Photosynthesis Stores Energy in Organic Compounds
Overview of Photosynthesis:
- Chlorophyll molecules and other pigments located within the chloroplasts
are able to absorb solar energy
- Once the energy is in a chemical form, it can be transported throughout the
cell and to other parts of the plant. It can also be stored.
- Pigments and chemicals are arranged in the chloroplasts to make tasks
operate efficiently.
Stages of Photosynthesis:
- Stage 1: Capturing solar energy and transferring it to electrons
- Stage 2: Using captured solar energy to make ATP and to transfer highenergy electrons to NADP+ (which yields) NADPH, which is then used as a
high energy electron carrier molecule.
o NADP+: (Nicotinamide Adenine dinucleotide Phosphate) also
participates in cellular reactions as energy
o At several places during photosynthesis, NADP+ accepts one
hydrogen atom and two electrons to form NADPH.
o NADPH may donate electrons to other molecules in the cell,
becoming NADP+ again.
- Stage 3: Using energy stored in ATP and high-energy electrons, carried by
NADPH to form energy-rich organic molecules, such as glucose, from CO2.
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Light Dependent Reactions:
- The first two stages must be in the presence of light (light dependent
reactions)
- These reactions require chlorophyll and occur on the thylakoid
membranes in chloroplasts.
- Chlorophyll absorbs the light energy that is eventually transferred to
carbohydrate molecules in the last stage of the process.
- The reactions of the third stage result in carbon fixation-incorporation of
carbon (CO2) into organic compounds like glucose.
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- Chlorophyll and other pigments are arranged in the thylakoid membranes
in clusters called photosystems.
o Chloroplasts of plants and algae have 2 photosystems:
 Photosystem 1 (PS1)
 Photosystem 2 (PS2)
 Named for when they were discovered, not for their sequence
in the process of photosynthesis.
 Each photosystem is made up of chlorophyll pigment
molecules, carotenoid molecules and a molecule that accepts
electrons (electron acceptor).
 All the pigment molecules pass the energy to an electron
accepting chlorophyll A molecule (the reaction centre).
 When the reaction centre (PS2) has received the energy, the
electron is known as “excited” (raised to a higher energy level)
 Before the PS2 can absorb more light energy to excite an
electron, this electron has to be replaced (a water molecule is
split, the oxygen molecule is released by the plant, and the
extra electron is released and replaced in the reaction centre).
 The first electron leaves PS2 and is then passed to an electronaccepting molecule (electron acceptor)
 From this molecule, the electron is passed through a series of
electron-carrying molecules (Electron Transport System).
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Each transfer releases a small amount of energy (which is used to push
hydrogen ions – from the water split – into the area inside the thylakoid.)
The energy is stored in the potential energy of the hydrogen ions. This
energy will be used to generate ATP and free phosphate groups.
The Electron Transport System passes the electron along to another
reaction centre (PS1) which is also absorbing light energy.
In order for PS1 to absorb more light energy to excite an electron, the first
electron needs to be passed to a high-energy electron-acceptor.
The electron is now used to reduce (think GER) NADP+ to form NADPH. The
reducing power of NADPH will be used in the light-independent reactions.
The light-independent reactions result in changing CO2 into organic
compounds like glucose.
Chemiosmosis
- During the light dependent
reactions, water molecules are split
into electrons (used during the
Electron Transport System), oxygen
atoms (released by the plant into
the air) and hydrogen ions
(temporarily stored in the thylakoid
membrane).
- Once the hydrogen ions are
transported into the inner space of
the thylakoid, they can’t diffuse
back across the membrane to the
stroma. (The membrane is impermeable to charged particles)
- ATP synthase (special structure embedded in the thylakoid membrane)
moves the hydrogen ions down the concentration gradient.
- The energy of the gradient is used to generate ATP molecules.
- Chemiosmosis: the connection of the movement of the hydrogen ions to
the production of ATP.
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Figure Below: H+ ions are released into the thylakoid lumen (space inside the
thylakoid) as electrons are removed from water. As these elections are passed
along an electron transport chain, additional H+ ions are pumped into the lumen.
H+ ions accumulate in the lumen, increasing the gradient charge and H+ ion
concentration across the thylakoid membrane. As the concentration and electrical
gradients begin to build, H+ ions move from the lumen to the stroma through
special protein channels (ATP Synthase) in the thylakoid membrane. The ion flow
drives enzymes that convert ADP and P into ATP.
http://www.youtube.com/watch?v=Oi2_n2wbB9o&feature=related
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