Getting it down!

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.

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!

Structure of a choloroplast

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.

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.

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.

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.

Light-dependant stage

• Takes place on thylakoid membranes…

L-D Stage

• PS I occurs mainly on the INTERGRANAL lamellae and PS II occurs almost exclusively on the GRANAL lamellae.

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.

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.

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.

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.

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.

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.

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.

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 = GP4. 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).

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

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.