Bio ch.11 notes
Photosynthesis
Photosynthesis is the process that converts light energy into chemical energy which occurs in plants, algae, certain other protists, and some prokaryotes.
Mode of nutrition:
a) Autotrophs (producers)
They make their own food to sustain themselves using light energy without eating anything derived from living organism producing CO2 and organic compounds where they are the source
of organic for all non-autotrophic organism. b) Heterotrophs (consumers)
They obtain their organic material from other organism by depending on photoautotrophs for food and O2, by either consuming the organism or the decomposing on the organism litter.
Note: The Earth’s supply of fossil fuels was formed from the remains of organisms that died
hundreds of millions of years ago so fossil fuels represent stores of solar energy from the distant past.
All the green parts of a plant have chloroplasts where the leaves are the major sites of photosynthesis in most plants.
Chloroplasts are found mainly in mesophyll cells where in light energy is absorbed by green pigment chlorophyll that resides in membranous sacs called thylakoid membrane which segregates in the stroma of the chloroplast.
The vein are used to deliver water from the roots to the leaves for photosynthesis and to deliver sugar to non-photosynthetic parts of the plant.
The stomata is used to exchange to enter CO2 and lets
oxygen out of the leaf and the epidermis protects the leaf.
6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O
At first scientist thought that the O2 was produced by the
breakdown of CO2 into C (that combines with water to make
sugar) and O2 until vin Niel proposed that all photosynthetic
organisms needs a hydrogen source which splits to provide a source of electrons, which was observed through bacteria that uses H2S instead of water where H2S was broken down into S
and sugar.
This was later proved by using a radioactive oxygen isotope (oxygen-18) in either carbon dioxide or water where the radioactive element was found in H2O when it was in water.
Photosynthesis is a redox process in which H2O is
oxidized and CO2 is reduced where it reverses the
direction of electron flow compared to respiration, it’s an endergonic process due to the increase in potential energy of the electrons as they move from water to sugar.
The stages of photosynthesis:
1. Light reactions (occurs in the thylakoid membrane) This step converts light energy to chemical energy in the form of two compounds NADPH and ATP.
It splits H2O providing a source of electrons and protons
and releasing O2 which then reduces NADP+ (nicotinamide
adenine dinucleotide phosphate), it generates ATP by
photophosphorylation.
Light, also called electromagnetic radiation, is a form of electromagnetic energy that travels in a rhythmic waves where its wavelength is the distance between crests of waves, which usually range from 10-5 nm (form gamma rays) to 103 nm (for
radio waves) in an electromagnetic spectrum
light behaves as though it consists of discrete particles that has a fixed amount of energy called photons where the amount of energy the photon is related inversely to the length of the wavelength such that the shorter the wavelength the more energy it has.
The sun radiates the full spectrum of electromagnetic energy and the atmosphere only allows visible light (380 nm-750 nm) to pass through while screening out the rest of the radiations. When light meet matter, it may be reflected, transmitted or absorbed
and substances that absorb visible light are called pigments which its color depends on the color that is most reflected and transmitted, and the rest of the wavelength is usually absorbed so chlorophyll is green because it absorbs and transmits green wavelength.
Spectrophotometer measures the ability of a pigment to absorb various wavelength of light, which directs a beam of light of different wavelength through a solution of the pigment and measures the fraction of the light transmitted at each wavelength.
An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength
where it provide clues to the relative effectiveness of different wavelengths for driving photosynthesis since light can perform work in the chloroplast only if it’s absorbed.
An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process by measuring the rate of the reaction over different wavelength.
The three types of pigments in the chloroplast are chlorophyll a, the key light capturing pigment, accessory pigment chlorophyll b, an accessory pigment called carotenoids.
The spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis since they are absorbed while green is the least effective pigment, which is confirmed by an
action spectrum that is prepared by illuminating chloroplasts with light of different colors.
The action spectrum is broader than the absorption spectrum due to the presence of accessory pigments with different absorption spectra that broaden the spectrum of colors that can be used for photosynthesis.
The slight structural difference between chlorophyll a and b is enough to cause them to absorb light at different wavelength (chlorophyll a appears blue green while chlorophyll b appears olive green)
Chlorophyll a and b absorbs short but high energy wavelength which makes a lot of this energy transmit as heat energy which can be damaging to the chlorophyll or it can interact with oxygen forming reactive oxidative molecules that are equally dangerous to the cell, so
the accessory pigment carotenoid is adapted to perform photoprotection where it absorbs and dissipate the excessive light energy.
Note: carotenoid in the human have a photo protective role which can be obtained through diet (ex. Carrot which is known for aid in night vision) unlike plants which synthesizes all the antioxidant they want.
When chlorophyll pigment (or any pigment in general) absorbs light (photon), it elevates from its ground state to high-energy unstable (like all high energy states) excited state, where the photons absorbed are those with energy to the energy difference between the ground state and the excited state which varies with different pigments thus each pigment will absorb photons with varying wavelength.
Since the excited state is unstable, the excited electrons will fall back to its ground state releasing the excess energy as heat and for some pigment (ex. Chlorophyll) light/photon afterglow called
fluorescence that appears red in the spectrum if chlorophyll was isolated from chloroplast.
Note: the fluorescence will be difficult to see against the green of the solution. Chlorophyll molecules are organized along with other organic molecules and proteins in a complex called photosystem which consists of reaction-center complex that is made of proteins holding a pair of special chlorophyll a pigment (they are only special due to their molecular environment which enables it to reduce the electron acceptor) and an electron acceptor, and light-harvesting complex consisting of different pigment molecules bounded to the protein which
enables the complex to harvest light energy at a larger surface area at a wider portion of the spectrum.
The energy from the photon will be absorbed by the pigment and is transferred from one pigment to another until reaching the special chlorophyll a where its electron will gets excited to higher level and the electron acceptor will reduce by accepting the excited electron of chlorophyll a.
This means the electron won’t be able to get back to its ground state and releasing the excess energy as heat and light, which why isolated chlorophyll will produce different observation than normal chlorophyll that converts the light energy to chemical energy.
The two types of photosystem in the thylakoid membrane are photosystem I and photosystem II where the only difference between them are their special chlorophyll a molecule where in PS I chlorophyll a called P700 absorbs wavelength of 700 nm while chlorophyll a called P680 in PS II absorbs wavelength of 680 nm.
Note: P680 and P700 are nearly identical but their association with different proteins in the thylakoid membrane affects their electron distribution.
The light reaction is divided into two process depending on the route the electron take, linear
and cyclic electron flow in which the reaction both synthesizes ATP and NADPH using light. In the linear electron flow:
1) The photon of light hits of the pigments molecule in the light-harvesting complex of PS II which excites its electrons to a higher unstable energy level, then as the electron falls to its ground state it releases the energy which excites the neighboring pigment molecules, this continues until the energy reaches the chlorophyll a in the reaction-center complex where it gets excited to a higher energy state.
Note: the photon of light excites the pigments of PS I and PS II at the same time.
2) The excited electron of P680 will be transferred to the primary electron acceptor making the resulting form of P680 is P680+
3) Since the P680+ is one of the strongest biological oxidizing agent known, an enzyme
catalyzes the splitting of the water molecule into 2 electrons that are supplied one by one to the P680+ each replacing an electron transferred to the electron acceptor, 2 H+ atoms
transferred into the thylakoid space and an oxygen atom which combines with another oxygen atom from another splitting of water molecule to form O2 that leaves the cell through
4) The excited electron moves through an electron transport chain that consists of an electron carrier called plastoquinone (Pq), a cytochrome complex, and a proteins called plastocyanin (Pc), where each component undergoes redox reaction as electron flow down the chain releasing free energy used to pump H+ into the
thylakoid space making a potential gradient across the thylakoid membrane 5) The potential gradient is then used to produce ATP in chemiosmosis.
6) The light energy also excite the pigments molecules in PS I making them keep transferring the energy to one another until it reaches to P700 in the reaction-centered complex where its excited electron would be accepted by the primary electron acceptor and then P700+ gets
reduced by gaining the electrons from the electron transport chain.
7) Electrons are then passed through a redox reactions from the primary electron acceptor to protein ferredoxin (Fd) down a second electron transport chain (it doesn’t produce a potential gradient so it doesn’t produce ATP)
8) Enzyme NADP+ reductase catalyze the transfer of the electrons from Fd to NADP+ to form
NADPH which needs 2 electrons to be reduced. Linear electron flow produces similar amounts of ATP and NADPH but for Calvin cycle to occur amount of ATP should be more than the amount of NADHP so the plant will undergo cyclic electron flow where it uses only PS I producing only ATP.
The electron cycle back to ferredoxin (Fd) to the
cytochrome complex then to plastocyanin molecule (Pc) to a P700 chlorophyll with no release of oxygen or production of NADPH.
Some organisms such as purple sulfur bacteria have PS I but not PS II making it depend on the cyclic electron flow only to generate ATP, this made scientists think that cyclic electron flow evolved before linear electron flow.
Note: Some organisms such as purple sulfur bacteria have PS I but not PS II so has the function of being photoprotective.
Note: as the hydrogen is pumped into the thylakoid space, the space pH decreases and becomes more acidic until the H+ travels down its electrochemical gradient through ATP
synthase which produces ATP in a process called photophosphorylation.
Mitochondria transfer chemical energy from food to ATP while chloroplasts transform light energy into the chemical energy of ATP.
In mitochondria protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix while in chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma.
Note: ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place.
2. Calvin cycle (occurs in the stroma)
The Calvin cycle forms sugar from CO2 using 9
ATP and 6 NADPH that’s why ATP is needed in more amounts than NADPH which applies the importance of the cyclic electron flow.
Note: Calvin cycle is called C3 cycle because the first main product of the cycle is a 3 carbon sugar, this cycle is used by C3 plants that live in moderate conditions.
The direct product of the Calvin cycle is a 3 carbon molecule called glyceraldehyde 3-phosphate (G3P).
a) Carbon fixation (the incorporation of CO2
into an organic molecule)
It’s the most important step, where each CO2 is
incorporated into the Calvin cycle by attaching itself to a 5 carbon sugar named ribulose bisphosphate (RuBP) which is catalyzed by RuBP carboxylase-oxygenase (rubisco) enzyme which is the most abundant protein on earth.
The 6 carbon sugar is highly unstable so it will quickly break down 2 carbon sugar called 3-phosphaglycerate per each CO2 molecule.
Note: each cycle fixates 3 CO2 molecules
b) Reduction
Each 3-phosphaglycerate receives a phosphate group from the breakdown of ATP becoming 1,3-Bisphosphoglycerate, then it’s lost when it is reduced by NADPH into G3P through the reduction of the carboxyl group on 1,3-Bisphosphoglycerate into an aldehyde group of G3P which stores more energy.
Then one molecule out of the six G3P molecule will be used by the plants as a starting material for metabolic pathways while the rest is used for the regeneration of RuBP where this step consumes 6 ATP and 6 NADPH.
c) Regeneration of CO2 acceptor (RuBP)
The carbon skeleton of the five G3P is rearranged into three RuBP molecule by consuming three ATP molecules.
The enzymes in the chloroplasts converts G3P into other many other organic compounds such glucose by combining two G3P molecules together, starch to store excess glucose in roots, tubers, seeds and fruits, sucrose by combining glucose and its isomer fructose together to transport the sucrose into other non-photosynthetic part of the plant or as cellulose by combining multiple glucose molecules in a chain.
