Tianyu’s Photosynthesis Notes:
Photosynthesis: “The conversion of light energy to chemical energy that is stored in
sugars or other organic compounds; occurs in plants, algae, and certain prokaryotes.”
10.1: The Process that feeds the Biosphere
-Entire Biosphere requires photosynthesis to sustain life
-Autotrophs are “self-feeders” and rely on only sunlight/nutrients/heat to survive and not
living things
-known as producers in an ecosystem
-nearly all plants are autotrophs, specifically ‘photoautotrophs’
-i.e. plants, protists (Euglina), mutlicellular algae (kelp), prokaryotes
(cyanobacteria), and other photosynthetic prokaryotes (purple sulfer bacteria)
-Heterotrophs are unable to make their own food, so they obtain organic food molecules
to gain energy from other living organisms
-known as consumers in an ecosystem
-some act as detritavores, and decompose the dead
-most fungi/prokaryotes are heterotrophs
Photosynthesis converts light energy to the chemical energy of food
“The process of photosynthesis most likely originated from a group of bacteria that had
infolded regions of the plasma membrane clusters of [photosynthetic enzymes and other
molecules]” securityfreak
Chloroplasts: site of photosynthesis in plants
-all green parts (of plants and other photosynthetic organisms) have chloroplasts
-found in cells of the mesophyll (contains 30-40 chloroplasts)
-chlorophyll is what makes the distinct green color
-helps convert solar  chemical energy
-many photosynthetic organisms also require CO2 to function (O2 exits)
-through microscopic passageways called stomata (on leaf)
-stroma is the dense fluid in the chloroplast used to surround the ‘thylakoids’
-thylakoids act as the machinery which converts the light energy to chemical energy
-interior called ‘thylakoid space’
-stacked in columns called ‘grana’
Tracking Atoms through Photosynthesis
Photosynthesis chemical equation:
Normal: 6CO2 + 12H2O + Light Energy  C6H12O6 + 6O2 + 6H2O
Simplified: 6CO2 + 6H2O + Energy  C6H12O6 + 6O2 (reverse of Cellular Respiration)
-both cellular respiration and photosynthesis occur in plant cells
Simplest: CO2 + H2O CH2O] + O2 (brackets mean that it isn’t a real sugar, but
represents a general formula for carbohydrate)
Splitting of Water
First discovery of how photosynthesis worked in plants was that:
-O2 is from H2O and not CO2
-the chloroplast splits the water into hydrogen and oxygen
-idea proposed by Van Neil of Stanford University since sulfur bacteria did it that
way too
Sulfur bacteria: CO2 + 2 H2S  [CH2O] + H2O + 2S
Plants:
CO2 + 2 H2O  [CH2O] + H2O + O2
General:
CO2 + 2 H2X  [CH2O] + H2O + 2X (X stands for element in area)
-20 years later discovered his hypothesis to be true by using a heavy oxygen isotope
-by using Oxygen-18 in the water molecules
Both photosynthesis and cellular respiration involve redox reactions
-cellular respiration involves electron transport chains, synthesizing ATP as electrons go
towards electronegative oxygen
-photosynthesis uses the opposite, and water is split and electrons go from the water ions
to the CO2 to create sugar, however, energy is required to do this, and that energy is
derived from light
The two stages of Photosynthesis
Light reactions (which convert solar energy to ATP, NADP+, and O2):
-it does this by splitting water, and then providing electrons and protons (energy)
-NADP+ temporarily stores the H+ ions
-uses solar power to reduce NADP+ to NADPH (by adding a pair of electrons and
H+)
-creates ATP during the process of photophosphorylation from ADP (by adding a
phosphate group)
Calvin Cycle (which “involves the fixation of atmospheric CO2 and reduction of the fixed
carbon into [a] carbohydrate.”):
-discovered by Melvin Calvin in 1940s
-first takes CO2 from the air by an autotrophic organism, called ‘carbon fixation’
-then reduces the CO2 to a carbohydrate by adding 2 electrons
-does this using the spare electrons from the Light reactions and energy
from ATP
-called “dark reactions” b/c they don’t directly require light to be performed
-however still requires the light reactions to power the cycle
-photosynthesis uses both reactions to create sugar and ATP to perform cellular functions
Light Reactions
and The Calvin
Cycle within a
plant Chloroplast
10.2: Light Reactions convert solar energy to the chemical energy of ATP and NADPH
The Nature of Sunlight
-light is a form of energy known as electromagnetic radiation
-wavelength determines the distance between the crests and the waves
-the entire range of electromagnetic radiation is known as the ‘electromagnetic spectrum’
-life on earth requires light wavelengths of 380 nm – 750 nm (known as ‘visible light’)
-sometimes light behaves as particles known as photons
-shorter wavelength = greater energy in each photon
-atmosphere blocks some of it
Photosynthetic Pigments: The Light Receptors
-light can be reflected, transmitted, or absorbed
-certain pigments absorb and reflect specific wavelengths of light
-i.e. white colored things have pigments that reflect all, black things absorb all
-spectrophotometers are used to measure the proportions of light in different wavelengths
-the range that a pigment can absorb is called the ‘absorption spectrum’
-chlorophyll has multiple pigments and is green b/c green is the least effective
wavelength, and also has pigments that absorb violet-blue and red light b/c it is most
effective in photosynthesis
-scientists can create a ‘action spectrum’ to graph and correlate different wavelengths and
their relationships to photosynthetic effectiveness
-‘chlorophyll b’ is olive green and ‘a’ is blue-green to absorb different wavelengths
-‘carotenoids’ help the photosynthetic cell absorb light that the pigments cannot
-also used in ‘photoprotection’ and stops the plant from absorbing too much light
Excitation of Chlorophyll by Light
-when the chlorophyll absorbs light, the electrons are excited into a higher energy level
and when the electrons “fall” back, the pigments emit energy as light or heat
-the energy levels can only go up in certain intervals, so only a photon with a certain
amount of energy will work with certain pigments
A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting
Complexes
A Photosystem is: “A light-capturing unit located in the thylakoid membrane of the
chloroplast or in the membrane of some prokaryotes, consisting of a reaction-center
complex surrounded by numerous light-harvesting complexes. There are two types of
photosystems, I and II; they absorb light best at different wavelengths.”
-is made up of a ‘reaction-center complex’, which acts as an electron acceptor
-also has a ‘light-harvesting complex’ which has ‘a’, ‘b’, and ‘carotenoids’
-a ‘primary electron acceptor’ does exactly what its name says
-uses the energy from light to boost an electron onto the ‘p.e.a.’ to create more energy
(only 10.1 and 10.2 up ‘til 193)