Download Chloroplasts

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BaConCell lecture notes
May 06, 2017, 11:22 PM
Page 1
Chloroplasts
Chloroplasts are essential to virtually all life - "we all burn by the power of the leaf",
a quotation I learnt years ago and have since forgotten the source. Of course we
use photosynthesis indirectly, either eating plants or eating animals that have eaten
plants. Whichever, the primary source of energy in the food chain for almost all life
on earth is photosynthesis. Thus, virtually all the organic materials required by living
cells have been and are still being produced by photosynthetic organisms.
In the plant cell, photosynthesis takes place in a specialised organelle, the
chloroplast. The products of photosynthesis are used directly for biosynthesis and
are also converted to low molecular weight sugars (usually sucrose) that is exported
to meet the metabolic needs of other non-photosynthetic cells in the plant. Much of
these 'photosynthates' are stored as starch, especially in seeds, roots, and
specialised storage organs, and we eat a lot of these to recover that energy source .
Unlike mitochondria, chloroplasts are members of a family of organelles called the
plastids, which are present in all living plant cells. They share many features,
including being enclosed by an envelope of two concentric membranes. All plastids
develop from pro-plastids.
Chloroplasts resemble mitochondria. Much like mitochondria, they have a highly
permeable outer membrane and a much less permeable inner membrane, which
surrounds an inner space called the stroma, which is analogous to the mitochondrial
matrix, and contains various enzymes, ribosomes, RNA and DNA. However, unlike
mitochondria, where the inner membrane is folded into cristae and holds the electron
transport chain, chloroplasts have one more compartment on a third membrane that
consists of flat sacs called thylakoids, which hold not only the electron transport
chain and ATP synthase, but also the photosynthetic light absorbing system that
harvests light energy. The thylakoids are joined to each other, and so the lumen of
the thylakoids is called the thylakoid space, and is thus separated from the stroma
by the thylakoid membrane.
Two main reactions occur during photosynthesis: the photosynthetic electrontransfer reactions (light reactions) and the carbon fixation reactions (dark reactions).
In the photosynthetic electron-transfer reactions, energy derived from sunlight
energises an electron in chlorophyll, enabling the electron to move along an electron
transport chain in the thylakoid membrane, similar to the chain in the inner
membrane of mitochondria. Chlorophyll obtains its electrons from H2O and liberates
O2. Thus, the reverse to the reactions in mitochondria, which utilise O 2 and liberate
H2O.
In the carbon fixation reactions, the ATP and NADPH produced by the photosynthetic
reactions serve as the source of energy and reducing power (respectively) to drive
the reactions that convert CO2 to carbohydrate. This carbon fixation is catalysed by
ribulose bisphosphate carboxylase, which catalyses the combination of CO2 from the
air with ribulose 1,5-bisphosphate to give two molecules of 3-phosphoglycerate,
which is used to form sugars, fatty acids and amino acids. Much of the 3phosphoglycerate is used to form starch, which is a carbohydrate reserve in plants,
much as glycogen is used in animals. For transport around the plant, the main form
of energy is sucrose, similar to animals in which glucose is transported around the
bloodstream.
Carbon fixation in plants is compartmentalised in different ways according to the
adaptation of the plant to different climatic and environmental regimes. Eg C3, C4
and CAM plants.
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BaConCell lecture notes
May 06, 2017, 11:22 PM
Page 2
The process of energy conversion begins when a chlorophyll molecule is excited by a
quantum of light (a photon) and an electron is moved from one molecular orbital to
another of higher energy. This excited molecule is unstable and can return to its
original state in one of three ways: by converting the extra energy into heat; by
transferring the energy (but not the electron) directly to a neighbouring chlorophyll
molecule (resonance energy transfer); by transferring the high energy electron to
another nearby molecule (an electron acceptor) and then returning to its original state
by accepting a low energy electron from another neighbour (an electron donor).
A photosystem consists of two closely-linked components, a photochemical reaction
centre (a complex of proteins and chlorophyll molecules) and an antenna complex
(pigment molecules that capture light energy and feed it to the reaction centre).
The antenna complex is important for capturing light and consists of several hundred
chlorophyll molecules linked together by proteins and held tightly to the thylakoid
membrane. When a chlorophyll molecule is excited by light, the energy is transferred
by resonance energy transfer to the reaction centre. The antenna molecules thus acts
as a funnel of energy.
The photochemical reaction centre is a trans-membrane protein pigment that consists
of a special pair of chlorophyll molecules that act as an irreversible trap for excitation
quanta because its excited electron is immediately passed to a chain of electron
acceptors.
By moving this high energy immediately away to a more stable
environment, the electron is more suitably positioned for subsequent photochemical
reactions that take time to complete.
Photosynthesis produces both ATP and NADPH directly by a two-step process called
non-cyclic photophosphorylation. The two photosystems enable the electron to be
passed all the way from water to NADPH. As the electrons pass through the
photosystems to generate NADPH, some of the energy is used for ATP synthesis. In
Photosystem II, the oxygens of two water molecules bind to a cluster of manganese
atoms in an enzyme that enables electrons to be moved one at a time to fill the holes
created by light in the chlorophyll molecules in the reaction centre. As soon as four
electrons have been removed (requiring four quanta of light) oxygen is released.
Thus Photosystem II catalyses 2H2O + 4 photons to give 4H+ + 4e- + O2. Quinone
molecules in the membrane pass their electrons to an H+ pump called the b6-f
complex, which pumps H+ into the thylakoid space across the membrane, and the
resulting electrochemical gradient drives the synthesis of ATP by an ATP synthase.
The final electron acceptor in this chain is Photosystem I.
Photosystem I accepts an electron into the hole created by light in the chlorophyll
molecule in its reaction centre. The electron is boosted to a very high energy level
that allows it to be passed into the iron-sulphur centre of ferredoxin, and then to
NADP+ to generate NADPH. This final step involves the uptake of H+ from the
medium.
The scheme is known as the Z scheme. By means of the two electron-energising
steps, an electron is passed from water to NADPH. The two photosystems produce
enough energy to pump H+ across the thylakoid membrane allowing ATP synthase to
harness some of the light energy for ATP production.
The chloroplast also carries out a number of other biosynthetic pathways, including
production of all of the cell's fatty acids and a number of amino acids, and also the
conversion of nitrite (NO2-) to ammonia (NH3) which is used for the synthesis of
amino acids and nucleotides.