the process that feeds the biosphere

• plants and other photosynthetic organisms contain organelles called chloroplasts

• photosynthesis is the process that converts solar energy into chemical energy within chloroplasts

  • carotenoids (the pigment) produces the red, orange and yellow colours in plant

• directly or indirectly photosynthesis nourishes almost the entire living world

• autotrophs - self-feeders that sustain themselves without eating anything derived from other organisms

  • producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules

  • almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules

• photosynthesis occurs in plants, algae, certain other unicellular eukaryotes and some prokaryotes

• heterotrophs - obtain organic material from other organisms

  • consumers of the biosphere

  • some eat other living organisms and others, decomposers, consume dead organic material or feces

  • almost all heterotrophs depend on photoautotrophs for food and O2

• earth’s supply of fossil fuels was formed from the remains of organisms that died hundreds of millions of years ago

• fossil fuels are being consumed faster than they are being replenished

• researchers are exploring methods of using the photosynthetic process to produce alternative fuels

photosynthesis converting light energy to food

• chloroplasts - structurally similar to and likely evolved from photosynthetic bacteria

• the structural organization of these organelles allows for the chemical reactions of photosynthesis

• leaves are the major locations of photosynthesis in plants

• chloroplasts are found mainly in cells of the mesophyll (the interior tissue of the leaf)

• each mesophyll cell contains 30-40 chloroplasts

• CO2 enters and O2 exits the leaf through microscopic pores called stomata

• a chloroplast has an envelope of 2 membranes surrounding a dense fluid called the stroma

• thylakoids are connected sacs in the chloroplast that compose a third membrane system

• lamella connects the thylakoids

• chlorophyll, the pigment that gives leaves their green color, resides in the thylakoid membranes

• photosynthesis is a complex series of reactions that can be summarized as the equation:

  6CO2 + 12H2O + light energy → C6H12O6 + 6O2 + 6H2O

• the overall chemical change during photosynthesis is the reverse of the one that occurs during cellular respiration

splitting of water

• chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product

photosynthesis as a redox process

• photosynthesis reverses the direction of electron flow compared to respiration

• photosynthesis is a redox reaction in which H2O is oxidized and CO2 is reduced

• photosynthesis is an endergonic process where the energy boost is provided by light

2 stages of photosynthesis

• photosynthesis consists of light reactions (the photo part) and the Calvin cycle (the synthesis part)

• the light reaction occurs in the thylakoids

  • splits H2O

  • releases O2

  • reduces the electron acceptor NADP+ to NADPH

  • (functionally the same as NAD and NADH)

  • generates ATP from ADP by photophosphorylation

• the Calvin cycle occurs in the stroma and forms sugar from CO2 using ATP and NADPH

• begins with carbon fixation, incorporating CO2 into organic molecules

the light reactions convert solar energy to ATP and NADPH

• chloroplasts are solar-powered chemical factories

• their thylakoids transform light energy into the chemical energy of ATP and NADPH

the nature of sunlight

• light is electromagnetic energy, also called electromagnetic radiation

• electromagnetic energy travels in rhythmic waves

• wavelength is the distance between crests of electromagnetic waves

• wavelength determines the type of electromagnetic energy

• the electromagnetic spectrum is the entire range of electromagnetic energy or radiation

• visible light consists of wavelengths (370nm to 750 nm) that produce colors we can see

• visible light also includes the wavelengths that drive photosynthesis

  • plants tend to like light on the ends of the spectrum (red or violet)

  • photon energy is used from those two ends of the spectrum

• light also behaves as though it consists of discrete particles called photons

• the majority of plants are green bc it uses everything but the green waves/photons so they get reflected back to us

  • in autumn, leaves are reddish orange because those photons provide less energy and since there is already not enough energy from the sun at that it, it will prefer using waves on the shorter sides ( more energy)

photosynthetic pigments: the light receptors

• pigments are substances that absorb visible light

• different pigments absorb different wavelengths

• wavelengths that are not absorbed are reflected or transmitted

• leaves appear green because chlorophyll reflects and transmits green light

• a spectrophotometer measures a pigment’s ability to absorb various wavelengths

• this machine sends light through pigments and measures the fraction of light transmitted at each wavelength

• an absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength

• there are 3 types of pigments in chloroplasts

  • chlorophyll a - the key light-capturing pigment

  • chlorophyll b - tan accessory pigment

  • carotenoids, a separate group of accessory pigments

    • do not produce as much sugar as chlorophyll

  • a and b differ by a functional group

  • the action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

• the action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann

• in his experiment, he exposed different segments of filamentous alga to different wavelengths

• areas receiving wavelengths favorable to photosynthesis produced excess O2

• he used the growth of aerobic bacteria clustered along the alga as a measure of O2 production

• the action spectrum for photosynthesis is broader than the absorption spectrum of chlorophyll

• accessory pigments such as chlorophyll b broaden the spectrum for photosynthesis

• the difference in the absorption spectrum between chlorophyll a and b is due to a slight structural difference between the pigment molecules

  • in chlorophyll a - methyl group

  • in chlorophyll b - carbonyl group

  • porphyrin ring - light absorbing head of molecule with magnesium at the center

  • hydrocarbon tail - interacts with hydrophobic regions of proteins inside the thylakoid membranes of chloroplasts

  • magnesium in the middle gives off electrons when light hits it which travel to the other parts of the molecule (replenishing electrons come from H2O)

• accessory pigments called carotenoids may broaden the spectrum of colors that drive photosynthesis

• some carotenoids function in photoprotection; they absorb excessive light that would damage chlorophyll or react with oxygen

a photosystem

• a photosystem consists of a reaction-center complex surrounded by light-harvesting complexes

• the reaction-center complex is an association of proteins holding a special pair of chlorophyll a molecules and primary electron acceptors

• the light harvesting complex consists of pigment molecules bound to proteins

• light-harvesting complexes transfer the energy of photons to the chlorophyll a molecules in the reaction-center complex

• these chlorophyll a molecules are special because they can transfer an excited electron to a different molecule

• a primary electron acceptor in the reaction center accepts excited electrons and is reduced as a result

• solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

• there are 2 types of photosystems in the thylakoid membrane

• photosystem II (PSII) functions first

• the reaction-center chlorophyll a of PS II is called P680 because it is best at absorbing a wavelength of 680 nm

• photosystem I (PSI) - is best at absorbing a wavelength of 700nm

• the reaction-center chlorophyll a of PSI is called P700

linear electron flow

• during the light reactions, there are 2 possible routes for electron flow

  • cyclic - electrons get recycled

  • linear - electrons leave the system

• linear electron flow - the primary pathway involves both photosystems and produces ATP and NADPH using light energy

• there are 8 steps to linear electron flow:

1. a photon hits a pigment in a light-harvesting complex of PSII and its energy is passed among pigment molecules until it excites P680

2. an excited electron from P680 is transferred to the primary electron acceptor and becomes P680+

3. H2O is split by enzymes and the electrons are transferred from the hydrogen atoms to P680+ reducing it to P680

   1. P680+ is the strongest known biological oxidizing agent (oxidizes water)

   2. the H+ are released into the thylakoid space

   3. O2 is released as a by-product of this reaction

4. each electron “falls” down an electron transport chain from the primary electron acceptor of PSII to PSI. Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane

5. potential energy stored in the proton gradient drives production of ATP by chemiosmosis

6. in PSI, transferred light energy excites P700 which loses an electron to the primary electron acceptor

   1. P700+ accepts an electron passed down from PSII via the electron transport chain

7. each electron “falls” down an electron transport chain from the primary electron acceptor of PSI to the protein ferredoxin (Fd)

8. NADP+ reductase catalyzes the transfer of electrons to NADP+, reducing it to NADPH

   1. the electrons of NADPH are available for the reactions of the Calvin cycle

   2. the process also removes an H+ from the stroma

• the energy changes of electrons during linear flow through the light reactions can be shown in a mechanical analogy

cyclic electron flow

• in cyclic electron flow, electrons cycle back from Fd to the PSI reaction center via a plastocyanin molecule (PC)

• cyclic electron flow uses only photosystem I and produces ATP but not NADPH

• no oxygen is released

• some organisms such as purple sulphur bacteria and cyanobacteria have PSI but not PSII

• cyclic electron flow is thought to have evolved before linear electron flow

• cyclic electron flow may protect cells from light-induced damage

a comparison of chemiosmosis in chloroplasts and mitochondria

• chloroplasts and mitochondria generate ATP by chemiosmosis but use different sources of energy

• mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP

• spatial organization of chemiosmosis differs between chloroplasts and mitochondria but shows similarities

• in mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix

• in chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma

• ATP and NADPH are produced on the side facing the stroma where the calvin cycle takes place

• in summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH

the calvin cycle uses ATP and NADPH to reduce CO2

• the calvin cycle, like the krebs cycle regenerates its starting material after molecules enter and leave the cycle

• the calvin cycle is anabolic; it builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH

• carbons enter the cycle as CO2 and leave as G3P

• for net synthesis of one G3P, the cycle must take place 3 times, fixing 3 molecules of CO2

• the calvin cycle has 3 phases

1. carbon fixation (catalyzed by rubisco)

   1. adding carbon onto a molecule

2. reduction

3. regeneration of the CO2 acceptor (RuBP)

   1. rubisco is an enzyme that performs carbon fixation on RuBP

• ATP, ADP and NADPH are produced

• must go through the cycle 6 times for one glucose molecule to be made

  • 6 CO2 used per glucose

alternative mechanisms of carbon fixation

• dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis

• on hot, dry days, plants close their stomata which conserves H2O but also limits photosynthesis

  • plants will make ATP and NADH during the day

  • at night, stomata will open and take CO2 to do the calvin cycle

  • cacti are acidic because they store CO2 as organic acid

• the closing of stomata reduces access to CO2 and causes O2 to build up

• having a lot of O2 causes an apparently wasteful process called photorespiration

photorespiration

• in most plants (C3 plants), initial fixation of CO2 via rubisco forms a 3-carbon compound called 3-phosphoglycerate

• because rubisco is such an old enzyme, it cannot tell the difference between O2 and CO2

• in photorespiration, rubisco adds O2 instead of CO2 in the calvin cycle, producing a 2-carbon compound

• photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

  • causes the whole cell to shut down

• photorespiration may be an evolutionary relic because rubisco evolved at a time where O2 barely existed

• photorespiration limits damaging products of light reactions that build up in the absence of the calvin cycle

• in many plants, photorespiration is a problem because on a hot, dry day, it can drain as much as 50% of the carbon fixed by the calvin cycle

C4 plants

• C4 plants minimize the cost of photorespiration by incorporating CO2 into 4-carbon compounds

• there are 2 distant types of cells in the leaves of C4 plants

  • bundle sheath cells - arranged in tightly packed sheaths around the veins of the leaf

  • mesophyll cells - loosely packed between the bundle sheath and the leaf surface

• sugar production in C4 plants occur in a 3 step process

1. the production of the 4-carbon precursors is catalyzed by the enzyme PEP carboxylase in the mesophyll cells

   1. PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when concentrations are low

2. the 4-carbon compounds are exported to bundle sheath cells

3. within the bundle-sheath cells, they release CO2 that is then used in the calvin cycle

• PEP carboxylase fixes CO2 with pyruvate (3C) to make malic acid (4C)

• in the C4 pathway, the PEP carboxylase and the calvin cycle occur in different cells to prevent the rubisco from having access to O2 (stops photorespiration)

• since the industrial revolution in the 1800s, CO2 levels have risen greatly

• increasing levels of CO2 may affect C3 and C4 plants differently, perhaps changing the relative abundance of these species

• the effects of such changes are unpredictable and a cause for concern

• suitable agricultural land is decreasing due to the effects of climate change, while the world demand for food continues to increase

• C4 photosynthesis uses less water and resources than C3 photosynthesis

• scientists have genetically modified rice, a C3 plant, to carry out C4 photosynthesis

• they estimate 30-50% increase in yield compared to C3 rice

CAM plants

• some plants including succulents use crassulacean acid metabolism (CAM) to fix carbon

• CAM plants open their stomata at night, incorporating CO2 into organic acids that are stored in the vacuoles

  • like pineapples

• stomata close during the day and CO2 is released from organic acids and used in the Calvin cycle

• the CAM pathway is similar to the C4 pathway in that they both incorporate CO2 into organic intermediates before it enters the calvin cycle

• in the CAM pathway, these steps occur in the same cell but are separated by time

life depends on photosynthesis

• the energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds

• sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells

• plants store excess sugar as starch in chloroplasts and other structures such as roots, tuber, seeds and fruits