Photosynthesis

A. Introduction 1. Location: chloroplasts (in plants and algae) or folds in cell membrane (in photosynthetic prokaryotes, cyanobacteria)

2. Stages in photosynthesis: §stage 2. light energy makes ATP + NADPH + H + ; occurs in thylakoid membrane stage 3. using ATP + NADPH + H + to synthesize organic compounds (glucose) from CO 2 (carbon fixation); process called Calvin cycle; occurs in stroma stage 1. capturing light energy; occurs in thylakoid membrane 2 H+ + NAD+ --> NADH + H+ in cell respiration 2 H+ + NADP+ --> NADPH + H+ in photosynthesis

6CO 2 + § the carbon + oxygen from CO 2 and the H from H 2 O produce the glucose § the oxygen from H 2 O is released as oxygen gas 3. Overall process: 12H 2 O +lightC 6 H 12 O 6 +6O 2 +6 H 2 O chlorophyll

4. Light energy: §light energy travels in wave packets called photons §visible light has a wavelength of 380 nm (violet) to 750 nm (red) (ROYGBIV) §long wavelengths [red] is low energy §short wavelengths [violet] is high energy

§clusters of photosynthetic pigments embedded in the thylakoid membrane absorb photons of a particular λ §energy is transferred to ADP + P i  ATP NADP + + 2H (from H 2 O)  NADPH + H + + ½ O 2

5. Chlorophyll: § pigment molecule has 2 parts: § long carbon-hydrogen tail; called phytol tail/chain (hydrophobic) § a ring of C, N, and lots of double bonds, Mg trapped in middle; called porphyrin ring

§ chlorophyll a (primary light absorbing pigment in all photosynthetic organisms) and chlorophyll b absorb photons of blue-violet λ and red-orange λ, respectively. § green λ are reflected (that is why leaves appear green) and green is transmitted so leaves look green from underneath

§ in fall, chlorophyll is no longer produced and is broken down, other pigments (e.g. red or yellow) show through and leaves change colour

B. Light Reactions 1. Photoexcitation § e - are normally stable in their ‘ground state’, the lowest possible energy level § when a photon strikes a chlorophyll molecule, the e - have energy added to them and are in a state of ‘excitation’ § an excited e - has more energy than one in a ground state, but it will return to the ground state in 1 billionth of a second

if left alone, the e - returns to the ground state and releases energy (heat + light) instead of the e- from chlorophyll releasing heat + light, they are captured by a primary e - acceptor (chlorophyll is oxidized, e - acceptor is reduced)

2. Photosystems § photosystems = a group of pigments that work together to capture light § the antenna complex consists of several hundred pigment molecules that capture light of different wavelengths and pass energy to the reaction center § the reaction center is a chlorophyll a pigment that takes all the energy and uses it to excite an electron § the excited e - is transferred to an e - acceptor (chlorophyll is oxidized, primary e - acceptor is reduced)

3. Sources of electrons: a) bacteria: § photosystem I contains chlorophyll P700 (max. absorption at 700 nm λ) § the photosystem I does not have enough energy to break water, § but H 2 S is rare; only found in sulphur springs § 6CO H 2 S + light energy  C 6 H 12 O 6 + 6H 2 O + 12S however it does have enough energy to break H 2 S

Sulphur springs

b) algae and plants § photosystem II contains chlorophyll P680 (max. absorption at 680 nm λ) § photosystem II does have enough energy to break water § electrons are then passed to photosystem I § 6CO H 2 O + light energy  C 6 H 12 O 6 + 6H 2 O+ 6O 2 c) photosystem I and II are used to produce ATP + NADPH + H +

4. Photolysis (the breaking of water by sunlight) § the Z protein (in the thylakoid space) uses energy from light to split H 2 O § 2 e - are given to photosystem II § 2 H + are released in the thylakoid space and create an electrochemical gradient § O 2 leaves the chloroplast as waste

5. Electron Transport System a series of proteins that move electrons and protons to produce high energy compounds

PROTEINS Z –protein 1.photosystem II 3. cytochrome b6-f 5. photosystem I 7. NADP reductase 2.PQ plastoquinone 4. PC plastocyanin 6. ferredoxin8. ATP synthase stroma z 6 H20H20 ½ O 2 + 2H + 2H + 2e- P680P700 2H + NADP + NADPH + H + ADP +Pi ATP 2H + 2e- 2H +

5. Electron Transport System b) photon strikes photosystem II and excites 2e - of chlorophyll P680, 2e - passed to electron acceptor called plastoquinone (PQ) c) 2e - from Z protein replace the missing 2e - from chlorophyll P680 d) PQ takes 2e - from photosystem II and moves with them to b 6 -f complex e) b 6 -f complex takes 2e - and passes them onto plastocyanin which allows 2H + to pass from the stroma into the thylakoid space; f) this creates an electrochemical gradient a) occurs in the thylakoid membrane

§g) P c takes 2e - and moves with them to photosystem I (these replace 2e - lost when photosystem I was struck by a photon) § h) photosystem I transfers 2e - to ferredoxin (Fd) (an e - acceptor) § i) Fd transfers 2e - to NADP reductase § j) NADP reductase passes on the 2 e - to NADP + § k) (NADP + + 2H + + 2e -  NADPH + H + )

6. Types of Electron Transport Systems a) non-cyclic phosphorylation § normal process as described in notes § involves both P700 and P680 and transfers e - from H 2 O to NADP + to produce 1 NADPH + H + and 2 ATP needed to make glucose § but demand for ATP and NADPH + H + is not always in a 2:1 ratio H 2 O + photons + 2ADP + 2Pi + NADP +  ½O 2 + 2ATP + NADPH + H +

b) cyclic phosphorylation § happens in all plants when extra ATP is needed § the process produces only ATP and not NADPH + H + § one of the proteins [Fd] in the thylakoid membrane shifts in order for the cycle to flow

stroma z 6 2e- P700 2H+ ADP +Pi ATP 2H+ 6 PROTEINS Z –protein 1.photosystem II 3. cytochrome b6-f 5. photosystem I 7. NADP reductase 2.PQ plastoquinone 4. PC plastocyanin 6. ferredoxin8. ATP synthase Cyclic phosphorylation

§ photosystem I (P700) is involved, but not photosystem II (P680) § electrons from P700 are excited but eventually return to P700 (H 2 O & NADP + are not involved) § P700  Fd  b 6 -f complex  protein C  P700 § b 6 -f complex pumps H + ions across the thylakoid membrane, creating an electrochemical gradient which is used to make ATP