Chapter 10 - Photosynthesis

Photosynthesis

• Series of energy-requiring redox reactions where light energy is converted into chemical energy stored in carbohydrates

• CO2 is reduced to form energy-rich C–C and C–H bonds and H2O is oxidized to produce O2 gas

• Occurs in chloroplasts

  • Light-capturing rxns = thylakoid membrane

  • Calvin cycle = stroma

• Primary energy source for ecosystems and produces atmospheric oxygen required for aerobic respiration

Light Reactions vs. Calvin Cycle

• Light reactions capture energy from sunlight to produce ATP and NADPH by exciting electrons in pigment molecules

  • Occur in the thylakoid membranes

• The Calvin cycle uses ATP and NADPH to reduce CO2 into glyceraldehyde-3-phosphate (G3P), a precursor to glucose

  • Occurs in the stroma of chloroplasts

• Stages are linked: Calvin cycle depends on ATP and NADPH and regenerates ADP, and NADP+, etc. for reuse

• Without light reactions supplying energy and electrons, the Calvin cycle cannot proceed

Chloroplast Structure

• Surrounded by a double membrane that regulates transport of molecules

• Thylakoid membranes contain chlorophyll + other pigments and ETC for light reactions

  • Arranged in stacks (grana) which increases SA available for light absorption

• Stroma contains enzymes (including Rubisco) necessary for the Calvin cycle

• Compartmentalization allows separation of light reactions and carbon fixation for efficiency

Why CO2 Diffusion Can Be Challenging

Trade-off between water conservation and carbon fixation

• The cuticle of plants reduces water loss but also limits gas exchange

• CO2 enters through stomata, which must open for diffusion, but opening the stomata increases risk of water loss

• Closing stomata reduces CO2 availability, limiting photosynthesis

Rubisco

The most abundant enzyme in nature; large but slow with 16 subunits and 8 active sites

• Catalyzes CO2 fixation in the Calvin cycle (RuBP + CO2 → 3PGA)

• It can bind O2 instead of CO2, leading to photorespiration

• Plants produce large amounts of Rubisco to compensate for inefficiency

Photorespiration

• Occurs when Rubisco binds O2 instead of CO2

• Produces 2-phosphoglycolate, which must be recycled using ATP

  • Consumes energy without producing sugars (reduces efficiency)

• Releases CO2, undoing carbon fixation

• More common in hot, dry conditions when stomata are closed

C3 Pathway

• Standard Calvin cycle pathway used by most plants

• CO2 is fixed directly by Rubisco into 3PGA

• Occurs in mesophyll cells

• Efficient in cool, moist environments

• Prone to photorespiration under low CO2 conditions

C4 Pathway

• CO2 is first fixed by PEP carboxylase into a 4-carbon compound in mesophyll cells

• The compound is transported to bundle-sheath cells

  • Less permeable to gases so CO2 [ ]s rise and promote carbon fixation by rubisco to make 3PGA

  • Also reduces photorespiration (so much CO2 around that O2 is less likely to bind to rubisco)

• CO2 is released and enters the Calvin cycle near Rubisco

• Increased efficiency of photosynthesis in conditions where stomata is mainly closed (hot environments; preventing dehydration

CAM Pathway

• CO2 is fixed at night when stomata are open, reducing water loss

• CO2 is stored as organic acids in vacuoles

• During the day, CO2 is released for the Calvin cycle while stomata are closed

• Separates carbon fixation and Calvin cycle in time

• Adaptation for arid environments

Calvin Cycle: Overview

• Occurs in the stroma of chloroplasts

• Reduces CO2 into carbohydrates using ATP and NADPH

• Produces G3P, a 3-carbon sugar used to form glucose

• Requires continuous input of energy from light reactions

• Central process of carbon fixation in plants

Calvin Cycle: Fixation Phase

• CO2 combines with RuBP (5-carbon molecule)

• Forms unstable 6-carbon that immediately splits into two molecules of 3PGA

• Reaction catalyzed by Rubisco and does not require ATP directly

Calvin Cycle: Reduction Phase

• 3PGA is phosphorylated by ATP

  • Consumes energy from the light reactions

• Reduced by NADPH to form G3P

• G3P contains high-energy electrons; used to regenerate RuBP in the next phase

  • Some G3P exits cycle to form sugars

Calvin Cycle: Regeneration Phase

• Remaining G3P is used to regenerate RuBP

• Requires ATP

• Ensures cycle can continue fixing CO2

• Maintains supply of starting molecule

• Completes the cycle

Use of Sugars from Photosynthesis

• G3P is used to synthesize glucose via gluconeogenesis

• Glucose can be converted into sucrose for transport

• Sucrose is used in respiration and growth

• Excess glucose is stored as starch

• Starch is broken down at night for energy supply