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cell bio lec 22-28 > lec 23 chloroplasts and photosynthesis > Flashcards

lec 23 chloroplasts and photosynthesis Flashcards

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Biology 101 flashcards
1
Q

autotrophs

A

utilize CO2 to manufacture their organic molecules

a. chemoautotrophs
b. photoautotrophs

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2
Q

chemoautotrophs

A
  • utilize the energy stores in inorganic molecules (eg hydrogen sulfide) to convert CO2 into organic compounds; eg found around thermal vent
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3
Q

photoautotrophs

A

-utilize the radiant energy of sun to convert CO2 into organic compounds
-includes:
1. higher plants
2. eukaryotic algae
3. various flagellated protists
4. a variety of prokaryotes (eg. blue-green bacteria)
all of the above carry out photosynthesis

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4
Q

photosynthesis

A

-sunglight is transformed into chemical energy and used to form carbohydrates
6CO2+6H2O—->light—-> C6H12O6+6O2
-photosynthesis in higher plants will be examined

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5
Q

what is photosynthesis

A
  • captures light energy by pigment molecules
  • chlorophylls and accessory pigments (carotenes and phycobilins [latter in cyanobacteria and red algae only])
  • its a redox reaction
  • produces oxidizing power (O2) by photolysis
  • captures electrons by cytochromes (plant version of electron transport chain proteins) and produces reducing power in form of NADPH
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6
Q

two fundamental mechanisms in photosynthesis

A

light reactions

dark reactions

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7
Q

light reactions

A

photochemical reactions

  • products are : ATP and NADPH
  • OCCurs in thylakoid membranes of chloroplasts
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8
Q

Dark reactions

A

thermochemical reactions

  • ATP and NADPH are used to synthesize carbohydrates
  • occurs in stroma of chloroplasts
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9
Q

a generalization

A

NADH is used for catabolism

NADPH is used for anabolism

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10
Q

chloroplast

A
  • the chloroplast is the organelles in which photosynthesis takes place
  • a typical plant cell (eg in the palisade layer of a leaf) might contain as many as 50 chloroplasts
  • the chloroplast is made up of 3 types of membranes
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11
Q

chloroplast is made up of three types of membrane

A
  • smooth outer membrane which is freely permeable to molecules
  • a smooth inner membrane which contains many transporters: integral membrane proteins that regulate the passage in and out of the chloroplast of
  • small molecules like sugars
  • proteins synthesized in the cytoplasm of the cell but used within the chloroplast
  • a system of thylakoid membranes
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12
Q

thylakoids

A
  • thylakoid membranes enclose a lumen: a system of vesicles (that may all be interconnected)
  • at various places within the chloroplast these are stacked in arrays called grana (looks like coins)
  • four types of protein assemblies are embedded in the thylakoid membranes
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13
Q

four types of protein assemblies are embedded in the thylakoid membranes:

A
  • photosystem I- includes chlorophyll and carotenoid molecules
  • photosystem II- which also contains chlorophyll and carotenoid molecules
  • Cytochromes b and f
  • ATP synthase

these carry out the so called light reactions of photosynthesis

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14
Q

Chloroplast stroma

A

-the thylakoid membranes are surrounded by a fluid stroma
-the stroma contains : all the enzymes needed to carry out the dark reactions of photosynthesis; that is, the conversion of CO2 into organic molecules like glucose
-a number of identical molecules of DNA, each of which carries the complete chloroplast genome. the genes encode some but not all-of the molecules needed for the chloroplast functions
the others are
*transcribed from genes in the nucleus of the cell
*translated in the cytoplasm and
*transported into the chloroplast

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15
Q

chloroplast genome

A

= total gene set found within the chloroplast

  • genome encodes about 100 chloroplast specific proteins (ribosomes, Rubisco subunit, etc)
  • chloroplast makes these 100 proteins using its own RNA polymerase (transcription) and its own ribosome proteins
  • Cells nucleus encodes about 900 chloroplast proteins that are made in the cytolasm and then are transported into the chloroplast
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16
Q

how do chloroplasts divide

A

-new chloroplasts arise from the division of pre-existing chloroplast
-before a cell divides, its chloroplasts also divide so they can be distributed to the daughter cells
-chloroplasts pinch themselves into two equal parts, just like bacteria (binary fission) which is not surprising given that they evolved from bacteria
-an important chloroplast and bacterial cell division protein is FtsZ
-FtsZ can polymerize and form contractile rings on the inside of the chloroplast
These rings constrict and divide the organelle
-there is also a ring on the outside of the chloroplast (called the outer plastid dividing ring) that helps squeeze the organelle from the outside during division. the outer ring proteins bay be derived from the host

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17
Q

absorption of light

A
  • NRG comes from sun in from of electromagnetic radiation
  • this radiation travels in discrete packets called photons
  • when photon is absorbed, compound is converted to a higher energy state (excited state)
  • ground-state may be re-established in three different ways
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18
Q

three different ways ground state may be re-established

A
  1. energy may be dissipated as heat
  2. energy may be re-emitted in form of longer wavelength (fluorescence)
  3. energy may be transferred to another molecule
    * this happens with photosynthetic pigments
19
Q

photosynthetic pigments

A
  • pigments are molecules that contain a chromophore
  • chromophore
  • absorption spectrum
  • action spectrum
20
Q

chromophore

A

chemical group capable of absorbing light of particular wavelengths

21
Q

absorption spectrum

A

plot of intensity of light absorbed vs. wavelength

22
Q

action spectrum

A

plot of physiological response vs wavelength

-action spectrum of photosynthesis follow absorption spectrum of the chlorophylls and carotenoids

23
Q

carotenoids

A

accessory pigments

  • long hydrocarbon chains containing alternating double bonds
  • increase efficiency by absorbing light in those regions where chlorophyll absorbs light inefficiently
  • absorb light of blue and green wavelength (400-550nm)
  • reflect yellow, orange and red wavelength (>550 nm,
24
Q

carotenoids in animals

A
  • 600 different types of carotenoids

- animals cannot make carotenoids- they obtain them from their diet (plants and algae)

25
Q

how do animals use carotenoids ?

A
  • ornamental colourings
  • pink colour of flamingos, pigments in bird feathers
  • pink colour of salmon
  • red colour of lobsters
  • antioxidants
  • risk of developing some cancers is reduced by eating diets enriched with carotenoids
  • e.g lycopenes in tomatoes linked to reduced lipid peroxidation, reduced heart attacks, improved male fertility, longevity
26
Q

photosystem II (PSII)

A
  1. reaction centre chlorophyll is referred to as P680
    P=pigment
    680=wavelength of light that his chlorophyll molecules absorbs most strongly
  2. when P680 absorbs photons, it gives up electrons to a primary electron acceptor( pheophytin) of higher reducing potential (=stronger affinity for electrons)
    3.P680 replenishes its electrons from H2O
  3. as H2O becomes oxidized, O2 is released
    -pheophytin eventually transfers electrons to a chain of electron carriers
27
Q

electrons are transferred from photosystem II to photosystem I

A

1,2. the excited electron from PSII is passed to the primary electron acceptor. Photolysis in the thylakoid takes the electrons from water and passes them to 680, to replace electrons passed to the primary electron acceptor
3. the electrons are then passed to photosystem I by the electron transport chain
4. this process produces ATP which is used later in the calvin cycle
5,6. P700 chlorophyll then uses light to excite the electron to its own primary acceptor
7. the electron is sent down another chain to create NADPH
8. NADPH is then used later in the calvin cycle

28
Q

ATP can be generated through a ______ using only _____

A

cyclic pathway

PSI

29
Q

excited electrons from PSI have two fates - they can pass down:

A
  • a short electron transport chain to NADP+ to form NADPH

- back to P700 to form ATP (cyclic photophosphorylation)

30
Q

in the non cyclic pathway,

A

electrons taken from chlorophyll a are not recycles back down to the ground state, the electrons end up on NADPH

31
Q

In the cyclic pathway, only photosystem I is used

A
  • the electrons used in the cyclic pathway are excited, pass down the chain (creating ATP) and then land back on P700 to be used again
  • neither oxygen nor NADPH are produced in the cyclic reactions
32
Q

Z scheme or pathway

A
  1. two photosystems acting in series
  2. electron flow occurs in 3 steps
  3. as electrons flow along Z-pathway, H+ ions are moved from stroma to inner compartment of thylakoids
  4. important end result is proton gradient proton concentration:
    a) high in lumen of thylakoid
    b) low in stroma
33
Q

3 steps of electron flow

A

a. b/w H2O and PSII
b. b/w PSII and PSI. electron transport chain
c. b/w PSI and NADP+

34
Q

photophosphorylation

A
  1. formation of ATP as result of electrons moving through photosystems I an II
  2. as in mitochondria:
    a. proton gradient drives ATP formation
    b. enzyme is ATP synthase
  3. ATP synthase embedded in thylakoid membranes
  4. two types of photophosphorylation
    a. noncyclic photophosphorylation
    b. cyclic photophosphorylation
35
Q

non cyclic photophosphorylation

A
  1. electrons move in linear path from H2O to NADP+
  2. uses photosystems I and II
  3. formation of ATP, NADPH and O2
36
Q

cyclic photophosphorylation

A
  1. electrons move from P700 to ferredoxin and back to P700
  2. involves photosystem I only
  3. formation of ATP

relative amount of non cyclic vs cyclic photophosphorylation is regulated

37
Q

CO2 fixation and formation of carbohydrate

A

C3 or Calvin cycle
1. Carboxylation
a)CO2 combines with ribulose-1, 5-bisphosphate (RuBP)
b)forms a transient 6-carbon compound
c) this breaks down to form 2 3-carbon molecules of 3-phosphoglycerate (PGA)
d. catalyzed by ribulose- 1, 5 bisphosphate carboxylase (Rubisco)
2. ATP and NADPH are used to form (in a couple of steps) glyceraldehyde 3-phosphate (G3P)
some G3P is used to make glucose, some gets cycled through to re-make more RuBP

38
Q

Origi of chloroplasts

A

-endosymbiotic theory suggests that chloroplast started as a photosynthetic bacterium, which later got assimilated by a larger cell

39
Q

how do chloroplasts resemble bacteria?

A
  • own circular genome
  • double membranes
  • share highly similar photosynthetic enzymes
40
Q

similarities of chloroplasts to mitochondria

A
  • method replication
  • generation of chemical energy (electron transport coupled to proton gradient)
  • Similar ATPases
41
Q

differences of chloroplasts to mitochondria

A
  • conduct opposing reactions - once uses glucose , the other makes it
  • different electron acceptors
42
Q

artificial photosynthesis

A

alternatives are sought to replace fossil fuels as energy sources

  • artificial photosynthesis attempts to replicate the natural process of photosynthesis, converting sunlight, water ,CO2 into H and H2O and carbohydrates
  • artificial light reaction hopes to generate H as a new energy source- a clean new fuel
  • one way: genetically alter cyanobacteria so that they release H instead of use it to make carbohydrates in bioreactors
  • another way: engineered enzymes on solar cells
  • artificial dark reaction-hopes to mop up excess CO2 in atmosphere
43
Q

heterotrophs

A
  • depend on an external sources of organic compounds
  • earliest life forms must have been heterotrophs
  • earliest life forms would have utilized organic molecules that had been formed abiotically

cell bio lec 22-28 (2 decks)

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