CHAPTER 8 PHOTOSYNTHESIS - Edwardsville School District 7 … … · · 2014-09-25Chloroplasts...
Transcript of CHAPTER 8 PHOTOSYNTHESIS - Edwardsville School District 7 … … · · 2014-09-25Chloroplasts...
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CHAPTER 8
PHOTOSYNTHESIS
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Photosynthesis – process by which plants use light to make food molecules from carbon dioxide and water
(chlorophyll)
6CO2 + 12H2O + Light C6H12O6 + 6O2 + 6H2O
Con. 8.1
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Autotrophs – plants that make their own food (also known as producers)
Photoautotrophs—organisms that use light as a source of energy to make food
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Chloroplasts
• All green parts of a plant have chloroplasts (leaves are major sites of photosynthesis)
• Green color of plants is due to green pigment w/in chloroplasts called chlorophyll
• Chloroplasts mainly found in mesophyll cells – green tissue in interior of leaf
p. 156
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• Consists of double membrane surrounding a thick fluid (stroma) – sugars are made from CO2 here
--CO2 enters leaf and O2 exits by way of
tiny pores known as stomata
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• Thylakoids (green sacs) are suspended in stroma – light energy is captured here
• Stack of these = Granum • Stacks of these = Grana
Thylakoid
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Redox Reactions
6CO2 + 12H2O C6H12O6 + 6H2O +6O2
-Photosynthesis STORES energy
-Carbon dioxide is reduced to glucose
-Water is oxidized to oxygen
-Water is first consumed then produced
ENDERGONIC
GAINING ELECTRONS
LOSING ELECTRONS
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-Plants make sugar from carbon and oxygen in CO2 and from some hydrogen in H2O
-O2 is released, coming from the H2O, not from CO2
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- Cellular respiration harvests energy by _______ the sugar & _______ O2 to H2O (respiration is energy-releasing, going from potential to kinetic by traveling down energy levels)
- Photosynthesis goes uphill (gaining potential and traveling up energy levels)
oxidizing reducing
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The Light Reactions
• Convert light energy to chemical energy & O2 gas is waste product
• Occur in thylakoid membranes
• Stored in ATP & NADPH
• No sugar produced
Con. 8.2
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Visible Light
• Sunlight is a type of energy called radiation (AKA electromagnetic energy)
• Travels in rhythmic waves
• Light reactions of photosynthesis only use certain wavelengths/colors of Visible Light p. 160
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• Light also behaves as discrete packets of energy called photons
• A photon is a fixed quantity of light energy (shorter the wavelength, the greater the energy)
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• A photon of violet light packs almost twice as much energy as a photon of red light
• Light may be reflected, transmitted, or absorbed (pigments are substances that absorb light)
**Read Fig. 8.8 & Fig. 8.9 p. 161
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• Pigments involved in photosynthesis are chlorophyll a and b and carotenoids
• The chlorophyll a is blue-green, chlorophyll b is yellow-green and carotenoids are shades of yellow and orange
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spectrophotometer - measures the ability of a pigment to absorb various wavelengths
absorption spectrum – graph plotting a pigment’s light absorption vs. wavelength
action spectrum – looks @ effectiveness of different wavelengths of radiation driving photosynthesis (wavelength vs. rate of CO2 use or O2 release)
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Photosystems
• Pigments are clustered in thylakoid membranes
• 2 Chlorophyll a molecule & a primary electron acceptor make up the reaction center of the pigment assembly
• Reaction center & other pigments function collectively as a light-gathering antenna that absorbs photons
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• Energy is passed from pigment molecule to pigment molecule until it reaches the reaction center
• Combination of the antenna molecules, the reaction center, and the primary electron acceptor make up the photosystem
• This is the light-harvesting unit of the chloroplast’s thylakoid membrane
Photosystems (cont.)
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Two Photosystems
• Photosystem I is called P700 because the light it absorbs best is red light w/a wavelength of 700 nm
• Photosystem II is called P680 because the light it absorbs best is orange shade of red light w/a wavelength of 680 nm
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ATP, NADPH, and O2
• First event in light reactions is the absorption of light energy
• Second event is the excitation of electrons by light energy
• Third event is formation of ATP & NADPH using energy made available by the cascade of energized electrons down electron transport chains
3 Steps of the Light Reaction
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Linear Electron Flow
photon relayed to PSII
P680 e- go to higher energy state
e- captured by primary e- acceptor
H2O is split into 2e-, 2p+, oxygen atom
e-’s go from PSII to PSI
e-’s fall down, helping to make ATP
excited PSI e-’s go down 2nd e- transport chain
NADPH made
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Cyclic Electron Flow
uses PSI but not PSII
doesn’t make NADPH
possibly controlled by concentration of NADPH to help w/supply & demand
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• Transport chains are similar to the one that functions in cellular respiration
• Consist of a series of electron-carrier molecules arranged in a membrane (the thylakoid of the chloroplast)
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Chemiosmosis
• ATP is synthesized by chemiosmosis
• Electron transport chains associated w/the chloroplast’s photosystems are arranged in thylakoid membranes
• Electron transport chain in the chloroplast drives the transport of H+ through thylakoid membrane
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• Flow of H+ back through the membrane is harnessed by ATP synthase to make ATP
• In photosynthesis this is called photophosphorylation
• H+ ions, along w/electrons from the electron transport chain, join w/NADP+ to form NADPH
p. 158
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The Dark Reaction (Calvin Cycle)
• Cyclic series of reactions that assemble sugar molecules using CO2 and energy-containing products of light reaction
• Takes place in stroma
Con. 8.3
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3 Phases
1. Carbon Fixation
- cycle must occur 3 times (3 CO2) to get sugar
- CO2 molecule from air attaches to 5C RuBP (rubisco)
- unstable 6C molecule so splits forming 2 molecules of
3-phosphoglycerate
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2. Reduction
- energy from ATP & high-energy electrons from NADPH
- help make one glyceraldehyde-3 phosphate (G3P)
- other 5 G3P are recycled
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3. Regeneration of RuBP
- these 5 G3P are rearranged into 3 molecules of RuBP
- ATP is used
Totals for Calvin
1 G3P molecule
9 ATP consumed
6 NADPH used
- G3P can now be used for many organic molecules needed in the plant
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Adaptation for Saving Water
• Most plants are C3 plants, which take carbon directly from CO2 in the air & use it in the Calvin cycle to build a 3-carbon molecule
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• Stomata in leaf surface usually close when the weather is hot & dry
• Minimizes water loss
• But CO2 and O2 are not exchanged as normal
• Calvin cycle is diverted to an inefficient process called photorespiration
• Consumes ATP
• No sugar made
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• Some plants have special adaptations that enable them to save water & avoid photorespiration ex: corn, sugarcane
• Special cells in C4 plants incorporate CO2 into a 4-carbon compound
• It’s broken down to release CO2 (this initiates the Calvin cycle)
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Characteristic C3 Plants C4 Plants
Origin Temperate Tropical
Examples Rice, Soybean, many tree
species Corn, sorghum, sugarcane
Carbon dioxide fixation 3 carbon molecule 4 carbon molecule
Site of photosynthetic cycle Mesophyll cells Bundle sheath cells
Carbon dioxide
concentration Regulated by diffusion
Elevated high
concentrations
Stomatal behavior Open for longer periods Open for shorter periods
Water use efficiency* Not very efficient Very efficient
Climatic adaptation Mostly cooler, moderate
climate
Mostly warmer, drier
climate
Carbon dioxide saturation High Low
Light saturation Low High
Photorespiration High Low
*The ratio of carbon dioxide fixed to water used per unit area of the leaf.
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CAM – Crassulacean Acid Metabolism • These plants are also adapted
to hot, dry climates ex: cacti, pineapples, succulents (aloe)
• Close stomata during day to prevent water loss & open at night
• Carbon compounds stored in vacuoles at night
• These compounds are broken down to release CO2 for photosynthesis during the day
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Global Warming • In atmosphere, CO2
retains heat from sun that would otherwise radiate back into space
• Burning of fossil fuels & wood releases excess CO2, which may be causing global warming
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Forest Replacement • Replace w/younger
growth of trees
• Increases photosynthesis, which reduces CO2, but burning at faster rate
• Older trees also remove CO2, but at slower rate