Energy Conversions Photosynthesis Chapter 10 Pg. 184 205 Life on Earth is solar-powered by autotrophs Autotrophs make their own food and have no need to consume other organisms. They are the ultimate source of organic compounds. Known as producers. Photoautotrophs use photosynthesis to make their own food from light and CO 2. This occurs in the chloroplasts of leaf cells. Heterotrophs live on compounds produced by other organisms. Known as consumers. Includes animals, fungi, and many prokaryotes. Depend on photosynthesis for food and oxygen. Heterotrophs use cell respiration to get energy. Plants Plants collect light and transform it to chemical energy, and obtain atoms from the environment to produce organic molecules needed for growth and survival. Structure Leaves: absorb sunlight Stomates: pores on underside of leaves; gas (CO 2 and O 2 ) and water exchange Roots: absorb water and nutrients Chloroplasts: contain pigments, perform photosynthesis Chloroplasts Specific sites of photosynthesis in plant cells Located in the green parts of plants, but most abundant in the mesophyll cells Structure Double-membrane Fluid-filled area called the stroma Vast network of interconnected membranous sacs called the thylakoids Inner area of the thylakoid is the thylakoid space Chlorophyll is located in the thylakoid membranes It is a light-absorbing pigment that drives photosynthesis and is responsible for plants green color Plant Pigments Light, or electromagnetic energy, is absorbed by plant pigments in photosynthesis. Two major types Chlorophyll: chlorophyll a and chlorophyll b absorb all wavelengths of light in the red, blue, and violet range, but not much green. Carotenoids: they absorb light in the blue, green, and violet range, but not much yellow, orange, or reds. Carotenoids include xanthophyll and phycobilins (in red algae). Chlorophyll b, carotenoids, and phycobilins are antenna pigments because they capture light in wavelengths other than those captured by chlorophyll a. They absorb photons of light and pass energy to chlorophyll a. The central atom in chlorophyll is magnesium. 1
Photosynthesis Reaction: 6CO2 + 6H2O + light energy C6H12O6 + 6O2 It is the reverse of cell respiration. All O2 that you breathe is provided by this process when water is split. The H2O molecule is split to provide electrons. These electrons go on to reduce carbon dioxide to sugar. Two stages occur: Light reactions: occur in the thylakoid, sunlight is required, and light is converted to chemical energy (ATP and NADPH). Calvin cycle: occur in the stroma, sunlight not required, and chemical energy in ATP and NADPH are used to convert CO2 into sugar Oxidation/Reduction Reactions (Redox Reactions) Oxidation Removes electrons from an atom Results in a positive charge Oxidation Is Loss (of electrons) Reduction Think: OIL RIG Adds electrons to an atom Results in a negative charge Reduction is Gain (of electrons) (reducing the charge) Reduction in Photosynthesis Splitting water releases electrons and hydrogen ions (H+). These bond to the carbon dioxide, reducing it to a sugar. Electrons increase the potential energy of the molecule ΔG is positive, making the reaction endergonic. Light provide extra free energy. Light Reaction Occurs in the thylakoid membranes Solar energy chemical energy Net products: NADPH (stores electrons), ATP, oxygen Main events: Light is absorbed by chlorophyll and drives the transfer of electrons from water to NADP +, forming NADPH. Water is split and O 2 is released. ATP is generated, using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. Light Light = electromagnetic energy made up of photons Substances that absorb light are called pigments different pigments absorb different wavelengths. An absorption spectrum shows which wavelengths of light a particular pigment absorbs. The action spectrum for photosynthesis graphs the effectiveness of different wavelengths of light in driving photosynthesis. 2
Photosystems Photons are absorbed by groups of pigment molecules in the thylakoids, called photosystems: Light-harvesting complex: many chlorophyll and carotenoid molecules that allow the complex to gather light effectively. When photons are absorbed, one of chlorophyll s electrons is raised to an orbital of higher potential energy it is said to be in the excited state. Reaction center: consists of two chlorophyll a molecules, which donate the electrons to the primary electron acceptor. This is the first step of the light reaction, a conversion of light energy to chemical energy, and what makes photoautotrophs the producers of the natural world. Thylakoids contain two photosystems, named photosystem I and photosystem II. Linear (Noncyclic) Electron Flow Overview Predominant route Sunlight energizes electrons, which drives the synthesis of ATP as they are passed from photosystem II to photosystem I. They are again excited in photosystem I and transferred to NADP+, forming NADPH. NADPH and ATP are used in the Calvin cycle to make sugar from CO2. Noncyclic Electron Flow 1) Photosystem II absorbs light, an electron is excited and lost, and chlorophyll now has an electron hole. 2) An enzyme splits a water molecule into 2 H+, two electrons, and an oxygen atom. The oxygen combines with another oxygen molecule, forming atmospheric O2. 3) The original excited electron passes from the primary electron acceptor of photosystem II to photosystem I through an electron transport chain (similar to that in cellular respiration). ATP is made through chemiosmosis. 4) The energy from the transfer of electrons down the electron transport chain is used to pump protons into the thylakoid space, creating a build up of H+. H+ ions diffuse through ATP synthase, generating ATP to be used in the Calvin cycle. 5) The electron from photosystem II ends up in the reaction center of photosystem I, which has just lost an electron due to light energy. Keep in mind that the ultimate source of electrons is water. 6) The excited electrons are passed to another electron transport chain, which transmits them to NADP +, which is reduced to NADPH. The high-energy electrons of NADPH are now available for use in the Calvin cycle. 3
Cyclic Electron Flow This type of electron flow uses PS I only. It uses a short circuit of linear electron flow by cycling the excited electrons back to their original starting point in PS I. Creates equal amounts of ATP and NADPH but the Calvin cycle requires more ATP than NADPH. Electrons are sometimes rerouted back to the electron transport chain from photosystem I to produce more ATP. Uses chemiosmosis to produce ATP, but does not produce NADPH, or oxygen, and water isn t used. Chemiosmosis Chloroplasts and mitochondria generate ATP by chemiosmosis. Basic steps: Electron transport chain uses flow of electrons to pump H + across thylakoid membrane. A proton-motive force is created within the thylakoid space that can be utilized by ATP synthase to phosphorylate ADP to ATP. It is generated in 3 places: H + from water H + pumped across the membrane by cytochrome complex Removal of a H + from the stroma when NADP + is reduced to NADPH. DARK REACTION (CALVIN CYCLE) Overview Occurs in the stroma of the chloroplast CO 2 from air is incorporated into organic molecules in carbon fixation Uses fixed carbon, NADPH, and ATP from the light reactions to form new sugars Actual product is glyceraldehyde 3-phosphate (G3P), which then forms glucose and other sugars 3CO + 9ATP + 6NADPH G3P + 9ADP + 9P + 6NADP 4
The Long Version 1) Three CO 2 molecules are attached to three molecules of the 5-carbon sugar ribulose biphosphate (RuBP). These reactions are catalyzed by the enzyme rubisco They produce an unstable product that immediately splits into two 3-carbon compounds called 3- phosphogycerate. At this point, carbon has been fixed the incorporation of CO 2 into an organic compound. Calvin Cycle The Short Version 1) Carbon Fixation 3 CO2 + RuBP RuBP = 5-carbon sugar ribulose bisphosphate Catalyzed by the enzyme rubisco (RuBP carboxylase) The Long Version 2) The 3-phosphoglycerate molecules are phosphorylated to become 1,3-bisphosphoglycerate 3) 6 NADPH reduce those molecules to six glyceraldehyde 3-phosphate (G3P). 4) One G3P leaves the cycle to be used by the plant cell. Two G3P can combine to form glucose, which is typically listed as the final photosynthetic product. The Short Version 2)Reduction Use 6 ATP and 6 NADPH to produce 1 net G3P G3P glyceraldehyde 3 phosphate The Long Version 5) Finally, RuBP is regenerated as the 5 G3P are reworked into three of the starting molecules, with the expenditure of 3 ATP molecules. The Short Version 3) Regeneration Use 3 ATP to regenerate RuBP Endergonic Reaction In the Calvin cycle, the formation of one net G3P requires the following: 9 ATP are consumed Replenished by the light reactions 6 NADPH Also replenished by light reactions One of the 6 G3P molecules produced is a net gain, and will be used for biosynthesis or the energy needs of the cell. Accounting The accounting is complicated: 3 turns of Calvin cycle = 1 G3P 3 CO 2 1 G3P (3C) 6 turns of Calvin cycle = 1 C 6 H 12 O 6 (6C) 6 CO 2 1 C 6 H 12 O 6 (6C) 18 ATP + 12 NADPH 1 C 6 H 12 O 6 Any ATP left over from light reactions will be used elsewhere by the cell 5
Photosynthesis Summary Light reactions Produced ATP, NADPH, and O 2 Consumed H 2 O Calvin cycle Produced G3P (sugar) Regenerated ADP and NADP Consumed CO 2 ALTERNATIVES Evolution of Alternative Mechanisms Problem with photosynthesis and C 3 plants: Stomata allow CO 2 to enter and H 2 O to exit. On hot, dry days, C 3 plants produce less sugar because declining CO 2 starves the Calvin cycle the plant must keep stomata closed to conserve water, thereby reducing CO 2 uptake. Additionally, rubisco can bind O 2 in the place of CO 2, causing oxidation/breakdown of RuBP, resulting in a loss of energy and carbon for the plant (photorespiration). This process can drain away up to 50% of the carbon fixed by the Calvin cycle. Adaptations to Arid Climates Metabolic adaptations reduce photorespiration: C 4 plants: the two steps of photosynthesis occur in different locations, which reduces photorespiration and increases sugar production. Physically separated CAM plants: keep stomata closed during the day to prevent excessive water loss; stomata open and CO 2 is absorbed at night; the Calvin cycle occurs in the early morning once the stomata are closed. Temporally separated Photosynthetic Pathways C3 C4 CAM Carbon fixation and Calvin cycle are together Rubisco Carbon fixation and Calvin cycle are in different cells PEP carboxylase Carbon fixation and Calvin cycle occur at different times Organic acid 6