Chapter 5 Flashcards
1
Q
Photosynthesis produces carbohydrate using
A
sunlight, carbon dioxide, and water
2
Q
In photosynthesis, oxygen is a
A
By-product
3
Q
6 CO2 + 12 H2O + light energy
A
C6H12O6 + 6 O2 + 6 H2O
4
Q
Photosynthesis contrasts with cellular
respiration:
A
– Photosynthesis is endergonic.
Reduces CO2 to glucose or other sugars
– Cellular respiration is exergonic.
Oxidizes sugars to CO2
5
Q
Two Linked Sets of Reactions in photosynthesis are
A
- Light dependent reactions or “light reactions”
- Light independent reactions or “dark reactions”
6
Q
Light dependent reactions or “light reactions” involve
A
Energy from photons causes release of electrons from chlorophyll.
These electrons replaced by splitting water; produce O2. Electrons
transferred to NADP+ forming NADPH. Electron transport also produces some ATP
7
Q
Light independent reactions or “dark reactions” involve
A
The Calvin cycle – NADPH and ATP is used to reduce CO2 to carbohydrates.
8
Q
Photosynthesis occurs in the
A
Chloroplast - green organelles of plants
9
Q
The chloroplast has how many membranes
A
Two
10
Q
Thylakoid membranes contain
A
Pigments
11
Q
the most common pigment is
A
Chloroplasts
12
Q
Why do plants appear green
A
Chlorophyll reflects (does not absorb) green light, thus plants appear green
13
Q
The fluid-filled space between the thylakoids and the inner membrane is the
A
stroma
14
Q
How many chloroplasts does a typical plant cell contain
A
40-50
15
Q
The thylakoids structure is
A
flattened sacs
16
Q
the granum are
A
stack of thylakoids
17
Q
The stroma is
A
Liquid matrix
18
Q
Photons are
A
discrete packets of electromagnetic energy
19
Q
Electromagnetic energy travels in
A
Waves
20
Q
Visible light (400 – 700 nm) is used by
A
photosynthetic organisms
21
Q
Pigments are molecules that
A
absorb only certain wavelengths of light
22
Q
There are two major classes of pigment in plant leaves:
A
chlorophylls and
carotenoids
23
Q
The chlorophylls (a and b) absorb
A
red and blue light and reflect and transmit green light
24
Q
The carotenoids
A
- accessory pigments because
the absorb light and pass energy to chlorophylls - absorb blue and green light and reflect and transmit yellow, orange, and red light
25
Different pigments absorb different
wavelengths of light
26
Each pigment has an absorption spectrum which is
Plot of the wavelength of light absorbed by that pigment
27
An action spectrum shows
The rate of photosynthesis at each wavelength
28
Which pigments are the most effective at driving photosynthesis
Pigments that absorb blue and red photons
29
The main photosynthetic pigments are
Chlorophylls
30
There Is a Strong Correlation between
the Absorption Spectra of Pigments and the
Action Spectrum for Photosynthesis
31
describe the structure of chlorophylls
- Chlorophylls have a long “tail” made of isoprene subunits, and a “head” consisting of a large ring structure with a magnesium atom in the middle.
– The tail keeps the molecule embedded in the thylakoid membrane.
– Light is absorbed in the head.
32
What occurs when a pigment
absorbs a photon and the electron gets
excited, but then falls back to its ground state. Some of the absorbed energy is released as heat and the rest is released as electromagnetic radiation (light)
Fluorescence
33
approximately how much blue and red light produce fluorescence
Only approximately 2% of red and blue
photons produce fluorescence. The
remaining 98% drive photosynthesis
34
Blue photons excite electrons to an even
an even higher energy state
35
Red photons excite electrons to a
high-energy state
36
If an electron is excited by photons but then returns to its ground state
the energy it absorbed is released as heat and fluorescence
37
Chlorophyll molecules work together in
groups, forming a complex called a
photosystem
38
Two major elements of the photosystem
1. antenna complex – accessory pigments and carotenoids
2. reaction centre – contains a chlorophyll a molecule
- as well as proteins that capture and process excited electrons.
39
Chlorophyll molecules transmit energy from excited electrons in the antenna complex to
a reaction center
40
At the reaction center, excited electrons are
passed to an electron acceptor
41
Chlorophyll at reaction
centre can not perform
another excitation reaction
until the
lost electron
is replaced
42
Replacement electron
comes from
hydrolysis
of water molecule
43
Energy released by excited electrons in
chlorophyll can
1. Drop back down to a low energy state, causing fluorescence
2. Drop back down to a low energy state, releasing heat
3. Excite an electron in a nearby pigment, inducing resonance
4. Be transferred to an electron acceptor in a redox reaction
44
Four fates for excited electrons in photosynthetic pigments
When sunlight promotes electrons in pigments to a high-energy state, four things happen: they can fluorescence, release heat, pass energy to a nearby pigment via resonance, or transfer the electron acceptor
45
There are two types of reaction centre
photosystem I and photosystem II
46
Photosynthesis is found to be
moderate at red light (680 nm) and at far red light (700 nm)
47
Photosynthesis greatly enhanced
when red and far-red light combined. This Led to recognition of two separate
photosystems that work in concert, one with absorption max at 680 nm and one with absorption max at 700 nm
48
how does photosystem II work
Energy reaches the reaction centre
* Chlorophyll is oxidized when an electron is donated to the electron acceptor pheophytin
* The electron is passed to an electron
transport chain (ETC) in the thylakoid
membrane
* Produces a proton gradient
ATP production
* Photosystem II triggers chemiosmosis and ATP synthesis in the chloroplast.
49
Photosystem II and the cytochrome complex are located in the
thylakoid membranes
50
Phrophytin
Higher electronegativity,
therefore accepts electrons
Missing magnesium
51
Oxygenic and Anoxygenic
Photosynthesis
This process is called oxygenic
photosynthesis. Photosystem II is the only known protein complex able to oxidize water in this way.
H2O --> 2 H+ + 2 e– + ½ O2
52
How Does Photosystem I Work?
Excited electrons from the reaction centre of photosystem I are passed down an ETC of iron- and sulphur-containing proteins to ferredoxin.
– Does not generate proton motive force and ATP
* A proton and two electrons from ferredoxin to NADP+, forming NADPH.
53
Photosystem II produces a
proton gradient
that drives the synthesis of ATP
54
Photosystem I yields
reducing power in the
form of NADPH
55
Photosystems I and II perform functions during
These occur during light-dependent
reactions
56
What happens after photosystems I and II have performed their functions
Ultimately, the ATP and NADPH will be used to reduce CO2 into glucose involving light- independent reactions
57
The Z scheme is a model of
how photosystems II and I interact
58
Describe the Z-scheme
First, the electrons of photosystem II will be replaced by electrons stripped from water, producing oxygen gas as a by-product.
* A special pair of reaction-centre chlorophyll molecules named P680 passes the excited electron to pheophytin
* At the end of photosystem II’s ETC, the electron is passed to a protein called plastocyanin.
* Plastocyanin carries the electron back across the thylakoid membrane and donates it to photosystem I,
* Electrons from PC replace electrons from the P700 pair of chlorophyll molecules in the photosystem I reaction centre.
* These electrons enter an ETC, then are eventually passed to ferredoxin and used to reduce NADP+ to NADPH.
59
The Z scheme explains which effect
the enhancement effect
60
What is the enhancement effect
Photosynthesis is more efficient when both 680- nm and 700-nm wavelengths are present
61
cyclic photophosphorylation is
Photosystem I occasionally transfers electrons to photosystem II’s electron transport chain to increase ATP production, instead of using them to
reduce NADP+.
This cyclic photophosphorylation coexists with the Z scheme and produces additional ATP.
62
The energy transformation of the light-dependent reactions and the carbon dioxide reduction of the Calvin cycle are two
separate but linked
processes in photosynthesis
63
ATP and NADPH are produced by photosystems I and II in
the presence of light
64
The reactions that produce sugar from carbon dioxide in the Calvin cycle are
light-independent. These reactions require the ATP and NADPH
produced by the light-dependent reactions.
65
The Calvin cycle has three phases
All three phases take place in the stroma of chloroplasts, they are
1. Fixation
2. Reduction
3. Regeneration
66
The CO2-fixing enzyme is called
ribulose 1,5- bisphosphate carboxylase/oxygenase
(rubisco). Has 8 active sites where CO2 is fixed
67
Rubisco is found
in all photosynthetic organisms
that use the Calvin cycle to fix carbon and is thought to be the most abundant enzyme on Earth
68
Rubisco is inefficient because
although it does catalyze the addition of CO2 to RuBP, it also catalyzes the addition of O2 to RuBP
69
Oxygen and carbon dioxide compete at the enzyme’s active sites, which
slows the rate of CO2
reduction
70
When O2 and RuBP react in rubisco’s active site, one of the products undergoes a process called
photorespiration
71
Photorespiration “undoes” photosynthesis because
it consumes energy and releases fixed
CO2
72
When photorespiration occurs, the rate of photosynthesis
declines drastically.
73
Carbon fixation is favoured over
photorespiration
74
Rubisco catalyzes competing reactions with
very different outcomes
- Reaction with carbon dioxide during photosynthesis:
RuBP + CO2 → 2 3-phosphoglycerate (used in Calvin cycle)
- Reaction with oxygen during “photorespiration”:
RuBP + O1 → 1 3-phosphoglycerate (used in Calvin cycle) + 1 2-phosphoglycolate (when processed, CO2 released and ATP used)
75
The leaf structures where gas exchange occurs
Stomata
76
Stomata consist of two guard cells that
change shape to open or close.
77
When a leaf’s CO2 concentration is low during photosynthesis, stomata
open to allow atmospheric CO2 to diffuse into the leaf and its cells’
chloroplasts
78
The calvin cycle maintains a concentration gradient favouring
Entry of co2, since the calvin cycle
constantly uses up the CO2 in chloroplasts.
79
How is CO2 concentration increased in plants from hot, dry habitats
The C4 pathway spatially separates carbon fixation and the Calvin
cycle.
– During carbon fixation, C4 plants incorporate CO2 into 4-carbon (C4) organic acids instead of
3-phosphoglycerate (performed by C3 plants)
80
In crassulacean acid metabolism (CAM)
plants, carbon fixation and the Calvin cycle are separated in time. These plants, which also live in hot,dry habitats keep their stomach closed all day and open them
81
G3P molecules produced by the Calvin cycle are often used to
make glucose and fructose, which can
be combined to form sucrose
82
In rapidly photosynthesizing cells where sucrose is abundant, glucose is
temporarily stored in the
chloroplast as starch
83
Because starch is not water soluble, it is
broken down at night and used to make more sucrose for transport throughout the plant
