Chapter 10 (Notes) Flashcards
Photosynthesis is
the process that converts solar (light) energy into chemical energy
Directly or indirectly, photosynthesis
nourishes almost the entire living world
Autotrophs
sustain themselves without eating anything derived from other organisms.
Autotrophs are the producers of the biosphere, producing
organic molecules from CO2 and other inorganic molecules
Almost all plants are
photoautotrophs, using the energy of sunlight to make organic molecules
Photosynthesis occurs in
plants, algae, certain other protists, and some prokaryotes.
These organisms feed not only themselves but also most of the living world
Heterotrophs
obtain their organic material from other organisms
Heterotrophs are the
consumers of the biosphere
Almost all heterotrophs, including humans,
depend on photoautotrophs for food and O2.
The Earth’s supply of fossil fuels was
formed from the remains of organisms that died hundreds of millions of years ago.
In a sense, fossil fuels represent stores of solar energy from the distant past.
Chloroplasts are structurally similar to and likely evolved from
photosynthetic bacteria.
The structural organization of these cells allows for the chemical reactions of photosynthesis.
Photosynthesis happens in
chloroplasts.
Chloroplasts aren’t found in
every plant cell.
Found in Mesophyll.
Leaves are the major locations of
photosynthesis.
Leaves green color is from chlorophyll,
the green pigment within chloroplasts.
Chloroplasts are found mainly in cells of the
mesophyll, the interior tissue of the leaf.
Each mesophyll cell contains
30-40 chloroplasts
CO2 enters and O2 exits the leaf through microscopic pores called
stomata
openings on leaf
The chlorophyll is in the membranes of
thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana
Chloroplasts also contain stroma,
a dense interior fluid
the goopy stuff inside
Chlorophyll absorbs
light.
Photosynthesis is a complex series of reactions that can be summarized/simplified into the following equation
6 CO2 + 6 H2O + Light energy —-> C6H12O6 + 6 O2
3 things in- 6 Carbon dioxide, 6 water, light
2 things out- 1 glucose, 6 oxygen
Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into
sugar molecules and releasing oxygen as a by-product.
In photosynthesis, the oxygen comes from the water molecules.
Photosynthesis reverses the direction of
electron flow compared to respiration.
Photosynthesis is a
redox process in which H2O is oxidized and CO2 is reduced.
Photosynthesis is an
endergonic process; the energy boost is provided by light.
Photosynthesis consists of two stages;
the light reactions (the photo part) and the Calvin Cycle (the synthesis part)
The light reactions (in the thylakoids)
Split H2O
Release O2
Reduce NADP+ to NADPH
Generate ATP from ADP by photophosphorylation
The Calvin Cycle (in the stroma)
forms sugar from CO2, using ATP and NADPH.
The Calvin cycle begins with carbon fixation,
incorporating CO2 into organic molecules.
The calvin cycle is Sometimes mistakenly called the “dark-cycle” but
it can occur at any time of the day.
The light reactions convert solar energy to
the chemical energy of ATP and NADPH
Chloroplasts are
solar-powered chemical factories
Chloroplasts’s thylakoids transform light energy into the
chemical energy of ATP and NADPH
Light is a form of
electromagnetic energy, also called electromagnetic radiation.
Like other electromagnetic energy, light travels in
rhythmic waves.
Light is a form of
kinetic energy.
Wavelength is the distance between
crests of waves
Wavelength determines the type of
electromagnetic energy
The electromagnetic spectrum is the
entire range of electromagnetic energy, or radiation
Visible light consists of
wavelengths (including those that drive photosynthesis) that produce colors we can see
Light also behaves as though it consists of
discrete particles, called photons (pieces of light)
Photosynthetic pigments:
the light receptors
Pigments are substances that
absorb visible light
Different pigments absorb different
wavelengths
Wavelengths that are not absorbed are
reflected or transmitted
Leaves appear green because
chlorophyll reflects and transmits green light
Pigments absorb all of the colors but the
one color that is transmitted that makes the thing look that color.
The spectrophotometer measures a
pigment’s ability to absorb various wavelengths
The spectrophotometer machine sends light through pigments and
measures the fraction of light transmitted at each wavelength
An absorption spectrum is a
graph plotting a pigment’s light absorption versus wavelength
The absorption spectrum of chlorophyll a suggests that
violet-blue and red light work best for photosynthesis
Chlorophyll a is the
main photosynthetic pigment.
the most important one
Accessory pigments, such as chlorophyll b,
broaden the spectrum used for photosynthesis
Accessory pigments called carotenoids absorb
excessive light that would damage chlorophyll.
When a pigment absorbs light,
it goes from a ground state to an excited state, which is unstable
When excited electrons fall back to the ground state,
photons are given off, an afterglow called fluorescence
If illuminated, an isolated solution of chlorophyll will
fluoresce, giving off light and heat
A photosystem:
a reaction-center complex associated with light-harvesting complexes
Chloroplasts excited by light in a leaf behave
differently than isolated chloroplasts
A photosystem consists of a
reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes
The light-harvesting complexes
(pigment molecules bound to proteins) transfer the energy of photons to the reaction center
A primary electron acceptor in the reaction center accepts
excited electrons and is reduced as a result
Solar-powered transfer of an electron from a
chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions
There are two types of photosystems in the thylakoid membrane
Photosystem II (PSII) Photosystem I (PSI)
Photosystem II (PSII) functions first
because the numbers reflect the order of discovery
Photosystem II (PSII) is best at absorbing a wavelength of
680nm
The reaction-center chlorophyll a of photosystem II (PSII) is called
P680
Photosystem I (PSI) is best at absorbing a wavelength of
700nm
The reaction-center chlorophyll a of photosystem I (PSI) is called
P700
The two photosystems work together to
use light energy to generate ATP and NADPH
During light reactions, there are two possible routes for electron flow:
linear and cyclic
Linear electron flow, the primary pathway, involves
both photosystems and produces ATP and NADPH using light energy
In linear electron flow,
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 (we now call it P680^+)
(((linear electron flow)))
P680^+ is a
very strong oxidizing agent
(((linear electron flow)))
H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms
to P680^+, thus reducing it to P680
(((linear electron flow)))
O2 is released as a
by-product of this reaction
(((linear electron flow)))
Each electron “falls” down an electron transport chain from the
primary electron acceptor of PSII to PSI
(((linear electron flow)))
Energy released by the fall of the electrons down the electron transport chain drives the
creation of a proton gradient across the thylakoid membrane
(((linear electron flow)))
Diffusion of H+ (protons) across the membrane drives
ATP synthesis
(((linear electron flow)))
In photosystem I (like photosystem II), transferred light energy excites
P700, which loses an electron to an electron acceptor
(((linear electron flow)))
P700^+ (P700 that is missing an electron) accepts an electron passed down from
photosystem II via the electron transport chain
(((linear electron flow)))
Photosystem I draws the electrons down from
photosystem II
(((linear electron flow)))
Each electron “falls” down an electron transport chain from the
primary electron acceptor of Photosystem I to the protein ferredoxin (Fd)
(((linear electron flow)))
The electrons are then transferred to
NADP+ and reduce it to NADPH
(((linear electron flow)))
The electrons of NADPH are
available for the reactions of the Calvin Cycle.
This process also removes an H+ from the stroma
Cyclic Electron flow uses only
photosystem I and produces ATP, but not NADPH.
No oxygen is released.
Cyclic electron flow generates
surplus ATP, satisfying the higher demand in the calvin cycle.
Some organisms such as purple sulfur bacteria have
photosystem I but not photosystem II
Cyclic electron flow is thought to have evolved before
linear electron flow
Cyclic electron flow may protect cells from
light-induced damage
Chloroplasts and mitochondria generate ATP by
chemiosmosis, but use different sources of energy
Mitochondria transfer
chemical energy from food to ATP
chloroplasts transfer
light energy into the chemical energy of ATP
Spatial organization of chemiosmosis differs between
chloroplasts and mitochondria but also shows similarities
In mitochondria, protons are pumped to the
intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix
In chloroplasts, protons are pumped into the
thylakoid space and drive ATP synthesis as they diffuse back into the Stroma
In chloroplasts, ATP and NADPH are produced on the side facing
the stroma where the calvin cycle takes place.
In summary, the light reactions generate ATP and
increase the potential energy of electrons by moving them from H2O to NADPH
The calvin cycle uses the
chemical energy of ATP and NADPH to reduce CO2 to sugar
The calvin cycle, like the citric acid cycle, regenerates its starting material after
molecules enter and leave the cycle
The calvin cycle builds sugar from smaller molecules by using
ATP and the reducing power of electrons carried by NADPH
Carbon enters the calvin cycle as CO2 and leaves as
a sugar named glyceraldehyde 3-phosphate (G3P)
For net synthesis of 1 G3P, the calvin cycle must take place
three times, fixing 3 molecules of CO2
The calvin cycle has three phases
- Carbon Fixation (catalyzed by rubisco)
- Reduction
- Regeneration of the CO2 acceptor (RuBP)
Alternative mechanisms of carbon fixation have evolved in
hot, arid climates
Dehydration is a problem for
plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis
On hot, dry days, plants close stomata, which
conserves H2O but also limits photosynthesis
The closing of stomata reduces access to
CO2 and causes O2 to build up
These conditions (closing the stomata) favor an apparently wasteful proces called
photorespiration
In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a
three-carbon compound (3-phosphoglycerate)
-rice, wheat, soybeans
In photorespiration, rubisco adds O2 instead of
CO2 in the calvin cycle, producing a two-carbon compound
Photorespiration consumes O2 and organic fuel and releases
CO2 without producing ATP or sugar.
Plants do not want to do
photorespiration that much.
Photorespiration may be an evolutionary relic because
rubisco first evolved at a time when the atmosphere had far less O2 and more CO2
Photorespiration limits damaging products of
light reactions that build up in the absence of the Calvin cycle
In many plants, photorespiration is a problem because
on a hot, dry day it can drain as much as 50% of the carbon fixed by the calvin cycle
C4 plants minimize the cost of photorespiration by
incorporating CO2 into four-carbon compounds in msophyll cells
-sugarcane, corn, grasses
This step requires the enzyme PEP carboxylase
PEP carboxylase has a higher affinity for CO2 than rubisco does; it can
fix CO2 even when CO2 concentrations are low
In C4 plants, these four-carbon compounds are exported to
bundle-sheath cells, where they release CO2 that is then used in the calvin cycle
C4 plants store CO2 in case
the stomata closes.
They store carbon in one cell, then can use carbon in another cell for the calvin cycle
The mesophyll cells of a C4 plant pump CO2 into the bundle sheath, keeping the CO2 concentration
high enough in the bundle sheath cells for rubisco to work
In the last 150 years since the industrial revolution,
CO2 levels have risen greatly
X
Increasing levels of CO2 may affect
C3 and C4 plants differently, perhaps changing the relative abundance of these species.
The effects of such changes are unpredictable and a cause for concern.
(X)
Some plants, including succulents, use
crassulacean acid metabolism (CAM) to fix carbon
CAM plants open their stomata at night (when it is cooler), incorporating
CO2 into organic acids
In CAM plants, stomata close during the day, and CO2 is released from
organic acids and used in the calvin cycle
C4 and CAM are plants solutions to
photorespiration
?
The energy entering chloroplasts as sunlight gets stored as
chemical energy in organic compounds
Sugar made in the chloroplasts supplies chemical energy and
carbon skeletons to synthesize the organic molecules of cells.
(what everything else needs to be alive)
Plants store excess sugar as starch in structures such as
roots, tubers, seeds, and fruits
In addition to food production, photosynthesis produces
the O2 in our atmosphere
