Biology
Mr. Camm
Chapter 6_Section 1
Review text pages: 112-129
Definition: “Light” + “Putting Together”
 The process by which photoautotrophic cells
containing chlorophyll in green plants
convert light energy into chemical energy
and synthesize organic compounds from
inorganic compounds, especially
carbohydrates from carbon dioxide and
water, accompanied by the simultaneous
release of oxygen.
Photosynthetic Equation:
6CO2 +6H20
Light Energy
C 6 H 12 06 + 602
Carbohydrate
 An essential structural component of living cells
(cellulose in plants), as well as storage and
transport of energy (starch & glycogen)
 Created by photosynthetic process of plants and
contain a combination of only carbon, hydrogen,
and oxygen
Saccharides are a group of carbohydrates that
include sugars and starches. Derived
from a Greek word meaning "sugar".
Almost all energy used by organisms to carry out the functions of
life comes directly or indirectly from the Sun.
Obtaining Energy: Two Classifications of organisms
o Autotroph “Self-Nutrition”
- An organism that uses energy from sunlight or from
chemical bonds in inorganic substances to make organic
compounds. (i.e. Plants & algae)
o Heterotroph “Another Nutrition”
- An organism that receives its energy from food instead of
directly from sunlight or inorganic substances.
(i.e. Humans, Animals, Fungi, and some Bacteria)
Green algae, mosses, ferns,
pine trees, oak trees,
shrubs, vines, grasses,
herbs
Eukaryotic cells with walls
of cellulose
Store carbohydrate as starch
Share particular
photosynthetic pigments
indeterminate growth
Means they can grow and
grow as long as a biotic
factors allow.)
Light Reaction
Light energy from the sun is absorbed by
autotrophs and is converted into chemical energy,
which is stored in ATP and the energy carrier
molecule NADPH.
Calvin Cycle
Organic compounds are formed using carbon
dioxide and the chemical energy stored in ATP and
NAPHD
First stage of
photosynthesis.
Requires Light to
begin reaction.
Occur in Thylakoid membranes or grana,
located in Chloroplasts.
Chloroplasts- specialized organelles in plants
and algae that absorb light.
oThylakoid
-are flattened sacs stacked to form
grana.
oStroma
-is a solution surrounding the grana.
Although the sun’s light appears to be white, it is
made of a visible spectrum of colors. (Page 115, Figure 6-4)
Light can be reflected, transmitted, or absorbed
when it hits an object.
Pigments- compounds that absorb light. Normally
absorbing specific colors more strongly than others.
(i.e. Chlorophyll & Carotenoids)
-When
colors are absorbed,
the remaining reflected or
transmitted colors from the
spectrum are visible.
Chlorophyll
is a green pigment located in the thylakoids of a plant
cell, that absorbs light most strongly in the blue and
red, but poorly in the green portions of the light
spectrum.
• Responsible for the green appearance of plant leaves because
it does NOT strongly absorb green light.
Two types:
•Chlorophyll a
•pigment directly involved in photosynthetic light reactions.
[absorbs less blue, but more red]
•Chlorophyll b
•an accessory pigment that helps capture light energy but
is not directly involved in photosynthesis.
Carotenoids- other accessory pigments [yellow,
orange, and brown] that are located in the thylakoids
and assist photosynthesis indirectly by absorbing colors
that chlorophyll a cannot, therefore capturing more
light energy. These are visible in the absence of
chlorophylls in the fall, as observable by vibrant fall
leaves.
A.
B.
C.
D.
E.
Pigments in chloroplast capture light energy.
Light energy is converted to chemical energy.
Chemical energy is stored in ATP and NADPH.
Oxygen (O2) is released
Accessory pigment molecules in Photosystems I
and II absorb light. Energy is passed to other
pigment molecules until it reaches a specific
pair of Chlorophyll a molecules.
Photosystem- cluster of pigment molecules and
proteins they are embedded in. There are two
types of photosystem (I and II) with similar
pigments types, but different light reaction roles.
http://www.cnr.vt.edu/dendro/forestbiology/photosynthesis.swf
Steps in Light to Chemical Reaction
1) Light energy forces electrons to enter a higher energy level in
the two Chlorophyll a molecules of photosystem II, causing
them to reach an excited state.
2) Excited electrons can leave Chlorophyll a molecules, making the
molecules undergo an oxidation reaction that must be
accompanied by a reduction reaction. The lost electrons are
accepted by the primary electron acceptor located in the
thylakoid membrane.
3) The primary electron acceptor donates electrons to the
electron transport chain, a series of molecules that transfer
electrons from one molecule to the next. This transfer causes
the electrons to lose most of their excited energy. The lost
energy is used to move protons into the thylakoid.
Steps Continued…
4. Light is absorbed by photosystem I and II simultaneously. Electrons move
from a pair of chlorophyll a molecules in photosystem I to another primary
electron acceptor. Lost electrons from this process are replaced by the
electrons that have passed through the electron transport chain from
photosystem II.
An enzyme inside the thylakoid splits water molecules into protons,
electrons, and oxygen. {For every 2 molecules of water split, 4 electrons
are available to replace those lost by chlorophyll molecules in
photosystem II. }
Protons are left inside thylakoid, and oxygen defuses out of chloroplast
and eventually the plant.
5. NADP+ is reduced to NADPH after the primary electron acceptor of
photosystem I donates electrons to a different electron transport chain that
brings them to the side of the thylakoid that faces the stroma. At this
location the electrons combine with a proton and NADP+.
NADP+ - an organic molecule that accepts electrons during oxidation/reduction reactions.
Electrons from photosystem II replace electrons that
leave photosystem I. Replacement electrons for
photosystem II are provided by the splitting of water
molecules.
Oxygen produced
when water molecules
are split diffuses out of
the chloroplast and
then leaves the plant.
2H20
4H + 4e + 02
Step 4
Therefore, in light reactions electrons flow from
H2O  PSII  PSI  NADP
one way electron flow, makes ATP & NADPH
Because ATP generated, called NON-CYCLIC
PHOTOPHOSPHORYLATION
An important part of the light
reactions is the synthesis of ATP.
During chemiosmosis, the movement
of protons through the enzyme ATP
synthase into the stroma releases
energy, which is used to produce ATP.
Relies on the concentration gradient
of protons across the Thylakoid
membrane
Text p. 118
Chapter 6_Section 2 p. 120-124
DARK REACTIONS
(Carbon Fixation)
Enzymatic reactions; temperature sensitive
Occur in STROMA of chloroplast
ATP and NADPH produced in light reactions
used to reduce CO2
Occurs via cyclical reaction = CALVIN CYCLE
also known as C3 photosynthesis
The ATP and NADPH produced in the light
reactions drive the second stage of
photosynthesis, the Calvin cycle.
In the Calvin cycle, CO2 is incorporated into
organic compounds, a process called carbon
fixation.
The Calvin cycle, which occurs in the stroma of
the chloroplast, is a series of enzyme-assisted
chemical reactions that produces a threecarbon sugar.
Most of the three-carbon sugars (G3P)
generated in the Calvin cycle are converted to a
five-carbon sugar (RuBP) to keep the Calvin
cycle operating. But some of the three-carbon
sugars leave the Calvin cycle and are used to
make organic compounds, in which energy is
stored for later use.
• The C4 Pathway
– Some plants that evolved in hot, dry climates fix carbon
through the C4 pathway. These plants have their stomata
partially closed during the hottest part of the day.
– Certain cells in these plants have an enzyme that can fix CO2
into four-carbon compounds even when the CO2 level is low
and the O2 level is high. These compounds are then
transported to other cells, where the Calvin cycle ensues.
• Light Intensity
– The rate of photosynthesis increases as light intensity
increases, because more electrons are excited in both
photosystems.
– However, at some point all of the available electrons are
excited, and the maximum rate of photosynthesis is reached.
The rate then stays level regardless of further increases in
light intensity.
• Carbon Dioxide Levels
– As with increasing light intensity, increasing levels of carbon
dioxide also stimulate photosynthesis until the rate levels off.