Photosynthesis Test (Ch 10)

Photosynthesis Equation

6Co2 + 6H2O + light → C6H12O6 + 6O2 + chemical energy

Light-dependent reactions overview

• Photosynthesis begins with the light-dependent reactions, which take place in the thylakoid membranes of the chloroplast. These reactions require light and water to produce ATP, NADPH, and oxygen.to generate energy for the light-independent reactions.

• Takes in water and splits into oxygen

Calvin Cycle overview

• Occurs in stroma, doesn’t require light directly

• ATP + NADPH + CO2 → sugar

  • ATP broken down to release energy

  • NADPH donates electrons to convert carbon dioxide into sugars

• Uses carbon dioxide to make G3P

Redox reaction

• Acronym: OIL RIG (oxidizing is losing, reducing is gaining)

• H₂O is oxidized → Becomes O₂ (loses electrons).

• CO₂ is reduced → Becomes C₆H₁₂O₆ (gains electrons).

Light dependent reaction pictures

Chemiosmosis

• the driving of ATP production as H+ ions flow down their gradient

  • Isn’t the specific process of production of ATP, but the process by which H+ ions flow

• H+ ions first flow into thylakoid, increasing concentration and decreasing pH —> creates proton gradient that creates potential energy

• H+ ions then flow back into stroma thru ATP synthase

  • Provides energy to synthesize ATP from ADP and inorganic phosphate (Pi)

• Separate from photophosphorylation

When do chloroplasts seem to switch from linear to cyclic electron flow?

when the ratio of NADPH to NADP+ is too high (when too little NADP+ is available to accept electrons)

• Especially since Calvin Cycle requires 18 ATP molecules, vs. the 12 NADPH, to create 2 G3P molecules (enough to make a glucose)

  • The 2:3 ratio of NADPH and ATP produced by linear electron flow may be insufficient

• may also prevent excess light from damaging photosystem proteins and promoting repair of light- induced damage.

Effect of absence of photosystem 1

Rate of biomass produced by photosynthesis will be lower b/c insufficient ATP/NADPH produced for the synthesis of carbohydrates

Chlorophyll purpose

absorbs light energy, receiving electrons from water

Step 1 of Light-Dependent Reactions: Photosystem II

• light energy (photons) excited electrons in PSII, which are transferred to ETC

  • This movement helps generate ATP through chemiosmosis, where protons (H+) flow through ATP synthase into stroma

Step 4 of Light-Dependent Reactions: Photosystem I

• Electrons from PSII re-energized in PSI and passed through another ETC (Fd → NADP+ reductase)

  • No proton gradient → no ATP production

• The enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+ (2 electrons gained for reduction to NADPH)

  • Electrons in NADPH at higher energy level than in water, so more readily available for reactions of Calvin Cycle

• H+ also removed from stroma

Step 2 of Light - Dependent Reactions: Photolysis

• To replace lost electrons, water molecules split

  • Produces electrons to replenish ETC

  • Produces protons to help create gradient for ATP production

  • Produces oxygen (byproduct)

Electrons and Protons purpose

• electrons transfer energy

• Protons help make ATP

Alternate Names for Photosystems

Photosystem II - P680 (pigment is best at absorbing light with a wavelength of 680 nm)

Photosystem I - P700 (pigment is best at absorbing light with a wavelength of 700 nm)

Role of NADPH in photosysnthesis

• carries high-energy electrons to Calvin Cycle for sugar production

Photophosphorylation

• process of ATP production in the light-dependent reactions using light energy

• Via chemiosmosis - process by which protons (H+ ions) flow across a membrane through an enzyme called ATP synthase

Non-cyclic vs. cyclic photophosphorylation

Non-cyclic - produces both ATP and NADPH (involves P1 and P2)

Cyclic - produces only ATP (only P1)

Enzyme responsible for carbon fixation in Calvin cycle

RuBisCo

Why do C4 and CAM plants have adaptations for photosynthesis?

• to reduce photorespiration and survive in hot, dry environments where stomates are often closes

What type of plants use the C4 pathway?

Grasses and plants with a specialized leaf anatomy (bundle sheath cells)

What type of plants use the CAM pathway?

Cacti, succulents, and pineapples

Step 2 of Light-Dependent Reactions:

Step 4 of Light-dependent reactions: H+ Gradient Formation

• the protons (H+ ions) built up inside thylakoid space after coming in from stroma

• Protons diffuse out through ATP synthase, leading to ATP production

  • Chemiosmosis

Plant anatomy

Thylakoids - membrane-bound compartments where light reactions occur

Grana - stacks of thylakoids

Stroma - fluid surrounding thylakoids (where Calvin Cycle occurs)

Mesophyll Cells - in plant leaf tissue, contain chloroplasts

First Step of Calvin Cycle: Carbon Fixation

• Technically occurs thrice to produce one G3P molecule from one CO2

• enzyme RuBisCo attaches 3 CO2 to 3 RuBP (a 5-carbon molecules), forming a short-lived, unstable 6-carbon compound (6 carbons with a phosphate on each end)

• Quickly splits into 6 3-carbon molecules (3-PGA) (3 carbons with a phosphate on one end)

Steps of Calvin Cycle

1. Carbon Fixation

2. Reduction Phase

3. Regeneration of RuBP

Carbon fixation definition

“ the initial incorporation of CO2 into organic material”

Step 2 of Calvin Cycle: Reduction Phase

• 6 ATP turns into 6 ADP, contributing a phosphate to form 6 Bi-PGA (now three carbons with 1 phosphate on each side)

• 6 NADPH contributed, leaving as 6 NADP+ and taking added phosphate with it

  • Removal converts 6 3-PGA into 6 G3P (a 3-carbon sugar)

• 1 G3P leaves cycle to become glucose and other carbohydrates

Step 3 of the Calvin Cycle: Regeneration of RuBP

• 5 G3P molecules stayed in cycle to be recycled

• Using 3 ATP, 5 G3P molecules turned into 3 RuBP molecules

  • 15 carbon total

  • ATP leaves as ADP

Photo of Calvin Cycle

Photorespiration

• process that occurs in plants when RuBisCo reacts w oxygen instead of carbon dioxide

  • Tends to occur in an oxygen-rich, carbon-dioxide-low environment

  • Leads to production of 2-carbon molecule (G2P) instead of normal 3-carbon molecule

• Inefficient because it doesn’t contribute to production of sugars and consumes energy

  • Reduces plant’s overall efficiency

  • Plant may draw on stored carbohydrates, reducing plant’s energy reserves, growth, and survival

First Step of C4 Pathway

• In mesophyll cells

• enzyme PEP carboxylase adds CO2 to PEP, forming Oxaloacetate, then Malate

  • Has much higher affinity for CO2 than rubisco and no affinity for O2

  • PEP carboxylase can fix carbon efficiently when rubisco can’t

2nd Step of C4 Pathway

• the four- carbon products moves into a bundle-sheath cell via plasmodesmata

Third Step of C4 Pathway

• The 4-carbon compounds CO2 is released

• CO2 re-fixed into organic material by rubisco and the Calvin Cycle

  • Same reaction regenerates pyruvate, which is transported to mesophyll cells

  • ATP used to convert pyruvate to PEP, which can accept addition of another CO2

Overview of C4 Pathway

• mesophyll cells pump CO2 into bundle-sheath cells, keeping CO2 concentration high enough for rubisco to bind CO2, not O2

  • O2 kept away from RuBisCo

• spends ATP energy to fix carbon dioxide, minimize photorespiration, and enhance sugar production

Overview of CAM

• crassulacean acid metabolism

• Mesophyll cells store organic acids they make during night in vaculoes until morning, when stomata close

• during day, when light reactions can supply ATP and NADPh, CO2 released from organic acids made night before to become incorporated into sugar

• Rather than structural/physical separation in C4 plants, CAM plants have came 2 steps but different times.

First Step of CAM Pathway

• In mesophyll cells

• enzyme PEP carboxylase adds CO2 to PEP, forming Oxaloacetate, then Malate

  • Has much higher affinity for CO2 than rubisco and no affinity for O2

  • PEP carboxylase can fix carbon efficiently when rubisco can’t

• Occurs during night, organic acids stored in vacuoles (stomates open during night)

Second Step of CAM Pathway

• Daytime (stomates closed)

• the four- carbon products moves out of vacuoles

Third Step of CAM Pathway

• The 4-carbon compounds’ CO2 is released

• CO2 re-fixed into organic material by rubisco and the Calvin Cycle

  • Same reaction regenerates pyruvate, which is transported to mesophyll cells

  • ATP used to convert pyruvate to PEP, which can accept addition of another CO2

C4 Leaf Anatomy

Bullseye

• inner tube of vein (vascular tissue)

• surrounding layer of bundle sheath cell (photosynthetic cells)

• surrounding layer of mesophyll cell (photosynthetic cells)

• stoma on top and bottom

Which photon color would carry the most energy?

Blue (the smaller the wavelength/closer to purple → the more energy)

What is the range of wavelengths pigments capture?

blue-violet and red-orange