Download Photosynthesis

Photosynthesis
• the process by which light energy is converted
to chemical bond energy and carbon is fixed
into organic compounds. The general formula
is:
Where does the energy to synthesise
ATP come from?
• Catabolic (breakdown) reactions
• Redox (reduction/oxidation) reactions
• The main way in which ATP is synthesised is by the removal of hydrogen
atoms from intermediate compounds in a metabolic pathway
• When two hydrogen atoms are removed from a compound, they are
picked up by a HYDROGEN CARRIER or ACCEPTOR
• We say the hydrogen carrier is reduced
• Electrons from the hydrogen atoms are passed along carriers (Electron
Transport Chain)
• When a component of the chain receives one of the hydrogen atoms, we
say it is REDUCED
• When a component passes an electron on, we say it is OXIDIZED
• Each of these redox reactions releases a small amount of energy and this
energy is used to synthesise ATP
PHOTOSYNTHETIC PIGMENTS-absorb light energy and use it
to provide energy to carry out photosynthesis
• Plants contain two major groups of
pigments, the chlorophylls and
carotenoids.
• Chlorophyll a whose double bonds
play a critical role in the light
reactions & are a source of the
electrons that flow through the
electron transport chains during
photosynthesis.)
• Chlorophyll b are green and
absorb all wavelengths of light in
the red, blue, and violet range.
PHOTOSYNTHETIC PIGMENTS cont.• Carotenoids are yellow, orange, and red. They
absorb light in the blue, green, and violet
range.
• Xanthophyll is a carotenoid with a slight
chemical variation.
Chloroplasts
• stroma, where the lightindependent reactions
occur.
• grana, where the light
reactions occur.
• The grana consist of layers
of membranes called
thylakoids- the site of
photosystems I and II.
• The chloroplast is enclosed
by a double membrane.
Two main processes of photosynthesis
• light dependent or light reactions-use
light energy directly to produce ATP that
powers the light-independent reactions
• light independent reactions-consist of
the Calvin cycle, which produces sugar.
• To power the production of sugar, the Calvin
cycle uses the ATP formed during the light
reactions.
Both reactions occur only when light is present.
PHOTOSYSTEMS
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Light-harvesting complexes in the thylakoid membranes of chloroplasts.
(a few hundred in each thylakoid)
Consists of a reaction center containing chlorophyll a and a region
containing several hundred antenna pigment molecules that funnel energy
into chlorophyll a.
Two types cooperate in the light reactions of photosynthesis, PS I and PS
II. (named in the order that they were discovered)
PS II operates first, followed by PS I.
PS I absorbs light best in the 700 nm range; also called P700.
PS II absorbs light best in the 680 nm range; also called P680.
LIGHT-DEPENDENT REACTIONS—THE LIGHT
REACTIONS
• Light is absorbed by the photosystems (PS II & I)
in the thylakoid membranes and electrons flow
through electron transport chains.
• there are two possible routes for electron flow:
noncyclic and cyclic photophosphorylation.
Noncyclic
Photophosphorylation
• During N.P.- electrons enter two
electron transport chains, and ATP
and NADPH are formed. The process
begins in PS II and proceeds through
the following steps:
• Photosystem II—P680. Energy is
absorbed by P680. Electrons from
the double bonds in the head of
chlorophyll a become energized and
move to a higher energy level. They
are captured by a primary electron
acceptor.
Photosystem II—P680 (cont.)
• Photolysis. Water gets split apart, providing
electrons to replace those lost from
chlorophyll a in P680.
• into two electrons, two protons, and one
oxygen atom.
• Two oxygen atoms combine to form one 02
molecule, which is released into the air as a
waste product of photosynthesis.
Photosystem II- P680 (cont.)
• Electron transport chain- Electrons from P680
pass along an electron transport chain
consisting of plastoquinone (PQ), a complex of
two cytochromes and several other proteins,
and ultimately end up in P700 (PSI).
• This flow of electrons is exergonic and
provides energy to produce ATP by
chemiosmosis.
• Because this ATP synthesis is powered by light,
it is called photophosphorylation.
Chemiosmosis
• The process by which ATP is formed during the light
reactions.
• Protons that were released from water during photolysis are pumped by the thylakoid membrane from
the stroma into the thylakoid space (lumen). ATP is
formed as these protons diffuse down the gradient
from the thylakoid space, through the ATP-synthase
channels, and into the stroma.
• The ATP produced here provides energy for the Calvin
cycle.
• NADP becomes reduced when it picks up the two
protons that were released from water in P680. Newly
formed NADPH carries hydrogen to the Calvin cycle.
• photosystem II
Photosystem I—P700
• Energy is absorbed by P700. Electrons from the
head of chlorophyll a become energized and are
captured by a primary electron receptor.( similar
to the way it happens in P680.)
• Two differences are:
1. the electrons that escape from chlorophyll a are
replaced with electrons from photosystem II,
instead of from water.
2. This electron transport chain contains ferrodoxin
and ends with the production of NADPH, not
ATP.
Cyclic Photophosphorylation
• The purpose of cyclic photophosphorylation is to produce
ATP. No NADPH is produced, and no oxygen is released.
• The production of sugar during the Calvin cycle consumes
enormous amounts of ATP, periodically, the chloroplast
runs low on ATP. When it does, the chloroplast carries out
cyclic photophosphorylation to replenish the ATP levels.
• Cyclic electron flow takes photo-excited electrons on a
short-circuit pathway. Electrons travel from the P680
electron transport chain to P700, to a primary electron
acceptor, and then back to the cytochrome complex in the
P680 electron transport chain