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Chapter 7

BIOL 1000 Chapter Notes - Chapter 7: Competitive Inhibition, Plastocyanin, Cellular Respiration

Course Code
BIOL 1000
Nicole Nivillac

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Chapter 7: Photosynthesis
Why it’s Important:
o The vast majority of earth’s energy enters the biosphere through a process called
o In the process of photosynthesis chlorophyll and other related pigments capture photons of
light energy, this captured energy is used to convert carbon dioxide into complex energy rich
7.1- Photosynthesis: An Overview
o Primary Producers: photosynthetic organisms, also called photoautotrophs.
o Primary producers use the energy harnessed from the sun and convert it into chemical energy
so that they may assemble simple inorganic molecules into more complex molecular structures
o Consumers: the animals that consume plants or other animals as sources of nutrients
o Decomposers: use the dead bodies of plants and animals as chemical energy (i.e. bacteria,
The Two Parts of Photosynthesis:
o 2 distinct stages of photosynthesis: light-dependent reactions, and light- independent reactions
o Light-dependent reactions: also called light reactions. Pigment molecules capture light energy,
and then use this energy to synthesize NADPH and ATP. This is executed via the electron
transport chain, which uses electrons donated from water. The splitting of water produces a
byproduct of oxygen, which is then released into the environment.
Formula for Light- dependent: 2H2O + light energy 4H+ + 4e- + O2
o Light- independent reactions: also called Calvin cycle. CO2 fixation occurs during this stage, CO2
is converted from the inorganic to organic form using the electrons carried by NADPH and the
energy from ATP. The added electrons, the H+ protons, and the CO2 molecule combine to
create a carbohydrate with the chemical formula (CH2O)n. The major product of this stage is 3
carbon sugars which can combine to make a 6 carbon sugar (glucose).
Formula for Light- independent: CO2+ H+ + e- (CH2O)n
o Overall Equation of Photosynthesis: 6CO2 +12H20 C6H12O6 + 6O2 + 6H2O
In Eukaryotes, Photosynthesis Takes Place in Chloroplasts:
o Eukaryotes which undergo photosynthesis perform both light- dependent and light-
independent processes in the chloroplast
o Chloroplast: an organelle with 3 membranes which define 3 distinct sections
- Outer membrane: covers the entire surface of the organelle
- Intermembrane compartment: compartment between the inner and outer membranes
- Innermembrane: lies just inside the outer membrane
- Stroma compartment: aqueous environment within the innermembrane. Houses the
enzymes responsible for catalyzing the Calvin cycle
- Thylakoid membrane: flattened closed sacs which house the molecules that carryout the
light reactions of photosynthesis (i.e. pigments, electron transfer carriers, and ATP
synthase enzymes)

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- Thylakoid lumen compartment: space enclosed by the thylakoid
7.2- The Photosynthetic Apparatus:
o A sophisticated photochemical apparatus is require to trap light and convert it into chemical
Electrons in Pigment Molecules Absorb Light:
o 2 important points about light and pigment molecules
- 1. When a pigment molecule absorbs a photon of light, one electron will move from ground
state to an excited state
- 2. The difference in energy levels between ground and excited state are equivalent to the
energy of the absorbed photon of light. If it does not match the pigment will not be
o 3 possible fates of an excited electron within a pigment molecule
- 1. The excited electron may return to its ground state from the excited state, releasing
energy as either heat or florescence (emission of light at a lower energy than absorbed)
- 2. The excited electron returns to grounds state, and the subsequent energy is transferred
to a neighboring pigment molecule, a process called inductive resonance
- 3. The high energy, excited state electron is transferred to a near by electron accepting
molecule called a primary acceptor
Chlorophylls and Carotenoids Cooperate in Light Absorption:
o Chlorophyll: the main photosynthetic pigment in plants. The two most dominant types of
chlorophyll are chlorophyll a and b (structurally similar)
o Carotenoids: second major group of pigments involved in photosynthesis
o Chlorophyll a is the most dominant molecule as during photosynthesis only chlorophyll a
becomes oxidized and donates an electron the primary electron acceptor. Chlorophyll b and the
carotenoids are considered accessory pigments, as after light absorption they donate excitation
energy to chlorophyll a via inductive resonance
o Absorption spectrum: plot of the absorption of light as a function of wavelength.
- Chlorophyll a most readily absorbs blue and red light, and the accessory pigments act to
extend to range of wavelengths that can be successfully captured for photosynthesis
o Action spectrum: plot of the effectiveness of particular wavelengths in driving photosynthesis,
determined by using a suspension of chloroplasts and measuring the amount of oxygen
released at different wave lengths
o When an action spectrum for a physiological phenomenon matches the absorption spectrum of
a pigment it is likely the two are connected
Photosynthetic Systems are organized into Photosystems:
o Pigment molecules are precisely bound to a number of different proteins, they are not simply
floating around the thylakoid membrane. The pigment-protein combinations are called
o Each photosystem is composed of a large antenna complex, and a central reaction center

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- The antenna complex: also called the light harvesting complex and surrounds the central
reaction center
- The reaction center: comprises a small number of proteins that bind a pair of specialized
chlorophyll a molecules as well as the primary electron acceptor
o 2 distinctly different kinds of photosystems: photosystem I and II (also called PSI and PSII)
- Photosystem I: at the reaction center of this photosystem is the specialized chlorophyll a
called P700 (bc it optimally absorbs light with a 700 nm wavelength)
- Photosystem II: at the reaction center of this photosystem is the specialized chlorophyll a
- P700 and P680 are structurally identical to other chlorophyll a molecules, their specific
absorption comes from interactions with particular proteins in photosynthesis
- The function of a photosystem is to trap photons of light and use its energy to oxidize the
chlorophyll, with an electron being transferred to the primary acceptor
- High rates of oxidation- reduction within reaction centers is caused by large antenna
complex of pigments absorbing a wide range of wavelength and efficiently using the energy
in reactions, this is done by inductive resonance
7.3- photosynthetic Electron Transport:
o Photosystems I and II are two major light trapping components involved in photosynthetic
transport in most photoautotroph
o The electron transport system consists of non-protein organic groups that switch between
being oxidized and reduced as electrons move throughout the system
The structure and Function of Photosystem II:
o Oxygenic allows plants to use water as a source of electrons
o When plants use the water, they split it causing oxygen to be released into the environment
o Oxygenic photosynthesis is a result of photosystem II
o The sequence of light harvesting and photochemical events is as follows:
- 1. Photons are absorbed by the antenna complex and the energy is funneled into the
reaction center, this results in an electron within the P680 being raised to an excited state
from ground sate (P680*= notation for excited state)
- 2. The excited P680* can easily be oxidized to P680+, by the primary electron acceptor of
photosystem II (molecule called pheophytin)
- 3. P680 is reformed when P680* gains an electron by oxidizing water
o P680+ is the strongest oxidant known in biology
o The reduction of P680+ to P680 by electrons from water is facilitated by an enzyme subunit of
photosystem II called the water splitting complex
o The reaction center in photosystem II Is very susceptible to damage by reactive oxygen species.
The proteins from the photosystem II reaction center are constantly being irreversibly
damaged, this is countered by a very efficient and complex repair system
Linear Electron Transport:
o Electron transport and ATP synthesis systems for light reactions are organize in the thylakoid
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