CHAPTER 6 - PHOTOSYNTHESIS
Photosynthesis – Process by which some
organisms capture light energy and store it in
organic compounds (mainly carbohydrates,
sugars)
Autotrophs – make their own food through
photosynthesis. They are at the start of every
food chain. Exs.) plants, algae, some
bacteria/protozoans.
Photosynthesis = “Biochemical Pathway” = a
complex series of chemical reactions in which
the product of 1 reaction is consumed in the next
reaction. It uses CO₂ and water to make organic
compounds + oxygen. Energy is stored!!!
Cellular Respiration – uses organic compounds and
oxygen from photosynthesis and releases CO₂ and
water. Energy is released!! See figure 6-1 on page 112.
Light Reactions – initial reactions in photosynthesis
(referred to as P.S. from this point forward). Begins with
the absorption of light in chloroplasts. See figure 6-2 on
pg. 112.
Thylakoids – flattened sacs inside the chloroplast…the
light reactions take place here.
Granum (Grana – Plural) – stacks of thylakoids.
Stroma – solution that surrounds the thylakoids…more
later.
Visible Spectrum of Light – from white light yields an array
of colors….each color represents a different wavelength
of light. See figures 6-3 and 6-4 on pg. 113.
Pigments – compounds that absorb light. Light that is
reflected is the color that the pigment appears to the
eye.
Chlorophyll a and b – green pigments located in the
membrane of the thylakoid.
 Chlorophyll a - directly involved in the light reactions of
P.S.
 Chlorophyll b – “accessory pigment” – assists
chlorophyll a.
 Carotenoids – other accessory pigments – yellow,
orange, red, brown.
Red is the best light for P.S. See fig. 6-4, pg. 113.
Photosystem – clusters of pigments in the thylakoid
membrane. 2 types – photosystem I and photosystem
II…..each has a different role in the light reactions. See
pg. 114.
Electron Transport
Electron Transport – (light reactions of P.S.) –
begin when accessory pigment molecules
absorb light in both photosystems. Energy
from the light is quickly passed to other
molecules until it reaches a specific pair of
chlorophyll a molecules. The rest of the
events are divided into 5 steps and are
outlined on the next slide. Refer to pages 114
and 115.
Electron Transport
Continued…
1. Light energy forces electrons (e-’s) to a higher
energy level (“excited”) in the 2 special chlorophyll a
molecules.
2. Excited electrons leave the chlorophyll a molecules.
They are picked up by a molecule in the thylakoid
membrane known as the Primary Electron
Acceptor.
3. Electrons then enter the Electron Transport Chain =
a series of molecules that move electrons through
the thylakoid membrane. As the electrons flow,
they lose their energy. This energy is used to pump
protons (H+) into the thylakoid.
Continued on next slide…..
At the same time, light is also being absorbed by photosystem
I…..e-’s become excited begin to flow through their own etransport chain. NOTE – e-’s lost by photosystem I are
replaced by the ones flowing from photosystem II.
5. e-’s from photosystem I flow to outside of thylakoid
membrane. e-’s combine with a proton (H+) and NADP+
(Nicotinamide Dinucleotide Phosphate) to yield NADPH.
NOTE – Special chlorophyll a’s, from both photosystems, lose
electrons, this is called OXIDATION. NADP+ accepts the e-’s,
this is called REDUCTION. The whole process is known as a
REDOX REACTION.
NOTE #2 – as photosystem II loses e-’s, they are resupplied by the
splitting of water molecules ( 2 H₂O → 4H+ + 4e- + Oxygen).
This is the oxygen that we breathe! See fig. 6-6, pg. 115.
Here ends the info about the Electron Transport System!
4.
Chemiosmosis – (also part of the Light Reactions)
– some protons are produced by the splitting of
water; others are pumped from stroma to the
inside of thylakoid. This sets up a “concentration
gradient” (High concentration of H+ inside
thylakoid; Low concentration of H+ outside
thylakoid). This represents Potential Energy like
that found in a car battery.
In Chemiosmosis: ATP Synthetase harnesses the
potential energy. H+’s flow through ATP
Synthetase from high to low concentration
(inside thylakoid to outside). As they flow they
produce energy. Energy is used to tack a
phosphate onto ADP forming ATP. See fig. 6-7,
pg. 116.
Summary of Light Reactions
As light causes e-’s to flow from photosystems I
and II, NADPH and ATP are produced. Both
molecules have had energy stored in them.
This energy will be used for the 2nd set of
reactions (Calvin Cycle) in Photosynthesis.
Light Energy → Light Reactions → NADPH + ATP
→Calvin Cycle.
Calvin Cycle
Calvin Cycle – (Melvin Calvin 1911-1997) – occurs
in the stroma. Carbon atoms from CO₂ are
bonded or “fixed” into organic compounds.
This is known as CARBON FIXATION.
** The Calvin Cycle has 3 major steps. Refer to
fig. 6-8, pg. 117 as you study the steps.
3 Steps of the Calvin Cycle
An enzyme combines CO₂ with a 5-carbon molecule called
RuBP (Ribulose Bisphosphate) making a 6-carbon
molecule that splits into two 3-carbon PGA molecules
(phosphoglycerate).
2. Each PGA receives a phosphate group from ATP. Then
NADPH donates a proton and releases a phosphate group.
The resulting molecules are known as PGAL
(phosphoglyceraldehyde).
3. It takes 3 turns of the Calvin Cycle. This yields 6 PGAL (5
are used to regenerate RuBP to keep the cycle going. The
one PGAL that remains is built up into organic molecules
that FEED THE WORLD!!!
NOTE – 3 turns of Calvin Cycle uses 9 ATP and 6 NADPH from
the light reactions and yields organic molecules
(carbohydrates) THAT FEED THE WORLD!!!
1.
Simple Equation for
Photosynthesis
 CO₂ + H₂O + Light Energy → Glucose +
Oxygen
Factors that Effect the Rate of
Photosynthesis
1. Light Intensity – P.S. goes faster (to a point)
with more intense light.
2. CO₂ Level – P.S. goes faster (to a point) with
more CO₂.
3. Temperature – P.S. goes faster (to a point)
with higher temperatures.