Photosynthesis Notes

Photosynthesis

6.2.1 Photosynthesis Equation

• Photosynthesis is the process by which plants make their own food using sunlight.
• Equation: 6CO2 + 6H2O + Sunlight \rightarrow C6H{12}O6 + 6O2
• 6 Carbon dioxide + 6 Water (from roots) + Sunlight → Glucose (Sugar) + Oxygen

6.2.2 Chloroplast: Structure & Function

• Chloroplast: The green part in plant cells where photosynthesis occurs.
  • Thylakoid: Flat green disk where the light reaction happens.
  • Stroma: Liquid area where the Calvin cycle happens.
• Function: The parts of the chloroplast help catch sunlight and turn it into sugar.

6.2.3 Pigments & Light Absorption

• Pigments: Special colors that catch sunlight.
• The main pigment is chlorophyll (green).
• Different pigments catch different colors of light.
• Chlorophyll catches blue and red light best, not green. That's why leaves look green - they reflect green light.

6.2.4 Light Dependent vs. Light Independent (Calvin Cycle)

6.2.5 Main Events in Light Reaction

• In the thylakoid:
  • Light splits water → gives electrons, protons, and oxygen.
  • Electrons go through the Electron Transport Chain (ETC).
  • Electrons help make:
    • ATP (energy battery)
    • NADPH (energy taxi)

6.2.6 Electron Transport Chain

• Electrons move step by step in the thylakoid membrane.
• Their energy helps pump protons (H^+) inside.
• This builds energy for ATP later.

6.2.7 Chemiosmosis

• Chemiosmosis is how plants make ATP.
• Protons (H^+) build up inside the thylakoid.
• They go through an enzyme called ATP Synthase.
• That spins and makes ATP from ADP + Pi (like charging a battery).

6.2.8 Main Events of Calvin Cycle

• In the stroma:
  1. Takes CO_2 from the air
  2. Uses ATP + NADPH (from light reaction)
  3. Makes glucose (sugar)
  4. Sends ADP + NADP+ back to the light reaction

6.2.9 Environmental Effects on Photosynthesis Rate

• Summary: Photosynthesis is like a two-part machine:
  1. Light reaction - collects energy
  2. Calvin Cycle - builds sugar
• Three factors affect the rate of photosynthesis: light intensity, CO_2 concentration, and temperature.

Overview of Photosynthesis

• Most autotrophs make organic compounds using photosynthesis.
• Photosynthesis converts light energy into chemical energy.
• The products and reactants of photosynthesis are identified in the overall chemical reaction for photosynthesis: 6CO2 + 6H2O + Light \rightarrow C6H{12}O6 + 6O2
• Photosynthesis is a complex series of reactions summarized as: 6 CO2 + 12 H2O + Light energy \rightarrow C6H{12}O6 + 6 O2 + 6 H_2O
• The overall chemical change during photosynthesis is the reverse of cellular respiration

Photosynthesis Phases

• Photosynthesis occurs in two phases:
  • Light-Dependent Phase: Light energy is captured and converted into chemical energy.
  • Light-Independent Phase (Calvin Cycle): Molecules formed during the light-dependent phase are used to make glucose.

Light Reactions (Thylakoids)

• Split H_2O, providing electrons and protons (H^+)
• Release O_2 as a by-product
• Reduce the electron acceptor NADP+ to NADPH
• Generate ATP from ADP by photophosphorylation

Calvin Cycle (Stroma)

• Makes sugar from CO_2, using the ATP and NADPH generated during the light reactions
• Begins with carbon fixation, incorporating CO_2 into organic molecules
• Reduces fixed carbon to carbohydrate by transferring electrons from NADPH

Phase One: Light Reactions - Absorption of Light

• The absorption of light is the first step in photosynthesis.
• Once light energy is captured, two energy storage molecules, ATP and NADPH, are produced.

Chloroplasts, Light Reactions, and Pigments

• Plants use light energy, carbon dioxide (CO2) and water (H2O) to grow.
• Light energy is absorbed in the chloroplasts and converted into chemical energy during photosynthesis.
• All plants need three basic reactants to grow: photons (light energy), carbon dioxide (CO2), and water (H2O).

Chloroplasts

• Chloroplasts are organelles that capture light energy.
• They contain two main compartments:
  • Thylakoids: flattened saclike membranes
  • Grana: stacks of thylakoids
  • Stroma: fluid-filled space outside the grana
• A chloroplast has an envelope of two membranes surrounding a dense fluid called the stroma.
• Thylakoids are connected sacs in the chloroplast that compose a third membrane system.
• Thylakoids may be stacked in columns called grana.
• Chlorophyll, the pigment that gives leaves their green color, resides in the thylakoid membranes.
• Analogy: If a stack of pancakes is a granum, each pancake would be a thylakoid.
• Label the Chloroplast:
  1. Inner membrane
  2. Intermembrane space
  3. Outer membrane
  4. Stroma
  5. Thylakoid
  6. Lamella

Pigments

• Light-absorbing colored molecules called pigments are found in the thylakoid membranes.
• Different pigments absorb specific wavelengths of light.
• The absorption spectrum of chlorophyll a indicates that violet-blue and red light will work best for photosynthesis, while green is the least effective.
• Three types of pigments in chloroplasts include:
  • Chlorophyll a: The key light-capturing pigment that participates directly in light reactions.
  • Chlorophyll b: An accessory pigment that helps capture additional light and passes the energy to chlorophyll a.
  • Carotenoids: A separate group of accessory pigments.

Electron Transport

• The thylakoid membrane has a large surface area, providing space for a large number of electron transporting molecules and two types of protein complexes called photosystems.
• Photosystems contain the light-capturing pigments and proteins that play important roles in the light reactions.
• Light energy excites electrons in photosystem II and causes a water molecule to split.
• Next, the excited electrons move from photosystem II to an electron acceptor molecule.
• Next, the electron-acceptor molecule transfers the electrons along a series of electron-carriers to photosystem I.
• Photosystem I transfers the electrons to a protein called ferredoxin.
• Finally, ferredoxin transfers the electrons to the electron carrier NADP+, forming the energy-storage molecule NADPH.

Chemiosmosis

• Chemiosmosis is the mechanism by which ATP is produced as a result of the flow of electrons down a concentration gradient.
• The breakdown of water is essential to this process.

The Calvin Cycle

• In the second phase of photosynthesis, called the Calvin cycle, energy is stored in organic molecules such as glucose.
• The first step of the Calvin cycle is called carbon fixation. CO_2 molecules combine with 5-carbon molecules to form 3-phosphoglycerate (3-PGA).
• In the second step, chemical energy stored in ATP and NADPH is transferred to the 3-PGA to form glyceraldehyde 3-phosphate (G3P).
• In the third step, some G3P molecules leave the cycle to be used for the production of glucose and other organic compounds.
• In the fourth and final step, an enzyme called rubisco converts the remaining G3P molecules into 5-carbon molecules called ribulose 1,5- bisphosphates (RuBP). These molecules combine with new CO_2 and continue the cycle.

C4 Plants

• C4 plants minimize the cost of photorespiration by incorporating CO_2 into a four-carbon compound as the first product of the Calvin cycle.
• C4 has evolved several times and is used by several thousand species in at least 19 different families; important agricultural examples include sugarcane and corn.

CAM Plants

• Some plants, including succulents, conserve water by using crassulacean acid metabolism (CAM) to fix carbon.
• CAM plants open their stomata at night and incorporate CO_2 into organic acids that are stored in the vacuoles.
• Stomata close during the day, and CO_2 is released from organic acids and used in the Calvin cycle.

Quiz Questions

1. What is a waste product of photosynthesis that is released into the environment?
   • Oxygen
2. Of which wavelength of light do carotenoids absorb the greatest percentage?
   • 400
3. Which is the internal membrane of the chloroplast that is organized into flattened membranous sacs?
   • Thylakoids
4. Which supplies energy used to synthesize carbohydrates during the Calvin cycle?
   • ATP and NADPH
5. Which best describes the role of rubisco?
   • It converts inorganic carbon dioxide into organic molecules that can be used by the cell.