Photosynthesis Flashcards
How do plants use solar energy to fix CO2 into carbohydrates
Photosynthesis converts the energy of sunlight into chemical energy stored in
sugars.
In plants and other eukaryotic autotrophs, photosynthesis occurs
in chloroplasts, organelles containing thylakoids. Stacks of
thylakoids form grana. Photosynthesis is summarized as
6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O.
Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of
hydrogen into sugar molecules. Photosynthesis is a redox process: H2O is
oxidized, and CO2 is reduced.
Decribe the light reactions of photosynthesis
The light reactions in the thylakoid membranes split water, releasing O2,
producing ATP, and forming NADPH.
Desribe the function of chlorophyll and the carotenoids
types of pigments in chloroplasts:
chlorophyll a, the key light-capturing pigment
that participates directly in the light reactions; the accessory
pigment chlorophyll b; and a separate group of accessory
pigments called carotenoids.
Other accessory pigments include carotenoids, hydrocarbons
that are various shades of yellow and orange because
they absorb violet and blue-green light (see Figure 10.9a).
Carotenoids may broaden the spectrum of colors that can drive
photosynthesis. However, a more important function of at least
some carotenoids seems to be photoprotection: These compounds
absorb and dissipate excessive light energy that would otherwise
damage chlorophyll or interact with oxygen, forming reactive
oxidative molecules that are dangerous to the cell.
Refer to textbook action spectrum for active wavelengths
Describe the Z scheme
1 - A photon of light strikes one of the pigment molecules in a
light-harvesting complex of PS II, boosting one of its electrons
to a higher energy level. As this electron falls back to its ground state, an electron in a nearby pigment molecule
is simultaneously raised to an excited state. The process
continues, with the energy being relayed to other pigment
molecules until it reaches the P680 pair of chlorophyll a
molecules in the PS II reaction-center complex. It excites an
electron in this pair of chlorophylls to a higher energy state.
2. This electron is transferred from the excited P680 to the primary
electron acceptor. We can refer to the resulting form of
P680, missing the negative charge of an electron, as P680+
3 An enzyme catalyzes the splitting of a water molecule into
two electrons, two hydrogen ions (H+ ), and an oxygen
atom. The electrons are supplied one by one to the P680+
pair, each electron replacing one transferred to the primary
electron acceptor. (P680+ is the strongest biological oxidizing
agent known; its electron “hole” must be filled. This
greatly facilitates the transfer of electrons from the split
water molecule.) The H+ are released into the thylakoid
space (interior of the thylakoid). The oxygen atom immediately
combines with an oxygen atom generated by the splitting
of another water molecule, forming O2.
Each photoexcited electron passes from the primary electron
acceptor of PS II to PS I via an electron transport chain,
the components of which are similar to those of the electron
transport chain that functions in cellular respiration. The
electron transport chain between PS II and PS I is made up of
the electron carrier plastoquinone (Pq), a cytochrome complex,
and a protein called plastocyanin (Pc). Each component
carries out redox reactions as electrons flow down the
electron transport chain, releasing free energy that is used to
pump protons (H+ ) into the thylakoid space, contributing
to a proton gradient across the thylakoid membrane.
5 The potential energy stored in the proton gradient is used to make ATP in a process called chemiosmosis
6 Meanwhile, light energy has been transferred via lightharvesting complex pigments to the PS I reaction-center complex, exciting an electron of the P700 pair of chlorophyll a molecules located there. The photoexcited electron is then transferred to PS I’s primary electron acceptor, creating
an electron “hole” in the P700—which we now can call
P700+. In other words, P700+ can now act as an electron acceptor, accepting an electron that reaches the bottom of the electron transport chain from PS II.
7 Photoexcited electrons are passed in a series of redox reactions from the primary electron acceptor of PS I down a second electron transport chain through the protein ferredoxin (Fd). (This chain does not create a proton gradient and thus does not produce ATP.)
8 The enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+. Two electrons are required for its reduction to NADPH. Electrons in NADPH are at a higher energy level than they are in water (where they started), so they are more readily available for the reactions of the Calvin cycle. This process also removes an H+
from the stroma.
see textbook for diagram
Describe how ATP and NADPH are made in photosynthesis
chemiosmosis- An electron
transport chain pumps protons (H+ ) across a membrane
as electrons are passed through a series of carriers that have
progressively more affinity for electrons. Thus, electron transport
chains transform redox energy to a proton-motive force,
potential energy stored in the form of an H+ gradient across a
membrane. An ATP synthase complex in the same membrane
couples the diffusion of hydrogen ions down their gradient to
the phosphorylation of ADP, forming ATP.
NADPH is a product of the light reaction as well and is made as part of the ‘z scheme’ linear electron flow - energising of two types of photosystems. The enzyme NADP+ reductase catalyzed the reduction of NAPH+ by transferring 2 electrons from Fd to NAPH+. This forms NADPH. Electrons in
pg 200 in textbook for diagram
Describe the dark reactions of photosynthesis
Calvin cycle Take place in the stroma • Use ATP and NADPH to convert CO2 to the sugar G3P • Return ADP, inorganic phosphate, and NADP+ to the light reactions
detail extra:
3 phases
Phase 1: Carbon fixation -Gaseous CO2 and the 5-carbon sugar ribulose 1,5-bisphosphate form two
molecules of 3-phosphoglycerate (PGA). The enzyme that catalyses this is rubisco.
Phase 2: reduction -
Phase 3: regeneration of the CO2 acceptor (RuBP)
In a complex series of reactions, the carbon skeletons of
five molecules of G3P are rearranged by the last steps of the Calvin cycle into three molecules of RuBP. To accomplish
this, the cycle spends three more molecules of ATP. The RuBP is now prepared to receive CO2 again, and the cycle continues.
What are the basic reactions in C3, C4 and CAM metabolisms
On dry, hot days, C3 plants close their stomata, conserving
water but keeping CO2 out and O2 in. Under these conditions,
photorespiration can occur: Rubisco binds O2 instead of CO2,
consuming ATP and releasing CO2 without producing ATP or carbohydrate.
Photorespiration may be an evolutionary relic, and it
may play a photoprotective role.
C4 plants minimize the cost of photorespiration by incorporating
CO2 into four-carbon compounds in mesophyll cells. These
compounds are exported to bundle-sheath cells, where they
release carbon dioxide for use in the Calvin cycle.
CAM plants open their stomata at night, incorporating CO2 into
organic acids, which are stored in mesophyll cells. During the day,
the stomata close, and the CO2 is released from the organic acids
for use in the Calvin cycle.
