Ch 10 NOTES (part 1):
Photosynthesis in Nature
10.1 – Photosynthesis converts
light energy to the chemical
energy of food.
PHOTOSYNTHESIS:
● PHOTOSYNTHESIS = the capture of
light energy & the conversion of this
energy to stored chemical energy (in the
form of sugar)
-AUTOTROPHS: “producers”; capable of
producing their own food
PHOTOAUTOTROPHS: use light
energy to synthesize food (e.g. plants)
● Photosynthesis
occurs in plants,
algae, certain
other protists, and
some prokaryotes
● These organisms
feed not only
themselves but
also the entire
living world…
thank you plants!

Plants
Unicellular protist 10 µm
Purple sulfur
bacteria
Multicellular algae
Cyanobacteria
40 µm
1.5 µm
● HETEROTROPHS: “consumers”; live on
compounds produced by other organisms
-animals that eat plants or other animals
● Almost all heterotrophs, including humans,
depend on photoautotrophs for food and
oxygen
**some heterotrophs are:
-decomposers: live on dead matter
Chloroplasts: The Sites of
Photosynthesis in Plants
● Leaves are the major locations of
photosynthesis
● Their green color is from CHLOROPHYLL,
the green pigment within chloroplasts
● Light energy absorbed by chlorophyll drives
the synthesis of organic molecules in the
chloroplast
● Through microscopic pores called
STOMATA, CO2 enters the leaf and O2 exits
CHLOROPLASTS:
● organelles in which photosynthesis occurs
● found primarily in leaves (about 500,000
per mm2)
● contain green pigment CHLOROPHYLL
(absorbs light energy)
● found mainly in mesophyll (tissue inside leaf)
● a typical mesophyll cell has 30-40
chloroplasts
Leaf cross section
Vein
Mesophyll
Stomata
CO2 O2
Mesophyll cell
Chloroplast
5 µm
Outer
membrane
Thylakoid
Thylakoid
Stroma Granum
space
Intermembrane
space
Inner
membrane
1 µm
● CO2 enters and O2
leaves through
STOMATA
● water is delivered to
leaves from the roots
via veins
CHLOROPLAST:
● enclosed by 2 membranes
● STROMA = fluid within the chloroplast
● THYLAKOID MEMBRANES are
interconnected & contain the thylakoid space,
or lumen (are usually in stacked columns
called GRANA)
● chlorophyll is stored in the thylakoid
membranes
PATHWAYS OF
PHOTOSYNTHESIS
● TOTAL equation for photosynthesis:
6CO2 + 12H2O + light  C6H12O6 + 6O2 + 6H2O
energy
● NET EQUATION:
6CO2 + 6H2O + light  C6H12O6 + 6O2
energy
The Splitting of Water
● Chloroplasts split water into hydrogen and
oxygen, incorporating the electrons of
hydrogen into sugar molecules
Tracking Atoms Through Photosynthesis:
Products:
12 H2O
6 CO2
Reactants:
C6H12O6
6 H2O
6 O2
Photosynthesis as a Redox Process
● Photosynthesis is a
redox process in
which water is
oxidized and carbon
dioxide is reduced
● like respiration, photosynthesis is a series of
redox reactions…BUT, the electron flow is
reversed!…the electrons increase in potential
energy as they move from water  sugar… an
uphill process!
(the required energy boost comes from light
energy)
Two Stages of Photosynthesis:
1) the LIGHT REACTIONS
2) the CALVIN CYCLE
(a.k.a. the “dark reactions”
or light-independent
reactions)
The Two Stages of
Photosynthesis: A Preview
● Photosynthesis
consists of the light
reactions (the photo
part) and the Calvin
cycle (the synthesis
part)
LIGHT REACTIONS:
● convert solar energy  chemical energy
● light is absorbed by chlorophyll and drives a
transfer of electrons and H from water to an
acceptor called: NADP+
● water is split and O2 is produced as a “waste
product”
● NADP+ is reduced to NADPH…ALSO, ATP is
generated via phosphorylation of ADP
(photophosphorylation!)
CALVIN CYCLE:
● CO2 from air is incorporated into
organic molecules by carbon fixation
● NADPH and ATP from the light
reactions power the production of sugar
● also called: the “dark” reactions or
light-independent reaction
Locations of the Photosynthesis Reactions:
● Light reactions occur in the THYLAKOIDS
● Calvin cycle occurs in the STROMA
H2O
Light
LIGHT
REACTIONS
Chloroplast
H2O
Light
LIGHT
REACTIONS
ATP
NADPH
Chloroplast
O2
H2O
CO2
Light
NADP+
ADP
+ Pi
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
Chloroplast
O2
[CH2O]
(sugar)