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CHLOROPLASTS
· Have a double membrane called the CHLOROPLAST
ENVELOPE.
· THYLAKOIDS are fluid-filled sacs that are arranged in
stacks called GRANA which are joined by LAMELLAE.
· The THYLAKOID MEMBRANES contain
PHOTOSYNTHETIC PIGMENTS which are arranged in
structures called PHOTOSYSTEMS.
· Photosystems contain a PRIMARY PIGMENT
(chlorophyll a) and ACCESSORY PIGMENTS (chlorophyll
b and carotene)
· Primary pigments are found at the centre of
photosystems and are known as the PRIMARY PIGMENT
REACTION CENTRE, where electrons are excited in the
light-dependant stage of photosynthesis.…read more

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CHLOROPLASTS
· Accessory pigments surround the primary pigment
and transfer light energy to it.
· There are two types of photosystem. Photosystem
1 has P680 chlorophyll a as its primary pigment
whereas photosystem 2 has P700 chlorophyll a.
· The small number corresponds to the wavelength
of light in nm that the chlorophyll absorbs best.
· The grana are surrounded by the STROMA which is
a fluid containing the enzymes required for the
light-independent stage of photosynthesis (the
Calvin cycle), aswell as the chloroplast DNA.…read more

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A CHLOROPLAST…read more

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THE LIGHT-DEPENDANT STAGE
· The light dependant stage of photosynthesis is the 1st stage and
takes place in the thylakoids. It can be cyclic or non-cyclic.
· NON-CYCLIC
1. Water is broken down into hydrogen ions, electrons and oxygen,
using light energy at PHOTOSYSTEM II. This is called
PHOTOLYSIS.
2. Electrons at PHOTOSYSTEM II gain energy from light and become
EXCITED. They are then emitted from the photosystem.
3. The electrons are accepted by ELECTRON ACCEPTORS and
passed to a series of ELECTRON CARRIERS in the ELECTRON
TRANSPORT CHAIN.
4. As they move along the electron transport chain the electrons
lose energy.
5. This energy is used by a COENZYME associated with each
ELECTRON CARRIER to pump hydrogen ions across the
THYLAKOID MEMBRANE into the THYLAKOID SPACE.…read more

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THE LIGHT-DEPENDANT STAGE
· 6. These hydrogen ions build up in the THYLAKOID SPACE
and a concentration gradient develops.
· 7. The only way that the hydrogen ions can diffuse out of
the thylakoid space is through an ATP SYNTHASE ENZYME
across the membrane, this is called CHEMIOSMOSIS (the flow
of ions across a membrane).
· 8. As the hydrogen ions (protons) flow through the enzyme,
the PROTON MOTIVE FORCE causes the enzyme to rotate,
driving the synthesis of ADP and inorganic phosphate to
produce ATP.
· 9. When the electrons reach photosystem I from the
electron transport chain, they gain energy from light again
and become excited.
· 10. These electrons are used in conjunction with the
hydrogen ions from the PHOTOLYSIS of water to REDUCE
NADP TO REDUCEDNADP.…read more

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