Chapter 7 PHOTOSYNTHESIS. Life Depends on Photosynthesis ] What if there is a nuclear winter? F What...
-
Upload
madlyn-nicholson -
Category
Documents
-
view
224 -
download
1
Transcript of Chapter 7 PHOTOSYNTHESIS. Life Depends on Photosynthesis ] What if there is a nuclear winter? F What...
Life Depends on Photosynthesis What if there is a nuclear winter?
What is itWhat would cause it?What are the repercussions?
What is a photosynthate?Glucose and other
carbohydratesRuns photorespirationMakes amino acids, starch, cellulose, rubber, quinine, spices, etc
Who came first: Autotrophs or Heterotrophs?Heterotrophs CREATED a heavy
CO2 atmosphere.Almost became extinct when resources disappeared.
Environmental pressure allowed heterotrophs to begin to photosynthesize
Autotrophes kick out TONS of O2
Who came first: Autotrophs or Heterotrophs?, con’t How did the rise of autotrophes change the
earth?Decrease CO2 in atmosphere so climate
got coldCreated polar ice capsLowered ocean levelsO2 levels increased to above today’s
levels Jurassic – much higher
ARE LEVELS STABLE NOW?????
A. Light
Visible light makes up only a small portion of
the electromagnetic spectrum.
Sunlight consists of:
4% Ultraviolet (UV) radiation
44% Visible light
52% Infrared (IR) radiation
Characteristics of Visible Light:• is a spectrum of colors ranging from violet to
red (ROY G BIV)• consists of packets of energy called photons • photons travel in waves, having a measurable
wavelength (λ) λ = distance a photon travels during a complete vibration [measured in nanometers (nm)]
A photon’s energy is inversely related to its wavelength......the shorter the λ, the greater the
energy it possesses.Which of the following photons
possess the greatest amount of energy?
Green photons λ = 530nm
Red photons λ = 660nm
Blue photons λ = 450nm
What happens to light when it strikes an object?
• reflected (bounces off)
Only absorbed wavelengths of light function in photosynthesis.
Visible light provides just enough energy to “excite” or energize molecules
• transmitted (passes through)
• absorbed
B. Photosynthetic PigmentsMolecules that capture photon energy
by absorbing certain wavelengths of light.
1. Primary pigments• Bacteriochlorophyll - green pigment
found in certain bacteria.• Chlorophylls a & b - bluish green
pigments found in plants, green algae & cyanobacteria.
Chlorophyll a is the dominant pigment in plant cells.
Central Mg atom with 4 N atoms –area where energy transfer occurs
Tail anchors molecule to chloroplast
2. Accessory Pigments• Carotenoids - red, orange, yellow pigments
found in plants, algae, bacteria & archaea.• Xanthophylls – red and yellow pigments
found in plants, algae & bacteria.• Fucoxanthin –brown pigment found in brown
algae, diatoms, & dinoflagellates• Phycoerythrin - red pigment found in red
algae.• Phycocyanin - blue pigment found in red
algae & cyanobacteria.• Bacteriorhodopsin – purple pigment found in
halophilic archaeaEach pigment absorbs a particular range
of wavelengths.
C. Chloroplasts (hyperlink chloroplasts)Sites of photosynthesis in plants & algae.Concentrated in mesophyll cells of most
plants.
Chloroplast structure:
• Stroma - gelatinous matrix; contains ribosomes, DNA & various enzymes.
• Thylakoid - flattened membranous sac; embedded with photosynthetic pigments.
D. Photosynthesis
Occurs in two stages:• Light reactions (pink hyperlink)- harvest
photon energy to synthesize ATP & NADPH.
• Carbon reactions (Calvin cycle) - use energy from light reactions to reduce CO2 to carbohydrate.
6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O
1. Light Reactions• require light• occur in thylakoids of chloroplasts• involve photosystems I & II (light
harvesting systems).
Photosystems contain antenna complex that captures photon energy & passes it to a reaction center.
LIGHT REACTION
•Uses light and water•Synthesizes ATP and NADPH•O2 released here (comes from water)•Begins in thylakoid membrane•4 clusters of proteins involved•2 clusters make 2 photosystems•other 2 clusters form electron transport chain (ETC)•notice different light lengths
Light Reaction, con’t
Photosystem II happens first
SUN Antenna Complex
Chlorophyll a in reaction center P680
e-
1st carrier molecule in ETC
Energy (electron) goes to Photosystem I
ATP, produced from ADP, comes out of thylakoid and into stroma for Calvin cycle
Powers Calvin
Collects photons sends electrons
Receives electrons sends electrons
Redox reactions
Oxygen is released here when H2O is split
Light reaction, con’t
Photosystem I is next
Sun
1st carrier molecule in new ETC
Antenna complex
Electrons change (reduce) NaDP+ to NaDPH
Chlorophyll a in reaction center P700
e-
Powers Calvin
2. Carbon Reactions (Calvin cycle; C3 cycle) • do NOT require light (occur in both
darkness & light as long as ATP & NADPH are available)
• occur in stroma of chloroplasts• require ATP & NADPH (from light
reactions), and CO2
CALVIN CYCLE (aka carbon cycle, aka C3 cycle)
•Found in plants that only use calvin: cereals, peanuts, tobacco, spinach, most trees and lawn grasses•C3 named for 3 carbon compound: phosphoglyceric acid (PGA)- 1st stable compound in pathway•Calvin – discovered this system•Goal of calvin cycle: Fix C from CO2 into organic compounds (glucose)•Occurs in both light and dark (as long as ATP and NADPH is available•CO2 enters through stoma (plural: stomata)•Powered by ATP/NADPH from light reaction•Diagram on pg 117 is good•Rubisco – most abundant and important protein in the world•85% of plants use this system
Calvin cycle, con’tRuBp (ribulose Biphosphate
CO2 from the atmosphere
Unstable 6 Carbon molecule
PGA
3-C
PGA
3-C
Combines with
(assisted by rubisco- an enzyme)
(uses ATP and NADPH as energy from LR)
Becomes
More ATP & NADPH used here
PGAL
C3H6O3
Converts to glucose then to other carbohydrate
Can be rearranged to form RuBP (starts cycle over)
(Phosphoglyceraldehyde: 1st carbohydrate product of Calvin) (C3H6O3)
Plants that use only the Calvin cycle to fix carbon are called C3 plants.
Ex. cereals, peanuts, tobacco, spinach, sugar beets, soybeans, most trees & lawn grasses.
Photosynthetic Efficiency Only .037% of atmosphere is CO2
Yet 200 billion tons of carbon (from CO2 used to make glucose
If each photosystem gets all the CO2 possible, efficency is only 30%- reality is much lower!
Cloudy days are only .1% efficient Cultivated plants only 3% efficient Greatest natural efficiency – evening primrose
– 8% Sugarcane – 7%
E. PhotorespirationProcess that counters
photosynthesis.Occurs when stomata close under
hot, dry conditions:• O2 levels in plant increase• CO2 levels in plant decrease
Under these conditions, rubisco fixes O2 (rather than CO2).Thus, PGAL is NOT produced.
Photorespiration Occurs when conditions are low CO2 or
high O2 Rubisco uses O2 instead of CO2 as
substrate Results in loss of Carbon for calvin
(carbon) reaction High temperatures complicate things:
fixing and releasing CO2 occurs at the same rate. No net C gain, no glucose produced
Stomata too open (trying to get more CO2) means dehydration
C4 Photosynthesis
Adaptations that allow certain plants to conserve water and reduce photorespiration at higher temperatures.
Found in sugarcane, corn, millet, sorghum, all flowering plants in hot, open environments (about .4% of all plants)
Use 4-carbon compound to concentrate carbon within special cells
CO2 is fixed in mesophyll cells first This keeps CO2 concentration 20-120 times greater than in
C3 cycle Not efficient in normal climates: used 2 ATP for every
carbon that is moved from the mesophyll cells Then it is fixed as normal via Calvin cycle These plants require about half as much water due to
recycling of compounds during the pathway
C4 Respiration
C4 plants reduce photorespiration by physically separating the light reactions and Calvin cycle.
C4 plants even fix CO2 when stomata begin to close – preserves water – plants require ½ the water of C3
CO2 malic acid migrates to bundle sheath cells and enters C3 glucose (instead of PGA)
Leaf anatomy of a C4 plant
C4 Photosynthesis:
• Light reactions occur in chloroplasts of mesophyll cells.
• Calvin cycle occurs in chloroplasts of bundle sheath cells.
2. CAM PhotosynthesisCAM plants reduce photorespiration
by acquiring CO2 at night.
Night:• mesophyll cells fix
CO2 as malic acid• malic acid is stored
in vacuoles.
Day:• malic acid releases
CO2 which enters Calvin cycle.
Malic acid
CAM (crassulcean acid metabolism)Respiration
Includes cacti, pineapple, Spanish moss, orchids, some firns and wax plants (10% of all plant species)
Take in CO2 at night Fix it in calvin cycle the next day Stomata stay closed during the day to
preserve water Plants acidic at night – more alkaline in
day Malic acid formed in large vacuoles in
same cells that contain chloroplasts Malic acid enters chloroplast during the
day where C3 begins