Unit 4 Notes
Photosynthesis
Photosynthesis
Converts light energy into food energy
6CO2 + 12H2O
light, enzymes, chlorophyll
C6H12O6 + 6O2 + 6H2O
Photosynthesis
 Two types of autotrophic organisms
 Photosynthetic—make food from sun’s energy
 Chemosynthetic—make food from other
inorganic molecules
 2 Steps to Photosynthesis
 Light dependent reactions—need light and
chlorophyll to make ATP and NADPH
 Light independent reactions—take ATP and
NADPH + CO2 to make glucose (or other
sugars)
Plant Structure
Leaf Structure
Plant Structure
 Leaf Parts
 Cuticle—waxy—provides protection from
insects, environmental damage, etc., and keeps
water from getting out
 Epidermis—defense/barrier—reduces water loss
 Palisade Mesophyll—structure (lots of
chloroplasts)
 Spongy Mesophyll—air flow (lots of
chloroplasts)
 Xylem—vascular tissue that moves water
 Phloem—vascular tissue that moves sugars
 Stomata—openings on bottom of leaf—lets CO2
in and O2 and H2O out
Plant Structure
 Leaf Parts (continued)
 Guard Cells—cells around the opening of the
stomata—open when water flows in by osmosis
 Open
 Triggers = Light / Decrease CO2 / Internal
Clock
 K+ enters cell  water potential decreases 
osmosis in  cells become turgid  stomata
opens
 Close
 Triggers = No Light / Increased Temperature /
Lose H2O / Increase CO2
 K+ leaves  osmosis out  cells become
flaccid  stomata close
Plant Structure
Plant Structure
 Chloroplasts
 Site of Photosynthesis
 The membranes (thylakoids) within the
chloroplast give it the ability to build up
concentration gradients for “whooshing”—
chemiosmosis
Light Dependent Reactions
Light energy (photons) is absorbed by pigments in
the thylakoid membranes
Pigment
Absorbs
Reflects
Chlorophyll a
Red/Blue
Green
Chlorophyll b
Red/Blue
Green
Carotenoids
Blue/Green
Orange/Yellow
Light Dependent Reactions
 Photosystems—Light Harvesting Units
 Photosystem I—has chlorophyll p700 (longer
wavelength)
 Photosystem II—has chlorophyll p680 (shorter
wavelength)
 Pigments absorb light energy and transfer the
energy to other pigments until it is absorbed by
a photosystem
 The photosystem gathers enough energy to
raise 1e- to a higher energy level (can be used
for ETS!)
Light Dependent Reactions
 Cyclic Photophosphorylation—Electron transfer with
PS I
 Electrons from PSI go through an ETS and
create ATP--electrons then goes back to PSI
 Electrons are cycling!
 Not enough energy for most plants
Light Dependent Reactions
 Non-cyclic Photophosphorylation—Electron transfer
with PS I & PS II
 PS II gathers light energy which raises electron to
ETS—ATP is made by chemiosmosis
 Electron goes to PS I and then is raised to a 2nd
ETS—NADP is reduced to NADPH
 Electrons in PS II are replaced by photolysis of
H2O
Light Dependent Reactions
Light Independent Reactions
 Reactions occur on the stromal surface of the
thylakoid
 Need:
Plant Makes
 Enzymes
 Ribulose Biphosphate (RuBP)—a 5-Carbon sugar
 CO2 (From Air)
 NADPH
 ATP
From Non-Cyclic
Photophosphorylation
Light Independent Reactions—
Calvin Cycle
 Carbon Dioxide Fixation:
 6CO2 + 6RuBP  6-6C molecules (unstable); so
they split into 12-3C molecules called PGA
(phosphoglyceric acid)
 Reduction
 Each PGA is phosphorylated by ATP and reduced
by NADPH into 12 molecules of PGAL
(phosphoglyceraldehyde)
 Regeneration of RuBP
 10 of 12 PGAL remake RuBP—need ATP to do it!
 2 of 12 PGAL make 6C sugar (glucose)
Light Independent Reactions—
Calvin Cycle
Light Independent Reactions—
Calvin Cycle
Cellular Respiration & Photosynthesis
Category
Purpose
Cellular Respiration
Photosynthesis
Convert stored
chemical energy
(glucose) to usable
energy (ATP
Convert solar energy to
stored chemical energy
(glucose)
ALL CELLS
Photosynthetic
Autotrophs
Where
Cytoplasm &
Mitochondria
Chloroplast
Steps
1.
2.
3.
4.
1. Light Dependent
2. Light Independent
Who
Glycolysis
Intermediate Step
Kreb’s Cycle
ETS
Cellular Respiration & Photosynthesis
Sun
Photosynthesis
Carbon
Dioxide
Work
ATP
Glucose +
Oxygen Gas
Cellular
Respiration
Photosynthesis in Dry Environments
 C3 Plants—Plants that fix CO2 with
Rubisco & make 3-C PGA
 In hot / dry environment, must close stoma
to conserve water
 Closing of stoma reduces access to CO2
 Rubisco fixes O2 instead of CO2—leading to
photorespiration—consumes oxygen gas
and makes carbon dioxide without making
ATP (actually uses ATP)
Photosynthesis in Dry Environments
 C4 Plants—Plants have an alternative mode of
fixing CO2 that is more efficient
 Leaf structure is slightly different
Bundle sheath cells packed around veins
Mesophyll cells loosely packed around
Calvin Cycle limited to Bundle Sheath Cells
 Process
PEP carboxylase fixes CO2 making a 4C product
4C product is sent to bundle sheath cells and
releases CO2 for Calvin Cycle
Process requires ATP but is more efficient than
CR
Photosynthesis in Dry Environments
Photosynthesis in Dry Environments
 CAM Plants—crassulacean acid
metabolism
 Plants open stomata at night & close during
the day—helps conserve water
 CO2 enters stomata at night & is
incorporated into organic acids stored in
central vacuoles
 CO2 is released from acids in the morning
to enter Calvin Cycle
Photosynthesis in Dry Environments