Biology, Section 3
Notes
Date Submitted: 12/8/11
Chapter 6 Section 1 Notes
Photosynthesis: Light Reactions
Obtaining Energy
A. Organisms can be classified according to how they get energy
B. Autotrophs: organisms that use energy from sunlight / chemical bonds in inorganic substances to
make organic compounds
a. Photosynthesis: process to convert light energy  chemical energy in form of organic
compounds, primarily carbohydrates
C. Heterotrophs: animals and other organisms that must get energy from food instead of directly from
sunlight / inorganic substances
D. Almost all organisms ultimately depend on autotrophs to obtain energy
E. Photosynthesis involves biochemical pathway: series of chemical reactions in which product of 1
reaction is consumed by next reaction
Overview of Photosynthesis
A. Autotrophs use photosynthesis to produce organic compounds
from CO2 and H2O
B. O2 and some organic compounds produced are used by cells in
cellular respiration (opposite of photosynthesis) to produce
CO2 and H2O
C. Photosynthesis divided into 2 stages:
a. Light Reactions – light energy is converted to
chemical energy, which is temporarily stored in ATP
and energy carrier molecule NADPH
b. Calvin Cycle – organic compounds are formed using
CO2 and chemical energy stored in ATP and NADPH
D. Equation: 6CO2 + 6H2O  C6H12O6 + 6O2
Capturing Light Energy
A. First stage of photosynthesis: light reactions
a. Require light to happen
B. Absorption of light in chloroplasts
a. Inside inner membrane is another system of
membranes, thylakoids (flattened sacs)
b. Thylakoids are connected and layered in stacks
called grana
c. Surrounding solution is stroma
C. How chloroplasts absorb light
a. Light from sun appears white, but is actually made of variety of colors
b. White light can be separated into its component array of colors
(red~violet) called visible spectrum by passing through prism
c. Component colors can be reflected, transmitted, or absorbed by
object
d. Many objects contain pigments: compounds that absorb light
e. By absorbing certain colors, pigment subtracts those colors from
visible spectrum
i. Ex: lenses in green-tinted sunglasses contain pigment that
reflects + transmits green light and absorbs other colors
 why looks green
D. Located in thylakoid membrane are several pigments, most important =
chlorophyll
a. 2 most common types:
i. chlorophyll a absorbs less blue light but more red light than chlorophyll b; vice versa
ii. Neither absorbs much green light; allow green light to be reflected / transmitted
iii. That’s why leaves and green plants contain large amounts of chlorophyll
b. Chlorophyll a is directly involved in light reactions
c. Chlorophyll b is called an accessory pigment b/c it assists chlorophyll a in capturing light
energy
d. Yellow, orange, and brown carotenoids are accessory pigments
i. By absorbing colors that chlorophyll a cannot absorb, the accessory pigment enables
plants to capture more light energy
e. Chlorophylls are generally much more abundant in leaves of plant; masks color of other
pigments
f. In nonphotosynthetic parts of plant (fruits, flowers), colors of other pigments may be visible
g. During fall, plants lose chlorophylls, and leaves take on hues of carotenoids
Converting Light Energy  Chemical Energy
A. Chemical energy is temporarily stored in ATP and NADPH
B. O2 is given off during reactions
C. Chlorophylls and carotenoids are grouped in clusters of few hundred pigment molecules in thylakoid
membrane
D. Each cluster of pigments is embedded in proteins = photosystem
a. Photosystem I and Photosystem II contain similar kinds of pigments but have different roles
1. Accessory pigment molecules in both photosystems absorb light
2. Molecules acquire some of energy carried by light
3. In each photosystem, acquired energy is passed quickly to next pigment molecule until it reaches a
specific pair of chlorophyll a
Chlorophyll a absorbs light. Next 5 steps:
1. Light energy forces electrons to enter a higher energy level in 2 chlorophyll a molecules of PS II.
Energized electrons are “excited”; have enough energy to leave chlorophyll a. B/C chlorophyll a
molecules lose electrons, they undergo oxidation reaction. Each oxidation reaction must be
accompanied by reduction reaction, so some substance must accept electrons.
2. Acceptor of electrons lost from chlorophyll a is molecule in thylakoid membrane called primary
electron acceptor
3. Primary electron acceptor donates electrons to first series of molecules called an electron
transport chain. As electrons pass from molecule to molecule, they lose most of energy since
they were excited. Energy they lose is used to move protons (H+) into thylakoid.
4. Light is absorbed by PS I.
a. Happens at same time light is absorbed by II.
b. Electrons move from pair of chlorophyll a molecules in photosystem I to another primary
electron acceptor.
c. Electrons lost by these chlorophyll a molecules are replaced by electrons that have passed
through ETC from PS II.
5. Primary electron acceptor of PS I donates electrons to a different ETC
a. Chain brings electrons to side of thylakoid membrane facing stroma
b. Electrons combine with a proton and NADP+ (organic molecule the accepts electrons
during redox reactions) Caused NADP+ to be reduced to NADPH.
Replacing Electrons in Light Reactions
A. If replacement fails to occur at step 4, both ETCs stop and photosynthesis would not happen
B. Replacement electrons for PS II are provided by water molecules
C. Enzyme inside thylakoid splits water
molecules into protons, electrons, and oxygen
2H2O  4H+ + 4e- + O2
D. For every 2 molecules of water that are split,
4 electrons become available to replace those
lose by chlorophyll in PS II.
E. Protons that are produced are left inside
thylakoid
F. Oxygen diffused out of chloroplast and can
leave plant (O2 not needed for photosynthesis,
but essential for cellular respiration)
Making ATP in Light Reactions
A. ATP is main energy $
B. Chemiosmosis: process of synthesis of ATP
a. Relies on [ ] of protons across thylakoid membrane
C. Mechanisms that build up [ ] gradient of protons ( [ ] of
protons inside thylakoid > [ ] in stroma)
a. Protons can be produced from splitting of water
inside thylakoid
b. Other protons are pumped from stroma to interior
of thylakoid
i. Energy required to pump protons is
supplied by excited electrons in ETC of PS
II.
D. [ ] of protons represents potential energy
a. energy is harnessed by enzyme ATP synthase
located in thylakoid membrane
i. makes ATP by adding phosphate group to
DTP
b. energy driving reaction is provided by movement
of protons from inside thylakoid to stroma through
ATP synthase complex
c. ATP synthase converts potential energy of proton [
] gradient into chemical energy stored in ATP
E. Some protons in stroma turn NADP+ into NADPH at end of ETC of PS I.
F. NADPH and ATP provide energy for 2nd set of reactions in photosynthesis