Ch 10.1: Photosynthesis Flashcards
What happens when a pigment molecule absorbs light?
one of its electrons goes from a ground state to an excited state, which is unstable
What happens to an excited chlorophyll molecule in isolation?
excited electrons fall back to the ground state, releasing excess energy as heat or light, an afterglow called flourescence
What is a photosystem?
consists of a reaction-center complex surrounded by light-harvesting complexes
What is a reaction-center complex?
an association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor
What is the light-harvesting complex?
it consists of various pigment molecules bound to proteins
What does the light-harvesting complex do?
is transfers the energy of photons to the chlorophyll a molecules in the reaction-center complex
How are these chlorophyll a molecules special?
they are special because they can transfer an excited electron to a different molecule
What accepts the excited electron?
a primary electron acceptor in the reaction center accepts the excited electrons and is reduced as a result
What is the first step of the light reactions?
solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor
What are the two types of photosystem in the thylakoid membrane?
photosystem II and photosystem I
Photosystem II
called P680 because its reaction-center chlorophyll a is best at absorbing light with a wavelength of 680 nm
Photosystem I
called P700 because its reaction-center chlorophyll a is best at absorbing light with a wavelength of 700 nm
What are the two possible routes for electron flow during the light reactions?
cyclic and linear
What is the linear electron flow?
the primary pathway, involves both photosystems and produces ATP and NADPH using light energy
First step in linear electron flow:
a photon hits a pigment in a light-harvesting complex of PS II, and its energy is passed among pigment molecules until it excites P680
Second step in linear electron flow:
an excited electron from P680 is transferred to the primary electron acceptor; we refer to the oxidized form as P680+
Third step in linear electron flow:
an enzyme catalyzes the split of H20 into two electrons, two hydrogen ions (H+), and an oxygen atom. The electrons are transferred to the P680+ pair, reducing it back to P680. The H+ are released into the thylakoid space. The oxygen atom combines with another oxygen atom generated by the splitting of a different H20 and forms 02
Fourth step in linear electron flow:
electrons are passed in a series of redox reactions from the primary electron acceptor of PS II down an electron transport chain to PS I
Fifth step in linear electron flow:
potential energy stored in the proton gradient drives production of ATP by chemiosmosis
Sixth step in linear electron flow:
in PS I, transferred light energy excites P700, which loses an electron to the primary electron acceptor, P700+ accepts an electron passed down from PS II via the electron transport chain
Seventh step in linear electron flow:
electrons are passed from the primary electron acceptor of PS I down a second electron transport chain to the protein ferredoxin (Fd)
Eighth step in linear electron flow:
the enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+
What is the cyclic electron flow?
photoexcited electrons cycle back from Fd to the cytochrome complex instead of being transferred to NADP+, uses only photosystem I, produces ATP, but no NADPH or oxygen results from this process
Similarities of chloroplasts and mitochondria
chloroplasts and mitochondria both generate ATP by chemiosmosis, some of the electron carriers, including iron-containing proteins called cytochromes, are very similar in mitochondria and chloroplasts, and the ATP synthase complexes are also very similar.
What are the differences between photophosphorylation and oxidative phosphorylation?
in chloroplasts, high-energy electrons drop down the transport chain from water, while in mitochondria, they are extracted from organic molecules. Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP
What happens during the calvin cycle?
the ca;vin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar
Is the calvin cycle anabolic or catabolic?
its anabolic, it builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH
What is the name of the made sugar in the calvin cycle?
glyceraldehyde 3-phosphate (G3P)
How many times must the calvin cycle go around for net synthesis of one G3P?
the cycle must take place three times, fixing three molecules of CO2, one for each turn of the cycle
What are the three phases of the calvin cycle?
carbon fixation, reduction, and regeneration of the CO2 acceptor
Phase 1: Carbon fixation
the binding of CO2 to a five-carbon sugar named ribulose bisphosphate (RuBP) is catalyzed by RuBP carboxylase-oxygenase, or rubisco, the six-carbon intermediate molecules is immediately split into two molecues of 3-phosphoglycerate
Phase 2: Reduction
each molecule of 3-phosphoglycerate is altered through phosphorylation by six ATP and reduction by six NADPH to ultimately produce a G3P sugar
Phase 3: Regeneration of the CO2 acceptor (RuBP)
the remaining five molecules of G3P are rearranged in a complex series of reactions yielding three molecules of RuBP and three additional molecules of ATP are used to facilitate the regeneration of RuBP
What does the calvin cycle consume for the net synthesis of one G3P molecule?
the calvin cycle consumes nine molecules of ATP and six molecules of NADPH
