SECTION 8.2

Photosythesis

  • The process by which the energy of the sun is used to convert H2O and CO2 into high energy sugars(glucose).
  • Importance-we need glucose and other carbs for energy but we cannot produce them ourselves.We also need oxygen and this is produced during photosynthesis by plants and algae.


Light Energy
  • Light is a form of energy that travels in the form of particles or waves.The waves are measured in wavelengths, which vary in length.Humans can see violet through red wavelengths.
  • Color- reflected light, we see color.
  • Pigment- a molecule that absorbs light.
  • Whatever the colors the pigment does not absorb are what we see.

Pigments
  • Chlorophyll A is a pigment that absorbs violet,red, and blue wavelengths of light.
  • Green is reflected by chlorophyll A. That's why most leaves appear green to us.
  • Other pigments called accessory pigments(chlorophyll b, xanophyll, and carotene) help chlorophyll A absorb a greater spectrum of light.

Chloroplasts
  • Photosynthesis takes place in chloroplast.
  • Chloroplast: Double membraned, disk-shaped organelle. Located inside this disk are stacks of sacks.
  • Each stack is called a thylakoid.
  • Inside each thylakoid is chlorophyll and other pigments.
  • Stacks of sacks are called grana.
  • Surrounding the grana is a fluid called stroma.
Balanced Photosynthesis Equation
  • 6CO2+6H2Olight> C6H12O6+6O2
  • Photosynthesis occurs in two phases.
-Light-Dependent reactions.
-Light-Independent reactions.
Phase One: Light Reaction
  • The absorbtion of light is the first step in photosynthesis.
  • Chloroplast capture light energy.
  • Goal: To convert light energy into chemical bond energy (ATP,NADPH).
  • What Goes In: H2O, light, pigments (chlophyll), ADP, P, NADP+.
  • What Comes Out: O2, ATP, NADPH.
  • Location: In the Thylakoids.
Electron Transport
  • Light energy excites electrons in Photo-system 2 and also causes a water molecule to split, releasing an electron into the electron transport system, H+ in to the Thylakoid space, and O2 as a waste product.
  • The excited electrons move from Photo-system 2 to an electron-acceptor molecule in the thylakoid membrane.
  • The electron acceptor molecule transfers the electrons along a series of electron-carriers to Photo-system 1.
  • As electrons are passed down Photo-system 2, energy is released to make ATP.
  • Light then strikes Photo-system 1 and electrons are captured by a 2nd electron acceptor.
  • The electron acceptor of Photo-system 1 sends the electrons down Its electron transport chain.
  • As the electrons get passed down the electron transport chain, their energy is used to "charge" an NADP+ into NADPH (adding two H+).
Phase Two: The Calvin Cycle
  • In the secound phase of photosynthesis, called the Calvin Cycle, energy is stored in glucose.
  • "Light Independent reactions"
  • Goal: Uses the ATP and NADPH from the light reaction to convert CO2 into carbohydrate (gluecose).
  • What Goes In: ATP, NADPH, CO2, RuBP (a suger).
  • What Comes Out:
-Glucose (stored by the plant)
-ADP, P, NADP+:(shuttled back to the light reactions to get "charged up" agin).
  • Step 1:Six CO2 molecules combine with six RuBP's to form twelve 3-carbon molecules called 3-PGA.
  • Step 2: ATP provides energy and NADPH provides H+ to transform the 3-PGA molecules called G3P. One molecule of G3P is essentially a (half-glucose). Two G3P's (half-glucose) leave the cycle to become glucose.
  • Step 3: An enzyme called rubisco converts the remaining ten G3P molecules back into 6 molecules of RuBP.
  • Step 4:The 6 RuBP's combine with 6 new CO2's to continue the cycle.
  • Factors Influencing The Rate of Photosynthesis:
  • 1.Availability of water.
  • 2.Intensity of sun light.
  • 3.Temperature.
Alternative Pathways- the H2O/CO2 Problem
C4 Plants
  • Adaptation to conserve water in hot regions.
  • Instead of 3-CArbon stages (3-PGA and G3P), these plants fix CO2 into a 4-carbon molecule
  • Able to keep stomata closed on hot days and Calvin cylce occurs only in special cells.
  • Used by sugar cane and corn
CAM Plants
  • Conserves water in dry regions.
  • Only open stomata for CO2 at night.
  • Holds the CO2 until daytime. Calvin cycle proceeds while stomata are closed.
  • Used by pineapple and cacti.
Bell Curve - optimum zone, tolerance zone