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Photosynthesis Flashcards

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AP® Biology flashcards Biology 101 flashcards
1
Q

What is photosynthesis

A

-most life forms on Earth rely directly or indirectly on photosynthesis
-photosynthesis is a physiological process used by plants, algae and some types of bacteria to convert light energy from sunlight into chemical energy
-organisms can use this chemical energy to synthesise large organic molecules, which form building blocks of living cells from simple inorganic molecules such as water and carbon dioxide - this is autotrophic nutrition

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2
Q

Define photoautotrophs

A

-organisms that photosynthesise are called photoautotrophs because they use light as energy source for autotrophic nutrition
-these organisms are also described as producers because they are at beginning (first trophic level) of food chain and provide energy and inorganic molecules to other non photosynthetic organisms

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3
Q

What is photosynthesis

A

-6CO2+6H2O+energy from photons_CHLOROPHYLL_ - C6H12O6+6O2
-a photon is a particle of light; each photon contains a quantam of energy
-the main product of photosynthesis is a monosaccharide sugar which can be converted to disaccharides for transport and then to starch for storage
-photosynthesis is an example of carbon fixation- process by which carbon dioxide is converted to sugars
-the carbon for synthesising all types of organic molecules is provided by carbon fixation

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4
Q

Describe carbon fixation

A

-carbon fixation is endothermic and so need energy
-carbon fixation also need electrons; the addition of electrons is a reduction reaction
-carbon fixation helps regulate the concentration of carbon dioxide in the atmosphere and oceans

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5
Q

What is respiration

A

-plants and other organisms that photosynthesise also respire
-during respiration they oxidise organic molecules that they have previously synthesised by photosynthesis and stored, releasing chemical energy - can drive the organisms metabolism
-during respiration, glucose and other organic compounds are oxidised to produce carbon dioxide and water
C6H12O6+6O2 - 6H2O+6CO2+energy

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6
Q

What are heterotrophs

A

-non-photosynthetic organisms such as fungi, animals, many protoctist and many types of bacteria are described as heterotrophs
-they obtain energy by digesting complex organic molecules of food to smaller molecules that they can use as respiratory substrates
-they obtain energy from the products of digestion by respiration

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7
Q

How do photosynthesis and respiration interrelate

A

-both photosynthesis and aerobic respiration are important in cycling carbon dioxide and oxygen in the atmosphere
-the products of one process are the raw materials of the other; aerobic respiration removes oxygen from the atmosphere and adds carbon dioxide while photosynthesis does the opposite
-sunlight energy - photosynthesis in autotrophs - carbohydrate+O2 - respiration in living organisms - CO2+H2O

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8
Q

Define compensation point

A

-plants respire all the time
-however, they only photosynthesise during daylight
-plants often compete for light
-the intensity of light has to be sufficient to allow for photosynthesis at a rate that replenishes the carbohydrates stores used up by respiration
-when photosynthesis and respiration proceed at same time, there is no net gain or loss of carbohydrates, the plant is at its compensation point

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9
Q

Describe the compensation period

A

-the time a plant takes to reach its compensation point is called the compensation period
-the compensation period is different for different plants
-shade plants can utilise light of lower intensity than sun plants can
-when exposed to light after being in darkness, shade plants reach their compensation point sooner (shorter compensation period) than sun plants, which require a higher light intensity to achieve their optimum rate of photosynthesis

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10
Q

Define granum

A

-inner part of chloroplasts made of stacks of thylakoid membranes where light dependent stage of photosynthesis takes places

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11
Q

Define photosynthetic pigment

A

-pigment that absorbs specific wavelengths of light and traps energy associated with light
-such pigments include chlorophyll a and b, carotene, xanthophyll

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12
Q

Define photosystem

A

-system of photosynthetic pigments found in thylakoids of chloroplasts
-each photosystem contains about 300 molecules of chlorophyll that trap photons and pass energy to primary pigment reaction centre, a molecule of chlorophyll a during light dependent stage of photosynthesis

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13
Q

Define stroma

A

-fluid filled matrix of chloroplasts where light independent stage of photosynthesis occurs

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14
Q

Define thylakoid

A

-flattened membrane bound sac found inside chloroplasts
-contains photosynthetic pigments/ photosystems and is site of light dependent stage of photosynthesis

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15
Q

Describe the structure of chloroplasts

A

-chloroplasts are the organelles within plant cells where photosynthesis occurs
-most plant chloroplasts are disc shaped and around 2-10um long
-each is surrounded by a double membrane, the envelope, with an intermembrane spaces of width 10-20nm between inner and outer membrane
-there are two distinct regions visible on electron micrograph inside chloroplasts: the fluid filled matrix called stroma and grana that consists of stacks of thylakoid membranes

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16
Q

What is a grana

A

-the first stage of photosynthesis, light dependent stage, takes place in grana
-chloroplasts have three distinct membranes - outer, inner and thylakoid giving three separate internal components - the intermembrane space, stroma and thylakoid space
-the thylakoids within a granum may be connected to thylakoids within another granum by intergranal lamellae

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17
Q

Describe structure of grana

A

-thylakoid membrane of each chloroplast is less permeable and is stacked into flattened disc like sacs called thylakoids that form stacks
-each stack is called granum
-one granum may contain up to 100 thylakoids
-proteins embedded in thylakoid membranes hold photosystems in place
-the grana are surrounded by the stroma so the products of the light dependent stage can easily be passed to the stroma to be used in the light dependent stage

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18
Q

Why do grana have a large surface area

A

-for the distribution of photosystems that contain photosynthetic pigments that trap sunlight energy
-electron carriers and ATP synthase enzymes needed to convert light energy to ATP

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19
Q

Describe structure of stroma

A

-stroma is fluid filled matrix
-it contains enzymes needed to catalyse the reactions of light independent stage of photosynthesis as well as starch grains, oil droplets, small ribosomes and DNA
-the loop of DNA contains genes that code for some of the proteins needed for photosynthesis
-these proteins are assembled at the chloroplasts ribosomes

20
Q

Describe photosynthetic pigments

A

-within thylakoid membranes of each chloroplast are funnel shaped structures called photosystems
-these photosystems contain photosynthetic pigments
-each pigment absorbs a light of a particular wavelength and reflects other wavelengths of light
-the energy associated with wavelengths of light captured is funnelled down to the primary pigment reaction centre consisting of a type of chlorophyll at base of photosystem

21
Q

Define chlorophylls

A

-chlorophylls are a mixture of pigments
-all have a similar molecular structure consisting of a porphyrin group, in which is a magnesium atom and long hydrocarbon chain

22
Q

Describe chlorophyll a

A

-there are two forms of chlorophyll a- both of which appear blue green
-both are situated at centre of photosystems
-both absorb red light but have different absorption peaks
-P680 found in photosystem II and its peak absorption is light of wavelength 680nm
-P700 found in photosystem I and its peak absorption is light of wavelength 700nm
-chlorophyll a also absorbs some blue light, of wavelength around 440nm

23
Q

Describe chlorophyll b

A

-chlorophyll b absorbs light of wavelengths 400-500nm and around 640nm
-it appears yellow green

24
Q

Describe accessory pigments

A

-cartenoids absorb blue light of wavelengths 400-500nm
-they reflect yellow and orange light
-xanthophylls absorb blue and green light of wavelengths 375-550nm
-they reflect yellow light

25
Q

Define electron carriers

A

-molecules that can accept one or more electrons and then donate those electrons to another carrier
-proteins embedded in thylakoid membranes are electron carriers and form an electron transport chain system
-ferredoxin, NAD and NADP are also electron carriers

26
Q

Define NADP

A

-nicotinamide adenine dinucleotide phosphate phosphate
-a coenzyme and electron and hydrogen carrier

27
Q

Define photophosphorylation

A

-generation of ATP from ADP and inorganic phosphate in presence of light

28
Q

Describe the light dependent stage

A

-the light dependent stage of photosynthesis occurs in the grana (thylakoids) of chloroplasts and involves the photosystem
-it involves direct use of light energy
1) light harvesting at photosystems
2)photolysis of water
3) photophosphorylation - production of ATP in presence of light
4)formation of reduced NADP
-oxygen, the by product of photosynthesis, is also produced in light dependent stage

29
Q

What are the two types of photosystems

A

-in photosystem I (PSI) the pigment at primary pigment reaction centre is a type of chlorophyll a, which has a peak absorption of red light of wavelength 700nm (P700)
-in photosystem II (PSII) the pigment at primary pigment reaction centre is a type of chlorophyll a but has a peak absorption red light of wavelength 680nm (P680)

30
Q

Describe photolysis

A

-in PSII there is an enzyme that, in the presence of light, splits water molecules into protons (hydrogen ions), electrons and oxygen
-splitting of water is called photolysis
-2H2O - 4H+ + 4e- + O2
-some of oxygen produced during photolysis is used by plant cells for aerobic respiration, but during periods of high light intensity the rate of photosynthesis is greater than the rate of respiration in plant so much of oxygen by product will diffuse out of leaves through stomata into surrounding atmosphere

31
Q

Describe the role of water

A

-source of protons (hydrogen ions) that will be used in photophosphorylation
-donates electrons to chlorophyll to replace those lost when light strikes chlorophyll
-is source of by product oxygen
-keeps plant cells turgid, enabling them to function

32
Q

Describe the two types of photophosphorylation

A

-photophosphorylation is generation of ATP from ADP and inorganic phosphate in the presence of light
-there are two types of photophosphorylation
-non-cyclic photophosphorylation involves PSI and PSII. It produces ATP, oxygen and reduced NADP
-cyclic photophosphorylation involves only PSI. It produces ATP but in smaller quantities than are made by non cyclic
-both involve iron containing proteins embedded in thylakoid membranes that accept and donate electrons and form an electron transport system

33
Q

List the steps of non cyclic photophosphorylation

A

1)when photon of light strikes PSII (P680) its energy channelled to primary pigment centre
2) light energy excites a pair of electrons inside chlorophyll molecule
3) energised electrons escape from chlorophyll molecule and captured by electron carrier, which is a protein with iron at its centre, embedded in the thylakoid membrane
4) these electrons replaced by electrons derived from photolysis
5) when this iron combines with an electron it becomes reduced (Fe2+) It can then donate the electron becoming reoxidised (Fe3+) to the next electron carrier in the chain
6) as electrons passed along chain of electron carriers embedded in thylakoid membranes at each step some energy associated with electrons is released
7)energy is used to pump protons across thylakoid membrane into thylakoid space
8) eventually electrons captured by another molecule of chlorophyll a in PSI. These electrons replace those lost from PSI due to excitation by light energy
9) a protein-iron-sulfur complex called ferredoxin accepts electrons from PSI and passes them to the NADP in the stroma
10) as protons accumulate in the thylakoid space, a protein gradient forms across the membrane
11) protons diffuse down concentration gradient through special channels in membrane associated with ATP synthase enzymes and as they do so, flow of protons causes ADP and inorganic phosphate to join forming ATP
12) as protons pas through channel they are accepted along with electrons by NADP which becomes reduced. The reduction of NADP catalysed by enzyme NADP reductase

34
Q

What is ATP and NADP used for

A

-light energy has been converted to chemical energy in form of ATP by photophosphorylation
-ATP and reduced NADP are now in stroma ready for light independent stage of photosynthesis

35
Q

Describe cyclic photophosphorylation

A

-uses PSI only (P700)
-as light strikes PSI a pair of electrons in chlorophyll molecule at reaction centre gain energy and become excited
-they escape from chlorophyll and pass to electron carrier system and then pass back to PSI
-during passage of electrons along electron carriers, small amount ATP generated
-however no photolysis of water occurs, so no protons or oxygens are produced
-no reduced NADP is generated
-chloroplasts in guard cells contain only PSI
-they produce only ATP which actively brings potassium ions into cells, lowering water potential so water follow by osmosis
-this causes guard cells to swell and open stoma

36
Q

What is Z scheme

A

summary of main events of cyclic and non cyclic photophosphorylation

37
Q

Define Calvin cycle

A

-metabolic pathway of light independent stage of photosynthesis occurring in stroma of chloroplasts where carbon dioxide is fixed with products of light independent stage to make organic compounds
-Calvin cycle also occurs in many photoautotrophic bacteria

38
Q

Define glycerate-3-phosphate (GP)

A

-intermediate compound in calvin cycle

39
Q

Compare ribulose biphosphate (RuBP) and triose phosphate (TP)

A

RuBP = five-carbon compound present in chloroplasts; carbon dioxide acceptor
TP= three-carbon compound and product of calvin cycle; can be used to make other larger organix molecules

40
Q

Describe the light independent stage

A

-light independent stage of photosynthesis takes place in stroma of chloroplasts
-although it does no directly use light energy it uses products of light dependent stage
-if plant not illuminated, light independent stage soon ceases because ATP and hydrogen not available to reduce carbon dioxide and synthesise large complex organic molecules

41
Q

Describe the role of carbon dioxide

A
  • carbon dioxide in air enters leaf through stomata and diffuses through spongy mesophyll layer to palisade layer into palisade cells through thin cellulose cell walls and then through chloroplast envelope into stroma
    -fixation of carbon dioxide in stroma maintains a concentration gradient that aids the diffusion
    -carbon dioxide that is a by product of respiration in plant cells may also be used for tis stage of photosynthesis
    -the series of reactions whereby carbon dioxide is converted to organic molecules is called calvin cycle
42
Q

List the Calvin cycle

A

1) carbon dioxide combines with carbon dioxide acceptor, a five carbon compound called ribulose biphosphate (RuBP). This reaction catalysed by enzyme ribulose biphosphate carboxylase-oxygenase (RuBisCO)
2) RuBP by accepting carboxyl (COO-) group becomes carboxylated forming an unstable intermediate six carbon compound that immediately breaks down
3) product of this reaction is two molecules of three-carbon compound GP (glycerate-3-phosphate). The carbon dioxide has now been fixed
4) GP then reduced using hydrogens from reduced NADP made during light dependent stage to triose phosphate (TP). Energy from ATP also made during light dependent stage is sued at this stage at the rate of two molecules of ATP for every molecules of carbon dioxide fixed during stage 3
5) in 10 of every 12 TP molecules the atoms are rearranged to regenerate 6 molecules of RuBP. This process requires phosphate groups. Chloroplasts contain only low levels of RuBP as it is continually being converted to GP but also continually being regenerated. The remaining two of 12 molecules of TP are the product

43
Q

Why does the Calvin cycles only runs during daylight

A

-the products of light dependent stage (ATP and reduced NADP) are continuously needed for Calvin cycle to run
-during light dependent stage, hydrogen ions are pumped from stroma into thylakoid spaces so concentration of free protons in stroma falls, raising pH to around 8 which is optimum for enzyme RuBisCO
-RuBisCO is also activated by the presence of extra ATP in stroma
-in daylight the concentration of magnesium ions increase in the stroma
-these ions attach to the active site of RuBisCO acting as cofactors to activate it

44
Q

What does ferredoxin do

A

-ferredoxin that is reduced by electrons from PSI activates enzymes involved in the reactions of Calvin cycle

45
Q

Uses of triose phosphate

A

-some TP molecules used to synthesise organic compounds e.g. some glucose converted to sucrose, some starch and some to cellulose
-some TP used to synthesise amino acids, fatty acids and glycerol
-rest of TP recycled to regenerate supply of RuBP
five molecules of three-carbon compound TP interact to form three molecules of five-carbon compound RuBP

Biology (9 decks)

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