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1
Q

Photosynthesis

A

is the process
that converts
solar energy into
chemical energy

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2
Q

Autotrophs

A

sustain themselves without eating

anything derived from other organisms.

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3
Q

Autotrophs are the producers of

A

the biosphere,
producing organic molecules from CO2 and other
inorganic molecules

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4
Q

Almost all producers are photoautotrophs

A

using
the energy of sunlight to make organic molecules
through photosynthesis

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5
Q 
Photosynthesis occurs in
plants, algae, certain other
protists (single-celled
eukaryotes), and some
prokaryotes
A

These organisms feed not
only themselves but also
most of the living world

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6
Q

Heterotrophs

A

obtain their organic material from

consuming all or parts of other organisms

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7
Q

Almost all heterotrophs, including humans

A

depend

on photoautotrophs for both food and oxygen.

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8
Q

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

A

Fossil fuels therefore represent stores of solar

energy captured by producers in the distant past.

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9
Q

The green color of plants is

from

A

chlorophyll, the green

pigment within chloroplasts

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10
Q

Although all green parts of a

plant contain chlorophyll,

A

leaves are the major site of

photosynthesis.

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11
Q

Chloroplasts are found mainly

in cells of the mesophyll

A

the

interior tissue of the leaf.

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12
Q

Each mesophyll cell contains

A

30–40 chloroplasts

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13
Q

stomata.

A

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

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14
Q

The chlorophyll is in the membranes of thylakoids (connected

sacs in the chloroplast)

A

thylakoids may be stacked in columns

called grana

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15
Q

The thylakoids are surrounded by stroma

A

a dense interior fluid

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16
Q

Photosynthesis is a complex series of reactions

that can be summarized as the following equation:

A

6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2O

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17
Q

Chloroplasts split H2O into hydrogen and oxygen.

A

The electrons of hydrogen are incorporated into

sugar molecules and oxygen is released.

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18
Q

Photosynthesis is a redox process in which H2O

A

is
oxidized and CO2
is reduced.

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19
Q

Photosynthesis is an endergonic process

A

the

energy boost is provided by light

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20
Q

Photosynthesis reverses the direction of

A

electron

flow seen in cellular respiration

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21
Q

Photosynthesis consists of

A 
The light reactions (the photo part) and the
Calvin cycle (the synthesis part).
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22
Q

The light reactions (in the thylakoids)

A 
– Split H2O
– Release O2
– Reduce NADP+
to NADPH
– Generate ATP from ADP by
photophosphorylation
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23
Q

The Calvin cycle (in the stroma)

A

– forms sugar from CO2
, using ATP and
NADPH

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24
Q

Chloroplasts are solar-powered chemical factories

A

Their thylakoids transform light energy into the

chemical energy of ATP and NADPH

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25
Light is a type of electromagnetic radiation wave
``` Light can also be considered discrete particles of energy called photons ```
26
Pigments are substances that absorb visible light
Leaves appear green because chlorophyll reflects and transmits green light
27
Chlorophyll a is
the main | photosynthetic pigment.
28
Accessory pigments, such as | chlorophyll b
broaden the spectrum | used for photosynthesis.
29
Accessory pigments called | carotenoids
absorb excessive light | that would damage chlorophyll.
30
A spectrophotometer
measures a pigment’s | ability to absorb various wavelengths
31
This machine sends light through pigments and
measures the fraction of light transmitted at each | wavelength.
32
An absorption spectrum
is a graph plotting a | pigment’s light absorption versus wavelength
33
An action spectrum
profiles the relative effectiveness of different wavelengths of radiation in driving a process
34
Absorption spectra | show that violet-blue
and red light work best | for photosynthesis
35
When a pigment absorbs light,
it goes from a | ground state to an excited state, which is unstable
36
When excited electrons fall back to the ground | state, photons are given off
an afterglow called | fluorescence
37
If illuminated, an isolated solution of chlorophyll
will | fluoresce, giving off light and heat.
38
In chloroplasts a photosystem consists of
1) a reaction-center complex (a type of protein complex) surrounded by, 2) light-harvesting complexes
39
The light-harvesting complexes (pigment | molecules bound to proteins)
transfer the energy | of photons to the reaction center.
40
A special pair of chlorophyll a molecules in the reaction center is
``` excited by photons from the light harvesting complex and donates electrons to the primary electron acceptor ```
41
There are two interconnected types of photosystems:
- Photosystem II (PS II) | - Photosystem I (PS I)
42
Photosystem II (PS II)
functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm (its chlorophyll a is called P680)
43
Photosystem I (PS I)
is best at absorbing a wavelength of 700 nm (its chlorophyll a is called P700)
44
``` Photosystem II A photon hits a pigment and its energy is passed among pigment molecules until it excites P680. ```
``` An excited electron from P680 is transferred to the primary electron acceptor. P680’s electrons are replaced by electrons from a water molecule. ```
45
``` Photosystem II Each electron “falls” down an electron transport chain from the primary electron acceptor of Photosystem II ```
The electron transport chain establishes a proton gradient across the thylakoid membrane that drives ATP synthesis
46
In Photosystem I
light energy excites P700 to pass an electron to its primary acceptor, this electron is replaced by the electron transport chain
47
photosystem 1 A second electron transport chain transfers electrons to NADP+ reducing it to NADPH
The result of the light reaction is that ATP and NADPH are supplied to the Calvin Cycle where they are used to produce sugars
48
The Calvin Cycle:
Making Sugar
49
The Calvin cycle was described in the 1950’s by Andrew Bensen, James Bassham and Melvin Calvin
The Calvin cycle uses ATP and NADPH (from light reactions) to convert CO2 to sugar
50
The Calvin Cycle
The cycle is similar to that in the citric acid cycle – the final product of the reaction is an initial reactant.
51
The Calvin cycle has three phases
1. CO2 acceptor fixes carbon 2. Reduction of the sugar 3. Regeneration of the CO2 acceptor
52
Phase 1: Carbon fixation:
``` In the first phase, the carbon in CO2 is attached to the 5-carbon sugar ribulose bisphosphate (RuBP) by the enzyme rubisco ```
53
(Rubisco)
Ribulose bisphosphate carboxylase | oxygenase
54
(Rubisco
Most abundant protein on Earth
55
Rubisco is relatively inefficient
``` fixing only 3 molecules/second and It is not very specific (occasionally binds O2 instead of CO2 ```
56
calvin cycle | Phase 2: Reduction
``` In the second phase, ATP phosphorylates the sugar and NADPH donates electrons to raise the potential energy of the sugar. One molecule of the sugar glyceraldehyde 3-phosphate (G3P) leaves the cycle ```
57
Phase 3: | Regeneration
In the third phase, additional G3P is rearranged into molecules of RuBP. For the net production of one molecule of G3P the cycle must fix three molecules of CO2
58
Alternative Mechanisms to Fix | Carbon in Hot, Dry Climates
Dehydration is a problem sometimes requiring trade-offs with other metabolic processes, especially photosynthesis.
59
On hot, dry days, plants close stomata, which | conserves H2O but also limits photosynthesis
Closing stomata reduces access to CO2 and | causes O2 to build up within cells.
60
Photorespiration
An apparently detrimental process called photorespiration occurs while the stomata are closed and O2 builds up in the mesophyll cells.
61
Photorespiration
``` • Rubisco binds O2 instead of CO2 • Additional CO2 is produced but lost from the leaf • ATP is consumed • Photosynthetic rate is reduced ```
62
The version of the Calvin cycle shown previously fixes carbon first in
a 3-carbon sugar –plants that use this method are called C3 plants
63
C4 plants
``` minimize photorespiration by using an enzyme, PEP carboxylase, with a higher affinity for CO2 than that of rubisco ```
64
In C4 plants PEP carboxylase fixes CO2 in 4-carbon compounds
These are exported from mesophyll cells to bundle sheath cells, where they release CO2 for the Calvin cycle
65
Comparison: C3 – C4 Plants
``` C3 - carbon fixed first in a 3-carbon sugar C4 - a 4-carbon sugar is the first product - CO2 fixed first by PEP carboxylase (instead of rubisco) - Calvin cycle occurs in bundle sheath cells ```
66
CAM Plants Some plants fix carbon by crassulacean acid metabolism (CAM)
``` 1. CAM plants open their stomata at night, incorporating CO2 into organic acids. ``` ``` 2. During the day, the stomata close and the CO2 is released from the organic acids for use in the Calvin cycle. ```
67
``` Each day the Earth’s atmosphere receives enough energy to supply all of humanity’s energy consumption for 25 years! ```
Only ~1% of the solar energy that reaches Earth is converted to chemical energy by plants
68
Significance of Photosynthesis
About 50% of sugars made in photosynthesis are consumed in the cellular respiration of the plant. The remainder are synthesized into proteins, lipids and other molecules for the plant. Some products are stored in roots, seeds and fruits

biology 171 exam 2 (4 decks)

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