Photosynthesis
College Board:
2.A.2 – Organisms capture and store free energy for use in biological processes
a. Autotrophs capture free energy from physical sources in the environment
Evidence of student learning is a demonstrated understanding of each of the following:
1. Photosynthetic organisms capture free energy in sunlight.
2. Chemosynthetic organisms capture free energy from small inorganic molecules present in their
environment and this process can occur in the absence of oxygen.
b. Heterotrophs capture free energy present in carbon compounds produced by other organisms
c. Different energy energy-capturing processes use different types of electron acceptors.
To foster student understanding of this concept, instructors can choose an illustrative example such as:
 NADP in photosynthesis and oxygen in respiration
d. The light-dependent reactions of photosynthesis in eukaryotes involve a series of coordinated reaction
pathways that capture free energy present in light to yield ATP and NADPH, which power the production of
organic molecules.
1. During photosynthesis, chlorophylls absorb free energy from light, boosting electrons to a higher energy
level in photosystems I and II
2. Photosystems I and II are embedded in the internal membranes of chloroplasts (thylakoids) and are
connected by the transfer of higher energy electrons through an electron transport chain
3. When electrons are transferred between molecules in a sequence of reactions as they pass through the
ETC, an electrochemical gradient of hydrogen ions (protons) across the thylakoid membrane is
established.
4. The formation of the proton gradient is a separate process, but it is linked to the synthesis of ATP from
ADP and inorganic phosphate via ATP synthase.
5. The energy captured in the light reactions as ATP and NADPH powers the production of carbohydrates
from carbon dioxide in the Calvin Cycle, which occurs in the stroma of the chloroplast.
X Memorization of the steps in the Calvin cycle, the structure of the molecules and the names of enzymes
(with the exception of ATP synthase) are beyond the scope of the course and the AP exam.
e. Photosynthesis first evolved in prokaryotic cells; scientific evidence supports that prokaryotic (bacterial)
photosynthesis was responsible for the production of an oxygenated atmosphere; prokaryotic photosynthetic
pathways were the foundation of eukaryotic pathways.
Notes: Photosynthesis:
Latin
Chemo - chemical
Chloro – green
Photo – light
Phyll – leaf
Plast - container
Stoma – mouth, opening
Terms
Accessory pigments
Aerobic
Anaerobic
Chemiosmosis
Chemoautotrophs
Chemosynthesis
Chlorophyll
Electrochemical gradient
ETC
Eukaryote
G3P
Grana
Light dependent
Light independent
NADP
NADPH
Mesophyll
Photoautotrophs
Photolysis
Phosphorylate
Photosynthesis
Photosystem I
Photosystem II
Primary electron acceptor
Prokaryote
RuBP
Rubisco
Stoma
Stroma
Thylakoid
Autotrophs: organisms that synthesize complex organic compounds (ie. glucose) from CO2 and H2O
CO2 + H2O → CH2O + 6O2
Photoautotrophs - organisms that synthesize organic molecules using free energy from light (plants, algae and some
prokaryotes (bacteria). Photosynthesis is the basis of most food chains on the earth.
Chemoautotrophs –some species of bacteria (domain Archaea) can synthesize organic molecules using free energy
released by breaking down chemicals in place of light. The process can be anaerobic (without oxygen). Prokaryotes
(bacteria) living near deep-sea vents are the basis of chains there.
Chemosynthesis - CO2 + 4H2S → CH2O + 4S + 3H2O – bacteria near deep=sea vents
Photosynthesis:
The Big picture: photosynthesis is anabolic and decreases entropy by converting low-energy CO2 (from the air) and H2O
into more organized G3P (most of G3P is converted into glucose). The glucose is used as the source of energy for all
other organisms.****
6 CO2 + 6 H2O
C6H12O6 + 6 H2O
Two steps: light-dependent reactions, light-independent reaction (Calvin cycle).
The structure of chloroplasts enables photosynthesis*****
Chloroplast structure: organelles in plant and algal eukaryotic cells. Chloroplasts are green due to chlorophyll
1. Outer membrane – phospholipid bilayer surrounding the chloroplast
2. Thylakoid – folded phospholipid membrane inside the chloroplast. The folding increases surface area so that
there is a lot of surface area stuffed into a small amount of space.
The light reactions of photosynthesis occur here and in the thylakoid lumen (space). The thylakoid
membrane contains the photosystems.
PURPOSE: to make ATP AND NADPH.
3. Grana – A stack of thylakoids. These create more surface area****
4. Stroma – watery space around the thylakoids. (water dissolves
substances which enables chemical reactions)
PURPOSE: Site of the light–independent reaction (Calvin Cycle)
which is where carbohydrates are actually produced.
The chemical reaction – takes place in two steps:
1. Light-dependent reactions - light energy is captured and used to produce ATP, NADPH and oxygen gas
2. Light-independent reactions - ATP is used as a source of energy. NADPH carries the H+ and electrons which
are added to CO2 to make G3P. G3P is converted into glucose or other organic molecules (amino acids,
nucleotides, etc.)
Light-dependent reactions:
1. The role of light. Light energy is captured by chlorophyll molecules and accessory pigments (proteins). Sunlight
travels in waves of different wavelengths. Visible “white” light can be split into the different wavelengths which
we see as color. Red Light has the longest wavelength, but has the least energy. Blue Light has the shortest
wavelength, but has the most energy. Light is also measured in units of energy called Photons.
2. The role of chlorophyll. Chlorophyll does not absorb all wavelengths of light. Chlorophyll absorbs and uses
mostly blue and red wavelengths and reflects most of the green
wavelength (which is why we see chlorophyll as ‘green’). Plants have
several other pigments that absorb light; chlorophyll a, chlorophyll b
and carotenoids. Accessory pigments capture light at other
wavelengths and pass the energy to one particular chlorophyll a
molecule. Chlorophyll molecules (and accessory pigment molecules)
are clustered together into groups called photosystems.
3. Photosystems. There are 2 different photosystems (I and II). They are
similar in structure but produce different results. Each photosystem
consists of 100 + chlorophyll and accessory pigment molecules which
are embedded in the thylakoid membrane. The thylakoid holds them
in place so that they work as an assembly line, making them more
efficient. The thylakoid is highly folded so there is room for thousands of each type of photosystem.
4. Absorption of light energy: light is absorbed by chlorophyll and the accessory pigments.
a. Photolysis – chlorophyll uses energy from light to break down water into: 2 H+, O2, and electrons. The
oxygen is waste and diffuses out of the chloroplast. The 2 H+ are ‘pulled’ through the thylakoid by the
electron transport chain and eventually are attached to NADP+. The electrons are ‘boosted’ to a higher
energy level.
b. ‘Boosted’ electrons – after receiving energy from light, 2 electrons in the chlorophyll molecules become
‘excited’ and move to a higher energy level. As they return back to their original energy level, they are
captured by a ‘primary electron acceptor’ molecule (a protein). The primary electron acceptor molecule
transfers the electrons to a series of other electron acceptor molecules called the electron transport
chain (also embedded in the thylakoid).
5. Electron transport Chain – electrons are passed down the chain of acceptor molecules. Each electron acceptor
in the chain is ‘stronger’ than the one in front of it so that the electrons are sequentially pulled ‘down’ the chain
to the next electron acceptor. As electrons move down the ETC, they lose energy (Law of Entropy)
a. Photosystem II is the ‘beginning’. It produces ATP by chemiosmosis.
1. Photolysis - light energy is used to split H2O into H+, O2 and e- -.
2. Electrons from Mg atoms (in the chlorophyll) are energized and captured by the PSII primary electron
acceptor. Mg electrons are replaced with electrons released by the splitting of water.
3. Accessory pigments also
capture light energy and pass it
to a chlorophyll a molecule
which ‘boosts’ the electrons.
4. As the electrons pass down the
Electron Transport Chain (ETC),
their kinetic energy ‘pulls’ H+
(protons) through the thylakoid
PSII
PSI
membrane into the thylakoid
lumen (space). Protons build up
inside the lumen and an
electrochemical gradient of +
charges is produced.
5. The electrons ‘travel downhill’
to the PSI photosystem
6. Chemiosmosis - the trapped protons (H+ ions), inside the thylakoid lumen, diffuse through ATP
Synthase. This enzyme is also embedded in the thylakoid membrane and as protons flow through the
ATP synthase, their kinetic energy is used to add inorganic phosphate to ADP to make ATP:
ADP + Pi ATP.
 Non-cyclic photophosphorylation – electrons flow to PSI and do not return to the ETC
 Cyclic photophosphorylation – electrons are
excited by PSI and go back through the ETC to
produce more ATP.
b. Photosystem I - electrons from PSII are again excited by
light captured by chlorophyll (+ accessory pigments).
The excited electrons are captured by PSI electron
acceptors, which passes them down to a different set of
electron acceptors. The last of these electron acceptors
passes them to NADP reductase, an enzyme that adds
them and H+ to NADP+ to form NADPH. NADPH and ATP (from PSII) are now used to make glucose in
the stroma part of the chloroplast during the light independent reactions (aka the Calvin cycle)
Some key points about the photosystems and chemiosmosis:
 Concentration gradients create ATP******
 PS II makes ATP, PSI makes NADPH****
 There are lots of photosystems in the thylakoids because the thylakoids are highly folded which increases
surface area. More surface area = more photosystems = more ATP
 Chloroplasts and mitochondria both produce ATP by the same method (chemiosmosis) using highly folded
membranes. This is evidence for evolution; mitochondria and chloroplasts evolved from bacteria.
 Bacteria lack mitochondria and chloroplasts and must do these processes on their cell membranes.
Calvin cycle - light-independent reactions - CO2 from the atmosphere/water, is reduced using H+ from NADPH and
energy from ATP.
A. Three steps:
Step 1: CO2 molecules enter the leaf through the open
stomata of the leaves. The CO2 molecules diffuse into the
cells by crossing the phospholipid portion of the plasma
membrane. Inside the cell, CO2 diffuses into the stroma
(watery) part of the chloroplasts. Inside the stroma, the
enzyme RuBISCO combines 3 molecules of CO2 and 3
molecules of RuBP. The result forms PGA.
Step 2: ATP is used to phosphorylate the PGA so that H+
(from NADPH) can bond to the carbon of PGA to (eventually)
form 1 molecule of G3P. G3P is the organic molecule produced by photosynthesis. Much of the G3p is
converted into glucose. Much of the glucose that is produced (and it actually requires 2 turns of the Calvin
cycle to make 1 glucose molecule) diffuses to the mitochondria and is used to make ATP to power the cell.
Most of the glucose is converted into cellulose for making cell walls. Plants may also use it for making all the
other organic compounds it needs (hormones, enzymes, amino acids, etc.).
Importance of prokaryotes (bacteria)
Photosynthesis first evolved in Prokaryotes and chemiosmosis which had to use their cell membranes (as they
do not have chloroplasts) to do the ETC and chemiosmosis. As a result of prokaryotic photosynthesis, the earth became
rich with oxygen and aerobic organisms (that could use glucose instead of light to produce ATP) then evolved. Evidence
comes from the fact that that type of bacteria still exists in large numbers.