Photosynthesis INTRODUCTION: metabolic process occurring in green plants, algae, some protists and cyanobacteria Photosynthesis is an PROCESS (building organic molecules which store radiant energy as chemical potential energy) Recall: Cellular Respiration is a PROCESS Summary Reaction for Photosynthesis: CO 2 : used in light reactions (sometimes called reactions) enters through stomata and goes to mesophyll cells H 2 O: used in light reactions (sometimes called reactions) enters through veins of leaf and goes to mesophyll cells chlorophyll: light absorbing coloured pigment that begins the process of photosynthesis found in primary function: convert energy into and in order to be used to convert to molecules 1
Chloroplast organelle involved in photosynthesis contains its own: (a) (b) Photosynthesis consists of two complex series of events; 1. (2 stages) 2. You should be able to: - label a diagram of a chloroplast - explain the structure/function of the chloroplast Light Dependent Reactions Capturing light energy occur within the membrane of the chloroplast produces: (1) (2) (nicotinamide adenine dinucleotide phosphate, a coenzyme; similar to NADH) Light Independent Reactions OR CARBON FIXATION DURING CALVIN- BENSON CYCLE occur in either the presence or absence of occur within the of the chloroplast uses ATP and NADPH from the light reactions to form organic molecules like glucose, from CO 2 produces: (1) (2) Stages 1 & 2 Stage 3 Note: Stage 3 used to be called dark reaction, but now we think that the enzymes needed in this reaction, need light 2
Light Electromagnetic (EM) radiation Travels in wave packets called photons Photons with short wavelength = high energy Photons with large wavelength = low energy Light is a mix of photons of diff energies Most of it we can t see If passed through spectroscope, photons can separate from one another Called Electromagnetic spectrum Photosystems Clusters of photosynthetic pigments embedded in thylakoid membrane They absorb photons of particular wavelengths Through light reactions they convert ADP to ATP and NADP + to NADPH Occurs in the stroma, but receives the energy and atoms to do this from the thylakoid Chlorophyll found within pigments contain: (1) (2) chlorophyll a: absorbs and wavelength photons (reaction centre of a photosystem) chlorophyll b: absorbs and wavelength photons (accessory pigment) **add this to your EM spectrum above Carotenoids and pigments (accessory pigments) absorb only in the end of the spectrum Excess energy absorber to protect chlorophyll and dissipate it as heat E.g. β-carotene from carrots 3
Chlorophylls a and b absorb blue-violet and red But...they reflect those with wavelengths between about 500 nm and 600 nm Different accessory pigments have different colours (e.g. Xanthophylls are yellow) In the fall, chlorophyll breaks down so other pigments are more visible Photosynthetically Active Radiation Ideal wavelengths for photosynthesis Ranges from 400 nm to 700 nm Combination of chlorophyll a and b, and all of the other accessory pigments that help out Question: page 154-155, #1-3, 7, 8, 11 The Details Light reactions begin when photons strike a photosynthetic membrane. Can be divided into 3 parts 1. Photoexcitation: absorption of a photon by an electron of chlorophyll 2. Electron transport creates an H+ reservoir 3. Chemiosmosis movement of protons to help phosphorylate ADP to ATP 4
In order to capture light energy, electrons must become excited and leave a molecule. Only chlorophyll a can pass electrons along to the primary electron acceptor. Pigments group together on the thylakoid membrane in photosystems. Antenna and accessory pigments capture light energy and pass it to chlorophyll a. What you were looking at... Location and structure of chlorophyll molecules in plants - consists of: Reaction centre (chlorophyll a) Antenna complex (hundreds of chlorophyll molecules and accessory pigments) Photosystem I Chlorophyll a molecule is called P700 (red light) Photosystem II Chlorophyll a molecule is called P680 (red light) Location of Photosystems: Thylakoid membrane pigment Wavelength (nm) There are two separate light powered systems in the membranes of the thylakoid disc: 1. Non-cyclic Photophosphorylation 2. Cyclic Photophosphorylation Used in photosynthesis of green plants Involves both (PS I) and (PS II) Chlorophyll a electrons are passed along to make NADPH and are replaced by water s electrons Produces ATP and NADPH 5
1. PS II absorbs light energy causing two electrons of chlorophyll P680 to become excited. 2. A Z protein, associated with PS II and facing the thylakoid lumen, splits water into oxygen, protons, and electrons Two electrons are used to replace the excited electrons in chlorophyll P680. Oxygen leaves the chloroplast as a byproduct. The protons remain in the thylakoid space add to the proton gradient that powers chemiosmosis. 3. The two excited electrons travel through a series of proteins within the thylakoid membrane 4. As the two excited electrons move from the excited reaction centre of PS II to plastoquinone (PQ), energy is lost. 5. This energy is used to move protons from the stroma to the lumen. This creates a proton gradient for chemiosmosis. 6. The increase in proton concentration within the thylakoid lumen drives protons out of the lumen to the stroma via a special protein called. This proton motive force is used to produce. 7. The electrons continue to move from PQ to other components of the electron transport chain (cytochrome b 6 -f complex to Plastocyanin) to PS I eventually replacing the 2 electrons that were lost by PS I when it was struck by photons. 8. The two excited electrons from PS I pass through another electron transport chain containing the protein ferradoxin (Fd). 9. The two excited electrons are then captured by NADP reductase which uses the two electrons and protons from the stroma to reduce to. 6
10. The and are used to drive light independent reactions. So... ATP is generated from reactions stemming from PS II NADPH is generated from reactions stemming from PS I (that gained e- from PSII) Photosystem II Photosystem I Photoexcitation Electron Transport Chemiosmosis OK the Light Reaction Details! Animation! Involves Photosystem I and II where the chlorophyll a electrons are passed along to make NADPH, and are replaced by H 2 O s electrons Used by prokaryotes Photosystem I only (so P700) the chlorophyll a electrons return to chlorophyll a (hence cyclic) Product = ATP (no NADPH is made) 1. PS I (P700) absorbs energy, causing the two to become excited. 2. The two excited electrons enter an electron transport chain 7
3. The two excited electrons are captured by the protein, causing some of their energy to be lost. 4. The two excited electrons are then captured by the cytochrome b 6 -f protein complex and lose more energy. 5. At this point, enough energy has been lost to add a P to ADP forming. 5. The passing of electrons from cytochrome b 6 -f complex to the next electron carrier, (Pc) results in further lowering of the energy in the electrons to almost the ground state 6. The passing of the two electrons to P700 returns the electrons to both the molecule and the ground state. Involves Photosystem I only where the chlorophyll a electrons return to chlorophyll a (no NADPH is made) Page 166 # 1-6 Light Independent Reactions Calvin Cycle A.k.a. C 3 photosynthesis Due to the first compound produced contains. reactions that convert into molecules occur in the of chloroplasts 8
Rubisco CO2 and RuBP unstable 6C intermediate which splits into 2 3C PGA Note that we start with 3 CO2 to produce 6 PGA 3 PGA is phosphorylated by an ATP 6 1,3-BPG Note: since 6 molecules, need 6 ATP 6 NADPH reduces 6 1,3-BPG 6 G3P or PGAL 1 G3P (1/2 a glucose) exits cycle as final product (5 move on in cycle) Remaining 5 G3P: rearranged 3 RuBP Use 3 ATP Cycle continues Note: 2 Pi is removed...count your phosphates! G3P molecules that leave are used to synthesize larger sugars (e.g.glucose) Rubisco (an enzyme) Ribulose biphosphate carboxylase/oxygenase Most abundant protein on Earth 3 CO2 must be fixed before 1 G3P molecule can be removed from the cycle 6 turns of the cycle makes one 6 C glucose molecule (looking at one CO2 at a time) Page 167 # 8-12 9
Do questions: page 166 167, # 2 9 Animation Websites: 1. http://faculty.nl.edu/jste/noncyclic_photophosphorylation.htm 2. http://www.stolaf.edu/people/giannini/flashanimat/metabolism/photos ynthesis.swf 3. http://www.johnkyrk.com/photosynthesis.html 10