Modern Biology Chapter 6: Photosynthesis. Plant cell.
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Transcript of Modern Biology Chapter 6: Photosynthesis. Plant cell.
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
• O2To air
• C6H12O6To plant
• CO2 From
Air
• H2OFrom Soil
• Light energy
From Sun
Overview of photosynthesis
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 parts2H2O 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 + 2NADPH2PGAL + 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