How Cells Acquire Energy Chapter 7 Carbon and
How Cells Acquire Energy Chapter 7
Carbon and Energy Sources § Photoautotrophs § Carbon source is carbon dioxide § Energy source is sunlight § Heterotrophs § Get carbon and energy by eating autotrophs or one another
Photoautotrophs § Capture sunlight energy and use it to carry out photosynthesis § Plants § Some bacteria § Many protistans
Linked Processes Photosynthesis Aerobic Respiration § Energy-storing pathway § Energy-releasing pathway § Releases oxygen § Requires carbon dioxide § Releases carbon dioxide
Chloroplast Structure two outer membranes stroma inner membrane system (thylakoids connected by channels) Figure 7. 3 d, Page 116
Photosynthesis Equation LIGHT ENERGY 12 H 2 O + 6 CO 2 Water Carbon Dioxide 6 O 2 + C 2 H 12 O 6 + 6 H 2 O Oxygen Glucose Water In-text figure Page 115
Where Atoms End Up Reactants Products 12 H 2 O 6 O 2 6 CO 2 C 6 H 12 O 6 6 H 2 O In-text figure Page 116
Two Stages of Photosynthesis sunlight water uptake carbon dioxide uptake ATP LIGHTDEPENDENT REACTIONS ADP + Pi NADPH LIGHTINDEPENDENT REACTIONS NADP+ P oxygen release glucose new water In-text figure Page 117
Electromagnetic Spectrum Shortest wavelength Longest wavelength Gamma rays X-rays UV radiation Visible light Infrared radiation Microwaves Radio waves
Visible Light § Wavelengths humans perceive as different colors § Violet (380 nm) to red (750 nm) § Longer wavelengths, lower energy Figure 7. 5 a Page 118
Photons § Packets of light energy § Each type of photon has fixed amount of energy § Photons having most energy travel as shortest wavelength (blue-violet light)
Pigments § Color you see is the wavelengths not absorbed
Variety of Pigments Chlorophylls a and b Carotenoids Anthocyanins
Chlorophylls Wavelength absorption (%) Main pigments in most photoautotrophs chlorophyll a chlorophyll b Wavelength (nanometers) Figure 7. 6 a Page 119 Figure 7. 7 Page 120
Accessory Pigments percent of wavelengths absorbed Carotenoids, Phycobilins, Anthocyanins beta-carotene phycoerythrin (a phycobilin) wavelengths (nanometers)
Pigments in Photosynthesis § Bacteria § Pigments in plasma membranes § Plants § Pigments and proteins organized into photosystems that are embedded in thylakoid membrane system
Arrangement of Photosystems water-splitting complex H 2 O thylakoid compartment 2 H + 1/2 O 2 P 680 P 700 acceptor PHOTOSYSTEM II pool of electron carriers stroma PHOTOSYSTEM I Figure 7. 10 Page 121
Light-Dependent Reactions § Pigments absorb light energy, give up e-, which enter electron transfer chains § Water molecules split, ATP and NADH form, and oxygen is released § Pigments that gave up electrons get replacements
Photosystem Function: Harvester Pigments § Most pigments in photosystem are harvester pigments § When excited by light energy, these pigments transfer energy to adjacent pigment molecules § Each transfer involves energy loss
Photosystem Function: Reaction Center § This molecule (P 700 or P 680) is the reaction center of a photosystem
Pigments in a Photosystem reaction center Figure 7. 11 Page 122
Electron Transfer Chain § Adjacent to photosystem § As electrons pass along chain, energy they release is used to produce ATP
Cyclic Electron Flow § Electrons § are donated by P 700 in photosystem I to acceptor molecule § flow through electron transfer chain and back to P 700 § Electron flow drives ATP formation § No NADPH is formed
Synthesis of ATP (chemiosmotic phosphorylation) H 2 O photolysis second electron transfer chain e– e– first electron transfer chain PHOTOSYSTEM II NADP+ PHOTOSYSTEM I ATP SYNTHASE NADPH ADP + Pi ATP Figure 7. 13 a Page 123
Chemiosmotic Model of ATP Formation § Electrons within the membrane of the chloroplast attract H+ protons § The H+ protons are pumped inside the chloroplast membranes § The Protons are allowed to pass out of the membrane through the CF 1 particle that is rich in ADP + P plus phosphorylating enzymes.
Chemiosmotic Model for ATP Formation Photolysis in the thylakoid compartment splits water H 2 O e– H+ is shunted across membrane by some components of the first electron transfer chain Gradients propel H+ through ATP synthases; ATP forms by phosphate -group transfer acceptor ATP SYNTHASE PHOTOSYSTEM II Figure 7. 15 Page 124 ADP + Pi ATP
Light-Independent Reactions § Synthesis part of photosynthesis § Can proceed in the dark § Take place in the stroma § Calvin-Benson cycle
Calvin-Benson Cycle § Overall reactants § Overall products § Carbon dioxide § Glucose § ATP § ADP § NADPH § NADP+ Reaction pathway is cyclic and Ru. BP (ribulose bisphosphate) is regenerated
6 Calvin. Benson Cycle CO 2 (from the air) CARBON FIXATION 6 6 Ru. BP unstable intermediate 12 PGA 6 ADP 6 12 ATP 12 NADPH 4 Pi 12 ADP 12 Pi 12 NADP+ 10 PGAL 12 PGAL Pi Figure 7. 16 Page 125 P glucose
The C 3 Pathway § In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA § Because the first intermediate has three carbons, the pathway is called the C 3 pathway
Photorespiration in C 3 Plants § On hot, dry days stomata close § Inside leaf § § Oxygen levels rise Carbon dioxide levels drop § The plant is in trouble because it does not enough Carbon dioxide to undergo photosynthesis § The plant still needs energy so it taps its own store of glucose
C 4 Plants § Carbon dioxide is fixed twice § In mesophyll cells, carbon dioxide is fixed to form four-carbon oxaloacetate § Oxaloacetate is stored as a crystal § When times get bad (drought conditions), the plant can now convert the crystalline form of oxaloacetate back to Carbon dioxide and undergo photosynthesis.
Summary of Photosynthesis light LIGHT-DEPENDENT REACTIONS 6 O 2 12 H 2 O ATP ADP + Pi NADP+ NADPH LIGHT-INDEPENDENT REACTIONS PGA 6 CO 2 Ru. BP CALVINBENSON CYCLE PGAL 6 H 2 O P C 6 H 12 O 6 (phosphorylated glucose) end product (e. g. , sucrose, starch, cellulose) Figure 7. 21 Page 129
Satellite Images Show Photosynthesis Atlantic Ocean Photosynthetic activity in spring Figure 7. 20 Page 128
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