CHAPTER 8 PHOTOSYNTHESIS
Con. 8.1 Photosynthesis process by which plants use light to make food molecules from carbon dioxide and water (chlorophyll) 6CO 2 + 12H 2 O + Light C 6 H 12 O 6 + 6O 2 + 6H 2 O
Autotrophs plants that make their own food (also known as producers) Photoautotrophs organisms that use light as a source of energy to make food
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
Consists of double membrane surrounding a thick fluid (stroma) sugars are made from CO 2 here --CO 2 enters leaf and O 2 exits by way of tiny pores known as stomata
Thylakoids (green sacs) are suspended in stroma light energy is captured here Stack of these = Granum Stacks of these = Grana Thylakoid
Redox Reactions 6CO 2 + 12H 2 O C 6 H 12 O 6 + 6H 2 O +6O 2 -Photosynthesis STORES energy ENDERGONIC -Carbon dioxide is reduced to glucose GAINING ELECTRONS -Water is oxidized to oxygen LOSING ELECTRONS -Water is first consumed then produced
-Plants make sugar from carbon and oxygen in CO 2 and from some hydrogen in H 2 O -O 2 is released, coming from the H 2 O, not from CO 2
- Cellular respiration harvests energy by oxidizing the sugar & reducing O 2 to H 2 O (respiration is energy-releasing, going from potential to kinetic by traveling down energy levels) - Photosynthesis goes uphill (gaining potential and traveling up energy levels)
Con. 8.2 The Light Reactions Convert light energy to chemical energy & O 2 gas is waste product Occur in thylakoid membranes Stored in ATP & NADPH No sugar produced
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
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)
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
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
photosynthesis (wavelength vs. rate of CO 2 use or O release) 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
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
Photosystems (cont.) 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
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
3 Steps of the Light Reaction ATP, NADPH, and O 2 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
Linear Electron Flow photon relayed to PSII P680 e - go to higher energy state e - captured by primary e - acceptor H 2 O 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 2 nd e - transport chain NADPH made
Cyclic Electron Flow uses PSI but not PSII doesn t make NADPH possibly controlled by concentration of NADPH to help w/supply & demand
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)
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
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
The Dark Reaction (Calvin Cycle) Cyclic series of reactions that assemble sugar molecules using CO 2 and energycontaining products of light reaction Con. 8.3 Takes place in stroma
3 Phases 1. Carbon Fixation - cycle must occur 3 times (3 CO 2 ) to get sugar - CO 2 molecule from air attaches to 5C RuBP (rubisco) - unstable 6C molecule so splits forming 2 molecules of 3-phosphoglycerate
2. Reduction - energy from ATP & highenergy electrons from NADPH - help make one glyceraldehyde-3 phosphate (G3P) - other 5 G3P are recycled
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
Adaptation for Saving Water Most plants are C 3 plants, which take carbon directly from CO 2 in the air & use it in the Calvin cycle to build a 3-carbon molecule
Stomata in leaf surface usually close when the weather is hot & dry Minimizes water loss But CO 2 and O 2 are not exchanged as normal Calvin cycle is diverted to an inefficient process called photorespiration Consumes ATP No sugar made
Some plants have special adaptations that enable them to save water & avoid photorespiration ex: corn, sugarcane Special cells in C 4 plants incorporate CO 2 into a 4-carbon compound It s broken down to release CO 2 (this initiates the Calvin cycle)
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 Carbon dioxide concentration Regulated by diffusion Bundle sheath cells 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 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. Mostly warmer, drier climate
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 CO 2 for photosynthesis during the day
Global Warming In atmosphere, CO 2 retains heat from sun that would otherwise radiate back into space Burning of fossil fuels & wood releases excess CO 2, which may be causing global warming
Forest Replacement Replace w/younger growth of trees Increases photosynthesis, which reduces CO 2, but burning at faster rate Older trees also remove CO 2, but at slower rate