorophyll a molecules

Light-dependent reactions a.k.a. light reactions photosystem II (PSII) electron transport E forces e- to increase E level (e- are said to be “excited”) excited e- leave chlorophyll a chlorophyll a is oxidized PEA donates e- e- reduces primary e- acceptor (PEA) (step 2) e- transported down ETC (step 3) each transfer, the e- loses some E E is used to move p+ into thylakoid

Light-dependent reactions a.k.a. light reactions photosystem I (PSI) light absorbed by PSI (step 1b) e- move from chlorophyll a to PEA (step 4) e- lost are replaced by e- from PSII PEA of PSI donates e- to NADP+ (step 5) brings e- to edge of thylakoid membrane by stroma e- combine with p+ and NADP+ NADP+ reduced to NADPH

Light-dependent reactions a.k.a. light reactions replacing e- (step 6) recall e- from chlorophyll in PSII replace e- that leave chlorophyll on PSI e- from PSII need to be replaced or both ETCs cease

Light-dependent reactions a.k.a. light reactions replacing e- (step 6) replacement e- come from water enzyme in thylakoid splits water molecule 2H2O  4 H+ + 4 e- + O2 p+ (H+) remain in thylakoid O2 diffuses out and leaves the plant replace e- lost by chlorophyll in PSII

Summary of Light Reactions Summary: what is produced during the light reactions p+ concentration gradient NADPH

Summary of Light Reactions Summary: what is produced during the light reactions p+ concentration gradient NADPH

Chemiosmosis potential E from gradient is harnessed by ATP Synthase in thylakoid membrane ATP Synthase serves two functions Catalyzes ADP + (P)  ATP Acts as carrier protein for p+ as H+ ions pass through ATP Synthase, ATP is produced

Section 6.2: The Calvin cycle

Stomata Open Closed

Section 6.2: The Calvin cycle Light-independent reactions Many names Calvin (or Calvin-Benson) cycle (men who first described cycle) Dark reactions (does not directly require light) Carbon fixation (incorporation of C into organic substances)

Section 6.2: The Calvin cycle sugars are long term energy storage (much more stable than ATP of NADP+) requires carbon dioxide (CO2 ) and water (H2O) CO2 enters plants through stomata (little tiny pores controlled by the plant) H2O enters plant through osmosis, capillarity or stomata

Step 1 after diffusing into the stroma from the cytosol, CO2 joins with a 5-C sugar (RuBP) to produce 2 3-C molecules of PGA (process is known as carbon-fixation)

Step 2 PGA is converted into PGAL 2PGA + 2ATP + 2NADPH2PGAL + 2ADP + 2NADP+ +2 phosphate

Step 3 and 3B Most PGAL converted back into RuBP 2PGAL + ATP  RuBP + ADP + phosphate + some fixed C Some PGAL leave Calvin cycle as fixed C (3B)

Balance Sheet Each turn of Calvin cycle results in fixation of 1 CO2 Three times around Calvin cycle results in 1 PGAL each turn requires 3 ATP and 2 NADPH 2 ATP from step 2 1 ATP from step 3 3 turns requires 9 ATP and 6 NADPH

… PGAL and other organic molecules like carbohydrates are formed and then used all over the cell for a variety of functions. 6CO2+ 6H2O + energy  C6H12O6 + 6O2 http://bcs.whfreeman.com/thelifewire/content/chp08/0802003.html

Alternative Pathways C4 pathway when CO2 is low, enables plants to continue fixing carbon grasses, corn uses less water, but moves much more slowly

Alternative Pathways CAM pathway when very hot and dry, enables plant to continue to fix carbon cacti, pineapples stomata open at night instead of during the day uses less water, but moves much more slowly