Finally got round to learning all this. Enjoy.

2. What is photosynthesis?
• A process in which light energy from the sun is
transformed into chemical energy.
• It is then used to make large ORGANIC
molecules from INORGANIC substances.

3. Where does it take place?
• In chloroplasts (2-10 nano metres long)
• They contain the photosynthetic pigments
(CHLOROPHYLL) which allows plants to carry
out photosynthesis.
• This is also what gives them the green
colouration, as they REFLECT green
wavelengths of light!

4. Structure of a choloroplast

5. Adaptations
• Inner membrane has TRANSPORT PROTEINS.
• (Many) grana provide large SA and
photosynthetic pigments are arranged into
PHOTOSYSTEMS = max light absorption!
• Proteins in grana hold photosystems in place
• Grana surrounded by stroma so products can
pass directly into stroma.

6. Photosynthetic Pigments
• Absorb certain wavelengths of light –
• In THYLAKOID MEMBRANES in a funnel
shaped structures called PHOTOSYSTEMS, held
in place by proteins.
• Primary pigment = chlorophyll a
• Accessory pigments = chlorophyll b and
CAROTENOIDS.

8. Chlorophylls
• Chlorophyll = mixture of pigments
• Light hitting chlorophyll causes two electrons
associated with Mg to become excited.
• 2 forms – Chlorophyll a (P680) and chlorophyll
b (P700)
• Both absorb red light at diff. wavelengths
• Both found at centre of photosystems =
PRIMARY PIGMENT REACTION CENTRE.

9. Accessory Pigments
• CAROTENOIDS – reflect yellow and orange
light, absorb blue light.
• Absorb wavelengths of light which are not
well absorbed by cholorophylls.
• They pass the energy on to them at the base
of the photosystem.
• Main ones = Carotene and xanthophyll.

10. Light-dependant stage
• Takes place on thylakoid membranes…

11. L-D Stage
• PS I occurs mainly on the INTERGRANAL
lamellae and PS II occurs almost exclusively on
the GRANAL lamellae.

12. The role of water
• PS II contains an enzyme which, in the
presence of light, can split water into protons,
electrons and oxygen = PHOTOLYSIS.
• Water = source of H ions used in
CHEMIOSMOSIS to produce ATP.
• Source of electrons to replace those lost by
oxidised chlorophyll
• Keeps cells TURGID.

13. Photophosphorylation
• When a photon hits a chlorophyll molecule, it’s
energy is transferred to two electrons and they
become excited (Mg).
• These electrons are captured by ELECTRON
ACCEPTORS and are passed along a series of
ELECTRON CARRIERS embedded in the thylakoid
membranes.
• Energy is released as electrons pass along the
chain as this pumps protons across the thylakoid
membrane and into the thylakoid space where
they accumulate.

14. Photophosphorylation continued…
• A proton gradient is formed across the thylakoid
membrane and the protons flow down the
gradient, through channels associate with ATP
synthase enzymes = CHEMIOSMOSIS.
• It produces a force which joins ADP and Pi to
make ATP. The kinetic energy produces from the
flow is converted to chemical energy – used in
the L-I stage.
• The making of ATP using light energy is known as
PHOTOPHOSPHORYLATION.
• There are 2 types, cyclic and non-cyclic.

15. Cyclic photophosphorylation
• Only uses PS I
• Excited electrons pass to an electron acceptor
and back to the chlorophyll molecule from
which they were lost.
• No photolysis but small amounts of ATP made.
• May be used in guard cells to bring in K ions,
lowering water potential…causing guard cells
to swell and open the stomata.

16. Non-cyclic photophosphorylation
• Involves PS I and PS II
• Light strikes PS II, exciting 2 x Mg electrons
which leave the PRIMARY PIGMENT REACTION
CENTRE >>> they pass along a chain of
electron carriers and the energy released is
used to synthesise ATP.
• Light also strikes PS I and a pair of electrons
has been lost, which, along with protons, join
NADP which becomes rNADP.

18. The light-independent stage
• Takes place in the stroma of chloroplasts.
• Can also be called the CALVIN CYCLE.
• Light is not directly used, but the calvin cycle
cannot work if light is not available.

20. The role of CO₂
• It is the source of carbon and oxygen for the
production of all large organic molecules.
• These molecules are used as structures, or act
as energy stores or sources.

21. The Calvin Cycle
1. CO₂ from the air DIFFUSES into the leaf via the
STOMATA and diffuses through air spaces until it
reaches the CHLOROPLAST ENVELOPE and
moves into the STROMA.
2. Combines with RuBP ( catalysed by RUBISCO)
3. Product = GP
4. GP is reduced and phosphorylated into TP. (This
process uses ATP and rNADP.)
5. 5/6 TP molecules are recycled by
phosphorylation, using ATP from the L-I Stage, to
3 molecules of RuBP (5C).

22. How the products are used
• Some GP can be used to make amino acids.
• Pairs of TP combine to make hexose sugars,
such as glucose.
• Hexose sugars can be polymerised into other
carbs such as cellulose and starch.
• Glucose (isomerised) = fructose
• Glucose + fructose = sucrose

23. Limiting factors
• Light intensity increases – stomata open, CO₂
enters leaves, trapped by chlorophyll, splits
water molecules to produce protons.
• Temp – above 25 degrees RUBISCO does not
work as well, more water loss from stomata =
stress response…stomata close, limiting the
availability of CO₂.