Metabolic Processes Photosynthesis

Leaf Structure You should be able to name the major parts of a leaf and identify where the major reactants and products of photosynthesis come from!

Sun vs Shade

Photosynthesis INTRODUCTION: metabolic process occurring in green plants, algae, some protists and cyanobacteria Photosynthesis is an anabolic PROCESS (building organic molecules which store radiant energy as chemical potential energy) Recall: Cellular Respiration is a catabolic PROCESS

Photosynthesis Summary Reaction for Photosynthesis:

Photosynthesis CO2: used in light independent reactions (sometimes called dark reactions) enters through stomata and goes to mesophyll cells

H2O: used in light dependent reactions (sometimes called light reactions) enters through veins of leaf and goes to mesophyll cells

chlorophyll: light absorbing green coloured pigment that begins the process of photosynthesis found in chloroplasts primary function: convert light energy into ATP and NADPH in order to be used to convert CO2 to organic molecules

Location of Photosynthesis Chloroplast organelle involved in photosynthesis contains its own: (a) DNA (b) protein making machinery You should be able to: - label a diagram of a chloroplast - explain the structure/function of the chloroplast

Overview of Photosynthetis Photosynthesis consists of two complex series of events; Photo Stage (2 stages) Synthesis Stage

Overview of Photosynthesis Light Dependent Reactions Capturing light energy occur within the thylakoid membrane of the chloroplast produces: (1) ATP (2) NADPH (nicotinamide adenine dinucleotide phosphate, a coenzyme; similar to NADH)

Overview of Photosynthesis Light Independent Reactions OR CARBON FIXATION DURING CALVIN-BENSON CYCLE occur in either the presence or absence of light occur within the stroma of the chloroplast uses ATP and NADPH from the light reactions to form organic molecules like glucose, from CO2 produces: (1) sugars (2) NADP+ (3) ADP

Stages 1 & 2 of Photo Stage Stage 3 Note: Stage 3 used to be called dark reaction, but now we think that the enzymes needed in this reaction, need light

Light

Light Light Electromagnetic (EM) radiation Travels in wave packets called photons Photons with short wavelength = high energy Photons with large wavelength = low energy

Light 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

Light – Photosystems (briefly) 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

Photosynthetic Pigments Chlorophyll found within chloroplasts Green pigments contain: (1) porphyrin ring (hydrophilic) (2) phytol tail (hydrophobic – allows it to embed into thylakoid membrane

**add this to your EM spectrum above  chlorophyll a: absorbs 430 and 660 wavelength photons (reaction centre of a photosystem) chlorophyll b: absorbs 450 and 640 wavelength photons (accessory pigment) **add this to your EM spectrum above 

Photosynthetic Pigments Carotenoids Red and yellow pigments (accessory pigments) Excess energy absorber to protect chlorophyll and dissipate it as heat E.g. β-carotene from carrots

So why are plants green? Chlorophylls a and b absorb blue-violet and red But...they reflect those with wavelengths between about 500 nm and 600 nm, which is.... green!

So why are plants other colours too? Different accessory pigments have different colours (e.g. Xanthophylls are yellow) In the fall, chlorophyll breaks down so other pigments are more visible

Ideal Wavelength of Photosynthesis 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” That’s why you can’t grow plants under regular lightbulbs 

Photosynthesis Question: page 159 # 3-6, page 165 #1, 2

Photosynthesis The Details

Photosynthesis: Light Reactions Light reactions begin when photons strike a photosynthetic membrane. Can be divided into 3 parts Photoexcitation: absorption of a photon by an electron of chlorophyll Electron transport – creates an H+ reservoir Chemiosmosis – movement of protons to help phosphorylate ADP to ATP

Photosynthesis: Photosystems 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.

Photosystem What you were looking at...... Location and structure of chlorophyll molecules in plants

Photosynthesis: Photosystems consists of: Reaction centre (chlorophyll a) Antenna complex (hundreds of chlorophyll molecules and accessory pigments) Location of Photosystems: Thylakoid membrane

Types of photosystems Photosystem I Photosystem II Chlorophyll a molecule is called P700 (red light) Photosystem II Chlorophyll a molecule is called P680 (red light) pigment Wavelength (nm)

Types of Photosystems There are two separate light powered systems in the membranes of the thylakoid disc: Non-cyclic Photophosphorylation Cyclic Photophosphorylation

Non-Cyclic Photophosphorylation Used in photosynthesis of green plants Involves both photosystem I (PS I) and photosystem II (PS II) Chlorophyll a electrons are passed along to make NADPH and are replaced by water’s electrons Produces ATP and NADPH

Non-Cyclic Photophosphorylation

Non-Cyclic Phos. - Description 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.

Non-Cyclic Phos. - Description 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 4 protons from the stroma to the lumen. This creates a proton gradient for chemiosmosis.

Non-Cyclic Phos. - Description The increase in proton concentration within the thylakoid lumen drives protons out of the lumen to the stroma via a special protein called ATP Synthase This proton motive force is used to produce ATP!

Non-Cyclic Phos. - Description 7. The electrons continue to move from PQ to other components of the electron transport chain (cytochrome b6-f complex to Plastocyanin) to PS I eventually replacing the 2 electrons that were lost by PS I when it was struck by photons.

Non-Cyclic Phos. - Description 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 NADP+ to NADPH.

Non-Cyclic Phos. - Description The ATP and NADPH 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)

The Two Photosystems & The 3 Light Dependent Processes Photosystem II Photosystem I Photoexcitation Electron Transport Chemiosmosis

Non-Cyclic….generally… 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 H2O’s electrons

Cyclic Photophosphorylation Used by prokaryotes Photosystem I only (so P700) the chlorophyll a electrons return to chlorophyll a (hence…cyclic) Product = ATP (no NADPH is made)

Cyclic Photophosphorylation

Cyclic Photophosphorylation: Description PS I (P700) absorbs photo energy, causing the two electron to become excited. The two excited electrons enter an electron transport chain

Cyclic Photophosphorylation 3. The two excited electrons are captured by the protein ferradoxin , causing some of their energy to be lost. The two excited electrons are then captured by the cytochrome b6-f protein complex and lose more energy. At this point, enough energy has been lost to add a P to ADP forming ATP.

Cyclic Photophosphorylation 5. The passing of electrons from cytochrome b6-f complex to the next electron carrier, plastocyanin (Pc) results in further lowering of the energy in the electrons to almost the ground state The passing of the two electrons to P700 returns the electrons to both the original molecule and the ground state.

Cyclic Flow...generally... Involves Photosystem I only where the chlorophyll a electrons return to chlorophyll a (no NADPH is made)

Homework  Page 165 # 4-6, 10-12, 15 Read your text up until the end of light independent reactions

Photosynthesis Light Independent Reactions

Light Independent Reactions Calvin Cycle A.k.a. C3 photosynthesis Due to the first compound produced contains three carbons reactions that convert carbon dioxide into sugar molecules occur in the stroma of chloroplasts

Stage One: Carbon Fixation CO2 and RuBP unstable 6 Carbon intermediate which splits into 2, 3 carbon PGA Note that we start with 3 CO2 to produce 6 molecules of PGA Rubisco

Stage Two: Reduction Reactions 3-PGA is phosphorylated by an ATP  molecule 1,3-BPG Note there are really 6 molecules, so we 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)

Stage Three: Regeneration of RuBP Remaining 5 G3P: rearranged  3 RuBP Use 3 ATP Cycle continues G3P molecules that leave are used to synthesize larger sugars (e.g.glucose) Note: 2 Pi is removed...count your phosphates!

Notes: 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)

Homework  Page 167 # 13-18

Do questions: page 166 – 167, # 2 – 9 Animation Websites: http://faculty.nl.edu/jste/noncyclic_photophosphorylation.htm http://www.stolaf.edu/people/giannini/flashanimat/metabolism/photosynthesis.swf http://www.johnkyrk.com/photosynthesis.html