Photosynthesis Quiz Review

Photosynthesis

What is photosynthesis?

• The process that converts light energy to the chemical energy of food

• Occurs in the chloroplast of plant cells

• Why it occurs: to convert light energy to chemical energy stored in glucose

Chloroplast (Found in leaves)

• Contains an inner and outer membrane

• Stroma - dense fluid

• Thylakoids - interconnected membrane sacs that separates the stroma from the interior thylakoids

  • Stacked thylakoids are called grana

  • Contains chlorophyll

Chlorophyll - Green pigment found in chloroplasts

• Made of a parphyrin ring (A light-absorbing head) and a hydrocarbon tail that anchors the chlorophyll

Mechanisms of Photosynthesis

• Formula - CO₂ + H₂O + Light Energy = O₂ + C₆H₁₂O₆

  • O₂ is derived from H₂O

  • Chloroplasts splits H₂O into Hydrogen and Oxygen

Light Reactions

• The convertion of solar energy to chemical energy in the form of ATP and NADPH

Wavelength = 380 (Purple) to 750 (red) nm

• Light acts as a particle called photons that contain a fixed amoung of energy

• Shorter wavelength = more energy

Pigments - substances that absorb visible light

• The color we see is the colour that is NOT absorbed by the pigment, but its absorbed

• In chloroplasts, chlorophyll absorbs violet-blue and red, and reflects green

• When molecule absorbs a photon, electrons get excited and jump from its ground state (low energy state) to an excited state (high energy state). They jump to a higher orbit level

  • Electron goes back down by losing energy, becoming released as heat

Absorption Spectrum - A graph plotting light absorption vs wavelength

• In Chlorophyll a, it absorbs best in red (662 nm) and blue (430 nm). It is NOT best in green

Photosystems - Contains a protein complex (Reaction center complex) surrounded by a light-harvesting complexes (Antenna Complex)

• Reaction Center Complex - Special pair of chlorophyll a molecules and a molecule that accepts electrons to be reduced by the primary electron acceptor

• Antenna complex - Has pigment molecules that boud to proteins

Photosystem II: Clorophyll a - P680 (Absorbed at 680 nm)

Photosystem I: Chlorophyll a - P700 (Absorbed at 700 nm)

• Electrons flow from water to PSII, then to PSI to NADP⁺ (Comes from the splitting of water)

Non-cyclic electron flow (Occurs in the stroma)

1. PSII absorbs a photon of light that strikes a pigment molecule that boosts the electrons. Electrons reach P680 which makes electrons excited, later transferred to the primary electron acceptor.

   1. 2 P680 - 2 P680⁺

2. A Z protein splits water, releasing oxygen

3. The photo-excited electrons passes from the primary electron acceptor to PSI via Electron Transport Chain (Plastoquinone, cytochrom complex, plastocyanin)

4. Electrons fall to a lower energy level for the synthesis of ATP

   1. Noncylic phosphorylation - process of ATP synthesis

5. Photon strikes a pigment molecule of PSI, boosting electron to a higher energy level by reaching P700

   1. 2 P700 - 2 P700⁺ (Now an oxidizing agent)

6. Primary electron acceptor passes electron down a second ETC (Ferredoxin) that does NOT have a proton gradient (No ATP is made). NADPH is made from NADP⁺ Reductase

   1. NADP⁺ + 2H⁺ = NADPH + H⁺

Cyclic Electron Flow

• Electron transport from PSI to Ferredoxin is NOT allowed by the electron donation to NADP⁺ Reductase

• The reduced Ferredoxin donates the electron back to Plastoquinone (Continually reduced and oxidized, moves protons accross the membrane)

• PSI works independently

Net Result: Energy from light is converted to ATP without the oxidation of H₂O or the reduction of NADP⁺ to NADPH

• This helps meet the demand for ATP

• Occurs when a plant has enought NADPH but still needs ATP

Clavin Cycle

• Uses ATP and NADPH to convert CO₂ to sugar (G3P)

• Anabolic process

• ATP is used as an energy source, NADPH is used as a reducing agent

• One glucose molecule = six cycles

• Three cycles = 1 G3P

3 Phases of Clavin cycle

1. Carbon Fixation - CO₂ attaches to Rubilose Bisphosphate (RuBP 5C), becoming catalyzed by rubisco. The product made gets splits in half to form two molecules of 3-Phosphoglycerate

2. Reduction - 3-Phosphoglycerate receives a phosphate group from ATP making 1, 3-Bisphosphoglycerate. This gets reduced by NADPH to form Glyceraldehyde-3-phosphate (G3P). Reduction results in the loss of a phosphate group

   1. 3 CO₂ = 1 G3P

Why only one G3P is made per CO₂

• Six molecules of G3P are made (18C) after fixation and reduction

• Five are used for the regeneration of three RuBP

3. Regeneration - Carbon skeletons of 5 G3P are rearranged into 3 RuBP. Three more molecules of ATP are catabolized to also regenerate RuBP

Adaptations to Photosynthesis

Photorespiration - when rubisco catalyzes O₂ instead of CO₂ into RuBP (Slows down Clavin cycle, uses ATP, releases O₂)

C₄ Plants (Four carbon plant/Malate)

• Light reactions and Clavin cycle are physically separated, making O₂ unexposed

• Uses a C₄ cycle that feeds CO₂ to rubisco

CAM plants (Crassulacean acid plants)

• Also has a C₄ Cycle but biochemichal pathways aren’t separated

• Opens stomata at night, allowing CO₂ to enter & minimize H₂O loss

• CO₂ is converted to make malate, which gets stored until daytime