Test 2 Notes - Cellular respiration, photosynthesis | BIOL 1107, Study notes of Biology

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Cellular respiration Photosynthesis -the process is exergonic -carbon and hydrogen atoms of the sugar are oxidized (lose in their sharing of e-) -oxygen atoms are reduced (gain in their sharing of e-) -e-^ lose energy as they travel down the e- transport chain -energy is used to produce ATP -water is formed from oxygen -electrons from sugar’s carbon and hydrogen atoms lose potential energy (oxidation) as they are transferred to oxygen -the process is endergonic (requires light energy) -carbon and hydrogen atoms from carbon dioxide and water are reduced (gain in their sharing of e-) -oxygen atoms of water are oxidized (lose in their sharing of e-) -e-^ gain energy from light energy as they travel up the e-^ transport chain -energy is used to produce sugar and ATP -oxygen is formed from water -e-^ from water’s hydrogen atoms and carbon dioxide’s carbon atoms gain potential energy (reduction) as they are transferred toward sugar Photosynthesis

• autotroph : producers of the biosphere, an organism that produces complex organic compounds from simple organic molecules using energy from light or inorganic chemical rxns -almost all plants are autotrophs -do not use organic cmpds as an energy source or a carbon source -reduce CO 2 to make organic cmpds -most use water as a reducing agent -converts physical energy from sun light into chemical energy in the form of reduced carbon -can be phototrophs or lithotrophs
  • photo : use light as an energy source
  • litho : oxidize inorganic compounds -photo and litho use a portion of ATP produced during photosynthesis or oxidation of inorganic compounds to reduce NADP+^ to NADPH to form energy rich organic compounds (such as carbs)
• heterotroph : consumers of the biosphere, unable to make organic material by photosynthesis, live on cmpds produced by other organisms -decomposers consume remains of dead organisms -most fungi and some prokaryotes get fuel this way -dependent on photoautotrophs for food and also for O 2

Chloroplasts -chloroplasts are the major site of photosynthesis in most plants -color of the leaf is from chlorophyll , a green pigment in the chloroplast -light energy absorbed by chlorophyll drives synthesis of organic molecules in a chloroplast -chloroplasts are found in the mesophyll , the tissue in the interior of the leaf -CO 2 enters the leaf, O 2 exits through stomata , pores -water absorbed by the roots is delivered to the veins

• stroma : dense fluid within the chloroplast, enclosed by an envelope of two membranes -stroma is separated from in the thylakoid space (interior of the thylakoids) by a system of interconnected membranous sacs called thylakoids Stages -two stages are light rxns (photo part) and the Calvin cycle (synthesis part)
• light rxns : converts solar energy to chemical energy -water is split, providing a source of protons (H+) and e- -light absorbed by chlorophyll drives transfer of e-^ and H+^ from water to an acceptor, NADP+, where they are temporarily stored -light rxns use solar power to reduce NADP+^ to NADPH by adding a pair of e-^ along with an H+ -they also generate ATP
  • photophosphorylation : use of chemiosmosis to power addition of phosphate to ADP -so light energy is initially converted to chemical energy as two compounds -NADPH, a source of e-^ for reducing that can be passed on to an e-^ acceptor (reducing it) -ATP -light rxns don’t produce sugar -thylakoids of the chloroplast are sites of light rxns -molecules of NADP+^ and ADP pick up e-^ and phosphate, then NADPH and ATP are released to the stroma, where they play an important role in the calvin cycle
• calvin cycle : begins by carbon fixation: incorporating CO 2 from the air into organic molecules already present in the chloroplast -then the fixed carbon to carbs by the addition of e- -steps of calvin cycle are sometimes called dark or light independent rxns because none of the steps require light directly -for most plants this process still occurs in daylight because then light rxns can provide the NADPH and ATP that the cycle needs -occurs in the stroma

Photosystems/Mechanism

• photosystem : units of protein complexes involved in photosynthesis that carry out absorption of light and transfer of energy and e- -found in the thylakoid membranes of plants, algae, and some bacteria
• rxn center : an enzyme that uses light to reduce molecules
  • inside the photosystem -surrounded by light harvesting complexes that aid absorption of light and transfers energy to rxn centers -each light harvesting complex consists of various pigment molecules -contains a molecule able to accept e-^ (being reduced) called the primary e-^ acceptor -light harvesting and rxn center complexes are membrane protein complexes -thylakoid membrane has photosystem I and photosystem II -photosystem II fns first in light rxns -contain different pigments, each absorbing a different wavelength of light -light drives synthesis of ATP and NADPH by energizing the two photosystems in the thylakoid membranes of chloroplasts -key to this is linear e-^ flow : flow of e-^ through the photosystems and other components built into the thylakoid membrane -linear e-^ flow occurs during the light rxns of photosynthesis
• cyclic e-^ flow : usues photosystem I but not photosystem II -e-^ cycle back from ferredoxin (Fd) to the cytochrome complex and then continue to PS I -no production of NADPH and no release of O 2 -does generate ATP

Linear e-^ flow Cyclic e-^ flow -excited e-^ from each of the photosystems can be donated to a 1̊ electron acceptor -both photosystems active -e-^ flow from one e-^ carrier to the next a. light excites electrons in PS II b. e-^ transferred to the 1̊ e-^ acceptor c. e-^ flow down the e-^ transport chain d. e-^ transferred to PS-I e. light excites e-^ in PS-I f. e-^ transferred to the 1̊ e-^ acceptor g. e-^ transferred to NADP+ -e-^ (and reducing power) are stored in NADPH -e-^ from the splitting of H 2 O replace the e-^ lost from PS II and generate O 2 as a byproduct -e-^ are gaining energy -the flow of e-^ and the splitting of water also creates a H+^ gradient across the thylakoid membrane, which then is used to synthesize ATP by chemiosmosis This process is known as linear photophosphorylation -only photosystem I is active -e-^ flow from one e-^ carrier to the next a. light excites electrons in PS-I b. e-^ transferred to the 1̊ e-^ acceptor c. e-^ flow down the electron transport chain d. e-^ transferred to PS I -e-^ from the electron transport chain replace electrons lost from PS-I -no H 2 O split and no O 2 generated -flow of e-^ creates a H+^ gradient across the thylakoid membrane, which is used to synthesize ATP by chemiosmosis ( cyclic photophosphorylation )

Chemiosmosis -mito and chloro both generate ATP by this method -an e-^ transport chain assembled in the membrane pumps protons across the membrane as e-^ are passed through a bunch of carriers that get progressively more electronegative -transform redox energy stored as an H+^ gradient across a membrane -ATP synthase is also in the membrane, it couples diffusion of hydrogen ions down their gradient with phosphorylation of ADP Respiration (mito) both Photosynthesis (chloro) -e-^ come from food by NADH -chemical energy from food -e-^ pumped across the inner mito membrane -e-^ transport chains move protons across membrane as e- are moving down the chain -ATP synthase is in the same membrane as the e-^ transport chain -diffusion of H+^ is coupled to ATP production -many components are very similar in structure -e-^ come from H 2 O or PS I -energy is light -e-^ pumped across the thylakoid space Calvin Cycle -uses ATP and NADPH to convert CO 2 to sugar -occurs in the chloroplast stroma -incorporates CO 2 into organic molecules by carbon fixation, and then reduces the fixed carbon to carbohydrate

• carbon fixation : -no direct light energy required -ATP and NADPH from the light reaction -NADPH provides the reducing power and ATP provides the chemical energy -cycle produces a 3-carbon sugar (glyceraldehyde 3-phosphate, G3P) -cycle has 3 phases:

1. Input phase -CO 2 is fixed by rubisco (enzyme that catalyzes the first step) -CO 2 is incorporated one at a time by attaching to ribulose biphosphate, RuBP -pdt is a 6 C intermediate that is unstable and splits in half -two molecules phosphoglycerate per CO 2 fixed
2. Reduction phase -ATP is used for phosphorylation -NADPH is used for reduction
3. Regeneration of CO 2 Acceptor (RuBP) -sugar carbons are shuffled around to make 3 5-carbon sugars (RuBP) from 5 3-carbon sugars -spends three ATP -RuBP can take CO 2 again, and cycle continues -to produce one molecule of glyceraldehyde 3-phosphate, 3 CO 2 molecules are fixed -two molecules of glyceraldehyde 3-phosphate are required to make one molecule of glucose

Adaptations -environmental conditions that promote photorespiration: hot, bright, dry days -in these climates, alternate modes of carbon fixation have evolved to minimize photorespiration. -the two most important of these adaptations are exhibited by C 4 and CAM C 4 plants -when their stomata close on hot days, C 3 plants produce less sugar because the declining level of CO 2 starves the calvin cycle -as CO 2 becomes scarce in the leaf, rubisco adds O 2 to the calvin cycle instead of CO 2 -pdt splits and a 2 C cmpd leaves the chloro -peroxisomes and mito rearrange and split that cmpd and release CO 2 -process called photorespiration because it occurs in the light and consumes O 2 while producing CO 2 -produces no ATP, consumes ATP -produces no sugar, decreases output because it steals organic material from the calvin cycle and releases CO 2 that would otherwise be fixed -it is suggested that plants do this because when rubisco first evolved, the atmosphere had less O 2 and more CO 2 than today -rubisco retains some affinity for O 2 , and since there is so much in the atmosphere, photorespiration is inevitable to some extent

• probability that RuBisCO reacts with oxygen vs. CO 2 depends on the relative concentrations of the two molecules at the site of the reaction C 4 -CO 2 is initially fixed to the 3 carbon sugar phosphoenol pyvuvate ( PEP ) forming a 4 carbon compound, oxaloacetate

-rxn catalyzed by PEP carboxylase which is better at distinguishing between O 2 and CO 2 than rubisco is -PEP carboxylase is only in mesophyll cells of C 4 plants -rubisco is more concentrated in cells of surrounding leaf veins, bundle sheath cells -CO 2 enrichment allows C 4 plants to sustain higher rates of photosynthesis than non C 4 plants, especially at higher temperatures -concentration of CO 2 relative to O 2 in bundle sheath cells is higher -mesophyll cells of a C 4 plant pump CO 2 into the bundle sheath, keeping the CO 2 concentration in the bundle sheath cells high enough for rubisco to bind carbon dioxide rather than oxygen -reactions with PEP carboxylase and the regeneration of PEP can be thought of as a CO 2 concentrating pump powered by ATP -rates of photorespiration in C 4 plants are lower than in C 3 plants -efficient use of CO 2 allows C 4 plants to grow better at higher temperatures and use less water -C 4 plants operate these two processes in separate cell types -CAM plants operate these two pathways at different times of day CAM -CAM plants close their stomata during the day and usually only open them at night -stomata being closed prevents CO 2 from entering leaves and also prevents water loss -CO 2 collected at night through the open stomata stored as organic acids until the day -during the day, the CO 2 is released from these organic acids inside the leaf and functions to drive the calvin cycle (carbon fixation) -CO 2 is fixed to RuBP -Light rections that produce the NADPH and ATP for the reduction of C and the regeneration of RuBP occur all day -CAM plants truly have light reactions and dark reactions Photosynthesis Summary -light rxns capture solar energy and use it to make ATP and transfer e-^ from water to NADP+, forming NADPH -calvin cycle uses ATP and NADPH to produce sugar from CO 2

-energy that enters the chloroplasts as sunlight becomes stored as chemical energy in organic compounds -sugar made in the chloroplasts supply the plant with chemical energy and carbon skeletons for synthesis of all major organic molecules of plant cells -sometimes there is a loss of photosynthetic products to photorespiration -technically green leaves are the only autotrophic parts of a plant -the rest of the molecule depends on organic molecules exported from leaves -carbs are exported as sucrose (disacc) -provides raw material for anabolic pathways that make proteins, lipids, and other pdts