Photosynthesis
 Van Helmont: a man who was curious about whether plants get all their
nourishment from the soil. He planted 5 lb willow tree sapling in a 200 lb
bucket of soil and gave it only water for 5 years. After this time, he saw the
tree gained 165 lb, while soil only lost 2 oz. He concluded that the plant
does not gain most of it substance from the soil. But he was correct when
he said the willow tree gained most of its mass from the WATER he gave it.
 Joseph Priestly: discovered that a candle burned out in a closed container,
but when he added a living mint plant to the closed container, the candle
continued to burn. He didn’t know of oxygen but concluded the mint
“restored” the air that the burning candle depleted
 Ingenhousz: finished photosynthesis mystery when he discovered that
plants require light.
 Overall equation for photosynthesis:
6CO2+ 12H20+ light 6O2+ 6H2O+ C6H12O6 Or
6CO2+ 6H2O+ light 6O2+ C6H12O6
 Photosynthesis: process by which autotroph (self feeders)- plants, algae,
some bacteria (ex: cyanobacteria), and some protist (volvox, euglena) use
light energy and convert it to chemical energy/ATP to make sugar molecules
from CO2 and H2O.
 All green parts of a plant have chloroplast and can carry out photosynthesis.
 Leaves have the most chloroplast and are the major site of the process.
 The green color in plants is from the chlorophyll pigment in the chloroplasts.
 Mesophyll: the interior layer of a leaf where chloroplasts are found.
 Stomata: tiny pores where CO2 and H2O enter
through.
 In the chloroplast below, the disk-like sacs are called
thylakoid.
 Grana: stacks of thylakoid.
 Stroma: thick fluid inside the inner membrane
 Chlorophyll molecules are found inside the
thylakoid membranes.
 Just like the cellular respiration, photosynthesis is
a redox process.
 As H2O is oxidized and carbon dioxide reduced
during photosynthesis, electrons gain energy by
being boosted up an energy hill.
 It is the light energy captured by chlorophyll
molecules in the chloroplasts that provides the
boost for the electrons.
 Solar/light energy  chemical energy/ATP
energy stored in sugar.
 Sunlight is a type of energy called radiation or
electromagnetic.
 Of the entire electromagnetic spectrum, we can
only see visible light that our eyes as different
colors.
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As light hits a chloroplast, pigment molecules in the thylakoids absorb different wavelengths of light.
Chlorophyll a absorbs mainly blue-violet and red light while reflects green light. ∴ it appears green in color.
Chlorophyll b absorbs mainly blue and orange light and reflects yellow-green. ∴ it appears yellow-green in color.
Carotenoids: other pigment molecules in the thylakoids; it is a family of orange-yellow colored pigments, readily visible in
tomatoes, carrots, and fall leaves.
 Chlorophyll and carotenoids do not participate DIRECTLY in the reactions of photosynthesis; they broaden the range of light
a plant can use and then pass the light energy they absorbed onto chlorophyll a which can put the energy to work in
photosynthesis.
 Photosynthesis has two stages:
1) Light-dependent reactions
2) Light-independent reactions
 Light-dependent reaction: (require light directly)
 Reactions that convert solar energy to chemical energy/ATP and produce O2 as a waste product.
 This is the “photo” part of photosynthesis since it takes the energy from light.
 Occurs in the phospholipid bilayer of thylakoid membranes of the chloroplast’s grana.
 Two types of Phtophosphorylation:
i) Noncyclic photophosphorylation
ii) Cyclic photophosphorylation
 Noncyclic photophosphorylation:
 Light can behave as either waves or as discrete packets of energy called photon
 When a pigment molecule absorbs a photon, one of the pigment’s electrons gains energy.
 The electron has been raised from the ground state to an excited state.
 The excited stage is very unstable and the electron loses the excess energy and falls back to its ground state almost
immediately.
 The excess energy that is lost can be:
a) Lost as heat
b) Emit light as electrons fall from an excited to ground
state called fluorescence
c) If within a chloroplast, the excited electrons can be
passed to a neighboring molecule called the Primary
electron acceptor (this is the first step in the light
reaction).
 All the pigments in the thylakoid membrane are
actually in clusters of 200-300 molecules.
 Reaction center: only one chlorophyll a molecule actually donates excited electrons to the primary electron acceptor
to trigger the light reactions.
 Chlorophyll a pigments and the other pigment molecules function together as light-gathering antennae that absorb
photons and pass the energy along the pigment molecules until it reaches the reaction center.
 Photosystem: combination of the antenna pigment molecules, the reaction center, and the primary electron
acceptor.
 Two types of photosystem:
a) Photosystem II
Note: they were named for the order in which they were discovered and not for the
b) Photosystem I
order in which they occur
 Photosystem II:
- The chlorophyll a molecule of the reaction center is called P680 because the wavelength of light it absorbs best is
680 nm (orangish red).
- Absorbs light energy (8 photons) and excited electrons pass from the reaction center chlorophylls to the primary
electron acceptors.
- Then each primary electron acceptor molecule is oxidized as it loses an electron down the Electron Transport
Chain (ETC).
- The energy lost down the electron transport chain is used to make ATP.
- Photolysis: lost electrons get replaced from the splitting of H2O leaves two H+ and ½ O2.
- Oxygen atom immediately combines with a second oxygen atom to form O2 that then diffuses out of the plant cell
and leaves through stomata (openings on the bottom of the leaf).
 Photosystem I:
- The reaction center is called P700 because the wavelength it absorbs best is red light with a wavelength of 700
nm.
- Absorbs light energy (8 photons) and excited electrons pass from the reaction center chlorophylls to the primary
electron acceptors get oxidized as it loses an electron down the ETC.
- The energy lost is temporarily stored in the coenzyme NADPH.
- Reaction: NADP+ + H+ +2e-  NADPH
- Every molecule of NADPH formed requires 2 electrons
- Photosystem I gets its electrons from the bottom of the first ETC.
 Products are:
1) ATP- formed by chemiosis/photophsphorylation
2) NADPH
Formed by redox reactions in the thylakoids
3) O2
 Equation for the light reaction:
H2O + ADP+ (P)i +NADP+ +2e- + H+ +light
½ O2 +2H+ +2e- + ATP +NADPH (not
balanced)
 Energy released during the ETC drives H+
from the stroma into the lumen of the
thylakoid generates a photon gradient
with potential energy to drive ATP synthesis.
Get Out of the jail Cell
Direction of H+ pump
Get Into the Photograph
 Cyclic Photophosphorylation:
 Much simpler
pathway to generate
ATP.
 Electrons in
Photosystem I are
excited like normal
and leave the
reaction center,
P7OO, only to be
passed along the ETC
of photosystem II
and returned back to
P700.
 At the end, ATP is produced.
 It’s not efficient as noncyclic because no NADPH is made.
 Use this method when there isn’t enough NADP+ around to accept electrons.
 Calvin cycle/ Calvin-Benson cycle, the light-independent reactions, dark reactions, or C-3 photosynthesis:
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Purpose: to make glucose
Happens in the stroma of the chloroplast
Require CO2 from the air, and ATP and NADPH
CO2= carbon source
Carbon fixation: Calvin cycle fixes CO2 from the air and converts it into carbohydrates.
The cycle must repeat 6 times in order to get a 6-Carbon glucose because it only takes in ONE CO2 with each spin.
It takes 3 CO2 / 3 pins to make one PGAL
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 PGAL+ PGAL= glucose
 During cycle:
1 CO2 enters combine with 1 5-Carbon RuBP with the aid of the enzyme rubiscoform 1 unstable 6-Carbon compound
quickly breaks down into 2 3-Carbons molecules called 3-PGA  consume 2ATP and 2NADPH energy (both oxidized)
2 3-Carbons PGAL/G3P formed  one of the PGAL leaves the cycle while other 5 remain 2 PGAL= 6-Carbon glucose 
remaining PGAL’s are cycled through a series of reactions and with the aid of ATP are converted back to the 5 carbon
RuBP continuing the cycle.
 2 PGAL could also have been used to make other sugars like sucrose or fructose.
 Many glucose molecules could be formed and linked together to form polysaccharides like starch or cellulose.
 Reactions don’t occur spontaneously.
 Enzyme catalyzed every product in photosynthesis and sometimes organic coenzymes like NADPH and metal-ion
cofactors were involved as well.
 Summary Calvin Cycle reaction:
6CO2 +18 ATP+ 12 NADPH  C6H12O6 +18 ADP+ 18 Pi +12 NADP+ + 24 e- + 12 H+
C3 plants: plants in which the Calvin cycle uses CO2 directly from the air and the first compound produced is the 3 carbon
compound—3PGA. Ex: rice, wheat, soybeans, and oats.
Photorespiration: on a hot day, C3 plants close their stomata reduce H2O loss (transpiration), but it also prevents CO2 from
entering and O2 from leaving. ∴ CO2 decreases in the plant and O2 increases. Rubisco fixes O2 instead of CO2, which yields no
glucose molecules. This decreases crop yields.
Photorespiration is believed to be an “evolutionary relic” when the atmosphere had much less O2 and more CO2 then it does
today.
C4 plants:
 Have adaptations to save water when the weather is hot and dry.
 Prevent photorespiration
 First product is a 4-carbon compound called oxaloacetate
 Keep their stomata closed most of the time thus conserving water while continuing making glucose by photosynthesis
 Have an enzyme called PEP carboxylase—fixes carbon into oxaloacetate. This enzyme can’t fix oxygen.
 Can still continue to fix carbon even when the carbon dioxide levels are low inside the plant.
 Oxaloacetate is converted into malate when acts as a carbon shuttle which donates CO2 to the Calvin cycle of a nearby
cell called bundle sheath cells—keeps making sugars even though the stomata are closed most of the time.
 Ex: corn, crabgrass, sugarcane—evolved in the tropics
 Crassulacean Acid Metabolism (CAM) plants:
 Ex: pineapple, cacti, and most of the succulent plants (with juicy tissues)
 Most are adapted to extremely dry climates (desert)
 Conserves water by opening it stomata and admitting CO2 ONLY at night.
 When CO2 enters its leaves, it is fixed into oxaloacetate just as was done in C4 plants.
 Fixes CO2 at night, releases it to the Calvin Cycle during the day--> keeps photosynthesis operating during the day even
though no more CO2 is entering.