The Green World

1

• 8
• The Green Worlds Gift
• Photosynthesis

2

• Photosynthesis and Energy (Section 8.1)
• 1. Try to name something you eat that isnt from
  a plant or from an animal that ate a plant.
• 2. All food comes from plants. Molecules of our
  bodies are made from food we eat, but plants make
  their own food from sunlight. Food is used for
• a) Creating macromolecules from monomers like
  glucose and amino acids.

3

• b) More importantly, food is used in respiration
  to generate cellular energy, ATP. Plants are the
  nearly universal source of energy for all living
  things.
• c) Plants take energy-poor reactants (water and
  carbon dioxide) and use solar energy to drive the
  uphill reaction of trapping those reactants in
  complex, ordered bonds of glucose.

4

• 3. Oxygen needed for respiration is produced as a
  by-product of photosynthesis.

5

• A. Nature of Light
• 1. Energetic rays have different wavelengths in a
  spectrum from gamma rays to radio waves, only a
  portion of which is visible light
• 2. Explain what it means to see a plant as red or
  green in terms of absorption and reflection, and
  why a black car is hotter on a sunny day than a
  white car. (Black absorbs all light and reflects
  none, white absorbs little and reflects almost
  all.) Explain what it means to absorb light by a
  pigment.

6

• 3. Photosynthesis driven by only part of the
  visible spectrum (blue and red) plant pigments
  in the green plant reflects green and absorbs
  blue and red.

7

• Tour of a leaf, where plants absorb light Figure
  8.3
• 1. Blade
• Leaf section, epidermis,
• stomata, mesophyll
• Chloroplasts, inner
• and outer membranes
• 4. Grana and stroma
• Thylakoid membrane,
• and compartment
• 6. Pigments

8
Stomates
9
Leaf cross section
10

• C. Photosynthesis occurs in two essential phases.
• 1. Light-dependentphoto of photosynthesis.
• a) Power of sunlight excites electrons in
  pigment molecules.
• b) Excited electrons are carried down transport
  chain of redox reactions like those in
  mitochondria.

11

• C. Photosynthesis occurs in two essential phases.
• 1. Light-dependentphoto of photosynthesis.
• c) Energy is used to make a gradient of H ions
  to drive synthesis of ATP, and electrons may be
  transferred by a carrier molecule like NAD,
  NADP.
• d) Pigment electrons are replaced by electrons
  stripped from water, making O2 gas.

2. Light-independent a) ATP and NADPH are not
good permanent storage molecules, so the plants
covert energy into several bonds in a glucose
molecule. b) Electrons from carriers are brought
together with CO2 and H2O to make this glucose.
12

• D. Photosystems are the working units that absorb
  solar energy.
• 1. Aggregates of hundreds of pigment molecules
  serve as antenna to absorb solar energy.
• 2. Reaction center of aggregate contains pair of
  chlorophyll molecules with electrons that absorb
  the energy and jump to electron carrier
  molecules Figure 8.4

13
(No Transcript)
14

• E. Energy transfer is possible using redox
  reactions.
• 1. One substance loses electrons (oxidized) while
  another gains electrons (reduced).
• 2. Electrons move down the energy hill, losing
  energy as they go (analogy of the passing of a
  hot potato warming each hand as it drops, giving
  off some heat as it goes. The last person to get
  the potato gets some heat and food as well.) The
  final recipient of the electron in this case is
  NADP Figure 8.5

15

• A. Follow the pathway
• 1. Photosystem II absorbs solar energy.
• 2. Electron jumps to the primary electron
  acceptor.
• 3. Chlorophyll is left without an electron,
  making it an oxidizing agent that grabs an
  electron from water, splitting it into H ions
  and O2.
• 4. Ejected electron falls back down the energy
  hill through a series of electron transfer
  molecules and a series of redox reactions until
  it reaches photosystem I (another reaction center
  also receiving solar energy).

16
(No Transcript)
17

• Light-Dependent Reactions
• A. Follow the pathway Figure 8.5 animated in the
  resources for this chapter as figure 8_05.
• 5. Again, energized electrons from photosystem I
  are transferred back down the energy hill, until
  they are received by NADP, an electron carrier
  that ferries electrons to the second stage, the
  light-independent stage of photosynthesis.
• 6. Travel took place from thylakoid to stroma
  Figure 8.6

18

• B. Importance of the light-dependent phase
• 1. Oxygen formation
• 2. Energized electrons being transferred, not
  just giving off heat of fluorescing, and ferried
  in NADPH.
• 3. Formation of ATP, which is used to power the
  second stage, the light-independent reactions.

19

• A. The Calvin (C3) Cycle the synthesis of
  photosynthesis, making food, trapping CO2 Figure
  8.7
• 1. Enzyme called rubisco brings together CO2 and
  sugar, carbon fixationthree low-energy molecules
  of CO2 from the atmosphere are combined with
  three five-carbon sugars (RuBP).
• 2. Six-carbon product is unstable and splits into
  two three-carbon products (3-PGA).

20

• 3. ATP places a phosphate group on each 3-PGA,
  and NADPH donates a pair of electrons yielding a
  high energy food, G3P.
• 4. Only one G3P exits the cycle the other five
  are used to regenerate the starting material RuBP.

21

• A. Glitch in the systemPhotorespiration
• 1. Rusbisco often combines O2 instead of CO2 with
  RuBP, unproductively.
• 2. Occurs one O2 for every three CO2.
• 3. Undercuts food production in crops that use C3
  cycle.
• 4. Especially problematic in hot weather because
  of evaporation of water. Plant closes stomata in
  leaves to prevent evaporation, but as water is
  kept in, CO2 is kept out. As the light-dependent
  reactions continue, O2 builds up, combining with
  RuBP unproductively.

22

• 1. Grasses, corn, sugarcane, and sorghum
• 2. Use a different enzyme located in
  bundle-sheath cells Figure 8.10
• 3. Costs ATP to shuttle CO2 to bundle-sheath
  cells in sunny climates this is not an issue,
  because with abundant sunlight, ATP is plentiful.
  
• 4. In northern climates, C4 plants are not as
  well adapted.

23

• CAM Plantsanother adaptation that saves water in
  hot climates (Section 8.8)
• A. Cactus, pineapple, mint, and orchid
• B. Close stomata during the day, open at night
• C. Start C4 metabolism at night by fixing CO2 but
  wait for day to use abundant ATP to finish.
• D. Comparison of three strategies Figure 8.12

24
(No Transcript)
25
Summary of Photosynthesis in most kinds of plants
26
The End