6.3 Overview of Photosynthesis

Transcription

1 6.3 Overview of Photosynthesis Chloroplast location of photosynthesis in plants and protists 3 membranes 2 make up the stroma Semifluid matrix Location of sugar production 1 makes up the thylakoid membrane Forms thylakoids Thylakoid membrane starts photosynthesis

2 6.3 Overview of Photosynthesis Photosynthesis proceeds in two stages Light-dependent reactions light energy converted to chemical bond energy (ATP) Light-independent reactions runs on ATP and NADPH and it drives synthesis of carbohydrates (glucose) Summary equation: 6H 2 O + 6CO 2 6O 2 + C 6 H 12 O 6

3 Summary - Light Dependent reaction Water molecules are split NADP+ (Nicotinamide adenine dinucleotide phosphate) accepts released hydrogen and electrons and becomes NADPH Oxygen escapes

4 Summary - Light independent reaction Runs on ATP and NADPH Energy forms carbohydrates (glucose) from carbon dioxide and water

5 Sites of Photosynthesis: Chloroplasts Light-dependent reactions occur at a muchfolded thylakoid membrane Forms a single, continuous compartment inside the stroma (chloroplast s semifluid interior) Light-independent reactions occur in the stroma

6 Sites of Photosynthesis

7 two outer membranes of chloroplasts stroma part of thylakoid membrane system bathed in stroma: thylakoid compartment, cutaway view b Chloroplast structure. No matter how highly folded, its thylakoid membrane system forms a single, continuous compartment in the stroma. Fig. 6.6b, p.97

8 Sites of Photosynthesis

9 Products of Light-Dependent Reactions Typically, sunlight energy drives the formation of ATP and NADPH Oxygen is released from the chloroplast (and the cell)

10 Key Concepts: OVERVIEW OF PHOTOSYNTHESIS Photosynthesis proceeds through two stages in chloroplasts of plants and many types of protists First, pigments in a membrane inside the chloroplast capture light energy, which is converted to chemical energy Second, chemical energy drives synthesis of carbohydrates

11 6.4 Light-Dependent Reactions Two types of photosystems In thylakoid membrane Light-harvesting complexes Absorb light energy and pass it to photosystems which then release electrons Electrons enter light-dependent reactions Figure 6.8

12 Noncyclic Photophosphorylation Electrons released from photosystem II flow through an electron transfer chain At end of chain, they enter photosystem I Photon energy causes photosystem I to release electrons, which end up in NADPH Photosystem II replaces lost electrons by pulling them from water (photolysis)

13 Noncyclic Photophosphorylation

14 Cyclic Photophosphorylation Electrons released from photosystem I enter an electron transfer chain, then cycle back to photosystem I NADPH does not form, oxygen is not released

15 ATP Formation In both pathways, electron flow through electron transfer chains causes H + to accumulate in the thylakoid compartment A hydrogen ion gradient builds up across the thylakoid membrane H + flows back across the membrane through ATP synthases Results in formation of ATP in the stroma

16 energy 6.5 Energy Flow in Light-Dependent Reactions excited P700 light energy a Energy from light-harvesting complexes causes photosystem I to lose electrons. P700 Photosystem I CYCLIC PHOTOPHOSPHORYLATION b Electrons give up energy as they pass through an electron transfer chain. The energy drives H+ across the thylakoid membrane, against its gradient. The electrons reenter photosystem I. Fig. 6.9ab, p.100

17 energy Photosystem II excited P Energy Flow in Light-Dependent Reactions excited P700 NADPH light energy light energy water P680 O 2 + H+ P700 Photosystem I NONCYCLIC PHOTOPHOSPHORYLATION c. Energy from a lightharvesting complex drives electrons out of photosystem II. Then, the photosystem pulls replacement electrons d. Electrons from photosystem II pass through an electron transfer chain. Energy lost at each step moves H+ across the thylakoid membrane. At the end of the chain, the electrons enter photosystem e. NADP+ combines with hydrogen and with electrons driven from photosystem II by energy from a light-harvesting complex. Fig. 6.9cde, p.100

18 Key Concepts: MAKING ATP AND NADPH In the first stage of photosynthesis, sunlight energy is converted to the chemical bond energy of ATP The coenzyme NADPH forms in a pathway that also releases oxygen

19 6.6 Light Independent Reactions: The Sugar Factory Light-independent reactions proceed in the stroma Carbon fixation: Enzyme rubisco attaches carbon from CO 2 to RuBP to start the Calvin Benson cycle

20 Calvin Benson Cycle Cyclic pathway makes phosphorylated glucose Uses energy from ATP, carbon and oxygen from CO 2, and hydrogen and electrons from NADPH Reactions use glucose to form photosynthetic products (sucrose, starch, cellulose) Six turns of Calvin Benson cycle fix six carbons required to build a glucose molecule from CO 2

21 f It takes six turns of the Calvin Benson cycle (six carbon atoms) to produce one glucose molecule and regenerate six RuBP. Light-Independent Reactions 6CO 2 a CO2 in air spaces inside a leaf diffuses into a photosynthetic cell. Six times, rubisco attaches a carbon atom from CO2 to the RuBP that is the starting compound for the Calvin Benson cycle. e Ten of the PGAL get phosphate groups from ATP. In terms of energy, this primes them for an uphill run for the endergonic synthesis reactions that regenerate RuBP. d The phosphorylated glucose enters reactions that form carbohydrate products mainly sucrose, starch, and cellulose. 6 6 ADP ATP 4 P i 6 RuBP 10 PGAL 1 P i Calvin-Benson cycle phosphorylated glucose 12 PGA 12 ATP 12 PGAL 12 ADP + 12 P i 12 NADPH 12 NADP + b Each PGA molecule gets a phosphate group from ATP, plus hydrogen and electrons from NADPH. The resulting intermediate is called PGAL. c Two of the twelve PGAL molecules combine to form a molecule of glucose with an attached phosphate group. Fig. 6.10, p.101