Download Chapter 10: Photosynthesis

Chapter 10: Photosynthesis
Photosynthesis in nature
Plants and other autotrophs are the producers of the biosphere
Two modes of nutrition
Autotrophs (plants are photoautotrophs, some bacteria are
chemoautotrophs)
Heterotrophs (includes decomposers)
Chloroplasts are the sites of photosynthesis in plants
Cells of the leaf mesophyll are where most chloroplasts are (ca 30-40/cell)
Gases enter and exit the stomata
Water enters from roots via xylem in veins
Sugars leave via phloem in veins
Review membrane structures of the chloroplast (page 178)
The pathways of photosynthesis
Evidence that chloroplasts split water enabled researchers to track atoms through
photosynthesis
Splitting water
Van Niels hypothesis from work with photosynthetic bacteria
(H2S)
Hypothesis supported with use of radioactive O (18O)
Oxygen output in photosynthesis was labeled only if it came from
labeled water, not labeled CO2
Most important is the extraction of water and its use in making
sugar
Photosynthesis is a redox process
Different than respiration
In respiration electron lose potential energy as they are pulled
down to oxygen
In photosynthesis increase in potential energy as they move form
water to sugar
Overview of photosynthesis
Light reaction
Transfer of electrons and H to NADP+ (makes NADPH)
Photophosphorylation makes ATP
Calvin cycle
“Carbon fixation”
Needs energy from ATP
Reduces CO2 to sugar with by adding electrons from
NADPH
The light reactions convert solar energy to the chemical energy of ATP and
NADPH
The nature of sunlight
Visible light drives photosynthesis
Photosynthetic pigments: the light receptors
Action spectrum (page 183) shows blue and red regions are most
effective in photosynthesis
Chlorophyll a is responsible for the light reaction
Chlorophyll b can capture light energy and store it for chlorophyll
a
Carotenoids function in photo-protection of chlorophyll
Excitation of chlorophyll by light
A photon of light is adsorbed by chlorophyll and an electron is
elevated to the next orbital
This is unstable but changes the potential energy of that electron
Photosystems: light harvesting complexes in the thylakoid membrane
A photosystem is a complex of pigment molecules designed to
collect light over wide areas
Chlorophyll in the reaction center passes an energized electron to a
primary electron acceptor
Primary electron acceptor in photosystem I is a chlorophyll called
P700
Primary electron acceptor in photosystem II is a chlorophyll called
P680
They are identical but have different absorption spectra
Noncyclic electron flow
This is the major pathway, uses both photosystems
Use figure 10.12 and follow the steps 1-6 (page 186)
Input: light, water, ADP, NADP
Output: O2, ATP and NADPH
ATP and NADPH in about same amounts
Cyclic electron flow
Uses only photosystem I
Makes only ATP (no O2 or NADPH)
Calvin cycle uses more ATP than NADPH so this is a way to make
up the extra ATP
Use figure 10.14 (page 187)
A comparison of Chemiosmosis in chloroplasts and mitochondria
The Calvin Cycle uses ATP and NADPH to convert CO2 to sugar
Similar to Krebs (starting material is regenerated)
Carbon enters as CO2, leaves as sugar
Glucose is not actually produced, G3P is
Three phases:
Carbon fixation
Enzyme (rubisco) adds C to a 5 carbon sugar (RuBP)
6 C intermediate immediately splits in half
Reduction
One molecule of G3P exits th cycle
Consumes ATP and NADPH
Regeneration of RuBP
Produces the starting material
Consumes ATP
Alternative mechanisms of carbon fixation have evolved in hot arid climates
C3 photorespiration
C4 plants
CAM plants
Photosynthesis is the metabolic foundation of the biosphere