mod 8 chap 8

Question 1

Photosynthesis

Answer 1

mjor entry points for energy into biological systems
it is the source of all food we eat, both directly when we consume plants and indriectly when we consume aniamks

photosyntehsis is also the source of all the oxygen that we breathe - as well as the fuels we use for heating, eelctricty generation and transportation

fossil fuels are legacy of ancient photosynthesis: oil has its origin in the bodies of marine algae and the organisms that grazed on them, wheeras coal reps the geologic remains of terrestrial palnst

Question 2

Photosynthesis is widely distributed

Answer 2

most evident to us is carried out by plants on land
trees, grasses, and shrubs are all ex of photosynthesc organisms
photosynthesis is carried out by prokaryotic as well as eukaryotic organissm on land and in sea
apporx 60% of global photosnthesis is carried out by terrestrial organisms, with the remainng 40% taking place in the ocean
the vast majority of photosyntehsis in the ocean is carried out by unicellular organisms
abt half of oceanic photosynthesis is carried out by single celled marine eukaryotes while otehr haldf i s carried out by photosyntehic bacteria
collectievly these organisms are called phytoplankton

photosyntehsis takes place almost eevrywhere sunlight is available to serve as a source of energy
in the ocean photosyntehsis occurs in the surface layer extending to abt 100m deep
on land, photosyntehsis occurs most readily in envrinments that are both moist and warm
tropical rainforests have high photosynteic productivty as do grasslands and ofrests in temperate zones

photosyntehic organisms have evolved adaptations that allow them to tolerate a wide range of enrinmental conditions
in dry regions, a combo of photosyntehic bacteria and unicellular algae forms an easily disturbed layer on the surface of teh soil known as the desert crust

photosynteic bacteria are also found in extreme heat
at otehr extreme, unicellualr algae can grow on suraces of glaciers causing snow to appear red

Question 3

2 stages of photosynthesis

Answer 3

2 processes
first stage is light capture in hwihc energy from sunlight is captured into usable chem forms
the second stage is carbon fixation in which this energy is used to synthesize carbs from co2
fixation is the process by which inroganic molecuels like co2 are converted into organic moelcules like carbs

carbs can be used as a starting point for the synthesis of other organic molecules
carbs are also considered fuel moelcules because they store energy which they can later release in cellular respiration

light capture begins with teh absoprtion of sunlight by pignents, moelcules that absorb some or all wavelengths of visible light
chlorophyll is principle pigment in photosyntehsis
light energy absorbed by chlrophyll drives e- throughh the photostnteic e- transport chain
the photosntehic e- transporat chain consists of a series of redox rcns - light eenergy absorbed by chlorophyll is used to power the movement of e-
in plants, algae and some photosyntehic bacteria, e- come from water, releasing oxyegn gas as a byproduct
the movement of e- through the e- chain leads to synthesis of nicotinamide adenine dinucleotide phoshate (NADPH) and ATP
bc these rxns are powered by sunlight they are sometimes refefered to as light rxns or light dependent rxns

like the e- transport chain in cellular respiration, the photosynetic e- transporta chain takes place in specilized membranes, trasnfers of e- from e- donor to acceptor hand harnesses the energy of e- trasnfoer to pump protons and syntehisze ATP
in cellular repsration, both the e- and the energy come from organic molecules and the final e- acceptor is oxygen which is reduced to water
in photosynethsis, the e- come form water and enegry comes from sun; the final e- acceptor is NADP+ which is reduced to NAPDH
the eneergy in NADPH and ATP is then used to build carbs from carbon dioxide
this means that light cpature and carbon fixation are linked by transfer of energy in form of NADPH and ATP
carbon fixation in plants, alage, and some photosyntheci bacteria involves series of chem rxns known as calvin cycle
in calvin cycle, carbs are produced usuing co2 are carbon source
calvin cycle doesnt usue sunlight directly - called light idnependent or dark rxns of photosyntehsis
pathway cannot operate without the enegry input provided by stead supply of NADPH and ATP produced in the light rxns
in a photosyntehic cells, the calvin cycle occurs in the light when the prdicts of the light rxns are produced

overall photosyntehsis is redox rxns
rduction rxns are when molecule accepts e- and gains energy wheereas oxidation is moelcule losing e- and releaisng energy
in photosnthsis co2 are reduced to form high energy carbs
in many photosynteic organisms, the e- used to redcue co2 come from water, rpdocuing oxygen as by product
oxidation of water is linked with redcution of co2 through the trasnfer of energy and e- by NADPH plus additional eenrgy from ATP
the source of enegry is sunlight
photosyntehsis is cellular respiration in reverse
in photosyntehsis, water occurs on both sides of eqn because of its role as an e- donor in light capture and as a by product of carbon fixation
can demonstrate that water is source of oxyegn released during the photosynetssi using isotopes which are same elemnts with diff number neutrons and tehrefore distinguished on the basis of their mass

Question 4

photosyntehsis in chlrosplasts of eukaryotes

Answer 4

photosyntehsis is carried out by many diff types of organisms
photosyntehsis in bacteria is remarkably diverse
in photosyntehic eukaryotes, both light capture and carbon fixation take place in organelles called chlroplasts
chloroplasts resemble mitochondria including the presence of internal emmebrane where e- transprot chain takes place

chlorplasts are bounded by two emmebranes
in the center of chlropslast is third highly folded memrbane that encloses a fluid filled space called lumen
the photosnetic e- transprot chain is this memrbane
the entire strcuture is called thylakoid
thylakoid is folded into grana
the complex folding of thylakoid membrane enhances light cpature because it rpovides a great deal of surface area for the components of photosyntehci e- trasnport chain
the region between inner mmemrbane and thylakoid membrane is called stroma
carbon fixation happens in the stroma

photosyntegic organisms are correctly described as autotrophs because they can form carbs from co2
they also need a constnatn supply of ATP to meet each cells energy needs
ATP is produced by photosyntehsis but only carbs can be exported from chrlosplasts
this explains why cells have mitochdonria as well
mitochodnria, teh carbs are broken down to generate ATP
celluylar respration is tehrefore one of several featrues that heterotrophic organisms share with photosyntehci organisms

Question 5

light dependent rxns

Answer 5

for sunlight to power the calvin cycle, cells must be ble to use light energy to produce both NADPH and ATP
in photosynthesis, light energy absorbed by pigment molecules drives the flow of e- through the photosyntehic e- transport chain and this flow of e- leads to teh formation of both NAPDH and ATP

Question 6

Chlorophyll is major entry point of light energy in photosynthesis

Answer 6

The sun produces a broad spectrum of electromag radiatio
each pint on the spectrum has diff energy level and corresponidng wavelength
visible light is the portion of the spectrum that we can see and it incldues range of wavelengths used in photosyntehsis
waveelnths of visible light range from 400nm-700nm
approx 40% of suns energt that reaches the earths surface is in this range

pigments are molecuels that absorb some wavelenths of visible light
pigment will look coloured because the relfect light in the wavelength they dont absorb
leaves appear green bc chlrophyll, the photosyntehic pigment, is poor at absorbing green wavelengths

chlorophyll consists of a lightasborbing region that contains a magnesium atom at its center and a along hydrocarbon side chain
the number of alternating single and double bonds surrounding the magnesium atom creates the overalpping of e- orbitals that allow chlorophyll to asborb visible light
chloroiphyll is not free to move throughout the chloroplast - its precisely posiitioned within integral membrane proteins
these protein pigment complexes work together as photosystems which are the structural and functiinal units that absorb light energy and drive e- transport
how chlrophyll is arranged in these protein pigment complexes is critical for efficient light capture

chlorophyll comes in several diff forms
small differences in structure between diff types results in differnces in their light absorbing properties
chlorophyll a is found in all photosyntehic eukaryotes and cycnanobacteria
green alage and land plants also produce chlrophyll b

photosystems contain pigments other than chrlophyll called accesory pigments
the most known are the orange yellow carotenoids which can absorb wavelenths of visible light that are poorly absorbed by chlrophyll
the presence of accesory pigments allows photosyntehic cells to absorb broader range of wavelenths than would be possible with chlriphyll alone
carotenoids also play a aorle in protecting the photosynthci e- transport chain from damage

Question 7

Antenna chlorophyll passes light energy to rxn centers

Answer 7

When chrlophyll absorbs light, one of its e- is elevated to a higher energy state - what happens next is dependents on how the chlrophyll is situated
when chlroiphyll is extracted from chloroplasts in a lab, it behaves like pigments in clothes or paint - light energy is absorbed and then rleeased as heat as the e- retruns to its intital state - with flueorecent pigments like chlrophyll, a small amount of absorbed energy is reemitted as light (fluorescence)

by constrast when chlrophyll is present in chlroplast of cell it can trasnfer eenrgy to an adjacent chlrophyll instead of losing the enegry as heat
energy is still released as teh e- returns to its ground state but the release of this energy raises the energy lvek of an e- in adjacent chrlophyll
this mode of energy transfer is effecient (little enegry lost as heat) but requires chrlophylls to be positioned at just the right distance one from anotehr in a photosynetsem
a chloropyll molecule that sborbs light enegry and passes it along to another chrlophylkl molecule is an antenna chrlophyll becasue it acts like an antenna for icnoming enegry

energy is trasnfered between antenna chrlophylls until it reaches a specially configured pair of chlrophyll molecules known as the rxn center
at rxn center the e- itself, not just the energy is trasnfered to an adjacent moelcule
rxn center chlorphylls have config distint from that of the antenna chrlophylls
this allows rxn center that has received eenrgy either by directly absorbing light or more commonly from anntenna chrlophyll, to trasnfer an e- to adjacent molecule
when this happens, the rxn center is oxidized and ajdecent e- acceptor molecyle is reduced
this e- trasnfer initiates a light driven chain of redox rxns that leads ultimaetly to formation of NADPH

once the rxn center has lost an e- it can no longer absorb light or cotnribute additional e- so for the photosynethic e- trasnprot chain to continue, another e- must be delivered to take its place - these replacement e- coem from water

division of labor among chrlophyll moleucles was discovered in 1940s by emerson and arnold who showed that only small fraction of chrlophyll molecules is directly involved in e- transport
most of the chlorophyll in thylakoid function as an antenna, funneling enegry to rxn centers
antenna chrlophylls allow the photosyntehic e- transprt chain to operate efficently
without the antenna chrlophylls to gather light energy, rxn centers would sit idle much of time even in bright sunlight

Because the number of chlorophyll molecules in their flasks was much greater the number of molecules produced per flash, Emerson and Arnold concluded that many chlorophyll molecules are needed to produce one molecule and that each photochemical unit must contain many chlorophyll molecules.

Question 8

The photosyntheic e- trasnprot chain connects two photosystems

Answer 8

water is an ideal source of e- for photosyntehsis
water is abundant in cells so its always available to be the e- donor
in addition, o2, the by product of pulling e- from water, diffuses readily away
water is a challenging e- donor bc it takes a great deal of eenergy to pull e- from water
the amount of energy that a single photosystem can capture usefully from sunlight isnt enough to allow e- to be pulled from water when the rxn center is oxidized and to produce an e- donoor capatble of reducing NADP+ to NADPH
the solution to this is using two photosystems with distinct chem properties arranged in series
the two photosystems are called photosystem I and photosystem II based on the order in hwich they were found
e- flow from photosystem II to I along the e- transprt chain
the eenegry captured by photosystem II allows e- to be pulled from water; the enegry captured by photsosytem I allows e- to be trasnfered to NADP+ to form NADPH

if u follow the flow of e- from water through both photosystems and on to NADP+ you can see large inc in energy as the e- pass trhough each of the two photosystems
you can see a dec in energy as the e- move between the two photosystems
this dec in energy explains why e- move in one direction trhough the photosysnteic e- transport chain
running these rxns in the opp direction would require an input of energy
this up down up pattern is called z scheme

the major protein complexes of the photosynetic e- trasnprt chain include photosystem II and photosystem I as well as the cytochrome b6f complex in between tehm
small mobile compoudns convery e- between the protein compelxes: plastoquinone diffuses trhough membrane from photosystem II to cytb6f and plastocyanin difusses through the thylakoiid lumen from cytb6f to photosystem I

water donates e- to one end of teh photosynteic e- transport chain whereas NADP+ accepts e- at the other end
the enzyme that pulls e- from water, releasing both H+ and o2 is located on lumen side of thylakoid at photosystem II
NADPH is formed when e- are passed from photosystem I to a memrbane associated protein called ferredoxin on the stroma side of the thylakoid membrane
ferredoxin-NADP+ reductase then catalyzes teh formation of NADPH by trasnfering two e- form two molecules of reduced feeredoxin to NADP+ as well as proton from surrounding solution

Question 9

accumulation of protons in thyalkoid lumen drives synthesis of ATP

Answer 9

the calvin ccyle requires ATP
in chloroplasts, as in mitcohdonria, atp is syntehsized by atp synthase a transmembrane protein powered by proton gradient
the light driven movement of e- through the photosyntehtic e- transport chain causes protons to accumulate in thylakoid lumen
in chlroplasts, atp synthase is orientated such that the syntehsis of atp is coupled to movement of protons from thyalkoid lumen to the stroma
the process by which the energy of sunlight is harnessed to move e- leading to accumulation of protons and synthesis of atp called photophosphrylation

two features of photosynteic e- transprt chain are responisble for buildup of protons in thylakoid lumen
frist the oxidation of water releases protons and o2 in lumen
second, pq and cytb6f produce a proton pump that moves protons from storma to lumen and function much like proton pumping in the e- transprt chain of cellular repsiratiion
together the mechanisms are powerful
when the photosysnetic e- transprit chain is operating at full capacty the cocn of protons in lumen can be more than 1000 times greater than the conc in the stroma

Question 10

cyclic e- transport inc the production of atp

Answer 10

the clavin cycle requires two moelcules of NAPDH and three moelcules of ATP for each co2 incorportaed into carbs
the movement of 4 e- through photosynteic e- transport chain which is needed to reduce two NADP+ does not transport enough protons into the lumen to produce the needed three atp
an additioanl pathway for e- that engage with proton pumping by p1 and cytb6f are needed

in cyclic e- transport, e- from photosystem I are redirected from ferredoxin back into the e- transport chain by being trasnfered to pq
because these e- eventually return to photosystem I this altnertive pathway is cyclic in constast to the linear movement of e- from water to NADPH

as the e- from ferredoxin are pcked up by pq, additional protons are transported from stroma to the lumen and as a result there are more protons in the lumen that can be used to drive syntehsis of atp by atp synthase

Question 11

calvin cycle

Answer 11

The calvin cycle is a series of enzymatic rxns that syntehsize carbs from co2
these rxns are divded into three phases:
- carboxylation in which co2 is added to a 5 carbon molecule
- reduction in which energy and electrons are trasnfered
- regeneration of the 5c carbon molecule needed phase 1

Question 12

incorportation of co2 is cataluyzed by enzyme rubisco

Answer 12

in first phase of calvin cycle, co2 is added to a 5c compoudn called ribuslose, 15, biphsphate RuBP
this is catalzyed by rubisco

before rubisco can act as carboylzase, RuBP and CO2 must diffuse into its active site - once active site is occupied, addition of co2 to RuBP proceeds spontaneoesly in the sense that no addition of eengry is required
the product is 6c cabron that breaks down to two moelcules of 3phophoglycerate (3PGA)

Question 13

NADPH is redcing agent of calvin ccyle

Answer 13

rubisco is responsible for the addition of the carbon atoms needed for the formation of carbs but rubisco alone doesnt increase teh amount of energy stored within the newly formed bonds
for this energy inc to take place, the c compounds formed by rubisco must be reduced

in calvin cycle, reduction of 3PGA involves two rxns: atp donates a phosphate group to 3PGA and NADPH transfers two e- and one proton which released one phosphate group
because two molecules of 3PGA are formed each time rubisco catalyzes inforption of one molecule of co2, two atp and two nadph are required for each moelcule of co2 inforpated by rubisco
nadph provides most of the enegry incropted in the bons of carb molecules produced by calvin cycle
atp still plays essential role in preping 3PGA for the addition of energy and e- form nadph

these enegrt transfer phases result in the formation of the 3 carbon dsugar moelcules g3p and dhap which are knonw as triose phosphates
triose phosphates are true products of the calvin cucle and the prinipal form of cabrs exported from teh chlropklast during photosytnetiss
however if every triose phosphate molecule produced by calvin cycle were exported form chlroplastm RuBP could not be regenrated and calvin cycle would grind to halt
most triose phosphate molecules must be used to regenrate RuBP
for every 6 triose phosphate moelcules prodicued only one can be withdrawn from calvin cycle

Question 14

regn of RuBP requires ATP

Answer 14

of chem rxns that makeup calvin cycle, most occur in last phase, regen of RuBP
these rxns rearrange the carbon atoms from five 3c triose phpsphate moelcules into three 5c RuBP
atp is required for the regen of RuBP raising the calin ccyles total energy reuqirements to two moelcules of NADPH and three molecules of ATP for each co2 incorportaed by rubisco
this 2:3 ration helps explain the function of the cyclic photophosphylation

Question 15

rxns of calvin cycle were identfiied using radioactive co2

Answer 15

In a series of experiments conducted between 1948 and 1954, American chemist Melvin Calvin and his colleagues Andrew Benson and James Bassham identified the carbon compounds produced during photosynthesis (Fig. 8.15). They supplied radioactively labeled to the unicellular green alga Chlorella and then quickly plunged the cells into boiling methanol, thereby halting all enzymatic reactions. In this short time, all of the carbon compounds of the Calvin cycle became radioactively labeled and could be identified by their radioactivity.

Additional experiments were required, however, to determine the chemical reactions that connected the labeled compounds. For example, by using a very short exposure to co2, Calvin and colleagues determined that 3-PGA was the first stable product of the Calvin cycle (experiment 1 in Fig. 8.15).
To determine what co2 reacts with as it enters the Calvin cycle, they first 14co2 supplied so that all of the components of the Calvin cycle became radioactively labeled. They then cut off the supply of before plunging the cells into boiling methanol. When they did this, the amount of RuBP increased relative to the amount observed in the first experiment. Based on this buildup of RuBP, they concluded that the first phase in the Calvin cycle was the addition of co2 to RuBP

Question 16

carbs are stored in form of starch

Answer 16

calvinc cyle is capable of producing more carbs than the cell needs or in a multicellular organism, more than the cell is able to export
if carbs were to accumulate, water would enter the cell and damge it
excess carbs are converted to starch, a storage form of carbs
bc starch forms insoluble granules, it provides a means of carb storage that doesnt lead to osmosis
staech produced during the day provides photosynthic cells with a source of carbs they can use during the night

cellular repsiration breaks down carbs in rpesence of o2 to supply energy needs of cell and prodces co2 and water as by products
by constrast photosynteis uses co2 and water in presence of sunlight to build carbs and releases o2 as by product

Question 17

Photosynthetic challenges

Answer 17

two major challenges prevent photosyntehsis from functioning efficiently
the first challenge is that if more light energy is absorbed than the calvin cycle can use, excess energy can damage the cell
the second challenge is that rubisco can catalyze the addition of either co2 or o2 to RuBP
the addition of oxygen instead of carbon dioxide results in loss of enegry and reduces the effeciency of photosynthsesi

Question 18

Excess light energy can damage cells

Answer 18

Photosynthesis is a dnagerous enterprise
unless photosynthtic rxns are carefully controlled, molecules will form that can damage cells by oxidizing lipids, proteins and nucleic acids indicriminetly

Under normal conditions, the photosynthetic electron transport chain proceeds in an orderly fashion from the absorption of light to the formation of NADPH. When NADP+ is in short supply, however, the electron transport chain “backs up” and leaves electrons and energy with no “safe” place to go. This greatly increases the probability of creating reactive forms of oxygen known collectively as reactive oxygen species. These highly reactive molecules can be formed either by the transfer of absorbed light energy from antenna chlorophylls to o2 or by the transfer of an electron to o2. Reactive oxygen species, however formed, can cause substantial damage to the cell.

NADP+ is returned to the photosynthetic electron transport chain by the Calvin cycle’s use of NADPH. Thus, any factor that causes the rate of NADPH use to fall behind the rate of light-driven electron transport can potentially lead to damage. Such an imbalance is likely to occur, for example, in the middle of the day when light intensity is highest.
Photosynthetic cells could speed up the resupply of NADP+ by synthesizing more Calvin cycle enzymes. This strategy, however, would be energetically expensive. When light levels are low, such as in the morning and late afternoon, Calvin cycle enzymes would sit idle. Thus, excess light energy is an everyday event for photosynthetic cells, rather than something that occurs only in extreme environments.

the rate at which teh calvin cycle can make use of NADPH is also infleucned by number of factors that are idnependent of light intensity
ex. at cold temps, teh enzymes of calbvin cycle function more slowly but temp has little impact on the asborption of light energy
on cold sunny day mroe light enegry is absorbed taht can be used by calvin cycle

photosyntehic organisms employ two lines of defense to avoid stesses that occur when the calvin cycle cant keep up with light harvesting
among these are antioxidant compounds that detoxify reatcive oxygen species
vitaminc and beta carotene and others neutralize the species
these compounds exist high in cocn in chlrosplasts
some of these antioxidant moelcules are brighjtly coloured
presence of antioxidant compoudns is one of many reasons why eating green vegetables if good for your health

A second line of defense is to prevent reactive oxygen species from forming in the first place. Xanthophylls are yellow-orange carotenoid pigments that slow the formation of reactive oxygen species by reducing excess light energy. These pigments accept absorbed light energy directly from chlorophyll and then convert this energy to heat, shown on the left in Fig. 8.17b. Photosynthetic organisms that live in extreme environments often appear brown or yellow because they contain high levels of xanthophyll pigments, as seen in Figs. 8.2a and 8.2b. Plants that lack xanthophylls grow poorly when exposed to moderate light levels and die in full sunlight.

Converting absorbed light energy into heat is beneficial at high light levels, but at low light levels it would decrease the production of carbohydrates. Therefore, this capability is switched on only when the photosynthetic electron transport chain is working at high capacity. How does this switch work? Recall that the photosynthetic electron transport chain leads to an accumulation of protons in the lumen. Xanthophyll pigments are activated by low lumen pH, such that their activity is coupled to the operation of the photosynthetic electron transport chain.

Question 19

Photorespiration leads to net loss of energy and carbon

Answer 19

second challeneg to effcoeny is rubiscos ability to use both co2 and o2 as substrates
if o2 instead of co2 diffuses into active site of rubisco, the rxn can still proceed although o2 is added to rubp instead of co2
an enzyme that adds o2 to another molecule is called oxygenase
rubisco is shorthand for rubp carboxylase oxygenase relfecting its abaility to catalyze two diff rxns

when rubsico adds o2 instead to rubp it results in a release of co2
this is refefred to as photorepsiration bc it requires light and consumes o2 while reelaisng o2
this consumes atp
atp is used to recycle the compounds formed when o2 combines with rubp
thuis photorepiration reps a net energy drain in two ways: it results in oxidation and loss in form of co2 of cabron atoms it already incorpoated and reduced by clvin cycle; second it consumes atp

the calbvin cycle orginated long ebfore the accumulation of oxygen in the atmosphere whihc provides epxlantion of why an enzyme with these proeprties might have initially evolved
the diffuclty is that for rubsico to favour the addition of co2 over o2 the enzyme must be highly selective and the price of high selectivty is speed
co2 and o2 are similar in size and chem strcuture and for this reason rubsico can acheive selctity by binding more tightly with transition ststa of cabroxylation rxn
as result the better rubisco is at disicrimating between co2 and o2 the slower its catalytic rate

the trade off between seletcivty and speed is constraint for photosyntehci organisms
rubiscos low cataltic rate means that photosynteic cells must produce huge amounts of the enzyme - why its the most abudant protein
at same time bc o2 is more abdunat than co2 as much as one quarter of reduced carbon formed in photosyntehis can be lost trhough photorepsiration

Question 20

Photosynthesis captures just a small precent of incoming solar enegry

Answer 20

typically only 1% to 2% of suns energy that lands on leafs ends up in carbs
photosntehsis is relevant to solving several pressing global issues: the reffects of rising co2 cocn; the search for reneable carbon neutral fuels to power trasnproationl; and agricultural demands for human pops

Photosynthetic efficiency is typically calculated relative to the total energy output of the sun (Fig. 8.19). However, only visible light has the appropriate energy levels to raise the energy level of electrons in chlorophyll. Most of the sun’s output (~60%) is not absorbed by chlorophyll and thus cannot be used in photosynthesis. In addition, leaves are not perfect at absorbing visible light—about 8% is either reflected or passes through the leaf. Finally, even under the best conditions, not all of the light energy absorbed by chlorophyll is transferred to the reaction center; some is given off as heat (also ~8%). This percentage increases when light levels are high because excess light is actively converted into heat by xanthophyll pigments.

the photosyntehic e- chain tehrefore cpatures at most 24% of suns enegry arrvinn at surface of leaf
although this number may appear low it is on par with efficny of high performance photovoltaic cells in solar panels
enegry is also lost at a later step
incorptaion of co2 into carbs results ine nergy loss- equiablent to 20% of teh total incoming solar radiation
some of this enegry loss is due to photorepriaton

in total the max enegry effcieny of photosyntehsis is calc to be 4%
effceincies acheived by relea plants are usually abt 1 to 2%

Question 21

evolution of photosynthesis

Answer 21

evolution of photosyntehsis has a profound impact on teh histroy of life on earth
it provides organissm with new source of energy and released oxygen into atmosphere

Question 22

abilitu to capture enegry from sunlight evelolved in steps

Answer 22

sunlight is valuable source of energy but it can cause damage
uc wavelenths can damge dna and other moelcules thus earliest inetrsactions with sun may have been evolution of uv absorbing compounds that could shield cells from suns damaging rays
overtime random mutations could have produced chem variants of tuv absorbing molecules
one or more vairnats may have been capable of using sun to meet energy needs perhaps by transfering e- to another moelcule as present rxn center does

first forms of light driven e- transpirt may have been coupled to net moevemnt of protons across the mmebrane allowing for the syntehsis of atp without requiring an e- donor
alt early rxn centers may have used light energy to drive moevement of e- from donor outside cell to an e- acceptor within the cell
in thsi way enegry from sun could have been used to syntehsize cabrs
the first e- donor could have been a soluble inorganic ion like reduced iron which is abudnat in early oceans

similarly its inliekly that these first photosynteic organisms used chlrosphyll as means of abrobing sun bc biosyntehic pathway for chlrophyll is compelx
some of inetrmeidate compounds leading to chrlosphylll are capabke of abrobing light
perhaps the intermediates were functional end product used as pigment by early photosnteic organism
the biosynteic pathway may have hained steps as chem variants produced by random mutations were selected bc they were more eeficnet or able to absorb new portions of visible spectrum
selection eventually resulted in chrlosphyll pigments that are used by photonstic organisms today

Question 23

the abiliuty to use water as e- donor in photosnteis evolved in cyanobacteria

Answer 23

the most ancient forms of photosyntis have only single photosyntem in their e- transport chain
a single photosyme cant capture enoughe enrgy to pul e0 from water and to raise their energy level so they can be reduced to co2
instead photosnetic rogansims with single system must use more easily oxidzed compoudns like hudrogen sulfide as e- donors
these organisms can only exist in enrviments where e- donros are abudant
bc tehs eorganism dont use water as e- donor they dont produce co2

major event was eveloutin of photosntheic e- transprt chains that use water as an e- donor
the first organisms to accomplish this feat were the cyanobacteria
these bacteria icprtaed two photosynstems into single e chain one to pull e- from water moelcules and one to raise the energy level of teh e- so that they can be used to reduce co2

hypotehis of that genetic material asociated with one photosyetm was tarnfered to abcterium that had the other photosystem resulting in single bacterium with geentic material to rpdocue both types of photosystems
anotehr hyoptehsis is that teh egentic material associated with one photsystem underwnet dupliction and they converged in sequence and fucntion igivng rise to two distnct but related photsystems

he ability to use water as an electron donor in photosynthesis had two major impacts on life on Earth. First, photosynthesis could now occur anywhere there was both sunlight and sufficient water for cells to survive. Second, water releases oxygen as it donates electrons. Before the evolution of oxygenic photosynthesis, little or no free oxygen existed in Earth’s atmosphere. All the oxygen in Earth’s atmosphere results from photosynthesis by organisms containing two photosystems.

Question 24

eukaryotic organisms are belived to have gained photosuyntesi by endosymbiosis

Answer 24

Photosynthesis is hypothesized to have gained a foothold among eukaryotic organisms when a free-living cyanobacterium took up residence inside a eukaryotic cell (right part of Fig. 8.20). Over time, the cyanobacterium lost its ability to survive outside its host cell and evolved into the chloroplast. The outer membrane of the chloroplast is thought to have originated from the cell membrane of the ancestral eukaryotic cell, which surrounded the ancestral cyanobacterium as it became incorporated into the cytoplasm of the eukaryotic cell. The inner chloroplast membrane is thought to correspond to the cell membrane of the ancestral free-living cyanobacterium. The thylakoid membrane then corresponds to the internal photosynthetic membrane found in cyanobacteria. Finally, the stroma corresponds to the cytoplasm of the ancestral cyanobacterium.

the proces in whcih one cell takes up residence inside another is endosymbiosis and the idea that chrlpslats and mitochodria arose in this way is called enodymsbuotic hypotehsis