AP Biology
Photosynthesis
I. Autotrophs – Organisms that can “produce” their own food. (“Auto” means “self”; “trophe” means “feeding”)
II. Heterotrophs – Organisms that “consume” other organisms (living or dead). (“Hetero” means “different”)
III. Chlorophyll – A green light-absorbing protein pigment found in plants, algae, and blue-green bacteria.
(“phyll” means “pigment”; “”chloro” means “green”)
IV. Chloroplast structure (“plast” means “container”) Remember, these ore organelles in some Eukaryotic cells.
A. Thylakoid – Little green discus shaped, membranous structures that actually contain the pigment chlorophyll.
1. Site of the light reaction of photosynthesis. (The thylakoid membrane contains the photosystems.)
a. PRIMARY PURPOSE IS TO HELP MAKE ATP AND NADPH. (CHEMICAL ENERGY MOLECULES)
B. Grana – A stack of thylakoids. These are necessary to create more surface area for making energy molecules.
C. Stroma – The watery space surrounding the thylakoids. (It holds the water needed for photosynthesis.)
1. Site of the light – independent reaction (a.k.a. the Calvin Cycle) of photosynthesis.
a. PRIMARY PURPOSE IS TO USE THE ATP AND NADPH TO MAKE SUGAR USING CO2.
V. Photosynthesis Chemical Reaction
A. Starts by taking radiant electromagnetic energy (sunlight) and converting it into the chemical energy
molecules ATP & NADPH.
1. Then takes the chemical energy (ATP and NADPH) and uses that chemical energy to power the
production of sugar. (A chemical energy storage (potential energy) molecule created by catabolism.)
B. 6 CO2 + 6 H2O (in the presence of sunlight)  C6H12O6 + 6O2 + Heat (KEY NUMBER IS 6 in balancing.)
C. H2O SPLITS using the energy of sunlight; CO2 does not split in this process.
E. There are two processes involved in the conversion of sunlight energy to sugar:
1. Light reaction (light dependent) – It converts sunlight into ATP and NADPH. (Usable chemical energy.)
2. Calvin cycle (light independent or Calvin cycle) – Makes sugar using CO2 and Energy (Carbon fixation)
a. Melvin Calvin discovered the working process.
VI. Sunlight (It is electromagnetic energy.) It is also HIGH QUALITY ENERGY. High quality means it can perform work.
A. Sunlight travels in waves with different wavelengths. The Electromagnetic spectrum shows all the
wavelengths found in sunlight.
1. Red Light– Has the longest wavelength. (It also has the least energy of “white light”.)
2. Blue Light- Has the shortest wavelength. (It has the most energy of “white light”.)
B. Visible “white” light – ROY G. BIV (red, orange, yellow, green, blue, indigo, violet) are the colors within.
C. Light travels in units of energy called Photons.
D. Absorption vs. Reflection
1. Absorbed –These colors are usable light energy.
a. Plants use the reds and blues; BUT NOT the green.
b. Chlorophyll A – Main protein pigment found in all plants and algae.
(It has a structure that looks like a Mg spider in carbon ring web.)
c. Chlorophyll B – Helps Chlorophyll A receive sunlight energy. (B funnels energy to A.)
d. Carotenoids – These are accessory pigments that help Chlorophyll A. (They funnel energy to
A too.) (These are red, orange, or yellow light absorbing protein pigments.)
e. Photosystem – Group of light absorbing pigments in the thylakoid membrane. Chlorophyll A
would be in the reaction center.(“system” means “group of”)
i. Photosystem I – It is responsible for ATP and NADPH production.
ii. Photosystem II – It is responsible for ATP production only.
3. Reflection – These colors are not usable. (They provide the COLOR of an object.)
a. This is why plants are green. Green light is reflected back toward your eyes.
Part II
I. Light Reaction of Photosynthesis
A. This process is used for turning sunlight into usable chemical energy. Those molecules are: ATP and NADPH
B. The process has two parts:
1. Non-cyclic electron flow – This part makes ATP (by non-cyclic photophosphorylation) and NADPH.
a. These electons start out in water, then they are released to Photosystem II as water lyses,
the electrons then make their way to Photosystem I by way of the primary electron
transport chain, and then ultimately end up going to NADPH. This is referred to as
non-cyclic because the starting point for the electrons is different from the ending point.
2. Cyclic electron flow – This part makes extra ATP. (by cyclic photophosphorylation)
a. These electrons start out at Photosystem I and then go down the secondary electron
transport chain to Fd (Ferrodoxin), but then are transferred over to the other path by going
to Cytochrome C. They then make their way back to their starting point, Photosystem I by
way of the primary electron transport chain. Hence the term cyclic, the starting point and
ending point are the same.
C. These two parts are occurring, in the presence of sunlight, at the same time on the Thylakoid membranes.
D. There are thousands of these Photosystems (I and II) on each Thylakoid membrane. (Surface Area? It’s
important again. More surface more photosystems  more energy production.)
II. The major steps to the light reaction of Photosynthesis:
Step 1: Sunlight hits the water in the stroma and also the Photosystems I and II at the same time.
A. The water in the stroma, using the high quality E of sunlight, lyses into O gas (a waste product), 2 H+ ions
(these stay in the stroma), and 2 free electrons. (These will be used to replace the 2 “excited” electrons lost
from the Mg atom of Chlorophyll A in Photosystem II.)
B. The Photosystem II Mg atom loses two electrons due to absorbing all of the E being funneled into
Chlorophyll A from Chlorophyll B and the Carotenoids. (These 2 excited electrons are collected by a primary
acceptor protein, also in the Thylakoid membrane, and moved toward Photosystem I along the primary
electron transport chain. They will be replaced by the 2 electrons from water lysing. This keeps the process
going.)
C. The Photosystem I Mg also loses two electrons due to absorbing all that E from the Chlorophyll and
carotenoid molecules. (These are also collected by another primary acceptor protein and moved toward
NADP+ along the secondary electron transport chain. These will be replaced by the 2 electrons coming down
the chain from Photosystem II in the primary electron transport chain.)
Step 2: Excited electrons travel down the electron transport chains. (This is a series of Redox reactions. A redox reaction
is basically two molecules exchanging electrons. One molecule receives them [called Reduction] and the other
molecule loses them [called Oxidation]. Hence the combined name of Redox.) This is associated with the Law of
Conservation of Mass…” Mass is neither created nor destroyed; only transferred and transformed.”
As the excited electrons go down the electron transport chain, by going through these series of Redox reactions,
their excited kinetic E (also called Free E) is being used to power the proteins called Proton pumps. As the
electrons go down their transport chain, their excited kinetic E decreases.
A. Photosystem II electrons (P.A. Pq cyto C Pc  Photosystem I)
1. Free E of the electrons is used to actively transport H+ ions (a.k.a. called protons) into the inner
thylakoid space The H+ ion concentration [H+] goes up inside the space. This causes the pH to
decrease and become more acidic. The [H+] goes down in the stroma. The stroma becomes more
basic. As this is occurring a concentration gradient is created. A concentration gradient is a source of
potential E now. (It would be like blowing air into a balloon. The pressure builds as more air is blown
inside the balloon. This is also an example of potential E.)
B. Photosystem I electrons (P.A.  Fd  NADP+) (Reduction occurs to create NADPH from NADP+.)(This is the
ending point for non-cyclic electron flow.)
OR
(P.A.  Fd Cyto C Pc Photosystem I) This would be for cyclic electron flow.
Remember this makes extra ATP.
C. Cytochrome C is an important molecule as ALL organisms possess it in their membrane that is used for
energy production. This supports common ancestory among ALL organisms.
1. Mitochondria and Chloroplast INNER membranes in eukaryotic organelles.
2. The plasma membrane of Prokaryotic cells.
Step 3: The trapped H+ ions, inside the Thylakoid, are released through the ATP Synthetase Complex. This is a group
of enzymes in the Thylakoid membrane that helps make ATP, by Anabolic Phosphorylation. Just look the
enzymes name. This release of kinetic H+ ions powers the phosphorylation of ADP ATP. (This would be like the
air coming out of the blown up balloon and turning a pinwheel.
A. This Kinetic movement of H+ ions produces a LARGE AMOUNT OF ATP.
B. This is an example of Energy Coupling (Two processes working together and involving energy.)The first
process was Active transport to pump the H+ ions into the confined space of the Thylakoid, using the Proton
pump proteins, to make the concentration gradient. The second process is diffusion, The H+ ions going
from high [ ] to low [ ]. The kinetic movement of the H+ fuels the production of ATP
1. This form of energy coupling, for making ATP, is referred to as Chemiosmosis.
Step 4: ATP and NADPH will now be used to power the fixing of CO2 into sugar in Calvin Cycle.
Part III
I. Calvin Cycle (A.K.A Light Independent reaction)
A. This part uses the ATP and NADPH, of light reaction, to perform Carbon fixation. (Making sugar using CO2.)
B. It has three steps:
Step 1: CO2 molecules enter the leaf through the open stomata. Then the CO2 molecules diffuse into the
cells by crossing the phospholipid portion of the plasma membrane. Once inside the cell, the CO2
molecules diffuse across the phospholipid portion of the chloroplast membrane. Once in the
stroma of the chloroplasts, 3 molecules of CO2 combine with 3 RuBP molecules using the
enzyme Rubisco. (RuBP is a 5 Carbon molecule.)
A. The resulting three 6 carbon molecules are unstable and break into two 3 carbon molecules
of 3 phosphoglycerate. (Six total carbon molecules still, just in two groups of three.)
Step 2: Use 6 ATP and 6 NADPH (1 ATP and 1 NADPH /molecule of 3 phosphoglycerate) to “bend” each
molecule twice into G3P. (G3P is half of a Glucose molecule.)
Step 3: Take out 1 G3P and recycle the other 5 G3Ps back into the original 3 RuBPs using 3 extra ATP
(Remember, from the cyclic electron flow.) This process basically takes 1 Carbon away from two
G3Ps (remember, there are 5 G3P left) and thus creating 2 two carbon molecules. Then one of
the two 2 carbon molecules is paired with one of the three remaining three Carbon molecules.
3 +2 = 5. The two single Carbons are added to the last G3P molecule to have 5 carbons again.
(3 + 1 +1 =5)So we end up with 3 five Carbon molecules of RuBP again. Thus, we have started
and ended at the same point… a cycle.
a. 1 G3P used to make glucose (So the cycle must go around TWICE to make 1 glucose
molecule. So repeat steps 1-3 to make the second half.)
C. Total Numbers needed: For each turn of the cycle – 9 ATP and 6 NADPH are needed.
: To make 1 glucose molecule (2 turns) – 18 ATP and 12 NADPH are needed.
D. These sugars will be needed to feed the whole plant or algae. Or they will be stored in the form of starch, a
complex carbohydrate. The sugars will be consumed in the process of cellular respiration. They could also
be utilized in the making of plant cell walls.