F214 OCR Biology - Photosynthesis

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Module 3 ­ Photosynthesis
Autotroph ­ Autotroph are organisms that can synthesise complex molecules like carbohydrates,
lipids, proteins, nucleic acid and vitamins from inorganic molecules as a source of energy. They also
use light energy and chemical energy. Heterotroph ­ are organisms that ingest and digest complex
organic molecules, to release the chemical potential energy stored within them.
Light energy is used during photosynthesis to produce complex organic molecules
Respiration in plants and animals depends upon the products of photosynthesis as Oxygen is
needed for aerobic respiration. It allows autotrophs and heterotrophs to release the chemical
potential energy in complex organic molecules which were made during photosynthesis.
Photosynthesis is a two-stage process taking place in the chloroplasts which is the light-dependant
stage and the light-independent stage
Chloroplasts carry out their functions as the thylakoids have a large surface area so they can absorb
more light in the light-dependant stage. The chloroplast envelope keeps the reactants close to their
reaction sites and in the stoma contains enzymes, proteins and organic molecules to be used in the
light-dependant stage.
Photosynthetic pigment is a colourless molecule which absorbs light energy, these photosynthetic
pigments are attached by proteins to the photosystem which are in the thylakoid membrane and
arranged in a funnel shape. There are primary and accessory pigments, the primary pigments are
chlorophyll a and absorbs the wavelength of 450nm and chlorophyll b at 500-640nm and appears
blue/green. Photosystem 2 absorbs 680nm and photosystem 1 absorbs 700 nm. The accessory
pigments include carotenoids and xanthophyll.
The importance of photosynthetic pigments is that they absorb different wavelengths of light and
convert it into chemical energy in the light-dependant stage. Light needs to excite the electrons in
non-cyclic photophosphorylation to get them to move to a higher energy level and move along the
electron transport carriers.
The light-dependant stage takes place in the thylakoid membranes and light-independent takes
place in the stroma.
In the light-dependant stage, light energy is absorbed by the photosynthetic pigments in the
photosystems and converted into chemical energy. This energy excites the electrons to move to a
higher energy level and moves along the electron transport chain to PS1, as they move they need to
be replaced in PS2, so photolysis of water occurs and splits it into electrons, protons and oxygen. As
the electrons move along the electron transport chain, they lose energy. The energy produced
pumps protons into the thylakoid membrane from the stroma, creating a proton gradient as the
concentration of protons is higher in the thylakoid membrane than in the stroma. As the protons
move back to the stroma, ATP synthase catalyses the reaction. The movement of protons across
the membrane is chemiosmosis and produces reduced NADP from NADP and ATP from ADP + Pi.
When the electrons reach PS1, they are excited to an even higher energy level.
In cyclic photophosphorylation the excited electron pass to an electron acceptor and back to the
original chlorophyll molecule, small amounts of ATP are made. No reduced NADP is produced but it
used from non-cyclic photophosphorylation.
In the light-dependant stage, water undergoes photolysis to produce protons, electrons and oxygen
needed. The protons are needed to produce ATP and reduced NADP as they are hydrogen ions.
Electrons are used to replace those that are oxidised.

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In the light-independent stage which is the Calvin cycle, CO2 enters the leaf via the stomata and
diffuses into the stroma of the chloroplast. The carbon dioxide binds with a 5 carbon molecule of
ribulose biphosphate and produces an unstable 6 carbon molecule. This reaction is catalysed by
the enzyme ribulose biphosphate carboxylase. This unstable 6 carbon molecule is then broken
down into 2 molecules of triose phosphate. The TP is then converted into another 3 carbon
molecule of GP.…read more


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